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R.J. Moiola1, J.E. Welton1, J.B. Wagner2, L.B. Fearn1, M.E. Farrell1, R.J. Enrico1, R.J. Echols1
(1) Mobil Exploration & Producing Technical Center, Dallas, TX
(2) University of Texas at Dallas, Richardson, TX

Abstract: Integrated analysis of the Ferron deltaic complex, Utah

The deltaic Upper Ferron Member (Turonian Ferronensis sequence of Gardner, 1995) of the Mancos Shale in Castle Valley, Utah has been subdivided into 5 to 7 stratigraphic cycles by various workers(e.g. Ryer,1981; Gardner,1993; Barton,1997). These cycles consist of seaward-stepping, aggradational, and landward-stepping successions that are thought to be bounded by regionally extensive marine flooding surfaces. Both river-dominated and marine- dominated cycles have been recognized.

Detailed sedimentologic, petrologic, and biostratigraphic analyses of outcrops and cores, however, indicate that the stacking pattern of the Ferron complex is controlled by autocyclic lobe-switching and by a decrease in sediment supply. In particular, multi-disciplinary biostratigraphy (i.e. palynology, micropaleontology, etc.) has shown that "flooding surfaces" only exhibit a brackish-water signature, and the abundance of distributary channels associated with all cycles suggests that the entire Upper Ferron is a river- dominated system that was subordinately influenced by marine processes. True marine flooding events are associated only with shales in the underlying Lower Ferron and overlying Blue Gate Members of the Mancos Shale.

Rock physics and seismic modeling of the Upper Ferron have demonstrated that its seismic response is strongly dominated by large acoustic impedance contrasts associated with coals/carbonaceous shales in the complex, in spite of the fact that many of the coals are thin and well below tuning thickness. Distributary channels, which are also below seismic tuning, show up as amplitude dim-outs.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana