STRATIGRAPHY OF THE MIDDLE MEMBER OF THE COALEDO FORMATION AT SUNSET BAY, COOS COUNTY, OREGON

Over the last century, the Middle Member of the Coaledo Formation has been researched by a small number of specialists. These studies broadly characterized the stratigraphy, topography, and mineral resources of Coos County. In the research my CHS colleagues and I did, we set out to develop a more detailed explanation of the geology of the Coos Bay basin, specifically the Coaledo Formation at Sunset Bay. The strata there are steeply inclined at a strike azimuth of 22 degrees and a dip of 52 degrees. Our study found mostly very fine sandstone and siltstone in which the sedimentary structures include laminations, cross beds, concretions, and load casts. The sequence of strata and sedimentary structures indicate deposition by a turbidite, most likely on the continental slope, an environment conducive to turbidite transport and deposition. On the continental slope sea-floor sediments are loose and turbidites are common. The strata we observed were alternating layers of laminated sandstone and laminated siltstone. The fine grain size of the turbidite indicates a low-density flow. Laminations in the sandstone divisions are parallel, indicating a planar flow regime. In the siltstone beds, laminations are wavy, indicating a turbulent flow regime, which could be the cloud of silt suspended in the water after the turbidity flow has reached the bottom of the slope. In the same strata and surrounding strata we found marine fossils, which also indicate a marine depositional environment. The fossils were mainly fragments but some were fully intact. Among those intact were gastropods and bivalves.