Mesoscale physical sedimentary structures and trace fossils in Holocene carbonate eolianites from San Salvador Island, Bahamas

Sedimentary Geology, 55 (1988) 163-184

163

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MESOSCALE AND FROM

TRACE SAN

PHYSICAL FOSSILS SALVADOR

SEDIMENTARY IN HOLOCENE ISLAND,

STRUCTURES CARBONATE

EOLIANITES

BAHAMAS

BRIAN W H I T E and H. A L L E N C U R R A N

Department of Geology, Smith College, Northampton, MA 01063 (U.S.A.) (Received March 30, 1987; revised and accepted June 8, 1987)

ABSTRACT

White, B., and Curran, H.A,, 1988. Mesoscale physical sedimentary structures and trace fossils in Holocene carbonate eolianites from San Salvador Island, Bahamas. In: P. Hesp and S.G. Fryberger (Editors), Eolian Sediments. Sediment. Geol., 55: 163-184.

Carbonate eolianites, less than 10,000 years old, are well exposed in sea cliffs and on rocky shore platforms along the northeast coast of San Salvador Island. These deposits formed when easterly trade winds blew carbonate sands landward from the beach zone as sea level rose over a previously exposed shelf during the Holocene transgression. Small, lobate, parabolic-like dunes coalesced laterally to form an elongate, transverse dune ridge oriented perpendicular to the prevailing wind direction. Detailed observations of small-scale sedimentary structures and laminations permit the distinction of sands deposited as climbing wind ripples, lee-side grainfalls, and lee-side sandflows. Micrite crusts and associated plant trace fossils characteristic of the dunal environment are c o m m o n in the Rice Bay Formation, and these can be compared directly with identical features and plants found in modern carbonate dunes on San Salvador. Some eolian laminations dip into the present-day subtidal zone, confirming a post-depositional rise in sea level along this tectonically stable coast. The rocks are lithified sufficiently by freshwater, vadose, low Mg-calcite cement to form wave-resistant sea cliffs. A distinctive feature of these terrestrial carbonate rocks is the occurrence of a variety of animal trace fossils. Skolithos linearis burrows, up to 30 cm in length and 0.5 cm in diameter, are quite common. Comparison with traces found in modern carbonate dunes suggests that these trace fossils were made by burrowing insects or spiders. The most a b u n d a n t and widespread trace fossil consists of closely spaced, irregular, small burrows up to 20 cm long in the horizontal plane and with uniform diameters of 0.3-0.4 cm. These burrows also extended downward as much as 3 cm into the sediments, and created a mottled texture in places. This trace fossil has been found only in lee-side sandflow and grainfall deposits. Such burrows probably were produced by insects or insect larvae, which favored the protected lee-side environment. The most unusual trace fossil in these Holocene eolianites is composed of a cluster of vertically oriented burrows. Each cluster consists of up to several hundred shafts, each with a diameter of 1 - 2 cm, that diverge upward from an approximately c o m m o n point of origin. These structures can be 1.4 m or more high and greater than 1 m in transverse section across the circular shape produced by the upwardly radiating burrow cluster. These trace fossils probably represent the escape pathways of the hatchlings of an infaunal insect or similar invertebrate.

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164 The preservation in Holocenerocks of characteristic mesoscale physical sedimentary structures and of animal and plant trace fossils analogous to those found in modem carbonate eolian dunes suggests that these features can provide useful criteria for the recognition of more ancient carbonate eolianites. heretofore sparsely recorded. Furthermore, the diagnostic physical sedimentary structures and trace fossils should permit detailed delineation of sub-environments within ancient carbonate dune complexes.

