Facies Criteria to Differentiate Between Wave-/Storm-Dominated Deltas and Strandplain Shorefaces, Lower Cretaceous Falher Member, Spirit River Formation, Alberta, Canada

Aaron J. DesRoches, James A. MacEachern, and Cameron R. Thompson
Earth Sciences, Simon Fraser University, Burnaby, BC, Canada

The lower cycles of the Albian Falher Member comprises facies successions indicative of mixed wave- and storm-dominated delta complexes and strandplain shoreface settings. Both successions are characterized by interbedded, moderately burrowed mudstones and sandstones, passing into weakly heterolithic intervals dominated by sporadically burrowed, stacked sandy tempestites with HCS and SCS. These pass upwards into amalgamated current ripples and trough cross-stratification, capped by low-angle planar-stratified sandstones, pebbly sandstones and conglomerates.

Establishing reliable facies criteria for the discrimination of non-deltaic strandplain shoreface systems and contemporaneous wave-dominated delta complexes is particularly challenging in the Falher, owing to the marked storm overprint typical of the unit. Distal settings display the greatest facies contrast. Prodeltaic intervals contain syneresis cracks, organic-rich mudstone drapes of fluid-mud origin, current and combined flow ripples, soft-sediment deformation, and carbonaceous detritus. Trace fossil suites are typified by reduced diversities and abundances of ichnogenera, lowered bioturbation intensities (BI 0-2), and a paucity of suspension-feeding structures in the tempestites. Offshore deposits of shorefaces, however, contain only rare fluid mud drapes, and show greater bioturbation intensities (BI 0-5), increased ichnogenera diversities, and the presence of suspension-feeding structures in sandy tempestites. Subtle ichnological and sedimentological features distinctive of deltaic influence may persist upwards into proximal facies, captured in the few fair-weather beds that survived storm amalgamation. The facies characteristics presented here may be useful in identifying other storm-dominated shallow marine deposits as deltaic, which hitherto have been interpreted exclusively as shoreface successions.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas