sequence stratigraphy of a glaciated basin with a focus on ... · outside the realm of glacial...

1478

ABSTRACT

A large integrated data set of cores, out-crop data, and seismic transects from the mud-buried Vars-Winchester esker in the Champlain Sea basin, Canada, was studied to gain insight into how muddy glaciated basins fi ll with sediment, and how esker sedi-mentary systems contribute to this process.

Three stratigraphic units—a till sheet over carbonate bedrock, the Vars-Winchester esker , and overlying Champlain Sea mud—are identifi ed in the data set. The till is mas-sive, mud rich, carbonate rich, and drum-linized. The esker is also carbonate rich, and rests erosively on till or bedrock. It consists of two elements, a narrow gravelly central ridge and a broad sandy carapace. Three units comprise the overlying mud package: gray carbonate-rich rhythmites, massive bio-turbated mud, and carbonate-poor, red-and-gray rhythmites.

A sequence stratigraphic model is proposed to explain these observations. Emphasis is placed on gradual ice-front translation super-imposed by rapid meltwater events. The esker is interpreted to have been derived from the underlying till by water that fl owed through a subglacial conduit (R-channel), within which the narrow gravelly central ridge was depos-ited. Most mud and fi ner sand bypassed the conduit and was deposited proglacially on the fl oor of the Champlain Sea, fi rst as sandy outwash and, farther basinward, as muddy carbonate-rich rhythmites. Gradual ice-front retreat superposed distal facies over proximal facies, generating the upward-fi ning succes-sion that starts with the esker gravel and ends with muddy rhythmites. Most esker sediment appears to have been deposited during rapid, jökulhlaup-like fl oods that punctuated grad-

ual retreat. Discharges are estimated to have been high, possibly on the order of several hundred to, perhaps more commonly, sev-eral thousand cubic meters per second. The chaotic and random-looking appearance of the resultant sedimentological signatures in the esker sensu stricto is sharply contrasted with the regularity of the muddy rhythmites. If the rhythmites are indeed correlative to the esker, which seems reasonable given their geochemistry and the fact that their volume scales to the volume of mud in the till, the fl ood events that deposited the esker must have been seasonally mediated, and the basin water must have attenuated the fl ood signal, resulting in a rhythmic “on-off” signature in more distal portions of the system. The regu-larity of the rhythmites does not betray the chaotic nature of the esker sensu stricto, and vice versa. Studying either one in isolation would lead to a very different “end-member” impression of how eskers form and how esker sedimentary systems operate during the infi ll-ing of glaciated basins.

INTRODUCTION

Stratigraphic patterns in sedimentary basins have been studied for hundreds of years for both fundamental and applied reasons: they can reveal how the Earth surface has evolved over time, and they can be used to fi nd natural resources, such as groundwater, mineral re-sources, and petroleum. Patterns generated by fl uviodeltaic–turbidite-fan systems—the classic depositional systems—are relatively well under-stood. One hundred years of research driven by petroleum exploration has resulted in the de-velopment of elegant process-response models that now form the de facto basis for interpret-ing the stratigraphic record and fi nding natural resources, both locally (facies models) and re-gionally (sequence stratigraphy). Stratigraphic

patterns in glaciated basins, by contrast, are less well understood, primarily because glacial de-posits contain fewer petroleum reserves. Glacial facies models have been developed, in some cases quite elegantly (e.g., Ashley et al., 1985), but as a whole they are less well constrained than their nonglacial counterparts, especially for subglacial environments (e.g., eskers, till, and tunnel channels) where quantifi cation of physical processes and stratigraphic responses is diffi cult to impossible. What is really lack-ing, however, is a glacial equivalent of sequence stratigraphy.

Sequence stratigraphy is process-oriented sedimentology on a large scale. Although it has been applied successfully to the analysis of gla-ciated basins (e.g., Corner, 2006), its terminol-ogy does not transfer well because of its overt implication that sea-level change is the main driver (cf. Brookfi eld and Martini, 1999). What has ostensibly been overlooked is the universal-ity of the central tenet of sequence stratigraphy, which can be paraphrased as follows:

Stratigraphic patterns and key surfaces in sedimentary basins are generated by the interplay between two variables, sediment supply and accommodation space (or “depositional space”) for sediment.

Provided this central tenet is framed appropri-ately and used judiciously, as we attempt to do in this paper, it can be used as a fl exible means to analyze glacial depositional systems just as it has been used for fl uvialdeltaic–turbidite -fan systems (Posamentier and Allen, 1999), carbonate depositional systems (Eberli et al., 2001), coal depositional systems (Bohacs and Suter, 1997), and aeolian depositional systems (Kocurek, 1999). This should not be seen as an attempt to usurp existing process-oriented glacial stratigraphic models (e.g., Clayton and Moran, 1974; Shilts et al., 1987; Boulton, 1996; Powell and Cooper, 2002), but rather to build on these models by capturing

For permission to copy, contact [email protected]© 2011 Geological Society of America

GSA Bulletin; July/August 2011; v. 123; no. 7/8; p. 1478–1496; doi: 10.1130/B30273.1; 15 fi gures.

†E-mail: [email protected]

Sequence stratigraphy of a glaciated basin fi ll, with a focus on esker sedimentation

Don I. Cummings1,†, George Gorrell2, Jean-Pierre Guilbault3, James A. Hunter1, Charles Logan1, Dmitri Ponomarenko1, André J.-M. Pugin1, Susan E. Pullan1, Hazen A.J. Russell1, and David R. Sharpe1

1Geological Survey of Canada, Ottawa, Ontario K1A 2A9, Canada2BGC Engineering Inc., Suite 500, 1045 Howe Street, Vancouver, British Columbia V6Z 2A9, Canada3BRAQ-Stratigraphie, 37 Chemin Cochrane, Compton, Quebec J0B 1L0, Canada

Glacial sequence stratigraphy, with a focus on esker sedimentation

Geological Society of America Bulletin, July/August 2011 1479

a key message : stratigraphic patterns in glaci-ated sedimentary basins are generated by the interplay between two variables, sediment supply and accommodation space. These two terms—sediment supply and accommodation space—offer glacial geologists a simplifi ed vocabulary with which to discuss sometimes-complex stratigraphic phenomena in glaciated basins, a function they have filled for decades outside the realm of glacial geology.

That being said, sequence stratigraphy must be acknowledged for what it is: a means to an end. Our end goal is not to formalize a rigid model replete with jargon. Rather, our objective is to use the central tenet as a vehicle to interpret litho-, chemo-, bio-, and seismic stratigraphic patterns in a large data set from the Champlain Sea basin near Ottawa, Canada (Fig. 1). Beyond this, dis-cussion is advanced on several fronts, most

of which pertain to the relative importance of slow, gradual processes versus rapid, stochastic processes in glaciated-basin fi lling. The geology of the Vars-Winchester esker is used as a platform to specifi cally address this question, and to gain insight into how esker sedimentary systems oper-ate during the infi lling of glaciated basins.

THE CHAMPLAIN SEA BASIN: A PRIMER

From a sedimentological perspective, the Champlain Sea was a giant mud trap, a huge epeiric buffer zone that fi ltered millions of cubic meters of mud from meltwater that drained into the Atlantic Ocean via the St. Lawrence Lowland. The inland sea formed when the ice sheet retreated suffi ciently into the Lowland, which allowed the ocean to in-

vade and mix with a large proglacial lake that had developed in the inner Lowland (Parent and Occhietti, 1988). The sea lasted from ~11,500–10,000 14C yr B.P. (Anderson, 1988), its level falling continuously as the crust re-bounded isostatically (Rust and Romanelli, 1975). During this time, the ice front contin-ued to retreat northward across and out of the basin, depositing moraines and eskers in the process, which then became blanketed by mud (Fig. 1). By the early Holocene, the land had emerged, and rivers had carved into the mud-rich landscape (MacPherson, 1968).

