Geology of Port Craig, Western Southland, New Zealand

McIntyre, Christine (Christine Mary)

An area of coastal exposure of approximately 2 km in length in the Port Craig area on the western side of the Waiau basin was mapped in detail. The geology of this area is dominated by the upper Miocene (upper TongaporutuanKapitean) Port Craig Formation. In the study area the Port Craig Formation unconformably overlies crystalline basement rock with affinities with eastern Fiordland basement and older Cenozoic sediments; the Hump Ridge Formation and the Blackmount Formation. The depositional environment deepens from the time of deposition of the Hump Ridge Formation which is seen in the study area as a nonmarine coarse sandstone or a shallow marine fine sandstone, to the Blackmount Formation turbidite sequence inferred to have been deposited in a outer shelf setting. The unconformity between these older rocks and the Port Craig Formation represents a period of either non-deposition or erosion.
Sedimentation recommences in the late Miocene with the deposition of the shallow marine Port Craig Formation. Movement along the Alpine Fault caused a t~ans_E~essi~~~l regime by the e~<:I _of _th~_ Oligocene, which resulted in thrusting of Hump Ridge over the Waitutu subbasin. This uplift was possibly responsible for shallowing on the western edge of the Waiau basin and the deposition of the Port Craig Formation. The Port Craig Formation is a coarse calcareous unit deposited in a high energy, shallow marine environment. The detritus found within the Port Craig Formation suggests the Hump Ridge area was major source of detritus for the Port Craig Formation. The Port Craig Formation shows evidence of deepening or a lower energy depositional environment.
Later tectonic activity has affected the Port Craig Formation in the form of faulting and small scale folding. The study area has been tilted as part of the ~and Hill Point- Port Craig basement block giving the sediments a relatively consistent 3QO easterly dip. The faulting within the study area is on a relatively small scale and includes a component of sinistral strike slip movement.

59 leaves (some folded) : col. ill., maps ; 30 cm.

1993McIntyre_CM

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