A paleoenvironmental interpretation of the Greycliffs Formation, Southern Wairarapa

Plio-Pleistocene marine sedimentary rocks in the Southern Wairarapa record water depth changes related to the complex interplay between tectonics and glacioeustatic sea level cycles (Hobbs, 2020; Nowland, 2011; Vella & Briggs, 1971). We focus on a 120m thick sequence of coarsening upwards siltstone and limestone near Martinborough. This sequence was deposited within a seaway (Ruataniwha Strait) that existed in the Hikurangi forearc basin at the onset of cooling global climate and the transition to the Quaternary. The focus of this study is the Greycliffs Formation that stratigraphically underlies the widespread Pukenui Limestone. The base of the Pukenui Limestone is used to mark the base of the Nukumaruan Stage, and is correlated with other limestones in the region based largely on the abundance of the cold-water scallop Psychrochlamys delicatula. This study aims to investigate the depositional environment of the Greycliffs Formation and reconstruct the paleogeographic setting of the southern part of the Ruataniwha Strait, using lithostratigraphy and palaeontological analysis. Furthermore, the timing and significance of P. delicatula occurrences in the region is investigated. Detailed lithofacies and biofacies analysis shows an overall coarsening up section through the Greycliffs Formation, reflecting slow tectonic shallowing of the Ruataniwha Strait. Smaller scale water depth changes between middle shelf and shoreface are superimposed on this record. The variations reflect the complex interplay of global sea level changes and the influence of local paleogeographic changes. The paleogeographic setting was generally open to the Ruataniwha Strait, tidal currents and south Pacific however a temporary barred embayment environment is indicated by foraminifera assemblage and a disproportionately fine period of siltstone deposition on the inner shelf.

New strontium isotope dates (87Sr/86Sr) from P. Delicatula shells show the base of the Pukenui Limestone is 2.44 Ma (Ben Hines, personal communication, August 2023), confirming that it is close to the established age of the Nukumaruan Stage (2.4Ma). However, 87Sr/86Sr dates on other P. delicatula bearing limestones in the East Coast Basin and the Whanganui Basin Hautawa Shellbed differ, indicating that the limestones are diachronous and cannot be correlated to mark the base of the Nukumaruan Stage or initial influx of cold water to the region. Furthermore, nine P. delicatula shellbeds are identified in the Greycliffs Formation. 87Sr/86Sr dates (Ben Hines, personal communication, August 2023) and sedimentation calculations produced in this study suggest that P. delicatula was present in the Ruataniwha Strait as early as 3.49Ma, well before the occurrence of the limestone formations.