Geology of Stephens Island and the age of Stephens formation

On Stephens Island, northernmost Marlborough Sounds, New Zealand, 1100 m of southeast-dipping sedimentary rocks, ranging from mudstone with limestone lenses to volcanogenic sandstone and conglomerate, comprise Stephens Formation, the youngest formation within the Maitai Group. The rocks form the most northerly exposure of the Nelson Regional Syncline; younging directions show that they are overturned and therefore are on the eastern limb of the syncline. They are well indurated with widespread metamorphic prehnite, pumpel1yite and lawsonite. No top or bottom to the formation is exposed. The formation is subdivided into Queens Beach, Incline, Takapourewa, and Titapua Members. The oldest rocks (Queens Beach Member) comprise 150 m of rounded cobble conglomerate which encloses angular blocks of coarse volcanogenic sandstone. One such block, 4 m long and up to 1.5 m thick, contains a diverse but indifferently preserved marine fossil assemblage dominated by brachiopods and pectinid bivalves of Late Permian age.If the fossiliferous sandstone and associated conglomerate formed penecontemporaneously, then the age of the conglomerate is Late Permian. However, the possibility of a younger (Triassic) age cannot be discounted. Late Quaternary deposits, including loess and windblown sand, occur locally.