Lobal affinity of Late Wisconsin tills at St. Marys in Southwestern Ontario, Canada

The exposures at the St. Marys Cement Inc. quarry at St. Marys, Ontario, have been studied by geologists since the mid-1950s. This paper summarizes previous and new information collected in 2009, 2010, and 2012 and discusses the record of sediments resting on the bedrock surface that have been exposed during quarry operations. The exposed sediments illustrate the dynamic behavior of the Laurentide Ice Sheet in this area during the Late Wisconsin. Evidence for subglacial meltwater activity, shifting ice-flow directions during till deposition, a local or regional erosion event, possible subaerial exposure, and ice-marginal sedimentation is observed in the sequence of tills and stratified sediments exposed in the quarries. It also highlights how rapid lateral facies changes, complex contact relationships, and steep erosional contacts can prove to be challenging for correlation and extrapolation of subsurface units into three-dimensional stratigraphic models. Fourteen units were identified of which the lower six were deposited during the Nissouri age (Catfish Creek Drift). This included five layers of till deposited by a glacier that alternated from flowing out of the Lake Huron basin to that of a regional flow to the south-southwest. An angular unconformity cuts into these sediments and separates them from a finer-grained sediment sequence (primarily of Port Bruce age) consisting of glaciofluvial gravel, two fine-textured till layers, and rhythmically bedded glaciolacustrine sediments, all overlain by the uppermost till, the Rannoch Till, and Mitchell Moraine ice-marginal fan sediments. Les geologues etudient les affleurements a la carriere de la St. Marys Cement Inc., a St. Marys en Ontario, depuis le milieu des annees 1950. Le present article resume des donnees deja acquises et de nouvelles informations recueillies durant les annees 2009, 2010 et 2012; il traite du profil geologique des sediments qui reposent sur le socle et qui ont ete exposes durant l'exploitation de la carriere. Les sediments exposes illustrent le comportement dynamique de l'Inlandsis laurentidien dans ce secteur au Wisconsin tardif. Des evidences d'activite d'eau de fonte sous-glaciaire, la variation de la direction de l'ecoulement glaciaire, un evenement d'erosion local ou regional, une possible exposition subaerienne et une sedimentation en bordure de la glace sont observes dans les sequences de tills et de sediments stratifies qui affleurent dans les carrieres. Tous ces points soulignent aussi la maniere dont les changements lateraux rapides de facies, les relations complexes de contact et les contacts d'erosion abrupts peuvent poser des defis pour la correlation et l'extrapolation des unites sous la surface dans des modeles stratigraphiques tridimensionnels. Quatorze unites ont ete identifiees dont six unites inferieures qui ont ete deposees durant le stade glaciaire Nissouri (drift de Catfish Creek). Ces unites comprenaient cinq couches de till deposees par un glacier qui s'ecoulait alternativement entre le bassin Huron et de maniere regionale vers le sud-sud-ouest. Une discordance angulaire recoupe ces sediments et les separe d'une sequence a grains plus fins (surtout d'age Port Bruce) qui est composee de gravier fluvioglaciaire, de deux couches de till a texture fine et de sediments glacio-lacustres a stratification rythmique; le tout est recouvert par le till superieur, le till de Rannoch et des sediments de lobe proglaciaire en bordure de la moraine de Mitchell. [Traduit par la Redaction]
Introduction With the onset of three-dimensional (3D) mapping of Quaternary sediments primarily for regional groundwater applications in areas surrounding the Great Lakes by State, Provincial, and Federal surveys, emphasis has [...]