Geological setting, sedimentology and biota of the estuarine late Oligocene Pomahaka Formation, New Zealand

Abstract

ABSTRACTPomahaka Formation, a late Oligocene succession of interbedded mudstone, lignite, shell beds and thin sands, was studied in small exposures along the Pomahaka River near Tapanui, Otago and in four adjacent sections in Waikoikoi Creek where about 17 × 0.25–4.0 m thick facies cycles were exposed. At the base of an ‘ideal’ cycle, thinly laminated sandy mudstone overlying lignite of the previous cycle records transgressive flooding of forest swamp by fresh or estuarine waters. Laminated shelly mudstone containing venerid bivalves (Hinemoana acuminata) forming the middle of several cycles represents maximum marine flooding. Above this, trough cross-stratified beds and sand with mud flaser laminae record progradation of distributary bars and/or intertidal sands. Finally, deposition of root-burrowed mudstone and lignite mark the re-establishment of swamp forest. Predominantly muddy sediment texture, the association of lignite, distinctive fresh- and brackish-water faunas and thin depositional cycles indicate a microtidal estuarine paleo-environment in southern Zealandia during late Oligocene time.