INTRODUCTION Apart from small amounts of hematite, the modern sediments and the exposed rocks of San Salvador Island are composed entirely of carbonate minerals. In this paper, the term eolianite is used to designate lithified carbonate sand that was demonstrably transported and deposited by wind action. Such rocks are an important component of the Holocene and Pleistocene rock record in the Bahamas, Bermuda, the Yucatan Peninsula of Mexico, and numerous other tropical locations world-wide. However, pre-Pleistocene carbonate rocks of eolian origin have been rarely, if ever, reported (McKee and Ward, 1983), although a recent report indicates that thin Pennsylvanian quartzose limestones from the Paradox Basin in Utah are eolian (Loope, 1986). It is not clear whether this rarity is real or results from failure to identify such rocks properly, perhaps because of the lack of diagnostic criteria. Carbonate eolianites are widely believed to contain few, if any, animal trace fossils (McKee and Ward, 1983). This view may contribute to the failure to identify ancient eolianites. In our experience, many modern carbonate dunes are difficult to study internally because of their extensive plant root systems. Roots are commonly so thick that attempts to trench or core through the dunes are either blocked or require such force that the roots are disturbed and adjacent sedimentary structures are distorted or destroyed. This is especially true for rather delicate animal burrows. In this context, lithified Holocene eolian carbonate dunes can serve as a bridge between the modern and the more ancient, and can provide criteria that may be useful for the identification of pre-Holocene eolianites. GEOLOGIC SETTING San Salvador Island is located some 650 km ESE of Miami, Florida, at the eastern edge of the Bahamas Archipelago (Fig. 1). The island is surrounded by a narrow shelf covered by up to 20 m of water. Beyond the shelf edge, a steep slope around the entire island plunges to depths of 2000 m or more. Thus, San Salvador is separated from the main Bahama Platform and rises as a pedestal from the ocean floor. Because of this and its tectonic stability, the island acts as a sea-level gauge. Glacial and interglacial fluctuations of sea level have had a major impact on the Pleistocene and Holocene rock record of the island (Carew et al., 1984; Titus, 1984; White et al., 1984; Carew and Mylroie, 1985). The structures and trace fossils described in this paper occur in eolianites exposed in the northeast corner of San Salvador on Cut Cay and N o r t h Point, and

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in the sea cliffs along Rice Bay and Hanna Bay (Fig. 2). Adams (1980), in his pioneering study of the geology of San Salvador, briefly described the rocks of the North Point area. He recognized their eolian origin and distinguished leeward and windward faces of small, lobate dunes. Hutto and Carew (1984), in their study of the eolianites of San Salvador, found that most are dominantly oolitic. However, this was not the case for samples from North Point, as these had a higher proportion of pellets and skeletal grains. Our studies show that the North Point eolianites are mostly pelsparites that also contain some ooids and skeletal fragments. Hutto and Carew (1984) also realized that the North Point eolianites are significantly younger than most of the eolianites on San Salvador, and they suggested that they are no more than 10,000 years old. A preliminary account of the physical sedimentary structures and the cluster burrow trace fossil of the North Point area was given by White and Curran (1985). In a recent paper, Carew and Mylroie (1985) established a stratigraphy for the Holocene rocks of San Salvador. They assigned all rocks lying above a prominent paleosol that marks the Pleistocene-Holocene boundary to the Rice Bay Formation and divided the formation into two members, based on the position of eolianite strata relative to sea level. Eolianites that clearly extend below present sea level were

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assigned to the North Point Member, while younger ones that were deposited after sea level reached approximately its present position were placed in the Hanna Bay Member. Whole rock radiocarbon analyses of North Point Member rocks from the west side of North Point revealed ages of 5250-5560 years (Carew and Mylroie, 1985). Features described in this paper from Cut Cay, North Point, and the Rice Bay cliffs occur in rocks of the North Point Member, and those from the H a n n a Bay cliffs occur in rocks of the H a n n a Bay Member. MESOSCALE PHYSICAL SEDIMENTARY STRUCTURES Introduction The rocks of North Point and Cut Cay clearly are lithified dunes, with dips of their leeward faces arcing through almost 180 ° , centered to the west. Individual dunes coalesced to form dune ridges oriented approximately north-south. They are thus similar to eolian dunes found elsewhere in the Bahamas (Ball, 1967) and on Bermuda (Mackenzie, 1964), that are classified by McKee and Ward (1983) as parabolic dunes that coalesced to form compound parabolic ridges. Preserved carbonate dunes of Pleistocene and Holocene age are widespread in m a n y tropical and subtropical parts of the world (see McKee and Ward, 1983, for a review of locations). These deposits are usually identified using a combination of criteria, summarized by McKee and Ward (1983) in their review of the eolian environment.

167

Fig. 3. Northern flank of a parabolic dune, west side of North Point. Note that eolian cross-beddingdips below present sea level. Top of the dune is about 7 m above the water level.

