Palaeomagnetism of red beds from the Kimberley Group, Western Australia: Implications for the palaeogeography of the 1.8 Ga King Leopold glaciation

Abstract

The 1.8 Ga King Leopold glaciation in northwestern Australia is recorded in the Kimberley Basin by subglacial erosional forms cut in an unconformity surface at the top of the Speewah Group and buried by glaciofluvial deposits at the base of the King Leopold Sandstone, the lowest formation of the Kimberley Group. The glaciofluvial deposits are conformably overlain by texturally mature, locally glauconitic quartzarenite of shallow-marine origin, implying that glaciation occurred near sea level. Palaeomagnetic data for five sites (29 specimens) in the hematite-bearing Elgee Siltstone of the upper Kimberley Group in the northeastern Kimberley Basin gave a dip-corrected mean direction of declination D = 95.4°, inclination I = 13.8° ( α 95 = 9.7°), indicating a palaeolatitude λ = 7.0 +5.3/−5.0°. Results for red beds from five sites (28 specimens) in the conformably overlying Pentecost Sandstone gave a dip-corrected mean direction of D = 94.1°, I = 13.0° ( α 95 = 11.7°), λ = 6.6 +6.4/−5.9°. Combining our data for the Elgee Siltstone and Pentecost Sandstone with results for the Elgee Siltstone in the southeastern Kimberley Basin [Li, Z.X., 2000. Palaeomagnetic evidence for unification of the North and West Australian cratons by ca. 1.7 Ga: new results from the Kimberley Basin of northwestern Australia. Geophysical Journal International 142, 173–180.] yielded a dip-corrected mean direction for 17 sites of D = 93.5°, I = 15.1° ( α 95 = 4.4°), palaeolatitude λ = 7.7 ± 2.3°, and an overall pole position at latitude λ p = 5.4°S, longitude φ p = 211.8°E, with confidence semi-axes d p = 2.3° and d m = 4.5°. A positive fold test at 99% confidence was obtained for the combined data for the 17 sites. The basinwide concordance of results for the Elgee Siltstone and the positive fold test argue for an early magnetisation acquired prior to late Palaeoproterozoic initial folding of the Kimberley Group and close to the time of deposition. Our results imply a low palaeolatitude for the Kimberley Group and the King Leopold glaciation. Palaeomagnetic data for the 1.822 Ga Plum Tree Creek Volcanics in the Pine Creek Orogen to the east of the Kimberley Basin are consistent with northwestern Australia being in low palaeolatitudes during the late Palaeoproterozoic. Hence the enigma of glaciation near sea level in low palaeolatitudes, which marks the Neoproterozoic and early Palaeoproterozoic, applies also to the late Palaeoproterozoic. Red beds from the Lansdowne Arkose of the Speewah Group also were studied, but only directions ascribable to Tertiary regolith weathering processes were found. Similar findings for the late Palaeoproterozoic McArthur Basin in northern Australia suggest some units in that basin also record Tertiary regolith processes.