Magnetic mineralogy of continental deposits, San Juan Basin, New Mexico, and Clark's Fork Basin, Wyoming

Abstract

Magnetic concentrates were obtained from nine bulk samples from the Late Cretaceous through middle Paleocene continental sedimentary section in the San Juan Basin, New Mexico, and from two bulk samples from the late Paleocene and early Eocene section in the Clark's Fork Basin, Wyoming. Strong‐field thermomagnetic (JS‐T) curves of almost all the San Juan Basin concentrates show only a single Curie temperature in the 175°–195°C range, indicating that the dominant ferrimagnetic mineral is detrital titanomagnetite (Fe3−xTixO4) of composition 0.51 ≤ x ≤ 0.54. The Clark's Fork Basin concentrates exhibited Curie temperatures of 200°C and 580°C, indicating a mixture of x = 0.5 titanomagnetite and magnetite, respectively. No evidence of hematite (or other ferric oxides or oxyhydroxides) was observed in the JS‐T data. Isothermal remanent magnetization (IRM) acquisition curves were determined for 56 samples from the Clark's Fork Basin and 114 samples from the San Juan Basin. Although dominated by IRM acquired in magnetizing fields of ≤300 mT, additional IRM was acquired in magnetizing fields from 300 to 700 mT. The IRM acquired above 300 mT is attributed to hematite (or other ferric oxides or oxyhydroxides) and is thought to be the result of minor oxidation, probably during recent weathering. These results indicate that IRM acquisition behavior may be used to monitor the hematite content in continental sedimentary rocks and may indicate stratigraphic intervals within which the natural remanent magnetization (NRM) could contain significant chemical remanence overprints. Although minor hematite content is indicated, the detailed sampling of IRM acquisition behavior for the San Juan Basin and Clark's Fork Basin sedimentary sequences did not reveal any significant correlation of IRM behavior with polarity of the characteristic NRM. These data thus support the previous conclusion, based on paleomagnetic data, that the characteristic NRM is a depositional remanence which provides a valid recording of the geomagnetic polarity sequence during deposition of these continental sedimentary sequences.