Formation of dolomite in the Coorong region, South Australia

Abstract

Cores of unlithified Holocene sediment from shallow ephemeral lakes in the Coorong region S.A., contain dolomite at various stratigraphic intervals. Two distinct dolomite types are recognized using stable carbon and oxygen isotope data, unit cell calculations, transmission electron microscopy (TEM), and bulk composition data. Detailed X-ray diffraction (XRD) analyses show that the dolomite is ordered, but that ordering is variable. Average unit cell parameters indicate that the crystal lattice of one type of dolomite (type A) is expanded in the c o direction (c o = 16.08 A ̊ ) and contracted in the a o direction (a o = 4.799 A ̊ ) relative to ideal dolomite (c 0 = 16.02 A ̊ and a o = 4.812 A ̊ , respectively). Type A dolomite occurs in the centers of the larger basins. The mole fraction of MgCO 3 is as much as 53 ± 1%. The unusual crystal chemistry and (TEM) data indicating a heterogeneous microstructure suggest that this dolomite is generated by rapid precipitation from brines which have highly elevated Mg Ca ratios. Dolomite ordering reflections are present in electron diffraction patterns but are weak. Stable oxygen and carbon isotope values are tightly grouped (ave. δ 18O = +7.6‰, s.d. = 0.6‰; δ 13C = +3.5‰, s.d. = 0.7‰) . The other type of dolomite (type B) is Ca-rich, has lower stable isotope values (ave. δ 18O ≈ +6.4‰, δ 13C ≈ −1.2‰) than type A dolomite, has a more homogeneous microstructure, is expanded in both the a o and c o axes, and is generally better ordered than type A dolomite. These data suggest type B dolomite is precipitated more slowly from less evaporitic brines than type A dolomite.