--> Abstract: Comparison of Shoreface Stratal Architectures in the Mesaverde Group (Upper Cretaceous) Northwest Colorado: Impact of Varying Transgression Rates on Progradational Trajectories, by Tara L. Benda and J. Michael Boyles; #90039 (2005)
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Comparison of Shoreface Stratal Architectures in the Mesaverde Group (Upper Cretaceous) Northwest Colorado: Impact of Varying Transgression Rates on Progradational Trajectories

Tara L. Benda1 and J. Michael Boyles2
1 ConocoPhillips, Houston, TX
2 Enhanced Oil Recovery Institute, University of Wyoming, Laramie, WY

Upper Cretaceous Iles and overlying William Fork Formations, Mesaverde Group, of northwest Colorado, are third-order clastic wedges composed of fourth-order progradational parasequence sets (Twentymile and Trout Creek Sandstones). Outcrop and subsurface work (100 km long regional Previous HitdipNext Hit line and several Previous HitstrikeNext Hit lines) document that higher frequency parasequences within these sequences were deposited in similar depositional settings (wave-dominated shorelines), however, the shoreface sandstones vary in thickness from 10 to over 50 m. Both thin units and thicker units stepped similar distances into the basin (~100 km) and both have progradational trajectories.

Anomalously thick shoreface sandstones (up to 50 m) were generated by vertically stacking several high frequency parasequences with progradational shoreline trajectories. Aggradational stacking occurred as progradational events were sequentially deposited and then truncated during transgressive erosion along a flat trajectory. This resulted in repeated removal of the upper part of previously deposited shoreface units tens of kilometers up-Previous HitdipNext Hit and subsequent amalgamation with the next parasequence. Each sand-on-sand parasequence boundary is defined by subtle deepening events (changes in sedimentary structures, decrease in grain size, increase in bioturbation). Thinner shoreface sandstones (10 m) were deposited during a time of lower sediment supply and rising transgressive trajectories, allowing most of the previously deposited shoreface to be preserved.

Observed differences in the stacking patterns and shoreface thickness variations of the Mesaverde Group are not simply a result of progradational Previous HitversusTop aggradational trajectories. Instead they are the product of periodic and rapid changes in the relative rates of sediment supply, sea level rise and subsidence during deposition.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005