Tara L. Benda¹, Ron J. Steel¹
(1) University of Wyoming, Laramie, WY

Abstract: Architecture, facies, and sequence stratigraphy of a fourth-order clastic wedge; Twentymile Sandstone, Mesaverde Group, NW Colorado

The Upper Campanian Williams Fork Formation contains two fourth-order clastic wedges, the uppermost being composed of the regressive Twentymile Sandstone (TMS) and overlying transgressive Holderness Member (HM) (Almond Formation equivalent). This upper fourth-order clastic wedge contains five high-frequency transgressive-regressive sequences separated by transgressive surfaces of erosion (TSE's) or amalgamated TSE's/sequence boundaries. These TSE's are recognized in outcrop by a landward shift in facies, intense marine bioturbation, and a lag deposit (often marine bi-valve shells).

The lower TMS (wave-dominated deltaic and interdeltaic shoreface deposits) consists of at least two stacked sequences; the overall low angle or downward trajectory of the shoreline suggests forced regression in places. The flooding surfaces separating parasequences show an increase in bioturbation abundance and diversity, tidal influence (bi-directional currents and carbonaceous drapes), and a slight change in grain size. The transgressive HM, composed of three sequences, contains more facies assemblages than the lower TMS. Estuarine facies, including flood tidal deltas, tidal inlets, bayhead deltas, crevasse splay complexes, and fine-grained deposits (central basin?) occur, along with thin, discontinuous shorefaces (often stacked), distributary channels, marine shales, and fine-grained coastal plain deposits.

This entire fourth-order clastic wedge shows evidence of high-frequency rises/falls in sea level. Flooding surfaces separating the regressive limb's stacked parasequences illustrate the pulses of transgression that occurred during the Western Interior Seaway's overall regression. The high-frequency regressive phases within the transgressive limb represent small pauses or slow downs during the sea's overall rise; these regressive phases resulted in deposition of the HM's complex facies architecture.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana