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Distinction betweeen HCS and antidune stratification; a key to detect process vs product changes


, : Distinction betweeen HCS and antidune stratification; a key to detect process vs product changes. Annual Meeting - American Association of Petroleum Geologists

Antidunes can easily be mistaken for hummocky cross-stratification (HCS) or sigmoidal cross-bedding, and are probably fairly common in various siliciclastic environments. Antidunes are difficult to identify because (1) their preservation potential is considered very low, and (2) their study is based on flume experiments and few outcrop studies of siliciclastic deposits (including so-called "HCS mimics") and pyroclastites. We have documented outcrops of Holocene antidune-bearing pyroclastites in Niijima, Japan and compared the facies architecture with antidunes and HCS of siliciclastic outcrops in the US and elsewhere. In the pyroclastic outcrops, antidunes occur in a wide range of scales (from 2 cm to 6 m high), forming compound antidunes. In 3-D, these antidunes in all scales have geometry of laterally coalesced domes trending in the strike direction. In 2-D, they have a convex-up geometry with internal upstream and downstream accretion surfaces, remarkably resembling the vertical cross-section of HCS. There are several criteria that can be used to distinguish antidunes, HCS and sigmoidal/low-angle cross-bedding, including (1) imbrications of individual grains, and (2) evolutionary patterns of 2-D bedform geometry along the dip section. Flume studies suggest that cross-bedding cannot form in grain sizes finer than upper fine sand, whereas HCS is found with fine to very fine sand. Some storm-generated HCS composed of medium- or coarser-grained sands were reported from shoreface, strait, embayment and tsunami deposits. However, these HCS might actually be current-generated antidunes or cross-bedding, recording a local/temporal process change, or product change induced by grain-size increase, without a need for process change.

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