Shelf and Deep-Sea Sedimentation in Eocene Forearc Basin, Western Oregon--Fan or Non-Fan?

Abstract

The lower Eocene Tyee and Flournoy Formations in the southern and central Coast Range of Oregon contain unusual associations of shelf and deep-sea facies deposited in an elongate portion of a forearc basin. The sandstone-rich nature and abrupt transition of sedimentary facies suggest that a variation of existing deep-sea fan models is present. The 6,500 ft (2,000 m) thick sequence calls for shelf sandstone virtually cascading into deep water along a line (shelf edge) rather than from a point source (submarine canyon) to feed a sand-rich system. Facies are described in terms of a sand-rich fan system. From south to north these facies include: deltaic, shelf, inner fan, mid-fan, outer fan, and basin plain. The deltaic facies contains coarse-grained, cross-bedded, distributary channel sandstone and interbedded coal. Some prograding deltaic sand extended to the edge of the narrow Eocene shelf and cascaded into the basin. Many complex, nested channels up to 1,200 ft (350 m) wide and 130 ft (40 m) deep were incised into fine-grained shelf and slope sediments at slope depths (line source). These channel and shelf-slope deposits comprise an inner fan facies. A single large canyon or feeder channel (point source) typical of some fan systems was not recognized. Mid-fan facies consist of broad, coalescing channels filled with thick, am lgamated sandstone beds. Sandstone/shale ratios in the mid-fan facies are from 85 to 15 up to 95 to 5. Outer fan facies have sandstone/shale ratios of 60 to 40 and contain well-developed, graded turbidite beds about 5 ft (1.5 m) or less in thickness, interbedded with shale. Basin-plain facies consist of fine-grained strata (sandstone/shale ratios of 30 to 70) deposited at the north end of the basin. These thin-bedded deposits also contain graded fine-grained turbidite sandstones. Paleogeographic reconstructions based on paleocurrent data, facies distribution, and sandstone composition indicate sediment derivation from the pre-Cenozoic Klamath Mountains terrane and a volcanic arc system to the south and southeast. The average sandstone composition is 19% quartz, 27% feldspar, 19% volcanic rock fragments, 7% other rock fragments, and 28% accessories and cement. Paleocurrent data from cross-bedding, and groove and flute marks indicate transport directions from south to north instead of a radially dispersed fan pattern. The stratigraphic succession of the Tyee-Flournoy system indicates a general shoaling trend and a progradation of deltaic facies across a narrow shelf and out over the basin fill during the early Eocene. The predominance of sandstone and confinemen within a narrow basin account for a turbidite facies differing from more familiar mixed sandstone and shale submarine-fan facies models.