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 facies of the Lower Miocene Waitemata Group. The sandstone-siltstone 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 Foraminifera in the siltstone, and traces of at least six types of burrowing, sedimenteating organism.The depositional environment is interpreted as a marine basin within a continental borderland, with weak bottom currents, receiving muddy, fine silt. South-eastflowing 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.

Sedimentary features and penecontemporaneous slumping in the Waitemata Group, Whangaparaoa Peninsula, North Auckland, New Zealand

“…2A and B. This evidence indicates turbidity currents flowed southeast-the same direction as reported by Ballance (1964b) from Takapuna. In Figs.…”

“…In these cliffs sub-horizontal and gently dipping alternating sandstone and siltstones of Otaian age are extensively exposed (Gregory, 1966). This sequence is identified as a turbidite facies of the Waitemata Group] similar to that of the Takapuna Section described by Ballance (1964b). Intercalated with this sandstone-siltstone sequence there are a number of thick volcaniclastic grits, believed to have been emplaced by submarine lahars and known locally *Present address: Geology Department, Dalhousie University, Halifax, N.S., Canada.…”

“…At Whangaparaoa Peninsula the basal interval of these grits is often a coarse volcaniclastic conglomerate or breccia. Recent workers (e.g., Kuenen, 1950;Brothers, 1954a;Searle, 1964;Ballance, 1965) are agreed that the Parnell Grits were emplaced by submarine lahars. There is evidence to the west of Huaroa Point that these submarine lahars were capable of transporting large "blocks" of basalt up to 90 ft across.…”

“…SEDIMENTARY FEATURES Ballance's (1964b) detailed study of normal Waitemata beds at Takapuna revealed that in thicker sandstone layers there is commonly developed a rhythmical repetition of primary features of the kind generally accepted as being characteristic of turbidites. This rhythm, which is closely comparable to that taken by Bouma (1962) as representative of the flysch facies, can also be recognised at Whangaparaoa Peninsula.…”

“…At Whangaparaoa Peninsula there are at least two and possibly three or more grit beds. It has been suggested (Ballance, 1964b) that the Waitemata Group accumulated within a continental borderland in a marine basin comparable in size to some modern offshore basins (herein called the Waitemata Basin). Ballance recognised four north-north-west-south-south-east -trending facies belts within this basin.…”