| Résumé | (disponible en anglais seulement)
As 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. |