Abstract

Samples were obtained at 13 sites in the Maringouin Formation (late Mississippian red sediments) in New Brunswick and subjected to routine thermal demagnetization. After thermal cleaning at 550 °C, the results fall into two groups of comparable stability, but with different mean directions, direction dispersions, and intensities. Group I directions have a small dispersion (mean 179°, + 21°; k = 46; pole 117° E 34° N) after correction for folding. The folding was early Pennsylvanian so that the magnetization was acquired before that time. Comparison with results from somewhat younger formations indicates that there was an apparent polar shift of about 14° between late Mississippian-early Pennsylvanian time and late Pennsylvanian-early Permian time. Group II directions, although thermally stable, have a large dispersion (mean k = 4) and the intensities are generally lower than those of Group I. The Group II results may be explained by supposing that the magnetization was acquired not at a single point in time, but by one or more processes operating over a considerable period as the rock became lithified. If the field reversed during this time, the resultant magnetization will be the vector sum of an early magnetization acquired at or soon after deposition and a later one, roughly anti-parallel, acquired during diagenesis. This hypothesis explains the present data. It may also provide an explanation of 'transitional' directions observed during reversals in deep ocean cores and other paleomagnetic observations yielding 'strung' distributions. It is possible that in sediments lithified during a period of single polarity, the integrating effect of the diagenetic magnetization may reduce the between-site dispersion so that estimates of secular variation will be systematically too small.

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