Evaluating the Shelf-Margin Eocene Coaledo Formation, Oregon: an Integrated Palynology, Geochemistry
The Coaledo Research Project is a multidisciplinary geological research effort that includes a team of 18 geoscientists from various research backgrounds, with the goal of reassessing the depositional history of the Eocene age sections of the Coos Bay Basin. Our contribution to this project includes the reconstruction of paleoclimate and paleodepositional history through a combined palynology, organic petrography and geochemistry study.
The Coos Bay Basin on the southwest coast of Oregon is a north-plunging synclinal trough that contains exceptionally preserved Eocene-Oligocene deltaic and shallow marine sediments. Dott (1966) and Dott and Bird (1979) interpreted a steep shelf to shelf-margin paleogeographic setting. The Eocene history of western Oregon includes multiple cycles of delta-fed gravity-flow fans that are separated by structurally enhanced unconformities. One of the targets of this study is the Middle Eocene to Oligocene unconformity bound sequence composed of the deep-water turbiditic Beds of Sacchi Beach and the deltaic to deeper marine Lower and Middle Coaledo Formations. The Coaledo Formation has been roughly dated by molluscs and benthic foraminifera, however, precise ages of this formation, the paleodepositional environments and associated flora have yet to be accurately determined. The new high-resolution sampling and multi-proxy analyses of these sections will be compared to sedimentologic lithofacies, to refine the paleoclimate and depositional models for the Coaledo Formation and Beds of Sacchi Beach.
Preliminary palynology work performed on Coos Bay Basin Cenozoic strata by Hopkins (1967) suggests the bulk of the flora indicate a warm to subtropical environment for the entire Paleogene section, but sampling was sparse and did not capture changes at the parasequence scale. The Lower Coaledo Formation alone is composed of roughly 20 coarsening-upward parasequences. Pollen, spore and dinoflagellate assemblages taken from these parasequences will allow us to identify discrete depositional settings. Paleontologic and magnetostratigraphy data suggest that each parasequence approximates 100,000 year-cycles (Armentrout 1981; Prothero and Armentrout 1985). However, large-scale paleowater depth changes have been attributed to either tectonic deformation or distributary avulsion (Rooth 1974; Ryberg 1978). Another focus of the study will be to evaluate the influence of the Middle Eocene Climatic Optimum (MECO, ~40 Ma) on the depositional system. Preliminary analyses of 10 samples selected from the Beds of Sacchi Beach and the Lower and Middle Coaledo Formations indicate a decent recovery of mixed kerogen types including both pollen and spores, and algae of both lacustrine and marine origin. Results so far indicate that samples are mostly of marine origin with varying degrees of fluvial influence.
Additionally, our study will combine the sedimentologic lithofacies with organofacies characterization. This approach will allow us to cross reference variations in kerogen types, thermal alteration and organic content (TOC%) with paleodepositional modeling of the Coaledo Formation. By creating an organofacies profile of the Coaledo, we can refine the shelf-margin hypothesis, estimate burial depths of the Formation and hypothesize the hydrocarbon yield potential of the Coaledo Formation. Organic petrographic analyses will allow further characterization of the kerogen into their maceral components, and will enhance the interpretations of paleoenvironment. Vitrinite reflectance will provide maturation data which will allow for calibration of the thermal alteration indices from pollen/spores, and confirm estimated burial depths.