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A shell concentration of the Middle Miocene Crassostrea gryphoides (Schlotheim, 1813) from Siwa Oasis, Western Desert, Egypt
Journal of African Earth Sciences 120 (2016) 1e11
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A shell concentration of the Middle Miocene Crassostrea gryphoides (Schlotheim, 1813) from Siwa Oasis, Western Desert, Egypt Ahmed M. El-Sabbagh a, Magdy M. El Hedeny a, b, * a b
Department of Geology, Faculty of Science, Alexandria University, Alexandria 21568, Egypt Deanship of Scientific Research, King Saud University, Riyadh, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 December 2015 Received in revised form 23 March 2016 Accepted 7 April 2016 Available online 20 April 2016
A concentration of heavy, thick-shelled, large-sized, and elongated population of the oyster Crassostrea gryphoides (Schlotheim, 1813) was recorded in shallow-marine deposits of the basal Oasis Member of the Middle Miocene Marmarica Formation exposed at Siwa Oasis, Egypt. The oyster assemblage is resedimented as a lens-shaped bank up to 80e100 cm thick and about 220 m long. Crassostrea gryphoides specimens are embedded in a yellowish green, soft marl matrix. This is the first documented occurrence of this lens at Siwa Oasis. The lensoid structure is bounded by a lower marl and an upper shale beds of about 2 m and 1.5 m thick, respectively. Assemblage within this lens is characterized by extreme variations of Crassostrea gryphoides, forming an almost monotypic assemblage. The shell packing was dense (shell percentages higher than 75%) at the base and the center of the lens, whereas it exhibits loose packing at the top and right and left sides of the lens (shell percentage less than 15%). Valves are poorly sorted and randomly orientated (both in surface and cross section views). Encrustation and bioerosion have observed on both sides of the left and right valves. The relatively limited varieties of encrusters together with moderate frequency of borings indicate moderate to high sedimentation rate. On the other hand, the low abundance of fragmented and abraded shells indicates good preservation and minimal transport. The studied lens concentration is interpreted as proximal tempestites assemblage. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Crassostrea gryphoides Shell concentration Middle Miocene Marmarica Formation Siwa Oasis Western Desert Egypt
1. Introduction Benthic macrofauna is known to be sensitive to environmental conditions such as temperature, salinity, substrate characteristics, energy, nutrients, etc. (e.g. Sanders, 1968; Fürsich, 1981; Staff and Powell, 1988; Pickerill and Brenchley, 1991; Kownacki et al., 2000; El-Hedeny, 2005). They exhibit signs of environmental stresses, such as high mortality, low diversity, shell abnormality, € hl, 1998). Among benthic dwarfism, etc. (e.g. Fürsich, 1981; Ro macrofauna, oysters represent an outstanding opportunistic example that tolerate a wide range of a highly dynamic and stressful environmental conditions (i.e. r-strategists; El-Sabbagh et al., 2011). Their concentrations are often mono-to pauci-specific and characterized by large-sized shells (e.g. Fürsich and Werner, 1986; El-Ayyat and Kassab, 2004). The response of their tolerance could also be observed in their morphological features,
* Corresponding author. Department of Geology, Faculty of Science, Alexandria University, Alexandria 21568, Egypt. E-mail address: [email protected] (M.M. El Hedeny). http://dx.doi.org/10.1016/j.jafrearsci.2016.04.007 1464-343X/© 2016 Elsevier Ltd. All rights reserved.
including general shape and outlines, ligamental area, beak, shellthickness, beak orientation, etc. (e.g. Newell and Boyd, 1970; Abdel Aal and El-Hedeny, 1998; El-Hedeny, 2005; Pufahl and James, 2006; El-Hedeny and El-Sabbagh, 2007). Shell concentrations are common phenomena of shelf and coastal areas (Kidwell et al., 1986; Fürsich, 1995; El-Ayyat and Kassab, 2004; Bressan and Palma, 2010; El-Sabbagh et al., 2016). Their formations are greatly affected by many biostratinomic processes. Accordingly, detailed biostratinomic analysis is essential for understanding of their formation (e.g. Brett, 2003; El-Qot et al., 2009; El-Sabbagh et al., 2016). In our recent study of the Middle Miocene benthic macroinveretbrates in Siwa Oasis, Western Desert, a lens concentration of the large shells of Crassostrea gryphoides (Schlotheim, 1813) is recorded for the first time in an outcrop, about 7 km north of the Siwa City (Fig. 1). Crassostrea gryphoides reefs were widespread and €r, 2008; common along the Miocene circum-Tethyan coasts (Hos¸go Harzhauser et al., 2015). Certainly, because of their abundance and higher fossilization potential of their shells, the present lens could provide significant paleoecological information as well as detailed
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Fig. 1. A Simplified geological map of the Siwa Oasis showing the section analyzed (modified after CONOCO and EGPC, 1988). The key map is modified after Abdel Fattah et al. (2013).
data on the paleoenvironment in which they were deposited (e.g. Brett, 2003; El-Sabbagh et al., 2015). Geographically, Crassostrea gryphoides (Schlotheim, 1813) is a long lived species. In the entire Western Tethys, it is appeared in the Oligocene and extended up to the Pliocene (Schultz, 2001). Other studies recorded it in higher stratigraphic levels (e.g. PliocenePleistocene, Khalil, 2011; Recent, Siddiqui and Ahmed, 2002; Ganapathi Naik and Gowda, 2013). The purposes of this paper are: 1) to register the occurrence of C. gryphoides in the Middle Miocene sediments of Siwa Oasis, 2) to describe and discuss the recorded C. gryphoides concentration, and 3) to interpret the postmortem alterations and the paleoenvironmental conditions prevailed during the formation of this concentration. 2. Location and stratigraphic context Siwa Oasis, a depression of about 10e17 m below sea level, is situated on the west of the Qattara Depression between latitudes 29 020 and 29 280 N and longitudes 25 120 and 26 020 E (Fig. 1). In the north, it is bounded by a Middle Miocene escarpment that rises about 100 m above the depression floor while it is bounded on the south by a low Middle Miocene scarp of about 20e25 m above the depression floor. The Marmarica Formation is widely exposed in this region. It was first introduced into the Egyptian lithostratigraphy by Said (1962) for a Middle Miocene sequence of about 78 m thick, at the northern scarp of Siwa Oasis. It is composed of alternating carbonate beds and greenish, bluish, and blackish shales and marls. The carbonates are made up of cross-bedded coquina and other organoclastic limestone. The rocks of the Middle Miocene
Marmarica Formation unconformably overlie the Lower Miocene Moghra Formation (Said, 1962). The studied section is located north of the Siwa City, at longitude 25 310 22.100 E and latitude 29 160 24.600 N (Fig. 1). At this area, the Marmarica Formation (about 78 m thick) is subdivided into three units. They are: a lower unit of marl, carbonate and shale, a middle unit of chalky and argillaceous limestone, and an upper unit of fossiliferous and non-fossiliferous chalky limestone. Based on the classification suggested by Gindy and El-Askary (1969), these units are named the Oasis, Siwa Escarpment and El Diffa Plateau members, respectively (Fig. 2). A remarkable feature of the studied rock unit is the predominance of carbonate rocks. They progressively decrease from top to bottom as shale and marl increase. Also, sandstones are completely absent. The Oasis Member attains a thickness of about 41 m. It is composed of fissile shale, marl, cross-bedded limestone, argillaceous limestone and coquina beds (Fig. 2). In places, shale and marl occurred as lenses that interfingering with limestones and coquinas. The cross-bedded carbonate rocks are tabular and mainly composed of mixed organic debris derived from several marine faunas (i.e. coquina). In case of well developed cross-bedding, the coquina composed of well-sorted fine to medium grains. In the other case, sorting and fabric of the cross-bedded rocks become complex, containing fragments of large fossils. In tops of some shale-marl beds, traces of horizontal bioturbation (mainly Thalassinoides isp.) and/or lag deposits of reworked shale/marl clasts are recorded. The Siwa Escarpment Member (about 21 m thick) consists mainly of thick beds of chalky limestones as well as thin beds of shale, marl and argillaceous limestone. Some bored hardground surfaces and lag deposits occur. Within this member, as
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Fig. 2. Lithological description of the studied section accompanied with a sketch showing the site of the Crassostrea gryphoides shell lens concentration (A) and a photograph of the outcrop, illustrating the studied lens (bed 25) underlain and overlain by marl and shale beds, respectively (B).
well as in the overlying El Diffa Plateau Member, cross-lamination is absent in the chalk beds and mostly look structureless. In addition, limestone and chalky limestone beds show oval to irregular solution scours and cavities, some of which are filled by breccia and fine-grained debris. The El Diffa Plateau Member attains a thickness of about 16 m and composed of relatively thick beds of chalky limestone cyclically rich and poor in fossils. Sediments of the studied area are characterized by a highly abundant invertebrate fauna, including mollusks, echinoids, bryozoans and few corals. In general, they represent the most characteristic index species in the Middle Miocene formations of Egypt. The rather fewer numbers of publications on the geology and stratigraphy of Siwa Oasis include those of Said (1962), Gindy and El-Askary (1969), Gindy (1970), Ziko et al. (2000); El-Shazly and Abdel-Hamid (2001), Abdel Fattah et al. (2013) and El-
Sabbagh et al. (2016). 3. Material The present study is based on 145 moderately to well preserved specimens of Crassostrea gryphoides (Schlotheim, 1813) collected from a lens within the upper part of the Middle Miocene Oasis Member (Fig. 2). This lens is located at the southern part of the sampled outcrop; at latitude 29 160 22.800 N and longitude 25 310 24.100 E. Crassostrea gryphoides were embedded in yellowish green, soft marl matrix. Whenever possible, way-up arrows were marked on shells at the time of collection. Their mode of fossilization ranges from exceptionally well preserved tests including microscopic surface characters to slightly abraded test material. The occurrence of well preserved oyster specimens allows an interesting
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paleoenvironmental, paleoecological and biostratinomic inferences. All specimens examined are housed in the Department of Geology, Faculty of Science, Alexandria University, Egypt. Specimen numbers are prefixed by NS25O.
