Graduate Thesis Or Dissertation
 

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  • Fossil foraminifers from the Coaledo and Bastendorff Formations near Coos Bay in southwestern Oregon were studied in an attempt to determine the age and environment of deposition. Fresh unweathered samples were collected from sea cliffs between Cape Arago and Charleston, Oregon. A large late Eocene delta is preserved in the rocks near Coos Bay, Oregon. The interpretations of previous workers, that the sandstones of the Coaledo Formation represent regressive offlap and the mudstones of the middle Coaledo and Bastendorff are the result of transgressive onlap of late Eocene seas, remain unchanged. The number of known species of fossil foraminifers from the Coaledo and Bastendorff Formations has been increased as a result of this study from 46 species to 148 species. The number of known planktonic species reported from the rocks of the region has been increased from 13 species to 28 species. The Coaledo Formation is middle late Eocene in age and is correlated in part with the Uvigerina garzaensis Subzone of the Bullmina corrugata Zone and in part with the Amphimorphina jenkinsi Zone of the standard West Coast Narizian Stage. The Bastendorff Formation has been considered in previous studies to be partly Eocene and partly Oligocene in age. Study of benthic foraminifers indicates that the Bastendorff contains a Uvigerina cocoaensis fauna, which can be correlated with the lower part of the Refugian Stage of the Tertiary of California and with the late Eocene Cocoa sandstone of the Gulf Coast. Planktonic foraminifers from the Bastendorff are entirely late Eocene in age and are correlated with confidence with the Runangan Stage of New Zealand and tentatively with the P17 (=Globigerina gortani- Globorotalia (T.) centralis) partial range zone of tropical regions. The lower part of the Refugian Stage of the West Coast is considered here to be late Eocene rather than early Oligocene in age. The entire Bastendorff Formation is considered to be late Eocene in age. The mudstones and siltstones of the middle Coaledo and Bastendorff have been previously interpreted as having been deposited in a shallow marine bay or upon the continental shelf. The present study of the fossil foraminifers of these formations has indicated that deposition of the middle Coaledo probably occurred at upper bathyal to lower neritic depths and that the Bastendorff Formation was deposited at lower bathyal to upper abyssal depths. High numbers of fossil radiolarians and planktonic foraminifers found in samples from the Bastendorff Formation suggest that deposition occurred in a region influenced by an open-ocean, mid-latitude water-mass, rather than on the continental shelf or in a restricted embayment. The presence of a normal marine, deep-water microfauna in mudstone units, located stratigraphically between non-marine deltaic sandstones, implies that strong vertical movements occurred during and immediately following the deposition of the Coaledo delta. The idea of a simple lateral shifting of a prograding deltaic complex is not supported by the study of benthic foraminiferal faunas. Vertical movements in southwest Oregon in late Eocene time may have been the result of underthrusting of an oceanic plate beneath the North American plate. The stratigraphic position, the sequence of sedimentary facies, and the occurrence of unique sandstone-filled channels with mudstone breccias all indicate that the beds at North and South Coves of Cape Arago should be correlated with similar strata of the Elkton Formation five miles south at Sacchi Beach rather than being considered a part of the Coaledo Formation.
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