A North West Shelf Triassic Reef Play: Results From Odp Leg 122

Abstract

Site 764 of the Ocean Drilling Program (ODP), drilled during Leg 122 in the Exmouth Plateau region, cored 200 m of Upper Triassic (Rhaetian) reef complex. This site, on the northern Wombat Plateau (northernmost Exmouth Plateau) represents the first discovery of Triassic reefal material near the Australian North West Shelf. Seismic reflection data through Site 764 show that the reef itself corresponds predominantly to a seismically poorly reflective zone. A number of regional unconformities appear to correspond, however, to traceable seismic horizons which pass with reduced amplitude through the reef, indicating stages of reef growth separated by erosion or non- deposition. Seismic facies around the edges of the reef are consistent with the deposition of wedges of prograding reef- derived detritus.Application of the seismic criteria for reef recognition established at ODP Site 764, to other seismic reflection data on the Wombat Plateau, demonstrates that a major Upper Triassic reef complex fringes the margins of the Wombat Plateau. The Wombat Plateau lies at the western end of the North West Shelf, which was part of the southern margin of a warm Tethys Ocean in the Late Triassic, at a palaeolatitude of 25- 30°S. Upper Triassic reefs are known from southeast Indonesia and Papua New Guinea, and now the Wombat Plateau, and may be common elsewhere along the outer margin of the North West Shelf. Upper Triassic reef complexes, with their associated reservoir, source and seal facies, could represent an exciting new petroleum exploration play for the entire North West Shelf. Facies analysis suggests that they are likely only on the outer shelf and slope. Shallow Triassic reef complexes are clearly identifiable using high resolution seismic reflection data. Seismic reflection data of lower resolution may well reveal the associated detrital carbonate wedges, which are more laterally extensive than the reefal core, deeper in the section.

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

“…This study drew attention to the following points: (1) the Late Triassic margin did not face northward (that is, toward the present northern edge of the Wombat Plateau and the Argo ocean) but northwestward, (2) the most recent deposits below the PRU (Rhaetian) were not significantly thickened or faulted during deposition approaching the present-day continent/ ocean boundary, and (3) the environments are different at Site 764 because at this site we recovered one of the numerous reefal buildups that are commonly found in the Rhaetian series where these series are preserved (northern Wombat Plateau; Williamson et al, 1989), but not because Site 764 was located on a reef belt at the Triassic platform edge (reefal buildups are also found southward from Site 761; Williamson et al, 1989).…”

“…During the Late Triassic, the platform faced toward the northwest, as shown also by Exon et al (1982), Williamson et al (1989), andvon Rad et al (1989b). North-northeast, southsouthwest to northeast-southwest faults were active at that time (i.e., the East Wombat fault or the Swan Canyon graben; Exon et al, 1982) but no actual evidence of activity along the South Wombat (east-west) fault trend has been found (Fig.…”

“…This iron-oxide banding could result from remobilization of iron-oxide of lithofacies i due to oscillations of fresh water bodies long after the deposition of the sediment, thus leading to the same interpretation than lithofacies j. Williamson et al (1989), and location of the ODP sites (vertical exaggeration 20 ×). Vertical hatching: post-rift series.…”

“…The seismic profiles across the Wombat Plateau show strong and fairly continuous reflectors in the Rhaetian sequence. These continuous reflectors are interrupted in some places, however, coinciding with patchy reefal buildup (especially at Site 764; Williamson et al, 1989). It is of major interest for the understanding of tectonic evolution and eustasy to correlate the Rhaetian beds-the most recent below the PRU-between Sites 761 and 764.…”

“…The only indication of syn-sedimentary tectonics is found at or below the very base of the Rhaetian series and consists of small, normal faults that delimit slightly upthrown blocks upon which coral patches have sometimes developed (Williamson et al, 1989). This extensional tectonic event is, however, of a lower order of magnitude than that which affected Carnian deposits and it is sealed by the lower Rhaetian beds.…”