T168 Initiation and Evolution of Arc-Forearc Systems in Cascadia and Beyond

Tuesday, 12 October 2021
1:30 PM - 5:30 PM
Portland Ballroom 257 (Hybrid Room)

COMBINING ORGANIC PETROGRAPHY, PALYNOMORPH ASSEMBLAGES AND LITHOLOGIC OBSERVATIONS TO RECONSTRUCT THE PALEOENVIRONMENT OF THE COOS BAY BASIN, SOUTHWEST OREGON, USA

Paper number: T168-12
Program: Technical Sessions
Time: 4:20 PM – 4:35 PM

Authors

Abstract

Eight rock formations identified in the Coos Bay Basin on the southwest coast of Oregon are exceptionally well preserved, they include the Sacchi Beach, Coaledo (Lower, Middle and Upper), Bastendorff, Tunnel Point, Tarheel and Empire formations. Lithologic characteristics of these formations, combined with fossil assemblages, indicate an extensive period of marine forearc sedimentation ranging from deep-water prodelta channel facies, overlain by shallower distributary channel complexes and lateral shoreface parasequence cycles. However, the rock formations in Coos Bay have had little to no palynological studies performed over the years, meaning they do not have detailed paleoenvironmental interpretations, and the paleofloral evolution of the region is not well understood in context of these environmental changes. In this study, organic petrography and palynological investigations are performed on 40 samples from Coos Bay Basin to determine the maceral composition and kerogen type of each rock. These findings are then integrated with lithologic interpretations for a more comprehensive geologic reconstruction of the Coos Bay region.

Results from organic petrography analysis indicate that all 40 samples are type III kerogen, and are rich in the humodetrinite maceral, with minor, variable amounts of liptodetrinite and sporonite macerals. This suggests that the majority of sediments from the eight rock units were deposited proximal to the terrestrial paleoshoreline, with influxes of strong to weak marine influences. Based on the palynomorph assemblages, the Middle Coaledo, Lower Coaledo and the Beds of Sacchi Beach formation contain an abundant, diverse assemblage of Eocene dinoflagellates, and therefore exhibit the strongest marine influence out of the eight formations. Lithologic interpretations for the Lower Coaledo suggest a distributary fed shelf with lower shoreface to distal prodelta depositional environments, as suggested by the palynomorph assemblages. For this research, we present the integration of various data and possible depositional models.

Geological Society of America Abstracts with Programs. Vol. 53, No. 6

doi: 10.1130/abs/2021AM-370942

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