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

By

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