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 /

by 1971- Carvajal, Cristian Rene

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 8 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.