Occurrence and estimated abundance of planktonic foraminifers at DSDP Leg 85 holes

Tropical planktonic foraminifers occur throughout the sequences at all sites of Leg 85, and the standard planktonic foraminiferal zonation of Blow (1969) is applicable to most of the recovered sequences. However, the abundance and state of preservation of foraminifers decline markedly in certain intervals because of the effects of dissolution. Although siliceous microfossils studied on this leg indicate recovery of nearly complete records for the Pleistocene to Oligocene interval, the planktonic foraminiferal biostratigraphy is interrupted by strongly dissolved sections at all sites. Particularly, faunas assignable to Zone N7 (early Miocene) and Zone N15-16 (early late Miocene) are almost totally unrecognizable throughout the eastern equatorial Pacific. Well-preserved and diverse planktonic foraminifers occur in the lower middle Miocene, where the evolutionary developments of Orbulina universa d'Orbigny and Globorotalia fohsi Cushman and Ellisor are well represented. The Orbulina first appearance datum is observed to be nearly coincident with the last occurrence level of the diatom Annellus californicus Tempère, thus .establishing an age of 15 Ma for this datum by using the paleomagnetic calibration of the diatom datum. Moderately well-preserved late Eocene planktonic foraminifers occur in the carbonate sediments immediately overlying the basalt basement at Sites 573 and 574. The Eocene-Oligocene faunal transition, however, is masked at both sites by an intercalation of metalliferous layers containing no planktonic foraminifers.

Planktonic foraminifers from Oligocene to Pleistocene sediments of DSDP Leg 92 sites

Leg 92 of the Deep Sea Drilling Project cored sediments containing calcareous microfossils at six sites along 19°S latitude in the South Pacific Ocean. Shipboard examination of these sediments revealed planktonic foraminifers of uppermost Oligocene through Pleistocene age that were identified and assigned to biostratigraphic zones according to the tropical zonation scheme of Blow (1969). Preservation of planktonic foraminifers in the sites from Leg 92 has been affected by the position of each site with respect to the lysocline and calcium carbonate compensation depth (CCD) at the time of deposition, depth of burial, and sediment accumulation rate (rate of burial). An additional factor may also be important, especially in the sediments deposited immediately above basement. Evidence of poor preservation in basal sediments of Holes 600C and 601, which have always been shallower than both the lysocline and the CCD, suggests that hydrothermal solutions circulating within young oceanic crust may penetrate the overlying sediments and affect the preservation of calcareous microfossils deposited there.

Early Cretaceous dinoflagellate cycsts from ODP Hole 123-765C

Results of a preliminary study of Early Cretaceous dinocyst assemblages from Site 765 on the Argo Abyssal Plain, off northwestern Australia, are presented. The palynological sequence is interpreted in terms of Australian zones and is, in descending order, the late Aptian Diconodinium davidii Zone (Cores 123-765C-33R to -39R), the middle to early Aptian Odontochitina operculata Zone (Cores 123-765C-40R to -49R), the Barremian Muderongia australis Zone (Cores 123-765C-50R to -54R), and the Berriasian lower Batioladinium reticulatum Zone (Core 123-765C-59R). The dating of the sequence as late Aptian to Berriasian on the basis of dinocysts is supported, in part, by data concerning associated foraminiferal, radiolarian, and calcareous nannofossil suites.