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Hummocky cross-stratification (HCS) has been reported from over 100 ancient units, but has never been unequivocally recognized in recent sediments. Consequently, little is known about hydraulic conditions under which HCS forms.
In Pleistocene lacustrine deposits of Lake Bonneville, both depth of formation and parameters of inferred generative waves may be defined accurately. HCS was formed in fine to medium-grained sand in water depths of 1.2-2.2 m (3.9-7.2 ft). The site of formation was exposed to a maximum fetch of 15 km (9 mi). Depths were great over this fetch, permitting calculation of conditions under which this example formed, assuming: (1) HCS is purely wave-formed; (2) HCS formed under maximum storm conditions; (3) maximum storm-generated wind speeds over Lake Bonneville were similar to speeds generated by continental storms today. Inferred wind-speed range is 16 m/sec (52 ft/sec, moderate gale force) to 34 m/sec (112 ft/sec, threshold hurricane force).
Resulting calculations indicate that HCS formed under waves with periods (T) of 3.8-5.7 sec and maximum orbital speeds (Um) of 0.9-2.5 to ~2-4 m/sec (3.0-8.2 to 6.6-13.1 ft/sec), respectively. This range falls within the wave-formed flat-bed field for fine to medium-grained sand. Independent corroboration is provided by the equivalence of wavelength of hummocky laminae in outcrop and calculated wave-orbital diameter (do). HCS has wavelengths of 3.1-4.2 m (10.1-13.8 ft) in this example; the 3-m and 4-m (10-ft and 13-ft) isograds of do run directly through the middle of the Um-T field calculated above.
It is generally agreed that HCS probably is formed by oscillatory flow or oscillatory-dominant combined flow. Consequently, this analysis serves at least as a guide to conditions of formation of HCS and suggests that HCS and wave-formed flat bed are at least partly equivalent bed configurations.
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