Fluvial and lacustrine successions in the youngest part of the Murihiku Supergroup, New Zealand

Abstract

Fossil-bearing Late Jurassic to Early Cretaceous volcaniclastic sandstones and siltstones from the Murihiku Supergroup are described from the Kaimango Syncline South Auckland, New Zealand. These constitute the youngest known sedimentary succession within the Murihiku Supergroup (Murihiku Terrane). We concentrate on the youngest Matira Siltstone Formation which has yielded a rich but relatively low-diversity macroflora of ferns and gymnosperms, diverse miospore palynofloras, with fair to good preservation, and sporadic occurrences of bivalve and gastropod faunas. Collectively, the paleontology and sedimentology of this formation is interpreted to represent paludal and lacustrine deposition, with intercalated sandstones thought to represent fluvial channelised or deltaic settings. U-Pb dating of detrital zircons (DZ) from sandstones in the youngest formations, Mangatara Measures and Matira Siltstone, have provided significant components in the age range 140 to 143 Ma, possibly straddling the Jurassic-Cretaceous boundary, although miospores suggest a Late Jurassic correlation. Lacustrine and paludal depositional environments have not previously been recognised from the Murihiku Supergroup and this raises the prospect of their enhanced petroleum source rock potential in frontier basins to the north and east of New Zealand.