Process Variability and Morphology of Mouth Bars: Examples From the Modern and Ancient

Abstract

Mouth bars develop where fluvial distributaries deliver coarse sediment to a shoreline. As a consequence, they are typically considered, by default, to be fluvial-dominated. Detailed studies of both modern and ancient shorelines show this to be an oversimplification. Mouth bars can have a complicated internal architecture strongly influenced by wave and/or tidal reworking and can, in fact, be wave- or tide-dominated. We show examples of end-member, wave- and tide-dominated mouth bars and compare these to the morphology of a classical fluvial-dominated mouth bar model. The examples show that the balance of fluvial, wave and tidal processes operating at the mouth bar location can result in highly contrasting mouth bar geometries, facies stacking patterns, preserved sedimentary structures and the internal architectures of baffles and barriers. Contrasting mouth bar growth patterns can also significantly affect the large-scale architecture of the system. We use geocellular modeling to illustrate the architecture of an ancient wave-dominated, tide-influenced mouth bar system from the Upper Cretaceous (Campanian) Horseshoe Canyon Formation, exposed near Drumheller, Canada. Near continuous outcrop exposure, in combination with four cored wells and seventy five wireline logs, allow detailed characterization of the mouth bar system in outcrop and subsurface. A hierarchy of depositional units are included in the modeling. At the largest scale is a Regressive Element Complex Assemblage Set (RECAS) up to 8 m thick. This consists of two wave-dominated, tide-influenced, fluvial-affected (Wtf) Mouth bar Element Complexes (EC). Each Wtf-Mouthbar EC can be subdivided into three to four Element Sets (ES), made up of multiple kilometre-scale mouth bar Element (E) lobes. Dipping, mud-draped clinoform surfaces are identified at two scales (bed- and bedset-scale), and tidal bundle sequences are recognized within clinoform bedsets. Geocellular models have been constructed at a scale sufficiently fine to enable the reconstruction of individual mouth bar lobes and intra-lobe baffles and barriers. A partial modern analogue from the Mitchell River Delta, Gulf of Carpentaria, Australia, is also used to help constrain planform geometries of the mouth bar sand-body and distributary channels. The resulting models can be used to assess the impact of intra-lobe architectures on reservoir connectivity and fluid flow characteristics.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015