Cristian Carvajal

Thick turbidite successions from supply-dominated shelves during sea-level highstand

Emphasis on the association between relative sea-level lowstand and the formation of sandy deep-water fans has tended to downplay the significance of high sediment supply and its potential to create deep-water fans, even during sea-level highstands. The Lance– Fox Hills–Lewis shelf margin in southern Wyoming suggests that high supply was critical in causing the accretion of this moderately wide Maastrichtian shelf margin, at a minimum rate of 47.8 km/m.y., and the generation of large, sand-rich fans during every shoreline regression across the shelf. It is surprising that fans developed from shelf-margin clinoforms that show systematically rising shelf-edge trajectories (proxy for rising relative sea level) as well as from those that show flat trajectories (stable to falling relative sea level). However, the latter, producing more sediment bypass, resulted in bigger and thicker fans, whereas the former produced somewhat smaller and thinner fans. We term the former highstand fans and suggest caution in using the lowstand model for high-supply systems.

Estimation of the paleoflux of terrestrial-derived solids across ancient basin margins using the stratigraphic record

A simple inversion scheme for estimating sediment flux from ancient shelf-margin successions is presented here by treating shelf-margin clinothems as the product of deposition associated with migration of a shelf-edge clinoform with constant shape at a rate equal to the shelf-margin progradation rate. Assuming sediment conservation, deposition can be broken into components of (1) response to subsidence and sea-level changes, and (2) basinward migration of the clinoform profile. Sediment flux can therefore be estimated with knowledge of progradation rate, subsidence/sea-level change rate, and clinoform dimensions. An advantage of this methodology is that it requires only two-dimensional data (i.e., dip-oriented cross sections) rather than three-dimensional volumes, making it ideal for use with sparse data sets as well as with outcrops. This methodology is also useful for analyzing areally limited data sets because it can predict the flux of sediment transported beyond the area of data coverage. The approach is able to accurately reproduce the sediment-flux estimates of previous workers from several margins (the Fox Hills–Lewis, Zambezi, New Jersey, and North Slope margins) using both volumetric and forward-modeling methods. Not only are the predicted distributions for sediment flux across ancient shelf-margins similar to distributions predicted by more data-intensive theoretical models, the estimated magnitudes for paleofluxes favorably compare with measured loads from modern rivers. Growth of continents is achieved in part by accretion of sediment on shelf margins. The rates and patterns of continental expansion are therefore partially dependent on the magnitude and distribution of mass transfer from eroding hinterlands to continental margins, fluxes which also play a critical role in global biogeochemical cycles. Flux estimates cast into a mass-balance framework suggest that approximately two-thirds of continental-margin sediments are exported past the shelf edge into deeper water at long-term geologic time scales. This finding implies that two-thirds of the terrestrial-derived, particulate organic carbon (POC) delivered from rivers to the ocean can be stored in deep water over geologic time scales. The observations presented here indicate that repetitive delivery of sediment to margins by shelf-edge deltas is fundamental to the long-term process of margin accretion.

Deltaic process and architectural evolution during cross-shelf transits, Maastrichtian Fox Hills Formation, Washakie Basin, Wyoming

The topset compartments of two Maastrichtian basin-scale clinothems are characterized, with focus on the function they played in constructing the Lance–Fox Hills–Lewis shelf-margin sedimentary prism in the Laramide Washakie Basin, south Wyoming. Approximately 1000 well logs were used to map the delta lobes and complexes on the Fox Hills shelf and to detail their depositional character, dimensions, and orientation as they autogenically shifted during transit from an inner-shelf to shelf-edge position. The regressive transits of the deltas initiated up to 40 km (25 mi) landward from the preexisting shelf-edge and preserved river and wave-dominated deltaic deposits that thicken and concentrate sand on the outer shelf. Tidally influenced deltas (now outcropping) also occur in localized areas along the paleoshelf edge, probably where wave influence was reduced along invaginated coastal segments. Net sandstone maps of the individual clinothem topsets show that (1) coeval delta lobes exist within each clinothem, suggesting multiple rivers; (2) delta lobes have a likely autogenic compensational stacking pattern; and (3) deltas thicken and storm-wave influence become dominant closer to the shelf edge. Our results support the ideas of (1) predictable increased wave influence and (2) change to strike-elongate architecture as deltas transit the shelf. In addition, along-strike changes in process dominance cause deltaic reservoirs to be highly variable in their orientation, external shape, and internal character. Some process changes are interpreted to be autogenic responses during overall shoreline progradation. The study also provides new data on delta-lobe and delta-complex thicknesses as well as on deltaic coastline versus shelf-edge progradation rates.

Quantifying sediment supply to continental margins: Application to the Paleogene Wilcox Group, Gulf of Mexico

Sediment supply to the ocean influences basin-margin growth and reflects upstream landscape evolution, including patterns of sediment routing, denudation, and tectono-climatic perturbations in source areas. Constraining sediment supply is useful for inputs to stratigraphic forward models and for predictions of reservoir presence and quality. Because of the importance of sediment supply, geoscientists have developed various methods to estimate it. Here, we apply Monte Carlo simulation (MCS) to the BQART model that is used to describe an empirical relationship between river catchment paleogeography, climate, and sediment load. We calculate a range of sediment supply from North American source areas to the Gulf of Mexico that suggests an overall decrease in median sediment supply from the late Paleocene to the early Eocene from 404–514 to 144–204 million tons per year, depending on the published paleogeographic model that we used to guide our selection of input variables. Comparison of these estimates with downstream sediment records shows that the subsurface depositional rates are within the 10th–90th percentile range of this BQART-MCS uncertainty model. The 50th percentile values of BQART-MCS results are overall larger than the published Wilcox sediment volume, which indicates that the size of Wilcox deep-water fans might be underestimated. We use source-of-change analysis to show the influence of each river-catchment input of the BQART model on change in sediment supply from the late Paleocene to the early Eocene. Integration of empirical-based methods, such as BQART, with physics-based experimental and modeling approaches might provide better constrains on sediment supply and deposition in frontier areas of oil and gas exploration.