Some late eocene depositional environments of the coal-bearing puget Group of western washington State, U.S.A.

Abstract

Plant-bearing sediments of the Puget Group in the area of detailed study accumulated in large flood basins, cutoff meanders, and along channel margins in point bars and crevasse-splay settings. In addition to the plant-bearing deposits, peat-swamp deposits and coarse-grained channel deposits are represented by stacked sandstone bodies and thick coal sequences. The depositional environments of the fluvial-deltaic deposits are distinguished by the presence and distribution of sedimentary structures, grain size, grain size variability, and the densityand layering of organic remains (both plant and invertebrate). The importance of organic matter distribution in the sediments to differentiation of subenvironments was unexpected and may prove to be a valuable sedimentary characteristics in depositional interpretations within a single basin. The sedimentary characterization of environments was made exclusive of the taxonomic composition of the plants preserved and is presented here without paleobotanical interpretation. The separation of depositional interpretation from taxonomic identity of the fossil plants allows an independent assessment of the paleoecology of the plants preserved. Two prominent coal-depositional models are discussed and evaluated in light of these data from the Puget Group. It is concluded that peat accumulation in this system can be attributed most easily to a domed peat-swamp model.