CHAMPLAIN SEA DELTAS IN SOUTHERN QUéBEC (CANADA): LINKING DEPOSITIONAL PROCESSES, STRATIGRAPHIC ARCHITECTURE AND GROUNDWATER RESOURCES

During deglaciation, an arm of the Atlantic Ocean invaded the isostatically depressed St. Lawrence valley up to an elevation of about 200 m, an incursion known as the Champlain Sea. While previous research has extensively dealt with the paleogeography, chronology and paleoeocology of the Champlain Sea, little research has addressed the issue of deltaic sedimentation patterns and their relationships with regional groundwater resources. As glacial isostatic unloading proceeded at a faster rate than eustatic sea level rise, early to middle Champlain Sea deltas consist of relatively simple offlap successions where deltaic lobes prograded on marine muds as relative sea level fell. Prominent, river-dominated sandy deltas were deposited at the mouth of meltwater-fed rivers and streams entering on the north shore of the deglacial marine basin while the wave-dominated deltas deposited on south shore of the marine basin were considerably smaller and thinner. The north shore deltas predominantly consist of a regressive succession of thick (> 30 m) deltaic lobes, consisting mainly of cross-stratified medium to coarse sand bodies and interpreted as mouth bar sediments. These generally unconfined deltaic aquifers are locally split by wedge-shaped layers of prodeltaic coarse silt deposited in adjacent low-energy settings and in relatively shallow embayments bordering the deltas. Such occurrences result in locally perched aquifer conditions. Because of the high rate of sediment supply, avulsion of deltaic lobes occurred exclusively in north shore deltas.

Wave-dominated deltas of the south shore are relatively subdued features where deltaic sediments supplied by normal fluvial systems were redistributed by longshore drift driven by strong catabatic winds blowing southwestward along the southern edge of the Laurentide Ice Sheet. Because of their moderate to low sediment supply rate, these deltas did not aggrade up to marine limit, hence their shallow thickness limits their usefulness as regional aquifer. Lastly, when eustatic sea level rise temporarily exceeded isostatic recovery around 6.5 ky (Laurentian transgression), estuarine muds deposited in the St. Lawrence macrotidal environment covered late Champlain deltaic sediments up to about 15 m ASL, thus concealing a large 30 m thick aquifer in an urban setting.