ABSTRACT

One hundred and eighty five feet of marine calcareous siltstones and fine calcareous sandstones (the Puriri Formation) contain an intercalation 25 feet thick composed of andesitic sand turbidites alternating with silty mudstones. At the base of many of the turbidites there are very fine examples of transverse ripples in mud, bent over and streaked out in the direction of current flow. Long axes of sand grains in the turbidites are oriented parallel to the length of ripples, and hence transverse to the turbidity current flow. Above the only layer in which individual ripples are streaked out in opposing directions, the grain orientation is perpendicular to the length of the ripples, suggesting that these may be longitudinal ripples. Other primary structures consist of high angle thrust ype faults with displacements up to 18 inches, which apparently were caused by drag exerted by turbidity currents; the name "drag thrusts" is proposed for them.

The Puriri Formation has a very restricted distribution, and is an early member of the strongly transgressive Waitemata Group of shallow water Miocene marine sediments. The turbidity currents were fed entirely by sand eroded from unweathered andesitic rocks, and appear to have flowed from both east and west into a very small, and presumably shallow, basin.