Marlborough Land District
A dominant feature of Marlborough geology is the presence of major active transcurrent faults which slice the region into blocks. The Wairau Fault, one of the branches of the Alpine Fault, divides Marlborough into two regions of contrasting geological structure. Northward lie the Upper Paleozoic greywackes and schists of the Richmond and Bryant Ranges and the Marlborough Sounds; southward lie basement greywackes and argillites of Triassic to Cretaceous age with younger sediments in tectonic depressions see section C of diagram 6). The fault pattern imposes an overall north-east-trending series of tilted mountain blocks, fault scarps, and faultangle valleys; drainage is parallel and to the northeast, the whole land district dipping toward Cook Strait, this being more pronounced north of the Wairau Fault.
The region north of the Wairau Fault is composed of greywackes and argillites of Permian and Carboniferous age, merging into a wide central belt of schists which are terminated abruptly to the south by the fault itself. The Alpine Fault, extending through the south-eastern part of Nelson district, in the Tophouse area, splits to give the Wairau Fault and (in Nelson district) the Eighty-Eight Fault and its continuation, the Whangamoa Fault, which bound the Richmond and Bryant Ranges to the north. The Marlborough Sounds, a complex system of embayments formed where these ranges dip into Cook Strait, have probably been formed by the drowning of entire valley systems during the worldwide rise of sea level which followed the melting of the Pleistocene ice. Tilting movements of the entire block between the Whangamoa and Wairau Faults may also be partly responsible see inset, diagram 8).
The region south of the Wairau Fault is split by further branches of the Alpine Fault, the major ones being the Awatere, Clarence, Kekerengu, and Hope-Kaikoura Faults. (These may be correlated with North Island faults; see inset, diagram 8.) The Mesozoic basement greywackes and argillites are thus faulted into a series of blocks, the most impressive of which are the Seaward and Inland Kaikoura Ranges, culminating in the peak Tapuaenuku (9,465 ft), of Jurassic and Cretaceous greywackes and argillites intruded by dyke swarms. See section C of diagram 6.) These geosynclinal deposits continued to accumulate throughout the Cretaceous period, so that a remarkably complete sequence of Cretaceous rocks is exposed, as, for instance, in the tectonic depression of the Clarence Valley, where may be seen the transition in later Cretaceous times to more massive, less indurated sandstones and mudstones of the Mata Series, which include flint beds, sulphurous mudstones, shales, and greensand. These are succeeded in many places by Tertiary marine sediments, the oldest being Paleocene flint beds, overlain by the Amuri limestone and bentonitic mudstones. This limestone forms dramatic white ridges which may be followed almost the length of the Clarence Valley, continuing around the “nose” of the seaward Kaikoura Range to form Ben More and other peaks. Amuri limestone also forms the resistant cliffs of the Kaikoura Peninsula and the small peninsula of Piripaua (Amuri Bluff), famous as the site of richly fossiliferous Upper Cretaceous strata, the source of some of New Zealand's large Cretaceous reptile fossils.
Volcanism is much in evidence in Marlborough. Intense volcanic activity took place late in the Rangitata Orogeny during mid-Cretaceous times when many dyke swarms and a few sills were intruded through the greywacke, particularly in the Inland Kaikoura Range (for example, Tapuaenuku). Terrestrial and marine basalt flows and tuffs were extruded on the surface, and the largest mass of these rocks forms Mount Lookout (5,933 ft) in the Upper Awatere Valley, towering over an area some 10 miles square of olivine-basalt lavas and interbedded tuffs up to 3,000 ft thick. The feeder dykes for these flows are well exposed.
The Kaikoura Orogeny (named from this region) began in Marlborough in the mid-Miocene. The Kaikoura Ranges and other Marlborough mountains were upthrust, and the Alpine Fault and its ramifications block-faulted the basement greywackes and their Cretaceo-Tertiary cover rocks, reaching a climax in the late Pliocene and Pleistocene. As the land rose, marine sedimentation became restricted to the depressions between rising mountain blocks and erosion stripped much of the cover of early Tertiary rocks, so that these are preserved only in fault-angle valleys. Blenheim stands on Quaternary alluvium which floors the Wairau Valley, a broad fault-angle depression bounded to the north by the Wairau Fault.
As may be seen in diagram 8, many active fault traces transect Marlborough and prominent scarplets are known along all of the major faults. Movements on these faults are continuing at the present day, giving many tremors and earthquakes. Precise annual surveys across some of them have disclosed horizontal displacements of over an inch per annum. The Wairau earthquake of 1848 caused subsidence of 5 ft in the lower Wairau Valley and was widely felt throughout the southern part of the North Island.
Marlborough's economic resources mainly consist of gravel, sand, and limestone, though alluvial gold, derived from the schists, has been won in the Wairau Valley.
by Geoffrey Conrad Shaw, B.SC., New Zealand Geological Survey, Lower Hutt and Graeme Roy Stevens, M.SC.(N.Z.), PH.D.(CANTAB.), Paleontologist, New Zealand Geological Survey, Lower Hutt.
- New Zealand Geological Survey Bulletin, n.s. 66, “The Geological Map of New Zealand 1:2,000,000”, Grindley, G. W., Harrington, H. J., Wood, B. L. (1959)
- Geomorphology, seventh edition, Cotton, C. A. (1958)
- geological maps, geological and palaeontological bulletins issued by New Zealand Geological Survey, Department of Scientific and Industrial Research; geological and palaeontological articles in theNew Zealand Journal of Geology and Geophysics, New Zealand Journal of Science and Technology and Transactions of the Royal Society of New Zealand
- Tuatara 10 (2), “New Zealand Biogeography”, Fleming, C. A. (1962)
- Descriptive Atlas of New Zealand, McLintock, A. H. (ed.), 1959.