34. Cretaceous Nannoplankton Biostratigraphy and Oceanography of the Northwestern Atlantic Ocean

34. CRETACEOUS NANNOPLANKTON BIOSTRATIGRAPHY AND OCEANOGRAPHY OF THE NORTHWESTERN ATLANTIC OCEAN Peter H. Roth, Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah ABSTRACT A tentative Cretaceous nannoplankton zonation is given and the biostratigraphic extent of classical European and newly proposed oceanic stages as well as radiometric ages for important paleontological events and geochronologic boundaries are presented. Calcareous nannoplankton distribution at Sites 390 and 391 is discussed. The evolutionary lineage of Lithraphidites carniolensis-L. praequadratus-L. quadratus is used for the biostratigraphic subdivision of the uppermost Bermudan Stage. A working hypothesis of Cretaceous nannoplankton evolution and oceanography of the Cretaceous oceans is discussed. One new genus (Haqius) and two new species (Lithraphidites praequadratus and Cyclagelosphaera bergeri) are proposed. The evolution of Retecapsa during the Early Cretaceous is discussed and light micrographs are provided for/?, neocomiana, R. angustiforata, and/?, brightonii. INTRODUCTION CRETACEOUS CALCAREOUS NANNOPLANKTON BIOSTRATIGRAPHY Cores recovered during Leg 44 provide one of the most complete Lower Cretaceous sections in the Western Recent improvements of calcareous plankton stratig- Atlantic. (Figure 1 shows location of the Leg 44 sites.) The raphy, summarized by van Hinte (1976a), have provided Lower Cretaceous limestones from Hole 391C contain rich the most detailed worldwide time-frame for the Cretaceous. assemblages of moderately well preserved nannofossil Planktic foraminifers and calcareous nannoplankton have assemblages similar to the ones studied by Thierstein (1971, a more widespread geographic distribution and show less 1973) in sections from central and western Europe. provincialism than nektic and benthic organisms. Mid-Cretaceous sediments—ranging in age from about 108 Planktic biocoenoses are mostly controlled by surface m.y. to 102 m.y. and belonging to the Magellanian and temperature distribution. The relatively equable oceanic Argusian stages—contain rich assemblages at the shallower climate during the Cretaceous resulted in a fairly wide tropi- Site 390, but slightly more poorly preserved assemblages in cal zone where calcareous planktic organisms thrived. the sapropellites at Site 391. Sediments ranging in age from The traditional guide fossils for the Cretaceous (nektic 102 m.y. to 72 m.y. were not recovered during Leg 44. All cephalopods and belemnites; benthic foraminifers and samples were studied in the light microscope and only rough echinoids) show more regional distribution. Thus, it has estimates of fossil abundance were made. The range charts been necessary to establish boreal and Tethyan ammonite presented are preliminary and show partly "filled" ranges; zonations (see van Hinte, 1976a). Planktic foraminifers if a sample lacked a species as a result of poor preservation, reappear in the upper Hauterivian. In the Berriasian and but the particular species had a more or less continuous Valanginian calpionellids serve as useful index microfos- range above and below the particular sample the range of sils. Benthic foraminifers are much more facies controlled the species is shown as continuous. and are therefore much less reliable age indicators. A sec- Lower Cretaceous calcareous nannoplankton ondary factor controlling calcareous plankton distribution is assemblages are rich in potentially useful species, especially nearness to shore or a "neritic effect." Nearshore groups related to Retecapsa, Cretarhabdus, and planktic foraminifer assemblages are dominated by hed- Zygodiscus. Studies with the electron microscope bergellids (Douglas, 1972). Neritic calcareous nannop- comparing the moderately well preserved assemblages with lankton in the Cretaceous include nannoconids and better preserved and admirably well described coccolith braarudosphaerids (Roth, 1973, in press; Thierstein, 1976). assemblages from the Speeton and Gault Clay of Great These forms are less reliable age indicators than oceanic Britain (Black, 1971, 1972, 1973, 1975) should provide a nannoplankton groups. more refined biostratigraphy for the Lower Cretaceous. In We presently do not know which physical and chemical the appendix to this paper I briefly discuss some of my factors are responsible for the neritic (nearshore) effect. species concepts for Retecapsa, Cretarhabdus, and some of Higher fertility, greater turbidity, or more variable salinity the species of Cyclagelosphaera. in the neritic environments seem to be the most important 731 P. H. ROTH 15°N 85° 80 65C 60°W Figure 1. Location of Leg 44 drill sites. factors. The neritic environment is often more variable and age indicators because they rarely occur