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