Usually, each criterion is equivocal by itself, and it is the combination of criteria that is diagnostic. The dunes on San Salvador are several tens of meters in width and up to 15 m high, and, as they are commonly preserved in their entirety, they are readily recognizable (Fig. 3). Such a criterion is probably less commonly preserved in pre-Pleistocene rocks, and it certainly would not be available from subsurface cores. Although the recognition of the occurrence of ancient dunes in northeast San Salvador was relatively straight forward, more detailed analyses of how individual dunes were initiated, grew, and moved, and determination of the paleowind directions proved more difficult. Several types of cross beds with well-developed internal laminations are splendidly preserved throughout these Holocene dunes, and it was tempting to assume that dip measurements of the laminations would reveal paleowind directions. Several hundred such measurements, however, gave no coherent pattern nor could any sensible dune history be unravelled. Seeking guidance from modern dunes, we examined some of the present-day dunes on San Salvador. For the most part, these are covered by vegetation, and the extensive root systems impede trenching and coring, and, in any case, the internal sedimentary structures

t68

commonly are destroyed, either by the growing roots or, as mentioned above, by efforts to reveal them. Other work on modern dunes, especially that of Hunter (1977), was. however. very illuminating. Hunter was able to distinguish different kinds of physical sedimentary structures formed in wind-deposited modern coastal sand dunes and to relate these to the processes that led to their formation. Similar structures were also produced in wind tunnel experiments by Fryberger and Schenk (198I), and this research provided further insight into the specific modes of formation of these features. Application of the results of this work on modern dunes and the experimental work led to the discovery of analogous sedimentary structures in siliciclastic rocks from the Pleistocene of Oregon (Hunter, 1980), the Pennsylvanian to Jurassic of the western United States (Hunter, 1981), the Permian of Arran. Scotland (Clemmensen and Abrahamsen. 1983), the Cambro-Ordovician of north-central United States (Dott et al., 1986), and the Middle Proterozoic of Scandinavia (Pulvertaft, 1985). No such analyses have been made for the rocks of carbonate terrains, and the small-scale structures diagnostic of eolian deposition described by Hunter (1977) are not mentioned as criteria for recognition of eolian carbonates, as listed by McKee and Ward (1983) in their review. A preliminary account of the use of Hunter's (1977) classification of small-scale eolian sedimentary structures in the study of carbonate rocks was given by White and Curran (1985). A more detailed discussion of this work is presented below.

Climbing wind ripples Sand commonly is transported by wind in the form of small ripples that have some distinctive characteristics that can aid in their identification in the rock record. Ripple crests are oriented perpendicular to the transporting wind direction, but they can have any orientation relative to the dip of the surface over which the sand is being transported. This feature is in sharp contrast to ripples formed by flowing water, and it is believed to be characteristic of eolian deposition (McKee and Ward, 1983). Good exposures of wind ripple crests are scarce in the Holocene eolianites of San Salvador, although one such example is shown in Fig. 4. Wind ripples have very low amplitudes, giving them characteristically high ripple indices (Tanner, 1967: McKee, 1979). Ripple indices measured for Rice Bay Formation ripple marks are shown in Fig. 5, and these clearly fall in the wind ripple category. For sediment accumulation to occur by migration of wind ripples, each ripple must climb relative to the next underlying one (Hunter, 1977). During the movement of wind ripples, coarser material is concentrated on the crests and finer material in the troughs, leading to the deposition of thin strata (Fig. 6), characterized by even thickness, sharp contacts, and in some cases by inverse size grading. Fryberger and Schenk (1981) reported that inverse grading is not always found in experimentally produced wind ripple strata. In the San Salvador eolianites

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Fig. 4. Upper surface of wind ripple deposits showing low amplitude wind ripples. North Point Member, Rice Bay.

this feature is not always readily visible in the field, although it commonly is evident under the petrographic microscope (Fig. 7). In these rocks the finer grained portions of the individual laminations are preferentially cemented by vadose calcite,

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F'ig. 6. Close-up view of wind ripple laminations showing coarse (dark) and fine (light) couplets. North Point Member, Rice Bay. Scale bar = 2 cm.

perhaps due to greater water retention in an unsaturated diagenetic environment, and differential erosion makes these finer parts stand out on the rock face. The laminations produced by climbing wind ripples were designated climbing translatent strata by Hunter (1977), but herein they are called wind ripple laminations. Such deposits make up much of the Rice Bay Formation eolianates. As the wind can blow wind ripples along, up, or down any of the faces of dunes or across interdune areas, the dip of the resulting wind ripple lamination is a function of the geometry of the underlying surface, and not the direction of the wind. Thus, wind ripple lamination cannot be used as a reliable indicator of paleowind direction. An exception to this might occur where the laminations dip at close to the angle of repose where they most likely formed on the slip surface on the downwind side of the dune.

