From Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.

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.

Lesli Wood

Abstract:

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 fourth–order 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.
Tidal bar show a systematic change in their geometry as you move basinward or landward, and as you move between regressive and transgressive depositional packages. Tidal bars developed in the slightly more distally located eastern outcrop window of the Sego study area show a taller geometry than those developed in the western outcrop window of the study area. Those bars developing during the falling stage of the Sequence 2 have a much broader geometry than those deposited during the falling stage of Sequence 1. Shallower water depths and increasing sand volumes likely resulted in the broad nature of the shelf tidal bars. In contrast to tidal bars developed in the falling stage systems tract, those developed during transgression are dominantly confined to the incised estuarine valleys and show a much narrower, confined geometry.