Diagenesis of Cretaceous limestones in the Edwards aquifer system of south-central Texas: A scanning electron microscope study

Sedimentary Geology, 21 (1978) 241--276

241

@)Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

DIAGENESIS OF CRETACEOUS LIMESTONES IN THE EDWARDS AQUIFER SYSTEM OF SOUTH-CENTRAL TEXAS: A SCANNING ELECTRON MICROSCOPE STUDY

MARK W. LONGMAN and PATRICIA A. MENCH

Cities Service Co., Box 50408, Tulsa, Okla. 74150 (U.S.A.) Department of Geological Sciences, The University of Texas, Austin, Texas 78 712 (U.S.A.) (Received November 7, 1977; revised and accepted January 24, 1978)

ABSTRACT Longman, M.W. and Mench, P.A., 1978. Diagenesis of Cretaceous limestones in the Edwards aquifer system of south-central Texas: ~ scanning electron microscope study. Sediment. Geol., 21: 241--276. Diagenesis in shallow cores from the Lower Cretaceous Edwards Limestone was investigated in thin sections and with the scanning electron microscope (SEM). The SEM is a particularly useful tool in the study of diagenesis in porous fine-grained carbonate rocks because of its good resolution and depth of field. The Edwards Group was deposited in shallow-marine environments and underwent normal early diagenesis. Dolomite and evaporite minerals such as gypsum formed penecontemporaneously in some tidal-flat sediments. Slightly later, when the carbonate sediments were flushed by fresh water, carbonate mud recrystallized to micrite and aragonite allochems altered to calcite or were leached. Some cementation by calcite occurred in a fresh-water phreatic environment. The Edwards Limestone was divided into two zones by Miocene faulting along the Balcones Fault Zone. On the upthrown side of the fault a circulating fresh-water aquifer developed, whereas relatively stagnant brackish water remained present on the downthrown side. Differences in the chemistry of the interstitial fluids in these zones resulted in different types of diagenesis. The presence of fresh water caused extensive oxidation, solution along fractures, recrystallization of micrite to coarse microspar and pseudospar, precipitation of equant sparry-calcite crystals in a variety of shapes and sizes, and extensive dedolomitization. The dedolomitization is thought to have been caused by the high Ca/Mg ratio of the circulating fresh water in a shallow subsurface environment. In the brackish-water zone, textures and fabrics related to deposition or early diagenesis, such as primary porosity, unoxidized organic material, framboidal pyrite, and evaporite minerals have been preserved. Some precipitation of authigenic dolomite, celestite, and kaolinite occurred in the brackish-water zone. In contrast to the fresh-water zone, precipitation of coarse calcite spar, dedolomitization, and recrystallization of micrite to microspar occurred only rarely in the brackish-water zone.

INTRODUCTION T h e scanning e l e c t r o n m i c r o s c o p e (SEM) has b e e n used with r e m a r k a b l e results to study recent carbonates. Details of carbonate muds, submarine

242 cements, and microorganisms that are barely discernible in thin section are clearly revealed with the SEM. In spite of this success, the SEM has n o t been used to the same extent on ancient carbonates. There are several reasons for this, including the tendency of limestones to become tightly cemented, dolomitized, or recrystallized. All of these diagenetic processes tend to obscure primary textures, b u t under favorable conditions they can produce interesting and characteristic secondary textures that are clearly revealed with the SEM. This study deals with subsurface samples of the Cretaceous Edwards Limestone in south-central Texas. Primary emphasis is on the use of the scanning electron microscope to complement thin-section petrography with the goal of interpreting the diagenetic history of the rocks. A variety of features that are barely visible in a "two-dimensional" thin section are clearly revealed with the addition of the third dimension and higher magnification provided by the SEM. Interpretation of carbonate diagenesis is often dependent on the fabrics of the fine-grained components of limestones and these can best be observed with the electron microscope.

Previous electron microscope studies The value of the scanning electron microscope in geologic work has been stressed by Gillott (1969), Pittman and Duschatko (1970), Timur et al. (1971), and others. The SEM has several advantages over the transmission electron microscope (TEM). It permits direct observation of the sample (after application of a thin film of a conducting metal, usually gold), whereas the TEM generally requires the preparation of a plastic replica for examination. Furthermore, the SEM can easily be used to examine porous carbonates which are difficult to prepare for TEM work. Within the past few years there have been a number of SEM studies on certain aspects of ancient carbonate rocks. Chalks have received considerable attention (Scholle, 1974; Mapstone, 1975; and many others) because of their interesting microtextures and h y d r o c a r b o n potential. Recrystallization in Pleistocene fossils was studied by Schneidermann et al. (1972) and Sandberg et al. (1973). Caliche has been examined by Kahle (1977) and Knox (1977). Microporosity and its importance to the oil industry has been discussed by Pittman (1971), Timur et al. (1971) and Wardlaw (1976). Rather than emphasizing primary textures and porosity as most of these papers have done, our paper emphasizes the recognition and interpretation of characteristic diagenetic fabrics in shallow-marine carbonates. Furthermore, because samples used in this study come from both fresh-water and brackish-water diagenetic environments in the shallow subsurface, the role of these fluids in diagenesis can be determined.

Purpose and study material The study area lies in south-central Texas near San Antonio (Fig. 1). The Edwards Limestone in this region is one of the most productive aquifers in

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the southwestern United States and supplies water to a b o u t 900,000 people in and around San A nt oni o ( A bbot t , 1975). A n u m b e r of studies have recently been u n d e r t a k e n to b e t t e r understand the geology, porosity, hydrogeology, and resource potential of this e n orm ous aquifer system (A bbot t , 1975; Maclay and Small, 1976; Pearson and R e t t m a n , 1976). To complem e n t this work, a detailed analysis o f diagenesis in the aquifer was undertaken as a thesis project at the University of Texas by the junior author. Twelve cores of the Edwards Limestone were chosen for study. Eight cores were taken by the United States Geological Survey, three by the Texas Water Dev elo p m ent Board, and one by the U.S. A rm y Corps of Engineers. Depths of the cores ranged from 20 to 400 m. The average length of the cores was a b o u t 100 m. Lithology, bedding characteristics, and fossils were described and over 500 thin sections were prepared. From the thin sections, 35 samples were selected for study with the SEM. In general, these were porous samples so t ha t porosity, crystal shapes, and ot her diagenetic textures could be examined.

Sample preparation Samples for SEM study were br oke n from the bulk specimen with diagonal cutting pliers and glued to aluminum stubs with polyvinyl chloride. Only fresh fracture surfaces were examined. Loose debris was removed just before coating by holding the specimen upside dow n and gently blowing air over the surfaces with a squeeze bulb. Because porous carbonates t end to charge u n d er the electron beam, a heavy conductive coating (about 500

244

thick) was applied. Sputter-coating gold was found to be superior to the standard method of evaporating gold--palladium, because the finer particles produced tend to diffuse better around corners. However, sputter-coated gold leaves a characteristic "orange-peel" texture visible at magnifications above 10,000 ×, and one of the X-ray emission peaks of gold can obscure the sulfur peak. In samples where these artifacts created problems, chrome was sublimed in a vacuum evaporator to form a conductive coating instead of gold. The SEM used in this work was a JEOL model JSM-2 equipped with a Kevex energy dispersive X-ray spectrometer for element identification. Samples were examined at magnifications ranging from 67 to 20,000 × and numerous stereo photo pairs were taken to clarify the three-dimensional aspects of grains and cements. Porosity analyses were obtained on selected specimens with a standard Boyle's Law Porosimeter. Samples analyzed were adjacent to chips mounted

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Geologic setting Regional geology of the Edwards Limestone has been discussed by Rose (1972). The Lower Cretaceous Edwards Group consists of 1 2 0 - - 2 0 0 m of porous limestone and dolomite that was deposited on a low-relief marine platform by alternatively transgressive and regressive seas. Regressive carbonate cycles consisting of subtidal marl grading upward through intertidal grainstones into supratidal micrites and dolomites are c o m m o n in outcrops in central Texas (Mueller, 1975) and similar but less well-developed depositional sequences occur in the cores from the San Antonio area. The carbonate platform was bordered on the south and east by the Stuart City Reef Trend, a series of rudistid bioherms. The shallow Maverick Basin lay to the southwest and a positive topographic element, the San Marcos Arch, extended through the northern part of the study area (Fig. 2). Depositional environments of the Edwards Group have been discussed elsewhere (Rose, 1972; Mueller, 1975). Fig. 3 presents the major environments recognized in this study along with some of their characteristics. Tidal-flat sediments typically consist of oxidized mud with variable amounts of evaporites, dolomite, and burrowing. Sediments deposited on a restricted shallow-marine shelf contain extensive burrowing and a limited marine

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Channel-lag deposits Channel-edge slumps Extensive burrowing Settle-out laminations Wispy structures Cyclic grain-size fluctuation Limited fauna--oysters, snails, 1-2 foram, spp. Mostly mud and pellets Darker-colored sediments Association with supratidal sediments

1. Limited marine fauna, more diverse than intertidal, no pelagic elements 2. Extensive burrowing 3. Wispy structures 4. Mixed grain size, mud and sand 5. Low-energy ripples and current-streaks 6. N o associated supratidal sediments

1. Diverse marine fauna without pelagic elements 2. C o m m o n l y coarse grainsize, good sorting 3. Evidence o f high-wave and current e n e r g y l b r a d e d shells, cross-bedding, ripple marks 4. Scattered local banks and mounds 5. N o associated supratida] features

10. Mostly mud and pellets l 1. Oxidized sediments-light-colored

Fig. 3. Characteristics of the major depositional environments. From Rose (1972, p. 51).

