Doctor of Philosophy
A. Guy Plint
Clastic, upper Albian-lower Cenomanian strata were deposited in a low-accommodation backbulge depozone of the Western Canada Foreland Basin in SW Alberta. These strata are lithologically very heterogeneous and encompass a spectrum of depositional environments along an alluvial to offshore transect. These rocks are assigned, in subsurface, to the Lower Colorado Group, and in outcrop to the upper Blairmore Group. Lithological heterogeneity, as a result of rapid lateral facies changes, resulted in diverse nomenclature that obscured genetic relationships between time-equivalent strata. The present study integrates wireline log, core, and outcrop data to establish a high-resolution allostratigraphic framework which allowed recognition of the Basal Colorado, Joli Fou, Viking and Westgate alloformations, in ascending order. Each alloformation comprises several allomembers and parasequences. The Basal Colorado alloformation is a NE thinning wedge that progressively onlaps lower Albian Mannville Group strata. Deposition took place in a brackish-water embayment open to the south. Basal Colorado strata correlate with the Lynx Creek Member of the Mill Creek Formation in outcrop. The Joli Fou and Viking alloformations have a broadly sheet-like geometry. Both alloformations comprise three allomembers, composed of a complex array of river-dominated and mixed-influence deltaic successions. The location of successive deltas was strongly influenced by differential compaction of underlying deltas. Sandstone-rich Joli Fou rocks pass northward into offshore mudstone. To the south and west, marine Joli Fou and Viking deposits grade laterally into alluvial facies. Only the upper part of the Westgate alloformation extends into SW Alberta. Tectonic tilting is indicated by the lowermost (alluvial) and uppermost (marine) Westgate parasequences, which are SW and NE thickening wedges, respectively. Paleo-valleys, typically ~20 m deep, are incised into the top surface of most allomembers. Different examples of valley incision and fill can be attributed to both sea-level changes, and also to climatically-controlled changes in the sediment load to discharge ratio. Two pulses of thrust-induced loading and a long term (>1 myr) eustatic cycle provided the overall control on stratal architecture, whereas climate-controlled cycles in the ~400 and 100 kyr band exerted an important control on the development of allomembers and parasequences. Eustatic changes are estimated to have been ~20 – 30 m.
Summary for Lay Audience
Cretaceous rocks in SW Alberta, about 100 Myr old, were deposited in a sedimentary basin that formed to the east of the actively rising Rocky Mountains. Sediment eroded from the mountains was deposited on floodplains and in deltas fringing a shallow seaway that extended across North America, from the Gulf of Mexico to the Polar Ocean. This shallow seaway, that existed for tens of millions of years, was due to 100-200 m of sea-level rise caused by the formation of new ocean spreading centres as the supercontinent Pangea broke apart. This study provides a very detailed analysis of the Cretaceous rocks and shows that they contain evidence for numerous cycles of rise and fall of sea-level, on a time scale much less than one million years. Such, ‘high frequency’ cycles can not be explained by large-scale plate tectonic processes, but instead, are due to cyclical changes in climate, controlled by changes in Earth’s orbit around the sun. Detailed correlation of transgressive and regressive sedimentary cycles has shown that the shallow-marine Cretaceous rocks are organized into larger packages that represent about 400,000 years, in turn composed of smaller packages that represent about 100,000 years. These time scales correspond to the natural eccentricity cycles of Earth’s orbit. The mechanisms that cause sea-level to vary in response to warming and cooling of climate are much debated: Growth and decay or polar ice caps is well-known to cause rapid sea-level change, but traditionally, the mid-Cretaceous is considered to have lacked ice caps. Alternatively, changing volumes of water stored in aquifers, caused by wetter and drier climate, may have effected sea-level change. Both mechanisms probably contributed to the observed sea-level cycles. The rocks also show that rivers alternately cut and then infilled valleys: this observation is best explained in terms of cyclical changes in river discharge from the Rocky Mountains, also linked to climate change. This study provides predictive insight into climate and sea-level changes in an ancient, apparently higher-than-present CO2 ‘greenhouse world’.
Galic, Slavena, "Allostratigraphy of the Lower Colorado Group (Cretaceous) in south-west Alberta" (2021). Electronic Thesis and Dissertation Repository. 8230.