Pectinid Coquinas and Their Palaeoenvironmental Implications Ð Examples from the Early Miocene of Northeastern Egypt

Palaeogeography, Palaeoclimatology, Palaeoecology 172 (2001) 171±191 www.elsevier.com/locate/palaeo

Pectinid coquinas and their palaeoenvironmental implications Ð examples from the early Miocene of northeastern Egypt

Oleg Mandica, Werner E. Pillerb,*

aInstitute for Palaeontology, University of Vienna, Geozentrum, Althanstrasse 14, A-1090 Vienna, Austria bInstitute for Geology and Palaeontology, Karl-Franzens University of Graz, Heinrichstrasse 26, A-8010 Graz, Austria Received 16 February 2000; accepted for publication 26 March 2001

Abstract The Early Miocene (Burdigalian) section at Gebel Gharra (SW Suez, Egypt) represents a mixed carbonate/siliciclastic regime and is composed of a series of ®ning- and deepening-upward sequences. Mass occurrences of scutellid echinoids and pectinid bivalves are remarkable features within this section. Five pectinid horizons, occurring in two superpositioned ®ning-upward sequences, are qualitatively and quantitatively studied in detail. The taxonomic composition of respective pectinid accumulations clearly support a deepening-upward interpretation with decreasing pectinid diversity, decreasing convexity, shell thickness, and sculpturing. The taphonomic parameters of the pectinid beds, in combination with the study of co-occurring biota and frequency and density of shell encrustations and bioerosion, clearly point to different mechanisms for shell accumulations, namely as `composite concentrations' and `reworked event-concentrations'. q 2001 Elsevier Science B.V. All rights reserved.

Keywords: Pectinidae; Coquinas; Taphonomy; Deepening-upward sequences; Burdigalian; Egypt

1. Introduction 1991) and the entire group is characterised by a distinct radiation during the Neogene (Demarcq, Considering the fossil record, pectinids are one of 1990). the most important bivalve groups due to their predo- Early Miocene shallow marine sections in the East- minantly calcitic shell mineralogy (e.g. Taylor et al., ern desert of Egypt (NW Suez) were recently studied 1969; Carter et al., 1998). These calcitic shells have a lithologically, micropalaeontologically and biostrati- high preservation potential and even in shallow water graphically by Abdelghany and Piller (1999). Within carbonate sediments where aragonitic mollusc shells this study, rich fossil occurrences particularly of echi- are usually dissolved, they are well-preserved. In noids and bivalves were discovered. On the base of addition to this high preservation potential, pectinids this information the best section Ð at Gebel Gharra occur frequently and are widely distributed in shallow (Fig. 1) Ð was selected and a detailed sedimento- marine environments. Moreover, distinct morphologi- logical and palaeontological study was conducted in cal groups developed different life strategies (Waller, order to provide a serious base for environmental reconstruction. The most striking fossils from a taxo- nomical, taphonomical, and palaeoecological per- * Corresponding author. Tel.: 143-316-380-5582; fax: 143-316- 380-9871. spective are pectinid bivalves and echinoids both E-mail address: [email protected] (W.E. Piller). occurring not only scattered in the sediment but also

0031-0182/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0031-0182(01)00268-1 172 O. Mandic, W.E. Piller / Palaeogeography, Palaeoclimatology, Palaeoecology 172 02001) 171±191

Fig. 1. Location and geological setting of the studied section. as mass occurrences in several coquinas (Piller et al., information on the depositional history of the 1998). Since the echinoids are currently under study sequence. (compare Nebelsick and Kroh, 1998) we focus in this paper on the pectinid bivalves. The relatively high abundance of pectinids in the 2. General setting studied section provides a good base for palaeoecological investigations of these bivalves. The occurrence Neogene rocks are well exposed in the Cairo±Suez of coquinas offers the opportunities to reconstruct the district and form the low hills of that area (Abdel- environmental conditions which led to the origin of ghany and Piller, 1999; Sadek, 1959). Sedimentation these Fossillagerstatten. around Suez is bound to two shallow tectonic basins The occurrence of a rich and diverse pectinid fauna (Gineifa-Gharra-basin in the north; Hagul-Sukhna- in the Suez district was already mentioned by Fraas basin in the south), which are ®lled with shallow (1867) and Fuchs (1877). A ®rst taxonomic descrip- marine, mixed carbonate-siliciclastic sediments. The tion of pectinids was carried out by Fuchs (1883), Neogene sequences overlay either shallow marine followed by Blanckenhorn (1901, 1903), and partially Eocene carbonates or non-marine Oligocene silici- by Deperet and Roman (1902±1928), and Roger clastics (Hermina et al., 1989). The Oligocene age is (1939). Additional taxonomic studies were performed not well dated but inferred from the regional geologi- by Said and Yallouse (1956) and Cherif (1974). More cal situation (Said, 1990). Due to rapid lateral facies recently, an attempt for biostratigraphic use of this changes lithostratigraphic classi®cation is widely rich fauna was made by Hamza (1992). unclear and controversial (Abdelghany and Piller, Within the studied section we want to: (1) docu- 1999). The Gebel Gharra is located approximately ment the diversity of pectinids; (2) detect pectinid 30 km WNW of Suez and the measured section is communities in 'normal' sediments; (3) study the on its east-facing side (base of measured section: pectinid coquinas qualitatively and quantitatively; 30810.60N/32818.40E; Fig. 1). Its study earlier in detail (4) reconstruct the environmental conditions which by Souaya (1961, 1963) and by Cherif (1974) gave rise to the coquinas on the base of sedimento- remained, however, biostratigraphically unclear. The logical and palaeontological data, and (5) add general recent dating by Abdelghany and Piller (1999) using O. Mandic, W.E. Piller / Palaeogeography, Palaeoclimatology, Palaeoecology 172 02001) 171±191 173 planktic foraminifera, calcareous nannoplankton and and sorting by shape were studied in quadrants normal the larger foraminiferal genus Miogypsina clearly to bedding plane of de®ned areas within the pectinid allowed a dating into the Burdigalian (M2±M4 after coquinas (Tables 1±5 and 7). Where possible, several Berggren et al., 1995; NN4 after Martini, 1971; upper quadrants were studied within one coquina to get an part of SB 25 after Cahuzac and Poignant, 1997). estimate of variance (Tables 1±5 and 7). In coquinas Since Miogypsina is found up to the top of the section, with preferred horizontal orientation of valves convex a Middle Miocene age can de®nitely be excluded. side up/down ratios of valves were also obtained.

