ANALYSIS OF SMALL-SCALE MELTWATER AND FLUVIAL EROSIONAL FORMS IN BEDROCK
IN WEST GREENLAND
James A., Environmental Earth Science, Eastern Connecticut State Univ,
83 Windham Street, Willimantic, CT 06226, firstname.lastname@example.org 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.
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.