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The Oscar Range is a Late Devonian reef complex that formed at the margin between the Precambrian Kimberley craton and the Canning basin. The range covers an area of 80 × 10 km (50 × 6 mi), and resembles a large atoll in that Frasnian and Famennian reef, marginal-slope, and back-reef subfacies grew around an exposed Precambrian core.
Frasnian reefs are dominated by stromatoporoids and Renalcis, and the reefs show periods of upbuilding, drowning, backstepping, and basinward progradation. Fault-controlled reef growth is developed locally. Marginal-slope deposits contain stromatoporoid debris, sponge boundstone, and crinoids. Back-reef deposits are generally Amphipora-rich biostromes, although fenestral oolite-intraclast-peloid beds are widespread in the southern Oscar Range. Dolomite is best developed in Frasnian back reef and, to a lesser extent, in reef-margin and reef-flat subfacies. In the area of the Precambrian core, conglomerates of Precambrian debris are interbedded with Frasnian limestones that are in depositional contact with basement. Hills of Precambrian rocks rise tens of meters above these peritidal ba k-reef deposits, indicating perhaps several hundred meters relief during the Frasnian.
The exposed Frasnian-Famennian contact is a disconformity. Famennian reefs are dominated by Renalcis and associated algal stromatolites; the equivalent marginal-slope is characterized by allochthonous reef blocks, sponge bioherms, and crinoidal debris. Steeply dipping (40°) basinward-prograding reef and marginal-slope tongues make up the shelf margin. The Famennian back reef is composed of fenestral oolite-peloid lithologic units with common teepee structures, flat-pebble conglomerates, and cryptalgal fabrics.
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