Depositional Environments and Diagenesis of the Devonian Dawson Bay Formation in Saskatchewan and Northwestern Manitoba
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The Devonian Dawson Bay Formation has been divided into the Second Red Bed, Burr, Neely and Hubbard Evaporite members from base to top. The purpose of this thesis is (1) to describe the sedimentary rocks of the formation from Saskatchewan and northwestern Manitoba, (2) to interpret the depositional environments and diagenesis, and (3) to discuss the implications of the new results. The Second Red Bed Member (SRBM) comprises mudstone and dolostone. This study shows that the mudstone was deposited in environments that ranged from saline- and dry-mudflat to distal alluvial-eolian plain, and that the dolostone formed in a Coorong-like environment. New evidence indicates that the SRBM was affected by pedogenesis, with the paleosol better developed in central Saskatchewan than in southeastern Saskatchewan. This suggests that the paleorelief in the former part of the basin was higher than in the latter, and that the marine waters came from the southeast. This conclusion not only has a scientific significance in reconstructing the paleogeography of the Williston Basin, but also sheds light on the future exploration of petroleum and other economic minerals in the basin. Three paleokarst horizons have been recognized in the contact zone between the SRBM and the Burr Member. The presence of the paleokarsts indicates that before the Williston Basin was completely flooded by seawater during early Burr time, two minor marine transgressions took place in the basin. New evidence shows that the lower Burr Member was deposited in an oxygen-restricted environment. In central Saskatchewan, parts of the lower Burr Member contain fine laminations, but the equivalent units in southeastern Saskatchewan lack these laminations. This indicates that the environment in central Saskatchewan was more oxygen-restricted. Thus, from central to southeastern Saskatchewan, the environment became less oxygen-restricted with increasing water depth. This contradicts the Rhoads-Morse-Byers model, which predicts that oxygen decreases with increasing water depth. In southeastern Saskatchewan, the upper Burr Member consists mainly of wackestone and lime mudstone, with a few thin packstone and grainstone interbeds. This reflects a low energy, relatively deep water environment. In contrast, the proportion of packstone and grainstone increases significantly in the upper Burr Member of central Saskatchewan and northwestern Manitoba. This suggests that the depositional environments in those regions had higher energy and shallower water conditions. From base to top, the depositional environment of the Neely Member changed from relatively deep, offshore settings, through higher energy, shallower water conditions represented by domical stromatoporoids, to intertidal and supratidal conditions. The Hubbard Evaporite Member was deposited in salt pan to saline mudflat environment, and the overlying First Red Bed formed in environments that ranged from saline mudflat, dry mudflat to distal floodplain. New evidence also shows that no significant sedimentary break is present between the Hubbard Evaporite Member and the First Red Bed, and that they belong to a single upward-shallowing succession.