246 fauna, t y p i c a l l y with miliolid f o r a m i n i f e r s . O p e n - m a r i n e shallow-shelf sedim e n t s c o n t a i n a diverse m a r i n e fauna, b u t usually have few pelagic f o r m s , because o f their p o s i t i o n b e h i n d t h e S t u a r t City R e e f T r e n d .

Hydrology H y d r o l o g y o f t h e E d w a r d s A q u i f e r has b e e n discussed b y A b b o t t ( 1 9 7 5 ) and Maclay and Small ( 1 9 7 6 ) . During early subaerial e x p o s u r e p r i m a r y p o r o s i t y was n o t c o m p l e t e l y filled and o n l y m i n o r a m o u n t s o f s e c o n d a r y p o r o s i t y f o r m e d in the limestones. Major faulting o c c u r r e d along t h e Balcones Fault Z o n e in t h e M i o c e n e (Weeks, 1 9 4 5 ; Ragsdale, 1 9 6 0 ) and raised the E d w a r d s L i m e s t o n e in t h e n o r t h and west relative t o sea level. Because o f t h e faulting, c o n d i t i o n s favorable for a circulating g r o u n d w a t e r s y s t e m develo p e d o n t h e u p t h r o w n side o f t h e fault a f t e r t h e Miocene. Circulation patterns o f t h e E d w a r d s A q u i f e r are s h o w n b y t h e arrows in Fig. 1. Fresh w a t e r m o v e s d o w n - d i p a f t e r it enters t h e aquifer until it a p p r o a c h e s t h e fault zone. Along the fault zone, f r a c t u r e s a p p a r e n t l y c o n t r o l t h e circulation o f t h e g r o u n d w a t e r and f u n n e l it t o w a r d t o p o g r a p h i c a l l y low areas w h e r e it is discharged in m a j o r springs. It is n o c o i n c i d e n c e t h a t a n u m b e r o f large cities,

Fig. 4. Cross-section through the San Antonio area showing the relationship of the freshwater and bad-water zones to faults in the Balcones Fault Zone. Location of line A--A' is shown in Fig. 1. The position of the bad-water line is not always controlled by a major fault as shown here. Data on depth of the Edwards Group and location of faults are from Maclay and Small (1976).

247 including San Antonio, Austin, Waco, and Dallas, grew up along this Balcones Fault Zone. On the down-dip side of the Balcones Fault Zone water circulation was (and is) inhibited. To the south and east of a fairly distinct "bad-water line" (shown in Fig. 1) that roughly parallels the fault zone, the Edwards Aquifer contains nonpotable brackish water with more than 1000 mg/1 dissolved solids including hydrogen sulfide (Sayre and Bennett, 1942). Pearson and Rettman (1976) have analyzed a number of samples from the bad-water zone. They found that most were saturated with respect to calcite and dolomite and that many were saturated with respect to gypsum, celestite, strontianite, and fluorite as well. Samples from the fresh-water zone are saturated only with respect to calcite. Diagenesis in the brackish water has been very different from that in the circulating fresh water on the upthrown side of the fault. Fig. 4 is a cross-section across the Balcones Fault Zone showing the relationship of the fresh-water and bad-water zones. Abbott (1975) believes the "bad-water line" became established shortly after the Miocene faulting as a random hydrologic flow boundary. Since that time it has become deeply engrained in the formation and it is apparently relatively stationary. In spite of the removal of water from the Edwards Aquifer in the past century, the "bad-water line" has not changed its position significantly (Maclay and Small, 1976), and it apparently has never been farther coastward than it is today. RESULTS AND INTERPRETATIONS Diagenesis in the bad-water z o n e

Rocks in the bad-water and fresh-water zones were deposited in the same environments. However, they now differ markedly. Rocks in the bad-water zone are typically medium gray or brown with unoxidized organic material, pyrite, dolomite, and celestite. Minor amounts of gypsum, strontianite, and halite are also present. Rocks in the fresh-water zone are typically light colored with little pyrite, celestite, or evaporites. Oxidized iron gives a faint rusty color to m a n y rocks in the fresh-water zone, particularly in the more permeable zones. Cavernous porosity, solution-enlarged fractures, recrystallization, pseudospar, and terra rossa are also c o m m o n in the fresh-water zone, but are rare or absent in the bad-water zone. Dolomite is c o m m o n in the bad-water zone, whereas calcite has replaced most dolomite in the freshwater zone (Abbott, 1974). Because rocks in the bad-water zone have undergone less diagenesis than those in the fresh-water zone, t h e y more closely resemble the original sediments and are described first here. Furthermore, since diagenesis in both the bad-water and fresh-water zones followed the same pathway until the Miocene, early diagenesis described from the bad-water zone also occurred in

248

the fresh-water zone (but it has been obscured there by more recent diagenesis). Micrite Rocks in the bad-water zone are dominantly micrite and dolomite, although fossils, pellets, and sparry-calcite cement are also present. Micrite occurs in three forms. Most c o m m o n is a mosaic of tighty interlocking calcite crystals (Plate l-A). Micrite also occurs as distinct subrounded crystals about 3 ~m in diameter (Plate l-B), and as elongate crystals that may be up to 5 or 6 pm long (Plate l-C). Porosity in micrites of the bad-water zone is variable. Relatively tightly cemented micrites such as that in Plate 1-A typically have between 5 and 10% porosity in irregular micropores less than 1 pm wide. Other micrites with less cementation have porosities ranging from 15 to 30%. Micropores tend to be interconnecting in these porous micrites, but permeability is generally low because of the small pore size. A correlation appears to exist between microporosity and depositional environment in the micrites of the bad-water zone in the Edwards Aquifer. Micrites deposited in tidal-flat environments and associated with dolomite

PLATE 1 T e x t u r e s of micrite and d o l o m i t e . A. Moderately well-cemented calcite micrite f r o m the bad-water zone. R o u n d e d shapes of f o r m e r micrite grains have been obscured by overgrowths. Sample has a measured porosity of 9.6%. Well R N D ; 1125.5 ft. B. Porous recrystallized micrite. R o u n d e d shape of micrite grains is readily apparent because there is little cement. Like m a n y relatively porous micrites, this is f r o m a tidalflat deposit. The measured porosity is 29%. Well TD-3; 371 ft. C. Porous micrite f r o m the bad-water zone which has recrystallized to elongate microspar grains with distinct crystal faces. During original n e o m o r p h i s m from aragonite to calcite, the elongate shape o f s o m e aragonite needles was apparently preserved. Precipitation of calcite overgrowths has n o t obscured the elongate shape. Measured porosity is 25%. Well R N D ; 1079 ft. D. Thin-section p h o t o m i c r o g r a p h of d o l o m i t e in micrite f r o m the bad-water zone. Centers of d o l o m i t e r h o m b s have been leached and s o m e have been refilled with calcite. D o l o m i t e that originally occupied the leached cores probably f o r m e d p e n e c o n t e m poraneously in a tidal-flat e n v i r o n m e n t . The m o r e stable rims were apparently deposited in schizohaline or lower-salinity water. Well R N D ; 677 ft. E. H o l l o w d o l o m i t e with irregular interior. The h o l l o w center is exposed because t w o unstable primary d o l o m i t e grains were in c o n t a c t w h e n m o r e stable o u t e r layers of d o l o m i t e formed. During sample preparation, the second r h o m b was r e m o v e d to expose the internal cavity f o r m e d by leaching inside the o t h e r r h o m b . An i n t e r b o u n d a r y sheet pore (ISP) just below the d o l o m i t e r h o m b and micritic inclusions are also visible. Well Ch; 581 ft. F. Zoning in h o l l o w d o l o m i t e . T w o d o l o m i t e r h o m b s grew in c o m p e t i t i o n with each other. Changes in the relative rate of growth caused the zoning. The zones may be the result of annual fluctuations in pore-fluid chemistry. If so, d o l o m i t e f o r m e d at a rate of a b o u t 0.3 p m / y e a r . Well SM; 504 ft.

