Hummocky stratification: Significance of its variable bedding sequences

Hummocky cross-stratification is an important structure formed on the shoreface and shelf by waves. It characterizes a wave-dominated facies. Attention to its variability can reveal much about sedimentary history and paleogeography. Diagnostic traits are antiformal hummocks and synformal swales defined by randomly oriented, even lamination with dip angles and truncation angles of < 15°. Hummocky stratification forms primarily in silt to fine sand. Although size grading of individual laminae is not characteristic, concentrations of mica and plant detritus in the tops of many laminae indicate a shape sorting. Parting lineation is common. Hummocky beds vary in thickness from a few centimetres to 5 or 6 m; bed sets may be tens of metres thick. Hummocky stratification apparently is formed most commonly by redeposition below normal fair-weather wave base of fine sand delivered offshore by flooding rivers and scour of the shoreface or shoals by large waves. Deposition involves both fallout from suspension and lateral tractive flow due to wave oscillation. There is evidence that, under intense oscillatory flow, large waves drape sand over an irregular scoured surface and also mold sand into roughly circular, unoriented hummocks and swales. We postulate that these circumstances are analogous to the transition to upper flat-bed conditions in unidirectional flow.

Hummocky stratification shows important variability. It occurs in both regressive (progradational) and transgressive strata in intervals a few centimetres to 175 m thick and may be interstratified with mudstone, sandstone, or conglomerate. Hummocky stratification commonly occurs in repetitive successions with the products of individual depositional events being clearest where mudstone separates hummocky beds. An idealized hummocky stratification sequence, which can serve a purpose similar to the Bouma sequence for graded beds, is as follows (bottom to top): first-orderscoured base (± sole marks); characteristic hummocky zone with several second-order truncation surfaces separating individual undulating lamina sets; a zone of flat laminae; a zone with well-oriented ripple cross-laminae and symmetrical ripple forms; all overlain by a more or less burrowed mud-stone or siltstone. This sequence reflects waning of storm waves followed by fair-weather sedimentation and burrowing. Variations from this idealized conceptual sequence involve omissions and/or expansions of one or more of the zones. The most common variant is amalgamation either by the stacking of successive hummocky zones or by intense bioturbation that obliterates original boundaries between depositional units. Other variations include units commencing with flat-lamination; units with predominant cross-lamination; and lenticular micro-hummocky lenses within shale. Combinations of relative sand supply, relative depth, tidal range, frequency, duration and magnitude of storms, and relative productivity for a burrowing benthos must account for such differences. Further documentation of variations in hummocky stratification should reveal important details about these factors.