SECONDARY MAGNETIZATIONS IN NORTH AMERICAN ORDOVICIAN ROCKS: OBSERVATIONS AND INFERENCES

A program of investigation of the natural remanent magnetization (NRM) of selected Ordovician rocks of North America was undertaken with the aim of determining primary remanence directions. In all cases, however, the rocks contained only secondary magnetizations ranging in age from the time of sample collection back to Late Paleozoic time.;Anisotropy of enhysteretic susceptibility measured on Trenton Limestone samples shows a magnetic fabric compatible with paleostress directions based on joint orientations and other structural indicators.;The Quimby's Mill Limestone from the Kentland cryptoexplosion structure yields a paleomagnetic pole of apparent Late Cretaceous age. The magnetization postdates tilting, and probably was acquired on cooling after shock heating. Thermal activation models for remanence and for conodont color alteration indicate peak temperatures in the sampling area of about 150(DEGREES)C, maintained for about 1000 years. A 1000 year cooling time indicates an overburden during cooling of at least 300 meters.;Syenite dikes and adjacent host rocks from the Pedernal Hills carry a characteristic southeasterly shallow remanence with a pole position suggesting a Late Paleozoic age for the remanence. Host rocks at a distance greater than one dike-width show no evidence of this magnetization, suggesting a positive contact test. Possible resolutions of these incompatible inferences include: (1) incorrect age assigned to the dikes; (2) local or regional rotations; (3) localized remagnetization by hydrothermal fluids.;Shakopee Dolomite and Camp Nelson Limestone samples were strongly overprinted during drilling and sawing. When samples with a controlled initial magnetization were cut in a field normal to the control magnetization, the resultant remanence was deflected by 7(DEGREES) to 70(DEGREES). The angle of deflection correlated strongly with the ratio of initial/anhysteretic susceptibility, which is a function of grain size. This supports a model in which the drilling-induced remanence resides in the grains at the sample surface which are stressed during cutting.;The El Paso Limestone carries two magnetic components of Late Mesozoic and Cenozoic age. The Cenozoic component appears to be a Brunhes-age thermoviscous component, but unblocking termperatures significanctly exceed theoretical model predictions. This may be due to transdomain processes not accounted for in the theory.