Abstract

Mouth bars can develop in locations where fluvial distributary channels deliver coarse sediment to the shoreline. As a consequence, they are typically considered, by default, to be fluvially dominated. However, depending upon the relative balance of fluvial, wave, and tidal energies at any given shoreline location, the preserved sedimentary structures, geometry, and internal architecture of the mouth bar will reflect the relative balance of these energies. Hence, mouth bars can be fluvial-, wave-, or tide-dominated and can also exhibit sedimentary structures formed by more than one of these processes. The ability to recognize, and to predict the architecture of these mixed-process systems is important for accurately interpreting the ancient record and has implications for hydrocarbon exploration and exploitation strategies.

A detailed description and interpretation is given herein of a mixed-process mouth-bar complex which forms part of an asymmetrical delta system from the Campanian of Alberta, Canada. This mouth-bar complex is interpreted as a mixed-influence, wave-dominated system that transitions laterally into a mixed-influence, fluvially dominated system. The mouth-bar complex represents a single progradational phase of deposition, is up to 8 m thick, and can be traced over an area of approximately 60 km² in both outcrop and the subsurface. It comprises multiple, kilometer-scale lobe geometries (elements, element sets, and element complexes) with internal dipping, mudstone- and carbonaceous-draped clinoform surfaces at two scales (intra-element and inter-element set). The lobe geometries are oriented subparallel to the coastline. The influence of tides on the system is illustrated by tidal bundle sequences in intra-element mouth-bar clinoform bedsets, networks of tidally overdeepened, mudstone-filled distributary channels, and the development of significant thicknesses (up to 3.5 m) of swash-zone deposits at the tops of the wave-dominated mouth-bar successions.

Depositional architectural units of the mouth-bar complex are estimated to have been generated by autogenic events from daily (tidal couplets), through decadal (elements) to centennial (element sets) and millennial (element complex and element complex set) time frames. A Holocene partial analog for these types of depositional systems is suggested from the Gulf of Carpentaria, Australia. Geometries, aspect ratios, and facies relationships of these mixed-process mouth-bar complexes vary significantly from the archetypal fluvial-dominated mouth bars described in the published literature.