Anatomy of siliceous hot springs: examples from Yellowstone National Park, Wyoming, USA

Sedimentary Geology 157 (2003) 71 – 106 www.elsevier.com/locate/sedgeo

Anatomy of siliceous hot springs: examples from Yellowstone National Park, Wyoming, USA Sean A. Guidry *, Henry S. Chafetz Department of Geosciences, University of Houston, SR-1 Rm. 312, University Park, Houston, TX 77204-5007, USA Received 7 June 2001; accepted 29 March 2002

Abstract Numerous siliceous hot spring systems in the Norris and Lower Geyser Basins of Yellowstone National Park, Wyoming, provide insights into spring geometries, depositional facies, and lithofacies associated with modern hot springs. Analyses of active (Cistern Spring, Octopus Spring, Deerbone Spring, and Spindle Geyser) and inactive (Pork Chop Geyser) siliceous hot springs have facilitated the construction of a facies model for siliceous hot spring deposits at Yellowstone. Yellowstone’s siliceous springs tend to group into four broad morphological categories: siliceous spires and cones, domal mounds, terraced mounds, and ponds. Siliceous spires/cones are subconical accumulations up to 5 – 7 m high and about 2 m in diameter, and are common deposits in Yellowstone Lake. Domal mounds are characterized by siliceous precipitates with a broad lens or shield geometry (2 – 3 m in vertical relief), discharge channels, and an areal accumulation of approximately 150 m2. In contrast, terraced mounds have a stair-step morphology, a substantial pool ( f 8 – 10 m in diameter), ‘‘shrubby’’ precipitates, and occupy areas of f 2000 m2. Siliceous ponds are variable in size, have little outflow, and exhibit low amounts of silica precipitation. Of these morphological varieties, domal mounds and terraced mounds are thought to have the best long-term preservation potential. The four spring morphotypes are composed of up to eight cumulative hot spring depositional facies: (1) vent ( > 95 jC), (2) proximal vent ( < 95 jC), (3) pool ( f 80 – 90 jC), (4) pool margin ( f 80 jC), (5) pool eddy ( < 80 jC), (6) discharge channel/flowpath ( < 80 jC to ambient), (7) debris apron (variable temperatures), and (8) geyser (variable temperatures). This facies model based on numerous springs facilitates our ability to interpret ancient hot spring deposits and to infer depositional conditions. Precipitation of siliceous sinter is the result of abiotic and biotic processes. Abiotic precipitational processes are dominant in the vent area, whereas biotic influences on the precipitate fabric become progressively more important downflow. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Hot springs; Siliceous sinter; Yellowstone; Microbes; Chalcedony

1. Introduction and significance Research into the sedimentological aspects of hot springs has recently gained impetus. The importance of these depositional environments is twofold: (1) *

Corresponding author. Tel.: +1-713-743-3417; fax: +1-713748-7906. E-mail address: [email protected] (S.A. Guidry).

there are significant gaps in our knowledge concerning this depositional environment and, therefore, our ability to properly interpret similar deposits in the rock record; and, (2) modern hot springs can provide insights into analogous environments on a primeval Earth and primitive biosystems. Establishing detailed facies relationships in modern hot springs has obvious significance in facilitating our ability to recognize relict deposits in the rock record. Distinct facies

0037-0738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0037-0738(02)00195-1

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patterns reflect specific temperature and hydrochemical regimes and, therefore, delineate the areal extent of hydrothermal activity. Fouke et al. (2000) demonstrated the utility of this facies approach in the carbonate systems of Yellowstone. Thus, a facies model for siliceous hot springs allows direct spatial and temporal comparisons with other hot spring systems. From a paleontological perspective, much of our knowledge of early life on Earth has been gleaned from bacterial fossils in Precambrian cherts (Schopf and Fairchild, 1973; Schopf and Packer, 1987; Westall et al., 1995). Some of these cherts are thought to have originated as thermal deposits analogous to the modern hot spring systems in Yellowstone National Park, Wyoming (Walter, 1976a). Thus, study of hot spring deposits may provide key insights into processes on a primeval Earth and their associated biofacies. Interest in hot spring deposits has also stemmed from economic mineral deposits associated with these features. Numerous studies have detailed the association of precious minerals (e.g., gold) with modern and ancient siliceous sinter deposits (Goldie, 1985; Cunneen and Sillitoe, 1989; Ewers, 1991; White et al., 1992; Fournier et al., 1994b; Zimmerman and Larson, 1994). Bacteria have long been recognized to bind metal ions in solution (Beveridge and Murray, 1980; Beveridge and Fyfe, 1985). Recently, sulfate-reducing bacteria have been documented to precipitate gold and zinc (sphalerite) from solutions containing very low concentrations of these metals (Labrenz et al., 2000). Thus, bacteria from siliceous hot springs may be crucial in actively mediating mineral precipitation and ore enrichment in shallow epithermal deposits. Worldwide, many siliceous hot springs have been documented, including numerous examples from the rock record (White et al., 1989; White, 1992; Hinman and Lindstrom, 1996; Konhauser and Ferris, 1996; Renaut et al., 1996; Walter et al., 1996; Jones and Renaut, 1997; Guo and Riding, 1998; Jones et al., 1998; Renaut et al., 1999; Allen et al., 2000). Thus, observations from Yellowstone’s modern siliceous hot springs can easily be compared with observations from numerous other hot springs to provide a broader, more encompassing, depositional model. An ideal place to investigate pristine hot spring systems is Yellowstone National Park, Wyoming, USA. Hot springs that precipitate amorphous hydrated

silica (opal-A) are the most abundant in Yellowstone (Fournier et al., 1994a; Bryan, 1995), and outnumber geysers by a ratio of approximately 10 to 1 (Rinehart, 1974). These siliceous precipitates, referred to as geyserite or siliceous sinter, form significant accumulations in some of Yellowstone’s geyser basins (Weed, 1889; White et al., 1988; Fournier et al., 1994a). Thermal features in Yellowstone are thought to have been active in the park for at least 100,000 years (White et al., 1988), thereby providing an intermittent historical record of hydrothermal activity in the region. A wealth of information is contained in these packages of siliceous hot spring sediments. In summary, the purpose of this investigation was to observe and document a wide variety of Yellowstone’s siliceous hot spring geometries, their associated precipitate fabrics and biota, and conditions of formation (e.g., temperature regimes) in order to construct a detailed facies model.

2. Silica precipitation mechanisms Although once largely regarded as abiotic precipitates, opal precipitated in hot springs is now recognized as one of many minerals and mineraloids in which biotic and abiotic processes control precipitation. The idea that organisms may play a profound role in mediating precipitation was first suggested for Yellowstone deposits by Weed (1889). After this initial proposal, a bacterially mediated origin was largely disregarded until the pioneering work by Birnbaum and Wireman (1984, 1985). During this approximately 100-year period, abiotic mechanisms for silica precipitation in hot springs were the dogma. Several of the abiotic mechanisms proposed include: (1) rapid cooling, (2) evaporative concentration, (3) a change in pH (Eugster, 1980; Rimstidt and Cole, 1983; Hinman and Lindstrom, 1996; Jones et al., 1997), and (4) cation effects (e.g., aluminum in the solution) (Ichikuni, 1970). Microbial mediation involves two main pathways: active and passive precipitation. In active precipitation, microbes actively induce the precipitation of amorphous silica through some vital mechanism. For example, Birnbaum and Wireman (1984) observed that sulfate-reducing microbes can induce the precipitation of silica by modifying the pH regime through

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their vital activities, thereby overcoming the affects of kinetic inhibitors for opal precipitation (Birnbaum and Wireman, 1984; Birnbaum et al., 1989). The mechanisms of this process are, at best, poorly understood. Precipitation may proceed by hydrogen bonding of amorphous silica and polymerization by siloxane bonds (Leo and Barghoorn, 1976; Iler, 1979; Birnbaum and Wireman, 1985), and is enhanced by cellular proton gradients (Birnbaum and Wireman, 1985). In passive precipitation, an organic substrate provides a template for precipitation. In nature, combinations of biotic and abiotic mechanisms are important in the precipitation of siliceous sinter. Silicification of microbes in a hot spring setting is very rapid. Schultz-Lam et al. (1995) and Renaut et al. (1998) recognized that silicification of the microbes was occurring while they were still alive. Recently, evidence indicates that silicification restricted to the exterior of the bacterial cell is not detrimental to all cells, and studies have shown that some silicified microbes can still be cultured (Phoenix et al., 2000). Other studies (e.g., Krumbein and Werner, 1983) recognized that cyanobacteria preferentially concentrate silica within their filaments (trichomes). This affinity for silica is important for mineralization, and has obvious significance for their long-term preservation potential as fossils. Exquisitely preserved, bacterial filaments have been recognized in Precambrian cherts (Schopf and Fairchild, 1973; Schopf and Packer, 1987; Westall et al., 1995), and may owe their preservation to this propensity to silicify.

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3. Geological setting

Fig. 1. Map of Yellowstone National Park, Wyoming, USA. Major areas of interest include the Norris Geyser Basin and Lower Geyser Basin. The White Creek Group is an eastern extension of the Lower Geyser Basin.

Yellowstone National Park is located in northwestern Wyoming, USA, and encompasses portions of Idaho and Montana (Fig. 1). The strata underlying the park are primarily composed of a thick (3 km) sequence of Paleozoic and Mesozoic conglomerates, sandstones, shales, and limestones deposited in marine settings (Harris et al., 1997). Several episodes of volcanic activity have been followed by caldera collapse (Eaton et al., 1975). Geysers and hot springs are believed to have been active since the retreat of the Pinedale (Pleistocene) glaciers and there is evidence of intermittent activity during earlier interglacial periods (White et al., 1975; Bargar, 1978).

Most of the hot spring water in Yellowstone National Park is simply meteoric water circulating through porous strata, and is derived from the mountains to the north and northwest of the caldera (Lewis et al., 1997). Based on isotopic measurements from each of the thermal basins in Yellowstone, estimates of contributed magmatic water are < 5%, if any (Craig et al., 1956; White, 1967; Fournier, 1989). Deuterium isotopic values closely match that of meteoric water (Craig et al., 1956; Leeman et al., 1977; Lewis et al., 1997). This meteoric water extensively interacts with rocks at temperatures as high as 300 jC (Lewis et al.,

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1997). Without this significant meteoric water component, the spectacular thermal features in Yellowstone National Park would not have developed.

4. Introduction to sampling sites Although numerous siliceous hot springs and geysers were observed over the course of the investigation, two major areas of Yellowstone, the Norris and the Lower Geyser Basin (Fig. 1), were chosen for detailed study. These localities contain a diverse suite of thermal features, encompassing Yellowstone’s oldest, hottest, and largest geyser basins. Detailed field and laboratory measurements were taken at selected sites that exhibited a wide variety of siliceous precipitate styles including Cistern Spring, Pork Chop Geyser, Deerbone Spring, and Spindle Geyser.

4.1. Norris Geyser Basin The Norris Geyser Basin (Figs. 1 and 2A) is Yellowstone’s oldest and hottest geyser basin and has been a site of many previous investigations (e.g., Rowe et al., 1973; White et al., 1975, 1988; Fournier et al., 1992). It is located at the intersection of a 0.6-Ma ring fracture and a north – south trending extensional fault (Norris – Mammoth Corridor); this area is seismically active (Lewis et al., 1997). Several cores (Y-9, Y-12, C-II) were drilled in the basin to assess the hydrothermal resources of the park (White et al., 1975). Despite widespread siliceous sinter at the surface, very little was detected in the cores (White et al., 1975). Thus, the Norris cores provide little insight into the diagenesis of hot spring siliceous sinter. The basin is underlain by the Lava Creek Tuff, a rhyolite deposited 600,000 years BP, which exhibits pervasive

Fig. 2. (A) Detailed map of the Norris Geyser Basin. Cistern Spring and Pork Chop Geyser are located in the Back Basin area of Norris. (B) The White Creek Group of thermal features in the Lower Geyser Basin of Yellowstone. This geyser group is located along Firehole Lake Drive. Tuft Geyser and Deerbone Spring are in very close proximity, whereas Spindle Geyser is located approximately 200 m further up the White Creek valley.

