Simulations of Nonmarine Extensional Basins and Their Fossil Records

Abstract As a common tectonic setting for nonmarine systems, extensional basins preserve an extensive fossil record. Although individual basins may have complex histories, extensional basins tend to display similar trends in accommodation and sedimentation that make facies development predictable. Spatial variations in accommodation and sedimentation across extensional basins mean that footwall and hanging-wall stratigraphy should differ, such that their facies stacking patterns will be out of phase. Patterns of fossil occurrence are therefore predicted to differ greatly across extensional basins, potentially complicating stratigraphic and paleontological correlations. Using sedimentary basin simulations in R, we evaluate how temporal and spatial changes in accommodation and sedimentation influence the distribution of high- and low-aggradation systems tracts (HAST, LAST) and the fossil record of nonmarine extensional basins. Trends in architecture, species diversity, and elevation in relation to accommodation emerge across the basin simulations. Footwall architecture is overall HAST-like, whereas hanging-wall architecture is LAST-like updip, becoming increasingly HAST-like downdip. On the footwall, species diversity and occurrences decrease downdip with decreasing accommodation, whereas on the hanging wall, they increase downdip with increasing accommodation. Overall diversity of the footwall is nearly twice as high as hanging-wall diversity, and footwall occurrences are four times greater than hanging-wall occurrences. Species’ elevation and water-depth preferences influence turnover patterns along with rates of change in accommodation and sedimentation, such that slow changes result in averaging of elevation or water-depth preferences in fossil assemblages, whereas rapid changes result in increases in first and last occurrences at downlap surfaces and subaerial unconformities.