Combined flow sedimentary structures and the genetic link between sigmoidal- and hummocky-cross stratification

This work is based on the comparison between facies tracts of flood-dominated fluvio-deltaic systems and basin- plain turbidites to the main combined flow experimental data available in literature; it discusses the possibility of a genetic link between sigmoidal and hummocky structures and their significance in facies analysis. Sigmoidal and hummocky-cross stratifications are large-scale sedimentary structures, usually considered indicative of tidal and storm deposits, respectively. However, facies analysis of flood-dominated fluvio-deltaic systems in tectonically active settings shows that these two types of structures are also typical of these depositional systems. In flood-dominated river-delta systems, coarse-grained mouth bars, which can be characterized by different types of sigmoidal-cross stratifications deposited by sediment-laden stream flows entering seawater, pass down-current into fine-grained delta-front sandstone lobes. That is to say, sharp based normally graded beds with hummocky-cross stratifications, deposited by flood-related hyperpycnal flows characterized by an oscillatory component, whose origin can be related to different processes. At a small scale, biconvex and rounded ripples and megaripples with sigmoidal-cross laminae are related to small- and medium-scale hummocky structures in basin plain turbidites, where ponding and rebound processes can transform the turbidity currents into combined flows. These field observations suggest a genetic link, at different scales, between these two types of structures, especially in terms of combined flows. This study, therefore, has prompted a re-examination of the combined-flow sedimentary structures produced in laboratory experiments, and has led to the proposal and discussion of some facies schemes (small- and large-scale) based not only on the ratio of Uu (unidirectional velocity) to Uo (oscillatory velocity) but also upon grain sizes, rates of fallout and frequency of oscillatory component (i.e. the period T).