Subarctic Glacial-Marine Sedimentation: A Model

Abstract

The classic concept of glacial sedimentation in the marine environment is based on Antarctic models and most commonly recognizes only the presence of coarse-grained particles in a muddy matrix (diamicton) as evidence of glacial-marine sedimentation. Studies in Alaska adjacent to active glaciers and recently deglaciated areas suggest that this classic concept encompasses only a small fraction of the total volume and types of glacial deposits in the marine environment. Consequently, the category of what many geologists recognize as evidence of glacial-marine sedimentation may be biased and incomplete, and excludes facies that may, by volume, represent the majority of glacially derived material deposited in the marine environment.

A search of the geological literature shows that a lack of consistency exists in the terminology and definitions used to describe the process of the introduction of glacial material to the marine environment. This paper describes the glacial-marine depositional picture that exists today in the northeastern Pacific Ocean (Gulf of Alaska and its fjords) and, then uses this picture as the basis for the development of a model and definition of glacial-marine sedimentation in a subarctic environment.

Today, almost one-third of the area draining into the Pacific Ocean’s northeastern Gulf of Alaska is covered by glaciers. This includes many fjord-locked tidewater glaciers which actively generate icebergs. Many non-tidewater glaciers discharge their sediment into lakes or streams, and in turn, much of this sediment is transported into the marine environment. Almost all of the glacier-draining streams have suspended sediment loads >1 g/l, with some >10 g/l.

The sediment facies distribution pattern in the offshore area of the Gulf of Alaska and its fjords is as follows: adjacent to tidewater glaciers are diverse areas characterized by diamicton, sand bodies, and lenses of fine sediment. Where the deposits are not massive, fine-grained thinly laminated sand and silt containing minor ice-rafted debris are common. Some sedimentary units appear to have formed in contact with the ice or from density flows. Seaward of the fjords and coastal glaciers, a progression of nearshore sand, grading into clayey silt or silty clay, exists. Ice-rafted pebbles are occasionally present in these units, but only in minor quantities. Bottom currents may locally concentrate the coarser material into lag deposits. During periods of high iceberg production, pebble and cobble transportation and input increases, but only a reduction in fine-grained sediment input leads to marine diamicton conditions; conditions where the pebble component exceeds trace quantities. Most fine-grained (rock flour) sediment is deposited on the mid-to-outer shelf and slope, and some by-passes the continental margin completely. Ice-rafted sediment shows up conspicuously in Pleistocene deposits collected many hundreds to thousands of kilometers beyond the shoreline, but is essentially absent in continental shelf surficial sediment due to an almost complete absence of present day icebergs reaching the open ocean.