Stratigraphic architecture and evolution of the early Paleoproterozoic McGrath Trough, Western Australia

Abstract Palaeocurrents, provenance and stratigraphic analysis of the upper part of the Mount Bruce Supergroup and the Wyloo Group in the southwestern part of the Hamersley Province of the Pilbara region of Western Australia have been used to define an early Paleoproterozoic foreland basin, the McGrath Trough, in front of the northwards-advancing Ophthalmian fold belt. The foreland basin succession commences with the Boolgeeda Iron Formation and includes the overlying Turee Creek and lower Wyloo Groups. The basin geometry is asymmetric, thinning northwards away from the Ophthalmian fold belt, and the trough axis trends east-southeast parallel to the Ophthalmian folds. Multiple intraformational unconformities in the upper Turee Creek Group record the advance of the fold belt into the axis of the McGrath Trough in the Hardey syncline area. East-southeast-trending folds and steep southward-dipping cleavage post-date deposition of the Beasley River Quartzite, a shallow-marine to fluvial sandstone that records an interval of tectonic quiescence during basin evolution. Deposition of the Beasley River Quartzite was followed by renewed uplift of the southern hinterland, resulting in continued deposition of fluvial siliciclastics into the foreland basin, which was eventually filled with the eruption of the Cheela Springs Basalt. The age of the McGrath Trough is constrained broadly between ∼2.45 Ga, the age of the underlying Woongarra Rhyolite, and ∼2.2 Ga, the age of the Cheela Springs Basalt at the top of the succession. The sedimentary fill of the McGrath Trough was derived mainly from the south, and becomes texturally and compositionally more mature upward, reflecting its recycled orogen provenance. Two main sources are identified: a dominant cratonic source interpreted to be similar to the granitoid-greenstone basement of the Pilbara Craton, and a volumetrically lesser source from banded iron formation (BIF), and volcanic and sedimentary rocks of the Mount Bruce Supergroup. Felsic volcanic detritus, likely to have been derived from the underlying Woongarra Rhyolite, is most common in the lower Turee Creek Group and becomes increasingly less common upsection, although it is reworked from underlying formations. Chert and BIF clasts appear first in the upper Turee Creek Group, and are important components of the most proximal alluvial fan to braided stream deposits in the upper Turee Creek and lower Wyloo Groups. Grains of microplaty haematite, identical in appearance to microplaty haematite in the giant Hamersley iron-ore deposits, appear in conglomeratic units from the base of the Beasley River Quartzite upwards. These fluvial sediments were derived from the south and record erosion of ore-grade deposits from the uplifted Ophthalmia fold belt during lower Wyloo Group times.