Nelson Rock and Mineral Club
Useful previous references include:
Campbell, J.D. 1955 The Oretian stage of the New Zealand Triassic System. Transactions of the Royal Society of New Zealand 82 (5), 1033-1047.
Campbell, J.D. 1974 Biostratigraphy and structure of Richmond Group rocks in the Wairoa River-Mount Heslington area, Nelson. New Zealand Journal of Geology and Geophysics 17 (1), 41-62.
Grant-Mackie, J.A. New Zealand Warepan (Upper Triassic) sequences – Murihiku Supergroup of South Island. Journal of the Royal Society of New Zealand 14 (2), 175-206.
Most of the Triassic bedrocks at Highfield consist of fine-medium sandstones and siltstones, with smaller quantities of coarse sandstone, gritstone and conglomerate, and layers of mudstone. Some of the rocks are tuffaceous, and within the conglomerates occasional pebbles of granite can be found.
The rocks are heavily jointed, and bedding is usually indistinct; bedding structures, such as sole marks, ripples or evidence of graded bedding are scarce. Occasionally,however, variations in rock type can be identified and followed across the landscape. The example below shows a bed of pebbly conglomerate that can be traced along the foot of the outcrop shown in the accompanying image, for several hundred metres. This helps to identify the line of the bedding.
The faults in this area appear to have only a small influence on the landscape. This is probably mainly because they run along bedding planes and thus do not bring together rocks of markedly different character, which erode at different rates. For this reason also, tracing faults across the landscape is not easy.
Clues to the distribution of the faults can be found, however, in some of the more subtle features of the landscape. These include:
Alipunctifera kaihikuana, a distinctive brachiopod that is restricted to the Kaihikuan stage of the Triassic. It is notable for the small pimples (punctae) that occur on the cast (above) and appear as small perforations the shell when it is present. These show where tubes extended through the shell. The purpose of these tubes has been much debated, but the most convincing explanation is that they were for respiration.
Monotis cf richmondiana: a bivalve, this species is distinctive, and marks the Warepan stage. It is often found in rich shell beds, where it is almost the only species present. Several species (or sub-species) have been identified, distinguished in part by the number and shape of the ribs on the shell (most species have larger primary ribs interspersed with smaller secondary or tertiary ribs).
Manticula problematica: a bivalve and one of the most readily identifiable and iconic fossils found on the farm, which helps define rocks of Otamitan age. Most specimens are tear-drop shape, like the one above, though some are rounder in outline.A few, however, are grossly inflated, so that the bottom shell (the more curved one) bulges outward like a balloon. Why this happens is not clear. Our preferred suggestion is that it is a growth response when the animal was living in rapidly accreting environment - a way of keeping itself above the sediment.
Eoheteropora maorica. This interesting species was once classified as a bryozoa, but is now attributed to the anthozoa - a class of marine animals that includes sea anemones and soft and stony corals. It is rarely found, and occurs mainly in gritty sediments of Kaihikuan or Otamitan age. The image above is considerably magnified - the original specimen shown here is only a few millimetres across.