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Eocene right-lateral displacements occurred on several fault zones in western and central Washington, including the Straight Creek fault (90-190 km or 56-118 mi of offset), the Entiat-Leavenworth fault system (amount of offset unknown), and possibly a major unnamed north-south trending fault through the Puget lowland. Within this framework, nonmarine sediments accumulated in several rapidly subsiding basins to form some of the thickest (more than 5,800 m or 19,000 ft) nonmarine sequences in North America. Two types of sedimentary basins are recognized. The Chiwaukum and Foss River grabens are small pull-apart basins that formed along the major faults. The Chuckanut, Swauk, and Puget(?) basins are much larger (up to 100 km or 62 mi wide) and formed between the major faults To varying degree, these larger basins display the characteristics of idealized smaller pull-apart basins: (1) high sediment accumulation rates; (2) rapid facies changes; (3) abrupt stratigraphic thickening and thinning; (4) partly internal drainage patterns; (5) irregular basin margins characterized by dip-slip faults and unconformities; (6) predicted deformational patterns; (7) rapid changes between extensional and compressional tectonics; and (8) interbedded and intrusive relationships with extension-generated(?) volcanic rocks. The difference in size and mode of occurrence between these basin types emphasizes the regional as well as local control that strike-slip faulting has on basin formation.
This extensional-basin province formed in a forearc setting between an obliquely subducting oceanic plate to the west, and the broad, diffuse Challis volcanic arc to the east. Eocene nonmarine basins in Washington should therefore be considered as end-member types of forearc basins.
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