3. Methods 4. Results

The palaeoenvironmental analysis has been carried 4.1. General description of the section out considering sedimentological and microfacial data, as well as taxonomic and taphonomic parameters The section at Gebel Gharra is approximately and functional morphology. 140 m in thickness (Fig. 2). The footwall is made up Taxonomy enables the de®nition of assemblages, of red siliciclastic sediments with abundant chert quantitative data give information on diversity allow- pebbles, which are interpreted as non-marine Oligo- ing calculation of several diversity indexes (e.g. cene deposits (e.g. Said, 1990). A direct contact Legendre and Legendre, 1998) (Plates 1 and 2). between these sediments and those under study in Since different indexes focus on different parameters the measured section was not found. The base of the we applied 2 indexes (Tables 1±5): Simpson index section consists of an alternation of cross-bedded ranges from 0 to 1, whereby higher values character- sands and silts, partly bioturbated and with rare ise higher diversities. Rare taxa have a low in¯uence plant remains, which are interpreted to represent a on its value. Therefore, it re¯ects mainly the distribu- ¯uvial±¯uviomarine depositional system. tion of the frequent species in a given sample. The Unequivocal marine sedimentation starts above an Shannon index is similar to the Simpson index, erosional and strongly bioturbated (Ophiomorpha) however, its value is strongly in¯uenced also by the level with (calcareous) sandstones with mud pebbles rare taxa of a sample. It characterises the diversity (beds 3,4) and three coquinas of densely packed irre- through number of occurring taxa and, additionally, gular echinoids (Parascutella) (Nebelsick and Kroh, by relative dominance of these taxa. This information 1998). Above the coquinas follows a poorly exposed is the base for the palaeoecological reconstruction sequence of bioturbated sands and marls (c. 15 m; using an actuopalaeontological approach. beds 5, 6) exhibiting a general ®ning-upward trend. Aspects of functional morphology were covered by The next package (beds 7±9) again starts with a analysing the following parameters: average shell bioturbated base, which is overlain by Planostegina size, convexity, shell thickness, and degree of orna- rudstones with mud pebbles. Above follow several mentation. For all parameters a rank scale was applied bioclastic sandstone beds with three distinct pectinid ranging from 1 to 3 (Table 6). Shell size was calcu- coquinas (for details see Section 4.2), which grade lated as length 1 height/2. A resulting value between into a more than 5 m thick unfossiliferous marl. 0 and 40 mm equals 1 on the rank scale, 41±60 mm This is overlain by another sediment cycle (beds equals 2, and .60 mm equals 3. For shell convexity a 10±12) with coarse-grained, fossiliferous, glauconitic differentiation into low (1), moderate (2) and high (3) sandstones with mud pebbles at the base, a conglom- was used. An exact calculation of the degree of in¯a- erate with frequent bryoids, and two pectinid coquinas tion as applied by Aguirre et al. (1996) was not (see Section 4.2). These sandstones grade into marls performed since most of the shells are disarticulated with intercalated silts and ®ne sands. (Table 7). A reddish, fossiliferous, coarse sand (30 cm; bed Taphonomic parameters were obtained by charac- 13) with bioturbations into the underlying marl is terising sediment matrix and fabric. Shell density, overlain by a coarsening-upward sequence starting articulation, orientation, left/right ratio of valves, with 13 m of grey marls, siltstones and ®ne sands 174 O. Mandic, W.E. Piller / Palaeogeography, Palaeoclimatology, Palaeoecology 172 02001) 171±191 Total Left Right Indet Total Left Right Indet 40 cm; Simpson: 0.64, Shannon: 1.03 30 cm; Simpson: 0.68, Shannon: 1.34 30 cm; Simpson: 0.69, Shannon: 1.18 000 19 80 11 500 2 13 000 52 19 0 1 1 17 1 1 11 11 19 0 1 20 0 11 5 5 80 10 0 0 28.80 1 2 0 2 0 16.7 16 12.1 19 0 1 12 4 16 1 1 0 13 3 1 11 5 0 0 22.6 0 0 2 5 0 0 7 1.19 1 0 0 13.1 1.19 100 4 16 2.380 100 0 100 0 13 8.333 26.3 12 0 13 1 25.0 0.0 0 68.4 0 15.4 100 0 11.5 28.6 62.5 0.0 5.3 100 12.5 1.0 89.5 100 100 12.5 100 100 71.4 0 0.0 5.26 45.5 0.0 0 100 0.0 0.0 0 5.26 0 18.2 0.0 100 100 0.0 100 0.0 36.4 100 100 100 100 71.4 25.0 0 0.0 0.0 0.0 41.7 100 0.0 0.0 33.3 100 0.0 Articulated Disarticulated Specimens Percentage of taxa Articulated Disarticulated 000002 54 1 38 1 31 0 14 3 0 10 29 1 0 12 0 2 0 3 0 54 0 9 1 1 0 31 29 1 21.3 31 3 0.39 12.2 0.39 12.2 1.181 0 0 0 0 100 0 6.45 100 70.4 100 100 93.5 100 25.9 0 32.3 0 0 0 38.7 3.7 100 0 0 29.0 100 0 100 93.5 0 £ £ £ ; Simpson: 0.67; Shannon: 1.23 Pecten fraasi M. ziziniae Amussiopecten Pectinidae indet.Total 0Quadrant 2; area: 65 Pecten fraasi P. cristatocostatus Ae. submalvinae M. 0 ziziniae 28F. schweinfurthi Amussiopecten Pectinidae 3 indet.Total 66 0 4Quadrant 3; area: 65 Pecten fraasi 10M. ziziniae 21F. schweinfurthi 22Amussiopecten 2 43Pectinidae 28 indet.Total 34 4 0Total 66 82 42.4 4 26 0 62 35 100 10 43 2 0 46 104 3 84 21 51.2 0 100 57 9 100 62 10.7 100 74 0 14.3 104 15.2 59.6 75.0 2.38 33.3 100 100 51.5 97.6 9.31 0 9.31 0 100 81.4 11.9 3.23 100 54.8 4.8 20.2 91.9 8.7 71.2 Table 1 Quantitative composition of coquina 7b Taxon NumbersQuadrant 1; area: 50 Percentages Pecten fraasi P. cristatocostatus Ae. submalvinae M. ziziniae F. schweinfurthi Amussiopecten Pectinidae indet.Total 0 2 133 9 11 252 57 113 41 133 154 254 52.4 100 0 0.79 100 6.77 99.2 22.4 8.27 85.0 16.1 60.6

O. Mandic, W.E. Piller / Palaeogeography, Palaeoclimatology, Palaeoecology 172 02001) 171±191 175

ecnaeo taxa of Percentage

Specimens

retto indet. Orientation

ovsd down Conv.side

ov ieup side Conv.

av indet Valve

Right

Left

Total

Articulated

retto indet. Orientation

ovsd down Conv.side

ovsd up Conv.side

Perpendicular

ovsd down Conv.side

ovsd up Conv.side

Perpendicular

ov iedown side Conv. ov ieup side Conv.

