Erosion In Bedrock

Erosion Forms

Fluvial erosion forms in bedrock created by meltwater flow from the Greenland Ice Cap at Kangerlussuaq, West Greenland
Gilbert R, Geography, Queen's University, Canada, gilbert@lake.geog.queensu.ca)
Hyatt JA, Environmental Earth Science, Eastern CT. USA, hyattj@easternct.edu)

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SPATIAL ANALYSIS OF SMALL-SCALE MELTWATER AND FLUVIAL EROSIONAL FORMS IN BEDROCK IN WEST GREENLAND
HYATT, James A., Environmental Earth Science, Eastern Connecticut State Univ, 83 Windham Street, Willimantic, CT 06226, hyattj@easternct.edu and GILBERT, Robert, Geography, Queen's University, Kingston, ON K7L 3N6, Canada
Abstract, 2006 Northeastern Sectional Meeting of the Geological Society of America - Harrisburg, PA.

Intense subglacial or proglacial discharge events may erode intricate forms in bedrock. These forms are often complex due to feedback with flow structure, interactions between adjacent forms, and the modification of relic forms by subsequent events. Thus, quantifying form is difficult, although new classifications provide a framework for developing form indexes and evaluating relationships between form and process controls (e.g. flow depths, bedrock properties, etc).

This study presents preliminary analysis of erosional surfaces on a bedrock sill near the mouth of Watson River, West Greenland. The site experienced subglacial meltwater discharge until about 7000 BP, and proglacial fluvial erosion since about 5000 BP, following isostatic uplift of the sill from the sea. Presently, normal fluvial discharge is confined to two artificially enlarged channels. However, jökulhlaups up to 1200 m3 s-1 from the nearby Greenland Ice Cap, which occupies >90% of the 6280 km2 drainage basin, periodically inundate portions of the sill.

Our analysis is based on detailed 2 m grid topographic surveys, rectified low-level (4 m) image maps with cm-scale resolution, and digitized form boundaries at three hydrodynamically distinct sites. Forms on the up-flow side of the sill are the most complex, including potholes (to >2 m deep), and elongate forms with both sharp and rounded edges. Conjugate forms have orientations that are clearly influenced both by flow and by structural elements in the underlying gneissic bedrock. On a site located on lateral benches that rise down-flow to the crest of the still, forms are preferentially convex to undulating but change with elevation. Complex hierarchical concave forms are found on the down-flow side of the sill in a zone of high-stage flow convergence. They exhibit en echelon relationships that change with elevation.

Analysis of form boundaries using medical imaging software generates quantitative measures that can be related to ideal forms defined in newly published classification schemes. The spatial distribution of these form indices can be compared with prevailing flow conditions (using elevation as a proxy for stage), and bedrock characteristics. Our goal is to distinguish present-day fluvial forms from those inherited from previous subglacial meltwater erosion.



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