Abstract

Heating of samples of the San Juan Cretaceous sandstones (NE Venezuela) results in large increases in their low field susceptibility, mostly above 650°C, which allows measurement of their low field susceptibility ellipsoids. Where measurements are available before and after heating, the same fabric is almost always enhanced. Samples, previously too weak for measurement, showed thermally induced fabrics which are consistent within each site, but vary between different localities. The susceptibility changes are all largely attributed to magnetite formation, probably from vermiculite at 450°C in the proximal sediments, and from illite at ≥850°C. In all areas, the induced magnetic fabrics can be interpreted in terms of either sedimentary structures or low grade metamorphism related to cleavage parallel to the fold axes. It is considered that this thermal enhancement technique offers a method for measuring low field magnetic fabrics in a wider range of geological conditions and lithologies than reviously, although the processes involved need considerable study.

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