Grainfalls Grainfalt occurs when winds move sattating grains and suspended sediment into a sheltered area, for example on the lee side of dune crests, where the sediment accumulates, commonly with a high initial dip (Hunter, 1981). On small dunes, grainfall can occur near the dune toe and give rise to low dip angles. Grainfull strata

171

deposited on lee slopes in wind tunnel experiments consistently wedged thinner downslope (Fryberger and Schenk, 1981), and this seems likely to be the case on natural dune lee slopes, too. Because they form on the lee of dunes, grainfall strata are a more reliable indicator of paleowind direction than wind ripple laminations. Shifting winds may cause the grainfall deposits to be overlain by wind ripple laminations, examples from the Rice Bay Formation are shown in Figs. 8 and 9, or, perhaps, to be eroded and reworked. Grainfall strata occur throughout the Rice Bay Formation eolianites, although they are markedly less abundant than strata produced by wind ripples. These grainfall strata commonly enclose sandflow lenses, as shown in Figs. 8 and 9.

Fig. 7. Photomicrograph of wind ripple laminations showing preferential cementation of finer layers by low-Mg calcite. Pelsparite, North Point Member~ North Point. Crossed polarizers. Scale bar = 1 mm.

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Sandflows Accumulation of sediment on the upper part of lee slopes, commonly by grainfall, may cause oversteepening and instability, resulting in downslope movement. Dry sands will flow non-cohesively and produce sandflows, but, if the sediments are crusted or partially lithified, they may founder as blocks and give rise to breccias. Sandflow strata are typically thicker than other wind-deposited strata, commonly exceeding 1 cm. They lie close to the angle of repose and tend to pinch out towards the lower part of a foreset (Hunter, 1981). Sandflows have sharp contacts and a distinctive lenticular shape when seen in strike cross-section (Fig. 9) or in horizontal exposure. As they form on the leeside of dunes, sandflows can be used to reconstruct dune morphology and to indicate prevailing paleowind directions. In the Rice Bay Formation eolianites, sandflow lenses are the least abundant of the three strata types discussed here, and they commonly are surrounded by grainfall deposits (Figs. 8 and 9).

Fig. 8. Sandflow lenses (dark) enclosed in grainfal! deposits, overlain with an erosional contact by wind ripple laminations. Arrow points to area of mottling by the irregular, small burrows. North Point Member, Cut Cay.

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Micrite crusts and associated plant trace fossils

In many eolianite outcrops in the study area, very thin, hard, brown micritic crusts cap bedding planes and lamination surfaces at intervals of approximately 10-15 cm (Fig. 10). On vertical faces these crusts stand out as thin, resistant layers, while their upper surfaces commonly have plant trace fossils and narrow micrite ridges up to 1 cm high on them (Fig. 11). Analogous crusts commonly occur on modern dune surfaces on San Salvador. Some of these crusts are hard enough to require a hammer to break them, and many of them have living roots and stems of trailing plants on and within them. In some cases, narrow low ridges of cemented sand form underneath long, exposed plant roots and stems, due, at least in part, to the protection from scouring winds provided by the plants. Analysis of the crusts by X-ray diffraction showed that sand grains of aragonite and high-Mg calcite are cemented by low-Mg calcite. Crust formation probably occurs following rainfall, by partial dissolution of aragonite and high-Mg calcite in the acidic freshwater followed by precipitation of low-Mg calcite as the rainwater evaporates in the hot sun.

Fig. 9. Close-up of sandflow lenses (dark) within grainfall deposits. Hammer liesjust above the erosional contact with the overlying wind ripple laminations. Rock face is almost vertical and close to a strike section. North Point Member, Rice Bay.

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Fig. 10. Flank of a parabolic dune showing micrite crusts (indicated by c) developed on lamination surfaces at 10-15 cm intervals. North Point Member. Rice Bay.

The micrite crusts and narrow ridges of the Rice Bay Formation eolianites are believed to have formed in the same manner. The plant trace fossils associated with the micrite crusts represent both lateral root systems and the long, trailing stems characteristic of plants such as railroad vine (Ipomoea pes-caprae) and bay geranium (Ambrosia hispida) that extend, commonly for several meters, across the surface of modern carbonate dunes. Trace fossils formed by plant roots are well known from Quaternary carbonates rocks of the Bahamas, where they were described by Northrop (1890) who introduced the term rhizomorph to designate them. K l a p p a (1980) reviewed the kinds of trace fossils produced by roots in Quaternary carbonates, and he introduced the general and genetic term rhizolith for structures that are demonstrably formed by the presence of the roots of higher plants. Such rhizomorphs, or rhizoliths, are c o m m o n in eolianites and other rock types on San Salvador, but a full description and discussion of them is beyond the scope of this paper. It should be noted, howeverL that land plants can colonize any sediment or rock that becomes exposed due to emergence. The presence of rhizomorphs in a carbonate rock does not necessarily mean that the sediments were deposited in a terrestrial environment, only that they were subaerially exposed at some time during or after deposition. The preservation