Left Right Valve indet.

oiinindet. Position ieposition Life 7.5 cm; Simpson: 0.77, Shannon: 1.37

£ o-lf position life Non- 420510211 810619649153125 00000000014300170017143017 0000171340611017 010110140 000172502508 0003100310 000000000 100010011001 0.00.0 0.0 12.50.0 0.0 0.00.0 0.0 0.0 12.5 0.0 0.0 12.5 0.0 0.0 17.6 0.0 0.0 58.8 0.0 12.5 0.0 0.0 50.0 0.0 17.6 0.0 0.0 0.0 5.88 0.0 0.0 0.0 0.0 82.4 0.0 0.0 17.6 0.0 0.0 0.0 0.0 0.0 12.5 100 0.0 0.0 87.5 100 0.0 25.0 0.0 62.5 100 0.0 0.0 76.5 0.0 0.0 0.0 23.5 100 100 25.0 62.5 0.0 0.0 82.4 0.0 17.6 35.3 64.7 100 0.0 0.0 100 0.0 100 4.1 100 100 8.7 0.0 8.7 0.0 100 0.5 Numbers´ Field estimations Totals 16.0 8.0 0.0 20.0 4.0 0.0 8.0 4.0 4.0 32.0 4.0 0.0 24.0 76.0 24.0 16.0 36.0 60.0 12.0 4.0 100 12.8 Table 2 Quantitative composition of coquina 8c Taxon Articulated DisarticulatedQuadrant 4; area: 200 Disarticulated F. schweinfurthi Amussiopecten O. josslingi Pecten fraasi Ae. submalvinae Pectinidae indet.Total 0Percentages 0 0F. schweinfurthi 4 11O. josslingi 3Pecten fraasi 7Ae. 0 submalvinae 2Pectinidae indet.Total 0.0 20 8 0.0 19 0.0 2 4 8.7 2.05 0 14 5.5 1.54 0.0 1.6 10 28 10.3 6.3 1 33 9.74 1.0 3.1 34 0.0 51 7.18 37 0 5.13 22.0 0.51 26.0 34 127 26.8 26.2 19.0 20 0.0 17.4 7 100 12 3.59 188 15.7 96.4 95 21.0 41 9.4 12.8 47 25 74.8 62.6 37.0 44 34.6 122 43.6 33.8 28.3 85 36 19.0 100 100 66 127 65.1 100 37 195 Amussiopecten

176 O. Mandic, W.E. Piller / Palaeogeography, Palaeoclimatology, Palaeoecology 172 02001) 171±191

ecnaeo taxa of Percentage

Specimens

retto indet. Orientation

ovsd down Conv.side

ov ieup side Conv.

av indet Valve

Right

Left

Total

Articulated

retto indet. Orientation

ovsd down Conv.side

ovsd up Conv.side

Perpendicular

ovsd down Conv.side

ovsd up Conv.side

Perpendicular

ov iedown side Conv. ov ieup side Conv.

Left Right Valve indet.

oiinindet. Position ieposition Life 20 cm; Simpson: 0.1, Shannon: 0.24 7 cm; Simpson: 0.01, Shannon: 0.04

£ £ o-lf position life Non- Numbers Field estimations120515231715125254532341063395939249237 0 0 0000 000000 0 100010011001 00000 000000 0 110020021102 00 45 110 1045 10 1000 Totals 0.42 0.840.0 0.00.0 0.00.0 21.5 0.0 0.0 21.9 0.0 0.0 1.27 0.0 5591111543716101362791165 0.0 7.17 0.0 0.0 6.33 0.0 0.0 0.42 0.0 0.0 0.0 10.5 0.062324130790 10.5 0.0 0.0000 0.0 19.0 000000 001010010011 0.0 0.0 1.27 0.0 98.7 0.0 0.0 40.0 44.7 0.0 50.0 100 13.93.64 10.0 40.1 50.0 0.0 50.0 1.212 0.00.0 39.2 1.82 38.8 100 0.0 14.5 0.0 0.0 20.7 0.0 100 0.0 7.88 0.0 0.0 100 0.0 100 0.0 94.8 0.0 100 4.24 0.0 0.0 0.0 5.45 0.0 40.0 0.0 0.0 0.0 50.0 100 100 100 10.0 0.0 3.0 100 50.0 50.0 3.0 0.0 0.0 4.0 0.0 55.2 0.0 0.0 100 6.67 100 93.3 0.0 22.4 0.8 0.4 0.0 9.67 100 61.2 21.8 0.0 16.4 100 55.2 100 0.0 0.0 99.4 100 0.0 0.0 100 100 0.602 Table 3 Quantitative composition of coquina 8d, differentiated in base and top Taxon Articulated DisarticulatedBase Quadrant 1205152317151313146324710633108999850250 5; area: 170 F. schweinfurthi Ae. Submalvinae C. multistriata Amussiopecten Total Percentages F. schweinfurthi Ae. submalvinae C. 5592111553716102362792166 multistriata Amussiopecten Total Disarticulated Top 62324130790 Quadrant 6; area: 135 0.4F. schweinfurthi 0.8Ae. submalvinae 0.0Total 20.4 20.8Percentages 1.2F. schweinfurthi 6.8Ae. submalvinae Total 6.0 0.4 12.4 12.4 3.61 18.4 1.2 1.2 1.81 98.8 14.5 42.4 13.2 7.83 0.0 43.2 39.6 4.216 39.2 5.42 0.0 20.0 100 100 3.0 3.0 55.4 6.63 93.4 22.3 9.64 61.4 21.7 16.3 55.4 100 100 O. Mandic, W.E. Piller / Palaeogeography, Palaeoclimatology, Palaeoecology 172 02001) 171±191 177

Table 4 Quantitative composition of coquina 10a

Taxon Numbers Percentages

Articulated Disarticulated Disarticulated

Total Left Right Indet Specimens Percentage of Articulated Total Left Right Indet taxa

Quadrant 7; area: 80 £ 40 cm; Simpson: 0.64, Shannon: 1.23 O. josslingi 2 20 4 16 0 22 15.6 9.09 90.9 18.2 72.7 0.0 O. revolutus 0 7 2 5 0 7 5.0 0 100 28.6 71.4 0.0 Ae. submalvinae 0 1 0 0 1 1 0.7 0 100 0.0 0.0 100 M. ziziniae 0 2 2 0 0 2 1.4 0 100 100 0.0 0.0 F. schweinfurthi 0 36 9 4 23 36 25.5 0 100 25.0 11.1 63.9 Amussiopecten 0 3 0 0 3 3 2.1 0 100 0.0 0.0 100 Pectinidae indet. 0 70 9 4 57 70 49.6 0 100 12.9 5.7 81.4 Total 2 139 26 29 84 141 100 1.42 98.6 18.4 20.6 59.6

(bed 14) which grades into a 3 m thick alternation of throughout. Other fossils are pectinids, Pinna, Isogno- internally graded sand(stone)s and siltstones with mon, echinoids, and larger foraminifera (Miogypsina, frequent bioturbation (Ophiomorpha). Over an Planostegina, ?Heterostegina). The uppermost bed of erosional base it is followed by a conglomerate, this sequence (bed 30) is a bioclastic quartzose which grades into red sandstone with bivalves (pecti- rudstone. nids, venerids, cardiids), echinoids (Parascutella), Beds 31±36 represent a siliciclastic sequence (c. balanids, and coralline algae (bed 15; 4 m). 6.2 m) with coarse sandstones, an oyster bed, a pecti- A carbonate sequence of about 39 m follows (beds nid layer (Amussiopecten sp.), rudstones with bivalves 16±30). It is predominantly made up of coralline algal and echinoids (scutellids, Echinolampas, Clypeaster) and coral limestones with bivalve coquinas (rudstones and a ®ning-upward calcareous sandstone, which and ¯oatstones); remarkable are two distinct Hali- terminates in a silt- to claystone with planktic for- meda±Acropora beds (beds 17 and 19). Besides aminifera (Abdelghany and Piller, 1999) (bed 36; 1 m). Acropora, corals are represented by Porites, Dendro- The uppermost part of the section is a carbonate phyllia, and Tarbellastrea. Oysters are abundant sequence (beds 37±42) with a conglomerate at the

