FORAMINIFERAL, RADIOLARIAN, AND DINOFLAGELLATE BIOSTRATIGRAPHY OF LATE CRETACEOUS TO MIDDLE EOCENE PELAGIC SEDIMENTS (MUZZLE GROUP), MEAD STREAM, MARLBOROUGH, NEW-ZEALAND

Integrated biostratigraphic study of foraminifera, radiolarians, and d inoflagellates substantially improves age control of well-exposed Late Cretaceous to late Middle Eocene (Haumurian-Bortonian) Muzzle Group s ediments at Mead Stream, inland Marlborough, New Zealand. The group, c omprising 650 m of well-bedded pelagic limestone, marl, and chert, con sists of two formations: Mead Hill Formation (257 m) and Amuri Limesto ne (393 m). The latter includes five distinctive, informal units: Blac k Siltstone, Lower Limestone, Lower Marl, Upper Limestone, and Upper M arl. Muzzle Group is in fault contact with mid-Cretaceous (Clarence Se ries) Split Rock Formation siltstones at its base, and is unconformabl y overlain by Oligocene (Landon Series) Weka Pass Limestone. The Creta ceous-Tertiary (K-T) boundary, within Mead Hill Formation, is marked b y a distinct lithologic change from pale limestone to dark laminated m udstone grading up into a 23 m thick chert unit. Basal Paleocene biota s suggest a relatively complete boundary sequence, similar to that obs erved in coastal Marlborough. The Late Paleocene Black Siltstone unit, considered laterally equivalent to distinctive transgressive or highs tand 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 Mang aorapan) in the upper part. The Lower Marl is Early Eocene, possibly a s old as Waipawan, at the base, otherwise it is Mangaorapan to perhaps earliest Heretaungan. The Upper Limestone is early Middle Eocene base d on Radiolaria; foraminifera and dinoflagellates have not been obtain ed 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 Mar l. Foraminifera indicate progressive transgression, with submergence f rom inner-mid shelf to mid-bathyal depths in the Cretaceous. Highly si liceous lithologies and an abundance of diatoms indicate a nutrient-ri ch upwelling setting. Significant shallowing, probably within the bath yal zone at the K-T boundary is inferred from an increase in total abu ndance of benthic foraminifera and greatly reduced numbers of planktic s. An influx of spumellarian radiolarians, together with increases in diatom abundance and whole-rock silica content, indicate enhanced leve ls of siliceous plankton productivity, probably due to regional upwell ing of cool, nutrient-rich waters in the earliest Paleocene. Benthic-d ominated foraminiferal faunas indicate bathyal to outer shelf depths p revailed for most of the Paleocene, and planktic foraminifera only ret urn to Cretaceous abundance levels in the latest Paleocene (Lower Lime stone). Bathyal conditions are maintained throughout the remainder of the sequence. Upwelling appears to have declined progressively, with s ilica content and siliceous biota abundance reaching very low levels i n Early-Middle Eocene (Lower to Upper Marl).