Glacial Sedimentary Environments and Deposits

 

Knik Glacier, AlaskaStudy of modern glaciers in places like Iceland, Greenland, Norway, and Alaska has revealed different settings in which glacial and near-glacial materials are formed. Geologists call these materials "sediments" and the settings in which they are deposited are referred to as "sedimentary environments". At and around glaciers are three broad sedimentary environments-beneath the glacier (subglacial), on top of or along the margin of the glacier (supraglacial/ice-marginal), and out in front of the glacier (proglacial). Sediments formed in these different environments have different characteristics (particle size and sorting) because of their mode of deposition.

The subglacial environment is the one most difficult to observe. Along the margins of some glaciers are places where one can crawl under the ice and see what is going on, but these places are few and far between. For the most part, scientists have to rely on ice cores and down-hole cameras to observe the subglacial environment. What we do know is that glaciers grind up and mix rock and soil debris in and beneath their base forming a mixture of material (rocks, sand, silt, and clay) that is called till when it is deposited.  Till is the most common subglacial deposit, but river and lake deposits also occur in channels and cavities beneath glaciers. When the glacier melts away lenses and pockets of water-sorted material are left within layers of till.

The supraglacial and ice-marginal environments can readily be observed along glacial margins. A dark, dirty-ice zone is not uncommon at a glacier's leading edge. Debris carried in the ice melts out and piles up on top of the thin ice at the glacier's edge. The supraglacial environment is a very unstable place because material deposited on top of ice is going to move when the ice melts. Rain and glacial meltwater wash some of this material off the glacier or deposit it in ponds on top of the glacier. Some of it sloughs off the ice front or collapses as buried ice melts away. The sediment that results once all the ice is gone is often a somewhat chaotic package. Till-like mixtures of material with a wide range of particle sizes, called "diamicton", are interspersed with waterlaid sediments from lakes and streams. Folds and faults commonly cut through the sediments and reflect a complex history of deposition.

Till of the Lemont Formation

The proglacial environment is even more dynamic than the subaglacial one. Here, glacial meltwater and summer rains carry debris away from the glacier or deposit it in lakes that come and go as the force of the water causes natural dams to give way and lakes to drain, sometimes catastrophically sweeping material away in the water. During winter when the proglacial environment dries out, glacial dust and sand storms carry fine particles (silt and sand) across the landscape. The sediments of the proglacial environment include materials sorted by water or wind, river sediment (called outwash), lake sediment, windblown sand, and windblown silt (called loess).

Because these sedimentary environments migrated across the landscape of Illinois as the glaciers came and went, their deposits are stacked up to form sequences of materials that record the glacial history of an area. For example, as a glacier advances across an area and later melts away, one might expect to find a glacial sequence from the base upward consisting successively of proglacial sediment, ice-marginal sediment, subglacial sediment, ice-marginal/supraglacial sediment, and proglacial sediment.

randomness