Ichnology and the Characterization of Brackish-Water Paleoenvironments within the Lower Cretaceous Mannville Group of East-Central Saskatchewan and West-Central Alberta, Canada
Preliminary ichnological-sedimentological study from 51 cored wells of the Lower Cretaceous (Early to mid-Albian) Mannville Group (Sparky, Waseca, and McLaren formations) in east-central Saskatchewan and west-central Alberta yield ten recurring facies that reflect conditions of reduced and fluctuating salinity, typical of brackish-water settings. The successions display physical sedimentological features dominated by abundant organic-rich mudstone drapes of probable fluid-mud origin, synaeresis cracks, soft-sediment deformation, carbonaceous detritus, normally graded mudstones, and wave, current and combined-flow ripples. Facies are generally heterolithic, attesting to autocyclic fluctuations in depositional processes.
Ichnologically, bioturbation intensities range widely (BI 0-5), and facies are characterized by abundant diminutive ichnogenera comprising low-diversity (locally monogeneric) trace fossil suites. Common ichnogenera include Gyrolithes, Teichichnus, Planolites, Palaeophycus, Cylindrichnus, Skolithos, Thalassinoides, Chondrites, and Lingulichnus. Rare but important elements include Asterosoma, Lockeia, Phycosiphon, Rosselia, and Rhizocorallium. Most suites consist of mixtures of elements characteristic of both the Skolithos Ichnofacies and the Cruziana Ichnofacies. Persistent ichnogenera constitute facies-crossing elements that record permanent to semi-permanent dwellings of deposit-feeding infauna. Additional behaviours include navichnia (sediment-swimming structures) and fugichnia (escape structures), consistent with rapid and episodic deposition of sand as well as mud (e.g., via flocculation). Variations in bioturbation intensity primarily attest to marked changes in sedimentation rates.
The integration of ichnological and sedimentological datasets indicates that deposition occurred within brackish-water embayments, bay-head deltas, tidal flats, and coastal plain environments. Variations in trace fossil diversities and the introduction of less markedly facies-crossing ichnogenera (e.g., Asterosoma, Phycosiphon, and Rhizocorallium) may suggest that elevated salinities occurred periodically within the study area.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009