or are absent in the less predictable. Calcareous nannoplankton which can en- much more widespread oceanic deposits. cyst or resort to benthic stages for part of their life cycle are Calcareous nannoplankton assemblages in the sediments generally more successful in unstable nearshore environ- are greatly influenced by preservation. Dissolution at the ments. Planktic organisms which respond rapidly to in- sediment/water interface preferentially removes the more creased fertility are also more common in the nearshore delicate forms (Mclntyre and Mclntyre, 1971; Roth and environment. However, neritic forms are generally poorer Berger, 1975). Diagenetic changes result in secondary cal- 732 CRETACEOUS NANNOPLANKTON cite deposition on the larger crystallites of coccoliths and Bukry and Bramlette (1970); Bukry (1973, 1975, 1976); dissolution of coccoliths mostly composed of smaller crys- Gartner (in Cita and Gartner, 1971); Manivit (1971); tallites (Adelseck et al., 1973; Roth et al., 1975). Thus, Perch-Nielsen (1968, 1969); Thierstein (1971, 1973, 1974, more deeply buried carbonates contain coccolith as- 1976); Verbeek (1976); Roth (1973); and Roth and Thiers- semblages biased toward more robust groups of coccoliths. tein (1972). Some zonal definitions were changed as indi- Secondary calcite overgrowth begins in surface sediments cated by the footnotes on Figure 2. The species used for the (Roth and Berger, 1975), but increases drastically in pelagic definition of zonal boundaries are shown under "biohori- carbonate sediments with more than 150 meters overburden zons." I have introduced a numbering system as a short- (Roth and Thierstein, 1972). Increased clay content in hand notation for these zone names which are taken from hemipelagic deposits seems to retard diagenesis because of long and cumbersome species names. No implication of a reduction in permeability of the sediments. High organic finality is implied by this ordinal sequence. A comparison matter content in anoxic sediments leads to increased pro- of the number of calcareous nannoplankton zones with the duction of CO? by bacteria that disintegrate organic matter, planktonic foraminifer zones listed in van Hinte (1976a) lower the pH, and therefore, increase dissolution. Thus, shows that there are 36 planktonic foraminifer/calpionellid green and black clays generally contain highly dissolved zones for the Cretaceous versus 23 calcareous nannop- coccolith assemblages. This is shown quite clearly by com- lankton zones. Thus the former zonation has slightly better paring the poorly preserved coccolith assemblages from resolution. Calcareous nannoplankton, however, are Hole 391C where mid-Cretaceous sediments were deposited slightly more likely to be incorporated into pelagic sedimen- under anoxic conditions with the much more diverse and tary record because they withstand greater diagenetic better preserved assemblages of the same age in Hole 390 changes than do planktonic foraminifers (Schlanger et al., which were deposited above the oxygen minimum layer. 1973). Most Lower Cretaceous sections recovered by When establishing a nannoplankton zonation, index deep-ocean drilling are dated on the basis of phytoplankton species that fulfill the following requirements should be zonations. In the Upper Cretaceous, planktonic foraminifers selected: — if well preserved — still provide more accurate bios- 1) Species should be oceanic and their distribution con- tratigraphic subdivisions. trolled mostly by surface water temperature; not so much by Paleontologists can distinguish at least 13 calcareous fertility. nannoplankton zones for the last 30 m.y. of the Cretaceous 2) They should have a wide latitudinal distribution. indicating a mean stratigraphic resolution of 2.3 m.y. 3) They should be resistant to preservational changes. In the Aptian-Albian interval, lasting about 15 m.y. five 4) They should have short stratigraphic ranges. nannoplankton zones are recognized resulting in a mean 5) They should be easy to recognize in order to avoid resolution of 3 m.y. Lower Cretaceous (Barremian to misidentifications. Berriasian) nannoplankton zones represent an average of 4 Unfortunately, these criteria are only fulfilled by few m.y. or somewhat longer. The species diversity of groups of Cretaceous coccolithophores. In the Lower Cre- calcareous nannoplankton is low in the Neocomian, taceous the placoliths (Watznaueria, Tubodiscus) and the increases rapidly in the mid-Cretaceous, and remains high genera Retecapsa, Cruciellipsis, Cretarhabdus, Podorhab- throughout the Late Cretaceous (Roth, in press). Therefore, dus, Parhabdolithus, onáLithraphidites fulfill criteria 1,2, much better stratigraphic resolution is possible in the and 3; however, criteria 4 and 5 are not fulfilled for many of

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