Oligocene unconformities and nodular phosphate — hardground horizons in western Southland and northern West Coast

Abstract

Oligocene strata generally occur as paraconformable or condensed sequences in the New Zealand region. Descriptions are given of well-exposed sections through Oligocene unconformities in the Waiau River (western Southland) and at Whitecliffs, Buller River (northern West Coast). In both cases the unconformities are preserved in sediment sequences of inferred basin-margin origin with respect to contemporaneous, Oligo-Miocene, fault-controlled basins. At Whitecliffs, an early Oligocene angular unconformity separates the regionally transgressive Maruia Group below from the more locally transgressive, mid-late Oligocene Cobden Group (Whitecliffs Formation) above; abundant fresh detritus from the Maruia Group occurs in breccias associated with the Cobden Group, which is separated from the overlying early Miocene Blue Bottom Group (Inangahua Formation) by a further unconformity, marked by a condensed sequence shellbed with rolled, phosphatized and glauconitized macrofossils and abundant pelagic microfauna. The shellbed has a hardground contact at its base, resulting in the incorporation of phosphatized intraclasts of Cobden Group limestone into the nodular phosphate shellbed. A similar nodular phosphate shellbed marks the contact between the Tunnel Burn limestone or Point Burn sandstone and the Garden Point mudstone in the Waiau River section in Southland, though diagenesis, particularly solution of the top of the limestone, has obscured some sedimentary detail. The phosphate present at both localities is in the form of shell replacements and mineral cements. The phosphatic nodular layers are difficult to data accurately, but probably both the Westland and Southland occurrences are mid-late Oligocene (Ld-Lw) and therefore of similar age to the Marshall Paraconformity. A type locality for the Marshall Paraconformity, at Squire Farm, south Canterbury, is proposed in an appendix.