Deepwater Fan Complexes in a Forearc Basin - Eocene Tyee Formation
The Tyee Formation in the Coast Range of Southern Oregon is an Early- to Mid-Eocene continental margin succession deposited in a forearc basin. The shelf-margin topsets (exposed mainly in the southern part of the basin) include fluvial, shelf and deltaic deposits, whereas the slope deposits include fine-grained turbidites (silt-mud) and nested slope-channel sandstones, leading down to the stacked basin-floor fan sandstones deposited in bathyal water depths. The basin floor fan component, represented by the Tyee Mountain Member, is a highly sand-prone unit with maximum thickness of 2000m and is distributed over an area of at least 4000Km2 in the northern part of the Tyee Basin. Although it is difficult to establish exact areal dimensions of individual fans, the stacked deep-water fan complexes of Tyee Formation, which were deposited over a period of 2 to 3 million years, are significantly large and appear to have a great volume of coarse clastic sediments. The possible causes of formation of such large fan complexes in the Tyee Basin include a) relatively large sediment yield influenced by Early Eocene greenhouse climate and easily eroded source rocks b) high-relief, tectonically active hinterland for the forearc basin contributing significantly to sediment yield at this time; and c) sustained presence of Tyee supply-system deltas close to the shelf edge (cross-shelf transgressions hindered by absence of high amplitude eustatic sea-level rises during Early Eocene greenhouse period). The proximal parts of the Tyee fan system tends to extend onto the lower slope as aprons, showing distinct, channelized, thick massive sandstone units that might originate from quasi-steady turbidity currents suggesting a close relation between the shelf-edge deltaic supply system and the deep-water fans of the Tyee Formation.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009