JURASSIC, CRETACEOUS, AND TERTIARY MEGAFOSSILS FROM THE RIYADH REGION OF EAST-CENTRAL SAUDI ARABIA SENIOR THESIS Presented in Partial Fulfillment • of the Requirements for the Degree, Bachelor of Science By Richard E. Mccutchen The Ohio State University Department of Geology and Mineralogy 1985 • TABLE OF CONTENTS • PAGE ABSTRACT-------------------------------------------- 1 INTRODUCTION----------------------------------------- 1 GEOLOGY OF THE AREA---------------------------------- 2 LOCALITY INFORMATION--------------------------------- 7 PRESERVATION OF THE MACROFOSSILS--------------------- 8 BIOSTRATIGRAPHY-------------------------------------- 8 FOSSIL DESCRIPTIONS---------------------------------- 12 PHYLUM MOLLUSCA CLASS GASTROPODA GENUS PLANORBIS PLANORBIS sp.----------------------------- 12 GENUS MITRA MITRA sp.------------------------------------- 13 GENUS ARCHITECTONICA • ARCHITECTONICA sp.---------------------------- 14 GENUS CYPRAEA CYPRAEA sp.----------------------------------- 15 GENUS BOLIS BOLIS LISBOA---------------------------------- 16 GENUS AMAUROPSIS AMAUROPSIS BULBIFORMIS------------------------ 17 GENUS TURRITELLA TURRITELLA sp.-------------------------------- 18 GENUS EPITONIUM EPITONIUM sp.--------------------------------- 19 GENUS GYRODES GYRODES CONRADI------------------------------- 19 GENUS NERITOMA NERITOMA (NERIDOMUS)sp.----------------------- 20 GENUS OLIVA OLIVA sp.------------------------------------- 21 • i PAGE • GENUS NATICA "NATICA" WILLIAMSI----------------------------- 21 GENUS POLINICES POLINICES (EUSPIRA) HALLI----------------------· 22 GENUS VALVATA VALVATA SCABRIDA------------------------------- 23 GENUS LUNATIA LUNATIA HALLI----------------------------------· 23 GENUS LAPPARIA LAPPARIA DUMOSA-------------------------------- 24 GENUS CYLICHNA CYLICHNA SECALINA------------------------------ 25 GENUS NATICOPSIS NATICOPSIS (NATICOPSIS)sp.--------------------- 25 CLASS CEPHALOPODA SUBCLASS AMMONOIDEA GENUS SPHENODISCUS • SPHENODISCUS (AUSTROSPENODISCUS)sp.---------- 26 GENUS LIBYCOCERAS LIBYCOCERAS sp.-------------------------------- 27 GENUS PSEUDOCENOCERAS PSEUDOCENOCERAS sp.---------------------------- 28 CLASS PELECYPODA GENUS AMBOCARDIA AMBOCARDIA sp.------------------------------- 29 GENUS CARDIUM (GRANOCARDIUM) DUMOSUM------------- 29 CARDIUM (GRANOCARDIUM) DUMOSUM------------------- 30 GENUS GRANOCARDIUM GRANOCARDIUM (GRANOCARDIUM)sp.----------------- 30 GENUS CARDIUM CARDIUM (PAUPERCULUM)sp.----------------------- 31 GENUS CARDIUM CARDIUM sp.------------------------------------ 31 • ii PAGE • GENUS VENERICARDIA VENERICARDIA (VENERICARDIA)sp.-------------- 32 GENUS CYMBOPHORA CYMBOPHORA sp.------------------------------ 32 GENUS DICTYOPTYCHUS DICTYOPTYCHUSsp.·---------------------------- 33 GENUS PHOLADOMYA PHOLADOMYA (BUCARDIOMYA)sp.----------------- 33 GENUS PHOLADOMYA PHOLADOMYA (PHOLADOMYA)sp.------------------ 34 GENUS PLATYCARDIA PLATYCARDIA sp.----------------------------- 35 GENUS MYTILUS MYTILUS sp.---------------------------------- 35 GENUS DENTONIA DENTONIA sp.-------------------------------- 36 GENUS PARAMEGALODUS • PARAMEGALODUS sp.--------------------------- 37 GENUS ALECTRYONELLA ALECTRYONELLA sp.---------------------------- 37 PHYLUM COELENTERATA CLASS ANTHOZOA GENUS CYCLOLITES CYC£0DITES sp. --------------------------- 38 GENUS CYCLOSERIS CYCLOSERIS sp.------------------------------ 39 GENUS TROCHAREA TROCHAREA sp.------------------------------- 40 GENUS LEPTOPHYLLASTRAEA LEPTOPHYLLASTRAEAsp.------------------------ 41 PHYLUM BRACHIOPODA CLASS ARTICULATA GENUS OLENEOTHYRIS OLENEOTHYRISsp.-------------------------- 41 • iii PAGE • GENUS TEREBRATALIA TEREBRATALIA TRANSVERSA--------------------- 41 GENUS TEREBRATULINA TEREBRATULINA FLORIDANA--------------------- 41 SUMMARY-------------------------------------------- 42 ACKNOWLEDGEMENTS----------------------------------- 43 REFERENCES----------------------------------------- 44 FIGURES AND PLATES--------------------------------- 46 • • iv • JURASSIC, CRETACEOUS, AND TERTIARY MEGAFOSSILS FROM THE RIYADH REGION OF EAST-CENTRAL SAUDI ARABIA RICHARD E. McCUTCHEN Department of Geology and Mineralogy, The Ohio State University, Columbus, Ohio 43210 ABSTRACT - Gastropods, bivalves, cephalopods, corals, and other fossils collected from Riyadh area, Saudi Arabia are significant for interpreting the age of the host rocks. The fossils range from Late Jurassic to Eocene in age. Descriptions are given of the significant components of the fauna. The state of preservation is generally good although the specimens are preserved as steinkerns. Stratigraphic • information is provided for the several fossil-bearing sections yielding these fossils. INTRODUCTION The Riyadh region in east-central Saudi Arabia was a platform