Controls on the deposition, accumulation and preservation of mixed fluvial and marginal-marine successions in coastal-plain settings.

Discerning the roles of autogenic and allogenic controls on the deposition, accumulation and preservation of sedimentary successions requires characterisation at a variety of scales. This is especially true for paralic environments where the preserved stratigraphic record is complicated by spatial and temporal interactions of fluvial, wave and tidal processes. The Campanian Neslen Formation (Utah) represents a marine influenced fluvial succession that accumulated in a humid, low-latitude coastal plain. Detailed lithofacies, architectural element and sequence stratigraphic analyses of the succession has involved the collection of 106 sedimentary logs, 194 architectural panels, 2000 paleocurrent readings, analysis of ichnofacies, and the tracing of key stratal surfaces in order to elucidate the relative balance of autogenic and allogenic processes. Outcrops in a range of orientations relative to the palaeoshoreline, enable the geometries of complicated architectural elements to be constrained. Mapping of channelised elements in three dimensions and the quantified analysis of their facies and geometry has been undertaken. Study sites, average spacing 3 km, have been used to produce a regional-scale correlation between sub-environments. The lower Neslen Formation accumulated as part of a high-accommodation, transgressive succession with variable influence from marine processes. Point-bar elements in the lower Neslen Formation are isolated and lithofacies assemblages within these elements deviate from widely used facies models as a result of the combined effects of low fluvial discharge and the presence of raised mires, which acted to modify channel dynamics. High-resolution correlation of strata have enabled identification of a several marine-influenced intervals and hence a refined sequence stratigraphic framework is proposed. The upper Neslen Formation is interpreted to represent part of a lower accommodation, highstand succession within which channelised elements became increasingly amalgamated upwards. This study demonstrates a rare example of the transfer of the fluvial-to-marine transition zone into the stratigraphic record and the implications of this for the distribution of reservoir heterogeneities. In contrast with previous studies that emphasise sea-level change as the dominant control on paralic successions; this stratigraphic dataset demonstrates the extent to which autogenic processes can modify the allogenic stratigraphic signature.