Abstract

Defining and dating the base of the Kiaman (the long period of reverse polarity that spans the time from the Late Carboniferous to the middle Permian) have proven difficult. In an attempt to do this an integrated stratigraphic, paleomagnetic, and geochronologic study was undertaken in Carboniferous rocks in the northern Tamworth Belt of the New England orogen, eastern Australia.

New paleomagnetic measurements at 145 horizons (sites) in the New England orogen of eastern Australia provide an accurate record of the late Namurian to Westphalian geomagnetic field. They have high unblocking temperatures, satisfy tilt, conglomerate, and slump tests, and show excellent agreement between a wide variety of igneous and sedimentary rock types, indicating that magnetism was acquired at, or soon after, deposition. Magnetite and, to a lesser degree, hematite are the carriers of magnetization. The record is contained in sequences of glaciogene sediments and arc-derived volcano-sedimentary units, folded into broad structures in the Late Permian to Triassic. New isotopic dates for the volcanic units indicate ages from 321 to 306 Ma (late Namurian to Westphalian). Magnetizations are steeply inclined downward (reversed polarity in the Southern Hemisphere) except within the Clifden Formation of the Rocky Creek syncline, where a change from normal to reverse polarity referred to as the Wanganui reversal records the onset of the Kiaman superchron.

Integrating complex stratigraphic, isotopic, and paleomagnetic data, we correlate the Wanganui reversal to the top of N6 in the Joggins section Nova Scotia, Canada, and estimate that its age is between 318 and 316 Ma. Further work may substantiate a younger normal zone in Australia that would support a slightly younger age, but within these limits. These considerations indicate that the Wanganui reversal occurs in the late Namurian (Marsdenian) and after the time of the Mississippian-Pennsylvania boundary. The mean direction of magnetization of the Upper Carboniferous Australian sequences is declination (D) = 195.3°, inclination (I) = 76.9°, κ = 25, α95 = 3.1°, with a paleopole at 51.9°S, 141.2°E, α95 = 5.7°, and a paleolatitude of 65.0° ± 5.6°S (P = 0.05). This paleolatitude is consistent with the glacial origin of some of the sedimentary units.

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