4. Description of the lens concentration Crassostrea gryphoides is resedimented in a lens-shaped bank up to 80e100 cm thick and about 220 m of lateral extension. The upper contact of the lens with the overlying marl bed (about 2 m thick) is sharp and sometimes erosive. Similarly, the contact with the underlying shale (1.5 m thick) is sharp and erosional (Fig. 2). The oyster assemblage is almost monotypic, composed mainly of C. gryphoides. In addition, rare small pectinids and dwarfed bivalves
are recorded throughout the lens. The C. gryphoides populations are composed of different growth stages, ranging from juveniles to adults (Fig. 3). Shells are mostly disarticulated (99.3%) and randomly oriented, though in many places convex-up valves seem to predominate over oblique, vertical and concave-down ones (Fig. 3). Assemblage is poorly sorted and fragmentation affects few specimens of C. gryphoides and does not exceed 10%. The shell packing shows positive gradation within each micro~ es and Kowalewski, 1998), stratigraphic unit (m.u., sensu Simo usually consists of a relatively dense packing at the base and the center of the lens (shell percentages higher than 75%) and loose packing occurred at the top and right and left sides of the lens (shell percentages less than15%).
Fig. 3. A. The eastern end of the Crassostrea gryphoides lens. B. Randomly oriented C. gryphoides valves. Note the dense shell packing at the base and the center of the lens relative to the top part. C. Surface view of the lens with valves of C. gryphoides concordant to the bedding plane. D. Some free valves of C. gryphoides both upwards and downwards convexes. E. C. gryphoides bouquet. F. C. gryphoides with convex-up and vertically preserved valves.
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Fig. 4. Crassostrea gryphoides (Schlotheim, 1813) from the Middle Miocene of Siwa Oasis, Western Desert. Note the wide variations in shell dimensions (length, width and thickness), ligamental area and beak curvature. All specimens are left valves except for F and G that represent the right ones. Scale bar represents 5 cm.
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Fig. 5. Characteristic features of the ligamental area of Crassostrea gryphoides.
5. Variability of the specimens Recent and ancient oyster shells display wide morphological variation, which is mainly caused by their life habit and growing conditions (e.g. Chinzei, 1986; Abdel Aal and El-Hedeny, 1998; Littlewood and Donovan, 1988; Machalski, 1998; Seilacher and Gishlick, 2014). They can cement themselves to a hard substrate to form banks or implants their individuals into the soft sedimentary substrate to form bunch, bouquet or cluster. Under these conditions, individuals modify their shell shape when restricted space is available for growing (Stenzel, 1971). Our specimens have large, heavy and thick-shelled with left valves of up to 30 cm in length, 20 cm width and up to 5 cm of shell thickness (Fig. 4). Right valves are smaller and flat. The C. gryphoides shells are highly variable, especially in their shell outline, ligament and beak. Shell outline. In general, the shell outline of the studied C. gryphoides specimens is elongated, varies from sub-trigonal, subovoidal to vertically-extended sub-rectangular (Fig. 4). In the examined specimens, and although there is no characteristic relationship between the shape of the shell and hinge area, no doubt that both are belonging to the same species. The shape of oyster shells and their proportions are highly variable and, therefore, are, in some cases, of little use for the identification of species (Galtsoff, 1964). Depending on the wide variation in shape outline, large number of previously described thick-shelled oyster species may mis-identified as a separate species (e.g. Abdel Aal and El-Hedeny, 1998; Machalski, 1998; Kosenko, 2014). This variation is strongly affected by the mode of life rather than ontogenic causality. According to Seilacher (1984), the elongated shape can be regarded as indicative either of the mud-sticking habit or of competition for space between specimens growing close to each other. In the study area, the large, elongated shells of C. gryphoides refers to the nature of their muddy substrate which considered as a strategy to prevent themselves from a complete burial into the soft substrate and from being transported by currents to unfavorable habitats. Ligament. It plays an important role for growth and survival of oysters (Galtsoff, 1964). In Crassostreinae, the ligamental area is composed of an elevated resilifer of the right valve and bulge-like bourrelets of the left valve articulating with opposing depressions (Fig. 5). This type is called the alivincular-arcuate ligament (sensu
Hautmann, 2004). This particular type of ligamental area occurs only in oysters and is probably an autapomorphy of this group (Malchus, 2004). In the studied C. gryphoides specimens, the ligamental areas have great variations in their appearances (Figs. 4 and 6). It is mainly triangular, higher than long; resilifer is wider than anterior and posterior bourrelets. Length and height of ligamental area are considerably varied in our assemblage, ranging from very narrow to wide, invariable length throughout the whole area to wide ventrally and narrow dorsally (Fig. 6). The present specimens exhibit a characteristic structure of risilifer, consisting of concave bottoms and elevated growth breaks (Fig. 5). The periodic pattern of growth breaks in each annual increment indicates that breaks are not caused by extreme environmental conditions (Andrus and Crowe, 2000). Fan et al. (2011) suggested that this breaks referred to spring time when the temperature was favorable to food supply and spawning. Meanwhile, Muller (1970) regarded any disturbance in ligament structure as an index of the environmental conditions that leads to affect the functions of the organism and morphology of its valves. On the other hand, Kirby et al. (1998) observed growth increment formation during the summer in Crassostrea virginica and interpreted these to reflect growth cessation due to high water temperatures (see also Lawrence, 1988; Andrus and Crowe, 2000). Beak. The beak curvature is the third characteristic feature in the morphology of C. gryphoides assemblage. The present specimens display wide variations in beak curvature of the left valves (Figs. 4 and 6). Most of the specimens have a considerable umbonal angle relative to the plane of commissure. They usually curved and directed toward the posterior end of the left valves, although in some specimens they may point toward the posterior. Very narrow, straight, or slightly curved beaks of these kinds are usually formed in oysters which grow on soft, muddy bottoms (e.g. Hallam and Gould, 1975; El-Hedeny, 2005). The extreme development of this type can be seen in the narrow and slender oysters growing under overcrowded conditions on reefs. 6. Paleoenvironment, paleoecology and biostratinomy Across the northwestern part of the Western Desert of Egypt, the depositional evolution of the Middle Miocene Marmarica Formation involved two stages, which are controlled by mild tectonics and eustatic sea-level changes (Gindy and El-Askary, 1969; Said, 1990; Abdel Fattah et al., 2013). The first stage comprises the deposition of the carbonate, marl and shale facies of the basal Oasis Member under a relatively oscillated sea level, low rate of clastic influx and warm climate. The second stage is represented by carbonate-dominated facies of the Siwa Escarpment and El Diffa Plateau members under fluctuating sea level. In the studied succession, the remarkable decrease in clastics in the two younger members and the absence of cross-bedding suggest a relative deepening of the original marine conditions with more restriction in clastic supply and water turbidity after the deposition of the Oasis Member. In the Oasis Member of the Marmarica Formation, the shale, marl, limestones and the commonly occurrence of mixed and/or transported assemblages of bivalves, gastropods, echinoids, bryozoans and corals suggest accumulation in moderate to high-energy shallow marine environments (Abdel Fattah et al., 2013; ElSabbagh et al., 2016). The concentrated and abraded nature of some macrofaunal groups in this member may suggest transportation by wave/tidal currents and storm events. In addition, high-energy shallow marine conditions can be deduced by the presence of cross-bedded coquina and other mechanical carbonate beds in the Oasis Member. In the study area, and generally at Siwa
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Fig. 6. Sketch of the ligamental areas and beaks of Crassostrea gryphoides illustrating the wide variations of their characteristic features. For all drawings, scale bars represent 1 cm.
