Seismogenic Deformation Structures in Microbialites and Mudstones, Silurian Lockport Dolomite, Northwestern Ohio, U.S.A.

Abstract

Syndepositional deformation structures are present at two different stratigraphic levels in the Silurian Lockport Dolomite at a quarry in Maumee, Ohio. Such structures are absent in this unit at twelve other quarries in northwestern Ohio. The structures at Maumee, present within stromatolites, thrombolites, and variably bedded mudstones, include convolute layers, homogenization of bedding and breccia clasts, clasts of stromatolites, boudinage structures, normal, reverse, and thrust faults, and discordant and concordant breccias. The distribution of deformation structures within beds suggests substantial control from substrate rheology, possibly resulting from differences in degrees of lithification and from variations in water content. The extensive variety of syndepositional deformation structures, juxtaposed evidence for nearly synchronous compression and dilation, proximity (4.5 km) of the structures to the largest fault in northwestern Ohio (the Bowling Green fault), and the restricted geographic distribution of the deformation structures suggest seismic shaking as a probable mode of genesis. Similarities between features produced by seismogenic deformation of siliciclastic sediments and deformation features in the Lockport Dolomite provide additional evidence that the latter features are seismogenic. It is postulated that two earthquakes, one with a magnitude 5, caused the formation of the syndepositional deformation structures present in the youngest facies in the Lockport Dolomite at Maumee quarry. The Bowling Green fault, 4.5 km west of Maumee, was a growth fault during formation of the Lockport Dolomite, and is considered to have been the source of earthquakes that created the syndepositonal deformation structures in this unit at Maumee quarry. Evidence from the Lockport Dolomite in northwestern Ohio suggests that seismogenic deformation structures created by two earthquakes along the Bowling Green fault with magnitudes of 5 did not form in this unit over large areas (i.e., all of northwestern Ohio). This evidence challenges the suggestion that syndepositional seismogenic deformation structures are always formed over a large area.