Stratigraphy, Sedimentology, and Ichnology of the Middle Cambrian to Lower Ordovician Deposits in Subsurface Western Canada

During the past decade, the economic importance of the preserved Cambrian-Ordovician succession in subsurface Alberta and Saskatchewan has increased considerably, mainly due to the development of geothermal, carbon sequestration, and helium exploration and extraction projects. Despite this, the construction of a depositional model based on the integration of sedimentologic, ichnologic, and sequence stratigraphic datasets has not been attempted before. For this purpose, eighty-six well-cores recovered across the Western Canada Sedimentary Basin that include this succession were analyzed in detail. In the area of study, the middle Cambrian to Lower Ordovician strata, has been subdivided into three units: the Basal Sandstone Unit (BSU), the Earlie Formation, and the Deadwood Formation. Numerous wells have been drilled through these units since the 1950s, however, just a group of these wells present relatively continuous core recovery available for analyses. The Basal Sandstone Unit is characterized by conglomerate and sandstone of fluvial origin. In southwestern Saskatchewan, BSU unique boulder conglomerate and conglomeratic sandstone facies represent fan-delta deposits that are associated with basement highs existing in the area. The Earlie Formation comprises sandstone, mudstone and local carbonate representing tidally influenced bay margin and proximal bay facies, as well as storm influenced distal bay to offshore facies. The Deadwood Formation consists mainly of sandstone, mudstone and flat-pebble conglomerate representing offshore to shoreface facies and wave-influenced prodelta to delta front settings. This succession provides a great opportunity to gain insight into the depositional processes and the endobenthic colonization of pre-vegetated and highly stressful settings in the Early Phanerozoic. Trace fossil analyses indicate the combinations and ranking of stress factors affecting the colonizing fauna is diverse throughout the succession. BSU fan-delta deposits display sparse to moderate bioturbation in mid- to outer-delta subaqueous deposits, indicating that endobenthic colonization was possible, even during short colonization windows, when stresses were ameliorated. BSU braided fluvial deposits lack bioturbation, indicating stressed settings with high energy conditions and is consistent with the worldwide absence of animal life in continental settings during the early Paleozoic. Earlie Formation bay margin and proximal bay deposits are distinguished by low to moderate bioturbation intensities, characterized by “impoverished” versions of the Cruziana and Skolithos ichnofacies, reflecting salinity as the main control of bioturbation in these brackish-water settings. High bioturbation intensities are attained in open-marine settings where relatively stable conditions are met. This is observed in distal bay and offshore transition moderate to intensely bioturbated deposits of the Earlie and Deadwood formations, formed under near-normal marine, low-energy, and well-oxygenated conditions and characterized by elements of the Cruziana ichnofacies. However, shelf and offshore facies also accumulated under very low energy, tend to be unbioturbated to sparsely bioturbated, indicating in this case, that oxygen and food supply might have been the main stress factors. In the Deadwood Formation shoreface deposits, bioturbation depends on the frequency and intensity of storms. Fairweather deposits display moderate to high bioturbation, while tempestites are reworked only by opportunistic colonizers. Wave influenced deltaic intervals of the Deadwood Formation are characterized by displaying sparse to no bioturbation and abundant soft-sediment deformation structures, reflecting the interplay of stressors in delta settings, most notably freshwater discharge and high sediment supply. The middle Cambrian to Lower Ordovician succession records a complex depositional history involving several relative sea-level changes. At least two third-order sequences have been recognized. The lower third-order sequence comprises the Basal Sandstone Unit, the Earlie Formation, and possibly the lower part of the Deadwood Formation. BSU fluvial to fan delta deposits were accumulated on top of the Precambrian basement, at the onset of a marine incursion over the North American craton, representing a lowstand systems tract. Subsequent transgression led to the replacement of continental settings by brackish-water settings. Earlie Formation embayment to offshore strata overlay the BSU, representing a transgressive systems tract. The highstand systems tract of this lower sequence is possibly recorded in central and southcentral Saskatchewan, where thin parasequences seem to show an aggradational pattern. In this same area, forced regressive shoreface facies, attributed to the Deadwood Formation, marks the upper boundary of the lower sequence. Regressive shoreface facies are in turn overlain by Deadwood offshore mudstones that represent the transgressive systems tract of the upper sequence. A second highstand systems tract is recognized in the uppermost interval of the Deadwood Formation in some areas in central and eastern Saskatchewan. The upper boundary of the upper sequence is marked by the erosional unconformity existent between the Deadwood Formation and the Ordovician Winnipeg and Red River formations.

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