Provenance of early Palaeozoic sandstones, southeastern Australia, Part 1: vertical changes through the Bengal fan-type deposit
Abstract Ordovician strata of the Lachlan Fold Belt in southeastern Australia are dominated by a turbidite succession that in its undeformed state was comparable in size to the modern-day Bengal Fan. The ancient fan was derived from the northern part of the Cambrian Ross–Delamerian orogenic belt that formed at a convergent boundary between plates containing Gondwanaland and part of the palaeo-Pacific Ocean, respectively. Substrate to the submarine fan consisted of mafic volcanic rocks with an immature island arc affinity similar to the Eocene Izu–Mariana–Bonin system. Sandstone compositions, determined from several stratigraphically constrained sections, demonstrate a uniform framework composition dominated by monocrystalline quartz with less abundant feldspar, polycrystalline quartz, metamorphic rock fragments, sedimentary rock fragments and mica. These indicate derivation from a low-grade metamorphic terrain. Up-sequence variation is limited to decreasing feldspar and lithic fragments and increasing quartz, indicating increased weathering and lowering of topography, in the source terrain over time. 25% of counted sandstones in the Late Ordovician interval contain a small component of mafic–intermediate volcanic rock fragments that accompanied a change in palaeocurrent direction from eastwards to northwards. In the Early Silurian, the Ordovician sequence was disrupted by the formation of several major deformation zones. Between these zones, developed several deep-marine sedimentary basins that contain a continuous succession from the Ordovician into the Early Silurian. Early Silurian sandstone is similar in composition to underlying Early Ordovician rocks except that the feldspar component is generally absent. Early Silurian sandstone was derived from uplifted portions of the Ordovician sequence and includes texturally inverted types with sandstone containing a bimodal mixture of coarse well-rounded quartz and fine angular quartz.