Stratigraphy of Tithonian (Ohauan-Puaroan ) marine beds near Port Waikato, New Zealand, and a redescription of Belemnopsis aucklandica (Hochstetter).

Abstract

Some 1500 m of marine Ohauan and Puaroan strata in the Port Waikato region of New Zealand are restudied in an attempt to resolve lithostratigraphic and biostratigraphic anomalies in published works. New correlations are proposed for some units of the Apotu Group. A lower 650 m siltstone, previously correlated entirely with Kinohaku Siltstone, is here regarded as including the interval Kinohaku Siltstone to Lower Puti Siltstone (new member). The Waiharakeke Conglomerate is not represented in the area by coarse sediments. A thin sandstone unit overlying the Lower Puti Siltstone and previously correlated with Waiharakeke Conglomerate is here correlated with the Ruakiwi Sandstone. The overlying 400 m siltstone, regarded previously as the entire Puti Siltstone, is here recognised as a new member, the Upper Puti Siltstone. The Coleman Conglomerate is much thicker in the region than mapped, and located higher in the sequence. Biostratigraphy is a key element in correlating the sequence with named units to the south. The sequence is mostly siltstone, but sandstone units provide some lithostratigraphic control. Belemnites are almost the only fossils found in the Late Ohauan and Early Puaroan part of the sequence and are important in the Late Puaroan. Formal belemnite zones, based in part on their stratigraphic distributions in other parts of southwest Auckland, are erected. Occasional ammonites provide tie points that correlate the succession with the standard stratigraphic column for the Tethyan realm. Belemnopsis aucklandica (Hochstetter) is redescribed and considered to be of full specific status as is its former subspecies, Belemnopsis trechmanni Stevens. Many pre-adult Belemnopsis aucklandica differ in form from adults, some have been previously identified in the field as Hibolithes. Some earlier identifications of belemnites from the Late Puaroan of Port Waikato are incorrect, most are now included in Belemnopsis aucklandica.