Cretaceous‐Tertiary foraminiferal succession at Flaxbourne River, Marlborough, New Zealand

Abstract

The stratigraphically and biostratigraphically most complete Cretaceous‐Tertiary (K‐T) boundary succession yet identified in New Zealand occurs in bathyal limestones of the Mead Hill Formation at Flaxbourne River, coastal Marlborough. It is one of three New Zealand K‐T sections with a well‐delineated “fireball” layer, containing anomalous iridium, soot particles, and fullerenes, and it contains a highly fossiliferous latest Cretaceous marl unit, not seen in other sections, immediately underlying the K‐T boundary clay. The c. 50 m thick Flaxbourne section is the only New Zealand locality known with an essentially complete sequence of earliest Paleocene foraminiferal zones. These include (K‐T boundary is zero datum): Guembelitria cretacea Zone (P0), 0.0–0.07 m; Parvularugoglobigerina eugubina Zone (P?), 0.07–0.24 m; P. eugubina‐Subbotina triloculinoides Subzone (P1a), 0.24–0.40 m; S. trilo‐culinoides‐Globanomalina compressa/Praemurica inconstans Subzone (P1b), 0.40–0.50 m; and Globanomalina compressa/Praemurica inconstans‐Praemurica uncinata Subzone (P1c), 0.50–11.5 m. Strata from 11.5 m to the highest exposed beds at c. 38 m are classified as undifferentiated Paleocene. Cretaceous planktic “survivor taxa” occur up to 0.40 m but are replaced rapidly above Zone P0 by Paleocene taxa. Latest Cretaceous strata are dominated by Heterohelix/ Globigerinelloides planktic foraminiferal assemblages, associated with sparse (typically <5%), mainly epifaunal benthic taxa and rare radiolarians. Commencing abruptly at the base of the boundary clay, maximum size of planktic and benthic specimens decreases sharply, while abundance of both radiolarian and benthic foraminifera increase about tenfold relative to planktics, features which typify the basal 0.4 m of Paleocene strata. Heterohelix/Hedbergella associations characterise planktic assemblages initially, but give way to Eoglobigerina /Chiloguembelina planktic assemblages. Epifaunal taxa dominate benthic faunas, but mfaunal forms show minor abundance peaks. Benthic abundance remains near 45% up through c. 0.3 m but then declines sharply to c. 25% at 0.40 m (highest detailed sample). Stratigraphically higher samples were studied in less detail. From 0.5 to 11.5 m they contain poor foraminiferal assemblages similar to those described above, but these are then replaced by planktic‐poor (<5% planktics) assemblages with normal‐sized benthic taxa, including Gavelinella beccariiformis and Nuttallides cretatruempyi, which occur to the top of the exposed section.