Fig. 7. Biostratinomic model for the formation of Crassostrea gryphoides lens concentration in the Middle Miocene Marmarica Formation of Siwa Oasis, Western Desert. A, C. gryphoides shells in life position; B, tilting of shells due to winnowing; C, C. gryphoides lens formed by storm-induced currents.
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Oasis, these directional structures are mostly bimodal (Gindy and El-Askary, 1969; Abdel Fattah et al., 2013). Direction of transport was predominantly west to northwest and less commonly was in the opposite direction (i.e. east to southeast). Many authors have discussed the paleoecology and biostratinomy of ancient oysters (e.g. El-Hedeny, 2005, 2007; Elnech Sabbagh, 2008; Ayoub-Hannaa and Fürsich, 2011; Dome et al., 2014; El-Sabbagh et al., 2015). Although they agreed that oysters live best in low intertidal or shallow subtidal, they found significant variations in their shell morphology (shape, thickness, size, ornamentation, etc.). Such variations are mainly referring to the environmental conditions (salinity, temperature, competition, etc.) prevailed during their life. As was mentioned, the studied C. gryphoides specimens are found as densely well preserved shells embedded in yellowish green soft marl. The fast growth and high density of C. gryphoides may reflect exceptionally high productivity and nutrient availability in the environment (Kirby, 2001). Owing to their elongated shells, their life habit is interpreted as a mud-sticker that strongly supported by the surrounding sediment in vertical position with the umbones directed downwards (Fig. 7a). This upright mode of life was probably reflecting growth in high-density populations (Machalski, 1998). According to several studies (e.g. Chinzei, 1982, 1986, 1995; Seilacher, 1984; Malchus, 1998; El-Hedeny, 2005), there are three strategies for the growth of Crassostrea populations; the first is based upon attachment of shells of living animals on former generations to form aggregates; the second is depend on the ability of the animal to form a thick, lightweight vesicular shell structure that prevents their shells to sink in soft substrate and the third mode is the ability of the animal to elongate their shell vertically to keep their soft parts alive above the soft substrate. Shells of the studied C. gryphoides are heavy, i.e. the strategy of formation of the lightweight vesicular or empty structure is not the case of the present assemblage. Instead, the present assemblage consists of shells that reflect the other two modes of life. The elongated shell forms are common in the studied assemblage. Meanwhile, we have some shells that have impressions of attachment with adjacent individuals (Fig. 4). The present concentration was produced by moderate to high energy event (Fig. 7). This can be confirmed by the densely packed, imbricated, and randomly oriented with few fragmented shells (Fig. 3). The majority of the left valves were positioned as convex up attitude, indicating that they removed apart from their positions and transported by currents. A few shells show a vertical position with the commissural plane more or less perpendicular to the bottom and the ligamental area pointing downwards. All these observations indicate that we are dealing with a little-transported, low-diversity autochthonous to parautochthonous lens concentration. This assemblage underwent transport, most likely by storm-induced currents (Fig. 7). Hence, our shell concentration is interpreted as proximal tempestites assemblage (e.g. El-Qot et al., 2009; Zuschin et al., 2015). Concerning the post-mortem signatures, our C. gryphoides specimens are mainly disarticulated (144 out of 145 specimens).
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The proportions of the collected left and right valves are greatly unequal (76.4% vs. 23.6%, respectively). The general degree of fragmentation and abrasion is uniformly low, whereas encrustation and bioerosion are common and mainly occur on the exterior surfaces of left and right valves. However, some traces are recorded in the interior sides of both valves (Fig. 8). In the studied assemblage, encrustation abundance is relatively low (25.5%). The common encrusters include congeners of C. gryphoides, cheilostome bryozoans, few serpulid worms and rare cirripeds (genus Balanus Da Costa). In general, limited reciprocal overgrowths were observed between different encrusters (Fig. 8). The C. gryphoides specimens are moderately affected by macroborings (48.3%). Borings are mainly represented by Entobia Bronn, Gastrochaenolites Leymerie and Maeandropolydora Voigt (Fig. 8). Few occurrences of Caulostrepsis Clarke and Rogerella SaintSeine are also observed. Of these, the ichnogenus Entobia is more common in the studied shells than other bioerosion traces. Clionaid sponges, the producer of the ichnogenus Entobia, prefer to live in nearshore, shallow, relatively low-energy marine environments (Calcinai et al., 2005). Nevertheless, their occurrence does not confirm that the shells of C. gryphoides are subjected to a prolonged post-mortem process. Many paleontologists regarded incorrectly that the occurrence of Entobia in fossil shells is an indication of pro-longed post-mortem process. Majority of clionaid sponges were observed in recent oysters (e.g. Carver et al., 2010; Lopes, 2011; Dunn et al., 2014). This could provide a compelling example for Entobia traces that they have a significant contribution both pre- and post-mortem stages. In addition, El-Hedeny and ElSabbagh (2005) have regarded that activities of bioerosion in radiolitid Eoradiolites liratus (Conrad) may be interpreted to have extended from a pre-death to the pre-final burial. 7. Conclusions The topmost part of the basal Oasis Member of the Middle Miocene Marmarica Formation contains a shell concentration of the oyster Crassostrea gryphoides (Schlotheim, 1813). Individuals are represented by different growth stages, ranging from juveniles to adults. The oyster concentration is a lens-shaped bank up to 80e100 cm thick and about 220 m long where thick-shelled, largesized, elongated and well preserved individuals are embedded in a yellowish green, soft marl matrix. The studied specimens of C. gryphoides are represented by monotypic community in moderate-to high-energy mud-supported deposits, with fast growth rates, indicating a relatively moderate to high sedimentation rates. Most of the shells are oriented both convex-up and convex-down, although few specimens show a vertical position with the commissural plane more or less perpendicular to the bottom and the ligamental area pointing downwards. The morphological variability of the soft-bottom C. gryphoides is interpreted in terms of ecophenotypic response to environmental parameters. The relatively limited varieties of encrusters together with moderate frequency of borings are also indicating a moderate to high sedimentation rate. On the other hand, the lower degree of fragmentation, poor sorting and scarcity of abrasion indicate good
Fig. 8. Boring and encrustation on Crassostrea gryphoides shells. A. Encrustation of large left valve of Crassostrea gryphoides on the exterior surface of another large left valve, NS25O443. B. Posterior side of a large left valve of C. gryphoides encrusted by juvenile C. gryphoides valves, NS25O61. C. A relatively dense cluster of a relatively equal-sized, disarticulated, small oyster shells on the exterior surface of left valve of C. gryphoides, NS25O59. D. Encrustation of small C. gryphoides shell on the interior surface of the left valve of the same species. Traces of Entobia isp. are occurred, NS25O75. E. Cheilostome bryozoans and Balanus sp. encrust the ligamental area of left valve of C. gryphoides. Note the reciprocal overgrowth between these two encrusters, NS25O77. F. Serpulid worm (arrow) on the exterior surface of left valve of C. gryphoides, NS25O130. G. Entobia isp. randomly distributed over the exterior surface of left valve of C. gryphoides. Note the traces of Maeandropolydora isp. in the central part, NS25O79. H. C. gryphoides left valve with Entobia isp. and Gastrochaenolites isp. traces, NS25O168. I. Gastrochaenolites isp. in the exterior surface of right valve of C. gryphoides, NS25O137. J. Maeandropolydora isp. in the interior surface of left valve of C. gryphoides, NS25O189. Scale bar for specimens ¼ 2 cm.