Johnston (1917) was the fi rst to compre-hensively identify the tripartite basin fi ll of till, eskers , and mud in the Champlain Sea basin near Ottawa (Fig. 2). The till, which is rarely well ex-posed in outcrop, is ornamented by north-south–trending drumlins, overlies bedrock striated in a

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Figure 1. The Vars-Winchester esker aquifer, eastern Ontario , showing major geomorphic features and location of seis-mic transects and wells. Landforms in inset map are from Parent and Occhietti (1988), Barnett (1988), Gorrell (1991), Gadd et al. (1993), and Simard et al. (2003). The diamicton ridge (arrows ) that underlies the terminal end of the Vars-Winchester esker is a newly identifi ed feature. Multiple wells at one site are represented by one symbol.

Cummings et al.

1480 Geological Society of America Bulletin, July/August 2011

north-south direction, and, as such, is generally interpreted to have been deposited by southward-fl owing ice (Richard, 1982a, 1982b). Eskers were studied in the 1970s and 1980s by Rust and his students (e.g., Rust and Romanelli, 1975; Sharpe, 1988), the Vars-Winchester esker being a notable exception. They coined the term sub-aqueous outwash, interpreted it to be a dominant component of the eskers, and suggested that the ice retreated northward during esker depo si tion. Gorrell (1991) extended this work by mapping all eskers near Ottawa using uncored wells and outcrops. Gadd (1986) studied the Champlain Sea mud using discontinuous cores from mul-tiple locations. He identifi ed three mud units—basal rhythmites (“varves”), massive mud, and red-and-gray stratifi ed mud—overlain locally by sand, and interpreted the entire succession to have been produced by deltaic offl ap during forced regression of the Champlain Sea. An increasing-then-decreasing porewater salinity trend was identifi ed in the mud (Torrance, 1988), which mirrors an increasing-then-decreasing trend in proglacial water-body paleosalin-ity inferred from micropaleontological data (Anderson et al., 1985; Rodrigues, 1988; Guil-bault, 1989). MacPherson (1968) investigated the large valley that is incised into the modern landscape within which the modern, under fi t Ottawa River resides.

Despite the extent of the previous work, two things are clear: analyses have rarely used inte-grated data sets, especially those that integrate continuous cores to bedrock with seismic data, and, either because of research mandate or data set used, studies have rarely investigated the en-tire basin fi ll—till, eskers, and mud—as an inte-grated whole. We aim to do both herein.

METHODOLOGY AND DATA SET

Although geographically restricted, the Vars-Winchester data set (Fig. 1) is one of the largest integrated data sets collected from a glaciated basin to date. It offers unprecedented insight into the geology of a mud-buried esker and, more generally, into the interlinked geological components that comprise the basin fi ll.

In 2006, eleven seismic refl ection profi les were collected across the inferred path of the mud-buried Vars-Winchester esker, an impor-tant groundwater aquifer on the eastern outskirts of Ottawa. The esker map of Gorrell (1991) was used as a working hypothesis of the location of the buried esker. Both compressional wave (P-wave) and horizontally polarized shear wave (SH-wave) methods were used (see Pullan et al. [2007] for details). Drilling targets were delin-eated based on the seismic data, and 18 wells were drilled in 2007, six of which were cored

continuously (Cummings and Russell, 2007). Cores were logged in detail and sampled at meter-spaced intervals for porewater salinity and grain size. In one of the cores, 24 micro-paleontological samples and 25 geochemistry samples were collected from the Champlain Sea mud succession. Fresh outcrop exposures along the esker at the Watson Road and Regimbald Road aggregate pits were studied. Approxi-mately 6000 public water-well logs were used to supplement the core, seismic, and outcrop data. While this number may seem impressive, in re-ality, water wells give at best a fi rst-order esti-mate of sediment texture (Russell et al., 1998), though they do generally provide accurate depth to bedrock. Black and white air photos and a digital elevation model (DEM) were used to analyze the geomorphology of the esker and surrounding landscape.

RESULTS

The tripartite basin fi ll of till, esker, and mud fi rst identifi ed by Johnston (1917) is also observed in the Vars-Winchester esker data set (Figs. 3 and 4). Each unit has a distinct morpho-logic, sedimentologic, and seismic stratigraphic signature. In this section, we describe the char-acteristics of the units and intervening contacts in detail and then, using inverse reasoning,

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Figure 2. Physical, chemical, and biological attributes of Quaternary strata in the St. Lawrence Lowland near Ottawa (idealized). Based on Johnston (1917), Richard (1982a, 1982b), Gadd (1986), Rust (1987), Torrance (1988), Guilbault (1989), Douma and Nixon (1993), Gorrell (1991), Shilts (1994), Hyde et al. (1997), Aylsworth et al. (2003), Hunter et al. (2007), and Cummings and Russell (2007).



reconstruct the processes that most likely gen-erated these characteristics and the depositional environments in which the processes most likely operated. Larger-scale, longer-term processes that generated the succession of stratal units are discussed in the subsequent section (Glacial Sequence Stratigraphy).

Diamicton (Till)

Massive sandy silt diamicton (average 1–3 m thick) was intersected above bedrock in most wells (Figs. 4 and 5). The diamicton displays distinct fi ssility in outcrop. Its matrix reacts strongly with dilute hydrochloric acid. Clasts are angular to subrounded, are rarely striated and faceted, and are predominantly derived from the underlying carbonate mudstone bed-rock (>90%), with a smaller percentage (<10%) of granite and gneiss clasts derived from the Precambrian Shield, which is located ~10 km

north of the upfl ow end of the esker. Water wells indicate that the diamicton unit drapes the bed-rock surface; it does not thicken considerably into bedrock lows or thin over bedrock highs. The diamicton is largely absent beneath the gravelly core of the esker, and the unit is also largely absent to the north end of the study area. North-south–trending drumlins ornament its surface. In seismic transects, the diamicton gen-erates strong, parallel refl ections that in places form the top of acoustic basement. Given these characteristics, the diamicton is interpreted to be subglacial till.

In the south of the study area near Winchester, the diamicton has been shaped into an ~3-m-high, several hundred–meter-wide ridge that extends east-west for ~45 km (Fig. 6). The Vars-Winchester esker terminates along this ridge in a large sandy fan, as do adjacent eskers. The terminal sandy fan of the Vars-Winchester esker overlies the ridge. The terminal-esker fans are in-

terpreted to be coeval (cf. Gorrell, 1991), and the diamicton ridge—a newly identifi ed feature—is interpreted to be a grounding line moraine.

Esker and Off-Esker Gravel

The architecture of the Vars-Winchester esker is clearly revealed by the new data (Fig. 7). Two key “building-block” elements are identifi ed, a narrow gravelly central ridge and a broad sandy carapace. Both are present in transects south of the Watson Road pit (Fig. 7B), whereas only the gravelly element is present in transects north of this (Fig. 7A). A third element, a discontinuous off-esker gravel layer, is observed locally adja-cent to the esker.

Gravelly Central Ridge (R-Channel Deposit)Where imaged in seismic transects and

inter sected in wells, the gravelly central ridge element is 5–20 m high (average 15 m) and 100–250 m wide (average 150 m) (Fig. 7A). Its fl anks dip between 10° and 40°. It is invariably buried by mud, at least in part, typically directly overlies bedrock, and generates a seismic facies that locally contains hyperbolic diffractions and is somewhat chaotic. In SH-wave transects, the gravelly central ridge typically scatters suffi cient sound energy to obscure the underlying bedrock refl ection. By contrast, in P-wave transects, the underlying bedrock refl ection is commonly vis-ible. Based on these characteristics, in addition to the well data, the gravelly ridge element is interpreted to be present in all seismic transects. Its continuity between seismic transects, how-ever, is diffi cult to ascertain, which leaves open the possibility that breaks may be present.

Pits excavated into the gravelly ridge land-form are narrow and elongate (0.5–2 km long, <150 m wide) and are fl anked by mud. The only fresh exposure is in the Regimbald Road pit. Here, the esker landform is relatively nar-row (250 m wide) and lacks a sandy carapace (Fig. 8A). Pit faces consist of well-rounded peb-bles and cobbles organized into thick (1.5–3 m), stacked, high-angle (20°–30°) dune-scale, cross-stratifi ed beds. Cross strata dip toward the south-southwest (±45°), which is roughly par-allel to the long axis of the esker. Medium to coarse sand typically fi lls pore space between gravel clasts. Rare subrounded boulders up to 1 m in diameter are observed. Like the under-lying till, carbonate mudstone clasts predomi-nate (>90%), with minor percentages (<10%) of granitoid clasts. However, in contrast to the till, clasts are well rounded and striae free. Beds commonly span the exposure, reaching widths of at least 10 m perpendicular to paleofl ow.

