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

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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

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