Sedimentology and Allostratigraphy of the Cardium Formation (Turonian-Coniacian) in southern Alberta, and equivalent strata in northern Montana
Doctor of Philosophy
Dr. Guy Plint
The Turonian-Coniacian Cardium Formation of southern Alberta consists of marine sandstone and mudstone, deposited in a foreland basin over ~2.3 m.y. The formation thins from 150 m in the western foredeep to 50 m 350 km to the east. Correlation of 10 regional flooding surfaces in >1200 well logs and 25 outcrops provides an allostratigraphic framework. Mapping of two previously-unrecognized erosion surfaces (E5.2 and E5.5) reveals additional complexity within sandstone of the Raven River Member.
Facies successions coarsen upwards from thinly-bedded and bioturbated mudstone, to heterolithic facies (either bioturbated or bedded), to clean sandstone. Where conglomerate unconformably overlies this succession, the base of the conglomerate marks a regional erosion surface. A tabular geometry, and wave-formed sedimentary structures even in distal settings, indicate deposition on a very low gradient shelf within the mud accommodation envelope (~70 m), and at times, within effective wave-base for sand (~40 m).
Isopach maps reveal two pulses of isostatic subsidence that were separated by a period of tectonic quiescence. The locus of flexural subsidence shifted southward by >200 kilometres in <1.2 m.y. during the first pulse of subsidence, indicating a southward shift in the position of the active load. Basement structures, including the Vulcan Low, Bow Island Arch, Red Deer High, and faults, caused differential subsidence and localized thickness changes in several allomembers.
The relative sea-level history of the Cardium Formation is characterized by two major relative sea-level falls (Late Turonian and Early Coniacian), separated by a major transgression (latest Turonian, Niobrara transgression). Higher-frequency relative sea-level changes are superimposed on this long-term curve.
Three regressive-transgressive cycles in the Cardium Formation can be correlated to similar cycles in the Bohemian Basin of the Czech Republic, based on biostratigraphic evidence, indicating a probable eustatic mechanism. In combination with previous studies, flooding surfaces that define Cardium allomembers have now been correlated >900 kilometres along strike, ~300 kilometres offshore, and between depocentres, suggesting a eustatic mechanism. Facies relationships across flooding surfaces suggest that eustatic fluctuations had a magnitude of ~12 m and a frequency of ~250 k.y., and are therefore best explained by glacioeustasy.
Shank, Joel, "Sedimentology and Allostratigraphy of the Cardium Formation (Turonian-Coniacian) in southern Alberta, and equivalent strata in northern Montana" (2012). Electronic Thesis and Dissertation Repository. 557.
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