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Revisiting the Utility of Hummocky Cross-Stratification in Paleohydraulic Study of Ancient Shelves and Sand Distribution Processes

Abstract

Cretaceous-age shelf sands form some of the most prolific hydrocarbon reservoirs in the world. However, the processes responsible for transporting sands across ancient shelves, and concentrating and reworking them into quality reservoirs are poorly understood. Shelf sand deposits are often characterized by hummocky cross-stratification (HCS), which through the concepts of Airy-wave Theory, are shown to indicate the nature and magnitude of processes responsible for their deposition. Morphometric data on these bedforms can be used to estimate the range of wave characteristics and water depths at which these bedforms deposited, allowing for a reconstruction of hydraulic conditions along paleo-shelves. Two example localities are used to illustrate the process and applicability of these analyses; Cape Sebastian Sandstone (CSS) in southwestern Oregon and the Tocito Lentil of the Mancos Shale (TL) in the southeastern San Juan Basin, New Mexico, both are Late Cretaceous marine transgressive deposits. Field studies were done to quantify the architecture of HCS in these units. HCS sands of CSS are thick, amalgamated and show few preserved hummocks. TL HCS are much muddier, showing mm-scale laminae of mud between thicker sands. HCS wavelengths in the CSS range from 2–3 meters and bed thicknesses vary from 30–40 cm. HCS beds in the TL vary in thickness from 15–30 cm with low-amplitude preserved hummocks exhibiting 2.5-3.5 m wavelengths. HCS grain sizes are fL–fU in CSS but slightly coarser (fU–mL) in the TL. Measurements of hummock spacing were used to estimate the range of orbital diameters. Maximum and median grain sizes were used to estimate range of orbital velocities. Possible combinations of wave heights and water depths were deduced using wave equations. Calculation using HCS show deeper water deposition for the TL compared to CSS. The deeper water setting at TL compared to the CSS is supported by higher percentage of mud and lesser dips preserved in the TL HCS. Results suggest that HCS at both locations are formed by long-period (10–30 sec) waves and orbital velocities ranging from 40 cm/sec to 140 cm/sec. One must keep in mind that the applied equations only calculate approximations of the real hydraulic processes and their application requires certain assumptions. Other geological evidence can be used to further refine paleohydraulic interpretations.