Testing for astronomical forcing of cycles and gamma ray signals in outer shelf/upper slope, mixed siliciclastic-carbonates: Upper Oligocene, New Zealand

We evaluate whether outer shelf/upper slope settings preserve a record of astronomical forcing using gamma ray logs from the upper Oligocene (upper Chattian) Tikorangi Formation, onshore Taranaki Basin, North Island, New Zealand. The formation is a siliciclastic‑carbonate succession deposited in outer neritic to upper bathyal depths at paleolatitudes of c. 45^oS. Gamma ray and sonic logs calibrated to core indicate four lithofacies based on percent carbonate. These are calcareous siliciclastic mudrock (highest gamma ray values), highly calcareous siltstones (high gamma ray values), very fine to fine siliciclastic muddy packstones (intermediate gamma), and winnowed very fine to medium grain-dominated packstones and grainstones (low gamma). Detrended gamma ray logs were tuned using minimal tuning methods (405 kyr long eccentricity cycle or, where this was poorly developed, the ~100 kyr short eccentricity cycle) with sedimentation rates tested statistically against astronomical forcing. The tuned data show well developed long- and short-term eccentricity, obliquity, and a weak precessional forcing. Obliquity modulation is observed at the 1.2 Myr scale (one well) and 175 kyr scale (two wells). The floating time scale was tied to the absolute time scale via published biostratigraphy, the La2004 eccentricity solution, the Atlantic ODP site 1264 δ¹⁸O record, and the Mi-1 isotope event at c.23 Ma. Sea-level fluctuations typically were moderate (20 to 40 m) estimated from the δ¹⁸O record. The high gamma siliciclastic-prone units formed during warmer and wetter phases (and possibly slightly higher sea levels), whereas the cleaner carbonates with abundant inner-shelf-derived fine carbonate developed during cooler, drier phases (and possibly slightly lower sea levels). Gamma ray logs from outer shelf/upper slope settings provide valuable data sets for documenting the effects of astronomical forcing on the sedimentary record. The study supports climate models for New Zealand which predict generally increased precipitation with warming over the next 100 years. The study appears to be the first to record astronomical forcing in the Oligocene marine record of New Zealand.