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von Rad, U., Haq, B. U., et al., 1992Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 12230. CALCAREOUS DINOFLAGELLATE CYSTS FROM THE LOWER CRETACEOUS OF HOLE761C, WOMBAT PLATEAU, EASTERN INDIAN OCEAN1Helmut Keupp2ABSTRACTLow-diversity calcareous dinoflagellate cysts were studied by SEM in 10 samples from the Lower Cretaceous ofHole 761C (256-235 mbsf). The ?late Aptian/Albian age interval can be recognized because of the calcispherespresent. The new orthopithonelloid genus Bitorus is introduced.INTRODUCTIONThe renewal of interest over the past 15 years in Mesozoiccalcispheres began with the first extensive SEM documenta-tion of a diverse calcisphere flora isolated from Jurassic andCretaceous sediments from the Indian Ocean by Bolli (1974).With the exception of a few papers on the ultrastructure ofPithonella (e.g., Andri and Aubry, 1973), this fossil group waspreviously known mostly from thin sections of limestones.The lack of detailed morphological definition of these small-sized fossils (between 10 and 100 µm) by optical methodsproduced taxonomic confusion. Descriptions and definitionsof the morphologic character became more objective with theaid of the SEM (cf. Villain, 1957, 1977; Keupp, 1981). Keupp(1979, 1980a, 1980b, 1987) succeeded in proving the peridini-acean affinity of the most prominent representatives of theMesozoic incertae sedis group Calcisphaerulidae Bonet, 1966,which is closely related to the modern Calciodinellaceae (cf.Wall and Dale, 1968). Fütterer (1976) and Tangen et al. (1982)also proved that the Cenozoic genus Thoracosphaera is ofdinoflagellate origin. Calcareous dinoflagellate cysts occurredthroughout the world in rock-forming abundance in shelf andslope sedimentary environments during the Jurassic and Cre-taceous (Villain, 1981). But despite their importance, calci-sphere morphology, paleobiology, and vertical and lateraldistribution, as well as environmental aspects, are still poorlyunderstood. The objective of this study of the calcispheresfrom Ocean Drilling Program (ODP) Leg 122 Hole 761C was tocontribute the following:1. documentation of morphological diversity;2. identification of possible environmental (facies) controlof calcisphere distribution;3. biostratigraphic interpretation of the vertical flora suc-cession.A preliminary outline of the Mesozoic to Cenozoic evolu-tion of the northwestern Australian continental margin, in-cluding the development of the Indian Ocean, was reported byvon Rad et al. (1989) based on the results of Legs 122 and 123.The site locations of Leg 122 were described and illustrated byvon Rad et al. (1989) and von Rad and Thurow (1989). Thelithologic sequence at Site 761 is characterized by at least two1 von Rad, U., Haq, B. U., et al., 1992. Proc. ODP, Sci. Results, 122:College Station, TX (Ocean Drilling Program).2 Free University Berlin, Institute of Paleontology, Schwendenerstr. 8,D-1000 Berlin, Federal Republic of Germany.hiatuses at the Rhaetian-Liassic/?late Berriasian-Valanginianand the Valanginian/Albian (von Rad and Thurow, this vol-ume). The clastic ferrugineous condensed sandstone sectionat the base of the Cretaceous sequence contains numerousbelemnite guards and represent the rapidly increasing subsid-ence rate. The light to dark yellowish brown calcisphere-richnannofossil chalk of the upper Lower Cretaceous (? Valangin-ian) sediment unit contains six bentonite layers. It seems tohave originated under hemipelagic conditions during the juve-nile phase of the ocean. The calcareous nannoplankton oozeand chalk of the upper part of the sedimentary column (Albianto Quaternary) represent pelagic conditions during the stage ofincreasing ocean maturity. The stratigraphic control of thesamples is based on calcareous nannoplankton (Bralower, thisvolume).MATERIAL AND METHODSTwelve samples of Lower Cretaceous light marls and marlychalks from Holes 761B and 761C (Fig. 1) were selected:122-761C-6R-1, 74-75 cm6R-CC, 13-14 cm7R-1, 72-73 cm7R-1, 106-107 cm8R-1, 24-25 cm8R-2, 0-1 cm9R-1, 69-70 cm9R-1, 92-93 cm9R-CC, 13-14 cm10R-1, 51-52 cm121-761B-28X-CC, 13-14 cm29X-2, 7-8 cmThe samples were washed using 20-µm mesh sieves. About100 calcispheres were picked from each sample using abinocular microscope at a magnification of 150 × to 200 × forstudy by SEM. Neither of the samples from Hole 76IB couldbe used for stratigraphic purposes. The well-cemented lime-stone of Sample 122-761B-28X-CC, 13-14 cm, contains manycalcispheres that are diagenetically completely destroyed.Only few determinable specimens of the stratigraphicallywide-ranging morphotype Orthopithonella cf. deflandrei(Kamptner, 1956) were found. The chalky calcisphere pack-stone of Sample 122-761B-29X-2, 7-8 cm, is also character-ized by a monospecific assemblage of Orthopithonella cf.deflandrei. The latter sample appears similar to ?upper Berri-asian/Valanginian Samples 122-761C-8R-2, 0-1 cm, and 122-761C-9R-CC, 13-14 cm.497H. KEUPPm230 H235-240-245-250-255-260-Hole 122-761C% of calcareousdinoflagellate cysts6R-1-6R-CC7R-19R-CC10R-1BitorusPithonellaObliquipithonellaOrthopithonellaVertical distribution of calcareous dinoflagellate cysts,T7I II I.1Inannoooze B1-6 bentonite layersclay/marl ° ° calcisphereslimestones «=»• belemnitessandstones Δ Δ porcellaneous foram.Figure 1. Lower Cretaceous vertical distribution of calcareous dinoflagellate cysts in Hole 761C. Lithologic column and stratigraphy after vonRad and Thurow (this volume).SYSTEMATIC PALEONTOLOGYAccording to Wall and Dale (1968) and Keupp (1981,1987),most of the Mesozoic/Cenozoic calcispheres can be classifiedin the peridinioid cyst family Calciodinelloideae Deflandre,1947. Fütterer (1976) and Tangen et al. (1982) demonstratedthat the modern planktonic calcareous nannoplankton Thorα-cosphαerα heimii represents a vegetative coccoid dino-phycean life stage. The cyst family Calciodinelloideae, knownfrom the Upper Triassic (?) and Jurassic to Holocene, issubdivided into three subfamilies based on the orientation ofthe calcite crystals, which form at least the outer calcareouswall layer (Keupp, 1987). The generally spherical representa-tives of the calciodinelloideans, which do not exhibit clearmorphological paratabulations, have been included in thefamily Calcisphaerulidae Bonet, 1966 incertαe sedis by theformer authors. Bolli (1974) named most of these Mesozoicand Cenozoic calcispheres Pithonella Lorenz, 1902. But thedifferent wall structures, each corresponding to


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