175

Fig. 11. Upper surface of micrite crust showing plant trace fossils interpreted as having been formed by runners of dune plants, Lens cap = 5.5 cm diameter. North Point Member, Rice Bay.

of plant trace fossils on and in the micritic crusts of the Rice Bay Formation rocks did occur contemporaneously within a dunal evironment, and such trace fossils do provide a specific criterion for the recognition of eolian deposition. The frequency and spacing of the micritic crusts and associated plant trace fossils in the Rice Bay Formation suggest that these dunes grew incrementally. Each relatively thin addition of wind-blown sand to a dune was followed by plant colonization and the development of cemented crusts. This mode of dune growth is in sharp contrast to what appears to have happened in the case of many of the Pleistocene dunes on San Salvador. In these, accumulation of much thicker windblown sand deposits preceded significant plant colonization. Soil development, caliche formation, and plant growth took place on the top of these dunes, eventually producing a rhizomorph horizon up to 2 m thick overlying unaltered dune sands. A N I M A L TRACE FOSSILS

Introduction Tracks and trails long have been recognized as occurring on siliciclastic eolianite bedding plane surfaces, and bioturbation effects resulting from the activity of

176 invertebrates in modern siliciclastic eolian deposits, mostly inland dune fields, have been studied by Ahlbrandt et al. (1978). These authors and Ekdale and Picard (1985) compiled useful tables listing animals that form traces and trace fossils in dunal environments. However, the effects of animals on and within coastal dune Complexes, and particularly carbonate dunes, have received little attention, Even a rather cursory examination of modern tropical and sub-tropical carbonate dunes reveals that many animals spend all or part of their lives in and on them. An incomplete list would include snails, land crabs, spiders, beetles, wasps, bees, and numerous other insects, snakes, lizards, birds, and small mammals (rodents). Many of these make tracks, or trails, or burrows of varying complexity. Nevertheless. animal trace fossils largely have not been described from carbonate eolianites, and they are generally considered to be rare or absent in such rocks (McKee and Ward. 1983). This is certainly not the case in the Rice Bay Formation, where animal trace fossils are both numerous and varied. We have not yet been able to establish precise modern counterparts for all of the trace fossils described below, and this may be a situation where study of the rock record will eventually lead to greater understanding of the distribution and behavior of animals in m o d e m carbonate dunes.

Skolithos The ichnogenus Skolithos is one of the geologically longest ranging, if not the longest ranging (late Precambrian to Recent), animal trace fossil known. This results from the fact that Skolithos is a simple form, consisting of a small diameter (1-15 ram), commonly lined, unbranched tube oriented vertical to bedding planes. Through time this type of tube, most often used as a dwelling burrow, has been constructed by many different types of invertebrates. The ichnogenus is one of the most typical representatives of the Skolithos Ichnofacies, characteristic of shifting substrates of lower intertidal to shallow subtidal environments (Frey and Pemberton, 1984). Nonetheless, Skolithos is known to have a much broader range of environmental occurrence, from floodplain deposits (Ratcliffe and Fagerstrom, 1980) to deep-sea fans (Crimes, 1977). Modern burrows with the form of Skolithos, made by insects and by arachnids, were reported as common by Ahlbrandt et al. (1978) in siliciclastic inland dune fields. Skolithos linearis burrows in association with irregular mazes of Ophiornorpha sp. were reported by Curran (1984) from shallow subtidal calcarenites of Pleistocene age on San Salvador. More recent field work showed that S. linearis burrows also are common in the Holocene carbonate eolianites of the Hanna Bay Member of the Rice Bay Formation, especially in the eohanite exposures in the sea cliffs at Hanna Bay (Fig. 12). These burrows consist of lined, unbranched shafts commonly 2 - 4 mm in diameter and up to 30 cm tong. We have seen similar S. linearis burrows in Holocene eolianites in the Yucatan and Pleistocene eolianites on Sand Cay, Turks and Caicos. These discoveries extend the already wide paleoenvironmental range of

177

Fig. 12. Specimens of Skolithos linearis. Hanna Bay Member, Hanna Bay cliffs. Scale on right in inches.

Skolithos to the carbonate coastal dune environment. By comparison with burrows in modern carbonate dunes on San Salvador and elsewhere, we suggest that the tracemaker animals were insects or arachnids, or both.

