Upper Triassic (Rhaetian) Sequences of the Australian Northwest Shelf Recovered on Leg 122: Sea-Level Changes, Tethyan Rifting, and Overprint of Indo-Australian Breakup

Abstract

During Leg 122, Upper Triassic shallow-marine sequences capped by an erosional post-rift unconformity with a 80-m.y. sedimentary gap were recovered in the northern part of the Exmouth marginal plateau (Sites 759-761 and 764). The continent/ocean boundary is located only a few kilometers north of the northernmost site (Hole 764B). The components and microfacies of the most recent sediments below the post-rift unconformity (Rhaetian; about 200 samples) were analyzed. These sediments were found in two sites about 20 km apart (Holes 761C and 764B). Based on microfacies analysis and chemical logs, a correlation between these two sites is proposed. It is suggested that the uppermost Rhaetian beds at Site 764 do not exist at Site 761 (later erosion), and that the part existing in both holes (about 170 m) is only 5% thicker at Site 764. Rhaetian tilting along a north-south direction was thus insignificant (0.02°). The Rhaetian series are composed of two shallowing-upward sequences which correspond to third-order depositional sequences (sensu Haq et al., 1987). The lack of evidence for Rhaetian syn-sedimentary tectonics in the study area, and the similarities between the northwestern Australian sedimentary sequences and the European series, indicate that global eustasy has played a major role during the Rhaetian. The analysis of Rhaetian beds, which were deposited upon a rather stable platform, and the geometry of underlying Triassic beds, do not document any causal link between the Late Triassic rifting and the breakup in the nearby Argo Abyssal Plain 80 m.y. later. This breakup is probably more related to the Neocomian separation between India and Australia, and cuts obliquely through the rim of marginal plateaus of the older Tethyan margin. The geodynamic evolution of this latter Gondwanan Tethyan margin around the Triassic-Jurassic boundary appears closely comparable to, and synchronous with, the evolution of the European margin of the Ligurian Tethys, presently outcropping in the Western Alpine nappes. 1 von Rad, U., Haq, B. U., et al., 1992. Proc. ODP, Sci. Results, 122: College Station, TX (Ocean Drilling Program).