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Glacially eroded sediments are introduced into the marine depositional environment by rivers, ice rafting, ice-contact deposit, or rarely by eolian transport. Glacial-marine facies range from well-sorted unimodal stratified deposits to bimodal or multimodal chaotic or massive deposits.
Position of the glacier terminus relative to the shoreline is the most important facies-determining factor in glacial-marine sedimentation. If the terminus is landward of the shoreline, as in the Gulf of Alaska, glacial-marine sediment is fluvially transported, in places rafted, and infrequently blown into the marine depositional environment. Deposits are generally unimodal, well sorted, stratified, and lack a coarse fraction. These deposits form in proximal environments, including beach, delta, and nearshore, and in a medial rock-flour-rich environment on the open shelf. If a glacier has an iceberg calving terminus in a fiord, or is grounded on the continental shelf, ice-contact deposition, ice rafting, and fluvial or current transport are the dominant processes. Ice-contact and ice- afted deposits are poorly sorted, whereas fluvial and current deposits are well sorted and stratified. Ice-contact deposits accumulate where glacier ice contacts the continental shelf and slope. If the glacier terminus is floating, as is the Ross Ice Shelf of Antarctica, then all proximal glacial-marine facies and most medial facies on the open shelf are absent. The resulting deposit is a distal facies, typically ice-rafted in origin, which is almost everywhere bimodal or multimodal. The distal ice-rafted component has commonly been incorrectly identified as the only glacial-marine facies. Variations in ice-terminus position can cause rapid, radical changes in the resulting deposits. Depositional rates in proximal or medial deposits may range from 1 to 30 mm/year; short-term accumulation rates may exceed 4 m/year. Accumulation rates in distal ice-rafted deposits average less than 0.1 mm/year.
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