ABSTRACT: Highly concentrated underflow deposits in a Late Wisconsinan subaqueous fan, Pointe St. Nicolas, Quebec: Possible depositional processes.

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Abstract

During the last deglaciation of the St. Lawrence Valley, the Laurentide Ice Sheet receded through deep waters of the Champlain Sea, depositing subaqueous fans from local meltwater effluxes. These subaqueous fans usually reflect intense underflowing, and often contain sedimentary faciesinterpreted as 'high-density turbidite' deposits. As the depositional processes of 'high-density turbidity currents' are poorly understood, the well exposed Late Wisconsinan subaqueous fans of the St. Lawrence Valley can provide useful outcrop data to better understand the possible range ofdepositional processes operational in highly-concentrated underflows. Two unusual underflow facies from a turbidite-dominated subaqueous fan at Pointe St. Nicolas, Quebec, are presented and discussed below in order to elucidate some possible depositional processes of highly-concentrated underflows. A predominant tabular gravel sheet runs across the length of the outcrop close to the top of the subaqueous fan. The lower contact of the sheet is flat and erosive. The top contact is sharp, but can be intercalated with glaciomarine muds or sandy turbidites. The sheet itself does not coarsen or fine upwards noticeably. The sheet is composed of beds (0.2-1.5 m) which are typically inverse toinverse-normal graded. The gravel is typically clast-supported, well to moderately sorted and matrix poor. Gravel clasts (pebbles to cobbles) are subrounded to rounded and unstriated. The matrix is usually muddy coarse sand (mud ~ <2%). At one location along the sheet, mud was present ingreater quantities in the matrix (~ <15%), larger 'floating' clasts were present (maximum d = 0.5 m) and bedding was massive. At another location, a large lenticular gravel is amalgamated to the top of the sheet, forming a convex-up 'hump'. The 'hump' pinches out laterally into and is overlain by sandy turbidites. Intercalation of the top contact of the gravel sheet with glaciomarine muds and subaqueous turbidites indicates that the gravels were deposited subaqueously. The flat unchannelized base of the gravel sheet and its tabular geometry is suggestive of a sheetflooding event, due possibly to a buoyant lift-off of the ice margin (cf. Powell, 1990). Indeed, the position of the gravel sheet at the top of the fan seems to indicate that the paleohydraulic event which deposited the gravels was coincident with the destabilization and decay of the ice margin, the end of sandy fan deposition, and the start of quietAlthough the gravels are much more coarse grained than deposits typically associated with grainflow-like 'traction'carpet' deposition, several data suggest that they were deposited in this fashion. The parental flows are interpreted to have been very energetic bipartite underflows which had highly concentrated traction-carpets developed in their basal regions on the basis of (1) the ubiquitous inverse grading, (2) the lateral gradation of sheet into massive muddier debris flow-likebeds and (3) the pinch out of the hump into sandy turbidites. Inverse grading may suggest that the flows had higher sediment concentrations. Inverse grading is often observed in deposits of cohesionless grain flows, and usually interpreted in terms of larger clasts moving to the top of the flow where shear stress is smaller, or to gravitational settling ('kinetic sieving') of smaller particles through holes created from vibrational grain movements (Bagnold, 1954; Savage and Sayed, 1984). The lateral gradation into muddier massive beds with floating clasts may suggest that locally the traction carpets acted more like cohesive debris flows (cf. Johnson, 1970). In these regions, higher percents of mud in the matrix would have provided buoyant support and possibly cohesive strength to the flow, allowing for larger clasts to be carried (Hampton, 1979; Coussot and Meunier, 1996). Evidence for bipartite flow in outcrop may a good indication that traction carpets developed in the parental flows (Sohn et al., 1997). The pinching-out of the lenticular gravel hump into turbidites is taken as evidence that the parental flows were bipartite. As angles of repose necessary for grainflowing are not present on the subaqueous fan (fan foresets dip 12o towards the NW), it seems more likely that the turbidity currents were providing shear energy to gravel traction carpets rather than trailing gravel grainflows as a turbulent sandy tail. The gravels are therefore interpreted astraction carpet deposits (sensu Sohn, 1997), and the parental flows would have been bipartite, with a fluidal, Newtonian turbulent upper portion, with grainflow-like gravel traction carpets at their base. Channels (maximum ~20 m wide by 5 m deep) which cut through the subaqueous fan at Pointe St.Nicolas are typically filled with fine sands which, from a distance appear massive, but upon closer inspection seem to have a 'diffuse' horizontal lamination. Texturally, the fine sands are very well sorted, with rare to occasional pebble clasts (maximum ~2 cm). The diffuse lamination could bedescribed as very subtle 'cyclical' thinning and coarsening of grain size of the laminae to thin bed scale (0.25 - 2 cm). Further evaluation of the nature of the diffuse stratification is difficult due to the subtle nature of the grain size changes. Lower contacts of the channels are erosive. Top contacts of the channels are either sharp, quickly gradational (<10 cm) into normally graded sandy turbidites, and also can be loaded. The faint diffuse internal stratification is somewhat reminiscent of bedload deposits formed during upper plane bed conditions, but the individual laminae are of centimetre rather than grain diameter thickness. Although the channel fills appear massive from afar, deposition from a single en masse event seems unlikely due to the internal stratification (Coussot and Meunier, 1996). The ungraded nature of the channel fills suggests that the underflows were quasi-steady underflowing throughoutthe duration of the channel filling events (Kneller and Branney, 1995). In the glacial subaqueous fan environment, steady currents which deposit thick well-sorted channel fills (up to 5 m thick) seem best explained as being associated with meltwater underflow processes. The loaded upper contact and the absence of cross-stratification may be taken as evidence that the channels were deposited relatively quickly from flows which had high sediment concentrations. Sediments deposited very quickly from suspension can often have metastable grain frameworks and are prone to soft sediment deformation. Absence of cross-stratification in the channel fills suggeststhat turbulent flow separation at the bed was inhibited, which may be due to increased apparent viscosity of the fluid under higher sediment loads. However, lack of outsized flow-rafted clasts seems to indicate that the underflows, despite having elevated sediment concentrations (at least close to their beds), may not have had significant apparent viscosities (outsized clasts are often observed in flows with sediment concentrations intermediate between stream flow and debris flow -e. g. Smith, 1986; Best, 1992). It therefore seems equivocal, in this case, whether the fine sand grains at the base of the underflows were transferring sufficient momentum during grain-to-grain collisions to be able to support traction carpets at the bedding surface (see Sohn, 1997; Sohn et al.1999). Other theories of how diffuse stratification is generated, such as migration of low-amplitude, long-wave length bedforms in hyperconcentrated flows (Smith, 1986; Best, 1992) also cannot not be verified at Pointe St. Nicolas. Features which have been used to identify deposits from low-amplitudelow-wave length bedforms, such as inversely graded laminae, occasional preservation of low-angle cross strata, or lenticularity of laminae normal to flow, were not able to be identified due to the well-sorted nature of the fine sands. It seems that a microfabric study of the diffusely laminated sands is necessary to further elucidate the process(es?) which operated during deposition. This study describes the sedimentological characteristics and interprets the depositional processes of two underflow facies. However, exactly how sediments are deposited from highly-concentratedunderflows continues to remain poorly understood. The depositional processes of highly-concentrated underflows will not become clear without continued experimental study (e.g. Postma et al., 1988; Vrolijk and Southard, 1997; Mohrig et al., 1998). These experimental results must be in turn coupled with field studies investigating the characteristics of various highly-concentrated underflow facies in order to gain geological significance.

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