Stratigraphy of Triassic-Jurassic boundary sequences from the Kawhia coast and Awakino gorge, Murihiku Terrane, New Zealand

Abstract

We have examined the stable carbon isotope stratigraphy, bio- and litho-stratigraphy of the Upper Triassic (Otapirian) - Lower Jurassic (Aratauran) boundary strata from the Awakino gorge and Kawhia coast sections, Murihiku Terrane, North Island, New Zealand. Successive occurrences of species of the bivalve genus Otapiria, O. dissimilis, O. marshalli and O. aff. marshalli, and also of age diagnostic ammonites indicate that the Hettangian strata in both sections are very thin compared with the Rhaetian sequences. Lamination structures are well developed at the Tr-Jr (Rhaetian- Hettangian) boundary transition interval. The δ: 13 values of organic matter from siltstone and carbonate nodules in the sedimentary rocks range from -28.5 to -25.5%o and there is no correlation between δ: 13C and C/N ratios. The excursion patterns of δ: 13Corg from both sections are similar, showing a negative spike at the uppermost Rhaetian and a positive spike immediately below the Tr-Jr boundary. A long-lived positive shift is also recognized in the Awakino gorge section, which coincides in pattern with those reported in Tr-Jr boundary successions from the Queen Charlotte Islands, British Columbia, Canada. These results suggest that although magnitude and absolute values are slightly different, the integrated pattern of stable carbon isotope signatures over the Tr-Jr boundary could be a valuable tool for correlation, at least between sedimentary rocks from similar depositional settings in the Panthalassa.