Stratigraphic Architecture and Sediment Facies of the Western Oak Ridges Moraine, Humber River Watershed, Southern Ontario*

Abstract

The Oak Ridges Moraine in southern Ontario is a ca. 160 km long east-west trending ridge of sand and gravel situated north of Lake Ontario. Study of the Oak Ridges Moraine in the Humber River watershed was undertaken to assess its role in the groundwater system of the buried Laurentian Valley. The Oak Ridges Moraine is interpreted to have been deposited in three stages. Stage I records rapid deposition from hyperconcentrated flows where tunnel channels discharged into a subglacial lake in the Lake Ontario basin. Low-energy basin sedimentation of Stage II was in a subglacial and ice-contact setting of a highly crevassed ice sheet. Stage III sedimentation is characterized by rapid facies changes associated with esker, subaqueous fan, and basinal sedimentation. Detailed sediment analysis challenges the concept that the Oak Ridges Moraine was deposited principally from seasonal meltwater discharges, climatic modulated ice-marginal fluctuations, or in an interlobate position. Instead it is interpreted to have formed in response to late-glacial ice sheet events associated with subglacial meltwater ponding, episodic and catastrophic subglacial meltwater discharge, and subsequent seasonal meltwater discharge. The moraine probably formed as the glacial-hydraulic system re-equilibrated to the presence of a thinned, grounded ice shelf and a subglacial lake in the Lake Ontario basin.La moraine de Oak Ridges, sud de l’Ontario, est une crête de sable et de gravier orientée est-ouest d’une longueur de 160 km au nord du lac Ontario. L’étude de la moraine de Oak Ridges dans le bassin de la rivière Humber permet de comprendre son rôle dans le système de drainage de la vallée Laurentienne. La moraine de Oak Ridges a été édifiée en trois phases. La phase I consiste en une sédimentation rapide par hyperconcentration des écoulements, où les chenaux en tunnel se déversent dans un lac sous-glaciaire du lac Ontario. Le bassin de sédimentation de faible énergie de la phase II est sous-glaciaire et touche à un inlandsis ayant d’importantes crevasses. La phase III se caractérise par un changement de faciès très rapide, par la présence d’eskers, de cônes aquatiques et de bassins sédimentaires. Les analyses sédimentaires détaillées ébranlent l’hypothèse que la moraine de Oak Ridges ait été formée par la fonte des glaces saisonnière, les fluctuations climatiques près des marges glaciaires, ou dans une position interlobaire. Notre interprétation indique plutôt qu’elle a été mise en place en réponse à des événements de fonte sous-glaciaire de nature épisodique et catastrophique, et par des apports subséquents d’eau de fonte saisonnière. La moraine s’est probablement formée lors de la ré-équilibration du système glacio-hydraulique en présence d’un inlandsis mince, en contact avec le substrat et alimentant un lac sous-glaciaire dans le bassin du lac Ontario

Reply to the comment by H.A.J. Russell on “Converging ice streams: a new paradigm for reconstructions of the Laurentide Ice Sheet in southern Ontario and deposition of the Oak Ridges Moraine”

“…1) and many other glacial landforms, especially recessional moraines. Furthermore, that work is underpinned conceptually by the hypothesis of Shaw and Gilbert (1990) that glacial landforms such as drumlins and most others in Southern Ontario are the product of gigantic regional-scale subglacial floods flowing under and lifting the ice sheet margin from its bed (e.g., Barnett et al 1998;Russell et al 2004Russell et al , 2005Sharpe et al 2004Sharpe et al , 2007Sharpe et al , 2013Sharpe and Russell 2016).…”

Comment on “Converging ice streams: a new paradigm for reconstructions of the Laurentide Ice Sheet in southern Ontario and deposition of the Oak Ridges Moraine”

“…The few statements pertaining to the ORM have no primary data support and commonly lack referencing, such as statements on paleocurrent directions in the ORM (p. 391). To the author's knowledge, there are limited publications with paleoflow information for the ORM (e.g., Duckworth 1979;Paterson and Cheel 1997;Russell and Arnott 2003;Russell et al 2004). Of these, Sookhan et al (2018) cite only one which is Russell et al (2004) and it is not cited in association with statements on paleocurrent directions.…”

“…To the author's knowledge, there are limited publications with paleoflow information for the ORM (e.g., Duckworth 1979;Paterson and Cheel 1997;Russell and Arnott 2003;Russell et al 2004). Of these, Sookhan et al (2018) cite only one which is Russell et al (2004) and it is not cited in association with statements on paleocurrent directions. There is also a lack of citations of much of the work on the subsurface of the ORM and where it is cited, Sookhan et al (2018) do not correctly apply the information.…”

“…Building on the recognition by Gwyn and Cowan (1978) that Newmarket Till extended beneath the ORM, the subsequent cumulative body of work has refined and thoroughly documented the stratigraphic units and relationships that clearly place constraints on any process orientated interpretation of the ORM. Regionally extensive, streamlined Newmarket Till (e.g., Pullan et al 2002;Sharpe et al 2002;Sookhan et al 2018) is incised by a series of valleys that are exposed north of the ORM (Brennand and Shaw 1994;Brennand et al 2006) and that are locally buried by ORM sediment (e.g., Barnett et al 1998;Sharpe et al 2002;Russell et al 2003aRussell et al , 2004Brennand et al 2006;Gerber et al 2018). To the south of the ORM, Halton Till extent and sedimentology has been refined and demonstrated to overlie and be gradational from ORM sediment (White 1975;Sharpe and Barnett 1997;Russell and White 1997;Russell et al 2004;Sharpe and Russell 2016).…”

“…As a surface expression of the landscape, valleys stop at the moraine because the ORM sediment overlies and infills the southward subsurface extension of the valleys (e.g., Barnett et al 1998). Data from seismic and drillcore are extensive and unambiguous in the documentation of valley extension beneath the ORM, in papers cited by Sookhan et al (2018) (e.g., Russell et al 2004;Brennand et al 2006;Sharpe et al 2007;Sharpe et al 2013) and in additional publications (e.g., Pugin et al 1996Pugin et al , 1999Barnett et al 1998;Sharpe et al 2002Sharpe et al , 2004Russell et al 2003aRussell et al , 2003b. Drawing upon technical groundwater reports, Sookhan et al (2018) reference Kassenaar and Wexler (2006) as supporting the absence of a regional network of tunnel valleys.…”