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250

tend to be porous, whereas micrites deposited in subtidal shelf environments tend to be more tightly cemented. A combination of t w o factors probably contributes to this relationship. Tidal-flat micrites may have had a higher proportion of "stable" carbonate mud (calcite and Mg-calcite) than the shelf micrites which contained more aragonite. The importance of calcite mud in inhibiting diagenesis is evident in chalks where porosity tends to be preserved because of the relative stability of the calcite components (Scholle, 1974; Mapstone, 1975). A second factor contributing to the porosity in the tidalflat micrites may have been the presence of brines rich in magnesium derived from evaporation or dolomite. Saline brines cause little, if any, micrite recrystallization although they may cause dolomitization (Illing et al., 1965). Fresh water low in magnesium is the major neomorphic fluid in aragonitic sediments (Friedman, 1964; Folk, 1974). Those tidal-flat micrites which are recrystallized and cemented are probably those most strongly affected by fresh-water diagenetic fluids. Oldershaw (1972) recognized a similar correlation between microporosity and depositional environment in some Ordovician micrites and suggested that the porosity in his tidal-flat micrites may have formed during leaching of an interstitial evaporite mineral. This explanation probably does n o t apply to the micrite in the Edwards because core samples were used. Remnants of interstitial evaporite minerals would likely still be present if they had played a role in preventing carbonate cementation in the micrites, b u t none were observed. The inferred path of diagenesis from primary carbonate mud to pseudospar is shown in Fig. 5. Based on analogy with recent carbonate mud (Cloud, 1962; Purdy, 1963), most mud deposited in the Cretaceous seas probably consisted originally of aragonite needles and tiny skeletal fragments

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251 produced by abrasion. While this sediment remained in a marine environment, few diagenetic changes occurred, although some additional aragonite and Mg-calcite may have formed authigenically. However, when exposed to fresh water, diagenesis was fairly rapid. Aragonite needles neomorphosed to calcite and became more equant. Magnesium ions were flushed from the Mgcalcite grains. In some cases the elongate shape of the needles was apparently preserved during recrystallization (Plate 1-C) and accentuated by precipitation of authigenic calcite as an overgrowth. Hathaway and Robertson (1961), in their experiments with artificial consolidation of Bahaman muds, showed that the elongate shape of aragonite needles could be preserved during recrystallization to calcite (see especially their Fig. 257.3, p. C-302). After recrystallization to calcite b u t before cementation, diagenesis in some tidalflat micrites of the bad-water zone in the Edwards Aquifer ceased (stages 2 and 3 of Fig. 5). Continued flushing by relatively fresh water in other micrites led to more complete recrystallization and the formation of an interlocking mosaic of small calcite crystals such as is characteristic of many subtidal micrites (stage 4 of Fig. 5). After this recrystallization and cementation, the stable mineralogy and low permeability of the micrite inhibited further diagenesis in the relatively stagnant waters of the bad-water zone. However, when the circulating groundwater system became established after the Miocene in the fresh-water zone, the extensive flushing created conditions favorable for further recrystallization of the porous micrites to microspar and pseudospar spherules. These are described in a later section.

Dolomite Dolomite is far more c o m m o n in the bad-water zone than in the freshwater zone (Abbott, 1974). Six types of dolomite have been recognized in rocks of the bad-water zone. Most c o m m o n is dolomite which formed in (and replaced) micrite. Also present are limpid dolomite as described by Folk and Siedlecka (1974), dolomite with irregular surfaces, dolomite with distinct internal zoning, and turbid, very fine-grained dolomite interpreted as a primary (or penecontemporaneous) supratidal dolomite. Gradations exist between these m o r p h o t y p e s and it is also c o m m o n to see one form overgrowing another. The sixth t y p e of dolomite is characterized by distinct leaching features, either externally or internally. The inferred diagenetic history of the various types of dolomite is summarized in Fig. 6. Supratidal dolomite. In thin sections, dolomite interpreted as forming penecontemporaneously in supratidal environments generally appears as small (less than 4 pm) rhombs with abundant inclusions inside clearer dolomite overgrowths (Plate I-D). Supratidal dolomite is difficult to observe directly with the SEM because virtually all crystals now have overgrowths which hide the primary rhomb. However, the chemical stability of the original protodolomite was apparently less than that of the surrounding overgrowths and

252

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Fig. 6. Sequence of types and textures in dolomite from the Edwards Aquifer.

hollow dolomite rhombs are c o m m o n in the Edwards Limestone. Plate 1-E shows one of these hollow dolomite rhombs as seen with the SEM. Thus, indirect evidence of the tidal-flat dolomites is easy to obtain. Sedimentologic evidence supports the interpretation that the dolomite is supratidal. Turbid, fine-grained dolomite rhombs occur in thin-bedded dolomitic sequences with desiccation features and local stromatolitic structures. Rose (1972) and Mueller (1975) have interpreted the dolomite in these sequences as having formed by penecontemporaneous replacement of soft aragonitic supratidal sediments. Similar Holocene dolomites are common in parts of the Andros Island and Persian Gulf tidal flats. Rather than well-ordered dolomite, Holocene tidal-flat dolomites are protodolomite with a Mg/Ca ratio of about 45/55 (Illing et al., 1965). The abundant inclusions in the tidal-flat dolomites of the Edwards suggest that these dolomites probably formed in a hypersaline environment and that they, too, were originally protodolomite. Zoned dolomite. Zoning of dolomite crystals has been discussed by Katz (1971). He states that zoning represents growth stages of the rhombs and reflects variations in the composition of interstitial brines with respect to their dissolved Ca, Mg, and Fe contents. Because Katz worked mainly with thin sections, the types of zones he describes reflect compositional changes in the dolomite rhombs. Under the SEM, some dolomite rhombs in the Edwards show a type of

253 zone unlikely any described by Katz (1971). These zones are typically only 0.3 pm thick (Plate l - F ) , and are impossible to see in thin sections. The zones are apparent only where t w o crystals have grown into each ot her in c o m p e t i t i o n for the same space. The zoning is apparently a reflection of relative rates of crystal growth rather than a change in composition, although this has n o t been checked with a microprobe. This "growth-rate zoning" is best developed in the first generation o f overgrowths on dol om i t e rhom bs f r o m tidal-flat deposits. Other types of zoning in d o l o m i t e similar to those described by Katz (1971) were observed in some thin sections but were n o t observed u n d e r the SEM and are n o t discussed here. Although detailed d e n d r o c h r o n o l o g y - t y p e analysis of the "growth-rate z o n es " in the dolomites of the Edwards has n o t been done, a cursory e x amin atio n of 10 r hom bs in one sample showed characteristic rhombicentric equizonation, i.e., consistent zoning patterns. This is n o t surprising since the zones almost certainly reflect small fluctuations in the saturation of the pore fluid and t h a t same fluid would be present t h r o u g h o u t the area of one SEM sample. It is t e m pt i ng to interpret the regularity of the zones (such as those shown in Plate l - F ) as the result o f a seasonal fluctuation of some sort. Murray (1969) d o c u m e n t e d a major annual reflux event on Bonaire and discussed its implications on dolomitization. Such an event could certainly cause zoning. Alternatively, seasonal rainfall could cause the chemistry of interstitial fluids in near-surface semiconsolidated sediment to change at regular intervals. Evidence presented by Mueller (1975) showed t hat much of the Edwards d o l o m i t e f o r m e d quite early in a schizohaline e n v i r o n m e n t such as would be significantly affected by seasonal rainfall. If the zones in the d o l o m i t e crystals really do represent annual "varves", measuring the thickness of the zones provides a way to det erm i ne the rate of growth of the dolomite. The zoning shown in Plate 1-F is typical. Seven zones occur in a lateral distance of 2 pm. This suggests t hat the zoned dolomite grew at an average rate o f a b o u t 0.3 p m / y e a r (the thickness of zones in o t h e r zoned dolomites is similar). Individual zones range in thickness from a b o u t 0.1 to 0.4 pm, and this suggests t hat dolomites have " g o o d " years and " b a d " years, just as trees do. Two notes of caution are necessary, however. First, this in ter p r e t a t i on of growth rate is largely speculative, and second, rates o f growth of u n z o n e d dolomites c a n n o t be det erm i ned and m ay be quite different f r om the inferred growth rate of the zoned dolomites. Dolomitized micrite. Much of the micrite in the Edwards has been dolomitized. Dolomite in micrite and biomicrite ranges from scattered " f l o a t i n g " r h omb s to packed r hom bs with very little micrite remaining. T he r h o m b s are typically 10--40 ~m on an edge. In thin sections it is clear from the distribution and lack o f abrasion t h a t this t y p e of dol om i t e is authigenic, n o t detrital. Authigenic fabrics are also clearly revealed with the SEM. Plate 2-A shows a d o l o m i t e r h o m b t h a t f o r m e d in micrite and i n c o r p o r a t e d micrite grains into its surface. A similar t e x t u r e is visible in Plate 2-B. Presumably, Ca and CO3

254 ions were m o b i l i z e d and m i x e d with Mg-ions d u r i n g this r e p l a c e m e n t . It is w o r t h n o t i n g t h a t at least t h e last stage o f d o l o m i t i z a t i o n o c c u r r e d a f t e r t h e micrite h a d recrystallized t o relatively spherical calcite grains (as s h o w n b y t h e shapes o f t h e crystals being i n c o r p o r a t e d into the r h o m b surface). R e c r y s t a l l i z a t i o n o f t h e micrite p r o b a b l y o c c u r r e d in fresh w a t e r (Folk, 1 9 7 4 ) , w h i c h indicates t h a t this t y p e o f d o l o m i t e is n o t c o n t e m p o r a n e o u s h y p e r s a l i n e tidal-flat d o l o m i t e , b u t r a t h e r a relatively late d o l o m i t e t h a t p r o b a b l y f o r m e d in a shallow-subsurface e n v i r o n m e n t .

