The Origin of Hummocky Cross-Stratification: An End to a Stormy Debate?

Peter J. McCabe, Keith W. Shanley

There has been considerable debate over the last fifteen years as to the origin of hummocky cross-stratification (HCS) within shoreface successions. HCS sandstones interbedded with mudstones have generally been interpreted to be deposited by storms below fair-weather wave base. Discussion has centered on processes, such as storm surge ebb, turbidity currents, and geostrophic currents, to transport sands a considerable distance offshore. However, there is a significant discrepancy in scale between the processes described from modern shorefaces and those interpreted from many ancient successions. Most recent storm deposits from modern shelves are less than 3 cm thick in water depths as shallow as 30 m and at only 25 km from the shoreline, whereas ancient HCS sandstone beds p to 1 m are commonly interpreted to have been deposited at depths >30 m and at sites many tens of kilometers offshore.

Related to the origin of HCS are estimates of the water depths of shallow marine facies. Estimates are often based on the position of a facies within a coarsening-upward succession - for example, by measuring the distance down from an overlying paleosol. Forward modeling of parasequence formation suggests that this may well produce erroneous results because of the angle of climb of facies and because stacking of successive parasequences juxtaposes identical facies within a vertical profile. Careful examination of ancient shoreface parasequences from the Cretaceous of the Western Interior suggest that many HCS sandstones previously interpreted to have formed in deep water were probably deposited in water depths <10 m at distances of <5 km from a shoreline. There is no need to inv ke storms to explain their origin. We suggest that most thicker HCS beds (>25 cm) were deposited above fair-weather wave base, and represent the sandy tops to minor parasequences. Such an interpretation is compatible with both the processes described from modern shorefaces and the parasequence geometries suggested by our modeling. This interpretation of HCS has profound implications for the prediction of lateral distances to coeval shoreface deposits and impacts our understanding of shoreface reservoir-architecture.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995