In a time of geological upheaval, Zealandia hardly shows its face
The Silurian has a special place in the history of geology, for it was the recognition of the British Silurian rocks by the geologist, Sir Robert Murchison, in 1830, that led to the idea of the geological timescale. In New Zealand, however, the Silurian has left relatively little mark on the geology or the landscape.
From evidence elsewhere, it was clearly a time of considerable upheaval. It started (or the Ordovician ended) with one of the largest extinction events to affect the planet, during which about a third of all brachiopod and bryozoan families, along with many conodonts, trilobites, and graptolites, were lost. After this, a more impoverished fauna and flora slowly developed. In the northern hemisphere, the collision of the proto-Europea and North American continents resulted in massive mountain-building (the Caledonian Orogeny) and tectonic activity. Rapid sea-level fluctuations also occurred.
What was happening to Zealandia during these events is less clear - so much so that most geology texts deal with the Ordovician and Silurian together. In most places the Silurian is simply a gap, giving an unconformity in the geological sequence. The Nelson area, however, boasts a large proportion of the Silurian rocks that do exist, and this is therefore one of the few parts of the country where it is possible to catch a brief glimpse of the elusive geology from this time.
Exactly what is Silurian is nevertheless not entirely certain, for there is no distinct boundary dividing it from the preceding era (surprising, given the mass extinction that was going on at the time), and dateable rocks are sparse. In fact, only one formation can be attributed to the Silurian with any degree of confidence: the Hailes Quartzite, which is part of the Takaka Terrane. As the name implies, it is composed predominantly of quartzite - black or grey in colour - but it also contains layers and lenses of sandy shale, slate and quartzose sandstone. Fossils are rare, but brachiopods, coral and crinoid fragments have been found in a calcareous lens near the top of the formation, and similarities with fossils found elsewhere have suggested a mid-Silurian date. Chemical analysis of the materials suggests that deposition occurred in much the same context as the late Ordovician rocks, with the accumulation of detrital sediments from a nearby land mass, and little or no direct volcanic influence. Subsequently, the rocks must have been heavily metamorphosed, perhaps as a consequence of igneous intrusion during the Devonian.
Other rocks of possible Silurian age are found in what has in the past been called the Ellis Formation. This has been mapped in association with rocks of Devonian age in several areas beside the Baton and Skeet rivers. It comprises quartzose sandstones and siltstones with rare fossils, including occasional trilobites. Its age, however, remains uncertain. In the Baton River it underlies, and grades into, mudstones of the Baton Formation, which are demonstrably Devonian in age. The Ellis Formation may thus be Silurian, as proposed by earlier studies, or early Devonian (or transitional between the two) as proposed in later studies by Bradshaw.
Between the Ellis Formation and the Devonian Baton River Formation, a conglomerate is also found. This has previously been interpreted as evidence of a major tectonic event, triggering erosion of coarse debris from the faulted hillsides. Again, however, the age and interpretation of the conglomerate have been disputed, and it may be no more than a local landslide deposit, dating from the early Devonian.
Access The classic locality to see the Hailes Quartzite is at Hailes Knob (NZTM: 1584440, 5451066). Access is not easy, however, and the best approach is probably via Waitui Road (from Upper Takaka) and the track that this becomes (a publicly accessible paper road) and then up through the conservation land to the ridge. Outcrops might also be visible in an area of similarly aged rock mapped along the Takaka River, which can be accessed via Cobb Valley Road (NZTM: 1582537, 5456384).
The Ellis Formation is accessible by following the tracks at the end of Baton River Road, either along the Baton River, for ca. 5km to the junction of the White Creek and beyond (NZTM: 1570783, 5428703) or along the Ellis River to the junction with Murray Stream (NZTM: 1574840, 5429818). Possible outcrops might also occur along the Wangapeka River, near the junction with Chummie Creek (NZTM: 1566655, 5413302)