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preservation and minimal transport. Consequently, the lens concentration is interpreted as proximal tempestites assemblage. Acknowledgments The authors would like to express their sincere appreciation to the Deanship of Scientific Research at King Saud University for funding this work through the Research Group Project No. RG‒ 1436‒022. Thanks to the editor (Damien Delvaux) and the two reviewers, Maria Helena Henriques and one anonymous reviewer, whose reviews and insights are helpful and aided in improving this manuscript. We also thank Mr. M. Rashwan for his careful photographic work. References Abdel Aal, A.A., El-Hedeny, M.M., 1998. On the variability of the Campanian Pycnodonte (Phygraea) vesiculare (Lamarck). N. Jb. Geol. Pal€ aontol. Mh. 1, 42e54. Abdel Fattah, Z.A., Kora, M.A., Ayyad, S.N., 2013. Facies architecture and depositional development of middle Miocene carbonate strata at Siwa Oasis, northwestern Egypt. Facies 59 (3), 505e528. Andrus, C.F.T., Crowe, D.E., 2000. Geochemical analysis of Crassostrea virginica as a method to determine season of capture. J. Archaeol. Sci. 27, 33e42. Ayoub-Hannaa, W., Fürsich, F.T., 2011. Functional morphology and taphonomy of Cenomanian oysters from eastern Sinai, Egypt. Palaeobiodivers. Palaeoenviron 91, 197e214. Bressan, G.S., Palma, R.M., 2010. Taphonomic analysis of fossil concentrations from n basin, Mendoza province, Argentina. La Manga formation (Oxfordian), Neuque J. Iber. Geol. 36 (1), 55e71. Brett, C.E., 2003. Taphonomy: sedimentological implications of fossil preservation. In: Middleton, G.V. (Ed.), Encyclopedia of Sediments and Sedimentary Rocks. Springer, Dordrecht, pp. 723e729. Calcinai, B., Bavestrello, G., Cerrano, C., 2005. Excavating sponge species from the Indo-Pacific ocean. Zool. Stud. 44, 5e18. riault, I., Malley, A.L., 2010. Infection of cultured eastern oysters Carver, C.E., The Crassostrea virginica by the boring sponge Cliona celata, with emphasis on sponge life history and mitigation strategies. J. Shellfish. Res. 29, 905e915. Chinzei, K., 1982. Paleoecology of oysters 1, 2. Kaseki (fossils) Paleont. Soc. Jpn. 31, 27e34, 32:19-27. Chinzei, K., 1986. Shell structure, growth, and functional morphology of an elongate Cretaceous oyster. Palaeontology 29 (1), 139e154. Chinzei, K., 1995. Adaptive significance of the lightweight shell structure in soft bottom oysters. N. Jahrb. Geol. Pal€ aontol. Abh 195, 217e227. CONOCO, Egyptian General Petroleum Corporation EGPC, 1988. Geological Map of Egypt, Scale 1: 500,000. NH 35 SW Siwa, 1 Sheet. nech, R., Farinati, E.A., Martinell, J., 2014. Crassostrea patagonica (d'Orbigny, Dome 1842) shell concentrations from the late Miocene of Río Negro province, NE Patagonia, Argentina. Span. J. Palaeontol. 29 (2), 165e182. Dunn, R.P., Eggleston, D.B., Lindquist, N., 2014. Oyster-sponge interactions and bioerosion of reef-building substrate materials: implications for oyster restoration. J. Shellfish Res. 33 (3), 727e738. El-Ayyat, A.M., Kassab, A.S., 2004. Biostratinomy and facies analysis of the upper Cretaceous oyster storm shell beds of the Duwi formation, Qusseir District, red sea region. Egypt. J. Afr. Earth Sci. 39, 421e428. El-Hedeny, M.M., 2005. Taphonomy and paleoecology of the middle Miocene oysobiol. 24 (2), 719e733. ters from Wadi Sudr, Gulf of Suez, Egypt. Rev. Pale El-Hedeny, M.M., 2007. Encrustation and bioerosion on Middle Miocene bivalve obiol. shells and echinoid skeletons: paleoenvironmental implications. Rev. Pale 26 (2), 381e389. El-Hedeny, M.M., El-Sabbagh, A.M., 2005. Eoradiolites liratus (Bivalvia, Radiolitidae) from the upper Cenomanian Galala formation at Saint Paul, eastern Desert (Egypt). Cret. Res. 26, 551e566. El-Hedeny, M.M., El-Sabbagh, A.M., 2007. Macro-borings on late Cretaceous oysters of Egypt. N. Jb. Geol. Pal€ aontol. Abh 244, 273e286. El-Qot, G., Abdel Gawad, G., Mekawy, M., 2009. Taphonomy of middle Jurassic (Bathonian) shell concentrations from Ras El Abd, west Gulf of Suez. Egypt. J. Afr. Earth. Sci. 54 (1e2), 31e36. El-Sabbagh, A.M., 2008. Biostratigraphy, taphonomy and palaeoecology of two tropical Coniacian-Santonian oyster species from Wadi Sudr, western Sinai, €ontol. Abh 249 (1), 47e74. Egypt. N. Jb. Geol. Pala El-Sabbagh, A.M., El-Hedeny, M.M., Rashwan, M., Abdel Aa,l, A., 2016. The bivalve Placuna (Indoplacuna) miocenica from the middle Miocene of Siwa Oasis, western Desert of Egypt: taxonomy, paleoecology, and taphonomic implications. J. Afr. Earth Sci. 116, 68e80. El-Sabbagh, A.M., Mansour, H., El-Hedeny, M.M., 2015. Taphonomy and paleoecology of Cenomanian oysters from the Musabaa Salama area, southwestern Sinai. Egypt. Geosci. J. 19 (4), 655e679. El-Sabbagh, A.M., Tantawy, A., Keller, G., Khozyem, H., Spangenberg, J., Adatte, Th, Gertsche, B., 2011. Stratigraphy of the Cenomanian-Turonian Oceanic Anoxic event OAE2 in shallow shelf sequences of NE Egypt. Cret. Res. 32, 705e722.
El-Shazly, S.H., Abdel-Hamid, M., 2001. Contribution to the middle Miocene megafossils from Siwa Oasis, western Desert, Egypt. Egypt J. Paleontol. 1, 263e297. Fan, K.Z., Chen, L., Cai, J., Liu, S.J., Li, Y.Y., Shen, Y.X., 2011. Preliminary study on oyster mushroom growth situation in 12 kinds of straw substrates. Chin. Agric. Sci. Bull. 27, 126e131. Fürsich, F.T., 1981. Salinity controlled benthic associations from the Upper Jurassic of Portugal. Lethaia 14, 203e223. Fürsich, F.T., 1995. Shell concentrations. Ecolog. Geol. Helv. 88, 643e655. Fürsich, F.T., Werner, W., 1986. Benthic associations and their environmental sig€ont. nificance in the Lusitanian basin (upper Jurassic, Portugal). N. Jb. Geol. Pala Abh 172, 271e329. Galtsoff, P.S., 1964. The American Oyster Crussostrea virginica Gmelin. Fishery Bulletin, vol. 64. United States Government Printing Office, Washington, D. C iii±480. Ganapathi Naik, M., Gowda, Gangadhara, 2013. Survey of oyster beds of Mulky estuary, south west coast of India. J. Acad. Ind. Res. 1 (10), 601e605. Gindy, A.R., 1970. The stratigraphic succession of the exposed coastal Miocene sediments north of Salum town, its diagenesis and correlation with that of the Siwa Depression, Western Desert of Egypt. Proc. Egypt Acad. Sci. 23, 9e28. Gindy, A.R., El-Askary, M.A., 1969. Stratigraphy, structure and origin of Siwa depression. West. Desert Egypt. Bull. Am. Ass. Pet. Geol. 53, 603e625. Hallam, A., Gould, S., 1975. The evolution of British and American middle and upper Jurassic Gryphaea; a biometric study. Proc. R. Soc. Lond. B 189, 511e542. Harzhauser, M., Djuricic, A., Mandic, O., Neubauer, T.A., Zuschin, M., Pfeifer, N., 2015. Age structure, carbonate production and shell loss rate in an Early Miocene reef of the giant oyster Crassostrea gryphoides. Biogeosci. Discuss. 12, 15867e15900. Hautmann, M., 2004. Early Mesozoic evolution of alivincular bivalve ligaments and its implications for the timing of the ‘Mesozoic marine revolution’. Lethaia 37 (2), 165e172. _ 2008. Presence of Crassostrea gryphoides (Schlotheim) from the lower € r, I., Hos¸go middle Miocene sequence of Kahramanmaras¸ basin (SE Turkey); its taxonomy, paleoecology and paleogeography. Bull. Min. Res. Explor. Inst. (MTA) Turk. 136, 17e28. Khalil, H.M., 2011. PlioceneePleistocene stratigraphy and macrofauna of the Farasan Islands, south east red sea, Saudi Arabia. Arab. J. Geosci. 5 (6), 1223e1245. Kidwell, S.M., Fürsich, F.T., Aigner, T., 1986. Conceptual framework for the analysis and classification of shell concentrations. Palaios 1, 228e238. Kirby, M.X., 2001. Differences in growth rate and environment between Tertiary and Quaternary Crassostrea oysters. Paleobiology 27, 84e103. Kirby, M.X., Soniat, T.M., Spero, H.J., 1998. Stable isotope sclerochronology of Pleistocene and Recent oyster shells (Crassostrea virginica). Palaios 13, 560e569. Kosenko, I.N., 2014. On late Jurassic and Early Cretaceous oysters (Bivalvia, Ostreidae) from northern Siberia. Paleontol. J. 48, 380e388. Kownacki, A., Galas, J., Duminicka, E., Mielewczyk, S., 2000. Invertebrate communities in permanent and temporary high mountain lakes (TatraMts). Ann. Limnol. 