Table 5 Quantitative composition of coquina 12a

Taxon Numbers Percentages

Articulated Disarticulated Specimens Percentage of taxa Articulated Disarticulated

Total Left Right Indet Total Left Right Indet

Quadrant 8; area: 170 £ 40 cm; Simpson: 0.70, Shannon: 1.35 O. josslingi 0 25 7 18 0 25 23.4 0 100 28.0 72.0 0.0 O. revolutus 0 8 1 7 0 8 7.5 0 100 12.5 87.5 0.0 Pecten kochi 1 32 17 13 2 33 30.8 3.03 97 51.5 39.4 6 F. schweinfurthi 1 7 2 0 5 8 7.5 12.5 87.5 25 0.0 62.5 Amussiopecten 1 3 0 0 3 4 3.7 25 75 0.0 0.0 75.0 Pectinidae indet. 0 29 3 1 25 29 27.1 0 100 10.3 3.4 86 Total 3 104 30 39 35 107 100 2.8 97.2 28.0 36.4 32.7 178 O. Mandic, W.E. Piller / Palaeogeography, Palaeoclimatology, Palaeoecology 172 02001) 171±191

Table 6 Estimated pectinid abundance within coquinas and shell morphology (thickness, sculpture, size, convexity) on a rank scale 1±3

Taxa Abundance in coquinas (%) Shell morphology (1: weak; 2: moderate; 3: strong)

7b 8c 8db 8dt 10a 12a Thickness Sculpture Size Convexity

Crassadoma multistriata (Poli, 0.0 0.0 0.4 0.0 0.0 0.0 1 2 1 1 1795) Aequipecten submalvinae 0.8 1.5 4.0 0.6 1.4 0.0 2 2 1 2 (Blanckenhorn, 1901) Amussiopecten sp. 25.6 25.0 0.8 0.0 4.2 5.1 2 1 3 1 Flabellipecten schweinfurthi 2.5 36.8 94.8 99.4 50.7 10.3 2 2 3 2 (Blanckenhorn, 1901) Pecten kochi Locard, 1877 0.0 0.0 0.0 0.0 0.0 42.3 2 3 2 3 Pecten fraasi Fuchs, 1883 44.6 25.0 0.0 0.0 0.0 0.0 2 3 2 3 Pecten cristatocostatus Sacco, 0.8 0.0 0.0 0.0 0.0 0.0 2 3 2 1 1897 Oppenheimopecten revolutus 0.0 0.0 0.0 0.0 9.9 10.3 1 1 1 3 (Michelotti, 1847) Oppenheimopecten josslingi 0.0 11.8 0.0 0.0 31.0 32.1 1 2 1 3 (G.B. Sowerby in Smith, 1847) Macrochlamis ziziniae 25.6 0.0 0.0 0.0 2.8 0.0 3 3 3 2 (Blanckenhorn, 1901) base, a coquina of pectinids with coral fragments, a 4.2.1. Lower sequence ®nely laminated, weakly cross-bedded, calcareous The lower sequence starts with a calcareous sandstone with mud clasts and glauconite and abun- Planostegina sandstone (rudstone) (30±50 cm) with dant fossils (heterosteginids, corals, pectinids, a sharp but irregular base of 15 cm relief (bed 7a). echinids). Bed 41 starts with an erosional and channel- At the very base angular, ill-sorted, mainly elongated- ized base grading from ¯oat- into packstones with platy (up to 15 cm long, 2±3 cm thick) mud- and silt- abundant bivalve fragments, a clypeastrid layer, clasts occur and also some sandstone fragments. bryozoans, and coralline algal rubble and rhodoliths. Ophiomorpha burrows, originating from this horizon, The uppermost bed of the section is a calcareous sand- reach 25 cm into underlaying marls. Beside abundant stone Ð ¯oatstone (5 m) which grades into a pack- Planostegina-tests also bryozoans, bivalve fragments stone (2 m). The basal 2 m are intensively bioturbated and echinoid remains are present. Larger skeletal (Thalassinoides), the top shows a `diagenetic cross components are common and dominated by echinoids bedding' due to secondary coloration. Fossils are (Parascutella sp.) and calcitic, disarticulated bivalves represented by coralline algae (rubble and rhodoliths), (pectinids, ostreids, spondylids). They are commonly gastropods, bivalves, and echinoids. The termination fragmented, bored by clionid sponges and encrusted of the section is erosional. by bryozoans and balanids. Increase in shell frequency and packing density 4.2. Pectinid-rich sediments gradually produces a 30±40 cm thick pectinid coquina (bed 7b; Fig. 4A). The matrix of the In the lower part of the section, between 39 and coquina is still a Planostegina rudstone. A 55 m, two ®ning-upward sequences occur. The lower, low density portion and an upper, high coarse portions of these sequences are rich in fossils, density part of the coquina can be separated. In repeatedly concentrated in coquinas characterized by the lower part occur subrounded, elongated, paral- pectinid bivalves (Figs. 2 and 3). All quantitative lel oriented mud-clasts (up to 7 cm in length). information for these coquinas is documented in Bivalves are the dominant fossils, however, due Tables 1±7. to aragonite dissolution only pectinids and Ð

O. Mandic, W.E. Piller / Palaeogeography, Palaeoclimatology, Palaeoecology 172 02001) 171±191 179

Convexity

Size Sculpture

arameters and average shell

Thickness

ovxsd pdw ratio up/down side Convex

etrgtvleratio valve Left/right

hno index Shannon

ipo index Simpson

ubro taxa of Number

riuae niiul e m per individuals Articulated

riuae individuals Articulated

dniidseie e m per specimen Identified

dniidseies(%) specimens Identified

ubro dniidspecimens identified of Number

pcmn e m per Specimens

ubro specimens of Number

) ufc (m Surface 2

2 0.1953 0.195 84 104 430.7 533.0 41 42 48.8 40.4 210.0 215.0 2 0 10.3 0.0 6 4 0.68 0.69 1.34 1.18 2.60 2.33 ± ± 2.27 2.29 2.59 2.36 2.46 2.62 2.27 2.07 Quadrant

