Depth of Burial of Monotis Belts
Judging from the youngest beds that have been preserved, the lower Mesozoic geosyncline continued to grow until the end of the Jurassic. It was then compressed and the beds folded. Erosion followed, and in most places a great thickness of the upper beds was eroded. Cretaceous and Tertiary beds were then deposited on the eroded surface cut across the upturned edges of the older beds. These younger beds have since been elevated and eroded so that the contact with the older beds below them is exposed at many places. Although the degree of compaction of the base of these younger beds is different at different places, at no
place is it as great as that of the older beds beneath. Evidently the thickness of the old rocks that had been eroded was greater in all places than that of the younger beds that were deposited and in part eroded later. Consequently these younger beds can have had no influence on the degree of compaction of the older, and if compaction and the associated features of low rank metamorphism are due to depth of burial, it is possible to estimate the depth of burial of the upper Triassic beds from their degree of compaction. As deposition in the geosyncline appears to have come to an end at the end of the Jurassic, the degree of compaction of the upper Triassic beds must be due to the thickness of the Jurassic, of which in most areas no trace now remains.
In the western belt the upper Triassic beds are only moderately compacted. They contain bituminous coal at Nelson and probably at Southland. They are not intersected by quartz veins. The degree of alteration along the alpine belt is considerably greater. Immediately underlying beds contain slates, and the upper Triassic beds are intersected by quartz veins. In the eastern belt the degree of compaction appears to be slightly less than in the alpine belt, but somewhat more than in the western belt. Fragments of coal from several localities have been analysed and shown to be of low volatile bituminous or of anthracite rank. The greater metamorphism of the alpine Triassic suggests that the alpine area was covered by a greater thickness of Jurassic beds than either the western or eastern areas, and hence that it continued to subside most rapidly during the Jurassic.