Grain Fabric in Hummocky Cross-Stratified Storm Beds: Genetic Implications

Abstract

ABSTRACT A detailed analysis of grain fabric in samples from hummocky cross-stratified (HCS) storm beds suggests that these shallow marine sandstones were deposited from waning combined (oscillatory/unidirectional) flows. Particle a-axis orientations in the deposits, in plan view parallel to visible lamination, vary widely but have a mean orientation approximately normal to unidirectional paleoflow indicators (sole marks) and subparallel to regional shoreline. This alignment of a-axes is attributed to deposition under combined flows with a multidirectional oscillatory component produced by wind-induced surface gravity waves with a modal crest orientation approximately parallel to the shoreline. Imbrication of particles varies in both the horizontally-laminated and HCS intervals of the HCS stor beds, due to waxing and waning of boundary shear stress caused by the oscillatory component of the flow. In the basal, horizontally-laminated interval of an HCS storm bed, grain imbrication angles vary about a mean of approximately 13° from the horizontal, dipping into the paleoflow inferred from sole marks. This preferred imbrication direction is attributed to boundary shear stress influenced by the unidirectional component of the combined flow. However, quasi-cyclic variation in imbrication angle over a broad range and periodic reversal of imbrication direction suggest that the oscillatory component was also significant during deposition of the horizontally-laminated interval. In HCS intervals, imbrication angles vary about a mean that is parallel to visible laminae, suggesting th t oscillatory flow had become predominant. The sequence of structures and changing patterns of fabric through these HCS storm beds is consistent with deposition under waning combined flows. Erosional scour, forming the sharp base and unidirectional sole marks, occurred in response to combined flows with an effective unidirectional component. The unidirectional component continued to influence the direction of boundary shear stress during deposition of the horizontally-laminated sands but waned and became negligible relative to a strong oscillatory component during formation of HCS.