Okains Bay Sea Caves (Canterbury)
The geological history of Banks Peninsula began more than 200 million years ago, however, much of what we see today is the product of various episodes of volcanism commencing some 12 million years ago and continuing over a period of some 5 to 6 million years. Ongoing fluvial and marine erosion has contributed to formation of harbours, as well as the numerous bays along the Banks Peninsula coastline. Part of this erosion process has created a unique set of 7 sea caves in Okains Bay, only one of which is the subject of this earthcache. The erosion processes which formed the sea cave have provided a unique opportunity to observe the geological layers and erosion from within.
Walls of layered basaltic pahoehoe lava flows bound either side of Okains Bay beach, interspersed with both brecciated and jointed layers enabling sea cave occurrence. Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, think of it as like gravel trapped in concrete. It can be of sedimentary or igneous origin, with the igneous type having subtypes of volcanic or intrusive. The breccia we see here is igneous volcanic, whereby small pieces of the cooling lava flow were reincorporated into the new lava, forming a brecciated layer. Scoria is a dark volcanic rock that contains small holes or vesicles which form when gases that were dissolved in the magma come out of solution as it erupts creating bubbles in the molten rock, some of which are frozen in place as the rock cools and solidifies.
Joints in the rock create permeable space where water can infiltrate and erode the surrounding rock, with both brecciated and scoria layers creating the same space between clasts and within the rock matrix. The contact planes between these layers appear to be ideal for the formation of sea cave ceilings, because the change in strength from the breccia to overlying pahoehoe lava inhibits marine erosion processes. The cavity ceiling cuts through mostly pahoehoe layers and also cuts into overlying breccia, indicating that the breccia is weaker than the pahoehoe and the water energy needed to erode pahoehoe will also erode breccia, but not vice versa (Countryman, M.).
Recent studies used coastal erosion features such as sea caves to show that Banks Peninsula did not undergo differential subsidence in the past 120,000 years (Bal, A.A). Based on the elevation of the caves and shore platforms, and the distance of the sea cave from sea level at high tide, is unlikely that present-day sea level coastal processes contributed to the major morphological features of the caves that are 2 to 5 m above MSL (mean sea level). During the Last Interglacial period, sea levels were at 6 m above present-day MSL Following previous assumptions of the tectonic stability of Banks Peninsula, it appears that the coastal features described here can be attributed to the high sea level of 120,000 years ago or earlier (Bal, A.A). .
Questions:
1. On the rear wall of the sea cave toward the left , there is a very distinctive series of rock layers visible. From bottom to top, how many distinct layers you see? Describe in detail the distinctive texture and colour of each layer, and the differences between them.
2. Along the right wall of the sea cave, there are how many smaller cave openings?
3. Back outside the sea cave, have a look at the present sea level. Is there evidence that the sea cave is subject to ongoing wave erosion?
It is expected that EACH TEAM wishing to log a find will SUBMIT THEIR OWN ANSWERS to the CO. Once your answers have been submitted, you are free to log your find. If there are problems with your answers you will be contacted. If you fail to submit answers, your log may be deleted.
References:
1. Bal, A.A. Sea caves, relic shore and rock platforms - Evidence for the tectonic stability of Banks Peninsula, New Zealand, New Zealand Journal of Geology and Geophysics, 40(3), 1997, pp. 299-305
2. Countryman, M. Formation of Okains Bay sea caves – original features and development: Eastern Banks Peninsula, Department of Geological Sciences, University of Canterbury, Christchurch, NZ.
3. Boffa Miskell Ltd. Banks Peninsula Landscape Study 2007
