An “Inverse Conglomerate” Paleomagnetic Test and Timing of In Situ Terra Rossa Formation at Bloomington, Indiana

Abstract

Directions of remanent magnetism in an ∼2-m interval of terra rossa claystone at Bloomington, Indiana, cluster around the current Earth magnetic field direction, a fact consistent with authigenic formation of the terra rossa over the last 0.78 m.yr., the date of the latest earth’s magnetic field reversal, and suggesting a minimum terra rossa formation rate of ∼2: 5m =m:yr: Paleomagnetic directions in limestone “floaters” within the terra rossa are indistinguishable from directions of the bedrock Mississippian Harrodsburg Limestone, indicating that the floaters are in situ and perfectly preserve their original stratigraphic position. Thus, the red claystone containing the floaters is interpreted as an authigenic, isovolumetric replacement of the Harrodsburg—the replacement of which the floaters escaped—and precludes the possibility that the floaters and the surrounding claystone are a mudflow that had carried limestone blocks in it. The implication that the claystone is authigenic independently confirms the petrographic observations reported previously that the red clay grew in situ, replacing the underlying limestone at a moving metasomatic front. Rock magnetic studies of the terra rossa clays show evidence of both goethite and maghemite as main magnetic carriers of this remanence. Both minerals are independently known to occur in pisolites of the terra rossa. The ferric-ironbearing red clays also are weakly magnetic.