Magnetization of the Cambrian Lamotte Formation in Missouri

Summary The Upper Cambrian Lamotte formation of South‐eastern Missouri in Central United States is a white sandstone which, though very weakly magnetized, is amenable to investigation by standard palaeomagnetic techniques. Out of the total of 150 oriented cores, 134 specimens were found to possess measurable intensities of the order of 1 times 10 ‐6 emu cm ‐3 . Though thermal cleaning does not produce reliable results, magnetic cleaning yields three groupings of significant directions of magnetization, two of which are associated with the lower coarse grained part of the formation and one with its fine grained upper layers. The first two are believed to be associated with secondary magnetization, one representing the direction of the Carboniferous geomagnetic field. The third group subdivides into two subgroups based on sampling of a columnar section near the top of the formation and is associated with the original magnetization of depositional origin. The palaeomagnetic poles of these two subgroups are situated at 160° W, 38° S and 155° E, 11·5° S respectively and are thought to represent two distinct phases of Upper Cambrian geomagnetic field. Mineralogical studies and investigations of the magnetic properties of the sandstone show that the magnetic minerals carrying the magnetization are members of the ilmenite–haematite series of the same average grain size (∼15μ) in both types of rock. The magnetization of the coarse grained part is mainly of chemical origin and is carried predominantly by an homogeneous ilmenite–haematite solid solution, while that of the fine grained part is associated with specular haematite and titanohaematite containing ilmenite lamellae. Both types of sandstone, but especially the coarse grained type, contain titanohaematite with microintergrowths, as well as finely divided haematite in a demagnetized state. Heat treatment of the fine grained sandstone for 6 hr at 735 °C changes its magnetic behaviour to that of the coarse grained type. The results suggest also that the stable part of remanence in natural haematite is that of the lattice defect moment with a ferrimagnetic Curie point at 725 °C. Comparison of the Upper Cambrian and Carboniferous Lamotte palaeomagnetic poles with the corresponding poles for European rocks suggests that North America and Europe have been drifting apart since Upper Palaeozoic. The results indicate that despite Post‐Cambrian re‐magnetization of the Lamotte sandstone a considerable part of the formation has retained its original Cambrian magnetization.