Recent Advances in Neogene Biostratigraphy of Los Angeles Basin

Gregg H. Blake

The stratigraphic record associated with continental margin basins represents, in varying degrees, the dynamic nature of the oceanographic, sedimentary and tectonic evolution of individual basins. With the high interest in the present and potential petroleum reserves contained in these deposits, there has been renewed interest in basinal correlations between known producing formations.

The Los Angeles basin is one of the most prolific basins in California. It is one of the Neogene basins along the California continental margin that was formed by tensional mechanisms related to complex wrench tectonism. These tensional mechanisms caused the pull-apart tectonics which resulted in rapid deepening of many of the southern California margin basins. Deposition in these basins was controlled by local and regional tectonics, in conjunction with climatic and oceanographic events of the late Tertiary.

For the last 60 years, the biostratigraphic framework for correlation between the oil fields of the Los Angeles basin has been based on benthic foraminiferal assemblages. These assemblages were used to correlate the Neogene clastic reservoirs across the basin. The reservoirs range in age from late middle Miocene to late Pliocene.

The basic problem with intrabasinal correlations, based on the appearance and/or disappearance of benthic assemblages, is that the occurrence of the assemblage within a stratigraphic sequence may reflect the effect of local environmental variations rather than evolutionary changes. However, with the understanding of the tectonic, sedimentologic, and oceanographic history of the clastic reservoirs, correlations can be established using the benthic foraminiferal assemblages which will provide stratigraphic correlations in addition to environmental interpretations. In addition, with the recent utilization of other microfossil disciplines, e.g., siliceous microfossils and calcareous nannoplankton, a more refined biostratigraphy is possible for the reservoirs across the basin.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.