| Popis: |
Seismostratigraphic studies on the Scotian and mid-Norwegian Continental Shelves have led to the development of a conceptual model for glacial deposition and erosion by ice sheets in a marine environment (till-tongue model). Some of the terminal deposits of the marine-ending glaciers are composed in part of acoustically incoherent, wedge-shaped bodies. The wedges are thought to be part of a grounding-line system associated with a floating terminus. According to the model, the sedimentary wedges are formed through the accumulation of glacial debris by subglacial processes proximal to the grounding line, together with a penecontemporaneous, proglacial contribution from sediment gravity flows. Sedimentation rates appear to be highest near and proximal to the grounding line; that is, within a grounding-line zone. Ice-proximal, stratified glaciomarine sediment accumulates from plumes within the water column and rain out of ice-rafted debris. The glaciomarine sediment interfingers with the till tongues, resulting from oscillations of the grounding line. Till tongues represent discrete stratigraphic units formed in both vertical and lateral successions, marginal to massive till deposits. Major till tongues can be correlated regionally for hundreds of kilometers and thus can provide a basis for stratigraphic correlation. On the mid-Norwegian Shelf, lateral and vertical successions of till tongues delineate several large-scale till units which represent major advances and retreats of the ice that probably were of both stadial and glacial rank. We refer to this type of succession as a "till-tongue stratigraphy." It appears that till-tongue formation is limited to intermediate water depths which range from 75 to 1,000 m (present water depth), but those formed in water depths >600-700 in may not be strictly subglacial in origin. We speculate that processes leading to the development of till tongues possibly give rise to the formation of subglacial undermelt till, together with thin flow tills near the grounding line. It is also possible that another type of subglacial till, embracing concepts inherent in the expression "deforming basal tills" as described by Alley and others (1989), may contribute to the formation of till tongues. Boundaries between the major till units of the mid-Norwegian Shelf are mainly depositional in the zone of maximum glaciation near the shelf edge and upper slope. Proximal to this zone of major deposition, intraformational erosional processes dominate and give rise to smooth, widespread, time-transgressive, erosional surfaces. Erosion progressively increases in a proximal direction, beveling the earlier glacial deposits, and ultimately reaches and modifies the bedrock surface. These regional surfaces are analogous to the Ross Sea unconformity which was also formed by glacial erosion. Morainic deposits within a till-tongue stratigraphy are termed "grounding-zone moraines" and are subdivided into advance and retreat varieties, with linear, tabular, and hummocky attributes. Fields of small morainic ridges (lift-off moraines) form in basinal areas during recession. |
