Hummocky Cross-Stratification--Indicator of Storm-Dominated Shallow-Marine Environments: ABSTRACT

Abstract

Hummocky cross-stratification (HCS), as formally defined by Harms and others in SEPM Short Course 2, is the preferred term for very distinctive, low-angle (2 to 15°), curved to undulating laminae which are broadly End_Page 460------------------------------ concave and/or broadly convex upward (hummocky). The swales commonly cut each other, giving rise to very low-angle, curved intersections of laminae. Laminae are broadly parallel over hummocks and swales, in sets 2 to 20 m thick. Wavelengths range from about 1 to 5 m, and heights range from about 10 to 40 cm. HCS is not a form of trough cross-bedding--dips are too low, stratification is as commonly arched upward as downward, and the hummocks and swales are elliptical to almost circular. HCS occurs both in thick (several meters) beds and in sharp-based thinner beds (tens of centimeters) interbedded with shales. In the latter, oriented sole marks commonly are present on the sandstone bases that indicate regional paleoslope. Shales between sandstones are bioturbated, but the HCS itself is not. Harms and others interpreted HCS as formed by storm waves, but below fair-weather wave base. Several other authors have defined similar stratification and have argued for a similar interpretation, but the widespread geologic occurrence and significance of HCS have been hidden by the multiplicity of different names. Our examples from the Fernie-Kootenay (Jurassic-Cretaceous) transition and from the Cretaceous Cardium Formation (both in southern Alberta) suggest by stratigraphic context with other facies that the HCS was formed below fair-weather wave base. Specifically, we suggest that water piled onshore during major storms returns seaward as a sediment-laden density current (as in Hurricane Carla, Texas Coast, 1961). The density current forms oriented sole marks but, instead of depos ting a Bouma sequence, the current deposits sediment onto a seafloor still under the influence of storm waves--forming HCS instead. End_of_Article - Last_Page 461------------