Ichnological and Sedimentologic Signatures of Mixed Wave-and Storm-Dominated Deltaic Deposits

Abstract The lithological characteristics of wave- and/or storm-dominated delta-front deposits are fundamentally similar to those of facies deposited on the wave-formed shorefaces of strandplain settings. Differentiating ancient shoreface deposits from those that record deposition in proximity to contemporaneous wave-dominated deltas, therefore, is challenging, especially where the facies represent deposits that are intermediate between end-member strandplains and delta fronts. To date, archetypal facies models are inadequate to describe and distinguish between such deposits. The challenge is further accentuated where studies are limited entirely to core and other subsurface data. Depositional processes typical of deltaic settings influence infaunal organisms in subtle but significant ways. The resulting ichnological signatures clearly reflect the innate differences in physicochemical conditions and paleoenvironmental stresses operating in these settings, such as variations in sedimentation rates, substrate consistencies, oxygenation, salinities, energy conditions, increased turbidity levels, and episodic deposition associated with river floods. Lower Permian successions of the Wasp Head, Pebbley Beach, and Snapper Point Formations of the southern Sydney Basin in southeastern Australia are spectacularly exposed in extensive coastal outcrops. The preserved lithologies and many of the primary sedimentary structures are virtually identical to those characteristic of offshore and strandplain shoreface deposits. Integration of the lithological, sedimentological, and subtle ichnological differences, however, demonstrate that these units were deposited under the influence of paleoenvironmental stresses. There is also considerable evidence of very cold climatic conditions and concomitant effects on the depositional environment from ice rafting, which imposed additional paleoenvironmental stresses. For the most part, fair-weather beds closely resemble strandplain shoreface deposits, with trace-fossil suites that are very diverse and contain a mixture of structures that reflect a variety of feeding strategies characteristic of the Cruziana and Skolithos Ichnofacies. Variations in the ichnological signatures, and departures from the archetypal ichnofacies expressions, in the form of sporadic bioturbation levels, reduced assemblage diversities, and reductions in ichnogenera sizes compared to their unstressed counterparts, suggest intermittent physicochemical stresses. Associated storm deposits display many of the sedimentological and ichnological characteristics associated with river influx and deltaic conditions, including: soft-sediment deformation structures and sediment-gravity-flow deposits, recording rapid sediment emplacement; mudstone drapes that are characteristic of hyperpycnally emplaced fluid muds and rapidly flocculated muds that are produced along the zone of mixing at the base of a hypopycnal (buoyant) mud plume; unbioturbated, carbonaceous mudstone interbeds with synaeresis cracks consistent with freshet-induced salinity fluctuations; an abundance of phytodetrital material, and allochthonous wood and large logs; and sandstone beds with “stressed” trace-fossil suites attributable to the Cruziana Ichnofacies, where ordinarily suites representative of the Skolithos Ichnofacies would be expected. These characteristics suggest that fair-weather beds reflect ambient wave shoaling, but during and immediately following storms, increased river discharge strongly influenced the depositional environment and thus the characteristics of the resultant event beds. Overall, the successions are therefore interpreted as wave- and storm-dominated prodelta to proximal delta-front deposits. Variations in storm signature throughout the successions reflect temporal and spatial variations in the preservation and, therefore, abundance of fair-weather beds. Such variations may represent changing storm climates, climatic seasonality, fluctuations in river discharge, increased amalgamation of beds by persistent storm activity, subtle changes in storm tracks with respect to delta-front orientation, and subtle shallowing or deepening along the delta front.

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