Given its ridge-like shape, fl uvial-like sedi-mentology, and erosive superposition over

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Figure 3. Interpreted seismic transects. Arrows point to a diamicton-cored grounding line moraine at the terminus of the esker. South of the tributary-like junction, the esker consists of a gravelly central ridge with a broad sandy carapace, whereas only the gravelly central ridge element is present north of this.

Cummings et al.


bedrock , the gravelly central ridge is interpreted to have been deposited in a meltwater stream that fl owed along the base of the glacier through an upward-arched conduit (R-channel) (Fig. 9). The lithologic similarity of esker and till clasts suggests the gravelly ridge was sourced from the till, with possible contributions from infl ow-ing, debris-rich basal ice.

Sandy Carapace (Subaqueous Outwash)Sand with variable amounts of pebble gravel

buries the gravelly ridge locally, forming a carapace that is an order of magnitude wider (0.6–4 km) and much more gently fl anked (1°–5°) than the gravelly ridge itself (Fig. 7B). In wells drilled through the sandy carapace into the gravelly central ridge, the sand-gravel con-tact is intercalated over several meters (Well H in Fig. 4). Perhaps for this reason, the contact between sandy carapace and gravelly ridge does not generate a distinct, high-amplitude

seismic refl ection. Nonetheless, the two ele-ments may commonly be differentiated in the seismic data because the gravelly ridge gener-ates more hyperbolic refl ections, is character-ized by a slightly higher seismic velocity, and, as mentioned above, can obscure or mask the underlying bedrock refl ection in SH-wave transects. In wells drilled through the sandy carapace adjacent to the gravelly central ridge (Wells F and J in Fig. 4), a thin (<1 m) basal layer of gravel was intersected above diamicton (till) or bedrock.

The only fresh exposure of the sandy cara-pace is at the Watson Road pit. The esker has a moderate width (~800 m) at this loca-tion (Fig. 8B). Pit faces consist of multiple upward-coarsening, mound-shaped units that are stacked compensationally on top of each other. Sharp-based units, commonly gravelly, interrupt this motif locally. Bedding is nearly horizontal or very gently dipping, and beds

are centimeters to decimeters in thickness. The upward-coarsening lobes are tens of me-ters wide perpendicular to paleofl ow. Over the course of fi eld work, a mound that was under-going progressive excavation became narrower and coarser over several tens of meters in an up-paleofl ow direction (Fig. 8B). The basal por-tions of the lobes are commonly defi ned by a bed of silty climbing ripples—the fi nest grained and most aerially extensive beds in the expo-sure. Climbing ripples, dunes (which also rarely climb), low-angle (antidune?) cross strata, and scours fi lled with diffusely laminated sand are common. Shells were not observed, as is typical of esker exposures in the basin (but see Rust, 1987). However, marine shell fragments (Port-landia arctica) were recovered at 2 and 11 m below the top of the sandy carapace in a nearby core at a location where the esker is buried by 10 m of Champlain Sea mud (Well I in Fig. 4). Shell fragments from the upper occurrence

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Figure 4. Well data. Numbers refer to (1) till, (2) esker and thin off-esker gravel layer, and (3) Champlain Sea mud with minor sand near base and/or top. Small numbers in the Champlain Sea mud unit refer to (i) gray carbonate-rich rhythmites, (ii) massive mud, and (iii) red-and-gray, carbonate-poor rhythmites. DEM—digital elevation model.



yielded a radiocarbon age of 11,450 14C yr B.P. (Beta-241003) (age uncorrected for reservoir effect). There was insuffi cient shell material in the lower occurrence to permit dating.

The sandy carapace is interpreted to be a subaqueous outwash fan complex deposited by sediment-laden, jet-plume pairs (Powell, 1990; Hoyal et al., 2003) that emanated from the R-channel onto the fl oor of the Champlain Sea (Fig. 9). The general impression gained from the outcrop exposures is one of rapid sedimentation from rapidly decelerating, unidirectional fl ows:

the presence of bedforms attests to the tractive nature of sediment transport, whereas climbing bedforms (ripples and some dunes) and diffuse stratifi cation attest to high rates of suspended-sediment rain-out (Ashley et al., 1982; Arnott and Hand, 1989). The upward-coarsening, mound-shaped depositional elements exposed at the Watson Road pit are interpreted to be in-dividual subaqueous outwash fan lobes. Com-pensational lobe stacking is interpreted to refl ect avulsion of the jet-plume depositional locus. Sharp-based, ungraded to upward-fi ning units

refl ect either progradation of proximal-fan chan-nels over distal-fan deposits or abrupt increases in discharge. In either case, these upward-fi ning units likely correlate downfl ow to fan lobes. Finally , although fl ow-expansion and jet-plume deposition has been suggested to occur subgla-cially under certain circumstances (e.g., Bren-nand, 1994), the presence of Portlandia arctica shell fragments in the sandy carapace (cf. Rust, 1987) supports the hypothesis of Rust and Romanelli (1975) that the subaqueous fans are proglacial features and that the proglacial water body at the time of esker deposition near Ottawa was the Champlain Sea, not a proglacial lake as has been suggested by most workers (cf. Ander-son et al., 1985).

The sandy terminal fan complex at the south end of the Vars-Winchester esker—the widest part of the esker landform—is poorly exposed. However, a similar-sized but better exposed terminal fan complex exists in the next esker to the west near Kemptville along the same grounding line moraine (Fig. 6). We use this fan as an analog for the terminal fan of the Vars-Winchester esker. The bed-scale sedimen-tology of the Kemptville fan is similar to that of the sandy carapace at the Watson Road pit: climbing ripples and diffusely laminated sand units are abundant, and beds are laterally ex-tensive, suggesting rapid deposition in an un-confi ned (proglacial) subaqueous fan setting. However, the scale of the fl ows appears to have been much larger. Beds are hundreds of meters wide perpendicular to paleofl ow as opposed to tens of meters wide (Fig. 10A). Steep-walled scours fi lled with diffusely laminated sand are large and ubiquitous (Fig. 10B); these con-tain rare, boulder-sized sand intraclasts, some several meters in diameter (Fig. 10C). Large synsedimentary dewatering structures are com-mon (Fig. 10D). This assemblage of features is not unique to the terminal fans of eskers along this grounding line, such as the Lanark fan de-scribed by Gorrell and Shaw (1991), but rather is observed along the eskers in the basin wher-ever the sandy carapace is abnormally wide (Gorrell, 1991). A positive correlation therefore appears to exist between esker width and dis-charge: in conjunction with avulsion of the jet-plume depositional center (Gorrell and Shaw, 1991), high discharge is suspected to have been integral in generating the wider (>1 km) parts of the sandy carapace of the Vars-Winchester esker. This point will be expanded upon in the Discussion section.

Off-Esker GravelIn off-esker wells, a thin (0.1–2 m) gravel

layer was commonly intersected between Champlain Sea mud and till or bedrock (Wells

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INTERPRETATIONS

Hydrotroilite-linedburrows

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Figure 5. Sedimentary facies observed in core and interpreted depositional environments. Dashed contacts are gradational or intercalated. Solid contacts are sharp and erosive. Sand and gravel units in the esker were typically homogenized during core collection. Insight into these units was obtained from aggregate pits (see Fig. 9).

Cummings et al.


