Marshall Unconformity

As you are looking out to the sea, you will notice rock sticking up at an angle on the left, and rock at a lower level on the right. The rock on the left (SE) is the younger Spy Glass Formation and on the right (NW) is the older Amuri Limestone. In between them is the Marshall Unconformity or Paraconformity as it is also referred to. An unconformity is a gap in the rock record formed by erosion of part of a sequence, followed by further deposition on top of the erosion surface . Erosion occurs by natural processes (e.g. wind/water) resulting in this ‘break in time’ where no deposition occurred. A paraconfomity is a specific type of unconformity where there is no obvious evidence for the break in time; the layers above and below are often parallel, and the presence of a time gap is only revealed by other evidence such as the absence of fossils.

The older rock (Amuri limestone) has formed on a continental slope, under the sea, during the Eocene Epoch (34-55 million years ago). During this time, Zealandia had separated from the Gondwana supercontinent and New Zealand was in a time of marine transgression (sea level rise); in fact, most of New Zealand was under water. This happened as the land was subsiding and formed the Amuri Limestone (limestone is formed from the remains of dead marine organisms containing calcium carbonate in their shells or skeletons).

Marshall Unconformity: During the early Oligocene there was a period of regional erosion for approx. 2-4 million years forming a significant unconformity across the top of the Amuri Limestone. Organisms also contributed to this and you will see bioturbation (burrows) in the Amuri Limestone, as well as large phosphate nodules (up to 3 cm diameter) in the base of the overlying beds. It is thought, however still debated, that this drop in sea level and exposure of land was due to the 'Eocene Terminal Event'. During this time, global temperatures cooled as there was a decrease in CO2 in the atmosphere and formed ice sheets on East Antarctica reducing sea level.

The younger rock (Spy Glass formation) forms part of the Motunau group and formed during the mid to late Oligocene. It is also limestone which formed on the outer continental shelf up to depths of 1500m. If you were to look at this rock through a microscope, you would see it contains foraminifera which are tiny single celled marine organisms - these are very useful to geologists to work out the environment in which some rocks were formed.

Both rock formations would have been deposited horizontally, however due to more recent tectonic activity, these have been folded.

See if you can put your finger on the unconformity. Follow along it with your eye to decide if any layers of Amuri Limestone have been eroded at a slight angle or not, to decide if this is an angular unconformity or a paraconformity? - what do you think?

Within the Amuri limestone you can see bioturbation (burrows and disturbed sediments caused by animal organisms). At this location, these are visible closer to the sea and appear pink as they have been filled in with sand. See photo for guidance on this.

You can also spot phosphate nodules in the Spy Glass formation formed originally on the continental shelf. These are 3 cm in diameter and are much darker than the surrounding rock. Again, see photo to help spot these.

Look carefully at the layers in the rock - can you spot folding or other structures?

Why do you think the Spy Glass Formation stands out as a prominent overhang along the outcrop?