Dolomite with irregular surfaces. In extensively d o l o m i t i z e d micrites the d o l o m i t e r h o m b s are p a c k e d t o g e t h e r . Micrite m a y or m a y n o t be p r e s e n t b e t w e e n the r h o m b s . In s o m e samples w h e r e the interstitial micrite has been r e m o v e d ( p r e s u m a b l y b y leaching), the d o l o m i t e r h o m b s are c h a r a c t e r i z e d b y r o u g h a n d irregular surfaces (Plate 2-C, D). T h e irregularities m a y be d i s t r i b u t e d as irregular b u m p s o n r h o m b surfaces or t h e y m a y f o r m " v " s h a p e d ridges e x t e n d i n g parallel t o the s h o r t diagonal o f the r h o m b . Similar " v " - s h a p e d ridges or " f l a m e s t r u c t u r e s " have been described b y Weaver ( 1 9 7 5 ) . Such a surface t e x t u r e m a y r e p r e s e n t s o m e sort o f surface e t c h i n g or it m a y be a g r o w t h p h e n o m e n o n as Weaver ( 1 9 7 5 ) suggests. If surface e t c h i n g was the cause, t h e flame s t r u c t u r e s m i g h t be e x p e c t e d to f o l l o w cleavage plains, w h i c h t h e y d o n o t . H o w e v e r , b e t t e r - d e v e l o p e d leaching features described in a later s e c t i o n do n o t f o l l o w cleavages either. Like Weaver, we i n t e r p r e t t h e flame s t r u c t u r e s as a c c r e t i o n a l features f o r m e d d u r i n g

PLATE 2 Textures in dolomite. A. Dolomite rhombs which have grown in micrite. Micrite has been incorporated into the rhomb surface making the surface slightly irregular. The micrite inclusions are rounded which indicates that the micrite recrystatlized before the last stages of dolomite growth. Well RND; 1125.5 ft. B. External leaching has produced etch pits and crevasses which are distributed irregularly over the surface of the dolomite rhomb. Recrystallized micrite and microspar surround the rhomb. The microspar has good crystal faces (see Plate 1-C from same sample) which may indicate that the microspar formed after the dolomite was leached. Well RND; 1079 ft. C. Dolomite with rough crystal faces. A micrite matrix was originally present but has been leached except for a few relict patches. Porosity in this sample has been measured at 29%. Well CH; 581 ft. D. Flame structures (v-shaped ridges) on dolomite rhomb surfaces indicated by arrows. These are accretional features related to crystal growth. Other surface irregularities are also visible. Magnified view of part of sample shown in C. Well CH; 581 ft. E. Limpid dolomite lining a cavity formed by the leaching of a mollusc shell. Note remarkably smooth crystal faces. Well SM; 504 ft. F. Pinnacle texture in a leached dolomite rhomb in microspar. Pinnacles apparently result from leaching around relatively "stable" calcite islands formed where micrite grains were incorporated into the dolomite surface. Well RND; 1079 ft.

255

PLATE 2

256 crystal growth. New layers of dolomite nucleated at rhomb edges and grew diagonally across the rhomb faces. Such ridges formed only where there was no competition for space with micrite. The relief of the flame structures may be an indication of the rapidity of crystal growth; limpid dolomite described below apparently grew very slowly from dilute solutions (Folk and Land, 1975) and developed no flame structures. It should also be noted that the flame structures could give rise to the zoning described in an earlier section.

Limpid dolomite. Surface roughness on dolomite rhombs within a single sample is usually fairly uniform, but between different samples it is quite variable. At one end of the spectrum are those dolomites with very rough surfaces formed by competition with micrite for space or by accretionary flame structures. At the other end are perfect dolomite rhombs with mirrorsmooth crystal faces {Plate 2-E). Such dolomite appears as very clear rhombs in thin section and has been called limpid dolomite by Folk and Siedlecka {1974). Limpid-dolomite crystals are t h o u g h t to grow as the result of slow, ordered precipitation from dilute solutions as shown by their greater stability in dilute hydrochloric acid and their crystal perfection (Folk and Land, 1975). A variety of evidence indicates t h a t the limpid dolomite in the Edwards Aquifer was a relatively late-stage dolomite. This includes its tendency to form as overgrowths on other forms of dolomite and also to line cavities and shell molds formed by leaching. It did not form readily in micrite, but formed best in open space where there were no obstructions. This suggests that leaching of micrite preceded the formation of limpid dolomite. Criteria for recognizing limpid dolomite are different depending on whether the petrographic microscope or the SEM is used. As defined by Folk and Siedlecka (1974) and Folk and Land {1975), any euhedral "water-clear" dolomite rhombs may be called limpid. However, much dolomite in the Edwards that appears limpid in thin section has rough surfaces when viewed with the SEM. As used in this paper, limpid dolomite must be both "waterclear" in thin section and have smooth crystal faces under the SEM. Leached dolomite. Dolomite with evidence of solution occurs in both the bad-water and fresh-water zones of the Edwards Aquifer. Complete solution of dolomite accompanied by calcite replacement is widespread in the freshwater zone and is described under the later section on Dedolomitization. Emphasis in this section is on leaching features in the bad-water zone. Both external and internal leaching were observed in dolomite from the bad-water zone. External etching produces pits, crevasses, and an unusual texture of "pinnacles". Pits and crevasses are shown in Plate 2-B and are distributed irregularly over the rhomb surfaces. Their distribution is apparently not controlled by cleavage planes. R h o m b edges seem more susceptible to attack than rhomb faces, probably because the leaching fluids can attack from two

257 directions rather than just one. Thus, m a n y crevasses appear to extend from rhomb edges toward the center of the rhomb faces. Similar etch pits and crevasses have been produced in the laboratory by etching limpid dolomite for short periods in dilute hydrochloric acid. This suggests that slightly acid solutions may have caused the leaching. A more c o m m o n form of dolomite leaching in the bad-water zone results in m a n y tiny "pinnacles" or "fingers" of the primary dolomite grain protruding out of an etched cavity in the rhomb surface (Plate 2-F). The dolomite fingers are consistently oriented perpendicular to r h o m b faces and terminate approximately in the plane of the original rhomb surface. The origin of this texture is unclear. The texture is most c o m m o n in dolomitized micrite and it is possible that micrite grains being incorporated into the surface of the rhomb created relatively stable "calcite islands" that resisted solution. With continued etching, the pinnacles formed in a manner similar to that which produces geomorphologic hoodoos. Plate 3-A supports this interpretation. In the photo, places where micrite has apparently been incorporated into the dolomite surface are not etched while adjacent areas show distinct incipient etch pits. Internal leaching of dolomite is c o m m o n in the Edwards Formation and hollow dolomite rhombs are widespread. All hollow dolomite in the Edwards is interpreted by us as having formed during leaching of unstable cores rather than as having formed by accretionary growth as Weaver (1975) suggested for some Miocene dolomite. Whole beds of dolomite with leached cores can be found in the Edwards, particularly in what are interpreted on sedimentologic evidence as supratidal deposits. A few rhombs in thin sections of these beds still have turbid cores such as might have been precipitated rapidly in a hypersaline environment. During diagenesis, the interstitial water changed from hypersaline to relatively fresh, but dolomite rhombs apparently continued to grow in spite of the change. In the later lower-salinity waters, the internal cores became less stable and eventually dissolved to form hollow dolomite boxes {Plate 3-B). Internal structure of some of these rhombs is clearly revealed with the SEM (Plates l-E, stereo pair 3-C and D). Elongate fingers of dolomite typically extend inward from the walls of the cavity. The presence of these internal projecting fingers is considered good evidence that the hollow centers formed by leaching rather than by constructional processes, as suggested by Weaver (1975). Differential etching is apparently responsible for the irregular fingers of dolomite that remain.