Irregular small burrows The most abundant animal trace fossil in the Rice Bay Formation, especially common in the North Point Member on North Point and Cut Cay, consists of small, irregular burrows. These are best seen on the upper surface of strata (Fig. 13), where they are revealed as irregularly meandering burrows that retain a uniform diameter of 3 - 4 mm along their length, which commonly exceeds 20 cm. The burrows consist of an outer wall that is noticeably paler than the enclosing sediment, and an interior sediment that is like the enclosing sediment matrix. The burrows are unbranched, but crossovers occur commonly, giving rise to an appearance of branching and radiate structure. The burrows are exposed most abundantly in vertical profile, where they can be seen to extend 2 - 3 cm into the

178

Fig. 13. Irregular, small burrows on upper surface of sandflow strata showing meandering form and crossovers. North Point Member, Rice Bay. Scale bar = 2 cm.

strata, in some cases with sufficient density to cause burrow mottling of the sediments (Fig. 14). An interesting feature of these burrows is that they only have been found in grainfaI1 and sandflow strata, although not all such strata contain them, This indicates that the tracemaker animals preferred the shelter of lee slopes. We have observed a preferential distribution of insects on the tee side of modern carbonate dunes, and we suggest that these burrows may have been made by insects or insect larvae. However, we have not yet identified a specific modern counterpart for this ancient tracemaker animal.

Cluster burrows The largest, most complex, and most distinctive trace fossil found in the Rice Bay Formation consists of a cluster of shafts radiating upward from a common area of origin, We.herein refer to this trace fossil by the informal name cluster burrow. The best preserved specimen of the cluster burrow (Fig. 15), occurs in a small sea cliff that begins some 60 m from the most southeasterly rock exposures on the Rice Bay side of the North Point peninsula (White and Curran, 1985). This is not an isolated specimen, however, and some fifteen other, similar trace fossils have so far been

179

Fig. 14. Vertical surface of sandflow and grainfall layers showing mottling produced by the same kind of organism that formed the irregular, small burrows illustrated in Fig. 13. Note the non-bioturbated texture of the overlying wind ripple laminations. North Point Member, Cut Cay. discovered in the Rice B a y F o r m a t i o n in n o r t h e a s t e r n San Salvador. A p p r o x i m a t e l y 50 m n o r t h w e s t of the figured specimen, a n o t h e r one is e x p o s e d in h o r i z o n t a l view, where it can be seen to c o n t a i n several h u n d r e d shafts. T h e s e two s p e c i m e n s o c c u r in a fossil d u n e that is a b o u t 85 m wide a n d u p to 5 m high, a n d it is enclosed in d u n e rocks that c o n t a i n micrite crusts, r h i z o m o r p h s , s a n d f l o w strata, grainfall strata, wind ripples e x p o s e d on b e d d i n g surfaces, a n d w i n d r i p p l e l a m i n a t i o n s . T h e cluster b u r r o w trace fossil consists of n u m e r o u s , straight to g e n t l y curved shafts that have n o lining. I n d i v i d u a l shaft d i a m e t e r s r a n g e from 1 to 2 cm (average 1.2-1.4 cm), a n d they can be 1.4 m or m o r e in length. In some cases shaft d i a m e t e r s n a r r o w slightly t o w a r d s their u p p e r ends. A few of the shafts b r a n c h u p w a r d s , a n d definite crossovers occur. Each trace fossil consists of a cluster of tens to h u n d r e d s of i n d i v i d u a l shafts that r a d i a t e u p w a r d f r o m an a p p r o x i m a t e l y c o m m o n p o i n t of origin, creating a c o n e - s h a p e d structure that m a y reach a d i a m e t e r of 1 m or more. This trace fossil does not r e s e m b l e a n y p r e v i o u s l y r e p o r t e d form. It b e a r s a superficial r e s e m b l a n c e to a b u r i e d shrub; however, closer o b s e r v a t i o n suggests that this is an unlikely e x p l a n a t i o n . T h e shafts are n o t p r e s e r v e d b y c o n c r e t i o n a r y micrite as is usually the case with true r h i z o m o r p h s , a n d their verticality a n d close p r o x i m i t y are unlike m o s t plants. T h e shafts show little b r a n c h i n g n o r m u c h

180

Fig. 15. Large cluster burrow trace fossil in vertical face of eolianite dune. The closely spaced shafts arc unlined, have diameters of 1-2 cm, with many continuous for the 1.4 m height of the specimen. North Point Member, Rice Bay. Scale = 10 cm.

variation in diameter, unlike most plants that show frequent branching and significant differences in diameter between older, thicker branches and younger, thinner twigs. We have never found any evidence of a root system radiating downward from the base of the upwardly radiating structure, nor any sign of a paleosol; both of which ought to be evident in a buried shrub. We suggest that this trace fossil represents the brooding and hatching activity of an invertebrate, perhaps a burrowing wasp, with each individual burrow being the escape pathway of a juvenile as it made its way to the sediment surface. We have observed small-scale burrowing by wasps in modern carbonate dunes, but, so far, we have found no modern c o u n t e ~ a r t with the large size and cluster form of this trace fossil. Again, we may learn something about modern animals from the fossilized traces of life activities of their ancestors.

