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The Eocene Chuckanut Formation of northwest Washington comprises as much as 6,000 m (20,000 ft) of alluvial strata and is one of the thickest nonmarine sequences in North America. It is exposed in several disconnected outcrop belts that are probably remnants of a regionally extensive fluvial system. Four distinct periods of sedimentation are represented in the main (50 × 20 km, 31 × 12 mi) Chuckanut outcrop belt near the town of Bellingham. These include: (1) early Eocene: rapid sedimentation in west-southwest-flowing fine-load meandering rivers; (2) early-middle Eocene: sedimentation in braided rivers draining northern fault blocks in the western part of the outcrop belt, synchronous with continued fine-load meandering-river sedimentation to the east; (3) middl Eocene: sedimentation in a south-flowing, coarse-load meandering river system in the western part of the outcrop belt, synchronous with continued sedimentation in west-flowing, fine-load meandering rivers of reduced size and competence to the east; and (4) middle to late(?) Eocene: sedimentation in alluvial fans and braided rivers in the eastern part of the outcrop belt draining uplifted pre-Tertiary basement north of the Boulder Creel fault. Following period 4, but still in the Eocene, the Chuckanut was first folded and then truncated by faulting.
It is proposed that the Chuckanut basin formed in an extensional zone of right lateral shear between major strike-slip faults. Consistent with this interpretation are: (1) rapid sediment accumulation rates; (2) rapid facies changes; (3) an irregular basin margin characterized by dip-slip faults and intraformational unconformities; (4) deformation consistent with predicted structural patterns; (5) rapid changes between extensional and compressional tectonics; and (6) interbedded and intrusive relationships with extension-generated(?) volcanic rocks. The Chuckanut basin is considerably larger than most pull-apart basins generally associated with strike-slip faulting, yet shares many of the same attributes. Similar large basins might be found in other continental margins characterized by strike-slip faults.
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