36, 181e188. Lawrence, D.R., 1988. Oysters as geoarchaeologic objects. Geoarchaeol 3, 267e274. Lopes, R.P., 2011. Ichnology of fossil oysters (Bivalvia, Ostreidae) from the southern Brazilian coast. Gaea 7, 94e103. Littlewood, D.T.J., Donovan, S.K., 1988. Variation of recent and fossil Crassostrea in Jamaica. Palaeontology 31, 1013e1028. Machalski, M., 1998. Oyster life positions and shell beds from the Upper Jurassic of Poland. Acta Palaeontol. Pol. 43 (4), 609e634. Malchus, N., 1998. Multiple parallel evolution and phylogenetic significance of shell chambers and chomata in the Ostreoidea (Bivalvia). In: Johnston, P.A., Haggart, J.W. (Eds.), Bivalves: an Eon of Evolution. University of Calgary Press, pp. 393e407. Malchus, N., 2004. Constraints in the ligament ontogeny and evolution of pteriomorphian Bivalvia. Palaeontology 47 (6), 1539e1574. € Muller, A.H., 1970. Zur funktionellen Morphologie, Taxiologie und Okologie von Pycnodonta (Ostreina, Lamellibranchiata). Mtsber. Dtsch. Akad. Wiss 11/12 (1), 902e923, 2, 936-950. Newell, N.D., Boyd, D.W., 1970. Oyster-like Permian Bivalvia. Bull. Am. Mus. Nat. His 143, 217e282. Pickerill, R.K., Brenchley, P.J., 1991. Benthic macrofossils as paleoenvironmental indicators in marine siliciclastic facies. Geosci. Can. 18 (3), 119e138. Pufahl, P.K., James, N.P., 2006. Monospecific Pliocene oyster buildups, Murray Basin, South Australia: brackish water end member of the reef spectrum. Palaeogeogr. Palaeoclimatol. Palaeoecol. 233, 11e33. €hl, H.-J., 1998. Hochauflo € sende palo € ko €logische und sedimentologische UntersuRo chungenim Posidonienschiefer (Lias epsilon) von SW-Deutschland. Tübinger Geowiss. Arb. 47, 1e170. Said, R., 1962. The Geology of Egypt. Elsevier, Amsterdam, p. 377. Said, R., 1990. The Geology of Egypt. Balkema Publishers, p. 734. Sanders, H.L., 1968. Marine benthic diversity: a comparative study. Am. Nat. 102, 243e282 (Chicago). Schultz, O., 2001. Catalogus Fossilium Austriae. Band 1. Bivalvia Neogenica (Nuculacea-unionacea). Osterreichische Akademie der Wissenschaften, Wien, p. 379. Seilacher, A., 1984. Constructional morphology of bivalves: evolutionary pathways in primary versus secondary soft-bottom dwellers. Palaeontology 27, 207e237. Seilacher, A., Gishlick, A.D., 2014. Morphodynamics. CRS Press, London, p. 551. Siddiqui, G., Ahmed, M., 2002. Oyster species of the sub tropical coast of Pakistan (northern Arabian Sea). Indian J. Mar. Sci. 31, 108e118. ~ es, M.G., Kowalewski, M., 1998. Shell beds as paleoecological puzzles: a case Simo study from the Upper Permian on the Paran a Basin, Brazil. Facies 38, 175e196.
A.M. El-Sabbagh, M.M. El Hedeny / Journal of African Earth Sciences 120 (2016) 1e11 Staff, G.M., Powell, E.N., 1988. The paleoecological significance of diversity: the effect of time averaging and differential preservation on macroinvertebrate species richness in death assemblages. Palaeogeogr. Palaeoclimatol. Palaeoecol. 63, 73e89. Stenzel, H.B., 1971. Oysters. In: Moore, R.C. (Ed.), Treatise on Invertebrate Paleontology, Part N, Volume 3, Mollusca 6, BivalviaGeol Soc Amer/University of Kansas Press, Boulder/Lawrence, pp. N953eN1224.
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Ziko, A., El-Sorogy, A.S., Zalat, A., Eweda, Sh, Saber, N., 2000. Middle Miocene bryozoa from Siwa Oasis, western Desert, Egypt. In: Youssef, E.A.A. (Ed.), Proc. 5th Intern. Conf., Geology of the Arab World, 3. Cairo Univ., pp. 1465e1496 Zuschin, M., Harzhauser, M., Mandic, O., 2015. Taphonomy and paleoecology of the lower Badenian (middle Miocene) molluscan assemblages at Grund (lower Austria). Geol. Carpathica 55 (2), 117e128.
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A shell concentration of the Middle Miocene Crassostrea gryphoides (Schlotheim, 1813) from Siwa Oasis, Western Desert, Egypt Ahmed M. El-Sabbagh a, Magdy M. El Hedeny a, b, * a b
Department of Geology, Faculty of Science, Alexandria University, Alexandria 21568, Egypt Deanship of Scientific Research, King Saud University, Riyadh, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 December 2015 Received in revised form 23 March 2016 Accepted 7 April 2016 Available online 20 April 2016
A concentration of heavy, thick-shelled, large-sized, and elongated population of the oyster Crassostrea gryphoides (Schlotheim, 1813) was recorded in shallow-marine deposits of the basal Oasis Member of the Middle Miocene Marmarica Formation exposed at Siwa Oasis, Egypt. The oyster assemblage is resedimented as a lens-shaped bank up to 80e100 cm thick and about 220 m long. Crassostrea gryphoides specimens are embedded in a yellowish green, soft marl matrix. This is the first documented occurrence of this lens at Siwa Oasis. The lensoid structure is bounded by a lower marl and an upper shale beds of about 2 m and 1.5 m thick, respectively. Assemblage within this lens is characterized by extreme variations of Crassostrea gryphoides, forming an almost monotypic assemblage. The shell packing was dense (shell percentages higher than 75%) at the base and the center of the lens, whereas it exhibits loose packing at the top and right and left sides of the lens (shell percentage less than 15%). Valves are poorly sorted and randomly orientated (both in surface and cross section views). Encrustation and bioerosion have observed on both sides of the left and right valves. The relatively limited varieties of encrusters together with moderate frequency of borings indicate moderate to high sedimentation rate. On the other hand, the low abundance of fragmented and abraded shells indicates good preservation and minimal transport. The studied lens concentration is interpreted as proximal tempestites assemblage. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Crassostrea gryphoides Shell concentration Middle Miocene Marmarica Formation Siwa Oasis Western Desert Egypt
1. Introduction Benthic macrofauna is known to be sensitive to environmental conditions such as temperature, salinity, substrate characteristics, energy, nutrients, etc. (e.g. Sanders, 1968; Fürsich, 1981; Staff and Powell, 1988; Pickerill and Brenchley, 1991; Kownacki et al., 2000; El-Hedeny, 2005). They exhibit signs of environmental stresses, such as high mortality, low diversity, shell abnormality, € hl, 1998). Among benthic dwarfism, etc. (e.g. Fürsich, 1981; Ro macrofauna, oysters represent an outstanding opportunistic example that tolerate a wide range of a highly dynamic and stressful environmental conditions (i.e. r-strategists; El-Sabbagh et al., 2011). Their concentrations are often mono-to pauci-specific and characterized by large-sized shells (e.g. Fürsich and Werner, 1986; El-Ayyat and Kassab, 2004). The response of their tolerance could also be observed in their morphological features,
* Corresponding author. Department of Geology, Faculty of Science, Alexandria University, Alexandria 21568, Egypt. E-mail address: [email protected] (M.M. El Hedeny). http://dx.doi.org/10.1016/j.jafrearsci.2016.04.007 1464-343X/© 2016 Elsevier Ltd. All rights reserved.
including general shape and outlines, ligamental area, beak, shellthickness, beak orientation, etc. (e.g. Newell and Boyd, 1970; Abdel Aal and El-Hedeny, 1998; El-Hedeny, 2005; Pufahl and James, 2006; El-Hedeny and El-Sabbagh, 2007). Shell concentrations are common phenomena of shelf and coastal areas (Kidwell et al., 1986; Fürsich, 1995; El-Ayyat and Kassab, 2004; Bressan and Palma, 2010; El-Sabbagh et al., 2016). Their formations are greatly affected by many biostratinomic processes. Accordingly, detailed biostratinomic analysis is essential for understanding of their formation (e.g. Brett, 2003; El-Qot et al., 2009; El-Sabbagh et al., 2016). In our recent study of the Middle Miocene benthic macroinveretbrates in Siwa Oasis, Western Desert, a lens concentration of the large shells of Crassostrea gryphoides (Schlotheim, 1813) is recorded for the first time in an outcrop, about 7 km north of the Siwa City (Fig. 1). Crassostrea gryphoides reefs were widespread and €r, 2008; common along the Miocene circum-Tethyan coasts (Hos¸go Harzhauser et al., 2015). Certainly, because of their abundance and higher fossilization potential of their shells, the present lens could provide significant paleoecological information as well as detailed
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Fig. 1. A Simplified geological map of the Siwa Oasis showing the section analyzed (modified after CONOCO and EGPC, 1988). The key map is modified after Abdel Fattah et al. (2013).
