Evidence for rapid sedimentation in a tunnel channel, Oak Ridges Moraine, southern Ontario, Canada

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Abstract

In south-central Ontario, a Late Wisconsinan regional unconformity consisting of tunnel channels and drumlinized till crops out north of Lake Ontario. The tunnel channels are locally infilled and the unconformity buried by sediment of the Oak Ridges Moraine. Based on seismic reflection profiling and drillcore, the tunnel channels are known to continue beneath the moraine. Detailed sedimentological analysis of ∼300 m of continuous core from two drillholes located ∼7 km apart in subparallel tunnel channels identified three facies. The gravel facies is 17 m thick and occurs in only one of the cores. It directly overlies the unconformity and may include a number of upward-fining units. Seismic reflection data suggests the gravel forms stacked gravel mesoforms or quasi-horizontal gravel layers (bed load sheets deposited from fluidal flows). Along the deep channel axis, the graded massive sand facies up to 37 m thick is the most common facies and consists of silty, medium sand with minor coarse sand. Strata are reverse-graded, normal-graded, or massive with isolated silt intraclasts and evidence locally for dewatering. This facies is interpreted to have been deposited from hyperconcentrated dispersions downflow of a hydraulic jump (or major channel confluence). The third facies consists of medium-scale and small-scale cross-stratified sand. Medium-scale cross-stratification occurs predominantly in the lower 37 m of the core and is interbedded with the graded massive sand facies; small-scale cross-stratified sand is progressively more common upward in core. Medium- and small-scale cross-stratified sands was deposited, respectively, by subaqueous dunes and ripples formed in dilute fluid flows. The complete succession is interpreted to have been deposited very rapidly within a subglacial tunnel channel before discharge ceased along the channel. Deposition followed closely, and in part coincided with rapid expansion of the channel by erosion in a hydraulic jump or at a major channel confluence along the grounding line of a subglacial lake. Scour into unconsolidated sediment contributed to the sediment flux and quickly overloaded the flow with suspended sediment, which in turn resulted in extremely high rates of sedimentation immediately downflow. Such depositional conditions support the notion that tunnel channels in the study area formed and/or served as conduits for subglacial jökulhlaup discharges, and that the Laurentide Ice Sheet most probably did not generally drain by steady state processes, but instead by short-lived catastrophic events.

Introduction

Erosional landscapes formed by large, catastrophic meltwater discharge events from subglacial drainage networks were once considered anomalous (e.g. Bretz, 1969). Episodic drainage processes are now recognized to have been common events during the waning of the Laurentide Ice Sheet (Shaw, 1996), and other continental glaciers (e.g. Piotrowski, 1994, Sawagaki and Hirakawa, 1997), and outlet glaciers (e.g. Fleisher et al., 1997). One element of this meltwater landscape is tunnel channel (valley) networks. These networks have been mapped from extensive regions once covered by Pleistocene ice sheets, particularly in North America and western Europe (e.g. Cofaigh, 1996). The Great Lakes basin of central North America (Fig. 1), the Scotia Shelf, North Sea and northern Germany are areas in which extensive channel networks have been reported. The plan and cross-sectional geometry of tunnel channels have been well defined by landform analysis (e.g. Brennand and Shaw, 1994, Fisher and Taylor, 1999), seafloor mapping (e.g. Huuse and Lykke-Andersen, 2000, Loncarevic et al., 1992) and seismic reflection surveys (e.g. Pugin et al., 1999). In contrast, the sedimentary fill of tunnel channels is poorly understood. In many cases, the stratigraphic detail of the thick fill of buried channels has been delineated by seismic facies analysis (e.g. Boyd et al., 1988, Mullins et al., 1996), and low-quality water well boring descriptions (e.g. Ehlers and Linke, 1989). Outcrop and continuous drillcore studies with detailed sedimentological data are comparatively rare (e.g. Ehlers and Linke, 1989, Eyles and McCabe, 1989, Ghienne and Deynoux, 1998, Piotrowski et al., 1999, Shaw and Gorrell, 1991, Wellner et al., 1996). Seismic data, for example, effectively resolve large-scale stratal geometries, but generally lack the resolution to identify small-scale sedimentary features necessary for depositional process interpretations. In contrast to architectural information provided by seismic data, tunnel channels mapped by water well records rely on textural changes from wash-boring records Ehlers and Linke, 1989, Patterson, 1994, Piotrowski, 1994. Accordingly, in the absence of downhole geophysics few data are available on either lithofacies detail or the three-dimensional geometry of the fill facies. As a consequence, although paleohydraulic models have been developed for some channels based primarily on water well data (Piotrowski, 1997), there are few detailed depositional models of channel fills. Where thick, deeply buried channel sediment has been cored, the full succession has only been partially intercepted (e.g. Boyd et al., 1988, Wellner et al., 1996). Similarly, outcrop studies have been restricted to near surface studies of late stage channel fills Eyles and McCabe, 1989, Shaw and Gorrell, 1991. The resultant scarcity of detailed lithofacies descriptions from the lower part of thick channel fills continues to constrain models of channel erosion and infill.