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hydrothermal alteration (White et al., 1988; Lewis et al., 1997). This lack of a thick sinter deposit in such an old geyser basin is probably due to glacial scouring of pre-existing deposits. Numerous sinter cobbles and clasts can be found throughout glacial kames at Norris, thereby providing evidence of glacial reworking (White et al., 1975). Thus, hydrothermal activity has been prevalent since the early stages of Pinedale glaciation (White et al., 1988). The spring waters of the Norris Geyser Basin are complex and their chemical compositions fluctuate over time (Fournier et al., 1986; Lewis et al., 1997). Hot springs in this geyser basin are fed by subsurface reservoirs with at least two different types of water. One type contains neutral pH waters with high chloride, high silica, and low sulfate concentrations (Fournier et al., 1986; White et al., 1988; Fournier et al., 1992; Bryan, 1995). In contrast, some springs are characterized by their low pH, moderate chloride, high silica, and high sulfate concentrations (Fournier et al., 1986; White et al., 1988; Bryan, 1995). As previously mentioned, both Cistern Spring and Pork Chop Geyser have been observed to fluctuate between different water chemistries (Fournier et al., 1986). These fluctuations have great significance in terms of the biota and resultant lithofacies. In addition to a dynamic hydrochemistry, springs in the Norris Geyser Basin have dynamic histories. Commonly, these springs abruptly begin and cease activity. ‘‘In a hot spring environment it only requires a minor change in the hydrothermal plumbing for a pool with a flourishing biota to be invaded with hot water’’ (Trewin, 1996, pp. 140 – 141). Periods of unusual fluctuations in discharge are common, and can precede sporadic changes in spring versus geyser activity. Hydrothermal explosions are a recurring event in the Norris Geyser Basin (White et al., 1988), and attest to the self-sealing nature of the vents. Therefore, the interpretation of any relict hot spring deposit must consider complicated depositional histories and facies distributions. 4.1.1. Cistern Spring Cistern Spring is a siliceous hot spring located in the Back Basin area of the Norris Geyser Basin (Figs. 2A and 3A,B). Although an inconspicuous precursor spring may have existed for as much as 80 years (White et al., 1988), vigorous spring activity was first

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documented in this previously forested area in 1966 (Fournier et al., 1994a). Intermittent geyser activity and fluctuations at Cistern appear to be related to eruptions of Steamboat Geyser (White et al., 1988). During this approximately 25-year period of activity, the spring constructed a terraced mound similar to those at many travertine-depositing hot springs (Chafetz and Folk, 1984). The rate of silica precipitation is f 5 cm/year (White et al., 1988), although true depositional rates vary considerably in different portions of all thermal systems. 4.1.2. Pork Chop Geyser Older siliceous sinter was collected from Pork Chop Geyser, a largely inactive deposit near Cistern Spring (Figs. 2A and 3C). This geyser has had a complex history including episodes of spring, fumarole, and geyser activity (White et al., 1988, 1992). In 1989, a hydrothermal explosion transformed the geyser into a quiescent pool (Fournier et al., 1994a). As a result of this explosion, large boulders of siliceous sinter ejected from the vent provide excellent examples of the otherwise inaccessible high temperature vent and proximal vent facies. Analyses of this older sinter have been coupled with observations from other active springs to construct a comprehensive lithofacies model of the hot spring. The variability in the type of geyser and spring activity complicates the interpretation of boulders strewn by Pork Chop’s hydrothermal explosion. Obviously, changes in pool flow regimes can markedly affect the type of precipitate at any given locality. The few studies of the hydrochemistry of Pork Chop Geyser (e.g., Thompson et al., 1975; Kharaka et al., 1990) have shown that the waters contain the highest chloride and silica concentrations in the Norris Geyser Basin (Bryan, 1995). One important factor controlling the distribution of lithofacies is water temperature. Measured water temperatures at Pork Chop have varied greatly over time. During the interval from 1960 to 1989, it ranged from 70 to 90 jC (Thompson et al., 1975; Kharaka et al., 1990). 4.2. The Lower Geyser Basin This geyser basin, the largest geyser basin in Yellowstone, lies south of the Norris Geyser Basin (Figs. 1 and 2B). The basin has been extensively

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studied in terms of its microbiology, especially at Octopus Spring in the White Creek area (Brock and Freeze, 1969; Sandbeck and Ward, 1982; Weller et al.,

1991; Stoner et al., 1994). In addition to springs, the basin is notable for its mudpots and associated acidic water activity. Springs in the basin exhibit many types

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of precipitates: siliceous sinter, travertine, and manganese oxides, and attest to the compositional variability of the subsurface fluids (Bargar and Beeson, 1981; Fournier et al., 1994a). Most of the basin consists of a veneer of siliceous sinter, glacial, and diatomaceous alluvial sediments (Muffler et al., 1982). However, significant (e.g., 11 m thick) amounts of hot spring travertine and siliceous sinter were detected in the Y-2 core taken in this area (Bargar and Beeson, 1981). Groundwater in the basin is derived from two reservoirs: an intermediate aquifer with fluids that are 170 jC and low in silica and chloride, and a hotter reservoir at a greater depth that is rich in sodium, silica, and chloride (Fournier et al., 1976; Bargar and Beeson, 1981). Based on its high alkalinity and calcium content, the water chemistry of the Lower Geyser Basin differs significantly from that of the Norris Geyser Basin. The variability in the mineralogy of the Y-2 core indicates that the fluid hydrochemistry has probably fluctuated over time (Bargar and Beeson, 1981). 4.2.1. Deerbone Spring and Spindle Geyser Deerbone Spring and Spindle Geyser are located in the White Creek area of the Lower Geyser Basin (Figs. 1 and 2B). Both are actively precipitating silica. Little is known about these springs. Thus, our observations provide new insights into these features. Deerbone, a previously unnamed spring, was named for the deer bones in its pool (located at

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UTM 516124E, 493106N; Fig. 3D,E). Spindle Geyser displays geyser activity every 1 – 3 min (Fig. 3F), when its water level rises and spills out of the spring pool into White Creek. The pool was filled by mud in 1989 during flashflooding, and the pool did not clear until 1993 (Bryan, 1995).

5. Methods 5.1. Field The water temperature and pH were measured using a Corning Checkmate Modular Testing System probe. Prior to usage, the instrument was calibrated using pH 7 and 10 buffer solutions. Sterile glass slides and copper squares were placed at several sites along the pool bottoms, pool margins, and discharge channels for a period of 5 days to study in situ precipitates. 5.2. Laboratory Samples for X-ray diffraction (XRD) were ground using a mortar and pestle and affixed to a plastic slide. A Siemens D-5000 Diffractometer (Cu-E), programmed to run over a 2h range of 5 –40j at a rate of 2j/min, was used to determine the mineralogy. A powdered fluorite standard was added to each sample prior to analysis as an internal standard. Petrographic examination included binocular, cathodoluminescence, fluorescent, scanning electron, and

Fig. 3. Field photographs showing a representative suite of the hot springs studied at Yellowstone. (A) Cistern Spring in the Norris Geyser Basin, an active siliceous sinter precipitating system. Main pool is approximately 9 m across. Terraced deposits extend downflow to the right of the pool in an area not shown by the photo. (B) The terraced mound morphology in Cistern Spring is well developed. Tree debris is instrumental in the construction of the rimstone dams (right arrow). These fallen logs have a readily apparent white, siliceous coating. Siliceous shrubs occupy the terracettes, especially in the foreground of the photo (left arrow). The terracette in the foreground is approximately 3.5 m wide, and the water is 5 – 10 cm deep. (C) Pork Chop Geyser. Large boulders are a result of a hydrothermal explosion that occurred in 1989. Moderately heated water occupies the pool area that is presently rimmed by the upturned blocks of sinter. Most of our knowledge about vent and proximal vent facies comes from boulders collected from this site. Boulders are up to 2 m wide. (D) Deerbone Spring, an active siliceous domal mound in the Lower Geyser Basin. The whole area shown in the photograph is part of the domal mound. The spring pool is small ( < 1 m wide). A discharge channel is present to the left of the pool. White siliceous sinter ‘‘levees’’ can be seen adjacent to the channels (left arrow). The channel contains hot water and mucilaginous microbial mats (right arrow). Boot in lower right-hand corner is 30 cm long. (E) View of Deerbone Spring exhibiting the domal morphology and abundance of microbial mats colonizing the discharge channel (white arrow). Hydrophilic vegetation is evident in areas not inundated by the hottest spring water (black arrow). Discharge channel in area of white arrow is f 50 m wide. (F) Spindle Geyser, a siliceous hot spring, in the Lower Geyser Basin of Yellowstone. An upturned rim of siliceous sinter is evident around the edge of the pool. Field of view is approximately 7 m wide. (G) Great Fountain Geyser near the Firehole River Drive in the Lower Geyser Basin of Yellowstone. Terraces are poorly developed. Circular rimstone dams (arrow) separate terraces at lower elevations. Total relief of the accumulation is 1 – 1.5 m. (H) Five Sisters Spring in the Lower Geyser Basin of Yellowstone. It is a siliceous pond morphotype and an active siliceous precipitating system. Notice the well-developed siliceous ‘‘rills’’ associated with the pool facies (arrow). These are sculpted by circulating currents as the water emerges from the vent. Siliceous pond is f 3 m wide and f 1.5 m across.

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standard petrographic microscopy. Fluorescence microscopy was done using an Olympus microscope equipped with excitor filters U/B. The ‘U’ filter produces excitation at 515-nm wavelengths, whereas the ‘B’ filter produces excitation at 420 nm. Samples chosen for scanning electron microscopy (SEM) were divided into etched and unetched splits. Samples designated for etching were placed on filter paper in a plastic desiccator that contained a shallow container of HF (about 2 ml) for 5– 10 min, so that the fumes could lightly etch the sample, using a technique developed by Westall (Westall, personal communication, 2001). All samples were affixed to an aluminum stub using carbon tape and lightly sputter coated (30 s) using gold or platinum. Two SEM units were used during the course of this investigation. The initial phase of microscopy was conducted using a Cambridge Stereoscan Model 250 SEM with an accelerating voltage of 30 kV. Other SEM analyses were using a Philips XL-40 Field Emission SEM. Operating conditions for the Philips machine were 2.5 – 5.0 kV and a working distance of 6.0 –16.0 mm. Qualitative elemental compositions were determined using an Oxford ISIS energy dispersive spectrometer (EDS) linked to the Philips SEM.

6. Morphology of accumulation The geometry of the siliceous deposits depends on many factors, the most important of which are probably initial topographic relief, water chemistry, and temporal changes in the volume of discharged water. No models can completely describe the tremendous variability observed in all siliceous systems. Whenever possible, details from other investigations into modern and ancient siliceous sinters were included to broaden the database for the descriptions, morphologies of accumulation, and facies models. Siliceous springs at Yellowstone group into four broad categories: spires and cones, domal mounds, terraced mounds, and ponds (Table 1). 6.1. Spires and cones Although spires and cones are frequently associated with subaerial geysers in Yellowstone, these morphotypes are also associated with hot springs.

