A 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
Authors: Hamblin A P 
Published: June 2016

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.