Continental margin origin for Cretaceous radiolarian cherts in Western Timor

Abstract

In Timor (Fig. 1) widespread outcrops of Cretaceous cherts belonging to the Wai Bua and Cretaceous Kolbano facies have been interpreted as deep-sea deposits that probably accumulated on the Australian continental rise¹. I consider here another but less well known group of Cretaceous cherts exposed in Western Timor at the base of the Palelo Group, which outcrops in mountain massifs with metamorphic rocks of the Mutis Complex. Although the observed contacts between the two units are faulted^2,3, the Palelo Group contains fragments of metamorphic rocks similar to those of the Mutis Complex^4,5, suggesting an original unconformable relationship. Geochemical data^6,7 from metasedimentary gneisses in the Boi Massif suggest that metamorphism occurred at pressures and temperatures likely to prevail deep in the continental crust. I suggest that the Cretaceous radiolarian cherts were deposited on a continental margin as the first sediments of a forearc basin. This is the first reported occurrence of radiolarian cherts deposited in this tectonic–sedimentary environment, and contrasts strongly with other Cretaceous radiolarian cherts exposed on the island. The evidence is crucial to understanding the tectonic evolution of the region because it is clear that the two groups of cherts were derived from different sides of the continent–island arc collision zone which makes up the Banda Arcs. The Palelo Group and the rocks it is associated with were derived from the South-east Asian side, which has had an active margin since at least the early Cretaceous.