Foraminiferal, radiolarian, and dinoflagellate biostratigraphy of Late Cretaceous to Middle Eocene pelagic sediments (Muzzle Group), Mead Stream, Marlborough, New Zealand

Abstract

Abstract Integrated biostratigraphic study of foraminifera, radiolarians, and dinoflagellates substantially improves age control of well‐exposed Late Cretaceous to late Middle Eocene (Haumurian‐Bortonian) Muzzle Group sediments at Mead Stream, inland Marlborough, New Zealand. The group, comprising 650 m of well‐bedded pelagic limestone, marl, and chert, consists of two formations: Mead Hill Formation (257 m) and Amuri Limestone (393 m). The latter includes five distinctive, informal units: Black Siltstone, Lower Limestone, Lower Marl, Upper Limestone, and Upper Marl. Muzzle Group is in fault contact with mid‐Cretaceous (Clarence Series) Split Rock Formation siltstones at its base, and is unconformably overlain by Oligocene (Landon Series) Weka Pass Limestone. The Cretaceous‐Tertiary (K‐T) boundary, within Mead Hill Formation, is marked by a distinct lithologic change from pale limestone to dark laminated mudstone grading up into a 23 m thick chert unit. Basal Paleocene biotas suggest a relatively complete boundary sequence, similar to that observed in coastal Marlborough. The Late Paleocene Black Siltstone unit, considered laterally equivalent to distinctive transgressive or highstand facies to the south and in North Island, forms the basal unit of Amuri Limestone. The overlying Lower Limestone is also Late Paleocene (late Teurian) at the base and Early Eocene (Waipawan to possibly Mangaorapan) in the upper part. The Lower Marl is Early Eocene, possibly as old as Waipawan, at the base, otherwise ii is Mangaorapan to perhaps earliest Heretaungan. The Upper Limestone is early Middle Eocene based on Radiolaria; foraminifera and dinoflagellates have not been obtained from this highly indurated unit, but indirect evidence suggesting a Heretaungan‐Porangan age is consistent with radiolarian results. All groups indicate a late Middle Eocene (Bortonian) age for the Upper Marl. Foraminifera indicate progressive transgression, with submergence from inner‐mid shelf to mid‐bathyal depths in the Cretaceous. Highly siliceous lithologies and an abundance of diatoms indicate a nutrient‐rich upwelling setting. Significant shallowing, probably within the bathyal zone, at the K‐T boundary is inferred from an increase in total abundance of benthic foraminifera and greatly reduced numbers of planktics. An influx of spumellarian radiolarians, together with increases in diatom abundance and whole‐rock silica content, indicate enhanced levels of siliceous plankton productivity, probably due to regional upwelling of cool, nutrient‐rich waters in the earliest Paleocene. Benthic‐dominated foraminiferal faunas indicate bathyal to outer shelf depths prevailed for most of the Paleocene, and planktic foraminifera only return to Cretaceous abundance levels in the latest Paleocene (Lower Limestone). Bathyal conditions are maintained throughout the remainder of the sequence. Upwelling appears to have declined progressively, with silica content and siliceous biota abundance reaching very low levels in Early‐Middle Eocene (Lower to Upper Marl).