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New Insights Into Lamina-Scale Sedimentary and Paleoecological Processes as Recorded in Diatomites from the Miocene Monterey Formation, California

L. D. White1, A. S. Chang2, and K. A. Grimm3
1Department of Geosciences, San Francisco State Univ, San Francisco, CA
2Department of Earth Sciences, Carleton Univ, Ottawa, ON, Canada
3Department of Earth and Ocean Sciences, Univ of British Columbia, Vancouver, BC, Canada

Laminated diatomaceous sediments from the Monterey Formation record the coupling of biological and sedimentary processes on subannual and subseasonal scales. The distinctive fabrics recorded in the laminated facies of the Monterey Formation in the Lompoc quarry not only reflect late Miocene paleoecological and paleoceanographic conditions, but record biologically mediated sedimentary and taphonomic processes as well.

Slabbed samples were imaged using light microscopy, Xradiography, and back-scattered scanning electron microscopy. This has resulted in a new classification of lamina and couplet styles based on couplet bimodality, lamina thickness, compositional domination, lamina spacing, and cyclicity.We recognize five distinct lamina types: (1) detrital laminae that consist of clay, silt, and robust diatoms that were deposited from continental runoff during rainy seasons; (2) thin biosiliceous laminae that consist of moderately preserved high-diversity diatom assemblages deposited during periods of rapid sedimentation; (3) thin biosiliceous laminae consisting of well-preserved monospecific diatom assemblages that likely experienced biologically-induced aggregation and rapid sedimentation without grazing; (4) thick discontinuous diatomaceous laminae consisting of either Thalassiothrix longissima mats or Chaetoceros setae and reflect vertically migrating diatom mats and high productivity, respectfully; (5) macerated biosiliceous laminae, consisting of highly fragmented biosilica from a variety of taxa, reflect intense zooplankton maceration and dissolution of diverse phytoplankton assemblages. Our results illustrate how different lamina types and associations can be used to proxy specific ecological and oceanographic conditions. This study provides a foundation for increased understanding of paleoenvironmental variability recorded by biological event strata in hemipelagic sediments.

AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California