| dc.description.abstract | The structurally disturbed area near Kentland, Indiana is a typical example of a so-called 'cryptoexplosion' structure which can be attributed to a large natural impact event. The structure includes a central uplifted area approximately 4 km in diameter, the most intense deformation being confined to a core slightly greater than 1 km in diameter. Stratigraphic uplift in the core region is probably greater than 600 m. The central uplifted area is surrounded by a ring depression 1.5 to 2.0 km wide, situated 3.2 km from the center of the uplifted area, and within which are preserved soft Pennsylvanian coals. At a radius of approximately 6.2 km from the center of the site, a subdued structural high is present and is believed to be the limit of the disturbance. Although most of the site is covered with a thin veneer of glacial till, a large quarry (0.4 km2 in area, 70-80 m deep) located on the northern flank of the central uplift exposes in detail some of the complexities which occur as a result of structural modifications of an original transient cavity. Quarry exposures are dominated by a large NNW plunging synclinal fold and numerous high angle normal and reverse faults. Megabreccia zones containing blocks 100 m or more long bound this synclinal fold on three sides. These zones are structurally very complex and characteristically seem to involve the more mobile sandstone and shale formations along with the competent carbonates. Recent drilling indicates that this pattern of large carbonate blocks and megabreccia zones also characterizes the structural pattern of the remaining covered portions surrounding the core of the central uplift.Shock deformational features produced by the impact event, including monomict and polymict breccias and shatter cones, are exposed in the quarry. Petrographic studies of quartz grains from the breccias and other sandy units show that brittle failure was common with irregular fractures, microfaulting, and cleavage the dominant features in almost all grains. Less than one percent of the grains studied contained basal deformation lamellae; multiple sets of other planar features in quartz grains have not been observed. These results suggest shock pressures in the range of 50 kb for the material in the quarry. Information gathered in this study indicates that the probable mechanism producing central uplift formation in impact craters in sedimentary rock is some type of rebound of the impact-compressed material. Most terrestrial impact structures in sedimentary rock are in various states of degradation, and information on the structural details at deeper levels in these impact sites is generally lacking. Nonetheless, comparison of similar-sized impact sites indicates that various factors, including target cha'acteristtics, projectile characteristics, and basement depth, could play important roles in determining final crater morphology. | en_US |
