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Abstract
Journal of Sedimentary Research, Section
B: Stratigraphy and Global Studies
Vol. 66 (1996)No.
4. (July), Pages 713-722
Complex Fluvial Response to Low Gradients at Maximum Regression: A Genetic
Link Between Smooth Sequence-Boundary Morphology and Architecture of Overlying
Sheet Sandstone
John M. Holbrook
ABSTRACT
The Mesa Rica Sandstone is an extensive fluvial sandstone sheet that is
separated from underlying marine strata throughout northeastern New Mexico
by a regionally smooth and laterally continuous sequence-bounding unconformity.
The Mesa Rica Sandstone consists mostly of channel-fill elements, with
lesser proportions of lateral-accretion elements, and almost everywhere
is of the same thickness as the largest channel fills. This broad sandstone
sheet reflects deposition on a coastal plain by frequent avulsion and minor
lateral migration of straight and low-sinuosity streams under nearly stable
base-level conditions, which are here attributed to rapid progradation
of the adjacent Mesa Rica deltaic shoreline relative to change in relative
sea level during maximum Kiowa-Skull Creek regressi n. Rapid deltaic shoreline
progradation under such conditions caused seaward extension and gradient
reduction in the adjacent Mesa Rica coastal plain. Overextension and resultant
loss of power in Mesa Rica rivers crossing this coastal plain forced both
stream straightening and storage of nontransportable coarser sediment in
coastal-plain channels. Because base level was stable, Mesa Rica channels
could aggrade only minimally, and were forced to spread excess sediments
mostly laterally by avulsion. Frequent avulsion and minor lateral migration
of Mesa Rica channels under these conditions caused both regional scouring
of a smooth sequence-bounding unconformity and deposition of a sheet sandstone
with minimal channel stacking.
The Mesa Rica Sandstone and the sequence boundary that underlies it
represent a distinctive genetic association of single-story fluvial sheet
sandstone and smooth sequence-boundary morphology that is probably not
unique to the Lower Cretaceous of northeastern New Mexico. Such unincised
sequence boundaries are also implied in conceptual models and flume experiments
of previous authors.
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