Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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Ichnotaxonomy, Paleoecology, Paleoenvironmental Analysis Of Paleogene Yager Complex Turbidite Deposits, Humboldt County, California

Abstract

Paleogene strata of the Yager Complex provide a vivid illustration of the problems encountered with accretionary terrains along continental margins. The complex was likely formed on a large submarine fan at bathyal to abyssal depths. It does not contain any known macro-fossils, which has formerly limited depositional facies analysis. However, trace fossil assemblages in examined turbidite sequences have distinct characteristics of proximal ‘mid-fan’ and distal ‘lower fan’ (terms are used in the broadest sense) depositional environments. Two outcrops, with five recurring ichnoassociations contain eighteen trace fossils, each representing disparate behavioral differences of organisms from these environments. The zonation of trace fossils is used in conjunction with lithologic and structural differences in outcrops to approximate two sedimentary environments. Deep Marine fan environments are often disturbed by strong ‘normal’ bottom currents and high- concentration turbidity currents. Communities respond in characteristic ways contingent on the nature of the disturbance and the prior environmental conditions. The turbidite sequences contain diverse communities of Graphoglyptid trace fossils, distinctive to deep ocean environments, as well as Ophiomorpha and Thalassinoides, facies jumping traces. Trace fossils from characteristic ichnoassemblages occurring in Yager outcrops are compared with theoretical trace fossil zonation schemes for deep marine fan environments. The successive differences in layered ichnoassemblages were studied to characterize the environmental response pattern of organisms in each environment. Abiotic conditions occurring at these outcrops are assessed based on abundance of Ophiomorpha burrows, often bounded to more oxygenated waters than occur in lower fan environments.