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TitleA Late Ordovician ancient loessite deposit in a glacially-influenced setting?: the Milton member of the Queenston Formation in southern Ontario, and synthesis of the background concepts behind a novel interpretation
AuthorHamblin, A P
SourceGeological Survey of Canada, Open File 7860, 2016, 42 pages (1 sheet), (Open Access)
PublisherNatural Resources Canada
Documentopen file
Mediaon-line; digital
File formatpdf
NTS40I; 40P; 40J/01; 40J/02; 40J/03; 40J/07; 40J/08; 40J/09; 40J/16; 40O/01; 30L/13; 30L/14; 30M; 41A; 41H/03
AreaWindsor; London; Waterloo; Kitchener
Lat/Long WENS -83.5000 -79.0000 45.5000 42.0000
Subjectssedimentology; stratigraphy; structural geology; tectonics; fossil fuels; Upper Ordovician; stratigraphic analyses; tectonic setting; lithology; depositional environment; tectonic elements; structural interpretations; structural features; paleolatitudes; paleoclimates; Caradocian; structural analyses; unconformities; sedimentation; lithofacies; facies; paleocurrents; paleocurrent analyses; oil; hydrocarbon potential; source rocks; hydrocarbon maturation; hydrocarbons; clay, commodity; Lindsay Formation; Collingwood Member; Blue Mountain Formation; Georgian Bay Formation; Queenston Formation; Ordovician; Paleozoic
Illustrationslocation maps; stratigraphic columns; photographs; rose diagrams; tables
Natural Resources Canada Library - Ottawa (Earth Sciences)
ProgramShale Reservoir Characterization, Geoscience for New Energy Supply (GNES)
Released2016 06 30
AbstractAs part of a larger regional study of Upper Ordovician strata in southern Ontario, 53 outcrops, 10 cores and 120 subsurface geophysical log traces of the Queenston Formation were analyzed. The Queenston Formation, of latest Ordovician age, consists of red non-calcareous siltstone to sandy siltstone with interbedded very fine to fine grained sandstone, and thins over about 250 km from 335 m in the southeast to 22 m in the northwest. The lower Streetsville member comprises a series of stacked thickening- and coarsening-upward shallow marine sequences arranged in an overall coarsening-upward trend, which culminates in the shoreline-related interbedded sandstone and siltstone of the middle Bronte Creek member. These units are overlain by up to 100 m of the upper Milton member, characterized by a thick, fining-upward succession of massive uniform red siltstone which is capped by deep desiccation cracks, a green diagenetic reduction zone and the regionally-extensive subaerial Cherokee Unconformity. In the past, this succession was interpreted as shallow marine to coastal deposits passing upward into supratidal mudflat/sabkha deposits. In this study, the following facies were identified: 1) red bioturbated mudstone to muddy siltstone (typical of the lower Streetsville member), interpreted as low-energy, shallow marine background deposits, 2) greenish to reddish very fine to fine grained sandstone (interbedded with facies 1 in. coarsening-upward sequences and typical of the lower Streetsville and middle Bronte Creek members), interpreted as higher-energy, nearshore to shoreline traction current deposits, 3) uncommon thin bioclastic calcarenite beds (present in the middle Bronte Creek member), interpreted as higher-energy, nearshore to shoreline traction current deposits, and 4) red, uniform, well sorted, pedogenically-altered siltstone (characteristic of the upper Milton member) and here interpreted as an ancient subaerial loessite deposit in a glacially-influenced setting. New paleocurrent data (175 direct and indirect indicators) suggest a regional shoreline trend of 20°/200°, with a generalized offshore paleoslope direction of 310°. The Milton member was deposited as an extensive, but rather thin, tabular blanket at ~ 15-20° S paleolatitude near the margin of the Gondwanan continent at the height of a brief, but potent, period of latest Ordovician glaciation. Well sorted, reddened, micaceous silt is the characteristic grain size in thick, massive, unbedded units. These units display uniform blocky/rubbly textures dominated by vertic features, desiccation cracks, fractures, peds and cutans, horizons of caliche nodules and scattered glaebules, gypsum crystals, evaporative crystal molds, weakly-developed calcisols, no fossils or bioturbation, and rare possible rootlets (all interpreted as the result of pedogenic processes). These observations have prompted the proposal of a new sedimentological interpretation for this unit: that of an ancient loessite deposit. If this interpretation is correct, the Milton member of the Queenston Formation represents the first ancient loessite identified in Canada, one of the oldest loessites in the world, and the first anywhere to be associated with the Late Ordovician glacial epoch.
Summary(Plain Language Summary, not published)
Interpretations of the Queenston Formation of southern Ontario have always been controversial, especially for the upper Milton Member which is composed of uniform red siltstone. This report suggests a novel sedimetologic interpretation for these deposits which were associated with a non-marine setting during a known glacial epoch. These enigmatic deposits are postulated to represent a thick deposit of ancient wind-blown silt, or loess. Multiple lines of evidence are brought to bear to support this new interpretation.