Lake-Type and Lacustrine Deposits of an Early Foreland Basin: An Example From Alberta, Canada

Abstract

Lake-types are typically interpreted from purely continental basins, but lake-type can also be recognized in early marine foreland basins where lacustrine packages are closely associated with brackish marine deltaic and estuarine systems. Our example from Alberta, Canada demonstrates that core- and log-based study of stratigraphic packaging, sedimentology, and ichnology, together with geochemistry and determination of kerogen type, can distinguish between intercalated marginal marine and lacustrine deposits. Thin stratigraphic units (~2-4 m thick) bounded by flooding surfaces represent lake flooding and shallowing. Black cemented and banded mudstones represent profundal lake deposits, lenticular- and wavy-bedded heterolithics represent sublittoral settings, and wavy-bedded and trough cross-laminated sandstones that represent littoral to eulittoral zones form an idealized succession. The packages are typically exposed, as evidenced by roots and terrestrial bioturbation (e.g., Taenidium barretti), and are capped by sharp co-planar exposure surfaces and flooding surfaces. The depth to water table during these exposure periods varies and is related to larger-scale packaging of stacked units. Rooting and pedogenesis of profundal facies is common, demonstrating rapid migration of the shoreline during periods of lake level rise and fall, and providing evidence for a dominantly underfilled, and/or shallow, stratified balanced-fill lake-type. Littoral to supralittoral facies assemblages vary by position within the lake basin, from low-amplitude wave-rippled quartzose sandstones, to plant-rich slumped quartzose sandstones, to rooted and bioturbated (Skolithos) quartzose sandstones, to lithic-rich ripple-laminated and trough cross-bedded sandstones. This variability highlights the pronounced heterogeneity of shoreline areas, and may indicate the presence of lake sub-basins within our study area. These deposits contrast with associated deltaic and estuarine settings primarily by their trace fossil assemblages and packaging of stratigraphic units, which are thin and bounded by distinct flooding surfaces in the lacustrine portions of the succession.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018