B and C in Fig. 4). This layer appears to be locally as permeable as the gravelly central ridge of the esker because circulation of drill-ing fl uids commonly decreased when it was penetrated, indicating that drilling fl uids were fl owing into the unit. Given its stratigraphic position, the off-esker gravel layer is tenta-tively correlated with the thin basal gravel layer that passes below the sandy carapace and into the gravelly central ridge of the esker. The composition of the off-esker gravel is similar to that of clasts in the esker and till: more than 90% are carbonate mudstone, with a subor-dinate amount of granite and sandstone. Be-cause of its thinness, the layer is not resolved as a discrete seismic unit. Sharpe and Pugin (2007), however, suggest that high-angle dif-fractions imaged locally above bedrock may attest to its presence.

The off-esker gravel layer is interpreted to have been winnowed from the till by fl owing subglacial meltwater. Other mechanisms, such as direct melt-out or “ablation” of debris from

the ice or widespread dropping of debris from icebergs fl oating in the Champlain Sea, seem in-compatible with its permeability and thickness, in addition to its common presence in areas where the till has been eroded.

Mud (Champlain Sea Deposits)

Mud buries the Vars-Winchester esker over 75% of its 50 km length (Fig. 1). In core, three units are identifi ed in the mud package (bot-tom to top): (1) upward-fining, carbonate-rich gray rhythmites, (2) massive gray mud, and (3) upward coarsening and thickening, carbonate-poor, red-and-gray rhythmites (Figs. 3 and 4). These units are commonly observed to be stacked on top of each other in “complete” Champlain Sea mud successions in the western Champlain Sea basin (Gadd, 1986; Shilts, 1994; Aylsworth et al., 2003; Ross et al., 2006). In seismic transects, the mud generates relatively transparent, continuous, parallel, nearly hori-zontal refl ections (Fig. 7). Refl ections onlap the

esker, and internal lap-out surfaces are absent between lithofacies units, at least at the scale of the seismic transects.

Basal Gray Muddy Rhythmites (Distal Subaqueous Outwash)

The gray rhythmite unit is <2 m thick, which is near the limit of seismic resolution. It is com-posed of multiple, thin (<1 cm) couplets, each sharp based and normally graded. The basal part of each couplet is composed of light-gray silt or very fi ne sand, and the upper part is dark-gray mud (Fig. 5). Light and dark layers are of near equal thickness. Faint horizontal laminae are visible locally in both light and dark layers. A maximum of 190 couplets are observed. The unit commonly fi nes upward. Rare pebble-sized dropstones are present. Bioturbation is negligi-ble to nonexistent. The unit reacts strongly with dilute HCl, with light layers reacting slightly more than dark ones, and it is enriched in ele-ments derived from the local carbonate bed-rock, such as strontium and calcium (Fig. 11).

Ottawa

U N I T E DS T A T E S

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Diamicton ridge

DrumlinsEskers


Large sandy fans at terminal end of eskers

Diamictonridge

Drumlins A

A′

Vars–Winchester

esker

Kemptvillefan

Figure 6. Diamicton ridge at the south end of the Vars-Winchester esker. The ridge, a newly identifi ed feature, is interpreted to be a ground-ing line moraine deposited in association with a reorganization of the subglacial meltwater system immediately prior to esker deposition. Esker traces from Gorrell (1991). m a.s.l.—meters above sea level.



Porewater salinity is low and increases upward. Microfossil samples contain rare Candona, a benthic freshwater ostracod, or a sparse mix of Candona and equally rare benthic forami-nifera (mostly Cassidulina reniforme). The gray rhythmite unit was intersected in all cores, with the exception of one drilled into the crest of the

esker (Well H in Fig. 4). Where intersected, the unit sharply overlies the esker. Previous authors identify a similar basal gray rhythmite unit throughout the western Champlain Sea basin, from south of the Canada–U.S. border (Pair and Rodrigues, 1993) to north of the Quebec-Ontario border (Gadd, 1986).

The gray rhythmites are interpreted to be distal subaqueous outwash fan strata (Fig. 10). The mix of cassidulinids with Candona pro-vides further evidence that the Champlain Sea existed proglacially during initial deglaciation near Ottawa (not an ice-ponded proglacial lake), although the benthic environment must

Sand

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atio

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Figure 7. Representative seismic transects that cross (A) the narrow northern portion of the esker and (B) the broad southern portion of the esker. In (A), the esker is narrow, steep-fl anked, and gravelly (interpretation: R-channel deposit), whereas in (B) the sediment body is much broader, has gently sloping fl anks, and consists of two elements, a narrow gravelly central ridge (R-channel deposit) and a broad sandy carapace with rare marine shells (subaqueous outwash fan complex). Both (A) and (B) are shear-wave (SH-wave) transects. Letters above wells pertain to letters in Figure 4. m a.s.l.—meters above sea level.

Cummings et al.


have been highly freshened by meltwater in-fl ux for Candona to have survived. This may have been facilitated by seasonal meltwater dis-charge variations and, consequently, seasonal migration of the salinity front (Cummings and Russell, 2007).

Massive Mud (Distal Subaqueous Outwash)The rhythmically laminated unit grades up

over ~10 cm into dark-gray massive mud (Fig. 5). The massive mud reacts with HCl, but less strongly than the underlying rhythmites. Its geo-chemical signature is transitional between that of

the underlying carbonate-rich mud and the over-lying carbonate-poor mud (Fig. 12). Porewater salinities reach a maximum level, as does biotur-bation intensity, macrofossil density (Portlandia arctica), and microfossil density and diversity (mostly Cassidulina reniforme and Islandiella spp., with Elphidium excavatum forma clavata becoming dominant near the top contact). Bio-turbation is commonly intense (Fig. 5), though it is not present throughout, especially where the massive mud is interstratifi ed with the overlying red-and-gray stratifi ed mud unit. Burrows are vertical to horizontal, 0.1–2 mm wide and <1 cm

long. They are defi ned by a black substance on freshly cut surfaces, possibly hydrotroilite (Barghoorn and Nichols, 1961; Guilbault, 1989), which disappears after several hours of exposure to the atmosphere. Rare pebble-sized dropstones are present. Freshly cut surfaces have a subtle sulfurous odor.

The massive mud unit is interpreted to have been deposited in a similar deep-water, basin-fl oor setting as the gray rhythmites it gradationally overlies (Fig. 9). However, the environmental conditions appear to have been slightly different. The near-fl oor salinity of the

W E

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Rubble

“Distal” cross section of same “mound” ~50 m down paleoflow

Paleoflow out of page (southward)

“Proximal” cross sectionof a “mound”-shaped unit

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Ottawa Riverincised valley

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SSWNNE

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SlumpCobbles

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A

Figure 8. Aggregate pit outcrops excavated into the esker landform. (A) Regimbald Road pit, where the gravelly central ridge of the esker is exposed (interpretation: R-channel deposit). (B) Watson Road pit, where the sandy carapace of the esker is exposed (interpretation: subaqueous outwash fan complex).



Champlain Sea was higher, possibly close to polyhaline levels (25‰–32‰), and the environ-mental stress on benthic biota was reduced, pos-sibly due to decreased turbidity, fewer salinity fl uctuations, and/or greater food availability. As a result, higher-salinity water became trapped in sediment pore spaces. A somewhat diverse benthic microfaunal community was able to fl ourish, and the seafl oor became intensely bioturbated.

Red-and-Gray Mud Rhythmites (Ice-Distal Basin-Floor Deposits)

Red-and-gray stratifi ed mud overlies the mas-sive mud unit (Fig. 5). Its lower contact with the massive mud can either be sharp or inter-calated over several meters. The unit consists of very faint couplets that grade upward from

light-gray mud to dark-gray or pinkish-red mud. Red and/or dark-gray layers are of simi-lar thickness or slightly thinner than light-gray layers. Light-gray layers are commonly but not invariably sharp based, and tend to grade into dark-gray and/or red bands. The light-gray bands dry out slightly faster than the dark-gray and/or red bands. In some cores (Wells H and I in Fig. 4), successive light-gray mud bands become sandier upward. Couplets increase in thickness upward, from <1 cm near the base of the unit to several tens of centimeters near the top of the unit. The thickest couplets in cores from the north end of the study area, where the mud package is thicker, reach a maximum thick-ness of 35 cm, whereas the thickest couplets in cores to the south, where the mud package is thinner, reach a maximum thickness of 15 cm.

