THE GEOCHEMISTRY AND MINERALOGY OF SHALE-HOSTED MIOCENE CONCRETIONS OF THE ASTORIA FORMATION

We conducted geochemical and mineralogical analyses of concretions in a marine shale of the Miocene Astoria Formation in northwestern Oregon. These concretions have three distinct layers that differ in thickness, color and grain size. The rust-colored outer rim or “crust”, 0.5 – 1 cm thick; maybe due to weathering. Beneath is a gray middle layer or “mantle” ranging in thickness from 2-5 cm. The brownish innermost portion or “core” ranges in thickness from 1 – 3 cm. In contrast to the clay-sized particles of the crust and mantle, the core is sandy. Back-scattered scanning electron microscopy (SEM) images and energy dispersive X-ray spectroscopy (EDS) analyses revealed that the core is composed of marine carbonates. Cross-sections of the elongate cores range from square to circular. Generally, the grainy core is the most easily weathered section as opposed to the other sections that more resistant to weathering and abrasion, suggesting that the core differs from the mantle and crust both geochemically and mineralogically.