Abstract

Goethite associated with dedolomite in the Morgan Creek Limestone in central Texas carries a chemical remanent magnetization (CRM). The pole position (88°N, 96°W; k=67, α 95=2.7), which is close to that of the modern pole, and the fact that only normal directions are present suggest that the component is a post-Brunhes (<700,000 yr) CRM. Dedolomite is common in the Morgan Creek and consists of calcite rhombohedra with relict dolomite. Goethite occurs surrounding the rhombs and in rhombic zones within the crystals. The goethite precipitated when iron was released from ferroan dolomite during the dedolomitization process. The paleomagnetic data, therefore, date the dedolomitization process as modern. Dedolomitization is probably related to surface weathering of the Morgan Creek.

In addition to goethite associated with dedolomite, the Morgan Creek contains hematite replacing fossils and glauconite, and detrital hematite. The Morgan Creek also contains three other magnetic components. One of these components is a viscous remanent magnetization aligned with the present magnetic field. Another component is a Triassic/Jurassic CRM residing in hematite. A fourth component in the Morgan Creek is an early Paleozoic magnetization residing in hematite and minor magnetite and represents the vector addition of a detrital remanent magnetization and one or more CRMs. The results of this study demonstrate that paleomagnetic and petrographic techniques can be used to date a diagenetic event and also illustrate the complex nature of magnetization in a sedimentary unit.

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