Upper flow regime sediment gravity flows in deepwater environments : spatio-temporal evolution of deepwater bedforms and the stratigraphy they build




West, Logan Mitchell

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Upper flow regime sediment gravity flows commonly described by the densimetric Froude number (i.e., Froude supercritical flows) are known to generate large ‘supercritical’ bedforms that build and evolve with successive sediment gravity flow events over geologic time periods. To date, expected characteristics of supercritical bedforms and the ability to identify them in the rock record remains incomplete, drawing heavily from physical experimentation and numerical modeling. Physical and numerical studies are highly beneficial but have limitations preventing them from being fully analogous to deepwater systems. This work aims to contribute to supercritical bedform and supercritical fan understanding through a detailed outcrop study, working from deposits out to bedsets and fan architectures. Detailed characterization of beds and bedsets of the Lycium Mbr outcrops in the Fish Creek-Vallecito Basin of southern California, USA reveals a 100 m-thick stratigraphy comprising sets of dozens of conformably aggrading sigmoidal turbidite event beds. Quantitative analysis of these bedsets indicates that deepwater, upslope migrating bedforms systematically preserve bedform geometry and sedimentary textural trends vertically within bedsets that represent the partial or full translation of large-scale, multi-event deepwater bedforms. Specifically, increasing-to-decreasing dip angles of dozens of separate sediment gravity flow event beds typically accompanied by increasing-to-decreasing trends in bed thickness and grainsize are established and tested as reliable indicators of migrating bedform preservation. In the Lycium Mbr outcrops, bedsets are classified into three types based on internal architectures and facies and interpreted to represent different bedforms along the spectrum of supercritical bedforms, namely cyclic steps and antidunes. Recognition and interpretation of bedform types further allows for quantitative constraints to be placed on paleoflow conditions, paleogeography, and relative depositional environments that are consistent with other observed field data. These observation point to a proximal to distal trend from cyclic step-like bedsets to antidune-like bedsets associated with observed decreases in grainsize and loss of any degree of flow confinement. The interpretation of antidunes represents the first recognition of deepwater, multi-event antidunes at scale with modern seafloor sediment waves, with implications for differences in subaqueous flow dynamics and environmental interpretation. This work also establishes that during the time of Lycium Mbr deposition, the Fish Creek-Vallecito Basin was conducive to repeat supercritical sediment gravity flows such that Lycium Mbr deposits can be reliably interpreted as a variant of a supercritical fan. Analysis of the large-scale Lycium Mbr depositional architecture reveals a deepwater fan composed almost entirely of bedforms with only subordinate channelization. The fan is distinctly not composed of sheet sands and exhibits depositional patterns in which deposits young both vertically and proximally as the fan retreats via upslope bedform migration until possible incipient channelization occurring as bedforms encroach toward the possible toe-of-slope helps to facilitate a fan-wide forward step out further into the basin. This pattern (step out, aggrade back, step out) is consistent across the outcrop and supported by other evidence (e.g., grainsize, bedform types, aggradation rates) of forward advance of the fan through time. This behavior and a propensity for abrupt facies changes along relatively short distances (tens of meters) are consistent with experimental expectations of supercritical fan behaviors


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