Table 1 Spring morphotypes and features associated with them Spring morphotype

Distinctive features

Example

Spire/cone

narrow vent, subconical form, no pool broad shield form, small pool diameter, channelized outflow stair-step morphology, sheet-flow conditions, shrubs low relief pond

Yellowstone Lake Spires Deerbone Spring

Domal mound

Terraced mound

Pond

Cistern Spring

Spindle Geyser

Siliceous spires/cones are high relief features at point-sourced spring sites (Bargar, 1978) (Fig. 4A). They have a wide base that narrows upward to a terminal point. Vents have a small radius (centimeters to tens of centimeters) and lack a terminal spring pool. Thus, in cross-section, the deposit has a conical to cylindrical form, with a narrow, central vent that extends up the length of the cone. Recently, inactive siliceous spires were found at the bottom of Yellowstone Lake near Bridge Bay (Shanks et al., 1997; Morgan et al., 1999, 2000; Shanks et al., 1999). Although initially reported as 35 m high and 50 m in diameter (Morgan et al., 1999; Morgan et al., 2000), recent refinements of the survey data show that the features are 5– 7 m high and up to 2 m wide (Shanks, personal communication, 1999; Klump, personal communication, 2001). Thousands of these siliceous spires have been identified from high resolution-multibeam bathymetric, seismic reflection, and magnetometer surveys (Morgan et al., 1999), and they may be the most numerous siliceous spring deposits in the park. In morphology, they resemble inactive ‘‘black smoker’’ chimneys from oceanic settings (Morgan et al., 2000), are found at water depths of f 15 m (Shanks, personal communication, 1999), and occur either alone or in ‘‘forests’’ of giant spires (Morgan et al., 1999). Little is known about their origin, but they presumably form at sites where hot water has discharged for a period during the last 12,000 years (Morgan et al., 2000). Some spires are found in groups associated with long (up to 400 m) fissures (similar to subaerial fissure ridge deposits); others surround possible hydrothermal explosion craters (Morgan et al., 1999). Even less is known about the microfabrics of these opaline precipitates, although they are rich in diatom remains with fewer

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Fig. 4. Generalized schematic cross-sections of four morphotypes of Yellowstone’s siliceous hot springs, i.e., spires/cones, domal mounds, terraced mounds, and ponds. Commonly recognized depositional facies are labelled on each, no scales shown. (A) Typical subaqueous siliceous spires/cones as described from Yellowstone Lake. They exhibit an inverted cone-shaped morphology and an elongate, narrow vent. Shaded areas between spires indicate lake sediments and siliceous debris. (B) Domal mound hot spring exhibiting convex surface and gently sloping discharge channel/flowpath facies; major depositional facies are indicated. (C) Terraced mound with its major depositional facies depicted. Inset details siliceous shrubs occupying the terracettes and rimstone dam with scalloped surface. (D) Pond morphotype. Ponds are generally depressions with little/no outflow. Low topographic relief and a lack of discharge channel/flowpath facies are distinctive of this spring morphotype.

preserved bacteria (Shanks, personal communication, 1999; USGS, 2001). Siliceous spires were discovered below Lake Taupo, New Zealand, in 1998 (Renaut, personal communication, 2002). A more detailed assessment of the mineralogy and microfabrics of the Yellowstone or New Zealand spires has yet to be conducted. Although the spires resemble black smoker chimneys in geometry, noteworthy distinctions exist. Black

smokers are typically composed predominantly of sulfide and sulfate minerals with lesser quartz or opal (Rona, 1984), whereas the Yellowstone Lake deposits are constructed mainly of opal; sulfate and sulfide minerals are not associated with the deposits (USGS, 2001). Siliceous spires and cones are also associated with some subaerial siliceous sinters, and exhibit a subconical form. An example is White Dome Geyser in

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the Lower Geyser Basin. The cone tapers upward and is f 6.5 m high (Bryan, 1995). It has a narrow vent (10 cm wide) (Bryan, 1995) and lacks a terminal pool. Precipitates along the flanks of the cone result from geyser spray and descending rivulets of water. Gravitational cements and draping beds that thicken laterally away from the vent are two criteria that help to distinguish subaerial versus subaqueous deposition. Subaqueous siliceous spires are unlikely to exhibit this style of precipitation and would likely contain isopachous cements. In contrast, precipitates in the subaerial cones are likely to exhibit continuous laminations that extend from the vent mouth down the flank. Unlike travertine cones, subaerial siliceous cones are shorter and generally have broader bases. Presumably, this is a function of lower precipitation rates in the siliceous systems. Subaerial spires have poorly developed microbial mats on their flanks due to shallow streams of water. Although geometries associated with these deposits are distinctive, preservation of these features in the rock record remains uncertain. Relatively thin bases make them susceptible to toppling, and their high relief facilitates erosion once hot spring activity ceases. No ancient well-developed siliceous spires/ cones have been documented. 6.2. Domal mounds Domal mounds are broad, gently sloping features entirely composed of subaerial siliceous sinter with a shield or lens morphology (Figs. 3E and 4B). Typically, these domal mounds develop on flat areas where hot spring waters emerge. Deposition of siliceous sinter in these areas results in a fan-shaped accumulation. Recognition of these deposits in the rock record hinges on the broad domal geometry and their gently sloping erosional surfaces and laminations. The erosional surfaces represent nondepositional surfaces or benches (cf. Chafetz and Folk, 1984). These laminations generally dip radially from high areas of the fan. Unlike siliceous spires, domal mounds have a slightly larger vent diameter and a terminal pool where the spring waters emerge. These spring pools tend to be small compared with those of terraced mounds. The interiors of some domal mound pools reveal irregularities or shelves that represent former

pool margins. Pools are rimmed by stromatolites at the water/air interface. These stromatolites prograde over the pool facies forming overhanging ledges. Directly adjacent to the pool margins are anastomosing discharge channels that are colonized by a wide variety of microbes. Only one or two discharge channels are active at a time; the remaining deposit is inactive. Channels are only a few centimeters wide ( f 30– 40 cm) and are thoroughly colonized by microbes. As precipitates accumulate, the spring discharge channel migrates by meandering or avulsion to occupy pathways with the steepest gradient, similar to channels of an alluvial fan. Abandoned outflow channels are covered by desiccated, silicified, microbial mats. Surrounding these mounded features are debris aprons. Aprons consist of platy siliceous intraclasts derived from the weathering of the sinter. Debris apron sediments are thin near the pool and have their maximum thicknesses in the more distal portions of the deposit. Dipping horizons of imbricated, platy siliceous intraclasts may also be present. These debris aprons can extend for many tens of meters away from the central pool. Domal mounds are common spring morphotypes. Deerbone Spring is a typical example of a hot spring deposit of this morphotype (Fig. 3D,E). The domal mound, which is asymmetrical, formed adjacent to a hill on the eastern edge of White Creek valley. The dome is between 2 and 3 m high and channelized flow from the spring extends for 7 m. The circular pool is small ( < 1 m2) compared to the mound area (150 m2). The sloping flanks of the deposit dip at angles of 3– 4j. The pool is 1.5 m deep and its interior has a beehive or cistern geometry: i.e., it is wider at the base near the vent pipe and tapers upward. The pool walls exhibit 3 – 4 irregular, vertically spaced shelves that represent former pool margins. The lack of a series of laterally continuous shelves indicates that accretion of these domal mounds occurs in discrete spurts rather than as continuous deposition. Unlike other siliceous pools, flow structures in the pool precipitates were not recognized. The pool margin is rimmed by an overhanging ledge of stromatolitic sinter. The stromatolitic columns are equidimensional with a width of 3 cm and a height of 3 cm. The internal laminae within these structures are convex downward. Thus, the apex of the stromatolite is oriented downward toward the vent. Internal stromatolitic laminations consist of

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alternating black and brown (organic-rich) and white (opal-A) couplets. Most pool water is restricted by this lip of well-indurated stromatolitic siliceous sinter. Low volumes of water discharge from only one break in the pool-margin facies, although numerous abandoned outflow channels are present. The active channel contains water 1 –2 cm deep, 15 cm wide, and is colonized by a thick (2 cm) green microbial mat (Fig. 3E). Mats are green at the top and stratified with orange layers below the surface, similar to those described at Octopus Spring (cf. Castenholz, 1984; Ward et al., 1989). The top layer has a rubbery, partially silicified surface with entrapped gas bubbles. Various insects and ephydrid fly larvae are part of the surface of the mat. Weakly indurated, platy siliceous intraclasts are commonly present directly below the mat. Changes in the style of the precipitate can be seen along a lateral transect of the channel. Extending 1 –2 cm on either side of the channel is a white, bleached siliceous sinter ‘‘levee’’ (Fig. 3D) that has a 1-cm elevation above the water level and contains vestiges of filaments within the precipitate. These filaments lack readily apparent preferred orientations. A zone of thin ( < 1 cm thick), orange-pink microbial mats extends up to 60 cm laterally from the levee, and is characterized by sheets of silicified microbial mat and white siliceous intraclasts. This zone is soft, spongy, and contains water just below the surface. Hydrophilic plants colonize the areas beyond this zone and act as a baffle, collecting desiccated mat material. Recently abandoned channels also contain white, crinkled, silicified, desiccated microbial mats in sheets up to 50 cm long. Where this desiccated mat has been stripped away, siliceous intraclasts are evident. These white intraclasts are platy, angular, and retain vestiges of filamentous microbes (e.g., elongate molds and tubules). Individual clasts are generally 0.5 –2 cm long (although dimensions range greatly). Much of the aureole surrounding the spring (extending beyond the 7 m of flow) consists of this debris apron of siliceous intraclasts. A modern domal mound at the Waikorohihi and Mahanga Geysers in New Zealand is briefly described by Jones et al. (2001b). The two siliceous geysers share a common mound (Jones et al., 2001b). An ancient domal mound is described by Walter et al. (1996, p. 516) in the Paleozoic Wobegong and Verbena siliceous sinters from the Drummond Basin of

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Australia. Although a domal mound was not described, sloping laminations in the sinter beds that crop out (Walter et al., 1996, Fig. 14b, p. 510) indicate a domal geometry. Similarly, the abundant erosional surfaces and breccias (clasts up to 3 cm, average 0.5 cm) (Walter et al., 1996) further substantiate a domal mound accumulation. The large areal extent of the Wobegong and Verbena deposits, however, implies that numerous springs were instrumental in their construction. 6.3. Terraced mounds Terraced sinter mounds are rare in Yellowstone National Park. In carbonate systems, terraced mounds start to form in the same manner as travertine domal mounds: irregularities along the outflow path create terracettes (cf. Pursell, 1985). The irregularities can be precipitates, debris that falls or washes into the deposit, or perturbations in the water flow, all of which facilitate mineral precipitation. Terracettes have raised rims (rimstone dams) and a shallow ( < 10 cm deep), dammed pool behind them (Figs. 3B and 4C). Each pool is fed by water from the spring orifice or a higher terracette. Pool bottoms tend to be bumpy and irregular and, at Yellowstone, are characterized by ‘‘shrubby’’ siliceous precipitates, whereas the rimstones have smoother surfaces. ‘‘Shrubs’’ consist of porous aggregates of spinose opaline silica that resemble miniature woody plant shrubs. ‘‘Shrub’’ is used here to describe a delicate branching morphology rather than an actual ‘‘woody plant’’ (cf. Chafetz and Folk, 1984; Chafetz and Guidry, 1999). Siliceous shrubs are very delicate and resemble shrubs in travertine-precipitating systems (Chafetz and Folk, 1984); these shrubs, however, are composed entirely of porous opal (Fig. 5A). Siliceous terraced mounds of Yellowstone exhibit a relatively wide pool (up to 8 – 10 m in diameter), considerably larger than those observed in the domal mounds. Whereas depositional relief for the two morphologies is similar, terraced mounds lack sloping surfaces on their flanks. Terraced mounds tend to exhibit sheet-flow water conditions, thus discharge channels are absent or poorly developed. Large volumes of water spill out of numerous areas of the pool. After the water exits the pool, it discharges into a series of terracettes at successively lower elevations

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Fig. 5. (A) Siliceous shrub from terracette at Cistern Spring. Observe the spinose terminations of the precipitate, the arborescent form, and abundant porosity. (B) Lilypad stromatolites (white arrow) at Cistern Spring. Scale (15 cm long) is resting on the exposed pool margin deposits. Water depth is 2 – 3 cm. The lilypad stromatolites prograde over the pool facies and form overhanging ledges. Additionally, lilypad ‘‘islands’’ form within the pool (black arrow) and grow upward to the water/air interface. Lateral progradation produces a compound horizon of solid lilypad stromatolites. (C) Detail of the top of a lilypad stromatolite from Cistern Spring. Observe the flattened appearance of the upper surface, and the highly irregularly swirled laminae (arrow). Scale bar at top of photo, each block is 1 cm.

separated by rimstone dams. The resultant spring, therefore, exhibits a crude stair-step morphology in cross-section, which can easily be distinguished from adjacent deposits and other spring morphotypes. Little is known about the deposits comprising vents and pools in these systems. Sculpted, smooth siliceous precipitates commonly line the pool and reflect pool flow currents. In areas where these terraced mounds exist, flow and precipitation rates ( f 5 cm/year) (White et al., 1988) are relatively high. These features can be recognized in the rock record by the distinctive stair-step cross-section of the accumulation. Cistern Spring represents an excellent active example of a siliceous hot spring with this terraced morphology (Fig. 3B). The total accumulation occupies an area of nearly 2000 m2 of which the pool occupies approximately 70 m2. Thus, the 9-m-wide pool occupies a more substantial area of the accumulation than in the domal mounds. The spring orifice is 2.5 m above the distal forested area. Terraced flow extends from the pool margin for a distance of 20 m. Little is known about the internal geometry of the vent or the pool, although periodic draining of the pool related to eruptions of Steamboat Geyser revealed a substantial pool 5 – 7 m deep (White et al., 1988). Distinctive sculpted siliceous sinters or rills were recognized within the pool, and the entire pool is lined with these precipitates. In springs with a rotational flowpath, such as Cistern Spring, pool bottoms display siliceous precipitates resembling ‘‘scales’’. These white scales are positive, subconical bulges up to 1– 2 cm high and oriented with their long axes parallel to flow direction. Tree limbs and other debris that fall into this part of the hot spring can also enhance the development of siliceous scales. Lilypad stromatolites are present along the pool margin and are typically 20 –30 cm wide. Stromatolites are flattened, no more than 2– 3 cm high, and prograde over the pool as ledges (Fig. 5B,C). The pool discharges water at multiple localities and flows through a series of terracettes. Terracettes and rimstone dams are well developed along the flowpath. The few dams are vertically oriented and commonly result from large trees that fall into the deposit thereby creating barriers to flow that are subsequently silicified. Many of the rimstone dams at Cistern Spring were observed to have a large tree as the ‘‘nucleating’’ substrate for the dam. The rimstone dams are up to 30 cm high, and have a vertical face