181 CRITERIA FOR T H E R E C O G N I T I O N OF A N C I E N T CARBONATE EOLIANITES

Numerous examples from the geologic record show that m a n y carbonate rocks represent accumulation during progressive shallowing of seawater, commonly leading to deposition in intertidal and supratidal zones (for review see James, 1984). It seems likely that, in some cases at least, such sequences would have a capping of eolianites. This is certainly the case in the Pleistocene rocks of San Salvador (White et al., 1984; Curran and White, 1985). Many other, similar Pleistocene occurrences are recorded by McKee and Ward (1983), although they note that pre-Pleistocene carbonate eolianites are virtually unknown, perhaps because they are unrecognized rather than being actually absent. As noted above, this situation is in sharp contrast to siliciclastic rocks, where examples of eolianites are known from as far back as Middle Proterozoic times. Our studies of the Holocene carbonate eolianites of San Salvador suggest that there are distinctive features of such rocks that should aid in their recognition in the more ancient rock record. Measurement of ripple indices of exposed wind ripples may be diagnostic where such ripples are sufficiently well-exposed and preserved. More likely to be commonly found in the rock record are the distinctive, inversely graded laminations produced by climbing wind ripples. For sediments formed by the migration of wind ripples to be preserved, they must be sufficiently lithified to prevent their later reworking by winds, streams, or marine waves and currents. In many cases, such consolidation would be accomplished by cementation in the freshwater vadose diagenetic environment. Such cementation occurs preferentially in the finer part of a wind ripple couplet, thus the finer and coarser parts of laminations will have different diagenetic histories that may be decipherable by careful petrographic analysis. Sandflow and grainfall strata are preserved in the Rice Bay Formation eolianites, and these distinctive features may be anticipated in older carbonate eolianites. Crusts formed by the precipitation of low-Mg calcite are notable features of modern carbonate dunes on San Salvador, where they commonly are associated with plants. Directly analogous micrite layers with characteristic plant trace fossils that represent the distinctive trailing stems and lateral roots of dune-dwelling plants are common in the Rice Bay Formation rocks, where they recur at intervals of 10-15 cm in many sequences. Animal trace fossils are common in some parts of the Rice Bay Formation, and their absence from ancient carbonate eolianites cannot be assumed. Skolithos linearis burrows are quite numerous in the H a n n a Bay Member, and they have been observed elsewhere in Holocene and Pleistocene eolianites. These simple burrows are not diagnostic of any particular paleoenvironment, but their presence cannot be used to exclude the possibility of wind-deposited sediments. Similar arguments can be made for the small, irregular burrows described above, although their restricted association with characteristic sandflow and grainfall strata may be useful corroboratory evidence. As fas as is known, the large, cluster burrow trace fossil is unique to

182 the Rice Bay Formation eotianites, and any future discoveries elsewhere in the fossil record may be indicative of an eolian depositional environment. CONCLUSIONS

(1) Inversely graded laminations produced by wind ripples are common in the Rice Bay Formation. These laminations cannot be used to determine paleowind directions, but they are characteristic of wind deposition, and they should be identifiable even in cores and small hand specimens from carbonate eolianites. (2) Sandflow and grainfall strata are preserved in the Holocene carbonate eolianites of San Salvador, and these distinctive structures should be present in analogous Pleistocene and pre-Pleistocene carbonate eolianites and should aid in their recognition. (3) Thin, hard crusts, in some cases associated with plants, form on the surfaces of modern dunes by dissolution in rainwater of aragonite and high-Mg calcite from dune sands and subsequent precipitation of tow-Mg calcite. Analogs of these crusts occur in the Rice Bay Formation as hard micritic layers, commonly with associated plant trace fossils, at vertical intervals of 10-15 cm. These crusts and associated plant trace fossils are characteristic of eolian dune environments, and they are recognizable in cores and hand specimens. Their presence in the Rice Bay Formation suggests relatively slow accumulation of the eolian dunes, with deposition of thin sand layers separated by periods of plant colonization and crust development. (4) Skolithos linearis burrows are common in the Hanna Bay Member. These trace fossils are not per se diagnostic of the eolian environment, but their presence in ancient carbonates should not be used to exclude the possibility of! eolian deposition. These S. linearis burrows are thought to have been produced by insects or arachnids, or both. (5) Small, rather irregular burrows that extend laterally and to some extent vertically through the sediments are numerous in the Rice Bay Formation eolianites. These trace fossils are not sufficiently distinct morphologically to be regarded as unique to eolian environments, although their restricted occurrence in sandflow and grainfall strata may supply supporting evidence for eolian deposition. These trace fossils are believed to have been formed by insects or insect larvae that lived on the protected lee-side of dunes. (6) The large cluster burrow trace fossils found in the North Point Member are unique and quite unlike any other described trace fossil. They are believed to have been formed by hatchlings of some invertebrate animal as they migrated to the surface. Their closest modern analogs are formed by the burrowing wasps, although there is a significant difference in scale, These trace fossils are confined t o t h e dune environment, and they may well be diagnostic of eolian sedimentation.

