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Sedimentary processes and paleoenvironmental significance of laminated diatomaceous sediments from the miocene monterey formation, California, USA Chang, Alice S.


Diatomaceous sediments from the Miocene Monterey Formation in Santa Barbara County, California, were investigated to gain a better understanding of sedimentary processes, diatom paleoecology, and the paleoenvironmental significance of laminated organic-rich hemipelagites. Sedimentary couplets and individual laminae from laminated intervals were described and classified to assess paleoenvironmental settings and interannual processes of hemipelagic sedimentation. Speckled beds, a type of non-laminated interval, were studied to understand physical properties of laminated diatomaceous sediments and slope failure processes. We utilized field observations, hand sample descriptions, x-radiography, optical microscopy and scanning electron microscopy to examine the composition and interpret the origins of laminae and speckled beds. Laminated intervals, sedimentary couplets and individual laminae were classified on the basis of bimodality, thickness, mud-diatom domination, spacing, and cyclicity. Five main lamina types were defined, based on composition and microfossil content. These laminae were termed detrital, thin biosiliceous, thick continuous diatomaceous, thick discontinuous diatomaceous and macerated biosilica. We focused on diatom paleoecology and biologically mediated sedimentary processes to interpret the origin of laminae and distinct couplet types, concluding that various diatom blooms—including selfsedimentation of conspicuous blooms and diatom mats—preserve a unique and high resolution paleoenvironmental record. A study of distinct, non-laminated intervals termed speckled beds gives insights to the physical properties of organic-rich hemipelagites. Speckled beds are sharply based and are abruptly overlain by undisturbed laminated diatomite. The speckled fabric is derived from discrete detritus-rich and diatomrich aggregates ranked in size and termed speckles, blebs and lozenges. Four types of speckled beds were classified: unimodal, bimodal, amalgamated, and speckled beds with laminated intraclasts. Failure of cohesive laminated sediments deposited on a slope within the oxygen minimum zone is prerequisite for speckled bed formation. We postulate a gravity flow hypothesis that describes the deposition of speckled beds from viscous, high-density turbidity flows. A thorough understanding of the processes by which individual lamina types and speckled beds are produced provides refining insights into Miocene paleoenvironments. The approaches developed in this study can also be applied to other subannually deposited hemipelagic sediments accumulating beneath productive coastal upwelling regions.

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