Total 1-3 0.590 254 430.5 121 48.0 206.6 2 3.4 6 0.67 1.23 1.39 ± 2.26 2.45 2.53 2.18 Coquina morphology Table 7 Quantitative composition of coquinas: quadrant areas, number of pectinid individuals, density, articulation, taxonomic diversity, taphonomic p 7b8c8d base 18d 0.200 top10a 6612a 5 4 0.119 330.0 0.150 6 250 38 0.270 195 7 166 2101.8 0.320 8 1300.0 58.0 250 0.680 141 68 614.8 191.4 139 100 166 0 440.6 35.0 2101.8 100 204.4 71 455.0 3 78 0.0 614.8 7 50.0 11 3 25.2 50.0 220.3 46.7 4 102.2 40.7 2 0.64 5 3 2 1.03 0.10 6.3 0.77 0.45 4.4 0.24 0.01 6 1.37 5 ± 3.21 0.04 1.64 0.64 1.01 2.31 2.21 0.70 1.29 1.23 2.00 1.33 2.42 1.35 1.88 0.90 1.99 2.00 2.50 0.77 2.00 ± 2.91 2.00 2.21 ± 2.49 1.99 2.99 1.62 2.12 1.58 2.00 1.89 2.27 2.15 1.73 2.37 2.79 180 O. Mandic, W.E. Piller / Palaeogeography, Palaeoclimatology, Palaeoecology 172 02001) 171±191 O. Mandic, W.E. Piller / Palaeogeography, Palaeoclimatology, Palaeoecology 172 02001) 171±191 181

Fig. 3. Outcrop photograph showing the position of the studied coquinas. The background hill represents the carbonate sequence of the top of the section. The drawing on the right margin corresponds to the detailed section of Fig. 2. more rare Ð ostreids remained. They are com- and cemented to one another. They are encrusted by monly encrusted by serpulids and bored by clionid bryozoans and bored by sponges and bivalves (Litho- sponges. Aragonitic bivalves (venerids, tellinids) phaga). One coral patch has been found associated are sparsely represented as casts. Other larger with these oyster assemblages. In total, bed 8a reaches skeletal components are irregular echinoids, bryo- a thickness of 40±50 cm and consists of yellowish/ zoans (branched and crustose), and balanids. brownish to greyish, calcareous, medium sandstone. Besides pectinids, also Parascutella sp. occurs The lower part is still a bioclast-supported rudstone, from base to top and is more abundant in the which grades upwards rapidly into a ¯oatstone. Skele- denser, upper part of the coquina where the coro- tal components are dominated by pectinid bivalves nae are oriented parallel to the bedding plane. (juvenile and adult, articulated and disarticulated At the base of the following bed (8a) small oyster Flabellipecten schweinfurthi and Amussiopecten assemblages (Hyotissa hyotis) commonly occur sp.), foraminifera (Planostegina sp.), and celeporid reaching 2 m in diameter and 20±25 cm in height. bryozoans; oysters and serpulids also occur. The These are attached to the underlying coquina top, larger biota are mostly fragmented. In addition, casts and occur predominantly articulated, densely packed of turritellid, naticid, and ®cid gastropods are found.

Fig. 2. Gebel Gharra general section and detailed part of section with position of studied coquinas. 182 O. Mandic, W.E. Piller / Palaeogeography, Palaeoclimatology, Palaeoecology 172 02001) 171±191

Fig. 4. Photographs showing two coquina types: (A) Coquina 7b is a `composite concentration' characterised by low pectinid density (detailed description in text). Moulds and casts indicate the dissolution of aragonitic shells. (B) Coquina 8d is interpreted as a `reworked event- concentration' near the storm wave base. It nearly exclusively consists of large specimens of Flabellipecten schweinfurthi.

Within the upper part of the bed pristine, articulated sandstone. In the lowermost 15 cm subparallel- pectinid shells commonly are present. Vertical burrow oriented pectinid valves occur frequently (matrix tubes occur throughout the bed ®lled with the reddish supported). The pectinids belong mainly to F. sediment of the overlying bed. schweinfurthi and Amussiopecten sp. In the upper The following bed (8b; 60 cm) consists of brown- part of the bed small-sized pectinid valves (1±2 cm) ish-reddish, Planostegina bearing, medium to ®ne occur. The uppermost 15±20 cm are intensively O. Mandic, W.E. Piller / Palaeogeography, Palaeoclimatology, Palaeoecology 172 02001) 171±191 183 bioturbated. Laterally, within several tens of meters, the thickness of 8b varies as does coloration. Bed 8c can be subdivided into three sub-units: the basal 15 cm consist of a pectinid coquina embedded in greyish to light brownish calcareous ®ne sandstone. The lower boundary is obscured by intensive gypsi®- cation. Pectinids (Fig. 5) are up to 10 cm in height. They are frequently horizontally oriented, rarely over- grown by oysters and/or clionid infested, but not encrusted. Within the next 40 cm, no changes occur in matrix, only the shell density suddenly decreases and small, mostly fragmented pectinids occur. Articu- lated forms mostly can be assigned to small, highly convex Oppenheimopecten josslingi, but also A. bara- nensis, F. schweinfurthi, and A. expansus occur. Echi- noid fragments are common but also whole schizasterids in life position occur fairly frequently. The sediment is matrix-supported, with decreasing skeletal density upwards. The uppermost 50±100 cm of this package are characterised by a slight coarsening terminating in a medium calcareous sandstone with abundant Ophiomorpha burrows. Skeletal components stem from the same pectinids and echinoids as below. In addition, fragmented dendroid bryozoans and scattered planosteginid foraminifera are present as well as a fairly rich planktic foraminiferal fauna (Abdelghany and Piller, 1999). Bed 8c is overlain by a dense, fabric-supported pectinid coquina (20±70 cm in thickness; bed 8d; Fig. 2) dominated by conspicuously large F. schweinfurthi. The matrix is made up of calcareous medium sandstone with rare Planostegina. Although the boundary to the underlying bed appears sharp because of the sudden increase in pectinid shells, the matrix remains unchanged between beds 8c and 8d. The coquina exhibits an uneven lower boundary with a relief up to 50 cm. The upper boundary is parallel to the usual bedding planes. The relief is produced by trough-like depressions. One large trough, 150 cm in width and 50 cm of maximum depth, was studied in detail (Fig. 4B). This resulted in the internal distinction of three coquinas, which are separated by horizons of lower packing

