AN INTEGRATED HIGH-RESOLUTION SEDIMENTOLOGIC, ICNOLOGIC, AND GEOCELLULAR MODELLING STUDY OF THE SEGO-NESLEN FORMATION TRANSITION, BOOK CLIFFS, UTAH

The Campanian Upper Sego and Neslen fms, deposited along the complex Cretaceous Western Interior Seaway coastline, were studied in outcrops near Harley Dome in the Book Cliffs, UT. Other studies of the Neslen fm have been either regional in focus or examined other areas. The goal of our study is to provide a high-resolution multi-proxy sedimentologic, ichnologic, architectural, and reservoir modelling study of the transition between the upper Sego and Neslen fms to quantify sandbody-shale geometries and model these highly heterogeneous paleoenvironments.

A composite measured section from East Canyon in the Book Cliffs reveals a series of marine sandbodies with dipping clinoforms (inclined heterolithic stratification) dominated by low angle planar and ripple laminated sandstone with abundant organic matter, lenticular bedding, mud drapes, double mud drapes, mud rip-up clasts, mud chips, and mud lag deposits as well as trough-cross bedded and high angle planar laminated sandstone. These sandbodies commonly overlie coal or organic siltstone and are interbedded with relatively thick packages of continental deposits that include coal, siltstone-mudstone-shale, and paleosols. The succession is capped by trough cross bedded and low to high angle planar laminated fluvial sandstones.

Ichnologic analysis differentiated between continental and marine deposits and clarified that most major sandbodies near the base of the Neslen are marine. Trace fossils in these sandbodies and other marine deposits include Cylindrichnus, Palaeophycus, Schaubcylindrichnus, Ophiomorpha, Planolites, Skolithos, Thalassinoides, Teredolites, Teichichnus, and Rhizocorallium.  

Combined facies, architectural, and ichnologic analyses identified bayhead deltas, estuaries, swamps, tidal channels, tidal bars, crevasse splays, levees, and distributary channels. The succession records interbedded tidally influenced bayhead deltas, channels, and coastal deposits overlain by fluvial coastal plain. A geocellular model will ultimately be generated that can be used as an analogue for these heterolithic complex coastal reservoirs.