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consisting of ‘‘scalloped’’ siliceous sinter. These ‘‘scallops’’ consist of protruding lips (0.5 – 1 cm wide) with a concave upward orientation, and closely resemble the rimstone microterracettes described for travertine deposits by Pursell (1985) and Fouke et al. (2000, p. 570). This vertical rimstone face is colonized by a surface veneer of brownish-pink microbial mats a few millimeters thick. Terracette pools associated with Cistern Spring range in size from decimeters to 2 – 3 m across, and contain water only a few centimeters deep (2 –15 cm). The most distinctive macroscopic feature associated with the terraces are white, siliceous ‘‘shrubby’’ precipitates (1 to 5 cm high) (Fig. 5A). Although delicate compared to well-indurated constituents comprising the vent facies, the shrubs are composed of a backbone of white, spicular opal with delicate, plumose terminations that make its survival during transport difficult. In hand-specimen, orangegreen filaments can be observed colonizing the blades or spines of the siliceous shrubs in the active systems. Another terraced accumulation in Yellowstone National Park is the Great Fountain Geyser (Fig. 3G). It exhibits terracettes of low topographic relief ( < 1– 1.5 m). Whereas tree debris was instrumental in constructing Cistern’s terracettes, the terraces at Great Fountain Geyser are constructed of white siliceous sinter lacking tree debris. Only three terraces are present along the flowpath and sheetflow conditions are prevalent. Rimstone dams are concentric and can be followed for tens of meters laterally. These dams are approximately 10 cm high and lack the ‘‘scalloped’’ texture of the well-developed rimstone dams at Cistern. For the most part, shrubs are poorly developed. Thus, the terraced morphology of the accumulation at Great Fountain Geyser is probably in a juvenile form. Several siliceous terraces have been described in the literature. Jones et al. (1998) describe a microterraced deposit in New Zealand (Ohaaki Pool), which exhibits carbonate and siliceous sinter precipitation. Unlike the terraced deposit at Cistern Spring, trees had little influence on the construction of the rimstone dams at Ohaaki Pool. In the Ohaaki system, relief between each successive terracette is on the order of a few centimeters (Jones et al., 1998). Walter et al. (1996) describe terraced deposits very similar to those of Jones et al. (1998) from sheet-flood deposits near the crest of Grand Prismatic spring in Yellowstone

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National Park. Walter et al. (1996) attributed the construction of the micro-terraces to the colonization of the slope by the cyanobacterium, Calothrix, and subsequent silicification. Thus, both of these examples of terraced deposits exhibit considerably lower relief than that observed at Cistern Spring. Terraced deposits were also described from the El Tatio Geysers in Chile (Jones and Renaut, 1997). Although most of the El Tatio accumulation consists of microterraces, a few well-developed terraces were observed (Jones and Renaut, 1997). Thus, terraced mound morphotypes have been described in many active siliceous sinter sites worldwide. 6.4. Ponds Siliceous ponds are geysers or hot springs morphotypes that occupy essentially self-contained depressions filled with hot water (Fig. 4D), and are in no way meant to be confused with cool diatomaceous marsh facies at the distal areas of some hot springs. Most of the area of the accumulation is occupied by the pool. For the most part, little water is discharged from these sites and, thus, discharge channels are absent or poorly developed. Precipitation of silica is minimal, and consists of smooth, sculpted pool precipitates and a rim of silicified stromatolites. Due to the low relief of these siliceous ponds, a great deal of detrital material is probably incorporated into these deposits. Recognition of these features in the rock record would be very difficult. Spindle Geyser is an example of a pond accumulation in the Lower Geyser Basin of Yellowstone National Park (Fig. 3F). Because of low relief, Spindle Geyser is periodically inundated by White Creek floodwaters and has been completely choked by stream sediments (Bryan, 1995). Although the spring has a distinct cycle of geyser activity which results in intermittent overflow into White Creek, most of the time the geyser is a self-contained pool. Very little precipitation of siliceous sinter occurs outside of the pool. Thus, most of the approximately 28-m2 area of the accumulation consists of the pool. The pool is irregular in cross-section, about 7 m wide, and about 1.5 m deep. Similar to Cistern Spring, the pool exhibits smooth siliceous sinter precipitates related to flow currents. Immediately above the air/water interface around the sides of the pool are clusters of

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small (1 – 2 cm) fan-shaped stromatolites approximately 1 cm in height. These stromatolites construct ledges prograding out over the pool facies similar to other siliceous hot spring accumulations. Several examples of siliceous ponds are scattered throughout Yellowstone National Park. In the Norris Geyser Basin, Emerald Spring is a siliceous pond with a morphology strongly resembling Spindle Geyser. Another example includes the Five Sisters in the Lower Geyser Basin (Fig. 3H). Few analogs to siliceous ponds have been described in the rock record.

7. Spring facies In addition to documenting the various morphotypes associated with Yellowstone’s siliceous hot springs, an attempt was made to rigorously define facies found in association with these features. As a

result, we present a continuum of observations from a variety of different scales: outcrop, hand-specimen, thin-section, and SEM. This range in scales provides a realistic and detailed facies model of these deposits. The following section builds upon the spring morphotypes previously described and introduces generic facies associated with Yellowstone’s siliceous deposits. Eight distinct depositional facies have been identified in the suite of samples collected (Table 2); not every facies was present at each locality. These facies include: (1) vent, (2) proximal vent, (3) pool, (4) pool margin, (5) pool eddy, (6) discharge channel/flowpath, and (7) debris apron (Fig. 4B). One other lithofacies (geyser facies) was recognized at Pork Chop Geyser and is related to its former geyser activity. Each depositional facies possesses distinctive lithologic characteristics and facies indicators, facilitating the construction of a facies model.

Table 2 Major depositional facies associated with siliceous hotsprings and a summary of salient characteristics Depositional facies

Temperature (jC)

Surface morphology

Crystal size

Mineralogy

Miscellaneous

Vent Proximal vent

>95 < 95

Dimpled hemispheres Laminated crusts

f 80 – 90

Pool margin

f 80

Euhedral to subhedral sulfur with opal drapes sculpted by convecting currents Lilypad stromatolites

Chalcedony Opal-A with chalcedony Opal-A with sulfur

Length-fast chalcedony Length-slow chalcedony

Pool

0.5 – 0.75-cm aggregates 0.5 – 1 mm (chalcedony), nm – Am spheres (opal) < 0.10 mm (sulfur), nm – Am spheres (opal)

Opal-A, Alunite, Fluorite, Calcite, Hematite

May exhibit fenestrae and gypsum nodules

Pool eddy

< 80

Hash consisting of eukaryotic debris

nm – Am spheres (opal), < 4 Am (alunite), nm – Am spheres (fluorite), < 1 Am (hematite), < 1 Am (calcite) nm – Am spheres (opal)

Opal-A

Discharge channel/ flowpath Debris apron

< 80 to ambient

Siliceous shrubs, silicified mats

nm – Am spheres (opal)

Opal-A

Variable

Siliceous intraclasts

1 – 2-cm clasts

Opal-A

Geyser

Fluctuating

Oncoids/pisoids

mm – cm

Opal-A

Abundant plant debris, eukaryotic remains, and microbial mat material Also, silicified microbial tubules. Diatoms and pollen present Platy reworked material from other facies. Opaline cements, local hematitic staining Irregular internal laminations. Cores either siliceous sinter fragments or weathered igneous clasts

Lacks chalcedony

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7.1. Vent facies The vent facies consists of siliceous precipitates lining the hydrothermal vent pipes. Erratic spring activity in any thermal basin is related to the migration and blockage of these vents over time. Relatively little is known about the petrographic characteristics of the vent facies because it is seldom accessible in modern springs and geysers. Welldeveloped examples of this facies can be seen in boulders from Pork Chop Geyser (Figs. 3C and 6A). Relict vent cavities are up to 50 cm in diameter in outcrop and highly irregular in cross-section. Each of these cavities is lined by hemispherical aggregates of frosted chalcedony; the aggregates are typically 0.5 to 0.75 cm in diameter (Fig. 6B). Crystal aggregates form a series of convex bumps lining the vent pipe. In hand-specimen, the aggregates form a rounded mass with a dimpled surface similar to that of a golf ball (Fig. 6B). Because of the smooth, round habit of the crystal aggregates in the vent, there is little indication of spring vent flow directions (e.g., no elongation in a particular direction). Longitudinal sections of individual chalcedony crystals have a crude fan shape with an approximate basal width of 20 Am, top width of 100 Am, and a length of 300 Am. Aggregates of these crystals are clear and have distinctive scalloped terminations (Fig. 6C,D). Under crossed-polars, radiating extinction bands are evident in the clear exterior (upper 0.15 mm) of each individual crystal. Transverse sections of crystal aggregates display irregular ‘‘sheaves’’ consisting of sub-rectangular (and less commonly diamond-shaped) mosaic patterns under crossed-polars (Fig. 6E,F). These sub-rectangular, transverse crystal sections have a width of 20– 40 Am. Each crystal of chalcedony is length-fast. No fluorescence in the 420or 515-nm wavelength light was observed. SEM images demonstrate that the crystal aggregate exteriors are composed of a number of tightly arranged, 100-Am botryoidal spheres. Solid, regular bands approximately 3 Am in thickness transverse the chalcedony (Fig. 7A). Each band maintains an even thickness except in a few areas where growth discontinuities occur. There is a sharp contact at the microscale between the vent chalcedony and the adjacent proximal vent facies (described in the next section).

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Because of the lack of accessibility of this facies in the active siliceous hot springs, temperature conditions can only be inferred from field measurements taken from the surrounding facies. Based on this distribution, it is likely that the temperatures of formation are in excess of 95 jC, near the boiling point of the water for the altitude at Norris Geyser Basin. 7.2. Proximal vent facies Adjacent to the vent facies are areas greatly influenced by the influx of hot water that extend a short distance along flow ( < 1 m) (Fig. 4B). Similar to the vent, this facies generally formed under hot water. Consequently, most of our knowledge about the facies comes from Pork Chop Geyser. In outcrop, it consists of regularly laminated, amber and white crusts resembling banded agate (Fig. 7B). Each lamination has a constant thickness in hand-specimen, commonly with millimeter-scale regularity (Fig. 7C). It is likely that there is a gradational relationship between the laminated chalcedony/opal crusts of the proximal vent facies and the opal comprising the pool bottom facies. These laminated crusts are predominantly opal with incipient crystals of length-slow chalcedony (Fig. 7D). Opal comprises up to 80% of the specimen and chalcedony the remaining 20%. Chalcedonic aggregates are much smaller than those comprising the vent facies ( f 0.5 to 0.2 mm in width) and can only be adequately discerned in thin-section. They occur in linear and curved bands. The lengthslow chalcedony is a marked deviation from the chalcedony lining the vent, and consequently, has a different origin. The chalcedony consists of radiating extinction bands similar to the features in the previous facies. Non-oriented, dusty inclusions are concentrated only in zones near the crystal bases and tops, defining discrete areas near the growth discontinuities. Within the chalcedonic bands, there are clear areas where the dusty inclusion laminae appear to have been partially obliterated by the dissolution and reprecipitation of chalcedony. Laterally continuous, micron-scale laminae can be observed within individual bands. Two major types of chalcedonic precipitates were observed in SEM images: chalcedony crusts alternating with bands of opal (Fig. 7E) and chalcedonic

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isopachous cements within the opaline crusts (Fig. 7F). In the chalcedony crusts, bands are typically 10– 15 Am in width. Laminae within the chalcedony are regular; bands, however, are influenced by the topography of the underlying opal. Spheres of opal are 4 – 5 Am in diameter and commonly hollow. The second type of chalcedony in the proximal vent facies consists of isopachous pore-occluding cements exhibiting numerous laminations parallel to the pore walls. The immediately adjacent opal appears ‘‘spongy’’, consisting of a network of 2-Am hollow tubules and spheres. No intermediate phases of silica were observed separating the opal and chalcedony, for example, no cristobalite lepispheres were observed. Conditions of formation for this particular facies are not well constrained. Temperatures were very likely a few degrees lower than the 95 jC threshold proposed for the previous facies (e.g., 92 or 93 jC) and low enough for predominantly opal precipitation. 7.3. Pool facies Extending from the proximal vent facies to the pool margin is the pool bottom facies; this occupies the greatest area of the hot spring pool (Fig. 4B). Observations from numerous siliceous hot springs were integrated to produce a composite description of this facies. Pools themselves can be highly variable in size, although most tend to have a cauldron morphology, ranging from 50 cm wide in domal mounds to many meters in terraced mounds. Although the depths of the pools can also vary, few attempts have been made to make detailed measurements. In Yellowstone, estimations of pool depths range from f 1 to f 5 m. In Yellowstone’s siliceous hot springs, many pool bottoms appear to have a smooth, sculpted (‘‘rilled’’) bottom of sinter (Fig. 3H). Rilled sinter can be in a

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variety of forms in these pool bottoms depending on flow conditions. The rilled opal results from deposition along the pool walls as the hot water emerges from the vent. In cross-section, the rill would resemble a sine wave or corrugated tin. The rills have individual widths on the order of 25 cm and variable lengths. Individual rill axes are typically elongated in the direction of flow. The depth of the rill trough commonly is f 5 –10 cm. Thus, rills are distinctive macroscopic features indicating a siliceous hot spring pool facies. Most details about the microfabrics were gleaned from precipitates on a series of sterile, glass slides immersed in Cistern Spring’s pool for a period of 5 days. Despite this relatively short period of time, glass substrates exhibited coatings of euhedral native sulfur and minor amounts of opaline precipitates. The slides are colonized by a delicate threadwork of microbial filaments composed of Gram-negative cocci exhibiting a streptococcal (chain) arrangement (and not of cyanobacterial origin). Sulfur crystals varied in size with the largest 0.10 mm long. Individual crystals appeared to be bound by clear siliceous drapings or silicified polymeric substances (biofilm). In SEM, the sulfur crystals display a tetragonal dipyramidal habit and range in size from 10 to 100 Am (Fig. 7G). The surrounding opaline precipitates appear smooth, with numerous native sulfur crystals enmeshed inside the silicified biofilms. The opal consists of submicron diameter spheres. No native sulfur crystals were identified in thin-sections or XRD patterns of Pork Chop Geyser precipitates. Thus, it is likely that native sulfur was not preserved in the deposits associated with Pork Chop Geyser. Temperature conditions for the formation of the sculpted sinter are probably variable, and range from 80 to 90 jC. A slightly acidic pH (for this particular temperature) of < 5.5 was recorded at Cistern Spring.

Fig. 6. (A) Field view looking down into a relict vent in a loose boulder at Pork Chop Geyser. Vent cavity is hollow, irregular in cross-section, and lined with chalcedonic quartz (arrow). Note the gradation into the laminated chalcedonic crust facies (‘‘cc’’). Hammer is 30 cm long. (B) Hemispherical aggregates of chalcedony line the relict vent shown in (A). Scale boxes are 1 cm. (C) Thin-section photomicrograph of a longitudinal portion of a spherulitic accumulation of chalcedony from the relict vent at Pork Chop Geyser, in plain light. The chalcedony exhibits a scalloped top surface (arrow). (D) Same field of view as shown in (C), but crossed-nichols. Chalcedony crystals display radiating extinction patterns. (E) Thin-section photomicrograph, crossed-nichols, of a portion of the spherulitic vent chalcedony showing transverse sections of the crystals’ borders. Individual sheaves tend to be sub-rectangular to diamond-shaped in cross-section. (F) Thin-section photomicrograph, crossed-nichols, showing the gradational relationship between the transverse section ‘‘mosaic’’ of the spherulitic vent chalcedony and the radiating fans comprising the exterior of the aggregate. Areas where chalcedony crystals are oriented perpendicular to the field of view are indicated by the left arrow and areas where they are oriented parallel to the field of view by the right arrow.

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7.4. Pool margin facies This facies is evident around the periphery of the pool at the air/water interface and exhibits the highest precipitation rates (Figs. 4B and 5B). Although stromatolites are ubiquitous in this facies, there is great variability in the morphotypes observed at each of the pool margins. Elongate lilypad stromatolites are the most common morphotypes, however, other digitate and branching morphologies were observed. Preservation of stromatolites in hand-specimens and thinsections from Pork Chop Geyser (Figs. 7H and 8A,B) indicates a high preservation potential for these features. These stromatolites prograde over the pool facies, thereby forming overhanging shelves and ledges at the air/water interface. Stromatolite genesis is likely the result of a myriad of bacterial taxa. Pool margin stromatolites at Spindle Geyser and Deerbone Spring exhibit colorful bacterial colonies associated with the stromatolites. These are easily observed in the field because of their color differences including: black colonies, orange colonies, green filamentous colonies, and still others that are composed of tan/pink streaming filamentous microbes and mucilage. Thus, construction of each of these colonial groups likely involves a community of organisms.

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At Cistern Spring, opaline, lilypad stromatolites are flat-topped and white with a faint yellowish hue due to native sulfur and/or alunite (Fig. 5B,C). The stromatolites average 20– 30 cm wide and 2 –3 cm high. Lilypad ‘‘islands’’ also form within the pool, growing upward to the air/water interface. Lateral progradation of these produces a compound horizon of solid lilypad stromatolites. Microbial colonization of the stromatolite at Cistern Spring is not as readily apparent as at Deerbone Spring or Spindle Geyser. However, endolithic communities can occasionally be observed in cross-sections, and include orange and green colored organisms. Internal stromatolitic laminations are composed of highly irregularly swirled white laminae that alternate with dark, organic-rich laminae, and are generally 0.1 cm thick (Fig. 8C). In cross-section, these stromatolites exhibit abundant fenestrate porosity (‘‘bird’s eye’’ structures) similar to many ancient microbial deposits (e.g., tidal flats). Individual fenestrae are up to 1.3 mm wide and 1 mm high. Much finer-scale laminae (up to 20 Am) can be observed in thin-section displaying an irregularly, swirled fabric. A different style of stromatolite can be observed at the pool margin of Spindle Geyser, a siliceous pond. The base of the stromatolite is generally about 1 cm wide and expands at the crest to a width of 1.5 to 2 cm. The lower and outer part of these stromatolites

Fig. 7. (A) Vent lining chalcedony from Pork Chop Geyser. This SEM photomicrograph gives an excellent view of the botryoidal surface morphology (‘‘B’’) as well as a cross-sectional view. Regular laminations are evident in the foreground (‘‘L’’). (B) The proximal vent facies consists of laminated opal and chalcedonic quartz (arrow) as seen in a large boulder adjacent to Pork Chop Geyser. The laminated crust has a discordant relationship with the displaced boulder (‘‘D’’). This displacive relationship of the large sinter boulder (‘‘D’’) indicates that an explosive event probably occurred prior to the 1989 hydrothermal explosion. Knife is 8 cm long. (C) Slab from Pork Chop Geyser illustrating the proximal vent facies and the regularity of each millimeter scale individual lamina. Scale boxes are 1 cm. (D) Thin-section photomicrograph, crossed-nichols, of proximal vent facies from Pork Chop Geyser. It consists of opal (dark areas) with incipient crystals of chalcedony. The distinctive banding in the chalcedony can be linear (left arrow) or curved (right arrow). (E) Backscattered SEM image of proximal vent facies from Pork Chop Geyser. The opal occurs as bulbous, hollow spheres (white arrow). Upper portion of photo exhibits banded chalcedony influenced by the topography of the underlying opaline spheres (black arrow). No intermediate phases of silica were observed between the chalcedonic bands and the hollow opaline spheres (e.g., no opal-CT lepispheres). Regularity of the bands in the chalcedony indicates that the chalcedony is primary and not replacement of these 4 – 5-Am opal spheres. (F) SEM image of ‘‘spongy’’ opaline precipitates and pore occluding isopachous chalcedony cements. Chalcedonic laminations within the filled pore have great regularity (arrow). Sample from Pork Chop Geyser. (G) Euhedral, tetragonal, dipyramidal native-sulfur crystal (arrow) that precipitated on a glass substrate which was submerged in the pool facies of Cistern Spring. Native sulfur is associated with the pool and pool margin facies at Cistern. The adjacent material consists of opal-A precipitates. Commonly, these sulfur crystals are enmeshed in silicified polymeric substances. (H) Slabbed hand specimen exhibiting stromatolites of the pool margin facies from Pork Chop Geyser. Two horizons of stromatolites are evident in the photo (white arrows). Basal portion of slab exhibits siliceous intraclasts (‘‘SI’’). Lower horizon of stromatolites exhibits a digitate morphology that widens upward. Irregular laminae within the stromatolites consist of grey and white couplets 1 mm thick. Flanking laminations indicate that the structures had synoptic relief. Between these stromatolitic columns is plant debris (e.g., silicified pine needles) (black arrow). Upper horizon of stromatolites consists of a 5-cm-long lilypad which has very fine white-grey internal laminations ( < 1 mm thick). Smaller, white, club-shaped stromatolites ( < 1 cm in height) are also present in the upper horizon (Fig. 8B). They also exhibit increasing basal diameters upward. White area below the identification number consists of opal-A exhibiting hollow tubules < 1 mm wide. Sample is 8 cm high.

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consists of tiny, siliceous stromatolitic spires ( f 10 mm high and f 15 mm wide) situated just below the surface of the water, colonized by an assortment of

microbes and mucilage (Fig. 8D). The stromatolitic spires grade into the fan-shaped stromatolites (Fig. 8E). This transition is marked by a series of succes-

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sively wider and larger stromatolites. The top surface of the stromatolites is smooth with an enterolithic pattern. A fresh fractured surface reveals two styles of siliceous precipitates: white powdery sinter comprising the stromatolites rim and base, and grey wellindurated sinter comprising the upper surface and occasional laminae within the stromatolite. This rim of stromatolites is highest at the pool margins and slopes away from the pool providing a sheltered area immediately in their lee that is protected from inundation of hotwater. Thus, the architecture of this pool margin is different from that observed at Cistern Spring. The stromatolites from all the environments shared a number of distinctive petrographic characteristics. First is the pronounced ‘‘crinkled’’ or irregularly swirled nature of the laminations of the stromatolite. Stromatolites fluoresced under 420-nm wavelength light, thereby accentuating these irregular laminations. Areas between the digits of the stromatolites commonly contain silicified pine pollen or rarely siliceous detritus. Although micro-cross stratification has been described within some of Yellowstone’s stromatolites (Walter, 1976b), none was observed in this suite of samples. SEM images of the stromatolites reveal a complex network of silicified cells and polymeric substances (Fig. 9A). They consist of silicified colonies of bacteria and associated biofilm that give it a grainy appearance. These microbial communities exhibit various stages of silicification. Bacterial morphotypes include bacilli (0.5 by 1 Am), cocci, and filaments (0.5 Am wide). EDS analyses of the cells routinely revealed elemental carbon, sodium, sulfur, and phosphorus in addition to silicon and oxygen from the enveloping precipitates. Several authigenic minerals are present in the pool margin facies including gypsum, hematite, alunite, fluorite, calcite, and native sulfur. Evaporites are especially common, and can be seen in a number of

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thin-sections. Gypsum tends to form poorly developed enterolithic nodules and ribbons within the stromatolitic heads. Alunite, a hydrous potassium aluminosulfate mineral (KAl3 (SO 4) 2 (OH) 6 ), is abundant throughout the facies (Fig. 9B). Alunite forms irregular masses and, in places, euhedral crystals approximately 4 Am in diameter, and appears to be restricted to this facies. A few of these crystals are twinned. Hematite is only observed as irregular micron-sized blebs enmeshed in relict organic matter; residues producing a strong carbon elemental peak in EDS. Fluorite and calcite are much less widespread. Fluorite was only identified in SEM; it is present as micronsize spheres as opposed to well-developed crystals. In contrast, calcite crystals tend to have a euhedral crystalline (e.g., rhombohedral) form consisting of aggregates of 500-nm crystallites. These calcite crystals are restricted in occurrence to silicified polymeric substances. Despite a high temperature environment, no aragonite was observed. Iron-rich clay minerals are less abundant than the other accessory minerals. At Cistern Spring, temperatures associated with this facies are typically around 80 jC, and the pH nears 5.5. These high temperatures imply that the predominant microbes involved in stromatolite genesis must be thermophilic bacteria. However, in many springs, water levels fluctuate due to a variety of factors, and true temperature conditions may be highly variable over time. In the samples from Cistern Spring, microbial remains are not always well preserved in this facies, probably due to high temperatures, abundance of oxygen, and fluctuating water chemistry. The best indicator of their former presence is porosity representing their former loci and highly irregular laminations in the siliceous sinter. 7.5. Pool eddy facies Immediately adjacent to the stromatolitic pool margin facies are the pool eddy deposits (Fig. 4B).

