Nature of the Updip-Downdip Linkage between Fluvial Sheetsands and Their Time-
Equivalent Shoreline and Shelf Sands in Some Mesaverde Clastic Wedges,
Greater Green River Basin
Ronald J. Steel and Jeff Crabaugh
University of Wyoming, Laramie, WY
Analysis of a 250km-long transect from the northwestern Green River Basin to the Sand Wash Basin, suggests that unconformity-based, amalgamated fluvial sands in the updip core of some Mesaverde clastic wedges pass downdip, through increasingly estuarine and mudprone coal-bearing strata, to a stacked series of marine-shoreline and shelf sands. Close examination of such large clastic wedges shows that they consist internally of higher frequency (4th-order) regressive-to-transgressive tongues with the following linkage characteristics:
1. The regressive signature of a tongue in its most updip reaches is an unconformity, with some overlying conglomeratic, fluvial deposits. This marked a time of sediment by-pass and sediment delivery to distant, time-equivalent shorelines, during periods of falling base level.
2. In downdip areas, by-passed sediment fed prograding falling-stage, and eventually lowstand deltaic shorelines. During early base-level rise and aggradation of these most distant shoreline deposits, there was some storage of fluvial deposits in the updip areas.
3. During transgression, and transit back across the shallow-water platform, there was commonly an enhanced tidal regime set up. Lowstand deltas in the distal areas were partly reworked into tidal sand ridges, on what now became a ‘shelf’. The broad distributary valleys of the previous regressive deltas were then back-filled by tide-influenced estuarine deposits, as relative sea level rose.
4. Transgression far back into the updip areas brought brackish-water shales, bayhead deltas and tide-influenced fluvial channels to drape the older, braided fluvial deposits.
5. In the most proximal areas there is no marine influence, unconformities simply alternate with coarse-grained, braided fluvial deposits.
AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming