Paleomagnetic and petrographic investigation of the Taum Sauk Limestone, southeast Missouri

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Abstract
Paleomagnetic results from 17 sites in one quarry of the Cambrian (Dresbachian Stage) Taum Sauk Limestone (member Bonneterre Formation) reveal two components of magnetization. On the basis of thermal demagnetization, coercivity spectrum analysis, and petrographic evidence, both components reside in hematite. Sites can be divided into three groups based on behavior during thermal demagnetization and petrographic criteria. The first group consists of massive pink limestones and contains a component with declination 270°, inclination −11°, corresponding to a Cambrian pole at 175.6°E, 3.6°S. The second group of sampling sites consists of specimens with numerous burrows filled with hematite intergrown with calcite. The sites are dominated by the second component of magnetization which has declination 147° and inclination 10° and corresponds to a reversed pole position at 133.1°E, 37.3°N. A significant fraction of this component, which is interpreted to be Carboniferous in age, resides in hematite intergrown with calcite in the burrows. Petrographic relationships indicate that the calcite and hematite were acquired before dolomite, which places a maximum age on dolomitization. The third group consists of massive pink limestones with variable amounts of burrowing and contains both directions of magnetization in roughly equal proportions, with closely overlapping blocking temperature spectra. The presence of two stable directions of magnetization from one location with neither being dominant or even present throughout the quarry indicates the significant effect that diagenetic and sedimentologic processes may have on the magnetization of sedimentary units.

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