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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 1003

Last Page: 1003

Title: Sedimentary Facies Within Coastal Belt Franciscan Complex, Garberville Quadrangle, Northern California: ABSTRACT

Author(s): Michael B. Underwood

Article Type: Meeting abstract


Mildly deformed shale and sandstone, exposed along the South Fork of the Eel River in the Garberville Quadrangle (northern California Coast Ranges), are thought to represent a distinct lithofacies of the coastal belt Franciscan complex. These rocks have been correlated with the so-called Yager Formation, exposed farther to the north. Near Garberville, the folded sandy turbidites and shales are probably Eocene or younger in age, and are in contact with three other tectonostratigraphic units: (1) melange of the Franciscan central belt; (2) deformed sedimentary rocks of the coastal belt and King Range, exposed to the west; and (3) younger (Miocene-Pliocene) shelf deposits, which resemble the Wildcat Group of the Eel River basin.

Regional mapping of turbidite facies (using the Ricci-Lucchi scheme) shows that a high percentage of the section consists of hemipelagic mudstone and shale; these facies G deposits are commonly interbedded with thin facies E silty turbidites. Locally, the fine-grained strata pass abruptly into sequences of thick-bedded to massive facies B sandstone and associated facies C turbidites. At one locality, thinner facies D turbidites are abundant within a thickening-upward and coarsening-upward sequence that is over 100 m thick. Small-scale thinning-upward cycles within this mega-sequence suggest localized channel migration and abandonment. The measured orientations of flute casts indicate that paleocurrents radiated toward the west and southwest, apparently at a high angle to the continent l margin.

Together, these data suggest that sediments exposed in the Garberville area were probably deposited within continental slope and restricted basin settings. If the general model associating the Franciscan complex with a Mesozoic-Cenozoic trench wedge is followed, then these strata can be interpreted as sediments deposited on the trench slope and within small trench slope basins above the deformed "accretionary prism" represented by much of the rest of the Franciscan.

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