Abstract

Cleavage is sporadically developed in Late Triassic redbeds of the Lepreau Formation in folds and in areas of uniform northwestward dip in the lower Lepreau River – Lepreau Harbour half-graben. The cleavage in shaly siltstone beds is a composite incipient fabric defined by patchily developed crenulated bedding laminae, pressure solution planes, and mineral orientation parallel to the cleavage direction; cleavage in sandstone beds is defined by alignment of submicroscopic cracks inferred from laboratory tests. The cleavage fabrics are subvertical trending northeast–southwest, and resulted from horizontal northwest–southeast compression. Closely spaced fractures subparallel to the cleavage fabric in both shaly siltstones and sandstones are interpreted as later structures formed during relaxation of tectonic stresses. The cleavage fabrics and fractures are also present in intraformational clasts of shaly siltstone and rare sandstone clasts within conglomerate beds. A distinct shape orientation of the shaly siltstone clasts parallel to their cleavage within relatively undeformed conglomerate is attributed to compression during folding of unconsolidated conglomerate beds. The mean cleavage orientation varies from coplanar to noncoplanar with fold axial surfaces. The folds and cleavage formed during lateral compression probably related to wrench faulting which postdates the formation of the Triassic grabens.

You do not currently have access to this article.