Sediment volume partitioning, topset processes and clinoform architecture understanding the role of sediment supply, sea level and delta types in shelf margin building and deepwater sand bypass : the Lance-Fox Hills-Lewis system in S. Wyoming /
Sediment volumes and their storage in shelf-margin compartments are used to
decipher the tectonics and accretion of a Laramide source-to-sink system. Logs from
some 520 wells were used to quantify volumes stored in the accreting, early
Maastrichtian shelf margin in southern Wyoming during a ~1.8 my time interval. Basin
development was monitored through the analysis of 15 clinothems; an approach suitable
for the greenhouse, high subsidence and high sediment supply conditions under which the
Washakie-Great Divide Basin filled.
Volume partitioning into topset, slope and basin-floor compartments averages
1.1:1.1:1.0 respectively. The topset is the sandiest compartment reflecting deposition
from fluvial and shorelines systems. The basin floor, where sand is concentrated in
submarine fans has the next largest sand volume, whereas the slope, which traps
turbiditic sand in channels, tends to be mud-prone. However, the slope contains the
largest volumes per compartment area reflecting its progradational nature. Topset and
slope volumes combined represents at least 2 thirds of total volume, and therefore they
serve as a reasonable proxy for total volume. Consequently, shelf-edge accretion rate, as
an indirect measure of topset and slope volumes, may serve as a reasonable proxy for
sediment supply in ancient margins where volumes are not available.
Volumes and clinothem architecture suggest a two-stage tectono-stratigraphic
model for basin development and infill. During stage 1, clinothems become aggradational
and thick with increasing volume and average rate of sediment supply; these clinothems
have more marine topsets and prograde into deepening basinal-waters. This indicates a
high and rising rate of relative sea level interpreted to result from increasing rates of
tectonic subsidence. Greater supply and subsidence in stage 1 suggest increasing thrustdriven
uplift and crustal loading. In stage 2, clinothems are progradational and thinner,
with decreasing volume, but high average supply rate; they developed more terrestrial
topsets and prograded into a basin of relatively stable to slightly deepening water depth.
This indicates lower rates of sea level rise due to lower tectonic subsidence rates, which
points to diminishing thrust loading. In addition, in the stage 2 supply rates are markedly
high and sea-level falls are also present suggesting continued mountain uplift and
possibly basin uplift through isostatic rebounding.
Through stages 1 and 2, average source uplift rate is estimated to have been high,
few mm/y. Uplift resulted in exposure and erosion of sandier rocks through time
accordingly rising the sand/mud ratio in basin deposits. Average catchment is estimated
at ~23,200 km
2 and from this catchment, average river load to the ocean is estimated at
~9 x 109 kg/y resulting in a yield of ~400 ton/km2/y and denudation rate of 0.15 mm/y.
These values suggest an average of ~1800 m for maximum hinterland relief, but at stage
1 climax, maximum relief likely was probably 2000-3000 m.
School:The University of Texas at Austin
School Location:USA - Texas
Source Type:Master's Thesis
Keywords:sedimentation and deposition sedimentary basins deltas washakie great divide basin wyo wyoming
Date of Publication: