AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Shelf-Slope Sedimentary Prism Development and Changing Deepwater Sand Distribution Pattern: Eocene Tyee Forearc Basin, Oregon
(1) Department of Geological Sciences, The University of Texas at Austin, Austin, TX.
(2) ExxonMobil Upstream Reseach Company, Houston, TX.
Deepwater deposits of Eocene Tyee Forearc Basin in Coast Range of Oregon records distinct change in distribution pattern of sandy turbidites associated with progressive development of a shelf-slope sedimentary prism at an active continental margin. Development of the Tyee Forearc Basin can be related to complex interaction between the subducting oceanic Farallon Plate and continental North American Plate. Tyee succession includes a wide range of continental, shallow marine, deepwater slope and basin-floor deposits. Siletz River Volcanics, which represents the basement for Coast Range of Oregon, has been interpreted as an oceanic terrane that got accreted to the North American Plate following an arrest in subduction in early Eocene and stepping out of the subduction zone to the present day outer shelf off Oregon coast. Magnetic data shows that the Siletz Terrane (along with the Tyee Forearc succession) experienced a clockwise rotation of greater than 60 degrees since early Eocene. At present Tyee Basin has a north-south orientation with continental deposits exposed in southern Tyee Basin and deep-water fan exposed in the northern part. Published paleocurrent data from turbidite deposits of Tyee Basin indicates an average south to north (in the present-day configuration) transport of sediment along the basinal axis. However, new paleocurrent data from continental and shallow marine deposits in the southern half of Tyee Basin indicates major sediment input transverse to the basin axis (from east). Detailed outcrop data along several strike- and dip-oriented transects in central and southern Tyee basin reveals varying sand distribution pattern at different stages of development of Tyee margin that can be related to increasing length and height of the Tyee shelf-slope sedimentary prism. At an initial stage turbidite deposits occur as thick sandy slope aprons exposed in southern Tyee Basin (described as ramp deposits in an earlier work). With the development of a larger shelf-slope prism at a later stage, well defined, large, deepwater slope channel systems developed on the Tyee slope that trapped coarse sediment in linear channel belts and supplied large volume of sand to the basin floor, forming extensive deep water fans. Increased wave influence observed in younger shallow marine deposits of the Tyee succession, as opposed to strong river and tidal influence in older shallow marine deposits, also indicates development of a wider shelf at a later stage.