PLIO-PLEISTOCENE CYCLOTHEMIC LAYER CAKE STRATIGRAPHY IN EASTERN NORTH ISLAND, NEW ZEALAND

Plio-Pleistocene sedimentary rocks crop out continuously over the 450 km2, tectonically undeformed, Tangoio black in eastern Hawke’s Bay, New Zealand. Strata within the Tangoio block are cyclothemic and dominantly shallow-marine. The sedimentary succession consists of two sandstone-mudstone and three temperate limestone-mudstone couplets; all were deposited within a gradually subsiding forearc basin during recurring sea-level changes. There are considerable lateral facies changes within each lithology that were ultimately controlled by uplift west of the study area. Nevertheless, each couplet is regionally correlative across the study area resulting in classic “layer-cake” stratigraphy. Each cyclothem is also a depositional sequence, and the sedimentary facies can be grouped into conventional transgressive (TST), highstand (HST), regressive (RST) and lowstand (LST) systems tracts.

The layer-cake disposition of the strata in the Tangoio block also controlled carbonate diagenesis within the sedimentary sequence. Alternating sequences of porous and non-porous formations developed a stacked sequence of confined aquifers that forced diagenesis to operate laterally rather than vertically. Multiple stages of cementation (limited marine, but extensive meteoric phases), and skeletal dissolution (primarily of aragonitic faunas) are recognized within these strata. Extensive cementation within the limestone portions of cyclothems produced well exposed and prominent scarps across the Tangoio block. Mudstones remain unlithified and are exposed in slumps between scarps .