Layers are typically horizontal but dip up to 20° where seismic refl ections are contorted (Well A in Fig. 4). Bioturbation levels are nil to low. Black residue, possibly hydrotroilite, is ob-served in light-gray bands near the base of the succession, and is also rarely present as discrete, diffuse layers in the pinkish-red bands. Rare, poorly preserved shells (Portlandia arctica) are locally present. Microfossils are rare, and are commonly absent in samples from the upper-most third of the succession (Fig. 11). Elphid-ium excavatum forma clavata is the dominant species. Compared to the underlying mud units, the red-and-gray stratifi ed mud unit reacts less strongly with dilute HCl, and commonly not at all, with red layers reacting slightly more than light-gray layers. It is enriched in Shield-derived elements and impoverished in elements derived from the local carbonate bedrock. The upper 1–5 m of mud below ground surface is orange-brown, is stiffer and dryer than the rest of the unit, and locally contains root traces, joint-like structures, and very fi ne sand beds (<20 cm thick) of stacked current-ripple or combined-fl ow–ripple cross sets with mud fl asers (Fig. 5).

Gadd (1986) interprets the red-and-gray stratifi ed mud unit to be a muddy ice–distal del-taic deposit. This seems supported by the core data: strata thicken and in some cases coarsen upwards; slumped and contorted deposits are common; and the unit thins southward and eventually pinches out over several tens of kilome ters moving away from the rivers that could have been meltwater charged (i.e., the paleo–Ottawa River and small rivers drain-ing the Shield). However, seismic data bring this interpretation into question: no obvious clinoforms are observed. Rather, refl ections are roughly fl at lying, horizontal, and are con-formable with the underlying mud, at least at the scale of the seismic transects. Deltas deposit clinoforms as they prograde basinward, albeit potentially very low angle ones (<0.01°), if the system is mud rich (Orton and Reading, 1993). If such very low angle clinoforms are present, it is possible that the seismic transects are not long enough to image them or that the transects are oriented perpendicular to the progradational axis of the delta(s). An alternative and perhaps more reasonable explanation is that the unit was deposited in a deep-water, basin-fl oor set-ting by areally extensive meltwater plumes that emanated from the smaller, higher-gradient riv-ers of the Precambrian Shield and/or the paleo–Ottawa River. Elsewhere in the basin, the upper mud unit is ponded and laps onto underlying strata (fi g. 4 in Ross et al., 2006), indicating that a hiatus preceded deposition. The domi-nance of microfossils tolerant to a wide range of salinities suggests that meltwater infl ux had

Champlain Sea

Jet-plume pair

ICE

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Sand bedforms and suspension rain-out

Mudsuspension rain-out

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Gravellycentral-ridge Muddy fan

Grain size

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Sedimentation rate

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Suspension settling

wave base

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Figure 9. Facies model cartoon for the Vars-Winchester esker depicting inferred depo-sitional environments for the two main elements of the esker, the gravelly central ridge (R-channel deposit) and the sandy carapace (subaqueous outwash). Inferred depositional environments for the muddy rhythmites and massive bioturbated mud units at the base of the overlying Champlain Sea mud package are also shown. The plume is shown as being negatively buoyant because Champlain Sea mud strata onlap the Vars-Winchester esker as opposed to draping it. Note that to fully understand the Vars-Winchester esker, this facies model, which is essentially a snapshot of the depositional system in time, must be integrated with the glacial sequence stratigraphic model (Fig. 12), which accounts for the longer-scale changes in sediment supply and accommodation space associated with ice-front retreat and subsequent isostatic rebound that generated the stratigraphic succession.

Cummings et al.


increased into the basin or, alternatively, that the connection between the basin and the ocean had become narrower. The reduced bioturba-tion levels, reduced faunal density and diversity, and upward increase in bed thickness suggest sedimentation rates increased over time (e.g., MacEachern et al., 2005).

GLACIAL SEQUENCE STRATIGRAPHY

We now interpret the longer-term processes that generated the succession of stratal units and intervening surfaces using sequence strati-graphic concepts and principles (Figs. 12 and 13). As stated previously, sequence stratigraphy holds that basin-wide patterns in sedimentary strata are produced by the interplay of two vari-ables, sediment supply (S) and accommodation space for sediments (A). In nonglacial sedi-mentary systems, shoreline translation forced by changes in S and A is the main process that mediates regional packaging of strata over long “Milankovitch-scale” time periods (e.g., Posamentier and Allen, 1999). Changes are as-sumed to be slow and gradual. Glacial sedimen-tary systems are different in two ways. First, the ice front is the main interface, not the shoreline.

Its translation during glacier advance-retreat cycles across different bedrock lithologies gen-erates long-term changes in S and A and causes depositional environments to migrate in time and space, which produces predictable, regional stratigraphic patterns in glaciated basins (Clay-ton and Moran, 1974; Shilts et al., 1987; Boul-ton, 1996; Powell and Cooper, 2002; Corner, 2006). Shoreline migration may be important in some cases, but its role in pattern genera-tion is typically subordinate to, and, because of glacio-isostasy and/or glacio-eustasy, typically tied to, climatically forced ice-front migration. Second, rapid meltwater events occur, both sub-glacially (Siegert et al., 2007) and proglacially (Bretz, 1923). These two traits—that ice margin translation generates the main, gradual S and A changes, and that meltwater events punctuate these gradual changes—are believed to be the hallmarks of glacial sedimentary systems.

Subglacial Erosion

The fi rst event recorded in the stratigraphic succession is erosional (Sharpe, 1979): the gla-cier advanced southward, eroded sediment, and striated the bedrock surface.

Subglacial Till Deposition

At some point, accommodation space be-neath the glacier switched from negative to posi-tive, and a massive till sheet was deposited. It is unclear what instigated this switch or when it occurred during the advance-retreat cycle. What is clear is that the glacier must have been strongly coupled with its substrate as it fl owed across the Precambrian Shield uplands and onto Paleozoic carbonate rocks in the basin: in addi-tion to wholesale pre-till erosion, the till itself changes from being carbonate poor, sand rich, and devoid of Paleozoic carbonate clasts over the Shield to being carbonate rich, silt rich, and containing almost exclusively (>90%) Paleo-zoic carbonate clasts on the fl oor of the basin less than 10 km downfl ow of the Precambrian–Paleozoic bedrock contact.

Subglacial Meltwater Erosion

Following till deposition, but prior to esker deposition, subglacial meltwater eroded the top of the till. The exact ice front position dur-ing this event, and the exact timing of the event, are unknown. Evidence for meltwater erosion is most obvious beneath the esker: the underlying till is commonly stripped and, in adjacent parts of the basin (e.g., Gorrell, 1991; Spooner and Dalrymple, 1993), the bedrock beneath eskers is commonly sculpted into forms (s-forms) that are identical to those found in bedrock river channels. The Vars-Winchester esker is therefore suspected to reside, at least in part, in a shallow channel eroded through the till to bedrock (cf. Gorrell, 1991), one that is perhaps akin to the erosional corridors within which well-exposed eskers commonly reside on the Precambrian Shield (e.g., Rampton, 2000). Off-esker gravel adds to the meltwater story, as do off-esker s-forms observed in adjacent parts of the basin (e.g., Gilbert, 2000). They suggest that meltwater erosion was more widespread. But how much more widespread, and by what mechanism(s) did the meltwater erosion occur ? Could esker deposition have been preceded by one or more jökulhlaup-like events? R-channels may increase in size by an order of magni-tude during jökulhlaups beneath modern gla-ciers, scavenging underlying till in the process (Fowler and Ng, 1996). Several authors have argued that meltwater fl oods beneath the Lau-rentide Ice Sheet were even more extensive, and that regional subglacial sheet fl oods eroded the top of the till, carving drumlins in the process, before devolving into channelized subglacial fl ows (Sharpe and Shaw, 1989; Shaw and Gil-bert, 1990; Sharpe and Pugin, 2007). Both of these ideas appear supported by the results of a

