Charles D. Howell1, Janok P. Bhattacharya1, Adam B. Robinson2
(1) University of Texas at Dallas, Richardson, TX
(2) University of Texas at Dallas
ABSTRACT: Scales of Facies Architecture in Top-truncated Lowstand Delta Lobes, Upper-Cretaceous Wall Creek Member, Frontier Formation, Powder River Basin, Wyoming, U.S.A
The Turonian Wall Creek Member has been interpreted as being deposited as an offshore bar, as storm deposited prodelta sand sheets, and as a wave-dominated delta. An outcrop investigation was initiated in order to evaluate these varying hypotheses. Fifty-nine vertical measured sections, including outcrop gamma ray scintillometry, were collected over a 900 square kilometer area.
The Wall Creek shows a mixture of different upward coarsening facies successions associated with distinctly different overlapping sandstone bodies separated by mudstones. These are interpreted as different delta lobes. The southern lobe grades upward from burrowed, to current rippled sandstones and mudstones into structureless to flat stratified and ripple cross-laminated sandstones interpreted as delta front turbidites. Photomontages show inclined beds dipping southeast, in the same direction as paleocurrents, suggesting that they are delta front clinoforms.
Farther north, dune-scale cross-stratified sandstones cap upward coarsening facies successions. Abundant mud drapes suggest tidal influence. To the east, the Wall Creek shows sharp-based mud-free cross-bedded sandstones, suggesting a wave-dominated shoreface. Pebbles occur at the sharp base and may indicate a forced regression. Pebbles also occur at the tops of the inclined beds, which are also truncated. We therefore interpret the Wall Creek as representing top-truncated lowstand delta deposits.
We identify several scales of facies variability within the Wall Creek. Variability of different sand bodies with length scales of several kilometers to tens of kilometers represents large-scale reservoir compartments. These compartments show varying internal reservoir heterogeneity, depending on the proportion of river-, tide- and wave-influence within lobes.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado