Cyclic Organic-Rich Sedimentation in the Marca Shale, Moreno Formation and the Onset of Late Cretaceous Upwelling along the California Margin
The Marca Shale Member of the Moreno Formation includes laminated diatomaceous and phosphatic sediments which constitute a suboxic upwelling facies deposited on the slope and outer shelf of the evolving Late Cretaceous margin of California. Sedimentologic observations, gamma ray spectrometry and bulk chemical analysis indicate the presence of 4 short term cycles during deposition of the Marca Shale. Such cycles are defined by the increase of terrigenous sediment into the basin and fluctuations in the amount of biosiliceous productivity with time. Cyclicity in productivity is also reflected by variations in the amounts (TOC) and type of organic carbon (HI) that were successfully preserved and stored in the basinal sediments. The cyclic nature of the Marca Shale suggests that Milankovitch band climatic oscillations influenced circulation during the latest Maestrichtian forcing 'dry' periods with high marine biosiliceous productivity and 'humid' periods with high terrigenous input, low productivity and bioturbated sediments. Periods with high productivity were characterized by high HI values (250-330), low terrigenous content and a large amount of biogenic silica (>30%). Periods with low productivity are marked by lower HI (<250), higher terrigenous content (>80%) and low marine biogenic components. The inverse correlation between detrital components and TOC suggests that organic matter content was 'diluted' as the fluvial input increased in the humid cycles. The hydrogen-rich organic matter, high amount of biogenic silica and diatom assemblage in the Marca Shale, indicate that latest Cretaceous coastal upwelling along the California margin resembles analog systems described from Recent and Neogene upwelling deposits along the Pacific coasts of North and South America.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California