The Ichnological Signatures of River- and Wave-Dominated Delta Complexes

Abstract Analyses of a number of Cretaceous intervals of the Western Interior Seaway of Alberta have led to ichnological and sedimentological criteria aiding in the identification and differentiation of river-dominated delta, wave-dominated delta, and non-deltaic shoreface successions. Dunvegan Allomember E constitutes a markedly river-dominated deltaic lobe in west-central Alberta, whereas Dunvegan Allomember D constitutes a strongly wave-dominated delta system. The Viking Formation, in contrast, displays characteristics typical of non-deltaic strandplain shoreface successions. Comparison of the sedimentary successions from these two stratigraphic units is valuable in establishing their unique ichnological characteristics. The principal differences lie in prodelta and lower delta-front deposits, and their analogous offshore and shoreface counterparts. Prodelta mudstones of river-dominated delta successions are largely devoid of burrowing, and contain low numbers of deposit-feeding and grazing structures that record low abundance and low diversity expressions of the Cruziana Ichnofacies. Current-generated structures, syndepositional deformational structures, and synaeresis cracks are abundant. Many intervals include beds of structureless mudstone, reflecting abundant deposition of fluid mud and development of soupground conditions. In contrast, wave-dominated systems yield prodelta mudstones with higher diversity, but low abundance expressions of the Cruziana Ichnofacies. Facies-crossing structures of trophic generalists and opportunists dominate the ichnological suites. Hummocky cross-stratification and storm-induced oscillation ripple laminated tempestites are abundant, though soft-sediment deformational structures and synaeresis cracks are commonly intercalated. Offshore units associated with non-deltaic shoreface intervals are typically intensely bioturbated and contain diverse, archetypal to distal expressions of the Cruziana Ichnofacies or more rarely, the Zoophycos Ichnofacies. Intervals display a close affinity with prodelta units of wave-dominated deltas and although there are subtle differences, discrimination can be difficult. Non-deltaic offshore deposits grade along-strike into wave-dominated prodelta units. The reduced numbers of burrows in prodelta mudstones correspond to significantly higher sedimentation rates and heightened physico-chemical stresses, compared to non-deltaic offshore deposits. Sediment-gravity deposits are restricted to the deltaic intervals exposed to hyperpycnal flows. Synaeresis cracks associated with these mass-flow deposits record freshet emplacement, and commonly accompany mud turbidites. The presence of structureless unburrowed mudstones, consistent with deposition of fluid muds and development of soupground conditions, are atypical of offshore settings but common to prodeltaic areas. Their recurring presence is diagnostic of deltaic influence. River-dominated delta-front deposits are largely characterized by structureless silty sandstones and sandstones with zones of intense synsedimentary deformation. Numerous intervals are devoid of bioturbation. Ichnological suites are typified by very low numbers of deposit-feeding and grazing structures with exceedingly rare suspension-feeding structures. Diversities are very low, reflecting a highly impoverished but proximal expression of the Cruziana Ichnofacies. Wave-dominated delta-front deposits, in contrast, mainly consist of stacked, hummocky and swaley cross-stratified tempestites, interbedded with oscillation-rippled sandstones and draped by largely unburrowed, black fissile mudstones. These tempestite-dominated intervals are commonly capped by trough cross-stratified, current rippled and low angle planar cross-stratified proximal delta-front deposits. Soft-sediment deformation features and structureless beds are less common than in river-dominated counterparts. Facies show low intensities of bioturbation, though diversities are moderately high. Suites are dominated by deposit-feeding structures, reflecting a proximal expression of the Cruziana Ichnofacies alternating with event beds low numbers of opportunistic (facies crossing) elements of the Skolithos Ichnofacies. The delta-front deposits contrast markedly with facies of non-deltaic shorefaces, which typically possess large numbers and diversities of both deposit-feeding and suspension-feeding structures, accompanied by lesser grazing structures. Intervals comprise a transition from proximal expressions of the Cruziana Ichnofacies (lower shoreface) to the archetypal Skolithos Ichnofacies (middle to upper shoreface). With increased storm reworking, differences between shoreface and delta-front deposits are obscured. This is due to erosion of the fairweather beds and/or reduction of the colonization window. Delta-front deposits lack abundant suspension-feeding structures, probably as a consequence of heightened water turbidity (which interferes with filter feeding apparatuses) and dilution of food resource concentrations with respect to the total sediment volume. Additionally, post-storm mantling of the substrate with carbonaceous muds shields the event beds from infauna, and depletes bottom-water O2 values during oxidation of the organic debris. Fluid muds mantle sandy substrates and yield soupground conditions. The combination of these unique ichnological and physical characteristics may be sufficient, in many cases, to differentiate deltaic successions from non-deltaic shoreface systems with relatively few cored intervals.

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