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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 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.
Inoceramid bivalves are the dominant invertebrate fauna of the Coniacian and basal Santonian of the Western Canada Foreland Basin in western Alberta. In the upper lower Coniacian through to basal Santonian, six successive faunas are recognized, which provide the basis for corresponding, formally defined inoceramid zones. From bottom upward these are the zones of: Cremnoceramus crassus crassus /C. deformis deformis, Inoceramus gibbosus, Volviceramus koeneni, Volviceramus involutus, Sphenoceramus subcardissoides, and Sphenoceramus ex gr. pachti. Particular faunas represent assemblages known widely from the Euramerican biogeographic region, although they characterize mostly its northern, boreal area. The inoceramid-based biostratigraphic scheme allows correlation with other parts of the North American Western Interior and with parts of the Euramerican biogeographic region.
The studied succession provides a good record of the Inoceramus gibbosus Zone, which characterizes the topmost lower Coniacian. This zone, first recognized from northern Germany, is usually absent, both in Europe and in North America, due to a stratigraphic gap resulting from a eustatic lowstand. The base of the middle Coniacian is marked by the abrupt appearance of the taxonomically variable Volviceramus fauna (V. koeneni (Müller), V. exogyroides (Meek and Hayden)), with associated Inoceramus undabundus Meek and Hayden and Volviceramus cardinalensis, newly described herein. Scaphites (Scaphites) ventricosus Meek and Hayden, the ammonite marker of the base of the middle Coniacian first appears in the late early Coniacian. The base of the upper Coniacian marks the first appearance of the characteristic northern inoceramid species Sphenoceramus subcardissoides (Schlüter), the appearance of which coincides with Scaphites (Scaphites) depressus Reeside, the ammonite marker of this boundary. Close to this boundary Volviceramus stotti also appears, which is newly described from the Canadian sections. The base of the Santonian corresponds to the abrupt appearance of Sphenoceramus ex gr. pachti (Arkhangelsky).
The studied sections demonstrate that the appearance of new inoceramid faunas (lowest occurrence of Cremnoceramus crassus crassus (Petrascheck), of various species of Volviceramus, Sphenoceramus subcardissoides (Schlüter) and of S. ex gr. pachti) takes place immediately above major marine flooding surfaces, suggesting a close correspondence between evolutionary and/or migration events and episodes of relative sea-level rise.
The Upper Cretaceous (Coniacian-Santonian) of the Western Canada Foreland Basin, contains a rich record of scaphitid ammonites (scaphites). We describe four species: Scaphites (Scaphites) preventricosusCobban, 1952, Scaphites (S.) ventricosusMeek and Hayden, 1862, Scaphites (S.) depressus Reeside, 1927, and Clioscaphites saxitonianus (McLearn, 1929). These are widespread index fossils that demarcate the upper lower-middle, middle, and upper Coniacian, and the lower Santonian, respectively. They occur in the lower part of the Wapiabi Formation, Alberta. The Coniacian part of the section has been divided into 24 informal allomembers based on the recognition of marine flooding surfaces, most of which can be traced through the >750 km extent of the study area. The most distinctive feature in the ontogenetic development of scaphites is the change in coiling during ontogeny. At the approach of maturity, the shell uncoils slightly, forming a shaft, which then recurves backward approaching the earlier secreted phragmocone. However, this sequence of scaphites shows an evolutionary trend toward recoiling, accompanied by an increase in size and degree of depression. These changes occurred against a background of changing environmental conditions resulting from the expansion of the Western Interior Seaway during the Niobrara transgression. This resulted in an increase in the area of offshore habitats, which may have promoted the appearance of larger species with more depressed whorl sections. Scaphites probably lived at depths of less than 100 m, and may have fed on small organisms in the water column.