Knowledge Bank

The Upper Ordovician Miamitown Shale exposed at the Crosby Road outcrop did not show an Upper Ordovician paleomagnetic pole position or shallow inclinations. Instead, Upper Tertiary magnetic data was observed. Thin section analysis shows large amounts of secondary limonite and hematite which have masked the primary Detrital Remanent Magnetization with an Upper Tertiary Chemical Remanent Magnetization. Alternating field cleaning techniques could not remove this component because the magnetization of hematite has a coercive field above 30,000 gauss, while the strongest alternating field used in this study was 600 gauss. Thermal cleaning techniques also failed because at 300°c the limonite converted to hematite and some of the pyrite oxidized to hematite creating a larger secondary magnetic component than was initially present. Saturation magnetization curves and thin section analysis prove that this transformation did occur. Because the Upper Tertiary secondary magnetization masks the other magnetic components, the Silurian secondary magnetization reported by Rowan (if it is indeed present in these rocks), has also been masked. Therefore, no conclusions can be reached regarding the validity of his results. However, there is no reason to doubt Rowan's conclusion that the Upper Ordovician strata in the Cincinnati area have had a long history of uplift along the Cincinnati Arch and have had many occasions to be infiltrated with iron oxide precipitating groundwater solutions. A polarity reversal within the Upper Tertiary secondary component of magnetization reveals that the incident causing this Chemical Remanent Magnetization in these rocks must have occurred at least 114,000 years ago during the Blake Event. Why the strata at Crosby Road was affected so recently, while Rowan's strata to the south was not, is not known. Further surface sampling studies should be attempted upon strata that has had persistent sedimentary cover in the past and has had a reduced chance of being infiltrated by groundwater.