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During Ocean Drilling Program Leg 122, 900 m of Upper Triassic (Carnian to Rhaetian) sediments on the Wombat Plateau (northern Exmouth Plateau) were drilled, an ideal sequence to study eastern Tethyan shallow-water carbonate environments. The early-rift series consists of fluviodeltaic siliciclastics and shallow-marine carbonates including reefal facies. By comparison with Upper Triassic carbonates of the western Tethys (e.g., 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-foraminifer detritus facies showing the reef-back-reef/fore-reef-lagoon transitions. The Carnian and Norian sequences are dominated by fluviodeltaic sediments that contain many carbonate intercalations. Their frequency and the amount of allochems increase upward in this shallowing-upward system. Overlying a global sequence boundary at the base of the Rhaetian ("Rhaetian transgression," 215 Ma), several shallowing-upward cycles from bioturbated wackestones to dolomitic algal bindstones suggest shallow subtidal to intertidal environments. Typical reef development was observed at the platform margin. 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. Later, calcisponge patch reefs were formed which developed into coral reefs. Several cycles characterize a catch-up carbonate system ("highstand systems tract"). The reef growth ended abruptly with the second sequence boundary, coinciding with a sea-level fall followed by the eustatic latest Rhaetian sea-level rise. The carbonates were affected by strong diagenesis. Mostly late-diagenetic alteration destroyed earlier diagenetic features. However, several phases can be distinguished which were formed in meteoric-vadose, meteoric-phreatic, marine-phreatic, and burial-diagenetic environments. The diagenetic successions of the Rhaetian carbonates in Leg 122 suggest that the locally uplifted Wombat Plateau horst was subaerially eroded, probably during the Callovian-Oxfordian.

Microfacies, Paleoenvironment, and Facies-Dependent Carbonate Diagenesis in Upper Triassic Platform Carbonates off Northwest Australia