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Title

Upper Triassic Tethyan Carbonates off Northwest Australia (Wombat Plateau, ODP Leg 122)

Authors
Röhl, Ursula
Dumont, Thierry
von Rad, Ulrich
Published in Facies. 1991, vol. 25, p. 211-252
Abstract Leg 122 of the Ocean Drilling Program (ODP) recovered Upper Triassic (Carnian to Rhaetian) sediments at the sediment-starved passive continental margin off Northwestern Australia. The early-rift series at the Wombat Plateau, a northern sub-plateau of the Exmouth Plateau, consists of Upper Triassic fluviodeltaic siliciclastics and shallow-marine carbonates including reefal facies. Twenty-five microfacies types could be distinguished. These sequences are capped by an erosional "post-rift unconformity" with a 70 m.y. hiatus during the Jurassic. The Wombat Plateau bears only a thin post-rift sedimentary cover of Cretaceous to Cenozoic age. The Carnian and Norian sequences are dominated by fluviodeltaic sediments that contain many carbonate intercalations. Their frequency and the kind and amount of allochems allow the reconstruction of a storm-influenced deltaic to prodeltaic environment with restricted estuarine (intradeltaic) lagoons and high-energy carbonate sand shoals in front of the delta lobes. The presence of the foraminifers Triasina berhauseri and Triasina hantkeni in Sites 761 and 764 indicate a Norian to Rhaetian age. The reefal platform can be differentiated in a lagoon to patch reef environment with abundant Aulotortidae, and a patch reef to shelf zone with smaller foraminifers. The 'Rhaetian' starts with a global sequence boundary. Several shallowing-upward cycles from bioturbated wackestones to dolomitic algal bindstones suggest a shallow-subtidal to intertidal environment at Site 761. Typical reef development was observed at the more' distal' Site 764. The limestone-marl alternations of the open marine shelf grade into local bioclastic and oolitic grainstones, which are the base for the incipient carbonate buildup. Calcisponge patch reefs developed into coral reefs. Several cycles characterize a 'catch-up' system grading into a 'keep-up' carbonate system. The reef growth ended abruptly with the second sequence boundary (211 Ma after HAQ et al., 1987), coinciding with the worldwide latest Rhaetian sea level fall, followed by renewed transgression. By comparison with Upper Triassic carbonates of the western Tethys (e.g., the Northern Calcareous Alps), several microfacies types could be combined to characteristic facies units: biolithite facies, different reef talus types, grapestone oncoid facies, and calcareous algae-foraminifera detritus facies showing the reef-backreef/fore-reef-lagoon transitions. Detailed investigations of microfacies, wireline logs and high-resolution seismics allow the determination of depositional sequences (sequence stratigraphy). We distinguish influences of regional or global tectonics and/or eustatic sea-level changes. The results show that regional tectonic movements are of minor importance in the Rhaetian and that the HAQ etal. (1987) cycle chart could also be used at the passive margin of Northwest Australia.
Keywords MicrofaciesForaminifersReefFacies ModelCarbonate Ramp – Sequence StratigraphyRift-TectonicsSea-Level ChangesPassive Continental MarginTethysWombat PlateauExmouth PlateauNw AustraliaUpper Triassic (Carnian-Rhaetian)
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Research group Sedimentology, Biostratigraphy and Micropaleontology
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RÖHL, Ursula et al. Upper Triassic Tethyan Carbonates off Northwest Australia (Wombat Plateau, ODP Leg 122). In: Facies, 1991, vol. 25, p. 211-252. https://archive-ouverte.unige.ch/unige:24413

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Deposited on : 2012-12-11

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