Outer Rocky Shores of the Mowanbini Archipelago, Devonian Reef Complex, Canning Basin, Western Australia

摘要

The Oscar Range in Western Australia's Canning Basin features inliers of folded Paleoproterozoic quartzite, quartzitic conglomerate, and phyllite that formed islands during the Late Devonian. Undisturbed strata of the Pillara Limestone (Upper Devonian, Frasnian Stage) surround individual paleoislands that rise above the former seabed with a maximum topographic relief of 90 m. On average, the Mowanbini Archipelago (aboriginal name for the Oscar Range) lies 15 km off the granitic and metamorphic mainland represented by the Kimberley Block to the north. Devonian facies on outer rocky shores were studied at three localities near the southeast end of the Oscar Range. At the west end of the study area, a Devonian reef margin sits unconformably on Paleoproterozoic phyllite, but to the east, the reef is separated from rocky shores by a wide lagoon. Massive clast-supported conglomerate and sandstone beds interfinger with backreef carbonates where the reef is closest to land. Across the lagoon 2.5 km eastward, differential erosion between steeply dipping quartzite layers interbedded with softer phyllite resulted in low sea stacks, or skerries. Tabular quartzite cobbles and quartz sand were shed by the skerries as a unidirectional apron of "breccia" under shoal water conditions. Nearby offshore stromatoporoid thickets reflect growth orientation aligned with paleocurrents compatible with the breccia apron. On a smaller island 2.5 km farther east, lateral transport of debris from a quartzite shore to a contiguous phyllite sector of the coast conforms to the same pattern of water circulation. Overall, the physical geography of outer rocky shores in relation to the lagoon and barrier reef suggests that waves crashed over the reef to energize a longshore current that moved west to east at one end of the Oscar Range. A rapid rise in sea level probably promoted the burial and preservation of the Mowanbini Archipelago, characteristics shared with other examples of quartzite paleoislands that date from Cambrian to Cretaceous times.