Fig. 5. Abundance of taxa (%), diversity (represented by Shannon and Simspon index; exact values in Tables 1±6) and morphologic variation (size, thickness, convexity, sculpture) (on a rank scale) of the pectinids in the studied coquinas. Coquina 8d is subdivided into the basal part (8db) and the top part (8dt). 184 O. Mandic, W.E. Piller / Palaeogeography, Palaeoclimatology, Palaeoecology 172 02001) 171±191 densities of shells. (1) The lowermost coquina follows components are commonly encrusted mainly by continuouslythe surface ofthe lower bedding plane with bryozoans; clionid borings are very rare. Towards a minimum thickness of 10 cm, increasing in thickness the top, pectinids become more common. The lower to about 20 cm in deepest parts of the depression. The part is characterised by scattered F. schweinfurthi valves are predominantly embedded subparallel to the while in the upper portion O. josslingi and O. revolu- lower bedding plane but preferred orientation is less tus are abundant. The latter is particularly near the pronounced than in the top-layer (see below). A few top. valves were found with bryozoans encrusting their inter- Bed 10b (25 cm) is made up by a brownish-reddish, ior surface. In addition, this basal coquina is charac- glauconitic, coarse sandstone. At the base yellowish terised by the common occurrence of isolated, mud, pebbles (25 cm) commonly occur. Biota are biplanar bryozoan colonies. (2) The middle coquina represented by foraminifera (small miliolids, Miogyp- also follows the relief, however, levelling the depression sina), bryozoans, serpulids, gastropods, and bivalves while attaining a maximum thickness of 20 cm in the (e.g. ostreids). center and laterally thinning out or disappearing respec- Bed 11 (30 cm) exhibits a signi®cant change in tively by fusing with the lower coquina (1). Composi- lithology representing a breccia to conglomerate tion and arrangement of valves is similar to (1). (3) The with a matrix of dark red, glauconitic, calcareous silt- top-coquina reaches about 7 cm in thickness and shows stone. The conglomerate components (25 cm) are already horizontal bedding. It is characterised by the yellowish to greenish mud-pebbles, which are most pronounced horizontal orientation of the valves commonly encrusted by thick layers of bryozoans of all concentrations. The distinction between the 3 (1±2 cm). In addition, horizontally oriented rip-up coquinas is very clear within the trough getting, clasts of muddy sediment occur. Encrusting celeporid however, more diffuse laterally (Fig. 4B). bryozoans cover skeletal as well as non-skeletal Bed 8e (35 cm) is a light-yellowish, calcareous components and produce independent aggregates medium sandstone with bryozoans (dendroid and (bryoids) in various shapes. Serpulid aggregates also globose celeporids), planosteginid foraminifera, and occur. Echinoids are represented by cidarids and frag- whole pectinids in life position (O. josslingi). The mented clypeastrids. Among bivalves anomiids and following 30 cm (bed 8f) are made up of a greyish, pectinids occur. Poritid corals and turritellid gastro- ®ne-bioclastic, medium calcareous sandstone with pod casts are present as well. Foraminifera, especially pectinids. The latter decrease upwards in abundance. Miogypsina, but also Planostegina and Amphistegina, Bed 9 separates the two pectinid-rich sequences. are abundant. Bed 9a (30 cm) is a brownish, fossiliferous (moulds The following package (bed 12) starts with a of smaller benthic and planktic foraminifera and 40 cm thick skeletal accumulation in glauconitic, micro-molluscs) sandy marl and bed 9b (c. 550 cm) coarse calcareous sandstone (bed 12a). Similar to is a grey, unfossiliferous, brown-spotted marl. bed 11, bryozoan encrusted mud pebbles are present. Fossils are dominated by pectinids (Fig. 4.2.2. Upper sequence 5, Table 6). Bryozoans (dendroid and globular), The second ®ning-upward sequence starts with solitary corals, serpulid aggregates (up to 5 cm in 65 cm of greyish-greenish, glauconitic coarse sand- diameter), echinoids (spatangids and clypeastrids), stone (bed 10a). The boundary towards the underlying crabs, and shark teeth (Carcharodon sp.) also marl is sharp and exhibits a bioturbational relief (up to occur. Toward the top fossils decrease in abun- 20 cm). Well rounded, bivalve-bored, mud-pebbles dance, however, articulated pectinids, with right, (210 cm) and coarse bioclasts occur in the lower highly convex valves (O. revolutus, O. josslingi, part of the bed with abundant globular celleporid Pecten kochi) become frequent. This horizon grades bryozoans and gastropod casts. Bivalves [pectinids, into a dark-greyish sandstone rich in benthic for- large oysters (Hyotissa hyotis), casts of aragonitic aminifera (15 cm; bed 12b), further upwards into shells], serpulids, balanids (building aggregates), dark grey to brownish planktic foraminiferan celeporid bryozoans, and irregular echinoids (e.g. marls. This sediment grades into non-fossiliferous, Parascutella sp.) occur throughout the bed. Skeletal greyish to brownish marl with intercalations of O. Mandic, W.E. Piller / Palaeogeography, Palaeoclimatology, Palaeoecology 172 02001) 171±191 185 1988). Internal ®ning- and deepening-upward sequences may be explained by local tectonics, particularly due to beginning Red Sea and Gulf of Suez rifting (Purser and Bosence, 1998). The pectinid coquinas are best developed in the middle of the transgressive part of the general sequence.

