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 ∼ 3 m to 100 + m stratigraphic thickness are discernible in the plots of minor constituents Br, MgO and K ₂O in extracted soluble salts and also in the plots of Na ₂O and total salts content. Fast Fourier transform (FFT) of the Br, MgO and K ₂O series gives principal spectral peaks in the 0.00-0.10 cycles/m frequency band at ∼ 250, 113, 35.4, 22.1 and 19.7 m, as well as peaks at ∼ 65 m and between 2.8 and 13.5 m. FFT of the Na ₂O 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 know low palaeolatitude ( < 15°) of northwestern Australia in Late Ordovician-Early Silurian time. The implied net rate of deposition of the Mallowa Salt of ∼ 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.