Paleoenvironmental Significance of Chert in the Franciscan Formation of Western California: Discussion Concerning the Significance of Chert in Timor

Abstract

In his recent paper, Chipping (1971), in support of his argument that cherts may be developed in a particular setting relative to an ocean ridge and a subduction zone, discussed the mode of occurrence of chert in some Asian mountain belts. He referred to the occurrence of chert in Timor where, following Grunau (1965), he said it is an “important constituent” of a melange. Chipping went on to claim that there has been subduction of oceanic crust beneath continental crust in the Timor region, although he cited no evidence in support of this interpretation. This reply challenges both of these statements on the basis of field evidence that a relatively insignificant amount of chert is associated with the melange in Timor, and in the absence of any evidence to support his suggestion that subduction of oceanic crust below continental crust has occurred in the Timor region since the early Permian. Another model is proposed, which relates the occurrence of chert in Timor in part to lack of supply of coarse terrigenous detritus rather than to any physical barrier to turbidity currents as Chipping (1971) suggested. The present model proposes that Chert formed in Timor above a sedimentary sequence on continental crust close to the outer margin of the continental slope, and it involves the northward subduction of oceanic crust below oceanic crust away from the northern continental margin of Australia.