Sedimentological data from two continuously cored drillholes supplemented with seismic data from Pugin et al. (1999) are discussed. The complete stratal succession of each core is reviewed and detailed lithofacies descriptions of the lower 20 and 50 m, respectively, of cores DH-Nob and DH-V-158 are presented. This paper focuses on the lower interval of channel fill sediment facies and facies assemblages that to date have not been observed in any other deep continuous cores logged by the Geological Survey of Canada and Ontario Geological Survey. The cores are located in a tunnel channel network previously defined by regional mapping (Sharpe et al., 1997), seismic profiling (Pugin et al., 1999), and borehole analysis (Russell et al., 2000) of the Oak Ridges Moraine area, southern Ontario. The implications of the lithofacies described for models of channel erosion and fill are discussed. The sediment facies described in this paper from southern Ontario may have implications for understanding the sediment fill of tunnel channels elsewhere in the Great Lakes basin, eastern Canada and northern Europe.

Section snippets

Geological setting

North of Lake Ontario, Paleozoic bedrock of south-central Ontario is overlain by a thick succession (<200 m) of Quaternary sediment deposited after the Illinoian glaciation (Fig. 2) (Karrow, 1974). Pre-Wisconsinan deposits and Lower Wisconsinan deposits are recognized based on their fossiliferous content, organic content and/or stratigraphic position beneath the Newmarket Till (e.g. Eyles and Williams, 1992, Karrow, 1967). The Newmarket Till is a regional unit that has a drumlinized and

Study site and methodology

The data set consists of two continuous, 9 cm diameter drillcores that are >300 m long from the western Oak Ridges Moraine, approximately 35 km north–northwest of metropolitan Toronto Fig. 1, Fig. 3. Both cores penetrate the complete Quaternary succession and terminate in bedrock. Drillcore recovery was >90%, but was significantly lower in gravel intervals. Drillhole DH-Nob was collared at 260 m above sea level (asl) and bedrock was intercepted 190 m beneath the surface at 70 m asl. Drillhole

Stratigraphy

Prior to discussing the detailed lithofacies of the lower part of the core from DH-Nob and DH-V-158, the complete strata succession is reviewed and assigned stratigraphically.

Lithofacies

Detailed lithofacies descriptions and interpretations are presented below for the gravel strata in DH-Nob and sand strata below 171 m asl in DH-V-158 that correspond with stage I of Oak Ridges Moraine sedimentation (Fig. 3b).

Depositional model

Regional mapping and seismic profiling indicates that the core sites are located within northeast–southwest-trending buried valleys interpreted as tunnel channels Brennand and Shaw, 1994, Pugin et al., 1999, Sharpe et al., 1997. The tunnel channels form a network of steep-walled, 0.5–6 km wide, 10–20 km long and up to 170 m deep valleys. Drillhole DH-Nob occurs near the eastern margin of the buried extension of the Holland Marsh valley tunnel channel. Site DH-V-158 is located ∼7 km to the

Discussion

This study has described lithofacies of tunnel channel fills intercepted in cores beneath the Oak Ridges Moraine. These buried channels are part of a regional network of tunnel channels that extend from Georgian Bay to the eastern end of Lake Ontario (Fig. 1). For the most part, these channels are incompletely filled and crop out as a northeast to southwest orientated channel system. The channel network is eroded into Precambrian and Paleozoic bedrock and unconsolidated Quaternary sediment

Conclusion

Sediment facies record the processes active at the time of deposition. From this broader interpretations of flow, processes and environments can be made and constrained by the stratigraphic setting, in this case in a well-defined tunnel channel network, and regional geological knowledge. Integration of these elements has lead to development of a depositional model involving rapid and voluminous jökulhlaup style sedimentation within and beyond the mouth of tunnel channels in the vicinity of a

Acknowledgements

Field assistance was provided by D. Cummings, W. Lewis, L. Maurice, and A. Wigston. G. Gorrell is thanked for managing the drilling program of DH-Nob. Core DH-V-158 was originally collected in 1994 by M. Gomer of Fenco McLaren. Database support for this work was provided by C. Logan. The program GRAN for plotting grain size was provided by E. Boisvert. The seismic trace presented in Fig. 4 was provided by S. Pullan. Review of an early version of this paper by P. Henderson, R. Rainbird, and R.

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