Seismic expression of depositional sequences associated with expansion and contraction of ice sheets on the northwestern Antarctic Peninsula continental shelf
Bart, P.J.; Anderson, J.B. (1996). Seismic expression of depositional sequences associated with expansion and contraction of ice sheets on the northwestern Antarctic Peninsula continental shelf, in: De Batist, M. et al. (Ed.) Geology of siliciclastic shelf seas. pp. 171-186 In: De Batist, M.; Jacobs, P. (Ed.) (1996). Geology of siliciclastic shelf seas. Geological Society Special Publication, 117. The Geological Society (London): London. ISBN 1-897799-67-5. 345 pp., more In: Hartley, A.J. et al. (Ed.) Geological Society Special Publication. Geological Society of London: Oxford; London; Edinburgh; Boston, Mass.; Carlton, Vic.. ISSN 0305-8719; e-ISSN 2041-4927, more |
| Abstract | We present results from seismic stratigraphical analysis of a regional grid of intermediate-resolution seismic profiles from the Antarctic Peninsula continental shelf that reveal greater complexity in the stratigraphical record than has been previously recognized using lower-resolution seismic records. Widespread unconformities and prograding stratal patterns are interpreted as subglacial erosional surfaces and proglacial depositional systems respectively. Two types of units are recognized, aggrading-shelf units and prograding-slope units. Aggrading-shelfunits display minimal slope progradation (less than 2 to 3 km) beyond the pre-existing shelf edge. Units of this type are composed of either acousticany layered prograding foresets or acousticany chaotic seismic facies. Unit thicknesses vary along the shelf from less than 20ms two-way travel time to more than 200 ms. Low-angle foresets downlap and partially fill palaeotroughs that were formed during prior glacial advances. Foresets are interpreted as proglacial depositional surfaces. The zones of active proglacial deposition were lobate in shape and ranged in width from a few kilometres to more than 30 km. Toplap truncations of foresets result from ice-sheet expansion over proglacial deposits. Glacial erosion on the shelf was concentrated along the axes of ice streams to form deep troughs. Slope progradation occurred mainly near the mouths of these troughs. Prograding- slope units exhibit local slope progradation in excess of 2 to 3 km beyond the location of the pre-existing shelf edge. Shifts in the position of troughs indicate lateral shifts in the positions of ice streams. Hence, sedimentation on the outer shelf and upper slope is point-sourced, not line-sourced, and individual units change from progradational to aggradational along the margin. It is not possible to interpret the ice-sheet grounding history of the shelf using changes in stratal stacking pattems without constraining these lateral changes. However, detailed mapping of prograding-slope units does reveal regional trends that imply glacial periods when greater quantities of sediment were eroded from the shelf and deposited on the slope. We interpret prograding-slope units as having been formed during relatively long-duration glacial expansions. Ice-sheet retreat from the shelf probably occurred rapidly, as indicated by a lack of backstepping proglacial deposits. |
|
