Depositional environments, basin evolution and tectonic significance of the Eocene Chumstick Formation, Cascade Range, Washington

Abstract

The Chumstick Formation represents a Paleogene, humid-tropical, alluvial-fan system which filled a wrench-fault basin in Washington State. Chumstick deposition was characterized by stream-flow processes; low ($$1 m/k.y.) accumulation of sublithic feldspathic sediments; and extensive vegetation which imparted bank stability even in proximal facies. Coarse-grained fluvial facies near major fault zones consist of laterally stacked braid-bar deposits with an overall sheet geometry. The bulk of the basin fill consists of vertically stacked multistory-channel deposits, sandy overbank deposits, and lacustrine deposits.Paleovegetation studies demonstrate the presence of broad-leaved, evergreen taxa typical of humid-tropical climatic conditions. The Chumstick flora can be grouped according to depositional sites as "upland" or "lowland" forest in either floodplain or channel-margin settings. Paleosols include entisols, inceptisols, (incipient Cca or K horizon development) and histosols. Evidence for periodic (seasonal?) dryness includes xerophytic flora, pedogenic calcretes, and episodically reoccupied fluvial channels with mud-draped trough cross-bedding.Three major phases of deposition can be discerned in the Chumstick Formation. Phase 1 ($>$51 Ma to about 42 Ma) consisted of westward-flowing streams in the western sub-basin, with no evidence for relief in the Leavenworth fault zone (LFZ). Phase 2 (about 42 Ma to 40 Ma) resulted from dextral faulting on all three fault zones. West-derived sediment sources resulted from uplift at two transpressive restraining bends in the LFZ. The eastern sub-basin opened as a transtensional step-over between the Eagle Creek (ECFZ) and Entiat (EFZ) fault zones. Tectonic disruption of drainage resulted in axial drainage systems which flowed SE, parallel to the LFZ and ECFZ, and a phase of internal drainage (lacustrine) in the eastern sub-basin. Phase 3 (about 40 Ma to 38 Ma) shows no relation to faults and may indicate tectonic quiescence. The basin was deformed in a zone of dextral transpression between about 38 Ma and 34 Ma. The stratigraphic thickness of the Chumstick Formation is about 12 km, however thermal maturity and geophysical data suggest an actual basin thickness of about 2 km. This discrepancy in thickness is explained by a conveyor-belt sedimentation model.