5.1. Lower sequence 0Figs. 5 and 6)

Taxonomic diversity. Six pectinid taxa were detected in coquina 7b, ®ve in 8c and four in 8d exhibiting a constant decrease in species number. The Simpson index shows a slight increase from 7b to 8c (0.67±0.77) and a strong decrease to 8d (0.1 and 0.01). A similar trend is re¯ected by the Shannon index (1.23, 1.37, 0.24 and 0.04). In 7b the dominating taxon is Pecten fraasi followed by M. ziziniae and Amussiopecten. In 8c the distribution is more even with F. schweinfurthi followed by Amussiopecten and P. fraasi and, additionally, O. josslingi.In8da total dominance of F. schweinfurthi with 97% occurs (base and top quadrant combined). Articulation. Only a very low number of articulated shells were discovered. The relatively highest amount of articulated valves was detected in bed 8b (top) (6.7%). Considering only the identi®ed individuals, the highest percentage of articulated valves appears in bed 8c (10.3%). Convexity. At the base, convex right shells are dominant, convexity decreases in the middle coquina Fig. 6. Density (absolute numbers of individuals per m2), articula- (dominance of F. schweinfurthi), and a total domin- tion (number of individuals per m2), sorting by shape (ratio of left/ ance of moderately convex F. schweinfurthi occurs in right valves) and orientation (ratio of valves with convex side up/ the upper coquina. In accordance with Aguirre et al., down) of the pectinid valves in the studied coquinas. Coquina 8d is subdivided into the basal part (8db) and the top part (8dt). 1996, decreasing convexity points to a deepening of the environment. Also the ability to swim is linked to convexity as highly inequivalve scallops are less siltstone and ®ne sand (500 cm; bed 12e) forming active swimmers then the more equivalve groups the top of this ®ning-upward sequence. (Cox, 1952, p. 20; Fleming, 1957, p. 12). Inequivalve shells are better adapted to a semi-sessile habit and can resist higher energy levels (Waller, 1969, 1991). 5. Interpretation Although the reason is unclear, the best swimmers occur in deeper waters. The general trend within the studied Gebel Gharra Sculpture. Highly sculptured, thick valves (Macro- section re¯ects a transgressive±regressive sequence. chlamis, Pecten fraasi) are dominant in the lower Since the biostratigraphic data place the section into coquina (7b); Macrochlamis occurs exclusively the narrow time-window of the latest Burdigalian, this herein. The decrease in sculpturing up-section trend probably corresponds to the highstand repre- points to an upward decreasing energy (Peres, 1961; sented in the global sea level curve (Haq et al., Kauffman, 1969). 186 O. Mandic, W.E. Piller / Palaeogeography, Palaeoclimatology, Palaeoecology 172 02001) 171±191 Fragmentation. The fragmentation index has been base probably facilitated the production of mud roughly estimated by the percentage of undetermin- pebbles and clasts. The shell concentration of 7b is able pectinids. Highest percentage in middle coquina composed of a great variety of fossils re¯ecting also points to high energy, the lower percentage in 7b is different life strategies (epifaunal and infaunal). An due to thick, highly sculptured valves. In the upper- increase in shell density (with increase of pectinids) most coquina fragmentation is weak, resulting in from base to top occurs. The increase in internal shell nearly 100% identi®ed shells. This decrease in frag- density can be interpreted as a result of winnowing of mentation can be interpreted in decreasing hydro- the sediment by currents. The relatively frequent dynamic energy. occurrence of Macrochlamis points to shallow Density. Upwards increase in pectinid density, water, relatively high energy conditions (e.g. Ctyroky, particularly considering only the unfragmented 1969; Mandic and Harzhauser, 1999). The frequent valves. This is in accordance with the observation shell encrustations and occurrence of globular celle- that mass occurrences of living pectinids do not porids, balanids, and serpulid aggregates also points to occur in very shallow water but in depths of a few a low sedimentation rate and time-averaging. These 10 s of meters (Fuchs, 1900; Dakin, 1909). shell concentrations (bed 7b) acted as a secondary Valve distribution. In all the three coquinas left hard substrate for the settlement of marine, mono- valves are predominantly found. The ratio increases speci®c oyster bioherms and rare coral patches. from 1.39 to 1.64, up to 3.21 and 2.31 in bed 8d. These patches are covered by coarse-grained sedi- Although the convexity differences between left and ments which contain an autochthonous fauna (e.g. right valves are stronger in the lower coquina, separa- articulated pectinids) in the upper part (bed 8a). The tion is more pronounced in the upper one. This may be following bed (8b) shows a slight ®ning-upward trend interpreted with a much higher hydrodynamic energy and a weak accumulation of pectinids at the base in the upper coquina and relatively quiet conditions in re¯ecting shallow water sedimentation. the lower two. Above a bioturbated base the next shell concentra- Shell orientation. In the lower coquina, chaotic tion follows. It represents the pectinid coquina with distribution prevails, in the middle and the upper highest diversity and a mixed assemblage of high and coquina a distinct (sub)horizontal orientation was moderately convex valves which are subhorizontally observed. The ratio of convex up/down shells is insig- oriented. Contrary to the lower concentration, addi- ni®cantly different with great variability concerning tional megafossils, except for some oysters, are rare different taxa in 8c. The preferred subhorizontal orien- and no encrustations of the pectinid shells occur. The tation clearly points to transport by current. The vari- high pectinid diversity points to a favourable habitat ability of orientation at the species level is due to for these bivalves, the lack of encrustations suggests a different hydrodynamic behaviour of valves as a result relatively rapid accumulation. This accumulation can of different valve morphology. be interpreted as caused by a single storm event with- The sequence generally exhibits a ®ning-upward out major transport of shells (`reworked event- trend, with coarse sandstones with mud-pebbles at concentration' sensu Kidwell, 1991). The occurrence the base and marl on top supporting also a deepen- of pectinids in life position above the coquina ing-upward tendency. The character of the three pec- supports this interpretation. This upper part of bed tinid coquinas is in accordance with these trends. In 8c is also characterised by a more open marine in¯u- particular, the decrease in diversity, in sculpture and ence due to the occurrence of planktic foraminifera. convexity are a clear indication of a decrease in This sandstone is interrupted by the occurrence hydrodynamic energy related to a deepening-upward of a composite, nearly monospeci®c pectinid trend. coquina composed of adult F. schweinfurthi.In The coarse-grained part of the lower sequence can fact, this coquina is built by three distinct shell be classi®ed as a `composite concentration' sensu concentrations. The lowermost one follows a Kidwell (1991). The occurrence of mud clasts at the pronounced relief with depressions down to base points to a reworking of the underlying bed due 50 cm. The thickness of the coquinas is highest to turbulent water. The intensive bioturbation at the in the depressions tending to reduce the relief. O. Mandic, W.E. Piller / Palaeogeography, Palaeoclimatology, Palaeoecology 172 02001) 171±191 187 The continuous sedimentary character below and Valve distribution. In both horizons a slight preva- within the coquina is clear proof of soft bottom lence of right valves was detected, with an increasing conditions below the coquina. The dominance of upward trend (0.90±0.77). This trend is due the left valves points to transport and sorting. The dominance of Oppenheimopecten which is predomin- restricted diversity and lowest convexity and orna- antly represented by right valves throughout the mentation indicate lowermost bathymetric position section. of the shell accumulations in the lower sequence. Shell orientation. In both beds chaotic distribution On the contrary, the pronounced relief suggests prevailed. high water energy. These conditions can easily Coarse-grained sediments with mud pebbles lie be explained by a tempestite, the dominant occur- above a sharp, intensively bioturbated boundary rence of large valves place it into a proximal with the marl of bed 9b. The biota in this bed are position. Above this interruption sedimentation diverse and a high percentage of encrusting organisms continues normally, re¯ecting the general ®ning- (bryozoans, balanids, serpulids) occurs. The strong and deepening-upward trend. This trend culmin- encrustations and occurrence of glauconite re¯ect a ates in the development of marl of bed 9b. reduced sedimentation rate. The frequent occurrence of Miogypsina and Parascutella indicates shallow 5.2. Upper sequence 0Figs. 5 and 6) water conditions; that of bryoids and balanid aggregates suggests high water energy as do the mud Taxonomic diversity. Six pectinid taxa were pebbles. The bed can be classi®ed as a `composite detected in coquina 10a and ®ve in 12a. The Simpson concentration' sensu Kidwell (1991). A vertical shift index shows a slight increase (0.64±0.70) as does the in pectinid frequency and composition is observed. Shannon index (1.23±1.35). In 10a the dominant F. The upward increase of thin-valved Oppenheimopec- schweinfurthi is followed by O. josslingi. O. revolutus ten in bed 10a points to a reduction in energy, perhaps is also abundant. In 12a P. kochi and O. josslingi related to a slight deepening. This may be the reason dominate with F. schweinfurthi and O. revolutus for a relatively good coincidence between this bed also reaching more than 10%. The main difference (10a) and Bed 8c of the lower sequence. between two horizons is the decrease of F. schwein- Up-section, the general depositional environment furthi which is accompanied by a sudden appearance remains similar with glauconitic sandstones (bed and dominance of P. kochi. 10b), and becomes somewhat higher energetic Articulation. Only a very low number of articulated with the conglomerates of bed 11. The biota are shells was discovered in both horizons. also similar (aggregates of bryozoa and serpulids). Convexity. In bed 10a moderately right convex taxa In addition, poritid corals occur. The lower part of dominate (F. schweinfurthi), but highly right convex bed 12a is again glauconitic at the base and species (O. josslingi and revolutus) are also abundant. contains similar fossils to those below. Upwards a In bed 12a the percentage of species with a highly general decrease in variety occurs with pectinids convex right valve increases distinctly with the becoming abundant and dominant. This bed shows dominance of P. kochi. The latter, however, is repre- the same characteristics as bed 10a and can also be sented by a dominance of left valves (1.31). classi®ed as a `composite concentration' sensu Sculpture. The two species of Oppenheimopecten Kidwell (1991). The taxonomic shift from F. are rather thin-valved, F. schweinfurthi and P. kochi schweinfurthi in bed 10a to P. kochi is accompa- are moderately thick-valved, P. kochi is more sculp- nied by a general decrease in shell size. The tured than the other species. This points to an increase increasing percentage of Oppenheimopecten higher in energy (Peres, 1961; Kauffman, 1969). up in both beds points to a reduction in water Fragmentation. No differences in the percentage of energy, ?deepening and establishment of average undeterminable pectinids occur between the two sedimentation rate. This interpretation is in contrast horizons. to the idea that increasing convexity and sculptur- Density. Pectinid density is generally low in both ing are accompanied by increasing energy and shal- beds and increases up-section. low water conditions (Aguirre et al., 1996). The 188 O. Mandic, W.E. Piller / Palaeogeography, Palaeoclimatology, Palaeoecology 172 02001) 171±191