extending across many hundreds of thousands of square miles during Phanerozoic time. In east-central Saudi Arabia sedimentary rocks making up this platform are exposed in a great curved belt flanking the eastern margin of the Arabian Shield. They reflect the attitude of the basement, dipping from 1° 00' (Pre-Cretaceous) to less than o0 30' • (Upper Cretaceous and Eocene) uniformly in a northeast, east, and southeast direction. This structural province of Saudi • Arabia is the Interior Homocline, which borders the Arabian Shield (Figure 1). Parallel, west-facing, strike escarpments supported by resistant limestone caps are characteristic of this province. In the valleys and plains between these escarpments there are sandstones and shales, partially hidden beneath Tertiary and Quaternary gravels and sands (Powers et al., 1966). The purpose of this project is to describe the megafossils in a collection gathered from an area west and northeast of Riyadh in east-central Saudi Arabia. The identified fossils will be used to determine the age of the rocks they came from. In this investigation I consider only major types of fossils in the collection and their significance. A serious • unsatisfactory limitation to this investigation is the state of preservation of many of the fossils that may intro­ duce errors in the determination of fossils from a particular locality. GEOLOGY OF THE AREA Below I will review the geologic history of the area, the lithology of the formations, and the depositional environ­ ments. During the time interval considered (Late Jurassic to Middle Tertiary) several distinctive rock units were deposited (Figure 2). West of Riyadh 50 miles and 75 miles respectively, along the road to Mecca are two fossil localities with the oldest rocks. They are located along • 2 Jabal Tuwayq, a sequence of nearly parallel west-facing • scarps of Jurassic rocks (Figure 1). The Tuwayq Mountain Limestone constitutes the backbone of Jabal Tuwayq (Powers et al., 1966). At the base of the Tuwayq escarpment in these areas is the Dhruma Formation, which is subdivided into the Lower, Middle, and Upper Dhruma (Figure 2,3). The Lower Dhruma is shale with some limestone. The Middle Dhruma is a limestone containing at some levels beds of clean current­ washed calcarenite, many of which have high lateral per­ sistance, and an upper oolitic bed that is cliff-forming. The Upper Dhruma consists of shale with some limestone in the lower part (Powers et al., 1966). Above the Dhruma Formation is the Tuwayq Mountain Limestone (Figure 2, 3). Its base • consists of soft calcarenitic limestone which overlies olive-green shale of the Dhruma. It is divided into three units: Tl, T2, and T3. Tl consists to a great part of interbedded calcarenite and yellow fossiliferous carbonate clay. The remaining part is a finely bioclastic, nodular, clayey limestone and calcarenite. It forms the base of the Tuwayq escarpment. T2 is a cream colored, very fine-grained, massive limestone, commonly very bioturbated, which has rare bioclastic layers. T3 at the top of the escarpment is made up of a fine-grained, granular, more-or-less clayey, bio­ clastic limestone (Vaslet et al., 1983). This is overlain by the Upper Jurassic Hanifa Formation (Oxfordian) of which only the Hawtah Member needs be mentioned (Figure 2, 3). It • 3 is made up of a lower white or yellowish calcareous clay, • fossiliferous clayey limestone, and rare beds of massive, vioclastic and pelletoid limestone containing corals. The upper part is a brown, bioclastic intraclastic limestone (Ibid.) . Northeast of Riyadh are three fossil localities with younger rocks. The one nearest Riyadh (10 miles) is located near the unconformable contact of the Sulaiy Formation (Lower Cretaceous) and the Kharj Formation (Miocene and Pliocene) (Figure 2, 3). Here, the Sulaiy Formation is a tan, chalky, massively bedded, aphanitic limestone. The Kahrj Formation is normally lacustrine limestone with included gypsum and gravel (Powers et al., 1966). A west-facing slump escarpment • (Hit) is present in this area and theKharj forms the top of this escarpment (Figure 1). The second locality, 20 miles from Riyadh, is located within the Yamama Formation (Lower Cretaceous) (Figure 2, 3). It consists of calcarenite, calcarenitic limestone, and aphanitic limestone (Ibid.). The third locality (65 to 70 miles northeast of Riyadh) is situated in the Al Arumah plateau region (Figure 1). The Aruma Formation makes up this plateau and it is made up of shallow-water limestone with massive dolomite and smaller amounts of shale and impure dolomite in the upper part (Ibid.) (Figure 2, 3) . The paleoenvironments of the area were as follows. In • 4 the Late Jurassic (Callovian Stage), at the time of deposition • of the Dhruma, Tuwaiq