Fig. 8. (A) Typical siliceous, columnar micro-stromatolite from Pork Chop Geyser that exhibits a branching morphology (arrow). These stromatolites started at a common horizon and grew upward. The micro-stromatolites merge near the center of the photograph. Irregularly swirled laminae are common within the stromatolite. (B) Detail of a siliceous micro-stromatolite from Pork Chop geyser from upper portion of Fig. 7H. Irregularity of laminae thicknesses within the stromatolitic head is evident (arrow) and individual laminae pinch out laterally. Scale bar is 1.5 mm. (C) Micro-stromatolite from interior of siliceous lilypad shown in Fig. 5C. (D) Pool margin at Spindle Geyser. Spiny stromatolitic precipitates and associated filamentous microbial colonies (arrow) typify the subaqueous pool margin facies. Hammer is 30 cm long. (E) Opaline stromatolites from pool-margin at Spindle Geyser shown in (D). Small spiny forms (bottom arrow) grade into larger fan-shaped stromatolites (top arrow). Up to the top of the page. Scale bars equal 1 cm.

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These form in shallow ponds (2 –3 cm deep) or in areas sheltered from a strong flow of hot waters by the stromatolites. In either case, complete inundation with the hottest spring water does not occur. A lack of winnowing currents in these areas allows much debris to accumulate. Despite the presence of hot water, a wide variety of opaline ‘‘allochems’’ are present in the precipitates. In hand specimen, pool eddy precipitates exhibit poorly developed layering approximately 4 mm thick. The precipitate has a grey and white color. The grey color is associated with fine sand-sized aggregates of opal in the cement, whereas the white color is restricted to preserved fossil remains (Fig. 9C). Despite lower temperature conditions, most of the preserved opaline allochems are external molds seldom exhibiting internal cellular preservation. The allochems are very loosely bound, and thus the rock is friable. Silicified fossil debris is abundant and includes reed stems, juvenile pine cones, bark chips, and rare animal remains. The precipitate is predominantly a hash of plant debris, most of which is not preserved in growth position. Reed stems are elongate segments up to 2.5 cm long and 0.2 cm wide, rarely with external stem ultrastructure preserved. Juvenile pine cones, typically 0.6 cm by 0.4 cm, are abundant. Preserved bark chips are dispersed throughout the precipitate and are rectangular (1.2 cm by 0.6 cm). At least three distinct horizons of moth remains are present in pool eddy deposits from Pork Chop Geyser. The typical silicified moth has a wingspan of 3.3 cm and a length of 1.2 cm. Vertebrate (reptilian) remains have also been recovered from pool eddy deposits at Pork Chop Geyser (Sturtevant, personal communication, 1999). Minor amounts of detrital quartz are also present and are probably transported into the deposit as windblown detritus. Thus, allochthonous material is an important contributor to these sediments.

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In this facies, temperature and pH conditions are variable depending on localized conditions including fluctuations in pool water level and variations in the ambient temperature. 7.6. Discharge channel/flowpath facies The discharge channel/flowpath facies occurs in areas where spring discharge occurs either as sheet or channelized flow (Fig. 4B). It is adjacent to the pool margin or pool eddy, and can extend for several tens of meters away from the pool. The degree of mineralization can be highly variable depending on flow regimes, from unsilicified microbial mat material to thoroughly silicified ‘‘shrubby’’ precipitates (Figs. 3E and 5A). Whereas shrubs tend to be associated with springs with high flow volumes, microbial mats appear to be best-developed in areas of low discharge. Although bacterial shrubs are the hallmark of this facies, other constituents are also present. For example, plant debris can also be swept into the flowpath, and spicular shrubs frequently encrust pinecones and tree limbs. Pollen grains and diatoms may also be present. In areas displaying low rates of silicification, shrubs tend to be poorly developed in the spring discharge channels. These springs also typically exhibit lower flow rates and depths of water (1 –2 cm deep). Microbial mats can become well developed in these settings and approach 2 cm in thickness. Where microbial mats colonize the discharge channel, a downstream direction ‘v’-shaped pattern develops with respect to the colonizing biota (Brock and Brock, 1971). Commonly, ‘v’ patterns are marked by profound changes in the color and constituency of the mat. This pattern develops in response to a decrease in temperature laterally away from the center of the channel.

Fig. 9. (A) Bacterial cells and silicified polymeric substances from the pool margin lilypad stromatolites at Cistern Spring. Arrows indicate bacterial cells analyzed by EDS. EDS patterns showed strong carbon, sodium, sulfur, and phosphorus elemental peaks. (B) Euhedral alunite crystal, an authigenic mineral associated with the pool margin facies at Cistern Spring. This crystal appears to be twinned (arrow). Adjacent precipitates are opal-A. (C) Silicified hash of local floral and faunal debris, including a silicified moth (black arrow), from the pool eddy facies (Pork Chop Geyser). Elongate white features are silicified reed stems. Included within this hash are juvenile pinecones (white arrow). Scale increments are 1 cm. (D) Photomicrograph of a siliceous shrub from Pork Chop Geyser with a radiating, arborescent form. White areas are opalA, darker areas are porosity and greenish organic matter. (E) Photomicrograph of a horizon of longitudinal and transverse silicified microbial filaments from Pork Chop Geyser. Transverse sections of filaments are present in the upper right-hand corner (arrow). The silica coats on the transverse sections give them a ‘‘micro-donut’’ morphology. (F) Photomicrograph of microbial filaments within a shrubby horizon of siliceous sinter from Pork Chop Geyser. Internal cellular structure (left arrow) and sheath (right arrow) are exquisitely preserved.

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As previously mentioned, shrubs have a radiating or arborescent form (Fig. 5A). Shrubby opal is most common at Cistern Spring and Pork Chop Geyser. Petrographic analyses of shrubs from Cistern Spring reveal bladed opaline precipitates (1 – 5 cm high). These siliceous shrubs exhibit irregular millimeterscale internal laminations sub-parallel to the edge of the blade, and have large vertically oriented pore spaces. A series of artificial glass substrates deployed at Cistern Spring for 5 days in this ‘‘shrubby’’ facies exhibited only minor amounts of silica precipitation. Orange-green filamentous microbes < 10 Am wide produced a dense tangle of cells across the surface of the glass slide. Shrubby siliceous sinters from Pork Chop Geyser contain hollow, tubular relict microbial filaments (Fig. 9A,E,F). At higher magnification, these filaments can be seen to contain hollow sheaths (trichomes), occasional preserved internal cellular remains (hormogonia), and cellular differentiation patterns (e.g., heterocysts in chains) (Fig. 9F). Division septa can be observed in a few of the thinsections. Where some degree of cellular preservation is evident, the filaments have an orange-green hue in transmitted light, and are 2.5 Am wide and variable in length (up to 100 Am); the sheath wall is approximately 0.75 Am thick. In most thin-sections, silicification appears to be restricted to the microbial filament. This opaline precipitate forms a clear halo around the filament in cross-section, producing a ‘‘micro-donut’’ form in transverse section (Fig. 9E). Siliceous coatings are typically 3.5 Am wide on either side of the filament. A cyanobacterial origin is indicated by the (1) preservation of a sheath surrounding the filament (trichome), and (2) preservation of cellular differentiation patterns. Shrubby horizons from Cistern Spring have a more diverse microbial assem-

blage preserved than those from Pork Chop. Noncyanobacterial filaments are present in Cistern Spring precipitates that are up to 4 Am in width and 80 Am long, and lack a pronounced sheath (Fig. 10A). Commonly, the margin of the filament (cell-wall) is poorly preserved, and a thin-line of porosity is developed along the flanks. These filaments lack any pronounced color in transmitted light. Similar features can also be observed in shrubby horizons from Pork Chop Geyser. These filamentous microbes are 2– 5 Am in diameter and exhibit highly variable lengths. Under 1000  magnification, other seemingly occult microbes become more readily visible; these smaller bacilliform microbes are much more numerous. Discrete brown, sausage-shaped rods (Fig. 10B) are on the order of 1 Am in length, a typical size for a bacterium, and commonly exhibit two cell chain (strep) arrangements. Other dense aggregates of micron-size coccoid cells can also be observed in shrubby horizons. These are typically greenish-grey in transmitted light, and form regular sheets of silicified cells (Fig. 10C). This pattern is typical of bacterial cells that divide in sheets. Shrubs were observed to fluoresce under 420-nm wavelength light. Pine pollen grains can also be observed interspersed among the shrubby precipitates and also exhibited very strong fluorescence. SEM images reveal that the siliceous shrubs are composed of a dense tangle of filaments and polymeric substances in various stages of silicification. The surface of the shrub is smooth. Where this smooth surface is broken, filaments are readily apparent. These silicified filaments commonly are 1 Am wide, and often exhibit pronounced division septae (Fig. 10D). Silicified polymeric substances, or biofilms, are very abundant, and some strands can be

Fig. 10. (A) Photomicrograph of filamentous microbial remains (arrow) from Cistern Spring preserved in opal-A. This sample comes from a shrubby horizon. These filaments do not exhibit a pronounced sheath and are clear in transmitted light. (B) Photomicrograph shows abundant bacterial cells from the shrubs at Cistern Spring preserved in opal. These cells are dark, straight or curved rods (arrow), and average 1 Am in length. These cells are very abundant. (C) Photomicrograph of a dense mass of micron-sized spherical bacterial cells arranged in a sheet (arrow). These are from a shrubby horizon at Pork Chop Geyser and are preserved in opal-A. (D) SEM image of a silicified microbial filament preserved in shrubby sinter from Cistern Spring. Background material is opal-A. The presence of division septae (white arrows) is an additional criterion to establish biogenicity of these features. (E) SEM image of silicified and partially lysed bacterial cells grouped in a chain (strep arrangement) (black arrow). These are associated with the discharge channel facies at Deerbone Spring. Background material is nanometer-scale aggregates of opal-A (white arrow). (F) Close-up of lower-right area in (E). Nanometer-scale spheres are abundant and closely associated with some of the silicified cells and mucous strands (arrows). It is difficult to tell whether the spheres are abiotic spheres of opal-A, silicified nanobacteria, or both. (G) Remains of a pennate diatom from the discharge channel facies at Deerbone Spring. Draping polymeric substances are attached to the diatom (left arrow) and a bacterial chain rests atop the diatom test (right arrow).

