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The Eocene Tyee and Coaledo Formations of southwest Oregon together illustrate vertical and lateral facies changes in delta-front and prodelta-to-shelf deposits, which prograded into a forearc basin. Prevalent coarse sandstone bodies with wood also contain large-scale, contorted cross-bedding; they show north-northwest paleocurrent transport directions. An abundance of hummocky stratification and symmetrical ripples in the prodelta and shelf sandstones indicate the dominance of wavy processes.
Conglomeratic, coaly, cross-bedded coarse sandstone, hummocky-bedded, and siltstone-mudstone facies are distinguishable in the Eocene strata. These five well-defined facies are interpreted to represent fluvio-deltaic, marsh-swamp, delta-front distributary, prodelta, and shelf deposits, respectively. Burrowed medium sandstone and shelly coarse sandstone facies in the Coaledo Formation are inferred to represent delta-margin deposits. Delta-front and prodelta deposits show soft-sediment deformation features, which indicate rapid deposition during erratic episodes of river flooding. Hummocky cross-stratification and burrowed beds with truncated tops indicate episodic storm events in the prodelta and shelf deposits.
Vertical successions in the Coaledo Formation typically show siltstone-mudstone facies at the base, overlain by the hummocky-bedded facies, and capped by coarse delta-front or delta-margin deposits. These upward-coarsening or progradational sequences number at least 15 in the Coaledo Formation alone, and indicate a complex history of delta-distributary switching coupled with subsidence and/or eustatic changes.
Wave-dominated deltaic deposition was a repetitive and persisting style of sedimentation throughout much of the Eocene period in southwest Oregon. This scenario is useful for distinguishing vertical and lateral relationships applicable to other ancient wave-dominated systems and to predictions in potentially economic units of the Pacific Northwest.
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