Landscape analysis, mapping, sedimentology, shallow geophysics, and borehole data are integrated to better understand the complex landform-sediment geometries and event sequences of the Oak Ridges Moraine, southern Ontario. A model for the origin of the Oak Ridges Moraine is based on the recognition that the moraine is built on a high-relief, erosional surface (unconformity) consisting of drumlin uplands and a network of deep, steep-walled, interconnected valleys (tunnel channels). The development of the moraine is thought to have occurred in four stages: I, subglacial sedimentation; II, subaqueous fan sedimentation; III, fan to delta sedimentation; IV, ice-marginal sedimentation. The model traces the transition from subglacial to proglacial conditions during moraine formation and examines the order and timing of sedimentation. It is thought that the early stages of moraine construction are better exposed in the east; in the west, these stages are buried by later stages.
Analysis of over 50 line-kilometres of land-based, shallow, seismic reflection profiles has provided a means of investigating the subsurface architecture and stratigraphic relationships of the glacial deposits in and beneath the Oak Ridges Moraine (ORM). The focus of this paper is the role of seismic reflection surveys, and the derived seismic facies and facies geometry, in the development of a well-constrained, regional, conceptual model of the subsurface stratigraphy in the area and the improved inferences these data allow regarding glacial event sequence and process interpretations. The data define four major seismic facies that characterize the complex glacial sequence of the ORM area. High-reflectivity facies (I) can be traced regionally and related to an eroded Newmarket Till surface. Medium (II) and low (III) reflectivity facies are generally associated with coarse-grained glaciofluvial deposits and laterally extensive, glaciolacustrine sequences of sand, silt, and clay, respectively. A chaotic facies (IV) is common within buried channels, and attributed to instability and (or) rapid channel-fill deposition. Seismic geometry (with borehole verification) shows that a broad surface network of channels extends below thick ORM sediments. The channel system is part of a regional unconformity formed on the Newmarket Till (facies I). The buried channels can have steep sides, and their fills frequently include tabular sheets, eskers, and (or) large cross-beds. The observations are consistent with the scenario of sheet flow and channel cutting by high-energy subglacial meltwater and filling with gravel, sand, and silt in succession (facies II and III) as the flows waned.
Seismic stratigraphy, geometry, and sediment facies within the Oak Ridges Moraine (ORM) area of Ontario record major structural elements and surfaces of the Quaternary sedimentary sequence. The derived stratigraphic architecture can be used to identify the key elements of a regional erosional surface, represented by an unconformity in the subsurface, and associated overlying channel sediments. The erosional surface unconformity forms a distinct time datum in the Quaternary sequence, which provides an important aid to lithostratigraphic correlation. The architecture also gives improved understanding of the effects of erosion on the late-glacial landscape. The surfaces of erosional drumlins and intervening troughs, and the beds and banks of meltwater channels in the ORM area, define the regional unconformity, highlighted by seismic profiles linked to continuously cored boreholes. These features are attributed to regional-scale, subglacial meltwater flow events. The sculpted surfaces, which are analogous to water-eroded forms, the presence of boulder lags and coarse-grained deposits on the regional erosional surface, and the channels with undulating profiles provide the vital supporting evidence for a meltwater interpretation. The inter-regional extent of the unconformity is inferred from the coherence of regional paleoflows and the extent of drumlinized uplands, tunnel channels, and scoured bedrock terrain across ~75% of the landscape from the ORM area east and south to the Finger Lakes, New York. The implied magnitude of erosion suggests a pressing need for directed sedimentological study in those ocean basins that were probable depositional sites for flood deposits.
Landscape analysis, mapping, sedimentology, shallow geophysics, and borehole data are integrated to better understand the complex landform-sediment geometries and event sequences of the Oak Ridges Moraine, southern Ontario. A model for the origin of the Oak Ridges Moraine is based on the recognition that the moraine is built on a high-relief, erosional surface (unconformity) consisting of drumlin uplands and a network of deep, steep-walled, interconnected valleys (tunnel channels). The development of the moraine is thought to have occurred in four stages: I, subglacial sedimentation; II, subaqueous fan sedimentation; III, fan to delta sedimentation; IV, ice-marginal sedimentation. The model traces the transition from subglacial to proglacial conditions during moraine formation and examines the order and timing of sedimentation. It is thought that the early stages of moraine construction are better exposed in the east; in the west, these stages are buried by later stages. Résumé : Les études de paysage, cartographie, sédimentologie, géophysique de subsurface et des carottes de trous des sondages ont été menées conjointement pour chercher à mieux comprendre les configurations complexes des formes de relief-sédiments, et les séquences des événements qui ont affecté la Moraine d'Oak Ridges, dans le sud de l'Ontario. Un modèle interprétant l'origine de la Moraine d'Oak Ridges est fondé sur le fait que cette moraine fut édifiée sur une surface d'érosion (discordance) en terrain à topographie accentuée, comprenant des drumlins sur terres élevées et des vallées à parois subverticales interconnectées (chenaux tunnels). Le modèle propose un développement de la moraine en quatre phases : I, sédimentation sous-glaciaire; II, sédimentation d'un éventail sous-aquatique; III, sédimentation de l'éventail évoluant vers un dépôt deltaïque; IV, sédimentation en bordure d'un glacier. Le modèle retrace la transition de conditions sous-glaciaires à proglaciaires durant la formation de la moraine, et traite de l'ordre et de la chronologie du sédiment. Il apparaît que c'est dans le secteur oriental que les premières phases de l'édification de la moraine sont le mieux exposées; tandis que dans le secteur partie occidental, elles sont enfouies sous le sédiment des phases plus tardives. [Traduit par la Rédaction] Barnett et al. 1167
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