Significance of the early foliation at Bermagui, N.S.W., Australia

Abstract

The first-generation mesoscopic folds, F 1, in the Ordovician turbidites at Bermagui, N.S.W., Australia, are tight to locally isoclinal and have a convergently fanning axial-surface foliation, S 1, which is a differentiated crenulation cleavage. S 1 overprints an earlier foliation, herein called S ∗ . In psammites, S ∗ is a strong alignment of mineral grains and rough, micaceous folia, and in pelites S ∗ is a microbanding of quartzose and micaceous layers. Depending on rock type and position in the F 1 fold profile, S ∗ may be (1) parallel to bedding, (2) rotated out of parallelism with bedding by the S 1 cleavage and (3) truly cross-cutting bedding. In many places, S ∗ cuts across bedding demonstrating that it is a superimposed tectonic foliation and not simply an inherited depositional fabric. In outcrops south of Bermagui, S ∗ changes vergence with apparent congruence with F 1 folds. In other outcrops near Bermagui, however, S ∗ maintains a constant vergence around F 1 folds, the geometry indicating a pre- F 1 anticline to the east. Since there is no evidence for large-scale pre- F 1 folds, the various relations of S ∗ to S 1 and F 1 are best explained by the rarely described process of late-stage hinge nucleation during a single protracted F 1 folding sequence. Other possibilities such as fold-hinge migration and pre-fold imbricate stacking are discussed but considered less likely.