MUD: THE FORGOTTEN COMPONENT OF ESKER SYSTEMS

Mud is rarely reported in eskers. However, several observations suggest it should in fact be the dominant grain size in many (most?) esker systems sensu lato. For example, mud constitutes between ½ and ¾ of the total load in modern glacial streams, and is abundant in till and basal ice, from which most esker sediment is thought to be derived. So where does all the mud go? Answering this question is not only key to understanding how sediment becomes partitioned in glacial systems, but is also prerequisite to developing models used to predict esker-aquifer properties locally (facies models) and regionally (sequence stratigraphy).

Using an integrated dataset (seismic, cores, well logs, outcrops), the Geological Survey of Canada is studying the Vars–Winchester esker, a major aquifer near Ottawa, Canada. From bottom to top, four units—bedrock, till, the esker and Champlain Sea mud—are observed in cores. The esker consists of two elements, a gravely central ridge interpreted to be an R-channel deposit, and a sandy-fan carapace with rare marine shells interpreted to be subaqueous outwash. Champlain Sea mud that overlies the esker fines up from muddy rhythmites to massive bioturbated mud, then becomes stratified and coarsens upward. The rhythmites and massive mud are interpreted to be distal subaqueous-outwash, and the upward-fining succession that starts with esker gravels and ends with bioturbated muds is interpreted to have formed as the ice-margin back-stepped through the Champlain Sea.

The absence of mud in the Vars–Winchester esker sensu stricto and its abundance in overlying outwash highlights two aspects believed unique to esker systems. First, because they lack floodplain-like sediment storage sites, R-channels should be very efficient at transferring mud to the basin—possibly up to 10 times more so than fluvial systems. This may in part explain the paucity of mud in esker aquifers. Second, unlike non-glacial basins, esker-fed basins may receive a “mud pulse” during deglaciation due to melting of debris-rich basal ice. If conditions are right, eskers should therefore “self-seal” with a layer of mud. Absence of mud where eskers are present (e.g., Keewatin) needs to be accounted for: mud may not have been abundant in the source material, was carried away by proglacial streams, or bypassed the area through long R-channels.