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Evaluating delta asymmetry using three-dimensional facies architecture and ichnological analysis, Ferron ‘Notom Delta’, Capital Reef, Utah, USA

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Abstract

Delta asymmetry occurs where there is strong wave influence and net longshore transport. Differences in the morphology and facies architecture between updrift and downdrift sides of asymmetric deltas are potentially significant for exploration and exploitation of resources in this class of reservoirs. Although delta asymmetry has been recognized widely from modern wave-influenced deltaic shorelines, there are few documented examples in the ancient record. Based on an integrated sedimentological and ichnological study, the along-strike variability and delta asymmetry within a single parasequence (Ps 6) is documented in continuously exposed outcrops of the Cretaceous Ferron Sandstone Member of the Mancos Shale Formation near Hanksville in southern Utah. Two intra-parasequence discontinuity surfaces are recognized which allow subdivision of the parasequence into three bedsets, marked as Ps 6-1 to Ps 6-3. Four facies successions are recognized: (i) wave/storm-dominated shoreface; (ii) river-dominated delta front; (iii) wave/storm-reworked delta front; and (iv) distributary channel and mouth bar. Dips of cross-strata within distributary-mouth bars and shorefaces show a strong downdrift (southward) component. Ps 6-3 predominantly consists of river-dominated delta-front deposits, whereas Ps 6-1 and Ps 6-2 show an along-strike facies change with shoreface deposits in the north, passing into heterolithic, river-dominated delta-front successions south to south-eastward, and wave/storm-reworked delta-front deposits further to the south-east. Trace fossil suites correspondingly show distinct along-strike changes from robust and diverse expressions of the archetypal Cruziana Ichnofacies and Skolithos Ichnofacies, into suites characterized by horizontal, morphologically simple, facies-crossing ichnogenera, reflecting a more stressed, river-dominated environment. Further south-eastward, trace fossil abundance and diversity increase, reflecting a return to archetypal ichnofacies. The overall facies integrated with palaeocurrent data indicate delta asymmetry. The asymmetric delta consists of sandier shoreface deposits on the updrift side and mixed riverine and wave/storm-reworked deposits on the downdrift side, similar to that observed in the modern examples. However, in contrast to the recent delta asymmetry models, significant paralic, lagoonal and bay-fill facies are not documented in the downdrift regions of the asymmetric delta. This observation is attributed to a negative palaeoshoreline trajectory during delta progradation and subsequent transgressive erosion. The asymmetric delta was induced by net longshore transport from north to south. The forced regressive nature of the delta precludes significant preservation of topset mud.

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