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EARLES,
Jennifer E., Geology and Physics, Univ of
Southern Indiana, 8600 University Blvd, Evansville, IN 47712,
jeearles@usieagles.org and MARIA, Anton H., Geology and Physics, Univ Southern Indiana, 8600 University Blvd, Evansville,
IN 47712
While the origin of the 1.85 Ga old Sudbury
Structure, Ontario, Canada,
was once controversial, evidence supporting an impact by an asteroid or comet
has grown since the discovery of shatter cones and shock metamorphic features
some 40 years ago. It is now widely accepted that the Sudbury Igneous Complex
represents an impact melt (wholly or partially) and that the overlying Onaping Formation represents fallback-flowback
breccias associated with the impact. However, our understanding of impact
processes is still limited. From the perspective that the shapes of melt
particles reflect the processes that form them, this study focuses on the
morphology of once-glassy (now completely recrystallized)
clasts still preserved within the Onaping Fm. The Onaping Fm. is
composed of four members (Basal, Gray, Green, and Black), all of which are
glass-rich breccias (suevite) and include clasts with shock metamorphic features. Here we document
the morphology of vitric clasts
from samples collected within the Black member, near Onaping Falls
off of Highway 144. The clasts are supported by a
carbon-rich, fine grained matrix, typically range in size from 0.1 to 10 mm,
and exhibit a variety of textures including fluid-form features, flow
banding, round vesicles, and stretched vesicles. The clasts
show no signs of rounding, and blocky, angular, and splintery shapes suggest
some form of quench fragmentation. Because of their morphological similarity
to volcanic glass shards, comparison with volcanic clasts
produced by well-documented styles of eruption can offer insight into melt
characteristics, and processes of fragmentation and transport during the
impact. At this point, the comparison is only qualitative. However,
additional samples will allow us to make quantitative comparisons in the
future using image-analysis techniques based on fractal geometry. Such a
study may be especially useful if we can obtain samples from similar suevites found at other large impact sites such as the Vredefort structure in South
Africa and the Ries
structure in Germany.
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