Interboundary sheet pores (ISPs). An interesting feature of m a n y dolomites is the presence of tiny cracks between crystals in apparent contact. Similar "cracks" have been described by Wardlaw {1976) who suggested the term "interboundary-sheet" pores. We propose shortening this rather cumbersome term to its acronym, ISP. ISPs are very thin uncemented planar pores between adjacent crystals. They are not fractures but rather a crystal growth

258 p h e n o m e n o n . ISPs are observed o n l y with t h e e l e c t r o n m i c r o s c o p e ( t h e y are t o o small to be seen in thin sections) and are widest b e t w e e n minerals o f d i f f e r e n t c o m p o s i t i o n s and crystals o f t h e same mineral o r i e n t e d at high angles to each other. ISPs are n o t well d e v e l o p e d b e t w e e n near-parallel crystal faces, p r e s u m a b l y because ions on o n e crystal face can " m a t c h " or " c e m e n t " with ions o n t h e o t h e r crystal. Thickness o f ISPs in the samples we e x a m i n e d was t y p i c a l l y b e t w e e n 2 0 0 and 1 0 0 0 ~ a l t h o u g h Wardlaw ( 1 9 7 6 ) r e p o r t e d t h a t m o s t o f t h o s e he observed averaged 2 p m in thickness. ISPs are n o t fractures a n d are q u i t e c e r t a i n l y n o t an artifact o f sample p r e p a r a t i o n . H o w e v e r , t h e y are planes o f weakness along w h i c h fractures can f o r m . Breakage along an ISP allowed t h e z o n i n g s h o w n in Plate 1-F to be seen; if t h e t w o crystals in the p h o t o h a d actually c e m e n t e d to each o t h e r , such z o n i n g (which f o r m e d via relative g r o w t h rates) w o u l d n o t be visible. ISPs were first observed b e t w e e n d o l o m i t e crystals. An e x c e p t i o n a l l y wide e x a m p l e is s h o w n in Plate 3-E. O t h e r e x a m p l e s are visible in Plates l - E , 2-D, a n d 2-E. F u r t h e r e x a m i n a t i o n revealed t h a t similar s t r u c t u r e s o c c u r b e t w e e n coarse calcite crystals (Plate 3-F) and b e t w e e n d o l o m i t e a n d celestite (Plate 4-F). ISPs were n o t observed in tightly c e m e n t e d micrite (Plate l - A ) , b u t d o exist in s o m e slightly c e m e n t e d micrite (Plate l-B). Similar s t r u c t u r e s also o c c u r b e t w e e n q u a r t z o v e r g r o w t h s in s a n d s t o n e (McBride, 1 9 7 7 , p. 56). ISPs f o r m because crystal g r o w t h is inhibited in very c o n f i n e d intergranular spaces. Wardlaw ( 1 9 7 6 ) r e p o r t e d o n e x p e r i m e n t s involving halite crystal g r o w t h in w h i c h g r o w t h o n a m a c r o s c o p i c scale was inhibited at intercrystal b o u n d a r i e s . He w e n t o n t o discuss t h e displacive g r o w t h observed f o r s o m e crystals a n d r e a s o n e d t h a t this indicates t h e presence o f a w a t e r film capable o f s u p p o r t i n g a shear stress. It a p p a r e n t l y is virtually impossible to

PLATE 3 Textures of dolomite. A. Contact of partly leached dolomite (below) and micrite (above). The inception of pinnacle formation is represented; leaching is concentrated between more resistant areas where micrite was incorporated into the dolomite. Well SLM; 659 ft. B. Thin-section photomicrograph of hollow dolomite rhombs in micrite. Projections of the original dolomite extend inward toward pore centers. Well RND; 923 ft. C, D. Stereo pair of hollow dolomite as seen with the SEM. Fingers of original rhomb remain in pore. Surrounding matrix is recrystallized micrite. Well SLM; 659 ft. E. An exceptionally wide interboundary sheet pore (ISP) between two dolomite rhombs. ISPs are a crystal growth phenomenon that form because mineral precipitation is inhibited in very confined spaces. Although ISPs are not fractures, they are planes of weakness along which fractures readily form. It is likely that the ISP in the picture was widened slightly during sample preparation. Well CH; 581 ft. F. Calcite polyhedra with some small dolomite rhombs on their surfaces. ISPs are visible between some of the calcite crystals (arrows). Calcite grows by adding sheets to crystal surfaces. Some of these accretionary sheets are visible near the top of the picture. Well RND~ 952 ft.

259

PLATE 3

260

replace this water film by crystalline material when t w o crystal grow into each other. Experimental growth of salol in capillary tubes reported b y Kirtisinghe et al. (1969) provides further clarification of reasons for the presence of ISPs. They found that the growth rate of free-growing salol slowed b y a factor of 100 to more than 1000 when the salol entered the confining space of a capillary tube. Dolomite crystal growth apparently also becomes slower and slower as the width of the intergranular space decreases until eventually crystal growth is completely inhibited. Unusually wide ISPs were observed between dolomite and celestite crystals where celestite had overgrown the dolomite (Plate 4-F). Inhibition of crystal growth in confined spaces may be partly responsible for these ISPs b u t does not explain their exceptional width. More likely the dolomite combined with the interstitial fluids to create a microenvironment surrounding itself that was n o t conducive to celestite precipitation. Future work with the electron microscope will almost certainly reveal that ISPs are widespread in sedimentary rocks. Such microcracks may provide a partial explanation for the puzzling problem of how fluids move through some apparently tightly cemented rocks.

Authigenic kaolinite To our surprise, authigenic kaolinite was found in several samples from the bad-water zone. Authigenic kaolinite is generally thought to form from dilute acidic solutions (Kittrick, 1969; Blatt et al., 1972, Fig. 7-13), and its

PLATE 4 K a o l i n i t e a n d celestite. All s a m p l e s are f r o m t h e b a d - w a t e r z o n e . A. B o o k l e t s o f a u t h i g e n i c k a o l i n i t e in d o l o m i t e . Well R N D ; 7 9 8 ft. B. E n l a r g e d view o f p a r t o f A s h o w i n g details o f t h e k a o l i n i t e plates. A p y r i t e f r a m b o i d (P) a n d small e q u a n t calcite m i c r i t e grains (c) are p r e s e n t o n t h e kaolinite. Well R N D ; 7 9 8 ft. C. Small c l u s t e r o f k a o l i n i t e flakes in m i c r i t e a n d m i c r o s p a r . E l o n g a t e crystals are c o m m o n in t h e m i c r i t e a n d crystal faces are u b i q u i t o u s . M i c r o s p a r f o r m e d a f t e r t h e kaolinite. Well R N D ; 1 0 7 9 ft. D. T h i n - s e c t i o n p h o t o m i c r o g r a p h o f t a b u l a r a n d a n h e d r a l celestite crystals in d o l o m i t e . S e d i m e n t o l o g i c e v i d e n c e suggest t h a t this s a m p l e was d e p o s i t e d in a tidal-flat e n v i r o n m e n t . I n c l u s i o n s o f d o l o m i t e c a n be seen in t h e celestite. Well R N D ; 9 5 2 ft. E. SEM p h o t o m i c r o g r a p h of t a b u l a r celestite (Ce) f r o m s a m e s a m p l e as D. Celestite is associated w i t h s o m e calcite c r y s t a l s (c) a n d a b u n d a n t small d o l o m i t e r h o m b s . Well R N D ; 9 5 2 ft. F. Cluster o f d o l o m i t e crystals b e i n g o v e r g r o w n b y celestite crystal ( d a r k b a c k g r o u n d ) . A large gap s e p a r a t e s t h e d o l o m i t e f r o m t h e celestite w h e r e it is b e i n g o v e r g r o w n . This gap is an u n u s u a l v a r i e t y o f i n t e r b o u n d a r y s h e e t p o r e (ISP) a n d p r o b a b l y f o r m e d b e c a u s e diagenetic fluids i n t e r a c t e d w i t h t h e d o l o m i t e t o c r e a t e a m i c r o e n v i r o n m e n t u n s u i t e d for celestite p r e c i p i t a t i o n . Well R N D ; 9 5 2 ft.

