Late Precambrian Paleomagnetism of Australia: The Adelaide Geosyncline

Paleomagnetic studies from sedimentary and volcanic rocks of the northern part of the Adelaide Geosyncline spanning the (?) 850-570 my period are described. The data are derived from coherent stratigraphic sequences where absolute isotopic age control is lacking, but fold tests and field relationships allow the construction of a relatively simple and consistent APWP segment for this time interval. Probable primary magnetizations have been isolated in four units ranging in age from oldest to youngest as follows: Wooltana volcanics (λ = 62S, Φ = 142E, dp = 16°, dm = 18°); Merinjina tillite (λ = 45S, Φ = 346E, dp = 06°, dm = 11°); Angepena Formation (λ = 33S, Φ = 344E, dp = 09°, dm = 16°); Brachina Formation (λ = 33S, Φ = 328E, dp = 12°, dm = 20°). Secondary magnetizations of probable early Cambrian to Cambro-Ordovician (Delamerian) age have been identified in the Wooltana volcanics (λ = 36N, Φ = 087E, dp = 13°, dm = 26°), Copley quartzite (λ = 36S, Φ = 010E, dp = 07°, dm = 15°), Merinjina tillite (λ = 01S, Φ = 065E, dp = 08°, dm = 14°), Tapley Hill Formation (λ = 33S, Φ = 344E, dp = 08°, dm = 15°), and Bunyeroo Formation (λ = 07S, Φ = 017E, dp = 09°, dm = 14°). Secondary magnetizations of probable Tertiary or Mesozoic age are present in many of the units studied. When combined with paleomagnetic results from the southern part of the Adelaide geosyncline, the new data show that no significant bending of the geosyncline has occurred since middle Proterozoic times, and that the characteristic sigmoidal structural trends in the geosyncline are original features. Apart from refining and clarifying the late Precambrian APW history for this southeastern structural block, the new data show that sedimentation in the Adelaide Geosyncline occurred in low to moderate paleolatitudes during Adelaidean time; thus a low latitude for late Precambrian glacial (or glacio-marine) deposition is indicated for this region.