Plate 1. Large-sized, moderately to weakly in¯ated pectinids of Gebel Gharra. Specimens are deposited in the palaeontological collection of the Natural History Museum in Vienna/Austria.

functional aspect of pectinid ornamentation, may balanid aggregates, also suggests reduction in water not only be related to withstand high energy condi- energy. Above this last pectinid accumulation a tions but may have other causes (e.g. protection distinct ®ning- and deepening-upward trend started. against predators) (e.g. Waller, 1969). The general Modern pectinid species have a well-de®ned depth decrease in biota, particularly of bryoids and range with optimum occurrences (e.g. Argopecten O. Mandic, W.E. Piller / Palaeogeography, Palaeoclimatology, Palaeoecology 172 02001) 171±191 189

Plate 2. Moderate- and small-sized, highly in¯ated pectinids of Gebel Gharra. Specimens are deposited in the palaeontological collection of the Natural History Museum in Vienna/Austria.

gibbus: 33±42 m; Pecten maximus: 20±45 m; Brand, larger foraminifera, scutellid echinoids, oysters) and 1991), which is used also in commercial exploitation. with sedimentological characters (coarse-grained Considering the studied Burdigalian pectinids a sediments, ®ning-upward sequences, mud-pebbles). comparable depth range appears to be most probable. The actualistic approach, however, bears some This is in accordance with the co-occurring biota (e.g. problems, because modern (e.g. Raf® et al., 1985) 190 O. Mandic, W.E. Piller / Palaeogeography, Palaeoclimatology, Palaeoecology 172 02001) 171±191 and also Pliocene (e.g. Aguirre et al., 1996) pectinid Stuewe (University of Graz) who critically read the faunas of the Atlantic and Mediterranean exhibit a manuscript and to F.T. FuÈrsich (WuÈrzburg) and an different taxonomic inventory (e.g. Ben Moussa and anonymous reviewer who enhanced the quality of Demarcq, 1992; Roger, 1939). the paper. The pectinid material is deposited in the palaeontological collection of the Natural History Museum/Vienna. The work was funded by the 6. Conclusions Austrian Science Foundation (FWF) Project P 11886-GEO. Quantitative and qualitative study of pectinid bivalves offers the opportunity for palaeoenvironmental reconstructions. Use of taxonomic and various morphological parameters, such as divers- References ity, sculpture, convexity, shell thickness and size, provides information on functional aspects related Abdelghany, O., Piller, W.E., 1999. Biostratigraphy of Lower Miocene sections in the Eastern Desert (Cairo±Suez district, to life style and habitat. The studied examples Egypt). Rev. Palaeobiol. 18/2, 607±617. clearly show that ornamentation, convexity and Aguirre, J., Braga, J.C., JimeÂnez, A.P., Rivas, P., 1996. Substrate- shell thickness are linked with water energy. related changes in pectinid fossil assemblages. Palaeogeogr., These data can therefore by used as proxies for Palaeoclimatol., Palaeoecol. 126, 291±308. reconstructing hydrodynamic regimes and water Ben Moussa, A., Demarcq, G., 1992. Temporal and spatial distribution of Neogene pectinid's communities in Western Mediterra- depth. Decrease in diversity seems generally to nean. Paleontol. Evol. 24±25, 175±183. be linked with increasing depth, except for shal- Berggren, W.A., Kent, D.V., Swisher III, C.C., Aubry, M.-P., 1995. low subtidal occurrences. Taphonomic parameters, A revised Cenozoic geochronology and chronostratigraphy. such as articulation, fragmentation, density, orien- Geochronology, time scales and global stratigraphic correlation, tation relative to bedding, and valve distribution Berggren, W.A., Kent, D.V., Aubry, M.-P., Hardenbol, J. (Eds.), SEPM, Spec. Publ. 54, 129±212. (right/left) provide information on post mortem Blanckenhorn, M., 1901. Neues zur Geologie und PalaÈontologie processes: in composite concentrations (bed 7a, Aegyptens III: Das MiocaÈn. Zeitsch. d. Deutsch. Geol. Gesell. 10a, 12a) pectinids co-occur with various benthic 53, 52±132. organisms of different life styles. Shell encrusta- Blanckenhorn, M., 1903. Die Vola Arten des aegyptischen und tions are sometimes abundant pointing to slow syrischen Neogens. Neues Jahrb. Miner. Geol. Paleontol., Beilage 17, 163±186. sedimentation and time-averaging. Change in Brand, A.R., 1991. Scallop ecology: distributions and behaviour. shell densities are explained by variable degrees Scallops: biology, ecology and aquaculture, Shumway, S.E. of winnowing or, as a primary reason, by change (Ed.), Dev. Aquacul. Fisher. Sci. 21, 517±584. in living condition. The pectinid accumulation of Cahuzac, B., Poignant, A., 1997. Essai de biozonation de l'Oligo- bed 8c is interpreted as `reworked event-concen- MioceÁne dans les bassins europeÂens aÁ l'aide des grands forami- nifeÁres neÂritiques. Bull. Soc. GeÁol. Fr. 168 (2), 155±169. tration' (Kidwell, 1991) due to a single storm Carter, J.G., Barrera, E., Tevesz, M.J.S., 1998. Thermal potentiation event causing high density of valves. Lack of and mineralogical evolution in the Bivalvia (Mollusca). J. encrustations points to relatively rapid sedimenta- Paleontol. 72 (6), 991±1010. tion, high dominance and density to highly favour- Cherif, O.H., 1974. Tertiary fauna from the Sadat area (south-west able conditions for the pectinids. of Suez). Bull. Inst. Egypt 53±54, 91±123. Cox, L.R., 1952. The Jurassic Lamellibranch fauna of Cutch (Kachh) No. 3, families pectinidae, amussidae, plicatulidae, Acknowledgements limidae, ostreidae, and trigoniidae. Pal. Ind., Ser. 9 3 (4), 1± 128 (Suppl.). Ctyroky, P., 1969. 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