CLARK, MICHAEL S., ARCO Oil and Gas Company, Bakersfield, CA

ABSTRACT: Depositional Controls on Clay Mineral Abundances in Slope and Basin-Floor Submarine Fans

Shales of Mesozoic deep-sea submarine fans exposed on the west side of the Sacramento Valley, California have clay mineral assemblages that are functions of the original depositional environment, although these assemblages may be modified or replaced during burial metamorphism and weathering. The ratios of fast-settling to slow-settling clays in shales frominferred original burial depths <6.7 km characterize specific sedimentary facies. For example, chlorite, a fast-settling clay, increases whereas smectite, a slow-settling clay, decreases in sand-rich lowstand fans from overbank to channel-fill to depositional lobe facies. Mud-rich fans (the basin plain facies) are chlorite-rich, whereas condensed sections are smectite-rich.

The clay distributions probably result from "flow-stripping" of channel-confined turbidity currents. During downslope movement, slow-settling clays concentrate in the upper portions of flows whereas fast-settling clays concentrate in the lower portions. As flows collide with channel walls, suspensions of slow-settling clays are "stripped" from the tops of flows and deposited in overbank environments. Thus, the fast-settling clays remain in the channels and are deposited as channel fill facies or, after additional flow-stripping, as distal unchannelized facies.

Shales subjected to deep burial (>6.7 km), irrespective of sedimentary facies, are illite-rich. Intensely-weathered shales, irrespective of sedimentary facies and burial depth, are smectite-rich. Thus, the depositional clay assemblages are preserved during eogenesis, but replaced by illite-dominated assemblages during deep burial (mesogenesis), and altered to smectite-dominated assemblages during uplift and weathering (telogenesis).

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.