Pit floor Car for scale

Thinly bedded fine sand

Slump

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Shovel for scale

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Climbing ripples

Figure 10. Traced photos of sandy subaqueous outwash strata from the large terminal fan complex in the adjacent esker to the west near Kemptville. This fan serves as an analog for the coeval but less well exposed terminal fan of the Vars-Winchester esker, which is located along the same grounding line moraine. (A) Thin sand beds deposited in a mid-fan setting, the predominant facies exposed in the pit. Paleofl ow is obliquely into the page. These beds are laterally continuous over hundreds of meters, suggesting that the jet-plume pairs that emanated from the R-channel were large, which in turn suggests the R-channel was large. (B) Diffusely laminated scour fi lls with walls approaching the angle of repose commonly dis-sect the thin sand beds. Paleofl ow is into the page. (C) Sediment intraclasts are rarely present in the diffusely laminated scour fi lls, some of which are the size of large boulders. This suggests that the associated fl ows were very powerful. Paleofl ow is into the page. (D) Large dewatering structures are common, especially in fi ner silty sand, suggesting that deposition was rapid.



small-scale analog experiment (Paola, 1977) in which unrestricted, sheet-like fl ow outside the confi nes of an R-channel is required to gener-ate a fi nal narrow R-channel deposit, in addition to observations made during the 1996 outburst fl ood at Skeiðarárjökull glacier, Iceland, where a subglacial sheet fl ood preceded esker deposi-tion (Burke et al., 2008). Transfer of meltwater from the surface to the base of the glacier (e.g., Sugden and John, 1976) may have played a role. In reality, however, it is diffi cult to gauge which of these models, if any, is applicable to the study area because the distribution of off-esker gravel near the Vars-Winchester esker is poorly constrained due to mud cover. Irrespective of its inferred origin, the off-esker gravel layer represents a newly identifi ed component of the Quaternary stratal succession, one that deserves further attention because similar gravel layers appear to be present elsewhere in the Cham-plain Sea basin (Gadd, 1986; Barnett, 1988;

Ross et al., 2006) and in other low-lying, mud-rich glaciated basins (e.g., Abitibi clay basin ; Veillette et al., 2007).

Ice Retreat and Esker Deposition

As deglaciation progressed, the ice sheet retreated northward, leaving a series of end moraines in northeastern United States and southeastern Canada (Parent and Occhietti, 1988; Ridge, 2004). When the ice front retreated into the Champlain Sea basin and reached a position near Winchester, possibly around 11,500 14C yr B.P. (Ridge, 2004), the glacial hydraulic system reorganized itself. Follow-ing melt water erosion, several equally spaced R-channels formed, and eskers, including the Vars-Winchester esker, started to deposit, in part in the R-channels and in part where the R-channels debouched into the Champlain Sea, which generated subaqueous outwash fans.

Given the superposition of the terminal fan over the grounding line moraine, moraine deposi-tion must have preceded esker deposition (cf. Ottesen et al., 2008). The moraine is straight and lacks invaginations, which suggests the ice sheet at the grounding line had a similar shape. Till was likely delivered to the grounding line by shearing of basal debris; meltwater only contrib-uted sediment to the moraine landform where eskers are present. Exactly what instigated mo-raine building and esker deposition is unknown. Possible mechanisms include a glacial surge fol-lowed by a meltwater purge (e.g., Kamb et al., 1985; Ottesen et al., 2008), lowering of base level associated with glacial lake drainage (e.g., Pair and Rodrigues, 1993), or climatic ameliora-tion and increased surface-to-base meltwater fl ux (e.g., Zwally et al., 2002).

Ice retreat is interpreted to have been the main process responsible for the two-tiered, sand-over-gravel stratal organization of the Vars-

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Figure 11. Micropaleontology, geochemistry, and in situ porewater salinity of Champlain Sea mud succession (Well A in Fig. 4).

Cummings et al.


Thin, permeableoff-esker gravel layer(extent poorly constrained;not present in drumlinzedtill uplands)

R-channelgravel

Proximal subaqueous outwash fan

Lodgement till Lodgement till

Stratified ice-distal basin floor mud

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Bedrock (carbonate mudstone or shale)

GRADUAL PROCESSESRAPID EVENTS

Subglacial meltwatererosion; extent poorlyconstrained outside ofimmediate vicinity ofesker

Jökulhlaup carvesOttawa River incisedvalley (early Holocene)

Subglacial erosion surface

Wave reworking (uplands) and tidal reworking (channels), tributary-stream incision,

and interfluve pedogenesis

Backstepping ice front

Forestepping shoreline

SandStiff diamicton Stiff diamicton

Red-and-gray stratified mud(upward coarsening)

Modern river

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Bedrock (carbonate mudstone or shale)

Massive mud

= Marine shells

= Roots

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K ~10–6 to 10–10 cm/s

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K ~10–6 to 10–10 cm/s

Esker well>500 gpmrelatively soft and low NaCl

Fractured-bedrock well<10 gpmrelatively hard and high NaCl

April–May

June–Oct.

River level

= Vertical fracture

The esker outcropslocally and, whereburied, is dissectedby several incisedstreams. There is therefore direct surface-to-groundwaterconnection locally.

A

Sequence-stratigraphic interpretation

Litho-, chemo-, and biostratigraphy

Hydrostratigraphy

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rbatio

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r salin

ity

Onlap surface in adjacent areas (not visible near VW esker)

HCl reactio

n

Figure 12. Summary of observations and a sequence stratigraphic interpretation. (A) Seismic stratigraphy, (B) lithostratigraphy, (C) hydrostratigraphy, and (D) sequence-stratigraphic interpretation. K—hydraulic conductivity; SH-wave—shear-wave; VW—Vars-Winchester esker.



Winchester esker. Initially, accommodation space was confi ned to R-channels. Poorly sorted, carbonate-rich sediment was entrained by the subglacial stream from till and from infl owing debris-rich basal ice. Most mud and fi ner sand (suspended load) bypassed the R-channel, leav-ing a narrow gravelly ridge with a coarse sand matrix (bedload). As the ice retreated and the environment switched from subglacial to pro-glacial, a large jump in accommodation space occurred. Broad submarine fans were deposited at the mouth of the R-channel. Sand with minor gravel was deposited in the proximal portions of the fans—these strata are exposed in sand pits in places along the esker landform—whereas the carbonate-rich mud bypassed the proximal parts of the subaqueous outwash fans and was depos-ited, largely off-esker, in the areally extensive, fl at-lying, fan toe sets as muddy rhythmites and, farther basinward, as massive bioturbated mud. Gradual backstepping of the ice front caused depositional environments to retreat, which, in accordance with Walther’s Law, superimposed distal over proximal facies. The sedimentary record of this gradual process is the upward-fi ning succession that starts with the esker gravel and ends with the massive bioturbated mud. Eskers in the Champlain Sea basin are therefore somewhat analogous to incised fl uvial valley fi lls (e.g., Zaitlin et al., 1994), although their upward fi ning trend was generated by re-treat of the ice front, not the shoreline.

Forced Regression and Ice-Distal Mud Deposition

As the ice retreated out of the basin, isostatic rebound, which had likely commenced ear-lier as the ice sheet started to thin, forced the Champlain Sea to regress. This caused a new sediment source—the ice-distal, meltwater-fed shoreline—to translate back into the basin. Melting of debris-rich basal ice, coupled with erosion of freshly exposed sediment covering the Shield, caused a carbonate-poor sediment pulse to be delivered to the fl oor of the now ice-distal basin. The fi ne-grained texture of the resultant deposit may in part refl ect trapping of coarser material upfl ow, possibly in proglacial lakes, which can function as highly effective sediment fi lters (Østrem et al., 2005). Contin-ued relative sea-level fall forced the salinity front basinward. This, possibly in conjunction with increased meltwater fl ux, caused the sea

to become fresher (cf. Guilbault, 1989; Shilts, 1994). Sedimentation rates increased as the shoreline translated basinward, generating the upward thickening and/or coarsening trend. Due to hypsometric effects, prolonged wave action preferentially reworked steep slopes, such as the tops of esker landforms, generat-ing sandy beach deposits, many of which ex-tend out over the top of the Champlain Sea mud package (Richard, 1982a, 1982b; Gor-rell, 1991). Tides also appear to have reworked sediment locally during the final stages of depo sition, given the presence of mud fl asers in some of the uppermost sand units (Well I in Fig. 4). However, because the basin was rela-tively wide, it is suspected that the effects of tidal currents were limited to shallow, channel-like constrictions along the shoreline, with more intense tidal sediment transport occur-ring at the topographic bottleneck near Quebec City (e.g., Occhietti et al., 2001), the exchange point between waters of the Champlain Sea and the Atlantic Ocean. Similar tidal dynam-ics are observed in modern, narrow-mouthed, microtidal seas such as the Mediterranean and Baltic (Howarth, 1982).

