29. CALCAREOUS NANNOFOSSIL BIOSTRATIGRAPHY AND SEDIMENT ACCUMULATION OF TURBIDITE SEQUENCES ON THE MADEIRA ABYSSAL PLAIN, SITES 950-952 1Richard W. Howe 2 and JoAnne Sblendorio-Levy 3 |
ABSTRACTDrilling on the Madeira Abyssal Plain during Leg 157 penetrated deep into the sequence of interbedded turbidites and pelagic sediments that underlie the plain. The turbidites generally have abundant and well-preserved calcareous nannofossil assemblages, with the interbedded pelagic sediments having variable preservation and abundance of nannofossils dependent on CaCO3 content. Poor preservation in pelagic sediments from the Paleogene and Miocene necessitated the use of nannofossil datums from turbidites to construct a consistent biostratigraphy. Site 950 records Neogene turbidite deposition of organic carbon-rich, volcaniclastic-rich, and highly calcareous turbidites from different sources around the plain with thin interbedded pelagic red clays, marls, and oozes. A disconformity separates the upper part of Zone CN3 from a single turbidite belonging to Zone CN2, below which a major disconformity separates Zone CN2 from Subzone CP19b, at the top of the Oligocene. Throughout the Oligocene and late Eocene, down to Subzone CP15a, the sequence comprises thin calcareous turbidites and thick pelagic red clays. Sites 951 and 952 only penetrated as deep as Zone CN3 and have similar sequences to the Neogene section of Site 951. Total sediment and pelagic sediment accumulation rates have varied dramatically with time, with low rates in the Paleogene section of Site 950 rising significantly throughout the Neogene. Sites 951 and 952 have much higher rates in the lower and middle Miocene than Site 950, above which all three sites have broadly similar rates. |
|