10.6: Glacial Environments
Glacial Influence
Glacial environments do not form a single, coherent depositional system but instead interact with and overprint existing systems such as fluvial, lacustrine, and eolian environments. While glacially-overprinted environments closely resemble warmer-climate versions of those same environments, diagnosis of glacial influence depends on recognizing the diagnostic features described below.
Glacial Indicators
One of the more widespread glacially influenced deposits is loess , composed of wind-blown silt. These fine-grained deposits accumulate downwind of glaciated regions, often forming extensive blankets of sediment. Once lithified, loess appears as a nondescript siltstone, making it difficult to distinguish from other silt-rich deposits unless it is found in association with other glacial indicators.
A more direct glacial deposit is glacial till , which lithifies into diamictite over time. This deposit consists of a poorly sorted mixture of mud, sand, and gravel, accumulating beneath or in front of a glacier. The lack of sorting and the mix of clast sizes distinguish it from other deposits, but care must be taken in interpretation, as similar textures can result from debris flows. A key diagnostic feature of glacial till is the presence of striated , faceted , or bullet-shaped clasts , which form due to abrasion within moving glacial ice, incorporation of faceted ventifacts, and dynamics within glacial ice, respectively.
Varves are annual couplets of sand and mud, recording alternating high-energy summer deposits and finer winter deposits when the lake is ice-covered. While varves provide a clear record of cold climate, similar rhythmic layering can occur in non-glacial settings such as tidal flats or fluvial systems with pulses of floodwater.
Ice-rafted debris serves as an additional indicator of glacial, or at least freezing conditions. As icebergs calve from sediment-laden glaciers, they can drift into open water and release debris as they melt. This process results in outsized clasts that are encased within fine-grained sediments. Mudrocks formed in cold climates might also contain glendonites , which are pseudomorphs of the mineral ikaite which forms only in near freezing conditions.