data on the paleoenvironment in which they were deposited (e.g. Brett, 2003; El-Sabbagh et al., 2015). Geographically, Crassostrea gryphoides (Schlotheim, 1813) is a long lived species. In the entire Western Tethys, it is appeared in the Oligocene and extended up to the Pliocene (Schultz, 2001). Other studies recorded it in higher stratigraphic levels (e.g. PliocenePleistocene, Khalil, 2011; Recent, Siddiqui and Ahmed, 2002; Ganapathi Naik and Gowda, 2013). The purposes of this paper are: 1) to register the occurrence of C. gryphoides in the Middle Miocene sediments of Siwa Oasis, 2) to describe and discuss the recorded C. gryphoides concentration, and 3) to interpret the postmortem alterations and the paleoenvironmental conditions prevailed during the formation of this concentration. 2. Location and stratigraphic context Siwa Oasis, a depression of about 10e17 m below sea level, is situated on the west of the Qattara Depression between latitudes 29 020 and 29 280 N and longitudes 25 120 and 26 020 E (Fig. 1). In the north, it is bounded by a Middle Miocene escarpment that rises about 100 m above the depression floor while it is bounded on the south by a low Middle Miocene scarp of about 20e25 m above the depression floor. The Marmarica Formation is widely exposed in this region. It was first introduced into the Egyptian lithostratigraphy by Said (1962) for a Middle Miocene sequence of about 78 m thick, at the northern scarp of Siwa Oasis. It is composed of alternating carbonate beds and greenish, bluish, and blackish shales and marls. The carbonates are made up of cross-bedded coquina and other organoclastic limestone. The rocks of the Middle Miocene
Marmarica Formation unconformably overlie the Lower Miocene Moghra Formation (Said, 1962). The studied section is located north of the Siwa City, at longitude 25 310 22.100 E and latitude 29 160 24.600 N (Fig. 1). At this area, the Marmarica Formation (about 78 m thick) is subdivided into three units. They are: a lower unit of marl, carbonate and shale, a middle unit of chalky and argillaceous limestone, and an upper unit of fossiliferous and non-fossiliferous chalky limestone. Based on the classification suggested by Gindy and El-Askary (1969), these units are named the Oasis, Siwa Escarpment and El Diffa Plateau members, respectively (Fig. 2). A remarkable feature of the studied rock unit is the predominance of carbonate rocks. They progressively decrease from top to bottom as shale and marl increase. Also, sandstones are completely absent. The Oasis Member attains a thickness of about 41 m. It is composed of fissile shale, marl, cross-bedded limestone, argillaceous limestone and coquina beds (Fig. 2). In places, shale and marl occurred as lenses that interfingering with limestones and coquinas. The cross-bedded carbonate rocks are tabular and mainly composed of mixed organic debris derived from several marine faunas (i.e. coquina). In case of well developed cross-bedding, the coquina composed of well-sorted fine to medium grains. In the other case, sorting and fabric of the cross-bedded rocks become complex, containing fragments of large fossils. In tops of some shale-marl beds, traces of horizontal bioturbation (mainly Thalassinoides isp.) and/or lag deposits of reworked shale/marl clasts are recorded. The Siwa Escarpment Member (about 21 m thick) consists mainly of thick beds of chalky limestones as well as thin beds of shale, marl and argillaceous limestone. Some bored hardground surfaces and lag deposits occur. Within this member, as
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Fig. 2. Lithological description of the studied section accompanied with a sketch showing the site of the Crassostrea gryphoides shell lens concentration (A) and a photograph of the outcrop, illustrating the studied lens (bed 25) underlain and overlain by marl and shale beds, respectively (B).
well as in the overlying El Diffa Plateau Member, cross-lamination is absent in the chalk beds and mostly look structureless. In addition, limestone and chalky limestone beds show oval to irregular solution scours and cavities, some of which are filled by breccia and fine-grained debris. The El Diffa Plateau Member attains a thickness of about 16 m and composed of relatively thick beds of chalky limestone cyclically rich and poor in fossils. Sediments of the studied area are characterized by a highly abundant invertebrate fauna, including mollusks, echinoids, bryozoans and few corals. In general, they represent the most characteristic index species in the Middle Miocene formations of Egypt. The rather fewer numbers of publications on the geology and stratigraphy of Siwa Oasis include those of Said (1962), Gindy and El-Askary (1969), Gindy (1970), Ziko et al. (2000); El-Shazly and Abdel-Hamid (2001), Abdel Fattah et al. (2013) and El-
Sabbagh et al. (2016). 3. Material The present study is based on 145 moderately to well preserved specimens of Crassostrea gryphoides (Schlotheim, 1813) collected from a lens within the upper part of the Middle Miocene Oasis Member (Fig. 2). This lens is located at the southern part of the sampled outcrop; at latitude 29 160 22.800 N and longitude 25 310 24.100 E. Crassostrea gryphoides were embedded in yellowish green, soft marl matrix. Whenever possible, way-up arrows were marked on shells at the time of collection. Their mode of fossilization ranges from exceptionally well preserved tests including microscopic surface characters to slightly abraded test material. The occurrence of well preserved oyster specimens allows an interesting
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paleoenvironmental, paleoecological and biostratinomic inferences. All specimens examined are housed in the Department of Geology, Faculty of Science, Alexandria University, Egypt. Specimen numbers are prefixed by NS25O.
4. Description of the lens concentration Crassostrea gryphoides is resedimented in a lens-shaped bank up to 80e100 cm thick and about 220 m of lateral extension. The upper contact of the lens with the overlying marl bed (about 2 m thick) is sharp and sometimes erosive. Similarly, the contact with the underlying shale (1.5 m thick) is sharp and erosional (Fig. 2). The oyster assemblage is almost monotypic, composed mainly of C. gryphoides. In addition, rare small pectinids and dwarfed bivalves
are recorded throughout the lens. The C. gryphoides populations are composed of different growth stages, ranging from juveniles to adults (Fig. 3). Shells are mostly disarticulated (99.3%) and randomly oriented, though in many places convex-up valves seem to predominate over oblique, vertical and concave-down ones (Fig. 3). Assemblage is poorly sorted and fragmentation affects few specimens of C. gryphoides and does not exceed 10%. The shell packing shows positive gradation within each micro~ es and Kowalewski, 1998), stratigraphic unit (m.u., sensu Simo usually consists of a relatively dense packing at the base and the center of the lens (shell percentages higher than 75%) and loose packing occurred at the top and right and left sides of the lens (shell percentages less than15%).
Fig. 3. A. The eastern end of the Crassostrea gryphoides lens. B. Randomly oriented C. gryphoides valves. Note the dense shell packing at the base and the center of the lens relative to the top part. C. Surface view of the lens with valves of C. gryphoides concordant to the bedding plane. D. Some free valves of C. gryphoides both upwards and downwards convexes. E. C. gryphoides bouquet. F. C. gryphoides with convex-up and vertically preserved valves.
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Fig. 4. Crassostrea gryphoides (Schlotheim, 1813) from the Middle Miocene of Siwa Oasis, Western Desert. Note the wide variations in shell dimensions (length, width and thickness), ligamental area and beak curvature. All specimens are left valves except for F and G that represent the right ones. Scale bar represents 5 cm.
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Fig. 5. Characteristic features of the ligamental area of Crassostrea gryphoides.
