AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
A Conceptual Framework for Wave-Dominated and Tidally Influenced Shorefaces
(1) Earth Sciences, Simon Fraser University, Burnaby, BC, Canada.
(2) Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada.
Tidal processes can have a significant impact on the sedimentological and ichnological character of wave-dominated shoreface deposits. As the influence of tides increases, the resulting shoreface successions begin to depart markedly from those postulated by the conventional, wave-dominated shoreface model, which was built upon essentially non-tidal shoreline settings. In shoreface settings subject to stronger tidal flux, tides can be manifest either directly or indirectly. Direct tidal effects refer to characteristics imparted by tidal energy (e.g., tidal currents), and are best expressed in the offshore and lower shoreface. Key evidence of direct tidal control includes uniform sediment calibres from the upper shoreface to the offshore, and little or no mud preserved in the lower shoreface. Additionally, sands in the lower shoreface and offshore tend to be intensely bioturbated. Where primary stratification is preserved, it largely comprises current-generated structures. Such shoreface deposits are referred to as tide-influenced shorefaces, and are expected in settings with low storm-wave input and strong tidal currents (e.g., straits). Indirect tidal influences are manifest by the lateral translation of wave zones across the shoreface owing to changes in water depth during the tidal cycle. This is best developed in macrotidal to megatidal settings. Indirect tidal influences are more pronounced in the upper and lower shoreface, and are recorded through the interbedding of sedimentary structures produced by shoaling waves, breakers and surf, swash-backwash, and surface runoff. The boundaries between shoreface subenvironments are correspondingly poorly defined. The foreshore in settings of elevated tidal range is also generally much thicker (typically 4 to 5 m). Bioturbation tends to be patchy in distribution across the shoreface, and dominated by vertical structures. Such systems are defined as tidally modulated shorefaces.
Using well-established sedimentological and ichnological criteria for recognizing wave-dominated (non-tidal) shorefaces - wherein sediment deposition is nearly wholly controlled by fair-weather wave and storm-wave processes - a conceptual model is developed for discriminating fair-weather (non-tidal) shorefaces, storm-influenced (non-tidal) shorefaces, and tidally influenced shorefaces. Five shoreface archetypes are defined: storm-affected, storm-influenced, storm-dominated, tide-influenced, and tidally modulated.