Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
SEPM (Society for Sedimentary Geology) Meeting, Grand Junction, Colorado, August 24-28, 2004
Systematic Architectural Trends in a Tidally-influenced Shelf and Shoreline System: Sego Sandstone, Eastern Utah, U.S.A.
The Sego Sandstone (Upper Cretaceous) of eastern Utah and western Colorado is an excellently exposed outcrop where one can view the facies to sequence scale architecture of a tidally-influenced deltaic and shoreline system. Even more impressive are the extensive subsurface well log and core data that exist immediately to the west and north of the outcrop enabling extension of outcrop observations into the subsurface for 30-40 km in the dip direction and over 100 km in the strike direction. These data (outcrop logs, gamma scans and petrophysical data, and subsurface gamma, spontaneous potential, density and sonic logs and whole core) have been used to create a three-dimensional architectural framework for the Sego whose elements clearly document a systematic architectural trend related to relative changes in shoreline.
The Sego is composed of four fourthorder genetic sequences bounded by regional flooding events. These flooding events enable regional correlation westward along outcrop exposures into more proximal facies and northwestward into the subsurface. Four key surfaces; the regressive surface of marine erosion (RSME), lowstand surface (LS), marine flooding surface (MAFS) and maximum flooding surface (MFS) bound the individual systems tracts. Oldest to youngest, sequence one is composed of falling stage and lowstand tidal bar complexes deposited across the RSME and overlain by a LS characterized by estuarine distributary channelization. Sequence two, the maximum lowstand sequence is thinner than sequence one and composed of thick falling stage tidal bar deposits deeply incised by sand-rich lowstand valleys. Sequence three, deposited on the backstepping limb of the third-order lower Sego cycle, is thinner than the previous two sequences and consists of thin tidal bar complexes incised by valleys containing fine-grained heterolithic fills. Transgressive and highstand oyster beds are regionally extensive and pervasively cemented.
The character of lowstand
erosional features in the Sego varies temporally and spatially in a continuum
as a function of changing accommodation space. Sandy distributary channels
characterize the fourth-order lowstand features on the third-order falling
limb, homogeneous-sandy, large valleys characterize the maximum lowstand
of the third-order sequence; and heterolithic valleys characterize the
fourth-order lowstand surfaces on the third-order rising limb. Valleys
average 1-4 km in width and are incised as much as 40 m in depth. Channels
average 200-700 m in width and are incised 2-4 m deep.