ORIGIN OF SEPTARIAN CONCRETIONS IN THE HURON MEMBER OF THE OHIO SHALE (DEVONIAN), DELAWARE COUNTY, OHIO
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Date
2019-05
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The Ohio State University
Abstract
One of the lesser understood features of concretions is a variety of heavily fractured concretions, named septarian concretions. By taking a new approach in the study of septarian fracture patterns, it is possible to connect the episode of fracturing to a wider tectonic event and better understand how and when they fractured. In March 2019, such a study was performed on the septarian concretions of Shale Hollow Park in Columbus, Ohio. The pattern of their fractures as well as observations made on the surrounding shale unit suggest the presence of two fracturing episodes. One episode appears in the concretions but fractures in the surrounding shale lack the pattern, suggesting the first episode of fracturing occurred after concretion formation but before the lithification of the surrounding shale. If the concretion was formed and brittle enough for tensile failure while the surrounding shale was not, this has implications for the time required to form a concretion. Contrary to the traditional view on concretion formation, this suggests that concretions formed synsedimentary. The second episode of fracturing is reflected in the joints of the surrounding shale and fractures that only pierced the outer ring of the concretions. This suggests a possible compositional similarity between the shale and the outer ring of the concretions. These conclusions could be tested by similar studies of septarian concretions in the corresponding Upper Devonian black shales in other localities. A geochemical analysis of the outer rings could prove insightful too.
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Keywords
Ohio Shale, septarian concretion, carbonate concretion, Acadian orogeny, geomechanics