NWEA Speaker Program for 2018-2019

October 25, 2018 Luncheon

The Marine Coaledo Formation: Progress report on an integrated study of an Eocene subtropical shelf-margin delta, Coos Bay, Oregon

John M. Armentrout, Laird B. Thompson, and David Blackwell

The Middle to Late Eocene Coaledo Formation and underlying Beds of Sacchi Beach record a marine history of forearc sedimentation aggrading from slope turbidites to shoreface deltaic sandstone encased in deepwater silty mudstone. This talk is a progress report on a multiyear, multidiscipline research program, testing the hypothesis that the Sacchi Beach-Coaledo succession represents a shelf-margin lowstand deltaic system.  

New insights include:

Confirmation of Dott’s (1966) depositional continuity of Beds of Sacchi Beach slope facies  grading upsection through increasingly sandy prodelta channel complexes into delta front and shoreface sandstones of the Lower Coaledo Formation.
Foraminifera and lithostratigraphic evidence supporting a shelf-margin setting with rapid subsidence accommodating a highy aggradational succession of shoreface sediments capped by a rapidly deepening mudstone ‘cap-rock’.
Radiometric dating of a Middle Coaledo tuff that challenges current regional and global correlations; and
Recognition of probable gravity-slide blocks of wave-worked facies into deeper water prodeltaic channel facies.

Additional contributions to this study include: detailed structural geology reported earlier at NWEA by Laird Thompson; stratigraphic correlations enhanced by a Drone-photo survey of coastal cliffs by Rocky Johnson; reassessment of benthic foraminiferal biofacies by Kristin McDougall; study of new molluscan fossils by Carole Hickman; analysis of shark and ray fossil biofacies by Bruce Welton; and a collaborative effort to reassess the paleomagnetic reversal and rotation history soon to be initiated by a team of Noel and Dave Blackwell with support from Ray Weldon and Scott Bogue.


John M. Armentrout is an Oregon native educated at the University of Oregon and University of Washington, focused on stratigraphy and paleontology. He worked for Mobil Oil Corporation 1973-2000 as an exploration geologist specializing in basin analysis and Petroleum Systems using sequence stratigraphic methods, especially for deepwater exploration plays. Upon ‘retirement’ from Mobil, John formed ‘Cascade Stratigraphic’, consulting 2000-2016 with significant efforts in California’s Great Valley, Suriname and offshore British Guyana and Columbia. He taught Petroleum Geology at the University of Oregon Winter Quarter 2014, 2016 and 2018. His current geologic interest is facilitating the integrated analysis of the Eocene Coaledo Formation near Coos Bay, Oregon.




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