FEAST TO FAMINE: SEDIMENT SUPPLY CONTROL ON LARAMIDE BASIN FILL

Major changes in Paleogene sedimentation in the Rocky Mountain region have traditionally been interpreted as responses to specific tectonic or climatic events, such as shifting regional stress fields or initial Eocene global warming. However, the distinctive unroofing history of Laramide-style uplifts also appears to have exerted an important but underappreciated influence on sedimentation. These uplifts elevated thick intervals of mudstone-rich Cretaceous (and older) foreland basin fill, which was quickly eroded and re-deposited into adjacent basins. Mesozoic sedimentary lithologies recycled from the older foreland basin deposits dominate Paleocene conglomerate clast compositions in many areas. To examine the volumetric significance of eroded Mesozoic rocks, we compared their inferred volume to that of preserved Paleocene strata between approximately 39-46º N. Volume calculations were based on a new regional Paleocene isopach map constructed from 1252 wells, interpolated using Inverse Distance Weighting. The volume of eroded Cretaceous and older rocks was based on previously published isopach maps, extrapolated across areas where these strata are now absent. Sediment volume for each time-slice was calculated using the cut/fill feature of the ArcGIS Spatial Analyst. The calculated volume of Cretaceous strata alone is approximately twice that of preserved Paleocene basin fill, suggesting that the basins where kept overfilled with sediment during this time. As a result, fine-grained clastic alluvial and paludal facies dominated, and were associated with relatively minor freshwater lake deposits. In contrast, several of the basins were occupied by large, long-lived, carbonate-producing lakes during the Eocene. Basement-derived clasts (granite, quartzite, and other metamorphic rocks) simultaneously became abundant in Early Eocene conglomerate. We propose that that Eocene lakes developed due to exposure of erosion-resistant lithologies within cores of basement uplifts. The resultant dramatic decrease in erosion rate is interpreted to have starved the adjacent basins of sediment, allowing them to be filled instead with water. Geomorphic evolution of the surrounding landscape therefore appears to have exerted an important first-order control on the character of Laramide basin fill.