March 1, 2020 at 5:30 pm

CANCELED | Geology Colloquium | How Meteorite Impact Driven Dolomitization Hid Evidence for Jeptha Knob’s Origin, March 27

an outdoor snapshot of Andrew Schedl in the woods

Andrew Schedl

Note: This event has been canceled.

The Geological Sciences Colloquium series presents Dr. Andrew Schedl discussing “The Jeptha Knob Structure: How meteorite impact driven dolomitization hid the evidence for Jeptha Knob’s origin” on Friday, March 27, at 2 p.m. in Clippinger 205.

Schedl is an Assistant Professor at West Virginia State University in the Department of Physics.

Abstract: Jeptha Knob is a 4.5 kilometer deformed structure composed entirely of carbonate rocks lying in the stable craton. Determining whether Jeptha Knob is of impact origin is problematic because of the absence of shock metamorphism, e. g. shatter cones.  In this study, calcite-twin analysis is used to test the impact hypothesis, and it provides the following, estimates of differential stress, incremental strain and temperatures at the time of twinning.  Calcite twinning shows that at depths of 200 to 400 m below the surface at the time of impact, differential stresses were 170 to 570 MPa, too high to be explained by tectonic forces.  Likewise, there is a differential stress anomaly, ≥60 MPa, which is roughly circular and approximately the dimensions of the Jeptha Knob structure.  Calcite strain data show two shortening events, ε3, that are almost perpendicular to one another, 86.2o.  The magnitude of these events is shown by the average twin intensities, 151.3 (twins/mm) [>1.5 X the highest recorded tectonic twin intensity] and 92.7 (twins/mm).  The highest intensity event probably records passage of a shock wave through the rock and the low intensity event records passage of a release/rarefaction wave through the rock.  Within Jeptha Knob are resurge deposits and some resurge clasts have thick twinned calcite (5.2±2.5 μm).  These mechanical twins indicate temperatures >170oC, which is best explained by meteorite impact.  The presence of resurge deposits indicates Jeptha Knob is a marine impact structure.

Here, meteorite impact created a hydrothermal system where seawater transformed a volume >2.0 km diameter and almost 1.0 km deep, from limestone into dolomite.  Water/rock ratios were >1.0 based on fluid inclusions and stable isotopes, so seawater was the source of magnesium.  We further argue that erosion, dolomitization and other mineralization removed evidence for meteorite impact, impact melt, shocked minerals and shatter cones, at Jeptha Knob.

Upcoming Colloquia

Katherine Fornash of Ohio University Geological Sciences on “Petrogenesis and Significance of Lawsonite-bearing Hybrid Rocks” on April 3 at 2 p.m. in Clippinger 205.

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