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PLATE 4

262

presence in limestone deserves explanation. Authigenic kaolinite in limestone is not widely reported in the literature, but this probably reflects the difficulty of observing it in thin section rather than a real absence. The high birefringence of the carbonate minerals easily masks the low birefringence of kaolinite and, unless the kaolinite occurs in large patches, it is virtually impossible to see. Even in samples of the Edwards where X-ray diffraction and SEM examination confirmed the presence of kaolinite, we had difficulty finding it in thin sections. Kaolinite has been reported from Pennsylvanian algal-mound limestones by Schroeder and Hayes (1968) where it occurs as a white powder filling vugs up to 2 cm across. It has also been reported by Keller (1976) in a Mississippian limestone from Missouri and in geodes with calcite crystals. Bartow and Sims (1975) reported kaolinite cement associated with carbonate cements in a Miocene sandstone of California. Kaolinite in the bad-water zone occurs in both micrites and dolomitic rocks. Plates 4-A and 4-B show pseudohexagonal booklets of kaolinite in dolomite. Plate 4-C shows a few kaolinite flakes in microspar. Examination of grain relationships in these figures suggests that the kaolinite formed after the dolomite and before the microspar. The excellent crystal shapes and relatively random distribution of the kaolinite booklets indicates that most of the kaolinite is near authigenic. Thin laminae of detrital kaolinite were found only near the base of the formation and these are n o t related to the authigenic clay. Fluctuating fluid chemistry such as that called on by Bartow and Sims (1975) to explain the association of carbonate cements with kaolinite may be applicable to the kaolinite in the bad-water zone. Slightly acid fluids have been reported from several wells in the bad-water zone by Pearson and Rettman (1976) and similar fluids could have contributed to the formation of kaolinite in the past. Since it formed after the dolomite, kaolinite almost certainly formed after shallow burial in the subsurface environment. Celestite Most celestite (SrSO4) in the Edwards Aquifer occurs in rocks deposited in intertidal and supratidal environments. It is typically associated with dolomite and minor amounts of gypsum, calcite, and length-slow chalcedony. Celestite has been f o u n d only in the bad-water zone but crystal molds after gypsum and/or celestite occur in parts of the fresh-water zone. The celestite assumes a variety of shapes ranging from large poikilotopic anhedral masses up to several millimeters in diameter to tabular euhedral crystals (Plate 4-D, E). Inclusions in the celestite are rare, but dolomite crystals are sometimes enclosed in larger celestite crystals. Some celestite clearly formed after dolomite (Plate 4-F}. Thus, although early precipitation of some celestite probably occurred in tidal-flat environments, further crystal growth occurred in the shallow subsurface environment. Celestite has been f o u n d in minor amounts in m a n y carbonate rocks. It is

263 present in algal mats and in areas of intense dolomitization in the coastal sabkhas of the Persian Gulf (Evans and Shearman, 1964; Kinsman, 1969a) where it forms an early diagenetic mineral. Late-stage authigenic celestite is also recognized and typically occurs in geodes and veins (Wood and Shaw, 1976). Two related sources of Sr for celestite have been suggested. The first involves the replacement of aragonite by calcite. Aragonite precipitated both organically and inorganically in marine environments typically has more than 8000 ppm Sr 2÷ (Kinsman, 1969b; Veizer and Demovic, 1974). Neomorphism of this aragonite in an open fresh-water system produces calcite with typical Sr values of less than 350 ppm (Kinsman, 1969b). Under certain circumstances which are not yet completely understood, the Sr 2+ released by this neomorphism can go into the formation of celestite. The second source of Sr involves dolomitization of aragonite or limestone (Wood and Shaw, 1976). Once again, the Sr in marine aragonite is the ultimate source of Sr ions. In sabkha environments, Mg-enriched brines react with aragonite to form dolomite (Illing et al., 1965). This dolomite typically has Sr values between 200 and 400 ppm (Kinsman, 1969b) and the Sr released from the aragonite during this process may also go into the formation of celestite. Because the celestite in the Edwards Aquifer is generally associated with dolomite in intertidal and supratidal rocks, the process involving dolomitization of aragonite described above probably provided much of the Sr for the celestite. However, a combination of the two factors cannot be ruled out. Most water in the bad-water zone is saturated with respect to celestite, whereas water in the fresh-water zone is strongly undersaturated (Pearson and Rettman, 1976). This probably explains the abundance of celestite in the bad-water zone and its paucity in the fresh-water zone. Diagenesis in the fresh-water zone

Prior to Miocene faulting, the rocks in the fresh-water zone are t h o u g h t to have closely resembled those in the bad-water zone (Abbott, 1974; Maclay and Small, 1976). Early diagenesis (pre-Miocene) of the rocks in the freshwater zone included neomorphism or leaching of aragonite fossils and other allochems, recrystallization of carbonate mud to micrite, and early cementation, predominantly in the meteoric phreatic and vadose environments (cf. Friedman, 1964; Bathurst, 1971). The similar pathways of diagenesis also led to the formation of the various types of dolomite described previously. After Miocene faulting, the distinct boundary between the fresh-water and badwater zones became established and a circulating fresh-water system developed on the u p t h r o w n side of the fault. Dominant diagenetic changes caused by this fresh water were recrystallization of micrite to microspar and pseudospar, dedolomitization, solution of evaporites, and precipitation of sparrycalcite cement. These are described in the following sections.

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Neomorphism of micrite Folk (1965) suggested the term neomorphism ("a comprehensive term of ignorance") for all transformations between one mineral and itself or a polymorph. Neomorphism may occur via inversion or recrystallization and may produce new crystals that are larger, smaller, or simply a different shape from the previous ones. An arbitrary boundary set b y Folk (1965) at 4 pm differentiates ordinary micrite from microspar. Grains more than 30 gm in diameter are termed pseudospar. Many types of neomorphism are described b y Folk (1965) and many of these occurred early in diagenesis of the Edwards Limestone (e.g. replacement of aragonite fossils by calcite). However, one t y p e called coalescive neomorphism is particularly significant in the fresh-water zone. After the original neomorphism of carbonate m u d (probably originally mostly aragonite) to calcite micrite, coalescive neomorphism in the fresh-water zone produced larger and larger microspar and pseudospar grains. Grains in neomorphosed porous micrite in the fresh-water zone typically are spherical to subspherical (Plate 5-A, B). In some samples all the grains are approximately the same size, whereas in others there is considerable variation. Diameter of the spherical grains varies from a b o u t 5 to 50 pm. Small crystal faces occur on parts of some spheres b u t most surfaces are covered by small irregular bumps and pits (Plate 5-C). In thin section, each microspar and pseudospar grain can be seen to consist of a single calcite crystal. Shoji and Folk (1964) suggested that there is no difference in crystal morphology between micrite and microspar except for grain size and that both micrite and microspar occur as subequant rounded to polyhedral blocks with slightly curved crystal faces. This is somewhat of an oversimplification and our work shows that both micrite and microspar occur in a wide variety

PLATE 5 T e x t u r e s of microspar in the fresh-water zone. A. Spherical microspar and pseudospar grains in a recrystallized porous micrite. Spheres have very irregular surfaces. Porosity in this sample was measured at 21%. Well TD-3; 107 ft. B. Enlarged view of microspar spheres. The very irregular nature o f the spheres is apparent. S o m e are even hollow. Well TD-3; 107 ft. C. Microspar and pseudospar spheres o f different sizes. Most o f the spheres have irregular surfaces but a few small well-developed crystal faces are present. Porosity in this sample is 23%. Well CH; 281 ft. D. Pore in a recrystallized micrite filled with calcite p o l y h e d r a in a variety of shapes. S o m e microspar grains have a p o o r l y defined r h o m b i c shape. Well SLM; 564 ft. E. Microspar and pseudospar associated with terra rossa. Grains have grown into interlocking mosaic, but a suggestion of spherical or bladed grains is present. Well CH; 367 ft. F. Thin-section p h o t o m i c r o g r a p h o f terra rossa. The c o n c e n t r a t i o n of clay (dark area in center o f p h o t o ) has strongly affected the recrystallization of microspar in adjacent areas. Same sample as E. Well CH; 367 ft.

