Early palaeozoic palaeomagnetism in Australia I. Cambrian results from the Flinders Ranges, South Australia II. Late Early Cambrian results from Kangaroo Island, South Australia III. Middle to early-Late Cambrian results from the Amadeus Basin, Northern Territory

Abstract

I. Cambrian results from the Flinders Ranges, South Australia A total of 460 samples from six sequences spanning the Cambrian succession of the Flinders Ranges (Adelaide “Geosyncline”, South Australia) has been analyzed through thermal demagnetization studies. All samples showed a recent field component, generally constituting more than 50% of the initial intensity, which in most cases was removed by 200–400°C. Two characteristic magnetic components have been identified: 1. (A) A secondary magnetic component of Cambro-Ordovician age (S-pole at 75.3°E 26.0°N, α 95 = 7.4°, N = 5 localities) interpreted as having been induced by thermochemical activity during a period of enhanced heat flux prior to the Late Cambrian-Early Ordovician diastrophistic phases of the Delamarian Orogeny. 2. (B) A primary magnetic component, which indicates rapid polar motion during the Early Cambrian and a much reduced polar motion during the Middle Cambrian. Representative palaeomagnetic pole positions for the primary component are: 1. (1) Basal Hawker Group (earliest Cambrian): S-pole at 2.3°E 26.7°S, d p = 8.1°, d m = 14.3°, N = 10 (sites). 2. (2) Billy Creek Formation— Wirrealpa Limestone— Aroona Creek Limestone (late Early Cambrian to early Middle Cambrian): S-pole at 20.1°E 37.4°S, d p = 7.2°, dm = 14.4°, N = 11(sites). 3. (3) Basal Lake Frame Group (Middle Cambrian): S-pole at 26.1°E 29.3°S, d p = 6.6°, d m = 13.1°, N =10 (sites). 4. (4) Pantapinna Formation (late Middle Cambrian?): S-pole at 29.2°E 36.4°S, d p = 8.4°, dm = 16.7°, N = 4 (sites). Available data suggest that deposition of the Lake Frome Group beds probably did not continue into the Late Cambrian. II. Late Early Cambrian results from Kangaroo Island, South Australia A total of 108 block samples from a late Early Cambrian red-bed sequence on Kangaroo Island (Adelaide “Geosyncline”, South Australia) has been analysed through thermal demagnetization studies. All samples contained a recent field component of considerable intensity. Two characteristic magnetic components have been identified: 1. (A) A secondary magnetic component of Late Cambrian—Early Ordovician age (S-pole at: 75.8°E 17.4°N, d p = 4.2°, d m = 1.9°, N = 54 specimens), attributed to thermochemical activity predating the main folding phases of the Delamarian Orogeny. 2. (B) A primary magnetic component corresponding to a S-pole position at 15.1°E 33.8°S (d p = 6.2°, d m = 12.3°, N = 16 sites). Both the primary and the secondary magnetic component are in very good directional agreement with the magnetization pattern from the correlated Billy Creek Formation of the Flinders Ranges (I). Consequently, noticeable rotational movement since late-Early Cambrian times between Kangaroo Island and the northwestern part of the Adelaide “Geosyncline” can be ruled out. III. Middle to early-Late Cambrian results from the Amadeus Basin (Northern Territory) A total of 328 samples from a Middle Cambrian red-bed succession and a Middle to early-Late Cambrian carbonate succession in the Amadeus Basin (Central Australia) have been analyzed through thermal demagnetization studies. All samples contained a recent field component of considerable intensity which became broken down, respectively below 200°C in the carbonate samples and between 300°C and 500°C in the red-bed samples. Another recent field component, broken down between 600°C and 675°C, was noted in some of the red-bed samples. Three characteristic magnetic components have been identified: 1. (A) A secondary magnetic component of Late Devonian—Early Carboniferous age (S-pole at 110.5°E 46.9°S, N = 2 localities) which predates the main folding phase of the Early Carboniferous Alice Springs Orogeny. 2. (B) Another secondary magnetic component (S-pole at 60.8°E 33.8°N, N = 2 localities) which is very similar to a thermo-chemically induced Cambro-Ordovician magnetic component, noted in rocks from the Adelaide “Geosyncline”. 3. (C) A primary magnetic component which suggests reduced polar motion during the Middle Cambrian (S-pole at 16.6°E 35.0°S, α 95 = 7.4°, N = 5 sites) and rapid polar motion during the early-Late Cambrian (S-pole at 34.9°E 14.9°N, d p = 6.3°, d m = 11.6°, N = 9 specimens).