183 ACKNOWLEDGEMENTS

We thank the staff of the College Center of the Finger Lakes, Bahamian Field Station for full logistical support of our field work on San Salvador. Valuable contributions to our eolianites research project were made by Smith College geology students: Eve Arbogast, Elizabeth Dole, Kate Japy, Elaine Kotler, Karen Kurkjy. Jean Lawlor, and Kim Pirie. We thank Constance Soja for her careful reading of the manuscript and Paul Godfrey for his help with the field identification of dune plants on San Salvador. The manuscript was improved by the helpful comments of R.C. Blakey and R.P. Swinehart, and we are grateful for their careful reviews. Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of our research through separate grants to Curran and to White. Generous financial support was also provided by the Merck, Pew, and Shell foundations, and we extend our thanks to each of them. REFERENCES Adams, R.W., 1980. General guide to the geological features of San Salvador. In: D.T. Gerace (Editor), Field Guide to the Geology of San Salvador. C C F L Bahamian Field Station, Ft. Lauderdale, Fa., pp. 1 66. Ahlbrandt, T.S., Andrews, S. and Gwynne, D.T., 1978. Bioturbation in eolian deposits. J. Sediment. Petrol., 48: 839-848. Ball, M.M., 1967. Carbonate sand bodies of Florida and the Bahamas. J. Sediment. Petrol., 37:556 591. Carew, J.L. and Mylroie, J.E., 1985. The Pleistocene and Holocene stratigraphy of San Salvador Island, Bahamas, with reference to marine and terrestrial lithofacies at French Bay. In: H.A. Curran (Editor), Pleistocene and Holocene Carbonate Environments on San Salvador Island, Bahamas, Guidebook for Geological Society of America Annual Meeting Field Trip. C C F L Bahamian Field Station, Ft. Lauderdale, Fla., pp. 11-61. Carew, J.L., Mylroie, J.E., Wehmiller, J.F. and Lively, R.A., 1984. Estimates of sea level high stands from San Salvador, Bahamas. In: J.W. Teeter (Editor), Proc. 2nd Symp. on the Geology of the Bahamas. CCFL Bahamian Field Station, Ft. Lauderdale, Fla., pp. 153-175. Clemmensen, L.B. and Abrahamsen, K., 1983. Aeolian stratification and facies association in desert sediments, Arran basin (Permian), Scotland. Sedimentology, 28: 311-339. Crimes, T.P., 1977. Trace fossils of an Eocene deep-sea fan, northern Spain. In: T.P. Crimes and J.C. Harper (Editors), Trace Fossils 2. Geol. J., Spec. Issue, 9: 71-90. Curran, H.A., 1984. Ichnology of Pleistocene carbonates on San Salvador, Bahamas. J. Paleontol., 58: 312-321. Curran, H.A. and White, B., 1985. The Cockburn Town fossil coral reef. In: H.A. Curran (Editor), Pleistocene and Holocene Carbonate Environments on San Salvador Island, Bahamas, Guidebook for Geological Society of America Annual Meeting Field Trip. C C F L Bahamian Field Station, Ft. Lauderdale, Fla., pp. 95-120. Dott Jr.. R.H., Byers, C.W., Fielder, R.S., Stenzel, S.R. and Winfree. K.E., 1986. Aeolian to marine transition in Cambro-Ordovician cratonic sheet sandstones of the northern Mississippi valley, U.S.A. Sedimentology, 33: 345-367. Ekdale, A.A. and Picard, M.D., 1985. Trace fossils in a Jurassic eolianite, Entrada Sandstone, Utah, U.S.A. In: H.A. Curran (Editor), Biogenic Structures: Their Use in Interpreting Depositional Environments. Soc. Econ. Paleontol. Mineral., Spec. Publ., 35: 3-12.

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