November 4, 2013 at 4:09 pm

Milam Presents on Jeptha Knob and Lunar Pit Craters

Ohio University faculty made an impact with 23 presentations at the Geological Society of America’s 125th Anniversary Annual Meeting & Exposition Oct. 27–30 in Denver.

Michael Fox & Dr. Keith Milam presented on AN ASSESSMENT OF SHOCK METAMORPHISM IN THE JEPTHA KNOB STRUCTURE:  A SUSPECTED COMPLEX CRATER IN NORTH-CENTRAL KENTUCKY. Milam is Associate Professor of Geological Sciences in the College of Arts & Sciences.

Impact craters in carbonate rock account for ~30% of all impacts on Earth yet little research has been conducted to study the effects of shock metamorphism on such targets. Unlike quartz, shock metamorphism in calcite does not produce planar deformation features but results in an increase in the density of mechanical twins, a feature that also results from non-impact related processes. However, under low to moderate shock pressures, calcite develops mechanical twins while dislocations dominate at higher pressures. With increasing shock pressures, x-ray diffraction peaks in calcite and dolomite broaden and peak intensities decrease as their crystal structures are affected by the shock pressures. Petrographic observations and XRD analyses of carbonate minerals have the potential to identify shock metamorphism in suspected structures and to constrain peak pressures in confirmed terrestrial impact sites all over the world such as the proposed impact site in north-central Kentucky, Jeptha Knob.

The Jeptha Knob structure is an approximately 4.26 km diameter circular structure situated just east of Shelbyville, Kentucky. Although Jeptha Knob has been proposed to represent the eroded remains of an impact crater, there is insufficient evidence to support this. This study examines the carbonate rocks at the Jeptha Knob structure for evidence of shock metamorphism and, if shock metamorphosed, to determine peak shock pressures.

A total of 65 samples were collected from two different drill cores, one from the deformed and uplifted interior of the structure (JK78-3) and another from an analog (non-deformed) section 21 km away (C-204). Analysis of the JK78-3 core, hand samples, and thin sections has shown that there is ~500ft of vertical displacement between of the same geologic units in each of the two cores. Carbonates in JK78-3 have been extensively fractured, faulted, and brecciated in a fashion similar to target rocks exposed in the central peaks of confirmed complex impact craters; however shatter cones were not identified. Ongoing XRD analysis of diffraction powders of analogous geologic units from both cores is being conducted in an effort to identify shocked dolomite or calcite in Jeptha Knob structure.

Peter Malinski & Keith Milam presented on LUNAR PIT CRATERS: A POPULATION STUDY.

There exists a type of impact crater called pit craters and debate still revolves around the formational habits of these features on different planetary bodies. Pit craters are defined as an impact crater with a divot (pit) in the central portion of the feature. Populations of these pitted craters have been observed predominantly on Ganymede and Mars; bodies that are considered to have a high concentration of volatiles. It has been suggested that there may be an intrinsic relation between the formation of pit craters and volatiles; however pit craters have been poorly modeled on volatile depleted bodies such as the Moon. This study located and observed pit crater populations on the Moon in an attempt to bridge the information gap between dry bodies and pit craters.

Populations of pit craters were identified on the relatively volatile poor Moon and compared to the total population set. The population study ranged from 20 – 60 km, where the lower end limit represents the transition from simple to complex craters and the higher end limit avoids complications from multi-ringed basins and peak ring structures. In addition to the population comparison, this study characterized the pit crater populations across the lunar surface in both maria and highlands to identify any relation to the lunar terrains. The main tool used to model the pit crater populations was the Lunar Orbital Laser Altimeter (LOLA) data set in conjunction with some visual data from the Lunar Reconnaissance Orbiter (LRO) and Apollo missions. Ultimately, the results reported here will compare and contrast the lunar pit crater population to those of well-studied bodies such as Ganymede and Mars.


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