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Comparison of Rates of Sedimentation
The fossil Monotis richmondiana occurs at several different areas in the South Island, but in the North Island is known only from West Auckland. This fossil has a restricted upper. Triassic time range and it is reasonably certain that the Monotis-bearing beds represent the same time span in all parts of New Zealand.
Figure 1 shows all the known localities of this fossil, the schist belt, and the Alpine Fault. It is convenient to consider these fossil localities as being parts of three belts: a western belt, a central or alpine belt, and an eastern belt.
The western belt is represented by the abundantly fossiliferous beds at Southland, Nelson, and West Auckland. At these places the upper Triassic beds form part of a fairly well known lower Mesozoic sequence, which at Southland and West Auckland extends up into the Jurassic. The structure is relatively simple, the Mesozoic beds being folded into a major syncline, the axis of which is shown on the map.
At most places along this belt the Monotis beds are abundantly fossiliferous (Marwick, 1935, p. 300) and the shells form a large part of well-sorted medium sandstone shell beds. It seems likely that the shell beds were concentrated from less abundantly fossiliferous material, and that the average rate of subsidence was small. The thickness of the Monotis-bearing beds is different in different parts of the western belt. The minimum at Nelson is a hundred feet or so, the average about a thousand feet, and the maximum two thousand feet.
The alpine belt is close to the eastern margin of the schist and has a known length of about 200 miles, Monotis having been reported from Macauley River at the south (Speight, 1921) and from Lake Rotoiti at the north. The Trent River locality is within this belt, and is the only area in which anything is known of the structure. As already mentioned, the structure is complex and the full thickness of the Monotis-bearing beds is not known. Although the proved thickness is only 500 feet, the full thickness is certainly more, and probably as much as 5,000 feet. Younger beds have been eroded from the Trent River area, and the original thickness may have been even greater. Subsidence and deposition were evidently rapid along this belt.
Fossils have been collected from only a few places along the alpine belt. In part, this apparent scarcity of fossils is due to lack of detailed examination. Nevertheless, the rocks at many places are extremely well exposed in bare mountains and if fossils are abundant, they should not be hard to find. Gravels derived from this belt are widely distributed over the lowlands on both sides of the Alps. That no fragments of shell beds have been found in these gravels increases the probability that shell beds are either rare or absent in the Alps. The extreme rarity of Monotis at Trent River has already been mentioned, and the fossil is probably no more abundant at the other localities along the alpine belt. It has been mentioned that in the western belt Monotis is found mostly in sandstone. In the alpine belt it has been found only in mudstone. This difference may be due to the different rates at which mudstone and sandstone accumulated in the
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western and in the alpine belt. In the western belt the fossiliferous sandstone bands probably accumulated slowly. In the alpine belt subsidence was probably much faster with fewer interruptions, so that the sediment was rarely resorted and concentrated by currents. Under such circumstances the lighter muds would have been deposited slower than the heavier sands, and a foot of mudstone may represent a much greater interval of time than a foot of sandstone. Most of the shells are single valves which may have been transported into the alpine area, perhaps along the surface of the sea. Other things being equal, the abundance of such fossils in any particular bed would depend on the rate at which that bed was deposited. This may explain why they have been found in the slowly deposited mudstone and not in the more rapidly deposited sandstone.
The eastern Monotis belt is about forty miles east of the alpine belt. Monotis has been reported from Malvern Hills at the south end by Speight (1920, p. 106) and from Okuku River at the north end by McKay (1881, p. 100). It has recently been found by one of the authors (H. W. W.) at Lees Valley between these two localities. This belt has not been examined in detail and nothing is known of the structure or thickness of the Monotis zone. The rocks are similar to those of the alpine belt, and consist of a monotonous succession of extremely compacted sandstone and siltstone. Submarine volcanics similar to those at Trent River are not uncommon, and were used by McKay as evidence for correlation between the two areas. Associated with the volcanics at Malvern Hills and Okuku River are lenses of limestone with the fossil Monotis. At Lees Valley the fossil is contained in medium sandstone, slightly more compacted, but otherwise similar to the Monotis-bearing sandstone of the western belt.