A Middle/Late Silurian paleomagnetic pole from limestone reefs of the Wabash Formation, Indiana, U.S.A

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Abstract
A Middle/Late Silurian paleomagnetic pole has been obtained from reefal limestones in north central Indiana. Eleven of 25 collected sites yielded well clustered characteristic directions after stepwise alternating field demagnetization. One additional site gave the same characteristic direction after great circle analysis. These 12 site‐level directions were used to calculate the mean paleomagnetic pole. Reef flank beds dip steeply (about 38°) away from reef centers. Field and petrographic studies demonstrate that part of this dip is a primary depositional dip and that part is due to a tilt caused by early postdepositional differential compaction of underlying mudstone beds. Present attitudes of geopetal surfaces indicate that the postdepositional component of the observed bedding dip is about 12°. A 12° tilt correction to paleomagnetic directions from oppositely dipping flank beds brings the mean directions into significantly better agreement, indicating that the magnetization was acquired early. The presence of both normal and reversed polarities also suggests early acquisition of the characteristic remanence. The mean of the site paleomagnetic poles (with sites from steeply inclined flank beds corrected for 12° of tilt) is 16.8°N, 124.9°E (K = 74, A95 = 5.1°), in good agreement with the Middle Silurian Rose Hill pole for North America.