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observed to desiccate under the electron beam. Some of the filaments appear to be smooth, whereas others have a heavy coating of opaline spheres 100 – 200 nm in diameter. A few of the spheres associated with silicified filaments and polymeric substances are hollow. In areas where siliceous shrubs are not apparent, desiccated mats along the discharge channels exhibit a strong biotic influence on the fabric of the siliceous precipitate. These microbial mats are composed of a complex community of microbes commonly having a filamentous form. Preservation of silicified cells is exquisite and consists of micron-sized streptobacillus and streptococcally arranged cells (Fig. 10E). Threads of silicified polymeric substances are also a major constituent (Fig. 10F). Siliceous precipitates appear to be preferentially associated with these organic residues, and are far less widespread in areas lacking readily apparent organic material. Other microscopic members of the mat community include pennate diatoms (10 –15 Am long and 2 –3 Am wide) (Fig. 10G) where temperature conditions ameliorate. These diatoms closely resemble Cymbella and Nitzschia (Jones et al., 2001a). Bisaccate pine pollen (20 –30 Am in diameter) is also common in many of the discharge channels. Because of the ephemeral nature of the spring discharge channels and their associated flow regimes, conditions in this facies also can be highly variable. Temperatures observed at Cistern Spring range from approximately 70 jC to ambient temperatures over a downflow distance of 20 m. Thus, this facies has the greatest temperature gradient in the entire siliceous hot spring facies model. 7.7. Debris apron facies Perhaps the most widespread lithofacies associated with modern siliceous hot springs is the debris apron (Fig. 4B). This facies consists of siliceous intraclasts

derived from all the other facies. Weathering processes are likely to act quickly on siliceous deposits that are no longer in areas of active mineral precipitation and produce these intraclasts. Much of the Norris Geyser Basin consists of a thin veneer of platy, reworked siliceous sinter fragments in various stages of lithification (Fig. 11A). Because of this great volume of platy, siliceous sediment, it is likely that much of the material beneath the geyser basins is chiefly composed of this rock type. Clasts are generally 0.5 –2 cm in length (though dimensions range greatly) and very angular. Although found throughout the Norris Geyser Basin in more distal areas of the thermal systems, fragmented sinters can be found in any area of geyser basins. Some of the horizons exhibit a pronounced red hematite stain. The clasts exhibit a wide variety in the size, shape, and degree of cementation. When lithified, individual clasts appear to be coated by a thin, clear isopachous coating of amorphous silica. This isopachous coating indicates that cementation occurred in the phreatic zone. Taken as a whole, this facies represents the lowest temperature regime of the siliceous hot spring model. 7.8. Geyser facies Intermittent geyser activity at a hot spring site can create a completely different lithofacies that will complicate the existing facies patterns. Good examples of geyser facies can be seen at Pork Chop Geyser and Veteran Geyser (Fig. 11B,C). Periodic agitation associated with water level fluctuations and eruptive activity can produce siliceous oncoids. Oncoids are either cemented in place or free to move about. They are up to 4.5 cm wide with an average of 2 cm. Where cemented, the crusts tend to be very well indurated. At Veteran Geyser, the oncoids are white/grey with a slight bumpy appearance. Smaller oncoids at Veteran Geyser tend to be more spherical than the larger

Fig. 11. (A) Brecciated fragments of siliceous sinter on the debris apron of Cistern Spring. Scale bar is 15 cm. (B) Veteran Geyser in the Norris Geyser Basin of Yellowstone. Siliceous oncoids are developing in the pool adjacent to the geyser orifice (arrow). Pool is about 2 m wide. (C) Close-up of the area shown in (B). Siliceous oncoids up to 4 cm in diameter are associated with the Veteran Geyser pool (arrow). (D) Thinsection photomicrograph of siliceous oncoid from Pork Chop Geyser. High irregularity of the laminations is attributed to microbial action. A distinct nucleus of reworked siliceous sinter is evident (arrow). (E) SEM image of silicified microbial filament (top arrow) preserved within a siliceous oncoid. The filament is hollow and apparently devoid of microbial organic remains. Internal cellular structures are preserved as external molds with septae. Transverse section of a filament reveals a ‘‘micro-donut’’ form (bottom arrow). Opaline precipitates are intimately associated with the presence of these microbial filaments.

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forms. They commonly occupy the bottom of a small pool or depression and are closely spaced around the geyser orifice. Water levels in the geyser pools fluctuate. The oncoids are stirred by undulating currents

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and geyser spray. Thus, these are good indicators of a geyser pool and its intermittent activity. At a microscale, these opaline-coated grains tend to have a nucleus of either an igneous rock fragment,

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rigid plant debris (e.g., pinecones), or reworked siliceous sinter (Fig. 11D). These interior clasts commonly have a platy form. For example, the interior of one siliceous oncoid from Pork Chop Geyser contained an interior clast 2.8 mm long and 0.75 mm in height. Commonly, the interior clast exhibits some signs of advanced diagenetic alteration not shown in the outer parts of the cortices. In reworked sinter nuclei, this includes partial dissolution, isopachous opal, or isopachous hematite cements. These oncoids exhibit a great deal of variability within their cortices. Generally, the oncoid nucleus is surrounded by a highly porous zone (0.35 mm thick) of silicified tubules. These filaments are orange/green (sheathed) or purple (non-sheathed) in transmitted light, 2– 3 Am in diameter, and up to 350 Am long. Orientation of the tubules within the cortices is variable; some are oriented perpendicular to the nucleus, whereas others are tangentially oriented. Opal is preferentially precipitated along the microbial filaments; the outer diameter of the thread plus precipitate is about 20 Am across. These filamentous layers commonly grade into another heavily silicified layer about 0.9 mm thick that contains dark irregular, discontinuous laminae (20 Am thick) which can be observed to pinchout laterally. Porosity is not as abundant in this layer of the oncoid as in the initial filamentous layers, but is present as occasional fenestrae up to 1.1 mm long and 0.1 mm high. The fenestrae are parallel to the oncoid laminations. Pine pollen can also be observed in this layer, usually preserved as external molds, arranged in quasi-concentric laminae 0.6 mm wide. Pine pollen laminae were observed to repeat several times within this zone of the oncoid and may represent seasonal laminae. The outer zone of the layer contains abundant, opaline micro-stromatolites. These micro-stromatolites exhibit a club-shaped morphology and fine dark laminations f 2 Am thick. Individual stromatolites are up to 1.4 mm wide and 1.8 mm high. Not all of these outer zone stromatolites are oriented perpendicular to the oncoid nucleus. A few of these appear to be oriented subparallel to the long axis of the nucleus. Stromatolitic laminations were observed to fluoresce. Thus, a significant biologic influence was probably involved in oncoid formation. Pollen grains can also be observed in pockets between the micro-stromatolitic heads and rarely as laminations within the stromatolite. Silt-sized, angular quartz grains are ran-

domly distributed throughout most of the oncoid cortices. SEM images show little detail about the laminae within the oncoid cortices, however, microbial filaments were evident (Fig. 11E). There is no preferred orientation to these microbial filaments. Tubules appear to be hollow and devoid of relict microbial organic remains. Opal appears to be preferentially precipitated around the microbial filaments. Because these oncoids are associated with geyser activity, pools must be periodically inundated with very hot water. However, when activity begins to subside, conditions soon ameliorate allowing the colonization by a wide array of microbes. Therefore, the geyser pools experience a highly variable temperature regime.

8. Discussion 8.1. Vent facies Length-fast chalcedony is a common constituent in this particular facies and, as a result, is a reliable facies indicator. Chalcedony has been documented as a cavity fill in a wide array of sedimentary rocks (White et al., 1956; Folk and Pittman, 1971; Pittman and Folk, 1971; Krainer and Spotl, 1998). Previous studies of Yellowstone’s hydrothermal systems, however, failed to identify chalcedony at locations near the surface, for example, ‘‘Chalcedony. . .has not been recognized in primary hot-spring sinter or near-surface veins’’ (White et al., 1956, p. 54). However, chalcedony is very likely to be one of the most common higher temperature precipitates lining near-surface and subsurface vents. In this study, no other forms of authigenic quartz were identified in the vent facies. Because chalcedony is restricted to the interior of hot spring vents, conditions of formation can be inferred. Although Murata et al. (1978) estimated a temperature of approximately 79 jC for the cristobalite/chalcedony transition, temperatures for chalcedony formation in this setting must be appreciably higher. Based on temperatures recorded at the pool margin at Cistern Spring, formation of chalcedony must be near the boiling point of water for the altitude (approximately 95 jC) or higher due to superheating in the shallow subsurface. As a result of these elevated

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temperatures and pressures, this particular type of chalcedony appears to be a primary precipitate rather than a secondary replacement of opal. The high temperatures associated with this facies exclude most prokaryotes and eukaryotes. Consistent with this, readily identifiable biological remains are absent and the fabric is abiotic. Thus, this facies represents the pure abiotic end-member of siliceous sinter precipitates. 8.2. Proximal vent facies The length-slow chalcedony associated with the proximal vent facies is a relatively uncommon form of chalcedony in most natural deposits and is a pronounced change from the length-fast chalcedony of the vent facies. Length-slow chalcedony is believed to form when either sulfate (Folk and Pittman, 1971) or magnesium (Kastner, 1979) is present in the precipitating solution (Knauth, 1994), and never has been documented from hot spring settings. Depositional fabrics indicate that the length-slow chalcedony in the proximal vent facies is a primary precipitate (Fig. 8E). Although observed sulfate concentrations range from 70 to 90 ppm in the solutions from Cistern Spring, it is unlikely that the sulfate model of Folk and Pittman (1971) adequately explains the close juxtaposition of length-fast (vent) and length-slow (proximal vent) varieties. Although evaporites are occasionally found in association with at least one of the other facies, there was no evidence of relict evaporites associated with the proximal vent facies. Measured magnesium elemental abundances were not sufficiently high to invoke the ‘‘magnesium’’ model proposed by Kastner (1979). Thus, the occurrence of length-slow chalcedony in this facies must be attributed to some other factor (e.g., precipitation rate?). Preserved biota in this facies is essentially absent. Abiotic precipitation must be the dominant mechanism of siliceous sinter precipitation in this facies. 8.3. Pool facies Opal precipitates are sparse in the pool and are markedly different from the two previously described facies. Identification of this facies in hand-specimen without adequate field relationships is difficult and chances for recognition in the rock record are low.

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The ‘‘rilled’’ characteristic of this facies was termed ‘‘flowline trends’’ by White et al. (1988, p. 22). Apparently, sculpting of pool bottom precipitates is achieved by convecting water currents containing colloidal silica (White et al., 1988). White et al. (1988) documented this facies immediately after Cistern Spring drained due to Steamboat Geyser’s major eruption. ‘‘Sinter ‘scales’ had deposited on logs and pool margins. . .pointing in direction of convective flow’’ (White et al., 1988, p. 46). Thus, convective flow indicators in the spring cauldron deposits are perhaps the best indicators of the facies. Most of the information about microfabrics in the pool bottom sediments has been gleaned from precipitates on the artificial substrates placed in the water. The presence of an abundant threadwork of microbial filaments composed of Gram-negative (non-cyanobacterial) streptococci indicates a strong biotic influence on the fabric. Whereas present in some hot springs, the distribution of native sulfur in the pool facies of Yellowstone’s hot springs is not ubiquitous. However, it has been documented in a few hot spring systems elsewhere in the world (Jones et al., 2000). Native sulfur has been associated with the actions of bacteria and various taxa can assemble and store native sulfur externally or internally as granules (Brock and Madigan, 1988; Prescott et al., 1996). A biologic origin seems especially appropriate based on the association of the sulfur crystals with silicified polymeric substances. Thus, the pool facies marks a pronounced increase in the amount of biologic activity in the environment and its influence on the resulting precipitates. 8.4. Pool margin facies Stromatolites are common around the margins of the pool in Yellowstone’s many travertine and siliceous sinter precipitating hot springs. Whereas many stromatolite morphotypes are observed, the lilypad forms are the most common. These stromatolitic precipitates resemble lilypads described by Renaut et al. (1999a). Cooling and evaporation were believed to be the dominant abiotic controls on silica precipitation; however, coccoid and filamentous bacteria were thought to play some passive role in the precipitation process as organic templates (Renaut et al., 1999a,b). Although Walter (1976a) and Grotzinger and Rothman

