Quantitative analysis of siliciclastic clinoforms: An example from the North Slope, Alaska

Abstract

Abstract Seismically-imaged clinoforms, constrained by well data, provide a unique opportunity to evaluate sedimentary processes using geometric considerations. The Lower Cretaceous of the Alaskan North Slope is a well-known example of large-scale (600 - 2500 m depositional relief), constructional, siliciclastic clinoforms. This study has depth-converted and decompacted a regional seismic profile in this area to evaluate the relationships among depositional slope angles, shelf-edge trajectories, seismic facies, sediment volume ratios, graded/erosional profiles, and lithologies. Insights from this analysis refute several assumptions about clinoform formation, and could help establish a quantitative methodology for clinoform analysis. Few published examples have been decompacted to estimate original depositional relief. In this study, seismically-imaged slopes of about 1.5° - 3.5° have approximately doubled in magnitude after backstripping, owing to their fine-grained nature and deep burial history. These slopes exceed the largest published estimates of constructional (“graded”) margins. North Slope clinoforms that show more evidence of slope erosion and bypass (out-of-grade) have, on average, lower slope angles than graded clinoforms. This observation is contrary to the assumption that higher slope angles correlate with more extensive erosion. In this study, we hypothesized that erosional modification has lowered the clinoform slope. On average, erosionally-modified clinothems have lower angle shelf-edge margin trajectories and thinner topsets relative to foresets/bottomsets. Additionally, erosionally-modified clinoforms are characterized by higher amplitude seismic facies. Well calibration indicates that the higher amplitude response results from interbedded sandstones and shales; low amplitude seismic facies, associated with graded margins, are more mud-prone. Out-of-grade clinoforms likely facilitate additional bypass/deposition of sediment downslope via sediment gravity flows. Flatter shelf margin trajectories indicate sediment supply exceeds accommodation; this may drive greater slope erosion and bypass. A predictive model, employing these clinoform geometric criteria, can assist in delineating intervals of enhanced bypass/submarine fan development. However, wide applicability beyond this example would require additional calibration and further testing.