The structure of the Palaeozoic sediments of the Mossman and Cooktown areas, North Queensland

Abstract

The Palaeozoic geosynclinal sediments of the Mossman and Cooktown areas consist of shallow‐water shelf sediments in the west, subdivided into two formations, overlain by a thick pile of deep‐water turbidites occupying the remainder of the Palaeozoic outcrop and constituting one formation. The sediments have been strongly folded in four phases, and are now characterised by widespread steep dips and steeply plunging folds. The first folds (B₁) were acute folds with steeply dipping axial planes striking north or northnorthwest, and with almost horizontal axes. Broad open B₂ folds, with steep axial planes of various trends, were superimposed on the steeply dipping beds of the B₁ folds. Few minor structures are associated with either of these phases of folding. The third phase of folding (B₃) resulted in tight, almost isoclinal, folds with steep axial planes trending between north and northwest, and with steep plunges. These folds were accompanied by a strongly developed axial plane slaty cleavage which, in many areas, led to a progressive transposition of the original sedimentary bedding into a new lamination. The final period of folding (B₄) was a minor one, affecting only a limited area in which the slaty cleavage of the B₃ folds is the dominant planar element. The cleavage is folded into sharp‐angled chevron folds with almost horizontal axes and gently dipping axial planes.