Stratigraphy and sedimentology of conglomerates in the Kidnappers Group, Hawke’s Bay

Abstract

The Middle to Late Pleistocene Kidnappers Group comprises 400 m of diverse lithologies deposited in fluvial, marginal marine and shallow marine environments. The stratigraphy and sedimentology of the greywacke conglomerates, the dominant lithology in the group, is investigated, in part using three new computer programmes. Five environmentally sensitive lithologies are recognised: Lithotype 1 - Imbricated, inverse to normally graded conglomerate; Lithotype 2 - Planar cross-stratified conglomerate; Lithotype 3 - Thin, massive to stratified conglomerate; Lithotype 4 - Planar cross-stratified, shelly, sandy conglomerate; Lithotype 5 - Thin, massive to low-angle cross-stratified, shelly conglomerate. On the basis of cross-stratification types, fossil content and the shapes, orientation fabrics and grain-size analyses of clasts, Lithotypes 1, 2 and 3 have a braided fluvial origin and Lithotypes 4 and 5 a shallow marine origin. Differences between Lithotypes 1, 2 and 3 identify a proximal-distal sequence within the braided fluvial environment. Lithotype 1 is interpreted as proximal channel and longitudinal bar facies, Lithotype 2 as mid-reach transverse and longitudinal/diagonal bar facies, and Lithotype 3 as channel and topstratum overbank deposits in the distal reaches of a braided river system. From the vertical succession of conglomeratic facies and marine fossiliferous units, a chronology of submergence and emergence events in the Kidnappers Group has been established. This succession is dominantly controlled by glacio-eustatic fluctuations in sea level which agree in frequency and period with sea level fluctuations derived from deep sea core data from 0.5 to 0.1 m.y.B.P. Tectonism has also influenced the supply of sediment to the group and reinforced the magnitude of the submergence and emergence events.