INVERSE RELATIONSHIP BETWEEN SEDIMENT ACCUMULATION RATE AND CHANNEL-BELT CONNECTEDNESS: AN EXAMPLE FROM LOWER CRETACEOUS STRATA OF THE SEVIER FORELAND BASIN, WYOMING, USA

Understanding controls on alluvial stratigraphy is critical to the effective development and management of hydrocarbon reservoirs and aquifers, and also to evaluating depositional responses to tectonics, eustasy, and climate. Quantitative alluvial stratigraphy models have provided much insight as to these controls. However, there is still considerable debate regarding the interrelationships between sediment accumulation rate, avulsion frequency, and channel-belt connectedness. Some studies suggest that high sediment accumulation rates cause an increase in avulsion frequency, resulting in high channel-belt connectedness. Other studies indicate that high sediment accumulation rates can cause deposition of a greater proportion of overbank material, resulting in lower channel-belt connectedness.

Lower Cretaceous fluvial strata throughout Wyoming provide an excellent opportunity to evaluate the interactions between sediment accumulation rate and alluvial stratigraphy. The rocks record nonmarine deposition in a cordilleran foreland basin east of the Sevier orogenic belt. The strata display the former existence of a thick foredeep depozone in western Wyoming (the Gannett Group), a thin forebulge depozone throughout central Wyoming (the Cloverly Formation), and slight thickening into a backbulge depozone farther east (the Lakota Formation). Compacted sediment accumulation rates for the foredeep are on the order of 10^-2 mm/yr. Foredeep deposits are dominated by mudstones, and conglomeratic channel deposits tend to be isolated. Sediment accumulation rates for the forebulge and backbulge depozones are on the order of 10^-3 mm/yr, an order of magnitude less than for the foredeep. Channel deposits in these areas are typically laterally and vertically connected. This field example supports alluvial stratigraphy models which suggest that channel-belt connectedness is inversely related to sediment accumulation rate. Implicit in these results is that, under certain conditions, avulsion frequency may be decoupled from large-scale, long-term sediment accumulation rate, or governed more by other parameters, such as peak annual discharge, relief or slope of the alluvial ridge, substrate composition, the distribution of floodplain channels, or processes that create crevasse channels.