COLLABORATIVE RESEARCH: Controls on Sequence Geometry and Bounding Surfaces in Ramp Settings

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Clastic sediments delivered to the margins of many sedimentary basins commonly form deltaic successions which forestep and backstep on evenly deepening ramps, rather than the more commonly modeled shelf-slope-basin floor bathymetry. The properties of sequence boundaries, and the geometry and stacking of sequence elements ("systems tracts") in such ramp settings are highly controversial and very few published examples are well constrained. This project is an integrated study of sequence geometry in ramp settings. The project will consist of conceptual and numerical modeling accompanied by rigorous field testing in one of the best exposed successions of clastic ramp sequences in the world: the Upper Cretaceous marginal marine deposits of the San Juan basin, New Mexico. The primary objectives of this project are to (1) identify, evaluate and map stratigraphic surfaces, which meet such objective criteria for a sequence boundary as downward shift in onlap and subaerial exposure of older marine strata, (2) test the concepts of instantaneous versus diachronous formation of sequence boundaries, (3) document stacking patterns and volumetric partitioning of sediment between different elements (systems tracts) of the sequences, (4) forward model variations in sequence geometry in response to different rates of tectonic subsidence, rates and amplitudes of sea level change, and sediment supply. Two-dimensional, flexural backstripping will be applied to reconstruct the geometry of the depositional surfaces through time. The modeling will be based on a combination of two numerical methods. One is a kinematic model using preset depositional slopes and sediment supply to generate the stratigraphic response to sea level and tectonic fluctuations. The second is a diffusion model that simulates sediment transport in a "foreland" basin setting. The overall goal of the project is to provide improved precision in models predicting the spatial and temporal distribution of lithologic units in basins with ramp margins, and to enable the extraction of more precise information about past sea level fluctuations from the stratigraphic record.