The Tongaporutuan sedimentation in Central Hawke's Bay

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Abstract
The Mesozoic and Tertiary strata of Hawke's Bay include many sequences of vertically graded, banded sediments, some of which are contemporaneous with sequences of massive sediments deposited in nearby localities. The distribution of the thickness of sediments in Central Hawke's Bay, indicates that a trough-like basin existed in the upper Miocene. Into this basin a thick sequence of banded' sediments was deposited, while at the same time a thin sequence of unstratified beds was laid down in the area surrounding the basin. An investigat10n of the grain sizes in the basin revealed a decrease in grain size in a westerly direction. The sediments were brought in from the east, presumably across a bar which is thought to have separated the basin from the open ocean. Kuenen and Migliorini's theory that graded sediments are deposited by turbidity currents cannot be applied satisfactorily. The banded sequences of the upper Miocene and their massive equivalents show that these beds must have developed by a different mechanism. Both the above mentioned basin and the bar subsided irregularly and intermittently, the basin subsiding more than the bar. As a result, the relation between bar-depth and basin-depth varied continually. Consequently there were changes in the speed ,and strength of currents across the bar. Grain size and the amount of sediment carried into the basin by these currents changed rapidly, causing banded sediments with vertical grading. The deposition of sediment continued in this way until the incoming current lost its power to transport coarse material. With the gradually diminishing carrying capacity cf the incoming currents, increasingly finer sediments were brought in until current flow ceased almost cntirely and only muds were laid down. With subsequent down-ward movement coarse material was 3.gain carried in. The system reflects an immediate lithologIcal response to tectonic movement. It is not suggested, however, that all occurrences of banded sediments can be explained in this manner, but from what has been observed in banded sequences elsewhere in New Zealand, both in the Tertiary rocks and the Mesozoic and Paleozoic greywackes, it is considered to be a possibility.

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