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Joint Meeting Pacific Section, AAPG & Cordilleran Section GSA April 29–May 1, 2005, San José, California

Turbidite, Densite, Debrite Or Gravite? Phytoclasts Can Help

Martha A. Woodward
Geology Department, Humboldt State Univ. & Scotia Pacific Co, P.O. Box 712, Scotia, CA 95565, [email protected]

Transportational and depositional environments as evidenced by phytoclast accumulations are compared with existing models of turbidite deposition to define the dynamic offshore setting of late Cretaceous-early Paleogene northwestern California. This study examines phytoclast concentrations within three depositionally related turbidite units of northwestern California: the Yager terrane, Yolly Bolly terrane and the Franciscan Central Belt Mélange.

Although literature on phytoclasts is abundant, studies of phytoclast deposition in the deep-sea are limited. A better understanding of the transportational and depositional history of turbidites is facilitated by an analysis of phytoclast concentrations. SEM images of phytoclasts are compared with images of in situ fossil wood fragments of the shallow marine Miocene Saint George Formation. This comparison illustrates degradation of phytoclast structural tissue resulting from transportation and subsequent processes. Details of flow events are captured in individual phytoclast accumulations. Preferred orientations of relatively long phytoclasts indicate possible flow direction. An examination of phytoclast laminae relative to coarse and fine grained sediments illustrates the usefulness of phytoclast laminae in the determination of upward direction in turbidite stratigraphic sections. Random orientations of phytoclasts adjacent to mudstone rip up clasts support debris flow origins for portions of these deposits. As sedimentology attempts to narrow the definition of turbidite and incorporate flow mechanics and rheology into more specific classifications such as densites, debrites and gravites, the importance of the phytoclast component should not be overlooked.

Posted with permission of The Geological Society of America; abstract also online (http://gsa.confex.com/gsa/2005CD/finalprogram/abstract_85804.htm). © Copyright 2005 The Geological Society of America (GSA).