Proglacial Meltwater Erosion

The last major geomorphic event in the Low-lands was erosional: meltwater traveling down the paleo–Ottawa River carved an anasto mosed valley into the top of the Champlain Sea mud package (Johnston, 1917; MacPherson, 1968; Catto et al., 1982; Shilts, 1994). The valley is huge. For 300 km upstream of Montreal, it retains a relatively constant cross section that is an order of magnitude larger than most modern rivers (only the Amazon comes close), and is equal in width to the Missoula outburst fl ood channel. The valley narrows and deepens at till-covered, bedrock-controlled constric-tions near Ottawa and Montreal. Large bar-like forms downfl ow of Ottawa are erosional mud paleo-islands, as are smaller streamlined forms within one of the individual channels. They necessitate simultaneous carving by a fl ow equal in width to the anastomosed valley. Coarse bedload is typically absent on the val-ley fl oor, although several streamlined ridges composed of meter-sized imbricated boulders are present near Ottawa (Keele and Johnston, 1913). An elongate, 30-m-deep scour oc-curs around a bedrock obstacle in a lake-like

Subglacial erosion

I C E

Subglacial erosion surface

Bedrock

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Subglacial till deposition

I C E

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Subglacial meltwater erosion(dimensions and timing poorly constrained)

Meltwatererosionsurface

3

Ice retreat andesker deposition

Isostaticrebound

4

Proglacial meltwater erosion

Isostaticrebound


6

Forced regression and ice-distal mud deposition

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Basin floor(?) mudEskerlandform5

I C E

I C E

C H A M P L A I N S E A

Wave and tidal reworked, pedogenically altered surfaceA I R

C H A M P L A I N S E A

Figure 13. Cartoon depicting the event sequence inferred to have deposited the Vars-Winchester esker and associated muddy strata. Numbers refer to steps outlined in text (see Glacial Sequence Stratigraphy section).

Cummings et al.


(bedrock-silled) reach just upfl ow of Ottawa (Shilts, 1994). The scour could not have been generated by falling base level because of the bedrock sill.

Given these observations, in addition to the absence of an obvious break in slope from which valley incision could have nucleated and the fact that the St. Lawrence River shows no comparable incision upstream of its confl uence with the Ottawa (Fig. 1), the incised valley is in-terpreted to have formed when discharge down the Ottawa River was much greater than today (cf. Shilts, 1994). The valley was carved in the early Holocene between ~9500 14C yr B.P., the age of the youngest Champlain Sea shells be-neath the erosion surface (Rodrigues, 1988), and ~7500 14C yr B.P., the age of the oldest peat overlying the erosion surface at the base of the incised valley (Aylsworth et al., 2000). Teller (1988) has suggested, based on shoreline evi-dence from former glacial-lake basins located upfl ow, that meltwater-outburst fl oods with discharges similar to that of the modern Ama-zon River (several hundred thousand cubic me-ters per second) traveled down the Ottawa–St. Lawrence corridor during the early Holocene. Given this evidence, in addition to the size of the valley, it is diffi cult to envision that the val-ley was generated by anything but a catastrophic glacial-lake outburst fl ood that traveled down the Ottawa–St. Lawrence river corridor and into the Atlantic Ocean during the early Holocene (cf. MacPherson, 1968; Shilts, 1994).

DISCUSSION

Catastrophism and gradualism have both fi g-ured prominently in the historical development of glacial geology. While most researchers to-day would agree that the former is not an alter-native to the latter—large-scale “catastrophic” events occur in modern environments—opin-ions still differ as to whether slower, frequent, lower-magnitude processes or rapid, infrequent, higher-magnitude processes control how the Earth is sculpted and which sedimentary signals become preserved in the stratigraphic record. In the sequence stratigraphic model proposed here, a mix of these two end-member models is in-voked. This duality is perhaps best refl ected in the geology of the Vars-Winchester esker.

The Vars-Winchester esker is in many ways a typical esker. Its gravelly central ridge (R-chan-nel deposit) is on average 150 m wide and 10 m high, its fans are of similar height but an order of magnitude wider, and it forms part of a group of eskers that run nearly parallel to each other and are spaced at a regular interval of ~10 km. Eskers throughout North America and Europe dis-play similar cross-sectional areas (Fig. 14) and spacings (Aylsworth and Shilts, 1989; Bolduc, 1992). This could be taken as an indication that eskers deposited during the last deglaciation formed under a narrow range of paleo hydraulic conditions. Previous esker paleodischarge esti-mates do not refl ect this; rather, they vary by fi ve orders of magnitude (Fig. 15). On the lib-

eral (“catastrophic”) side, authors have invoked esker-forming discharges similar to those of the largest rivers on Earth (e.g., Brennand, 1994). R-channels are envisioned to have been equal in cross section to the esker itself (tens to hundreds of meters; see also Shreve, 1985a, 1985b), and deposition is thought to have occurred rapidly, over the course of few events. On the conser-vative (“gradual”) side, eskers are envisioned as having built up slowly and incrementally over time under gradual, astronomically forced (sea-sonal, diurnal) variations in meltwater discharge (e.g., Clark and Walder, 1994). Discharges are envisioned as having been small—similar to that of a small stream (e.g., Hooke and Fastook , 2007)—and R-channels are thought to have been likewise small (tens of centimeters to meters in diameter).

In the Vars-Winchester esker, gradualism is refl ected in the sand-over-gravel, upward-fi ning trend of the esker, which is interpreted to have been generated by slow, gradual ice-front retreat. Gradualism is not obviously refl ected, however, in the sedimentology of individual channel–fan building block elements that comprise this upward-fi ning succession. Rather, these building blocks appear to have been deposited primarily during rapid, high-magnitude discharge events. For example, beds in the gravelly central ridge exposed at the Regimbald Road pit are at least 10 m wide perpendicular to paleofl ow. The R-channel must have therefore been at least 10 m in diameter (cf. Clark and Walder, 1994),

1

10

100

1 10 100 1000 10,000 100,000

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Hei

gh

t (m

)

Vars-Winchester esker(sandy fan)

Vars-Winchester esker(gravelly ridge)

12

34

5

6

7

8 2

34

56

1. Gravelly ridgeelements

2. Sandy fanelements

Figure 14. Average size of eskers from across North America and Eurasia. Note similarity between esker dimen-sions, irrespective of geographic location. Gravelly central ridges tend to be on average 15 m high and 100 m wide, whereas associated sandy fans tend to be of similar height but an order of magnitude wider. Inset cartoon modifi ed from Bolduc (1992). 1—Labrador eskers, Canada (Bolduc, 1992); 2—Tarlow esker, Poland (Radlowska, 1969); 3—Deep Rose esker, Nunavut, Canada (Shilts, 1984); 4—Rooskagh esker, Ireland (Delaney, 2001); 5—Katahdin esker, Maine, USA (Shreve, 1985a); 6—Skane eskers, Sweden (Hebrand and Amark, 1989); 7—Casement Glacier eskers, Alaska, USA (Price, 1966); 8—Kaneville esker, Illinois, USA (Lukert and Winters, 1965).



and may have been up to an order of magni-tude larger, if dune–fl ow-depth scaling relation-ships for fl uvial systems apply (see Leclair and Bridge, 2001). Given that the strata consist of cobbles with rare small boulders, the fl ows likely traveled at several meters per second (cf. Costa, 1983). Discharge is therefore estimated to have been at least several hundred cubic meters per second, and was perhaps more likely several thousand cubic meters per second (Fig. 15). This latter, liberal estimate is reinforced by the substantial width of beds and the huge sediment intraclasts in the sandy terminal fan near Kempt-ville (Fig. 10). If scaling relationships for ex-perimental subaqueous fans apply (Hoyal et al., 2003), the width of the R-channel during deposi-

tion of the wide sandy fan complexes (e.g., the terminal fan) may have been similar to width of the gravelly central ridge (~150 m). To put this into perspective, R-channel discharge may have at times been similar to, or possibly greater than, the discharge of the modern Ottawa River (~2000 m3/s). This falls in the middle of paleodis-charge estimates previously invoked for similar-sized eskers (Fig. 15).

