Eocene-Oligocene diatoms in the western Indian Ocean

The occurrence of diatom species in the Eocene-Oligocene sections of Ocean Drilling Program (ODP) Leg 115 sites and Deep Sea Drilling Project (DSDP) Sites 219 and 236 in the low-latitude Indian Ocean are investigated. Diatoms are generally rare and poorly preserved in the Paleogene sequences we studied. The best-preserved assemblages are found close to ash layers in early Oligocene sediments. The low-latitude diatom zonation established for the Atlantic region by Fenner in 1984 is fully applicable to the Paleogene sequences of the western Indian Ocean. Correlation of the diatom zones to the calcareous nannofossil stratigraphy of the sites places the Coscinodiscus excavatus Zone of Fenner within calcareous nannofossil Subzone CP16b. For the Mascarene Plateau and the Chagos Ridge, the times when the sites studied, together with the areas upslope from them, subsided to below the euphotic zone are deduced from changes in the relative abundance between the group of benthic, shallow-water species and Grammatophora spp. vs. the group of fully planktonic diatom species. The Eocene section of Site 707, on the Mascarene Plateau, is characterized by the occurrence of benthic diatoms (approximately 10% of the diatom assemblage). These allochthonous diatoms must have originated from shallow-water environments around volcanic islands that existed upslope from ODP Site 707 in Eocene times. In Oligocene and younger sediments of Sites 707 and 706, occurrences of benthic diatoms are rare and sporadic and interpreted as reworked from older sediments. This indicates that the area upslope from these two Mascarene Plateau sites had subsided below the euphotic zone by the early Oligocene. Only Grammatophora spp., for which a neritic but not benthic habitat is assumed, continues to be abundant throughout the Oligocene sequences. The area of the Madingley Rise sites (Sites 709-710) and nearby shallower areas subsided below the euphotic zone already in middle Eocene times, as benthic diatoms are almost absent from these Eocene sections. Only sites located on abyssal plains, and which intermittently received turbidite sediments (e.g., Sites 708 and 711), contain occasionally single, benthic diatoms of Oligocene age. The occurrence of the freshwater diatom Aulacosira granulata in a few samples of late early Oligocene and late Oligocene age at Sites 707, 709, and 714 is interpreted as windblown. Their presence indicates at least seasonally arid conditions for these periods in the source areas of eastern Africa and India. Three new species and two new combinations are defined: Chaetoceros asymmetricus Fenner sp. nov.; Hemiaulus gracilis Fenner, sp. nov.; Kozloviella meniscosa Fenner, sp. nov.; Cestodiscus demergitus (Fenner) Fenner comb, nov.; and Rocella princeps (Jouse) Fenner comb. nov.

Calcareous nannoplankton of DSDP Leg 43 holes

During Leg 43, six holes (Sites 382-387) were drilled in the western part of the North Atlantic Ocean; locations of sites are shown in Figure 1. Lower Cretaceous to Quaternary calcareous nannofossils were found in 127 of 189 cores recovered during the leg. The ages and zonal assignments of these fossiliferous cores based upon light-microscopical observation are given in Table 1. An almost continuous succession of nannofossil assemblages of the lower Maestrichtian to upper Paleocene is present at Site 384. A detailed investigation was conducted on samples at this site, and the evolution of approximately 50 species is documented through almost the entire Paleocene epoch.

Calcareous nannofossil abundances in Mesozoic sediments of ODP Leg 129 holes

Calcareous nannofossils were studied from Jurassic and Cretaceous sediments drilled in the western Pacific during Ocean Drilling Program Leg 129. Mesozoic sediments at Sites 800, 801, and 802 are dominated by volcaniclastic turbidites, claystones, porcellanites, and radiolarites. Pelagic limestones are limited to the middle Cretaceous, and a few calcareous claystones were recovered in the Upper Jurassic section at Site 801. We documented the distribution of nannofossils, their total abundance, preservation, and relative species abundance based on semiquantitative and qualitative studies. Preservation of the calcareous nannofloras is poor to moderate, and the total abundance fluctuates from rare to very abundant. Marker species proposed for the middle and Late Cretaceous were recognized, allowing the application of standard nannofossil biozonations. At Site 800 calcareous nannofloras are abundant and moderately preserved in the Aptian-Cenomanian, and nannofossil biostratigraphy constitutes the basic stratigraphic framework for this interval. Radiolarians are the most abundant and persistent group throughout the sequence drilled at Site 801. Long intervals are barren of nannofloras and assemblages are usually characterized by low abundance and poor preservation. Nannofossil biostratigraphy was applied to the upper Aptian-Cenomanian interval and a few marker species were recognized for the late Tithonian. At Site 802 Cretaceous biostratigraphy is mainly based on calcareous nannofossil biozones corroborated by radiolarian and palynomorph events in the late Aptian-Coniacian age interval. A hiatus was indicated between the Santonian and the late Campanian, and another is suspected in the interval between the Cenomanian and the Coniacian.

Distribution and abundance of nannofossils in DSDP Site 89-585

Late Aptian through middle Eocene nannofossil assemblages were recovered from a continuously cored section at Site 585. Poorly preserved assemblages of low diversity were observed in samples taken throughout both upper Aptian and/or lower Albian sandstone and mudstone and middle Cenomanian to lower Turonian claystone at the base of this section. A 70-m interval barren of nannofossils separates these poorly preserved assemblages from those recovered from an upper Campanian chalk farther uphole. This chalk marks the most significant change in carbonate deposition at this site, and deposition of interbedded zeolitic claystone and sediment of varied nannofossil content proceeded without major interruption until the early Paleocene (Fasciculithus tympaniformis Zone, CP4). A middle Eocene chalk (dated by nannofossils) unconformably overlies lower Paleocene sediment in both Holes 585 and 585A. Only a few interbeds of zeolitic claystone are present within 100 m of nannofossil-rich sediment above this unconformity. This entire interval is cautiously assigned to the Discoaster sublodoensis Zone (CP 12), which indicates a sedimentation rate almost an order of magnitude higher than expected from normal pelagic sedimentation. The most obvious feature of the assemblages examined from these cores is the amount of reworked material. Rare Nannoconus elongatus and Braarudosphaera sp. in several upper Campanian to middle Eocene samples demonstrate the contribution of pelagic material from upslope and, along with other reworked species throughout the Upper Cretaceous samples examined, provide evidence contradictory to an excursion of the calcium compensation depth to deep basinal settings in the western Pacific during the Campanian-Maestrichtian time (Thierstein, 1979). The overwhelming dominance of reworked species in all middle Eocene samples examined and the persistence of these assemblages throughout such a large thickness of sediment suggest that currents that redeposited material intensified at this time and may be associated with the formation of the lower Paleocene/middle Eocene unconformity at this site. A single surface core of calcareous ooze taken from Hole 585A dated as early Pleistocene contains abundant and well-preserved late Miocene and Pliocene species.