OIL SEEPAGES AT MILFORD

Southland Times, Issue 23587, 15 August 1938, Page 11

OIL SEEPAGES AT MILFORD

Appearance At Minor

Faults

A paper on “Some Geological Notes on the Coastal Strip between Big Bay and Milford Sound” was given by Mr J. Healy at a meeting of the Geological Section of the Wellington Philosophical Society. The paper was based on fieldwork done by J. Healy and R. W. Willett, of the New Zealand Geological Survey, in February of this year, when a geological examination was made in the vicinity of reported oil seepages. The inner margins of Big Bay and Martin’s Bay comprise long stretches of sandy beach separated by rocky headlands, and into the bays flow the Awarua and Hollyford rivers respectively. The larger of these, the Hollyford, flows for some distance through Lake MeKerrow and emerges from it three or four miles from the coast. South of Martin’s Bay is a succession of steep limestone cliffs and small beaches to Madagascar Beach, where the limestone runs out to sea. The coastline is impassable round several of these bulffs, and the party had to climb up through the bush to get round them. Bordering the coast is a plateau which reaches a maximum height of 1900 feet above sea-level, and which at Big Bay slopes gradually down towards the sea to 300 or 400 feet. ® s ®“ where wave-action has cliffed the plateau back for large distances until its seaward margin is much higher, though south of Martin’s Bay are numerous flat-topped spurs of 600 to 800 feet in height which suggest the presence of a bench carved at that . height in the higher one. The plateau is covered with gravels and boulders up to a diameter of 50 feet. These are of glacial origin and come from further inland. The plateau has been well dissected and many of _ the boulders concentrated in the gullies and along the coast by stream and wave action. . Lake McKerrow, from its U-shaped cross section and the presence of hanging valleys throughout its length, was formerly a glacial valley and was at one time an arm of the sea similar to the sounds further south. Beach sands containing salt-water shells of recent origin are found at the head overlain by gravels deposited by the Hollyford River. Gravel brought down by the streams draining the loosely-capped coastal plateau, and a large supply of waste from the rapidly-cliffed headlands, blocked up the mouth of the sound to form the lake now known as McKerrow. . The evidence along the coast indicates the most recent movement to have been one of uplift to the extent of five to ten feet. This occurred in comparatively recent times. The oldest rocks of the district are a series of quartz-mica sub-schists and schists. They vary from an almost pure sandstone at Madagascar Beach to finely-corrugated mica-schist up Jerusalem Creek at Martin’s Bay, and the schistosity apparently increases inland. Between Martin’s and Big Bays they are much disturbed.

Seepages of oil at Madagascar Beach and Wolf river wei'e examined and samples obtained. The structure of the rocks and their unsuitability as reservoir beds preclude the existence of an oilfield of economic importance, added to which the seepages occur close to the schist basement. The explanation offered for the presence of the oil is that certain carbonaceous strata, known to exist in surrounding localities but not actually located in this district, in undergoing crushing movements yielded small quantities of oil which is now escaping to the surface along minor fault cracks. The seepages actually occur along just such cracks. Gold has been found in small quantities on several beaches, where it has been concentrated from widespread sporadic occurrence in the loose covering of the coastal plateau by the combined action of streams and waves into beach leads near the mouths of the streams. Lodes similar to those at Glenorchy were found, but no trace of scheelite or other minerals of economic value could be determined.