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PLATE 5

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of shapes. The rounded shapes of microspar grains shown in Plate 5-C are only one end of a large spectrum (cf. Plate 5-D and Plate 1-C). The neomorphism of micrite to microspar results in an increase in grain size and a decrease in surface area. Smaller grains dissolve more easily because of their large surface area to volume ratio and provide CaCO3 for enlargement of other grains. However, this does n o t explain w h y many microspar grains are so spherical. Crystal faces were free to form where there was no interference with surrounding grains in the porous micrite but seldom did so. This may be because of competition for space with the surrounding matrix as suggested by Fischer et al. (1967), b u t this is unlikely in light of the high amount of porosity in most of the microsparites. Alternatively, it may be that fresh water, undersaturated with respect to calcite, has dissolved and rounded the crystal faces, although this is also unlikely because of the similarities of shapes and sizes in most samples and the presence of some distinct crystal faces. Most likely, the conditions of microspar formation such as sporadic wetting, local solution of micrite and rapid reprecipitation of the calcite combined to produce the spherical shapes. Some microspar grains do have distinct crystal faces (Plates 5-D and 1-C) and these occur in a wide variety of shapes from simple rhombs to complex scalenohedra. Microspar grains with crystal faces are most c o m m o n in relatively nonporous micrites of the fresh-water zone (whereas more porous micrites have subspherical grains) and the more porous micrites of the badwater zone (where less porous micrite has n o t recrystallized to microspar). Porosity, water chemistry, and rate of fluid flow apparently combine to determine the morphology of microspar crystals in the relatively pure micrites of the Edward Aquifer. Clay minerals also play an important role in the formation of microspar, apparently because of their tendency to attract magnesium ions (Longman, 1977). Several clays including montmorillonite are known to absorb magnesium ions from seawater (Dunoyer de Segonzac, 1970) and apparently can also absorb magnesium in the subsurface environment during diagenesis. By acting as a Mg-ion sump, the clay "liberates" the calcite micrite from a "cage" of Mg-ions and allows it to recrystallize to coarser size. Although clay is sparse in the Edwards Aquifer, patches of Pleistocene terra rossa occur in several cores from the fresh-water zone. These patches are associated with much microspar and pseudospar in grains up to 0.5 mm long. Microspar grains are tightly packed in these terre rosse and crystal boundaries are difficult to distinguish with the SEM (Plate 5-E). However, thin sections show the microspar--pseudospar to be loafish or bladed in shape. In areas of higher clay concentration, crystal size greatly increases and the pseudospar becomes more bladed (Plate 5-F). Clayey impurities have been segregated during neomorphism. In hand specimen the orange patches (7.5 YR 7/6 on the Munsell Soil Color Chart) appear to be largely clay, b u t in thin section they appear to consist primarily of microspar and pseudospar grains which have displaced small amounts of clay. X-ray analysis shows that

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kaolinite is the d o m i n a n t clay although lesser amounts of montmorillonite and illite are also present. The orange color is probably caused by minor amounts of hematite or limonite. The clays in these terre rosse are mainly detrital as shown by their morphology and occurrence.

Timing of microspar formation. Microspar associated with the clay minerals in the terra rossa can be more accurately dated as to the time of its formation than the rounded microspar spherules formed by the action of interstitial fluids in the porous micrite. Caves along the Balcones Fault Zone contain m a n y vertebrate fossils of Mid-Pleistocene and younger age but no vertebrate remains older than Pleistocene have yet been found (Lundelius and Slaughter, 1971, p. 20). This suggests that the terra rossa in the caves must be younger than Pliocene and t h a t the microspar in the terra rossa probably formed in the Pleistocene or Holocene. Microspar spherules formed by action of interstitial fluids are restricted to the fresh-water zone. None were observed in the bad-water zone. This indicates that the formation of the spherules is controlled by fresh-water circulation and t h a t it postdates the establishment of the fresh-water aquifer in the Miocene. Neomorphism of micrite to microspar is clearly a relatively late diagenetic event. The effects of outcrop weathering can be ruled out as a cause of this neomorphism, because core samples were used in this study. Instead neomorphism must have occurred in a shallow-subsurface environm e n t at depths of 25--200 m, because core samples were taken from these depths. Precipitation of sparry-calcite element Many grainstones in the bad-water zone are very porous. Some have been partially dolomitized. Others are cemented with equant or bladed sparry calcite and these apparently were cemented early in diagenesis in a fresh-water phreatic environment before the bad-water zone became established. In the fresh-water zone, on the other hand, equant-calcite cement is ubiquitous. This biased distribution of sparry calcite is clearly the result of differential cementation by fresh water late in diagenesis after the establishment of the fresh-water zone. Sparry calcite in the fresh-water zone takes m a n y different shapes. Many rocks are tightly cemented and calcite crystals .assumed the shape of the pore space t h e y filled. Partly cemented rocks are also c o m m o n and these typically contain pores lined with calcite crystals in a remarkable variety of shapes. Rhombic crystals are c o m m o n (Plate 6-A) as are m a n y more complex shapes (Plate 6-C, 6-D). Sparry calcite can even be found growing over microspar spherules and calcite rhombs (Plate 6-E). The rhombic-calcite cement is particularly interesting. Several limestone beds in the fresh-water zone of the Edwards Aquifer consist of neomorphosed pellets containing and cemented by rhombic calcite (Plate 6-B). These rhombs resemble dolomite in thin section but are stained by Alizarin Red S.

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When examined under the SEM (Plate 6-A), the calcite rhombs differ from good dolomite rhombs in appearing slightly bloated, i.e. they have more rounded edges and less regular crystal faces. Similar rhombic calcite has been described by Perkins (1968) from rocks of a wide variety of ages and locations. Folk (1974) suggests that this t y p e of calcite forms in fluids with a low salinity and a low Mg/Ca ratio. Recrystallization experiments involving aragonite oolites (Longman, in prep.) have produced textures similar to those shown in Plate 6-B. Fresh Bahaman oolites were placed in distilled water in a sealed metal container and heated to 150°C for 15 days. The resulting textures are shown in Plates 7-A and 7-B. The oolites have partly recrystallized to rhombic calcite and are also partly cemented by rhombic calcite. These experimental textures suggest that the porous pelletal limestones in the Edwards were originally aragonite and were diagenetically altered in very low-salinity water in a meteoric phreatic environment. In addition to salinity and Mg/Ca ratio, there are probably a number of other factors that control the shape of authigenic calcite crystals. Saturation with respect to calcite may be important. It is not u n c o m m o n to see coarse equant calcite crystals growing over much smaller bladed or scalenohedral crystals. The different crystal sizes probably indicate that the crystals formed at different times and at different rates from fluids of different composition. Very local differences in fluid chemistry apparently also had an effect, because crystals in neighboring pores can have quite different shapes. Outstanding examples of this are seen in the lining of crystals in a leached miliolid foraminifer shell (Plate 6-F) and in a leached mollusc shell (Plate 7-F). Interestingly, temperature and pressure apparently had little effect. These factors, which were probably constant within a single sample during a single stage of diagenesis, did not prevent a variety of crystal shapes from forming in a single pore.

PLATE 6 T e x t u r e s of calcite cement. All samples are f r o m the fresh-water zone. A. R h o m b i c calcite cement. These r h o m b s are less well f o r m e d than m o s t d o l o m i t e r h o m b s and have r o u n d e d edges. Well LH; 350 ft. B. Thin-section p h o t o m i c r o g r a p h of same sample as A. Many large calcite r h o m b s have f o r m e d on recrystallized pellets. Well LH; 350 ft. C. Calcite crystals b e t w e e n micritic allochems at t o p and b o t t o m of picture. The calcite occurs in a variety of shapes. Well DX-2; 338 ft. D. Enlarged view of part of C showing details of crystal shapes. N o t e the slightly irregular crystal faces. Well DX-2; 338 ft. E. Coarse e q u a n t calcite c e m e n t caught in the act of engulfing r h o m b i c calcite crystals f o r m e d during an earlier stage of c e m e n t a t i o n . Again n o t e slightly irregular crystal faces. Well SLM; 564 ft. F. Cavity of a leached miliolid foraminifer lined with calcite crystals in a variety o f shapes f r o m r h o m b s to m o r e c o m p l e x polyhedra. Well DX-2; 338 ft.

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PLATE 6

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Dedolornitization

Most dolomite in the fresh-water zone of the Edwards Aquifer has been dedolomitized. Although the term dedolomitization has been attacked b y some as being ambiguous and inconsistent (see Smit and Swett, 1969), it has been widely used in the literature for the process of replacement of dolomite by calcite in carbonate rocks and it is used in this paper. Dedolomite is the rock t y p e produced b y dedolomitization. Criteria for recognition of dedolomitization in rocks have been discussed by Lucia {1961), Shearman et al. (1961), Goldberg (1967) and Folkman (1969). These include: (1) relicts of dolomite in incompletely replaced rhombic crystals, (2) polycrystalline rhombic pseudomorphs of calcite after dolomite, (3) rhombohedral structures or palimpsest textures in which the outlines of the original dolomite crystals are emphasized by iron oxides or other substances in calcite, and (4) observations in the field of transitions from dolomites to their dedolomitized equivalents. Dedolomite may take many forms. One of the most distinctive is a mosaic of fine-grained calcite crystals replacing an individual dolomite r h o m b (Plate 7-D}. Dedolomites of this t y p e generally occur in porous micrite in the Edwards Aquifer. Another c o m m o n form of dedolomite in the Edwards is calcite in the centers of dolomite rhombs. Plate 1-D shows leached dolomite rhombs whose centers have been filled with calcite although some rhombs in this

