A Comparative Study of the Sedimentary Organic Matter of the Monterey Formation in Two Different Basins by Compound-Specific Isotope Analysis

Schouten Stefan, Jaap S. Sinninghe Damste, W. Irene C. Rijpstra, Jan W. De Leeuw, and Martin Schoell

An organic geochemical study of two time-equivalent sections of outcrops [Naples Beach (NB) and Shell Beach (SB)] of the Monterey Formation from different basins was undertaken. A similar difference in ¹³C-content of the S-bound C₃₅ hopane (cyanobacteria) and cholestane (algae) was found in the SB section compared to the time-equivalent part of the NB section. This indicates that the differences in the carbon isotopic composition of these compounds is caused by a general phenomenon (an influx of cool, CO₂-rich bottom waters in the Miocene Pacific ocean) and not by local changes. The carbon isotopic composition of dinorhopane in SB sediments varies in a similar manner as that of the dinorhopane of NB sediments though in different time-secti ns, indicating different bottom water conditions in the two basins at different time intervals. Compounds derived from diatoms in SB sediments are isotopically very different compared to those in NB sediments indicating the presence of different species with different physiological behaviour. The carbon number predominance and the carbon isotope data of the free n-alkanes suggests a terrestrial source in NE sediments in contrast to SB-sediments indicating that the terrestrial input in SE sediments was low. The results demonstrate that the depositional environments of two Monterey basins were affected in the same way by global climatic changes but that significant differences in organic matter input occur.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California