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SPECIAL REPORTS.

1. Dargaville Subdivision. (By 11. T. Ferrar.) Introduction. The systematic survey of North Auckland has been continued during the past season, which extended from the 12th October, 1922, until the 20th May, 1923. During this period an area of some 484 square miles was geologically mapped, and an excursion to Silverdale was made in order to report, upon an alleged occurrence of mineral oil in that locality (see Special Report No. 4). During various portions of the season assistance was rendered by Mr. W. H. Cropp, A.0.5.M., Field Assistant; Mr. E. 0. Macpherson, Assistant Geologist; Mr. P. T. Cox, 8.A., New Zealand University Senior Scholar ; and Mr. H. E. Fyfe, Otago University. A strip of country extending from Bream Tail on the east coast to Kaipara North Head on the west coast was mapped in detail on a scale of 20 chains to the inch. This area, which is immediately to the south of the strip surveyed last year, includes the survey districts of Mangawai, Waipu, Matakohe, Tokatoka, Kopuru, Te Kuri, North Head, part of Hukatere, and part of Okaka, a closely settled but hitherto geologically unexplored district. Physiography. The district mapped during the past season displays five types of country —namely: (I) tilted fault-blocks of greywacke which in part have been stripped of their covering of younger sedimentary rocks ; these fault-blocks rise to heights of 1000 ft. or so, and are confined to the neighbourhood of Waipu : (2) hills of dacite and andesite which cover a belt of country stretching from Mangawai to Maungaturoto and rise to heights of 800 ft. and 900 ft. above sea-level; the conspicuous rocky peaks of Tokatoka and Maungarahu also belong to this typo : (3) a dissected peneplain of Notocene limestones, claystones, and sandstones, with an average height of 500 ft. above sea-level, which occupies the greater part of Matakohe and Tokatoka survey districts : (4) a belt of consolidated sand-dunes, 400 ft. to 500 ft. high, which occupies the peninsula extending south-westward from Dargaville to Kaipara North Head ; on the sea-board of this peninsula a fringe of moving dunes, 20 ft. to 30 ft. high, expands to form the North Spit at the entrance to Kaipara Harbour : (5) the recent alluvial flats of the Wairoa, Waipu, and Mangawai estuaries and of the inland valleys. With the exception of the Waipu and Mangawai, all the larger streams of the area flow to Kaipara Harbour, by way of its numerous ramifications. Some of the streams, notably the Piroa, have maintained their courses across uplifted fault-blocks ; others, such as the Pukekaroro, have cut their way through ranges of volcanic rocks ; and others again, such as the Mangonui, a tributary of the Northern Wairoa, have entrenched themselves in the peneplain of Notocene sediments. General Geology. The most comprehensive report upon the district is that by S. H. Cox in " Reports of Geological Explorations during 1879-80," No. 13, 1881. In 1885 (R.G.E. No. 17) and again in 1887 (R.G.E. No. 18) Park examined portions of the area with a view to locate coal which was supposed to be present in the district. He rightly pointed out that if coal existed it would crop out on the surface owing to the high angles of dip of the strata, and recommended surface prospecting rather than boring. Since 1885 no geological map of the district has been published. The account of North Auckland, stratigraphy, as outlined in the Annual Report for 1920, still holds good for the area under review, although uncertainty exists as to the precise age of some of the strata. The oldest rocks found in the district are the greywackes belonging to the Waipapa Formation, which is- regarded as of Trias-Jura age. Such rocks form the hill country surrounding the Waipu basin, and they underlie the central portion of the area, but westward are exposed only at Whakahara, one mile move the wharf at Tokatoka, on the left or east bank of the Northern Wairoa. The argillaceous limestones and claystones of the Onerahi Formation have not yet yielded any internal evidence as to their age. These beds are correlated with the Waiparan of other parts of New Zealand, and are tentatively regarded as Cretaceous. The formation covers large areas in the Waipu, Matakohe, and Tokatoka survey districts, the claystones giving place to argillaceous limestones towards their southern boundaries. The Whangarei Formation is represented by brown sandstone and, locally, by crystalline limestone. The formation is correlated with the Oamaruian of the South Island, the sandstone being provisionally placed in the Waiarekan and the limestone in the Ototaran stage. In the eastern portions of the district the sandstone rests upon the denuded surfaces of the Waipapa greywackes, in the northern and western portions it overlies the argillaceous Onerahi Formation, and in the southern and south-western parts of the district it covers but small areas. Coaly partings are fairly frequent in the sandstone, and near Waipu a thin coal-seam of no commercial value occurs in a sandstone outlier resting on greywacke. Beds of crystalline limestone of small extent are found scattered over the whole area. The long tongue of land between the Wairoa River and the Tasman Sea, extending from Dargaville on the north' to Pouto Pilot-station on the south, consists of consolidated and unconsolidated windformed sandhills interrupted by swampy valleys. A similar though smaller belt of dune-sands occurs in Mangawai Survey District on the east coast. The sand-dunes are of Pleistocene and Recent age. The Horehore and Raupo flats are alluvial plains of recent origin.

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