The Norwest Bend Formation: Implications for the evolution of Neogene drainage in southeastern Australia

Strontium isotope data indicate that sediments of the Norwest Bend Formation were deposited within a large estuarine system during the Latest Miocene to Early Pliocene (probably > 5 Ma). The Norwest Bend Formation is divisible into two members; a lower sand-dominated member and an upper oyster coquina member. These two units are separated by an irregular, erosion surface that has no correlative within the adjacent Loxton-Parilla Sands. Tectonism in the western Murray Basin adjacent to the Adelaide Foldbelt has exerted a considerable control over the formation of these estuarine sediments. The Lower Norwest Bend Formation occupies a paleodrainage system that was restricted to a region west of the Hamley Fault and was presumably incised following uplift of this western block. Flooding of this paleodrainage system produced the Lower Norwest Bend estuarine system. Later tectonism on the Hamley Fault resulted in the formation of the erosion surface within the Norwest Bend Formation. We suggest that the Norwest Bend estuarine system owes its existence to marginal tectonism associated with the young uplift of the Mt. Lofty-Flinders Ranges. The paleodrainage system incised prior to deposition of the Norwest Bend Formation differs markedly in geometry from the present Murray River-gorge system. This suggests that the modern Murray River system developed after deposition of the Late Miocene-Pliocene Norwest Bend Formation.