Late Paleocene to middle Eocene foraminiferal biostratigraphy of the Hampden Beach section, eastern South Island, New Zealand

Abstract

The Hampden Beach section is a well-exposed and near-complete 256.5 m thick Paleogene succession of moderately to well-exposed claystone, siltstone, and glauconitic sandstone of Moeraki, Kurinui, and Hampden Formations that spans over 20 m.y. from Teurian to Kaiatan Stages (Paleocene to late Eocene). The sequence was deposited on a passive margin at shelf (Moeraki Formation) to deep upper bathyal (Kurinui Formation) and outermost shelf to upper bathyal paleodepths (Hampden Formation). Paleocirculation was restricted to the north, placing the site under a marginally neritic watermass. The lowest 30 m of the section consists of non-calcareous mudstone of upper Moeraki Formation which is dated as Teurian (Paleocene) by an assemblage of long-ranging, agglutinated foraminifers. Kurinui Formation (30-134.8 m) comprises non-calcareous basal greensand overlain by a fining-upwards succession of slightly calcareous siltstone and claystone. The formation spans the Waipawan, Mangaorapan, and Heretaungan Stages (early to early middle Eocene). The first occurrence of Elphidium hampdenense at 59.7 m marks the base of the Heretaungan Stage. Hampden Formation (134.8-256.5 m) consists of a moderately calcareous basal greensand, grading up into a calcareous siltstone. Hampden Formation contains the Bortonian boundary stratotype which overlies an incomplete or very condensed Porangan interval within the basal greensand, as defined by the presence of the key index species Elphidium saginatum. Bortonian strata are identified by the first appearance of Globigerinatheka index 0.7 m above the base of the formation. The Bortonian/Kaiatan boundary is identified 5.5 m below the top of the section (at 251.0 m) by the highest occurrence of Acarinina primitiva. Revised age control on Acarinina primitiva implies that the top of the Bortonian Stage may be lowered by 2 m.y. to 39 Ma, which represents a significant reduction in the duration of the Bortonian and a tripling in the duration of the Kaiatan Stage-from 1 to 3 m.y. Other abundant microfossils in the section provide scope for further integrated biostratigraphy.