ABSTRACT
The Tyee and Coaledo Formations of southwestern Oregon contain lateral and vertical transitions of delta to shelf facies within an Eocene forearc basin. Fluvial, marsh-swamp, delta-distributary channel, delta-front, prodelta-shelf, and delta-margin facies (wave-reworked deposits) are identified. Coarse-grained, plant-rich sandstones characterized by large-scale, contorted cross-bedding and northwest-trending paleocurrent indicators suggest fluvial contributions (during river-flood stages) within delta-distributary channels. In marine deposits, abundant hummocky cross-stratification, flat laminations, and symmetrical ripples indicate dominance of wave processes.
Two types of vertical shoaling sequences are distinguished by their capping lithology. Type One sequences formed in moderate wave energy and are capped by delta-distributary sandstones. Type Two sequences are capped by delta-margin facies and indicate reworking during higher wave-energy conditions. Both successions record variations in shoreline progradation, basin subsidence, sea level change, or a combination of these processes.
Wave-dominated deltaic deposition was a persistent style of Cenozoic sedimentation in western Oregon. This study may be useful for predicting facies relationships and geometries in similar Eocene systems, such as the gas-producing Cowlitz Formation (Mist field, northern Oregon) and may enable a better assessment of hydrocarbon accumulations in the Pacific Northwest.
