Mesozoic Facies Evolution of Northwest-Australian Margin (ODP Legs 122-123): Does It Document Indo-Australian Breakup or Neotethys History?

P.O. Baumgartner, J. P. Marcoux

The transect from the Exmouth Plateau to the Argo Abyssal Plain drilled during ODP Legs 122 and 123 documents a margin comparable to Tethyan Mesozoic margins, but major evolutionary steps, dated in Neotethys as Early to Middle Jurassic, took place here in the Early Cretaceous.

Oceanic basement in the Argo Abyssal Plain is overlain by lowest Cretaceous red-brown quartzose siltstones and bentonites, which, in turn, are overlain by Neocomian red claystones with subordinate radiolarite interbeds. This indicates sea-floor spreading began in the earliest Cretaceous and not in the Callovian-Oxfordian, as had been postulated. Although the Neocomian sedimentary facies compare well to coeval Tethyan analogs, the well-preserved Neocomian radiolarian assemblages of the Argo basin have little in common with Tethyan assemblages. This suggests the Argo basin was paleo-oceanographically distinct from Neotethys due to paleogeographic separation from Tethys and/or owing to a higher Neocomian paleolatitude.

Our observations do not seem to document the breakup history of Neotethys (Gondwana breakup) but instead indicate a much younger secondary breakup that occurred possibly continentward of an older ocean basin. Whereas the Carnian-Norian sedimentation history on the Exmouth Plateau may reflect Neotethys rifting, the unconformity is not associated with breakup but is due to starvation by sediment trapping in continentward marginal basins. We speculate Argo basin seafloor spreading is related to an early phase of the Early Cretaceous Indo-Australian breakup, situated near a hypothetical northeast Indian-northwest Australian-Neotethyan triple junction.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.