Abstract

The middle Eocene Tyee Formation records the deposition of a sandstone-rich deep-water system in a fore-arc basin. Poor outcrop exposures and a lack of high-resolution biostratigraphic data cloud the details of correlation within the Tyee Formation. However, the broad outline of the Tyee deep-water depositional system is clear. In the south, deep-water channels cut into what we interpret as slope mudstones. Farther to the north and down depositional dip, there is a transition over about 20 km (12 mi) from these channel-fill deposits to highly amalgamated, tabular sandstone bodies that we interpret as proximal-fan deposits. About 100 km (62 mi) to the north of these slope systems, poorly amalgamated sandstones with interbedded mudstones that we interpret as distal-basin-floor-fan deposits are found. Given that the Tyee Formation is more than 1 km (0.6 mi) thick, it is unlikely that it represents the deposits of one submarine fan, but rather the deposits of multiple sandstone-rich fans that are stacked in a progradational pattern.

The distal-basin-floor-fan deposits located west of Corvallis, Oregon, are the focus of this paper. These rocks are significant because they show an excellent strike exposure through such a deposit. Moreover, the facies seen in these deposits are different from those described in many classic models of distal fans (e.g., Mutti, 1977) in that they contain a dearth of thin-bedded, ripple-laminated sandstones. Instead, they are composed of thicker bedded, massive sandstones and mudstones. Many of the massive sandstone beds are capped by muddy, organic-rich sandstones that we interpret as slurry deposits (sensu Lowe and Guy, 2000), a facies that has been described as forming in waning-flow conditions.