5. Variability of the specimens Recent and ancient oyster shells display wide morphological variation, which is mainly caused by their life habit and growing conditions (e.g. Chinzei, 1986; Abdel Aal and El-Hedeny, 1998; Littlewood and Donovan, 1988; Machalski, 1998; Seilacher and Gishlick, 2014). They can cement themselves to a hard substrate to form banks or implants their individuals into the soft sedimentary substrate to form bunch, bouquet or cluster. Under these conditions, individuals modify their shell shape when restricted space is available for growing (Stenzel, 1971). Our specimens have large, heavy and thick-shelled with left valves of up to 30 cm in length, 20 cm width and up to 5 cm of shell thickness (Fig. 4). Right valves are smaller and flat. The C. gryphoides shells are highly variable, especially in their shell outline, ligament and beak. Shell outline. In general, the shell outline of the studied C. gryphoides specimens is elongated, varies from sub-trigonal, subovoidal to vertically-extended sub-rectangular (Fig. 4). In the examined specimens, and although there is no characteristic relationship between the shape of the shell and hinge area, no doubt that both are belonging to the same species. The shape of oyster shells and their proportions are highly variable and, therefore, are, in some cases, of little use for the identification of species (Galtsoff, 1964). Depending on the wide variation in shape outline, large number of previously described thick-shelled oyster species may mis-identified as a separate species (e.g. Abdel Aal and El-Hedeny, 1998; Machalski, 1998; Kosenko, 2014). This variation is strongly affected by the mode of life rather than ontogenic causality. According to Seilacher (1984), the elongated shape can be regarded as indicative either of the mud-sticking habit or of competition for space between specimens growing close to each other. In the study area, the large, elongated shells of C. gryphoides refers to the nature of their muddy substrate which considered as a strategy to prevent themselves from a complete burial into the soft substrate and from being transported by currents to unfavorable habitats. Ligament. It plays an important role for growth and survival of oysters (Galtsoff, 1964). In Crassostreinae, the ligamental area is composed of an elevated resilifer of the right valve and bulge-like bourrelets of the left valve articulating with opposing depressions (Fig. 5). This type is called the alivincular-arcuate ligament (sensu
Hautmann, 2004). This particular type of ligamental area occurs only in oysters and is probably an autapomorphy of this group (Malchus, 2004). In the studied C. gryphoides specimens, the ligamental areas have great variations in their appearances (Figs. 4 and 6). It is mainly triangular, higher than long; resilifer is wider than anterior and posterior bourrelets. Length and height of ligamental area are considerably varied in our assemblage, ranging from very narrow to wide, invariable length throughout the whole area to wide ventrally and narrow dorsally (Fig. 6). The present specimens exhibit a characteristic structure of risilifer, consisting of concave bottoms and elevated growth breaks (Fig. 5). The periodic pattern of growth breaks in each annual increment indicates that breaks are not caused by extreme environmental conditions (Andrus and Crowe, 2000). Fan et al. (2011) suggested that this breaks referred to spring time when the temperature was favorable to food supply and spawning. Meanwhile, Muller (1970) regarded any disturbance in ligament structure as an index of the environmental conditions that leads to affect the functions of the organism and morphology of its valves. On the other hand, Kirby et al. (1998) observed growth increment formation during the summer in Crassostrea virginica and interpreted these to reflect growth cessation due to high water temperatures (see also Lawrence, 1988; Andrus and Crowe, 2000). Beak. The beak curvature is the third characteristic feature in the morphology of C. gryphoides assemblage. The present specimens display wide variations in beak curvature of the left valves (Figs. 4 and 6). Most of the specimens have a considerable umbonal angle relative to the plane of commissure. They usually curved and directed toward the posterior end of the left valves, although in some specimens they may point toward the posterior. Very narrow, straight, or slightly curved beaks of these kinds are usually formed in oysters which grow on soft, muddy bottoms (e.g. Hallam and Gould, 1975; El-Hedeny, 2005). The extreme development of this type can be seen in the narrow and slender oysters growing under overcrowded conditions on reefs. 6. Paleoenvironment, paleoecology and biostratinomy Across the northwestern part of the Western Desert of Egypt, the depositional evolution of the Middle Miocene Marmarica Formation involved two stages, which are controlled by mild tectonics and eustatic sea-level changes (Gindy and El-Askary, 1969; Said, 1990; Abdel Fattah et al., 2013). The first stage comprises the deposition of the carbonate, marl and shale facies of the basal Oasis Member under a relatively oscillated sea level, low rate of clastic influx and warm climate. The second stage is represented by carbonate-dominated facies of the Siwa Escarpment and El Diffa Plateau members under fluctuating sea level. In the studied succession, the remarkable decrease in clastics in the two younger members and the absence of cross-bedding suggest a relative deepening of the original marine conditions with more restriction in clastic supply and water turbidity after the deposition of the Oasis Member. In the Oasis Member of the Marmarica Formation, the shale, marl, limestones and the commonly occurrence of mixed and/or transported assemblages of bivalves, gastropods, echinoids, bryozoans and corals suggest accumulation in moderate to high-energy shallow marine environments (Abdel Fattah et al., 2013; ElSabbagh et al., 2016). The concentrated and abraded nature of some macrofaunal groups in this member may suggest transportation by wave/tidal currents and storm events. In addition, high-energy shallow marine conditions can be deduced by the presence of cross-bedded coquina and other mechanical carbonate beds in the Oasis Member. In the study area, and generally at Siwa
A.M. El-Sabbagh, M.M. El Hedeny / Journal of African Earth Sciences 120 (2016) 1e11
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Fig. 6. Sketch of the ligamental areas and beaks of Crassostrea gryphoides illustrating the wide variations of their characteristic features. For all drawings, scale bars represent 1 cm.
Fig. 7. Biostratinomic model for the formation of Crassostrea gryphoides lens concentration in the Middle Miocene Marmarica Formation of Siwa Oasis, Western Desert. A, C. gryphoides shells in life position; B, tilting of shells due to winnowing; C, C. gryphoides lens formed by storm-induced currents.
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Oasis, these directional structures are mostly bimodal (Gindy and El-Askary, 1969; Abdel Fattah et al., 2013). Direction of transport was predominantly west to northwest and less commonly was in the opposite direction (i.e. east to southeast). Many authors have discussed the paleoecology and biostratinomy of ancient oysters (e.g. El-Hedeny, 2005, 2007; Elnech Sabbagh, 2008; Ayoub-Hannaa and Fürsich, 2011; Dome et al., 2014; El-Sabbagh et al., 2015). Although they agreed that oysters live best in low intertidal or shallow subtidal, they found significant variations in their shell morphology (shape, thickness, size, ornamentation, etc.). Such variations are mainly referring to the environmental conditions (salinity, temperature, competition, etc.) prevailed during their life. As was mentioned, the studied C. gryphoides specimens are found as densely well preserved shells embedded in yellowish green soft marl. The fast growth and high density of C. gryphoides may reflect exceptionally high productivity and nutrient availability in the environment (Kirby, 2001). Owing to their elongated shells, their life habit is interpreted as a mud-sticker that strongly supported by the surrounding sediment in vertical position with the umbones directed downwards (Fig. 7a). This upright mode of life was probably reflecting growth in high-density populations (Machalski, 1998). According to several studies (e.g. Chinzei, 1982, 1986, 1995; Seilacher, 1984; Malchus, 1998; El-Hedeny, 2005), there are three strategies for the growth of Crassostrea populations; the first is based upon attachment of shells of living animals on former generations to form aggregates; the second is depend on the ability of the animal to form a thick, lightweight vesicular shell structure that prevents their shells to sink in soft substrate and the third mode is the ability of the animal to elongate their shell vertically to keep their soft parts alive above the soft substrate. Shells of the studied C. gryphoides are heavy, i.e. the strategy of formation of the lightweight vesicular or empty structure is not the case of the present assemblage. Instead, the present assemblage consists of shells that reflect the other two modes of life. The elongated shell forms are common in the studied assemblage. Meanwhile, we have some shells that have impressions of attachment with adjacent individuals (Fig. 4). The present concentration was produced by moderate to high energy event (Fig. 7). This can be confirmed by the densely packed, imbricated, and randomly oriented with few fragmented shells (Fig. 3). The majority of the left valves were positioned as convex up attitude, indicating that they removed apart from their positions and transported by currents. A few shells show a vertical position with the commissural plane more or less perpendicular to the bottom and the ligamental area pointing downwards. All these observations indicate that we are dealing with a little-transported, low-diversity autochthonous to parautochthonous lens concentration. This assemblage underwent transport, most likely by storm-induced currents (Fig. 7). Hence, our shell concentration is interpreted as proximal tempestites assemblage (e.g. El-Qot et al., 2009; Zuschin et al., 2015). Concerning the post-mortem signatures, our C. gryphoides specimens are mainly disarticulated (144 out of 145 specimens).
