The Sedimentology Of The Waitemata Group In The Takapuna Section,Auckland

Abstract

Alternating sandstones and muddy siltstones at Takapuna are interpreted as a marine turbidite fades of the Lower Miocene Waitemata Group.The sandstone-silt-stone alternation is described in terms of the following rhythmic sequence:a turbidite consisting of (1) massive sand,with a very abrupt base on siltstone, and occasional erosional sole structures, grading upwards into (2) laminated,(3) convoluted, and (4) ripple-drift-bedded sand;followed by interturbidite beds consisting of a variable number of alternations of (5) fine, muddy siltstone with (6) ripple-drift-bedded,very fine sand. Siltstone inclusions are common in the turbidites,and carbonised plant matter, in all stages of comminution, is abundant in all lithologies.Other sedimentary structures include intraformational slumping. Grain-size analyses are presented.All lithologies are positively skewed and very poorly sorted.Petrographically the turbidites are lithic subgreywackes, with abundant fine-grained sedimentary rock fragments and subordinate quartz and plagioclase feldspar, and a fine detrital matrix.They are mostly uncemented and highly porous.The interturbidite rippled sand is lithologically identical with, but finer-grained than, turbidite sand; it is interpreted as turbidite sand redistributed by bottom currents.Diagenetic features include carbonate nodules in the turbidites, and limonite staining The rocks are unfossiliferous,apart from plant debris, a few planktonic Fora-minifera in the siltstone, and traces of at least six types of burrowing, sediment-eating organism The depositional environment is interpreted as a marine basin within a continental borderland, with weak bottom currents, receiving muddy, fine silt. South-east-flowing traction currents periodically moved thin, rippled layers of fine sand, posisibly derived from turbidites deposited elsewhere in the basin. South-east-flowing, high-density turbidity currents deposited sand beds averaging 18 in. in thickness; they arrived at irregular intervals, and occasionally may have flowed north-north-east A preliminary discussion of some regional aspects of Waitemata sedimentation is given