Abstract

The flysch succession of the Balangbaru Formation unconformably overlies a basement accretionary complex which is highly tectonised. The flysch itself is not internally deformed, but slightly tilted to the east. This suggests that by Upper Cretaceous times, when the flysch succession was deposited, Early Cretaceous subduction had ceased.

A facies analysis of the Balangbaru Formation has resulted in the recognition of six facies classes. A through F, on the basis of lithology, bed thickness and geometry, sand to shale ratio, grain size and internal sedimentary features. Facies association and vertical sequence analyses indicate that the flysch succession was deposited by sediment gravity flows, including high- and low- density turbidity currents, and debris flow processes in a submarine fan environment, ranging from lower bathyal to abyssal (below CCD) environments. In order to distinguish the historical development of the environment, the Balangbaru Formation is divided into three members, on the basis of lithostratigraphy and sequence. From bottom to top, these comprise: (i) the Allup Member, characterised by a chaotic fabric of debris flow deposits representing an inner fan facies association; (ii) the Panggalungan Member, characterised by 'distal turbidite' features representing an outer fan to basin plain facies association; (iii) the Bua Member, characterised by 'proximal turbidite' features representing a mid fan facies association.

Palaeocurrent data show bimodal-oblique with flows predominantly from the northwest to southeast and west to east, with subsidiary flows from northeast to southwest. The predominant current indicators suggest that the basin morphology may have been asymmetrical, being relatively steeper on the western side, since the currents were mainly derived from the northwest and west.

The composition of the sediment in the Balangbaru Formation shows progressive changes with time from the lower to upper part of the succession. Petrographic, heavy mineral and geochemical studies suggest that the sediments in the lower part were mainly derived from erosion of the basement accretionary complex, but the upper part was more influenced by a magmatic arc provenance.

The tectonic setting of the Balangbaru Formation is interpreted as a small fore-arc basin, on the trench slope. The basement complex, which is present in the Bantimala and Barru areas, was uplifted, by thrusting from significant depth, prior to the deposition of the Balangbaru Formation. Thus, it is acted as a barrier to prevent transport of volcaniclastic sediments into the basin, and as a source for the Allup Member.