Paleomagnetism of the Ordovician redbeds of Bell Island, Newfoundland

Rao, Kunduri Viswasundara (1970) Paleomagnetism of the Ordovician redbeds of Bell Island, Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

A thermomagnetic study of oölitic hematite sandstone from the gently dipping Lower Ordovician Bell Island and Wabana groups near St. John's revealed three to four superposed magnetic components. The rocks were collected from a 150-meter section at 24 sites up to 6 km apart, plus one site 180 m below that section. -- The natural remanence (NRM) directions occur in two nearly antiparallel groupings (A, B) close to published late Paleozoic directions. Two or more specimens each from 116 samples were stepwise thermally demagnetized in air, with field-nulling carefully controlled to ≤ 25 γ. Two new vector groupings (C, D), diverging sharply from the NRM, appeared after the 600°C and 685°C steps. -- Up to 600°C the intensity (J) increased, sometimes by factors of 2 or more, compared with the NRM (10⁻⁵ - 10⁻⁴ emu/cm³), before dropping sharply to a minimum at 685°C. Powder measurements in a magnetic balance to 750°C showed this to be mainly due to formation of a new, strongly magnetic component with Curie point 610 - 620°C, which x-ray diffraction tests suggest is γ-Fe₂0₃. This process had no observable effect on the high-temperature groupings (C, D), probably associated with α-Fe₂0₃. -- Mean directions of the thermally discrete group D (685°C) component were significantly better aligned (k = 160) than expected from the present secular variation. Assuming reversed polarity, a possibly Ordovician pole calculated from this direction is in the central Pacific (27.8°N, 168.2°W, dp = 1.8°, dm = 3.6°, N = 11 vertical zones) and constitutes the first paleomagnetic result from laboratory-treated Ordovician rocks in North America. The published evidence is still inadequate to interpret this result in terms of Lower Paleozoic paleogeography around the North Atlantic.

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