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

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 Troug h, 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 asy mmetric, thinning northwards away from the Ophthalmian fold belt, and the t rough axis trends east-southeast parallel to the Ophthalmian folds. Multipl e 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 c leavage post-date deposition of the Beasley River Quartzite, a shallow-mari ne to fluvial sandstone that records an interval of tectonic quiescence dur ing basin evolution. Deposition of the Beasley River Quartzite was followed by renewed uplift of the southern hinterland, resulting in continued depos ition of fluvial siliciclastics into the foreland basin, which was eventual ly filled with the eruption of the Cheela Springs Basalt. The age of the Mc Grath Trough is constrained broadly between similar to 2.45 Ga, the age of the underlying Woongarra Rhyolite, and similar to 2.2 Ga, the age of the Ch eela Springs Basalt at the top of the succession. The sedimentary fill of t he McGrath Trough was derived mainly from the south, and becomes texturally and compositionally more mature upward, reflecting its recycled orogen pro venance. Two main sources are identified: a dominant cratonic source interp reted to be similar to the granitoid-greenstone basement of the Pilbara Cra ton, and a volumetrically lesser source from banded iron formation (BIF), a nd volcanic and sedimentary rocks of the Mount Bruce Supergroup. Felsic vol canic detritus, likely to have been derived from the underlying Woongarra R hyolite, is most common in the lower Turee Creek Group and becomes increasi ngly less common upsection, although it is reworked from underlying formati ons. Chert and BIF clasts appear first in the upper Turee Creek Group, and are important components of the most proximal alluvial fan to braided strea m deposits in the upper Turee Creek and lower Wyloo Groups. Grains of micro platy haematite, identical in appearance to microplaty haematite in the gia nt Hamersley iron-ore deposits, appear in conglomeratic units from the base of the Beasley River Quartzite upwards. These fluvial sediments were deriv ed from the south and record erosion of ore-grade deposits from the uplifte d Ophthalmia fold belt during lower Wyloo Group times. (C) 2000 Published b y Elsevier Science B.V. All rights reserved.