Doctor of Philosophy
Within a 90,000 km2 study area in SW Alberta, Santonian – basal Campanian strata of the upper Wapiabi Formation are dominated by marine rocks that thin westward from ~300 ~180 m over ~300 km, reflecting apparently spatially uniform flexural subsidence driven by the static load of the orogen. A high-resolution allostratigraphic framework established ten allomembers grouped in four informal tectono-stratigraphic ‘units’, three of which were previously recognized in NW Alberta. In the north and east, the formation is dominated by mudstone organized in numerous metre-scale siltier- and sandier-upward parasequences. A lack of clinoforms and abundant wave-formed sedimentary structures suggest that deposition took place on a shallow, low-gradient, wave-graded ramp where sediment transport was affected mainly by storm-generated combined flows. In the south and west, marine mudstone passes laterally into nearshore sandstone and heterolithic coastal plain facies. Nearshore sandstone facies represent a broadly linear (although with some local irregularities), NW-SE-trending strandplain. Towards the SW, shoreface sandstones interdigitate with coeval coastal plain facies that represent an array of channel, crevasse splay, lake and floodplain environments. A net aggradationalprogradational stacking pattern with limited updip alluvial aggradation reflects minor tectonic subsidence in the SW. Subsequent increase in accommodation suggest minor eustatic rise recorded by subsequent backstep backstep of the shoreface. Integration of present and previous work shows that three tectono-stratigraphic ‘units’ can be recognized over a strike distance of > 800 km. The flexural depocentres of units 1 and 3 extend across the present study area, but unit 2 is confined to a northern depocentre; in the study area, unit 2 time is recorded by thin bioclastic condensed facies that formed on a contemporaneous forebulge. Isopach mapping on an allomember scale (order of 100- 200 kyr) revealed subtle differential subsidence across Precambrian basement domain boundaries resulted in the distortion of the wedge-shaped rock bodies typical of foreland basins. The clearest expression of this deep-seated control is along the boundary between the Vulcan Structure and the Medicine Hat Block. Differential movement between blocks may be attributable to subtle changes in in-plane stress, related to far-field forces resulting from tectonic activity in the Cordillera.
Mohammed, El Mahadia I., "High-resolution allostratigraphy and sedimentology of Santonian - early Campanian Wapiabi Formation in southern Alberta, Western Canada Foreland Basin" (2018). Electronic Thesis and Dissertation Repository. 5351.
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