Although evidence for high-magnitude fl ow is predominant in the Vars-Winchester esker, at other times meltwater discharge appears to have been lower and the R-channel much smaller. As noted by Brennand (1994) during her work on eskers in southern Ontario, lower-magnitude fl ow events appear to be preferentially preserved

in the fans, which represent a more distal, higher-accommodation part of the esker system than the gravelly central ridge. The subaqueous outwash fan lobes that comprise the sandy carapace at the Watson Road pit are only several tens of me-ters wide perpendicular to paleofl ow. Over the course of excavation, one of these lobes tapered up-paleofl ow to a width of ~10 m (Fig. 8B), sug-gesting that the associated R-channel must have been 10 m or less in diameter. The pebbly texture of this deposit suggests fl ows traveled ~1 m/s (Costa, 1983). These data, in concert, suggest that discharge was on the order of 50–100 m3/s, similar to that of a small stream in fl ood.

Theoretically, variation in discharge of sev-eral orders of magnitude might be expected over

R-channel width (m)

Dis

char

ge

(m3 /

s)

Smalle

r dis

charg

es, m

any events

Larger d

isch

arges,

few

er events

Eurasian eskersBoulton et al., 2008

IcelandicjökulhlaupeskerBurke et al., 2008

Katahdinesker

Shreve, 1985b

0.1 10 1001 10001

100

1000

10

10,000

100,000

Vars–W

inch

ester

esker

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Mississippi

Ottawa

Severn

Nigardsbreen

Norwoodesker

Brennand, 1994

KatahdineskerHooke and Fastook, 2007

Esker

R-channel

Esker

R-channel

Conservative end-member model

Liberal end-member model-Huge discharges-Large R-channels-Few discharge events-Discharges commonly random

-Small discharges-Small R-channels-Multiple discharge events -Discharges seasonally mediated

Vars–Winchesteresker

-Larger discharges (mostly)-Larger R-channels (mostly)-Multiple discharge events-Discharges seasonallymediated (mostly)

Figure 15. Estimated depositional conditions for the Vars-Winchester esker plotted against estimates for other, similar-sized eskers. The average discharge of several rivers is shown for comparison.

Cummings et al.


the course of deposition of an esker. Melt water fl ux should vary in response to astronomic forc-ing (daily and seasonal insolation cycles) and episodic events (jökulhlaups), and the size of the R-channel and associated proglacial fan should shrink and grow accordingly (Rothlis-berger, 1972; Hoyal et al., 2003). However, the prevalence of high-magnitude fl ow signatures in the Vars-Winchester esker, like those ob-served in other similar-sized eskers (e.g., Gor-rell and Shaw, 1991; Brennand, 1994), may not be surprising, given that discharge spikes are common in glacial meltwater systems (Østrem , 1975), and that sediment transport rates in-crease nonlinearly with discharge (Julien and Simons, 1984).

The meltwater events that deposited the Vars-Winchester esker may have been high magnitude, but were they random (e.g., Burke et al., 2008)? In discussing this question, it is instructive to compare proximal and distal parts of the esker system sensu lato. Unlike the proxi-mal parts of the esker system, where random-looking, high-magnitude signatures abound, the carbonate-rich muddy rhythmites that com-prise the distal part of the esker system sensu lato are surprisingly regular in their spacing and thickness. How is this possible? One po-tential explanation is that, contrary to popular thinking (e.g., Gadd, 1986), the rhythmites are not varves, but are rather pulse-like underfl ow deposits emplaced during random jökulhlaups. This seems unlikely for several reasons. In ad-dition to the regularity of the rhythmites, inter-vening fi ne-grained interfl ood layers might be expected, if a random process governed deposi-tion, and these are not observed. An alternative and perhaps more reasonable explanation is that the high-magnitude discharge spikes occurred primarily during summer, a common observa-tion in modern glaciers (cf. Østrem, 1975), and that this spiky discharge signal was fi ltered and smoothed out by the basin waters, resulting in the preservation of a gradual, periodic, on-off signal associated with the seasonal melt cycle. In other words, a gradual process—the seasonal melt cycle—modulated a “catastrophic” one, namely jökulhlaup-like meltwater fl oods from the glacier. If this is true, it serves as a caution-ary tale for glacial geologists: evidence for the role of gradualism and catastrophism is likely strongly biased by the data set investigated. Sig-natures from catastrophic fl oods may become more muted in the distal parts of the system, whereas the preservation potential of gradual (seasonal) processes may decrease in the op-posite direction (cf. Brennand, 1994). A very different impression of how glacial sedimentary systems work may therefore be reached depend-ing on whether basinal deposits (mud) or glacio-

fl uvial feeders (e.g., eskers) are investigated, even within the same basin. Clearly all data sets must be evaluated in order to fully understand how glacial sedimentary systems work.

CONCLUSIONS

(1) Stratigraphic patterns in the Champlain Sea basin, like stratigraphic patterns in all sedi-mentary basins, were generated by the interplay between two variables, sediment supply and accommodation space. Ice-front translation caused gradual, long-term changes in these variables, and generated the main stratigraphic theme. Rapid meltwater events punctuated this gradual change, causing rapid changes in sedi-ment supply and accommodation space that left distinct signatures in strata and on surfaces.

(2) The geology of the Vars-Winchester esker refl ects both gradual and “catastrophic” processes : gradual ice-front retreat generated the net upward fi ning trend, whereas rapid high-magnitude fl oods that punctuated gradual retreat were responsible for the bulk of the sediment erosion, transport, and deposition.

(3) The esker-depositing fl oods are estimated to have had discharges of several thousand cubic meters per second. The associated R-channels are interpreted to have been large, commonly similar in cross section to the gravelly central ridge itself. These estimates fall in the mid to upper range of previous estimates for similar-sized eskers elsewhere.

(4) Though high in magnitude, the esker-depositing fl oods were not obviously random. Moving proximally to distally through the sys-tem, high-magnitude fl ood signatures in the esker become attenuated, and are eventually replaced by “on-off” signatures in the distal muddy rhythmites, the regularity of which is highly suggestive of a periodic, astronomi-cally forced process. Flooding is therefore suspected to have been modulated by the sea-sonal melt cycle.

(5) The Champlain Sea existed proglacially when the Vars-Winchester esker was being de-posited, not a glacial lake. The presence of Can-dona may suggest highly freshened conditions, but not necessarily marine-disconnected con-ditions. A glacial lake may have preceded the Champlain Sea in the basin, but only in areas south of the Vars-Winchester esker.

ACKNOWLEDGMENTS

This study was instigated by the Source Water Protection Act, a piece of legislation passed in 2004 that encouraged municipal governments in Ontario to establish science-based strategies to manage their drinking-water sources. Joint partners included the South Nation Conservation Authority, the Ontario

Geological Survey, and the Geological Survey of Canada. Don Cummings held a National Science and Engineering Research Council Visiting Fellowship during the research. Roger Hooke, Woody Thomp-son, and Tom Weddle are thanked for their hospitality and scientifi c openness during three trips to Maine, where eskers similar to the Vars-Winchester esker are well exposed in outcrop. Internal reviews by Isabelle McMartin and Serge Paradis at the Geological Sur-vey of Canada helped improve the manuscript prior to journal submission. The two anonymous reviewers are thanked for their particularly constructive critiques, as are Joni Mäkinen and Cathy Delaney for their reviews of an earlier version of the manuscript.

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