Mixed Autogenic and Allogenic Origin of Stratigraphic Complexity in Paralic Environments: Examples From the Neslen Formation, Mesaverde Group, Utah

Abstract

Discerning the roles of autogenic and allogenic controls on the accumulation and preservation of sedimentary successions requires characterization at a variety of scales. This is especially true in paralic environments for which the preserved stratigraphic record is complicated by spatial and temporal interactions of fluvial, wave and tidal processes. The Campanian Neslen Formation (Book Cliffs, Utah) represents a tide- and wave-influenced, fluvial coastal plain. Detailed lithofacies, architectural element and sequence stratigraphic analyses of the 100-m thick succession has involved the collection of 150 graphic logs (3.5 km total length), 132 architectural panels (representing 33.5 km), 4150 paleocurrent readings, analysis of ichnofacies, and the tracing of key stratal surfaces over a 30-km transect from relatively more landward to basinward locations. Studied outcrops expose the succession in a range of orientations relative to the paleoshoreline, enabling complicated architectural-element geometries to be mapped. The formation comprises architectural elements representative of the following sub-environments: (i) fluvial and tidally influenced point bars that are sandstone dominated and heterolithic, respectively; (ii) distributary channels; (iii) wave-influenced shorefaces; (iv) mouth bars and bay-head deltas; and, (v) fine-grained, commonly coal prone, floodplains and overbanks. Lithofacies analysis has enabled the internal architecture of individual elements to be discerned; regional mapping of these elements demonstrates their geometry and relationship to each other. The application of a decompaction routine to the coal-prone succession has enabled regional-scale correlation between sub-environments and hence correlation of architectural elements within an established sequence stratigraphic framework. Spatial and temporal variations in sub-environments within this framework can therefore be explained. For example, sand-prone point bar elements decrease in abundance down-dip in response to increased avulsion frequency; some mouth-bar elements occur anomalously far up-dip and record delta lobe auto-compaction following maximum transgression. Results demonstrate the need to undertake analysis at multiple scales (bed-set, element, reservoir and sequence) for the development of realistic depositional models that account for spatial and temporal stratigraphic complexity driven by mixed autogenic and allogenic processes.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016