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(1996) discuss an abiotic origin for some modern and ancient stromatolites, we believe that bacteria play a profound role in the construction of these features through either passive or active precipitational processes. A bacterial origin is especially appropriate based on several lines of evidence, such as: (1) the stromatolites are closely associated with bacteria which were observed colonizing their surfaces on the outcrop, (2) filamentous remains within the stromatolites can be routinely identified in SEM images, (3) irregular laminations in the stromatolites display fluorescence, (4) internal micro-stromatolites occasionally exhibit pronounced changes in growth direction which could not be attributed to simple ‘‘wicking’’ or ‘‘cornice’’ precipitation, and (5) the presence of residues in the stromatolites that produce a strong carbon elemental peak in EDS. Thus, abundant evidence indicates that the stromatolitic features observed in these environments are indeed biological in origin. What has not been determined is whether precipitation of the silica is biologically induced or is just using the biota as a hospitable template. Accessory or authigenic minerals may be good micro-indicators of this facies. Alunite was recognized only in this facies, and has also been identified in association with Yellowstone cores (Bargar and Beeson, 1985) and ancient siliceous sinters (Walter et al., 1996). It is formed by sulfuric acid solutions acting on rocks containing potassium feldspar (Klein and Hurlbut, 1977) and has been documented in many other settings that display hydrothermal alteration (Bove and Hon, 1990). Spherical aggregates of fluorite were also present in this part of the hot spring deposit. Fluorite with a similar anhedral, spherical habit has also been documented in pool settings in some of Yellowstone’s travertine hot springs (Allen et al., 2000), thus this facies indicator also exists in carbonate systems. The presence of gypsum, an evaporite mineral, within some of the stromatolites indicates that there is some abiotic influence on mineral precipitation in the pool margin facies. 8.5. Pool eddy facies Pool eddy deposits are less likely to be recognized in the rock record than accumulations from the other facies. Isolated occurrences of this facies may occur in discrete pockets interspersed among the lilypad stro-

matolites. Because of the abundant eukaryotic remains associated with the facies, ancient pool eddy deposits may shed light on the higher taxa of the hot spring community. 8.6. Discharge channel/flowpath facies Though not universal in their occurrence, shrubs are a reasonably good indicator of a discharge channel setting. The biologic affinity of these shrubs is unknown, however, much work has been done in the travertine systems to suggest that the shrubs are indeed bacterial in origin (Chafetz and Folk, 1984; Chafetz and Guidry, 1999). Similar shrub morphologies have also been produced by laboratory grown bacterial colonies, supporting the interpretation of their biotic origin (Ben-Jacob et al., 1994; Ben-Jacob, 1997; BenJacob and Levine, 1998; Ball, 1999). Thus, siliceous shrubs have a great biotic influence on their fabric. Spicular shrubs, first described by Walter (1976b), are spinose with sharp terminations (Walter, 1976b, Figs. 4 and 5, pp. 493 – 494). Walter (1976b) attributed the formation of these spicular shrubs to photosynthetic cyanobacteria, predominantly Chloroflexus aurantiacus and Synechococcus sp. However, the radiating terminations in the shrubs studied here look more like those attributed to Conophyton weedii (Walter et al., 1976, Fig. 12, p. 286) or Phormidium sp. (Walter et al., 1996). Another reason to infer a microbial origin for these shrubs is the presence of abundant preserved microbial filaments. A filamentous cyanobacterium similar to Phormidium sp. (Walter, 1976b; Walter et al., 1996) may be involved in their genesis. Additionally, sausage-shaped cells observed in petrographic thin-sections of siliceous shrubs closely resemble the cells of Synechococcus lividus described by Ward et al. (1989, Fig. 2, p.7). Other features observed in the rock record with a strong resemblance to shrubs are the ‘‘palisade’’ precipitates of Walter et al. (1996), which were originally recognized in carbonates (Hardie and Ginsburg, 1977). These features contain abundant silicified, hollow tubules within the shrubby precipitates. Walter et al. (1996) attributed these features to the action of cyanobacteria (e.g., Calothrix) in shallow micro-terracettes. We propose that a community of microbes is probably involved in the construction of these precipitate fabrics as either organic templates or

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possibly biotically induced precipitates, and the palisades may represent the erosionally truncated bases of the siliceous shrubs. As a testament to the influence of the biota on the precipitate fabric, no shrubby precipitates were observed in the higher temperature regimes believed to represent the purely ‘‘abiotic’’ end-member precipitates (e.g., vent). Microbial mats along the discharge channels of Deerbone Spring are similar to those described by Castenholz (1984) and Ward et al. (1989) at Octopus Spring. Because of close proximity and similar water conditions, it is reasonable to suspect that the matforming organisms are probably very similar. Crosssections of extant Deerbone microbial mat closely resembled those of Ward et al. (1989, Fig. 1A, p. 4), who determined that the predominant microbes involved in mat construction were S. lividus and C. aurantiacus. Synechococcus is a cyanobacterium, whereas Chloroflexus is filamentous green, non-sulfur, phototroph. Heterotrophic bacteria were also thought to be present in the top few millimeters of the mat (Ward et al., 1989). Growth rates of the mat were estimated to be 18 –45 Am/day (Ward et al., 1989). Laminated mats colonizing the discharge channel are distinctive features restricted to this facies. Although abiotic processes may also contribute to silica precipitation (e.g., evaporation), we believe the biota has the greatest influence on the precipitate fabric in the discharge channel facies. This strong biological control over the fabric of the precipitate has been recognized for quite some time (see Walter, 1976b). Birnbaum and Wireman (1984, 1985) have documented that sulfate-reducing bacteria are capable of decreasing dissolved silica in solutions, thereby demonstrating that bacteria can have a role in inducing the precipitation of silica. If bacteria are capable of actively mediating the precipitation of silica in the laboratory, it is very likely that they are indeed capable of the same in nature. With such a strong relationship between the precipitate morphology and the biological constituents, it is possible that organisms play an active role in the precipitational process, and therefore the precipitates are interpreted to be biotically induced. 8.7. Debris apron facies The clasts that comprise this facies originated due to: (1) mechanical weathering induced by erosion,

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desiccation, or frost action, and less commonly, (2) physical fragmentation induced by grazing ungulant trampling (Walter, 1976b). Analyses of cemented clasts reworked from this facies may be the only preserved record of hydrothermal spring activity and have provided a wealth of information about the early history of the Norris Geyser Basin (White et al., 1988). Based on observations of numerous cores drilled in the thermal areas of Yellowstone National Park, this is the most common facies. 8.8. Geyser facies Geyser deposits are best characterized by their association with siliceous oncoids. Siliceous coated grains have been recognized in association with geysers elsewhere in the world (Renaut et al., 1996; Jones and Renaut, 1997) and have long been recognized in carbonate depositional environments (Chafetz and Meredith, 1983; Nickel, 1983; Peryt, 1983). In carbonate systems, most oncoids owe their origin to the action of algae and bacteria (Peryt, 1983), although abiotic pisoids have also been well documented (Folk and Chafetz, 1983). According to Folk and Chafetz (1983), the two different varieties of coated grains are relatively easy to distinguish in the travertine precipitating systems. Abiotic coated grain (pisoid) accumulations commonly display cross-stratification, and individual grains tend to have very fine, uniform concentric laminae, smooth external spherical form, and a general paucity of bacterial colonies. In contrast, accumulations of biotic coated grains (oncoids) lack cross-stratification, and individual grains tend to exhibit highly irregular cortical layers, a crudely radial structure, lumpy forms, and radially arranged bacterial clumps (Folk and Chafetz, 1983, p. 475). A biologic origin is favored for all coated grains observed in the course of this investigation. Observations of numerous thin-section always revealed irregular laminations in the cortices that pinched and swelled laterally within each thin-section. Filamentous microbes associated with these features exhibited marked pigmentation in transmitted light. These filaments probably represent a diverse community of sheathed cyanobacteria as well as purple filamentous sulfur bacteria. Fluorescent characteristics associated with the siliceous oncoids further support a biotic origin for the stromatolites and the filaments.

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Oncoids were observed in boulders from Pork Chop Geyser, as well as in the modern at Veteran Geyser, Norris Geyser Basin. Siliceous oncoids are trapped in a depression surrounding the geyser orifice. According to the model presented by Jones et al. (1998, p. 431), these form at spillpoints along the spring outflow. Observations based in Yellowstone, however, substantiate a geyser pool origin for these features. Other siliceous oncoids similar to those described here were observed by Jones et al. (2001b) and referred to as ‘‘geyser eggs’’. At other localities, the predominant microbe observed in association with oncoids is believed to be Calothrix (Renaut et al., 1996; Jones and Renaut, 1997). At Yellowstone, it is difficult to say exactly which microbe(s) are present in the oncoids because most microbial activity involves communities of bacteria. A model for siliceous oncoid formation devised by Jones and Renaut (1997) has micro-stromatolitic growth as the first phase to appear on the nucleus. The last phase of precipitation involved concentric laminae that coat the zone of micro-stromatolites. However, we observed oncoids in which the first phase of precipitation involved the formation of a concentric layer of distinct microbial filaments lacking a club-shaped micro-stromatolitic form. The second phase included concentric laminae with occasional dark wisps lacking well-preserved filaments, and the third phase was a club-shaped, micro-stromatolitic growth. Thus, the micro-stromatolites are the last phase in the development of oncoids studied from the geysers at Yellowstone. Similar siliceous oncoids were observed by Walter et al. (1996, p. 511). Although they were recognized to have formed in turbulent, shallow pools near geysers, they were described as sinter pisoids. Externally, they closely resemble the oncoids of Veteran Geyser and Bead Geyser in Yellowstone. However, Walter et al. (1996) provide no petrographic descriptions of the microfabrics associated with the Verbena ‘‘pisoids’’. 8.9. Biotic/abiotic controls on facies These microfabrics shed light on the mechanisms governing siliceous sinter precipitation as well as aid in the construction of a conventional facies model for siliceous hot springs. Precipitates form a natural pro-

gression between those that are little influenced by the extant biota to those that have a strong biotic influence on the microfabric. Chalcedony and chalcedonic crusts represent the high-temperature regimes where precipitation is dominated by abiotic processes within the siliceous hot springs. Away from the higher temperature regimes, biotic influence on precipitate fabrics becomes more apparent. In the pool facies, the precipitates are most influenced by currents, however, microbes are present and may have the greatest effect on the precipitation of native sulfur. Silicified polymeric substances binding the enmeshed native sulfur indicates that microbes play a role in the formation of the precipitate. A pronounced biotic influence, whether passive or active, is evident in the microfabric of the pool margin stromatolites (lilypads). Although abiotic processes cannot entirely be discounted, the discharge channel/flowpath facies’ shrubs and silicified microbial mats display the greatest biologic influence on the architectural fabric of the precipitates.

9. Conclusions Siliceous hot springs exhibit a wide variety of distinctive morphologies: siliceous spires/cones, domal mounds, terraced mounds, and ponds. Many of these morphologies are analogous to those in the travertine precipitating systems. It is these distinctive morphologies that make these deposits easy to discern from the adjacent deposits. Investigation of a number of these siliceous hot springs yields a relatively simple facies tract. Most siliceous hot springs at Yellowstone can be divided into eight lithofacies, although all may not be present at any given locality or outcrop; these facies include: vent, proximal vent, pool, pool margin, pool eddy, discharge channel/flowpath, debris apron, and geyser. Recognition of these depositional facies in hot springs is paramount for us to recognize these deposits in the rock record. A number of characteristics can be used to successfully identify relict hot spring deposits or facies. Macroscopic length-fast, chalcedonic quartz crystals are the best indicator for the vent facies. Laminated opal with incipient length-slow chalcedony indicates the proximal vent facies. These characteristics of the chalcedony make them ideal facies indicators in these

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systems. Pool bottoms are best identified by outcrop patterns of the ‘‘rilled’’ opaline siliceous sinter, exhibiting some remnants of flow structures. Lilypad stromatolites are the best indicators for the pool margin facies and may also contain fenestrae and/or evaporites. The eddy facies is characterized by a hash of eukaryotic remains. Bacterial shrubs and silicified filaments are the best indicators for discharge channel/flowpath facies. Debris apron facies are an admixture of reworked fragments from other facies. Geyser facies may/may not be present, but are indicated by siliceous oncoids. Hot spring precipitates form a natural progression between those with little biotic influence on their fabric to those with a predominantly biotic contribution. Vent chalcedony represents the pure abiotic endmember, whereas opaline shrubs represent precipitates with a large biotic influence on precipitate fabric and architecture. Although, in nature, it is difficult to ascertain the relative degree of biotic/abiotic influences, some of these hot spring siliceous precipitates (e.g., siliceous shrubs) may indeed be mediated by the vital activities of bacteria.

Acknowledgements This research is supported by a NASA-Johnson Space Center Astrobiology Grant (to HSC), GSA Student Grant #6239-98 (to SG), SEPM Robert J. Weimer Student Grant (to SG), and The Explorers Club-Rocky Mountain Chapter Grant (to SG). The authors would like to thank Bennie Guidry, Janis Guidry, Tamara Kneen, and Yonqiang Wu for field assistance in Yellowstone over the course of the investigation. We also thank the National Park Service (Ann Deutch, John Varley) for permission to sample in the park. We greatly appreciate the assistance of Dr. Frances Westall with microscopy. The authors also wish to thank Bruce Sellwood, Bruce Fouke, and Robin Renaut for insightful reviews of the manuscript.

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