Abstract
Lower to upper Coniacian rocks in the foredeep of the Western Canada Foreland Basin are dominated by mudstone and subordinate sandstone and were deposited on a very low-gradient, storm-dominated marine ramp. The rocks are organized into several scales of upward-coarsening, upward-shoaling succession, bounded by marine flooding surfaces. Abundant, publicly available
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wireline log data permit flooding surfaces to be traced for hundreds of kilometers in subsurface. Flooding surfaces can be considered to approximate time surfaces that allow the subsidence history of the basin to be reconstructed. Particularly widely traceable flooding surfaces were chosen, on pragmatic grounds, as the boundaries of 24 informal allomembers, most of which can be mapped along the foredeep for >750 km. Allomembers can also be traced westward into the fold-and-thrust belt to outcrop in the Rocky Mountain Foothills. Some flooding surfaces are mantled with intra- or extrabasinal pebbles that imply a phase of shallowing and, potentially, subaerial emergence of part of the ramp. The rocks yield a rich and well-preserved molluscan fauna dominated by inoceramid bivalves and scaphitid ammonites. Several major inoceramid speciation events are recognized. The lowest occurrence of Cremnoceramus crassus crassus, various species of Volviceramus, Sphenoceramus subcardissoides, and S. pachti all appear immediately above major flooding surfaces, suggesting that speciation, and dispersal of new inoceramid taxa were closely linked to episodes of relative sea-level rise. Thus, the boundaries of biozones can be shown to coincide with physical stratigraphic (flooding) surfaces. The generally rare species Inoceramus gibbosus is abundant in the upper part of the lower Coniacian; the preservation of this zonal form may be attributed to rapid subsidence of the foredeep that outpaced a major eustatic? sea-level fall that took place at the end of the early Coniacian and that is marked by a hiatus in most epicontinental basins. Regional mapping shows that allomembers, which have a neartabular geometry, can be grouped into “tectono-stratigraphic units” that fill saucer-shaped, flexural depocenters. Individual depocenters appear to have been active for ca. 0.5 to 1.5 m.y., and successive depocenters are offset laterally, probably reflecting episodic shifts in the locus of active thickening in the Cordilleran orogenic wedge and related subsidence in the foreland basin. Preliminary carbonisotope results from one section are tentatively correlated, using biostratigraphic tie-points, to the English Chalk reference curve: the Light Point, East Cliff, and White Fall carbon-isotope events (CIE) are recognized with some degree of confidence. The astronomically calibrated succession of CIE in the English Chalk suggests that the 24 mapped allomembers in Alberta each had an average duration of about 125,000 yr. Because allomembers can be traced for hundreds of km, an allogenic control, probably eustasy, appears to be the most likely genetic mechanism.
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