--> Abstract: The Origin of Hummocky and Swaley Cross-Stratification - the Controlling Influence of Aggradation Rate and Unidirectional Current Strength, by Simone Dumas, R.W.C. Arnott, and John Southard; #90039 (2005)

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The Origin of Hummocky and Swaley Cross-Stratification - the Controlling Influence of Aggradation Rate and Unidirectional Current Strength

Simone Dumas1, R.W.C. Arnott2, and John Southard3
1 Queen's University, Kingston, ON
2 University of Ottawa, Ottawa, ON
3 MIT, Cambridge, MA

Over the past few decades two sedimentary structures have been widely reported from the shallow-marine sedimentary record: hummocky cross-stratification (Harms et al. 1975) and swaley cross-stratification (Leckie and Walker, 1982). The origin of both these structures, however, remains the source of a still unresolved debate. Experiments reported here investigated potential genetic differences between hummocky cross-stratification and swaley cross-stratification. A hummocky bed was aggraded "synthetically" at various rates in order to investigate the resultant stratification. At a high aggradation rate (4.2 mm/min), stratification resembled hummocky cross-stratification, whereas at a low aggradation rate (1 mm/min), the stratification was more akin to swaley cross-stratification. Addition of a weak unidirectional current (~ 5-10 cm/s) to a long-period, moderate oscillatory flow (~ 50-90 cm/s) formed anisotropic hummocky cross-stratification. More elevated unidirectional currents (> 10 cm/s) formed high-angle cross-stratification resembling unidirectional dune cross-stratification. On a cross-shelf depositional profile, the optimal preservation and depositional conditions for hummocky cross-stratification is suggested to be above (but near) storm wave base under oscillatory-dominant combined flows with moderate aggradation rates. Swaley cross-stratification, on the other hand, is postulated to be formed at shallower depth, by similarly oscillatory-dominant combined flows, but under lower deposition-to-transport conditions that resulted in lower aggradation rates. These data, therefore, illustrate not only the possible genetic link between hummocky and swaley cross-stratification, but provide an important advance in our ability to reconstruct depositional and paleogeographic conditions from the shallow-marine sedimentary record.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005