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Sedimentology of late Pleistocene (Laurentide) deglacial-phase deposits, eastern Maine; An example of a temperate marine grounded ice-sheet margin

By
Gail M. Ashley
Gail M. Ashley
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Jon C. Boothroyd
Jon C. Boothroyd
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Harold W. Borns, Jr.
Harold W. Borns, Jr.
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Published:
January 01, 1991

The major deglacial-phase deposits of eastern Maine include stratified end moraines, single and amalgamated ice-margin delta complexes, extensive segmented esker systems that link with marine deltas and fans, and a fossiliferous glacial marine mud (the Presumpscot Formation) that is interbedded with, or drapes, the other deposits. Previous work has determined that deposition within the 40-km-wide swath, which is now the modern coastal zone, occurred between 13,500 and 12,500 B.P.

Detailed mapping, measurement of stratigraphic sections and paleocurrents, and calculation of drainage basin area and volume of sediment bodies have led to a better understanding of lithofacies assemblages, depositional processes, sediment sources, and regional geometry of these deposits.

The “stratified end moraines,” better described as assemblages of marine fans, measure as much as 15 m high, 100 m wide, and 16 km in length, and were deposited along ice lobe fronts. Proximal vertical sequences are dominated by 15- to 30-cm-thick beds of poorly sorted, pebble-to-cobble gravel locally interbedded with gravelly diamict. Well-sorted, matrix-less (“open-work”) pebble/cobble gravel occurs in channel forms that can be traced tens of meters laterally. Distal sequences consist of laminated beds of medium-to-fine sand capped by silt drapes; climbing ripple cross-stratification is uncommon, and some diamict beds are present. Lithofacies of the delta front, prodelta, and esker core (ice-tunnel deposits) are remarkably similar to those of the fans, differing only in greater degree of dip (deltas), and clast size (ice-tunnel deposits). Paleocurrents of fans show a large dispersion, in some instances indicating flow parallel to the inferred ice margin; proximal beds are deformed, indicating an active ice margin.

The ice-margin fan and delta complexes have a combined area of 250 km2 and total volume of nearly 5 billion m3, which strongly suggests that the 2,400-km-long ice-tunnel system (including tributaries) was active along its entire length in order to supply needed sediment. Ice-tunnel deposits filled the distal ends of tunnels as the active ice margin retreated.

The geometry and volume of deltas, fans, and ice-tunnel deposits, together with the time constraints, indicate deposition into marine waters at the margin of a complex, multi-lobate, grounded, warm-based ice sheet.

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GSA Special Papers

Glacial marine sedimentation; Paleoclimatic significance

John B. Anderson
John B. Anderson
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Gail M. Ashley
Gail M. Ashley
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Geological Society of America
Volume
261
ISBN print:
9780813722610
Publication date:
January 01, 1991

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