9
The proportions of the collected left and right valves are greatly unequal (76.4% vs. 23.6%, respectively). The general degree of fragmentation and abrasion is uniformly low, whereas encrustation and bioerosion are common and mainly occur on the exterior surfaces of left and right valves. However, some traces are recorded in the interior sides of both valves (Fig. 8). In the studied assemblage, encrustation abundance is relatively low (25.5%). The common encrusters include congeners of C. gryphoides, cheilostome bryozoans, few serpulid worms and rare cirripeds (genus Balanus Da Costa). In general, limited reciprocal overgrowths were observed between different encrusters (Fig. 8). The C. gryphoides specimens are moderately affected by macroborings (48.3%). Borings are mainly represented by Entobia Bronn, Gastrochaenolites Leymerie and Maeandropolydora Voigt (Fig. 8). Few occurrences of Caulostrepsis Clarke and Rogerella SaintSeine are also observed. Of these, the ichnogenus Entobia is more common in the studied shells than other bioerosion traces. Clionaid sponges, the producer of the ichnogenus Entobia, prefer to live in nearshore, shallow, relatively low-energy marine environments (Calcinai et al., 2005). Nevertheless, their occurrence does not confirm that the shells of C. gryphoides are subjected to a prolonged post-mortem process. Many paleontologists regarded incorrectly that the occurrence of Entobia in fossil shells is an indication of pro-longed post-mortem process. Majority of clionaid sponges were observed in recent oysters (e.g. Carver et al., 2010; Lopes, 2011; Dunn et al., 2014). This could provide a compelling example for Entobia traces that they have a significant contribution both pre- and post-mortem stages. In addition, El-Hedeny and ElSabbagh (2005) have regarded that activities of bioerosion in radiolitid Eoradiolites liratus (Conrad) may be interpreted to have extended from a pre-death to the pre-final burial. 7. Conclusions The topmost part of the basal Oasis Member of the Middle Miocene Marmarica Formation contains a shell concentration of the oyster Crassostrea gryphoides (Schlotheim, 1813). Individuals are represented by different growth stages, ranging from juveniles to adults. The oyster concentration is a lens-shaped bank up to 80e100 cm thick and about 220 m long where thick-shelled, largesized, elongated and well preserved individuals are embedded in a yellowish green, soft marl matrix. The studied specimens of C. gryphoides are represented by monotypic community in moderate-to high-energy mud-supported deposits, with fast growth rates, indicating a relatively moderate to high sedimentation rates. Most of the shells are oriented both convex-up and convex-down, although few specimens show a vertical position with the commissural plane more or less perpendicular to the bottom and the ligamental area pointing downwards. The morphological variability of the soft-bottom C. gryphoides is interpreted in terms of ecophenotypic response to environmental parameters. The relatively limited varieties of encrusters together with moderate frequency of borings are also indicating a moderate to high sedimentation rate. On the other hand, the lower degree of fragmentation, poor sorting and scarcity of abrasion indicate good
Fig. 8. Boring and encrustation on Crassostrea gryphoides shells. A. Encrustation of large left valve of Crassostrea gryphoides on the exterior surface of another large left valve, NS25O443. B. Posterior side of a large left valve of C. gryphoides encrusted by juvenile C. gryphoides valves, NS25O61. C. A relatively dense cluster of a relatively equal-sized, disarticulated, small oyster shells on the exterior surface of left valve of C. gryphoides, NS25O59. D. Encrustation of small C. gryphoides shell on the interior surface of the left valve of the same species. Traces of Entobia isp. are occurred, NS25O75. E. Cheilostome bryozoans and Balanus sp. encrust the ligamental area of left valve of C. gryphoides. Note the reciprocal overgrowth between these two encrusters, NS25O77. F. Serpulid worm (arrow) on the exterior surface of left valve of C. gryphoides, NS25O130. G. Entobia isp. randomly distributed over the exterior surface of left valve of C. gryphoides. Note the traces of Maeandropolydora isp. in the central part, NS25O79. H. C. gryphoides left valve with Entobia isp. and Gastrochaenolites isp. traces, NS25O168. I. Gastrochaenolites isp. in the exterior surface of right valve of C. gryphoides, NS25O137. J. Maeandropolydora isp. in the interior surface of left valve of C. gryphoides, NS25O189. Scale bar for specimens ¼ 2 cm.
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preservation and minimal transport. Consequently, the lens concentration is interpreted as proximal tempestites assemblage. Acknowledgments The authors would like to express their sincere appreciation to the Deanship of Scientific Research at King Saud University for funding this work through the Research Group Project No. RG‒ 1436‒022. Thanks to the editor (Damien Delvaux) and the two reviewers, Maria Helena Henriques and one anonymous reviewer, whose reviews and insights are helpful and aided in improving this manuscript. We also thank Mr. M. Rashwan for his careful photographic work. References Abdel Aal, A.A., El-Hedeny, M.M., 1998. On the variability of the Campanian Pycnodonte (Phygraea) vesiculare (Lamarck). N. Jb. Geol. Pal€ aontol. Mh. 1, 42e54. Abdel Fattah, Z.A., Kora, M.A., Ayyad, S.N., 2013. Facies architecture and depositional development of middle Miocene carbonate strata at Siwa Oasis, northwestern Egypt. Facies 59 (3), 505e528. Andrus, C.F.T., Crowe, D.E., 2000. Geochemical analysis of Crassostrea virginica as a method to determine season of capture. J. Archaeol. Sci. 27, 33e42. Ayoub-Hannaa, W., Fürsich, F.T., 2011. Functional morphology and taphonomy of Cenomanian oysters from eastern Sinai, Egypt. Palaeobiodivers. Palaeoenviron 91, 197e214. Bressan, G.S., Palma, R.M., 2010. Taphonomic analysis of fossil concentrations from n basin, Mendoza province, Argentina. La Manga formation (Oxfordian), Neuque J. Iber. Geol. 36 (1), 55e71. Brett, C.E., 2003. Taphonomy: sedimentological implications of fossil preservation. In: Middleton, G.V. (Ed.), Encyclopedia of Sediments and Sedimentary Rocks. Springer, Dordrecht, pp. 723e729. Calcinai, B., Bavestrello, G., Cerrano, C., 2005. Excavating sponge species from the Indo-Pacific ocean. Zool. Stud. 44, 5e18. riault, I., Malley, A.L., 2010. Infection of cultured eastern oysters Carver, C.E., The Crassostrea virginica by the boring sponge Cliona celata, with emphasis on sponge life history and mitigation strategies. J. Shellfish. Res. 29, 905e915. Chinzei, K., 1982. Paleoecology of oysters 1, 2. Kaseki (fossils) Paleont. Soc. Jpn. 31, 27e34, 32:19-27. Chinzei, K., 1986. Shell structure, growth, and functional morphology of an elongate Cretaceous oyster. Palaeontology 29 (1), 139e154. Chinzei, K., 1995. Adaptive significance of the lightweight shell structure in soft bottom oysters. N. Jahrb. Geol. Pal€ aontol. Abh 195, 217e227. CONOCO, Egyptian General Petroleum Corporation EGPC, 1988. Geological Map of Egypt, Scale 1: 500,000. NH 35 SW Siwa, 1 Sheet. nech, R., Farinati, E.A., Martinell, J., 2014. Crassostrea patagonica (d'Orbigny, Dome 1842) shell concentrations from the late Miocene of Río Negro province, NE Patagonia, Argentina. Span. J. Palaeontol. 29 (2), 165e182. Dunn, R.P., Eggleston, D.B., Lindquist, N., 2014. Oyster-sponge interactions and bioerosion of reef-building substrate materials: implications for oyster restoration. J. Shellfish Res. 33 (3), 727e738. El-Ayyat, A.M., Kassab, A.S., 2004. Biostratinomy and facies analysis of the upper Cretaceous oyster storm shell beds of the Duwi formation, Qusseir District, red sea region. Egypt. J. Afr. Earth Sci. 39, 421e428. El-Hedeny, M.M., 2005. Taphonomy and paleoecology of the middle Miocene oysobiol. 24 (2), 719e733. ters from Wadi Sudr, Gulf of Suez, Egypt. Rev. Pale El-Hedeny, M.M., 2007. Encrustation and bioerosion on Middle Miocene bivalve obiol. shells and echinoid skeletons: paleoenvironmental implications. Rev. Pale 26 (2), 381e389. El-Hedeny, M.M., El-Sabbagh, A.M., 2005. Eoradiolites liratus (Bivalvia, Radiolitidae) from the upper Cenomanian Galala formation at Saint Paul, eastern Desert (Egypt). Cret. Res. 26, 551e566. El-Hedeny, M.M., El-Sabbagh, A.M., 2007. Macro-borings on late Cretaceous oysters of Egypt. N. Jb. Geol. Pal€ aontol. Abh 244, 273e286. El-Qot, G., Abdel Gawad, G., Mekawy, M., 2009. Taphonomy of middle Jurassic (Bathonian) shell concentrations from Ras El Abd, west Gulf of Suez. Egypt. J. Afr. Earth. Sci. 54 (1e2), 31e36. El-Sabbagh, A.M., 2008. Biostratigraphy, taphonomy and palaeoecology of two tropical Coniacian-Santonian oyster species from Wadi Sudr, western Sinai, €ontol. Abh 249 (1), 47e74. Egypt. N. Jb. Geol. Pala El-Sabbagh, A.M., El-Hedeny, M.M., Rashwan, M., Abdel Aa,l, A., 2016. The bivalve Placuna (Indoplacuna) miocenica from the middle Miocene of Siwa Oasis, western Desert of Egypt: taxonomy, paleoecology, and taphonomic implications. J. Afr. Earth Sci. 116, 68e80. El-Sabbagh, A.M., Mansour, H., El-Hedeny, M.M., 2015. Taphonomy and paleoecology of Cenomanian oysters from the Musabaa Salama area, southwestern Sinai. Egypt. Geosci. J. 19 (4), 655e679. El-Sabbagh, A.M., Tantawy, A., Keller, G., Khozyem, H., Spangenberg, J., Adatte, Th, Gertsche, B., 2011. Stratigraphy of the Cenomanian-Turonian Oceanic Anoxic event OAE2 in shallow shelf sequences of NE Egypt. Cret. Res. 32, 705e722.
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