MILANKOVITCH-BAND CYCLICITY IN BEDDED HALITE DEPOSITS CONTEMPORANEOUS WITH LATE ORDOVICIAN - EARLY SILURIAN GLACIATION, CANNING BASIN, WESTERN AUSTRALIA

The Mallowa Salt of the early Palaeozoic Carribuddy Group, Canning Basin, Western Australia, is a halite-mudstone-anhydrite-dolomite evaporite sequence of Late Ordovician to Early Silurian age deposited in barred marginal-marine to ephemeral saltpan and saline mudflat environments. BHP-Utah Minerals' potash exploration well Gingerah Hill No. 1 obtained drill core of a 477 m stratigraphic interval of the Mallowa Salt, which permitted the study of geochemical variations in a thick halite sequence contemporaneous with Late Ordovician-Early Silurian glaciation on other continents. A strip of uniform width was continuously ground from the core and the powdered rock bagged at 1 m intervals for geochemical assay. The resulting geochemical stratigraphic series display conspicuous cyclicity: several orders of cycles ranging from approximately 3 m to 100 + m stratigraphic thickness are discernible in the plots of minor constituents Br, MgO and K2O in extracted soluble salts and also in the plots of Na2O and total salts content. Fast Fourier transform (FFT) of the Br, MgO and K2O series gives principal spectral peaks in the 0.00-0.10 cycles/m frequency band at approximately 250, 113, 35.4, 22.1 and 19.7 m, as well as peaks at approximately 65 m and between 2.8 and 13.5 m. FFT of the Na2O and total salts series shows dominant spectral peaks at 220-259 and 109-114 m. If the strongest spectral peak at 113 m is taken to represent the relatively stable eccentricity period of 100 ka and a constant net rate of accretion assumed for long sections, the other main periods in the Mallowa Salt as exemplified by the Br spectrum would be 31.3 +/- 3.0, 19.6 +/- 1.1 and 17.4 +/- 1.1 ka. These figures are consistent with predicted Late Ordovician-Early Silurian (440 Ma) periods for obliquity (30.5 ka) and precession (19.3 and 16.4 ka) based on evolutionary change in the Earth-Moon system. Additional spectral peaks identified with implied periods of approximately 206-233, 57.5, 8-12 and 2.5-3.5 ka also may be of palaeoclimatic relevance. The relative amplitudes and structure of spectral peaks in the 0.00-0.10 cycles/m frequency band support the identification of climatic oscillations forced by orbital cycles. The data indicate a precession-eccentricity-dominated pattern, which accords with the known low palaeolatitude (< 15-degrees) of northwestern Australia in Late Ordovician-Early Silurian time. The implied net rate of deposition of the Mallowa Salt of approximately 1.13 m/ka is consistent with net rates determined for other Palaeozoic evaporite sequences. The study indicates that for bedded halite deposits the content of elements such as Br, Mg and K that accumulate in residual bitterns is a more sensitive recorder of climatic variability than is Na or total salts content. The Mallowa Salt is one of the oldest sequences to provide substantive evidence of Milankovitch orbital cycles.