SEQUENCE ARCHITECTURE OF EXPOSED LATE MIOCENE BASIN FLOOR FAN AND CHANNEL-LEVEE COMPLEXES (MOUNT MESSENGER FORMATION), TARANAKI BASIN, NEW ZEALAND

Abstract

A unique section through late Miocene basin floor fan, channel-levee complex, and prograding complex units of several lowstand systems tracts, occurs in the coastal region of north Taranaki, New Zealand. Strata dip gently and are superbly exposed in coastal cliffs as much as 200 m high. The overall succession exposed along the coast is progradational. The major controls on sedimentation were relative changes in sea-level (base-level changes), influenced by high rates of sediment supply. Broad intervals dominated by one or other lowstand systems tract component are identified in outcrop, and can be correlated into the subsurface using seismic reflection profiles and SP logs. Exposed basin floor fan and channel-levee successions within the Mount Messenger Formation form the basis of the present study. Five distinct sequences are recognized. Relatively thick, fine-grained intervals constitute the upper portions of individual sequences. Paleobathymetric deepening trends and high gamma-ray counts have been recorded in some of these intervals. Sequence boundaries occur as erosional contacts. The basin floor fan (lowstand fan) units comprise meterthick massive and convolute-bedded sandstone with minor mudstone, and thin-bedded massive sandstone, horizontal and ripple-laminated sandstone, and mudstone. More proximal early lowstand (basin floor fan equivalent) lithofacies include conglomerate and thick-bedded sandstone, within nested channels that erode as much as 30 m into underlying strata. Channels almost invariably occur above slumped intervals, and we suggest that retrogressive slumping created sea-floor topographic depressions within which the channels preferentially formed. Channel-levee complexes are characterized by interleaving packages of thin-bedded siltstone and sandstone. Levee overbank siltstone is prevalent in lower, more distal parts of channel-levee complexes. In upper, more proximal parts, coalescing channelized units are common, and within these, welldeveloped Bouma sequences and climbing ripple-laminated sandstones are particularly distinctive. Thick-bedded sandstones (basin floor fan) have lowest outcrop gamma ray values, of about 125 counts per second (cps). Sandstone gamma ray values generally range up to 180–190 cps, both in the basin floor fan and channel-levee lithofacies. Mudstone gamma ray counts are generally higher (up to 250 cps), though there is considerable overlap with sandstone values, particularly in thin-bedded lithofacies. Sandstone permeabilities measured in outcrop are mainly in the range 100mD–800mD. Thick-bedded sandstones have the highest permeabilities, demonstrating a primary facies control on reservoir quality.