--> Upper Cretaceous and Lower Eocene Conglomerates of Western Transverse Ranges: Evidence for Tectonic Rotation, by Walter E. Reed and Robert G. F. Krause; #91024 (1989)

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Upper Cretaceous and Lower Eocene Conglomerates of Western Transverse Ranges: Evidence for Tectonic Rotation

Walter E. Reed, Robert G. F. Krause

Stratigraphic and paleomagnetic studies have suggested that the western Transverse Ranges (WTR) microplate is allochthonous, and may have experienced translational and rotational motions. Present paleocurrent directions from the Upper Cretaceous Jalama Formation of the Santa Ynez Mountains are north-directed; these forearc sediments (Great Valley sequence) contain magmatic arc-derived conglomerate clasts from the Peninsular Ranges in southern California. Paleocurrents in the lower Eocene Juncal and Cozy Dell Formations are south-directed. This juxtaposition is best explained by 90° or more of clockwise rotation of the WTR microplate, so that Upper Cretaceous forearc sediments sourced from the Peninsular Ranges magmatic arc were deposited by west-directed currents. Eo ene sediments were derived from an uplifted portion of the western basin margin and deposited by east-directed currents. Franciscan olistoliths in the Upper Cretaceous sediments indicate deposition adjacent to an accretionary wedge; conglomeratic clasts recycled from the Upper Cretaceous sequence, and radiolarian cherts and ophiolitic boulders in the Eocene strata indicate derivation from an outer accretionary ridge.

The western Traverse Ranges microplate was detached from the continental margin adjacent to the Peninsular Ranges by a boundary fault, along which the microplate translated northwestward and rotated clockwise; the suture with the continental margin is the Nacimiento fault zone. The Santa Maria basin, therefore, is part of the WTR microplate, with a decollement marking the tectonic boundary between the Franciscan basement of the Santa Maria basin and the Great Valley sequence of the Santa Ynez Mountains.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.