PLATE 7 Dedolomite and miscellaneous textures. A. T h i n - s e c t i o n p h o t o m i c r o g r a p h o f artificially r e c r y s t a l l i z e d B a h a m a n a r a g o n i t e oolites. R e c r y s t a l l i z a t i o n is o n l y partially c o m p l e t e , b u t r h o m b i c calcite has f o r m e d in a n d b e t w e e n oolites. T h i s t e x t u r e is c o m p a r a b l e t o t h a t s h o w n in Plate 6-B. Oolites were h e a t e d t o 1 5 0 ° C for 15 d a y s in distilled w a t e r in a sealed c o n t a i n e r . B. SEM p h o t o m i c r o g r a p h o f s a m p l e s h o w n in A s h o w i n g well-defined r h o m b i c calcite b e t w e e n p a r t l y r e c r y s t a l l i z e d a r a g o n i t e oolites. T h e s e r h o m b s were p r o d u c e d artificially in t h e lab in distilled w a t e r a n d suggest t h a t t h e r h o m b s s h o w n in Plate 6-B also f o r m e d in very fresh water. C. T h i n - s e c t i o n p h o t o m i c r o g r a p h o f p o i k i l o t o p i c calcite e n c l o s i n g a n d r e p l a c i n g d o l o m i t e r h o m b s ( t i n y b r i g h t speckles). T h i s is a fairly c o m m o n t y p e o f d e d o l o m i t i z a t i o n in p a r t s o f t h e f r e s h - w a t e r z o n e . Well SB; 4 6 4 ft. D. T h e relict o u t l i n e o f a d o l o m i t e r h o m b is visible b u t t h e d o l o m i t e has b e e n r e p l a c e d b y a m o s a i c o f e q u a n t calcite crystals. S u c h d e d o l o m i t i z a t i o n is c o m m o n in t h e p o r o u s m i c r i t e o f t h e f r e s h - w a t e r z o n e . Well SB; 4 0 3 ft. E. Coarse p o i k i l o t o p i c calcite h a s o v e r g r o w n a n d p a r t l y r e p l a c e d d o l o m i t e r h o m b s ( o n l y r h o m b i c relicts or holes r e m a i n ) . T h i s is t h e same t y p e o f d e d o l o m i t i z a t i o n s h o w n in pict u r e C. Well LH; 2 7 8 ft. F. C o m p l e x calcite crystals lining a l e a c h e d m o l l u s c shell in t h e f r e s h - w a t e r z o n e . N o t e t h e impressive v a r i e t y o f s h a p e s a n d sizes for s u c h a small area. T h e s e crystals are assoc i a t e d w i t h t h e d e d o l o m i t e t e x t u r e in E. S a m p l e LH; 278 ft.

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PLATE 7

272 p h o t o g r a p h are still hollow. The calcite centers in these rhombs are single crystals in near optical c o n t i n u i t y with the dolomite. T h e y are best recognized by artificial staining o f the thin section with Alizarin Red S. A third t y p e of d e d o l o m i t e is p r o d u c e d when d o l o m i t e is replaced by a mosaic o f calcite crystals coarser than the original dolomite. This t y p e of d o l o m i t e has a luster-mottled appearance with relict dol om i t e crystals poikilotopically enclosed in large crystals of calcite. With time, the edges of the d o l o m i t e r h o m bs are attacked and removed by the calcite and eventually t h e whole r h o m b may be replaced. Armstrong (1967) described dedolomites of this t y p e as having a p s e u d o m e t a m o r p h i c t e x t u r e and interlocking crystal boundaries. The characteristic appearance o f this t y p e o f d e d o l o m i t e in thin section is shown in Plate 7-C. Under the SEM, the d e d o l o m i t e t e x t u r e is less apparent, but relicts of d o l o m i t e r hom bs and partially calcitized r h o m b s can sometimes be seen (Plate 7-E). The several ty pes of d e d o l o m i t e in the fresh-water zone are all believed to have f o r m e d by the same general mechanism. Extensive fresh-water flushing moved relict Mg-rich brines f r om the fresh-water zone while bringing in a b u n d a n t Ca 2÷ ions f r om the limestones in the recharge area. This c o m b i n e d with the dissolution of gypsum in the fresh-water zone to raise the Ca/Mg ratio o f the pore waters. The high Ca/Mg ratio and relatively rapid fresh-water flushing caused the calcite to replace dolomite. The abundance of dedolomite in the fresh-water zone and its paucity in the bad-water zone indicates t h a t d ed o lo mitizat i on occurred after the fresh-water aquifer system was established in the Miocene. Ded o lo mite in the Edwards Aquifer is n o t directly related to recent outcrop weathering since cores were used in this study. Neither is it related to buried unconformities. Rocks in b o t h the fresh-water and bad-water zones were deposited in the same environments and u n d e r w e n t the same diagenetic history until the establishment of the fresh-water zone in the Miocene. D ed o lo mite related t o subaerial exposure prior to the Miocene would have f o r m e d in b o t h zones but none is f o u n d in the bad-water zone. F u r t h e r m o r e , d e d o l o m i t e occurs t h r o u g h o u t the 100--200 m of the Edwards Group in the fresh-water zone and is n o t c o n c e n t r a t e d in a few zones as would be e x p e c t e d if it f o r m e d during subaerial exposure. In summary, we believe t h a t t he extensive d e d o l o m i t i z a t i o n found in the fresh-water zone of the Edwards Aquifer was caused by fluids with a high Ca/Mg ratio and t hat it occurred after Miocene faulting created the circularing fresh-water system. The d e d o l o m i t i z a t i o n occurred in a shallow-subsurface e n v i r o n m e n t at depths of fifty to a few hundred meters. SUMMARY AND CONCLUSIONS Rocks o f the L ow e r Cretaceous Edwards G roup were deposited in shallowmarine and peritidal environments in what is now t he San A nt oni o area of south-central Texas. Samples f r om shallow wells in b o t h the bad-water zone

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and the fresh-water zone of the Edwards Aquifer were studied with thin sections and the scanning electron microscope. Rocks in both zones underwent similar diagenesis after deposition until the Miocene. During this time, carbonate mud n e o m o r p h o s e d to calcite micrite, aragonite and Mg-calcite allochems were altered to calcite or leached, and evaporites, particularly gypsum and celestite, formed in some tidal-flat sediments. Dolomite is widespread and formed {1) penecontemporaneously in some tidal-flat sediments, (2) as zoned crystals in what were probably slightly schizohaline very shallow-subsurface environments, (3) b y replacement of micrite after the micrite had recrystallized in a somewhat deeper-subsurface environment, and {4) as pore-lining limpid crystals after leaching of aragonite fossils. Miocene faulting resulted in a circulating fresh-water aquifer system to the north and west of a fairly distinct "bad-water line" which roughly parallels the Balcones Fault Zone. To the south of this "bad-water line", interstitial fluids remained relatively stagnant and contain over 1000 mg/1 dissolved solids. Water in this bad-water zone tends to be saturated with respect to halite, calcite, dolomite, gypsum, celestite, strontianite, and fluorite, whereas the water in the fresh-water zone is saturated only with calcite. Because of differences in the chemistry of the interstitial fluids, post-Miocene diagenesis in the two zones has been very different. Rocks in the bad-water zone maintain most of the textures associated with deposition or early diagenesis: evaporites are present; porous micrite and dolomite are ubiquitous; and pyrite and unoxidized organic material occur locally. Some authigenic dolomite, kaolinite, and celestite formed in the bad-water zone. Rocks in the fresh-water zone, on the other hand, have been oxidized and contain extensive secondary solution cavities, particularly along fractures. Recrystallization of micrite to coarse microspar and bladed to spherical pseudospar is widespread. Extensive dedolomitization has occurred in waters with a high Ca/Mg ratio in a shallow-subsurface environment. Sparry calcite cement is widespread and occurs in a variety of relatively equant shapes. It completely fills many pores and reduces porosity to essentially zero in some areas.

Study of the relationship between diagenesis and interstitial fluids in the Edwards Aquifer demonstrates the p r o f o u n d effects of pore-fluid chemistry on diagenesis in shallow-subsurface environments. The importance of these effects is increased when it is realized that the carbonate rocks under discussion had already reached a so-called "stable" mineralogy of calcite and dolomite when the circulating fresh-water aquifer developed. In spite of this, considerable additional diagenesis occurred in the fresh-water zone because of the relatively high rates of water circulation. ACKNOWLEDGEMENTS

Special thanks are extended to Mr. Allen White for his excellent assistance on the SEM. Julie McClellan also helped on the microscope, and Dave Darm-

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stetter did much of the photographic work. We would like to thank Drs. R.L. Folk (University of Texas at Austin), R.D. Harvey (Illinois State Geological Survey), R.G. Loucks (Texas Bureau of Economic Geology), and R.T. Terriere (Cities Service Co.) for their critical reading of the manuscript. The senior author thanks Cities Service and particularly Dr. Frank Wantland for permission to publish and the use of the SEM.

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