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When the Wave-Dominated Shoreface Model Breaks down: Observations from from Ancient Wave-Dominated, Tide-Influenced Shoreline System

Vakarelov, Boyan 1; Ainsworth, Bruce 1
1 Australian School of Petroleum, CO2CRC, Adelaide, SA, Australia.

Existing wave-dominated facies models are in essence microtidal and do not account for wave-dominated environments significantly influenced by tides. Observations from modern systems show that such systems are abundant along tide-influenced shorelines facing wide shelves and large embayments, suggesting that such deposits should be well represented in the ancient record. Such sedimentation styles can result in the poor development of upper shoreface, cross-stratified intervals due to the daily re-working of subaqueous bars and dunes, and the formation of an anomalously thick horizontal planar stratified interval due to the daily migration of swash waves across the intertidal zone. The association of many wave-dominated, tide-influenced environments with shallow shelves also suggests that these environments can be affected by sediment reworking and erosion during major storms significant distances from a shoreline.

We present an ancient example of a wave-dominated, tide-influenced shoreline from the Campanian Bearpaw to Horseshoe Canyon Formation transition near Drumheller, Alberta, Canada. The coarsening-up succession, examined in both outcrop and core, shows the following differences from a typical wave-dominated, shoreface facies model:

(i) The coarsening-up succession shows abundant double carbonaceous/mud drapes and tidal rhythmites, which can also co-exist in the same strataset, suggesting tidal influence during sedimentation.

(ii) The boundary between the mudstone-dominated and sandstone-dominated portions of the succession is abrupt and scoured, which is interpreted to indicate a shallow receiving basin during deposition rather than evidence of forced regression.

(iii) Even though there is a well-defined swaley cross-stratified interval, indicating storm influence, hummocky cross-stratification is lacking. Tidal influence in the swaley cross-stratified interval is restricted to tops of swale bedsets and probably represent remnant fair-weather deposits.

(iv) Conformably overlying the swaley cross-stratified facies association is a four meter thick horizontal planar stratified, carbonaceous layer-bearing interval, which in turn is overlain by a sub-regionally distributed rooted horizon. The planar stratified interval is interpreted as an intertidal beach deposit.

We present a new facies model for wave-dominated, tide-influenced systems that accounts for the facies variations described above.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009