• DOI:10.2973/DSDP.PROC.20.111.1973
• Corpus ID: 54664552

Cretaceous Benthonic Foraminifera, Leg 20, Deep Sea Drilling Project

@inproceedings{Krasheninnikov1973CretaceousBF,
  title={Cretaceous Benthonic Foraminifera, Leg 20, Deep Sea Drilling Project},
  author={Valery A. Krasheninnikov},
  year={1973}
}

• V. Krasheninnikov
• Published 1 November 1973
• Environmental Science, Geography, Geology

Lower Cretaceous deposits consist of white, gray, yellow, and greenish chalk; siliceous porcelaneous limestones; marls with intercalations of radio larian clay stones; and red, brown, pinkish, black, and gray cherts (Holes 195, 195A, 195B, 196). Nannofossils and radiolarians are the predominant microfossils in these sediments. Identifiable representatives of calcareous nannofossils constitute at least 10 to 15% of the sediment and indicate an assignment to the lowermost part of the Neocomian (Valanginian, early Hauterivian) (see Nannofossil Biostratigraphy, Hekel, this volume). The radiolarians are usually preserved and suggest an early Cretaceous age for these sediments (see Foreman, this volume). Benthonic foraminifera occur much less frequently than nannoplankton and Radiolaria and were found only in the limestones of Hole 195B (Core 3). Included are both calcareous and agglutinated species; they are characteristically small and not abundant. The most common species in the assemblages are: Lenticulina kugleri Bart., Bett., and Bolli; L. muensteri (Roem.); L. crassa (Roem.);/,. subulata (Reuss); Frondicularia hastata Roem.; Lingulina praelonga ten Dam; Spirillina neocomiana Moull.; Dorothia praeoxycona Moull.; D. zedlerae Moull.; D. ouachensis (Sig.); and D. aff. hauteriviana (Moull.). They are accompanied by rare specimens of a number of other species of Lenticulina, Dentalina, Lagena, Nodosaria, Ramulina, Lingulina, Frondicularia, and Pseudonodosaria. Minute, poorly preserved planktonic foraminifera (Gubkinella sp.) were found together with the benthonic species. The above assemblage of benthonic foraminifera from Hole 195B has much in common with the associations at Sites 49 and 50 of Leg 6 on the western slope of Shatsky Rise. The age of the Site 49 and 50 deposits with Lenticulina kugleri, L. ouachensis, Frondicularia hastata, Dorothia praeoxycona, etc., was originally interpreted as Late Jurassic (Tithonian)-Early Cretaceous (Neocomian) (Fisher et al., 1970). Subsequent study of the benthonic foraminifera of Holes 49 and 50 by Douglas and Moullade (1972) led to the conclusion that they are somewhat younger—late Hauterivian-early Barremian. There is also a discrepancy in defining the age of deposits based on benthonic foraminifera on the one hand (upper Hauterivian-lower Barremian) and nannoplankton on the other (Valanginian-early Haute rivian). However, it should be kept in mind that the zonal scale of the Neocomian based on nannoplankton, and the stratigraphic subdivision of the Neocomian of the northwestern part of the Pacific area based on benthonic foraminifera, have not been sufficiently perfected. This seems to be the cause of the contradiction in age of thesç deposits, though the discrepancy is not large. Thus, limestones, marls, and radiolarian claystones with interbeds of variegated flints of Neocomian age are developed over a considerable area of the extreme northwestern Pacific. They have been penetrated by holes, not only on the western slope of the Shatsky Rise, but also on the abyssal plain towards the west and southwest. At present, the depth of the ocean exceeds 6000 meters here (Holes 195B, 196). Calcareous sediments with flinty interlayers seem to occur on basaltic basement (Hole 197) and underlie pelagic red clays. The deposition of carbonate was naturally proceeding above the level of carbonate compensation; thus, water depth in the Neocomian must have been considerably less than at present. Douglas and Moullade (1972) believe, on the basis that benthonic foraminifera from the Neocomian deposits of the Shatsky Rise display a great similarity with the shallow-water microfauna of the Neocomian of Europe, that the depths of the Neocomian Pacific Ocean were measured in hundreds rather than thousands of meters. However, the composition, morphological peculiarities and preservation of microfossils from the limestones and flints of Hole 195B testify to an abyssal origin for these rocks. Among foraminifera, representatives of the Lagenidae are numerically predominant; the tests of calcareous and agglutinated foraminifera are thin-walled and small; and the walls of agglutinated foraminifera are fine-grained. On this basis, water depth should be estimated to have been on the order of at least 1-1.5 km. The limestones, marls, and variegated flints of the Neocomian of "the northwestern part of the Pacific ocean resembles, in lithological and micropaleontological character, abyssal limestones, radiolarites, and flints of the Mesozoic (Triassic, Jurassic, Lower Cretaceous) of the Alpine belt. As it is known, many workers regard these deposits as oceanic, formed on basaltic crust of an oceanic type.