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Abstract: Lamination-Scale, Multi-tracer Geochemistry of Organic-Rich Diatomites of the Monterey Formation: Implications for Organic Carbon Richness and Speciation


Although many changes in the Monterey Formation (MF) are manifested as changes in lamination style, most researchers have limited their focus to decimeter-to-meter scales in outcrops and well-based studies. Lamination-scale research permits highest resolution and accuracy of paleoenvironmental reconstruction. We find that differences in ecological, environmental and depositional factors produce distinct laminae and lamina associations. Ongoing research focuses on lamination-scale geochemical variations to understand how changes in organic carbon richness and speciation are related to differing physico-chemical, biological, and diagenetic processes in diatomites of the MF.

We employed novel methods for collecting representative samples from a 59 meter section of organic-rich laminated diatomite at the Celite diatomite quarry, Lompoc, California. Individual laminae, and packets of laminae were subsampled for geochemical analyses: Rock-Eval pyrolysis and elemental analysis (C, N, S, TOC and OC source); opal content, delta{15}N and delta{13}C of organic matter primary production.); XRF and XRD (mineralogy, redox conditions). These analyses enable the integration of lamina-scale changes representative of subannual variations, with outcrop-scale variations attributable to changes over geological time scales.

Initial results show lamina-scale variations in TOC (4.48-1.41%) and biosilica (18-87%) with clay-rich, diatom-lean intervals having the highest TOC. This distribution is not intuitive and may be attributable to 1 ) electrostatic adsorption of OM onto clay particles; 2) porosity-dependent factors such as migration of OM out of diatom-rich laminae and/or the inability of biodegrading enzymes to access clay-rich interstices. Understanding these factors is essential to understanding organic carbon enrichment and primary migration in the MF.

Search and Discovery Article #90945©1997 AAPG Pacific Section Meeting, Bakersfield, California