Late Miocene to recent evolution of the Awatere Basin, Medway and Middle Awatere Valleys, Marlborough

Abstract

The Awatere Basin is a Late Miocene to Pliocene dominantly marine sedimentary basin in northeast Marlborough. The development of part of this basin from the Late Miocene to the Present has been determined by detailed field mapping in the Medway and middle Awatere Valleys. Late Miocene lithological units have been distinguished, described, and dated using foraminifera and molluscs. Ages indicated by these fossils were refined by correlation with palaeomagnetically dated sections in the lower Awatere Valley. Major structures within and bordering the basin in the Medway/middle Awatere Valley area have been located and examined and cross and strike sections constructed in order to examine the three dimensional structure. The post-Miocene evolution of the Awatere Basin has been considered in the light of current knowledge of aspects of the regional geology. Sedimentation in the Medway Valley area began in the early Tongaporutuan, ~10.5 Ma ago. Throughout the early Tongaporutuan and continuing into the mid Tongaporutuan, muds and sands of the Medway Formation were deposited near a land mass, but mostly at outer shelf-upper slope depths, in northeast trending fault angle depressions of the developing Medway fault system. Angular clasts of basement greywacke derived from active fault scarps were deposited in the fault controlled basins by debris flows. The Medway faults were low angle northwest dipping thrust faults when they were active. They are buried by progressively younger Medway sediments from southeast to northwest, showing that the Medway thrust system propagated northwestward in an overstep sequence. A change in depositional and deformational regime occurred later in the mid Tongaporutuan, ~8.5 Ma ago, and the Upton Formation was deposited over the Medway faults and basins. In the middle Awatere Valley the Upton Formation is dominantly conglomeratic. The conglomerate is weakly bedded and composed almost entirely of rounded clasts of basement greywacke which mostly range from 0.5-25 cm in diameter. This conglomerate was deposited at outer shelf-upper slope depths in a basin over 1500 m deep that was separated from the main Awatere Basin to the northeast by a basement sill. The sill formed as a fault propagation fold above the Flaxbourne and Haldon Faults, two of the major faults of the Medway thrust system. Land closely surrounded the conglomerate sub-basin in the northwest, southwest and southeast. Rivers transported a continuous supply of gravels to the basin margins. These gravels were transported into the depths of the basin by sediment gravity flows. Northeast of the basement sill, the Awatere Basin opened and deepened and muds and sands were deposited. The Upton conglomerate sub-basin was not created in a strike-slip regime. It is not a pull-apart basin, as an eastern bounding fault can not be identified, and it is unlikely to be related to transpression on the Awatere Fault as the conglomerate does not become coarser or more angular towards that fault. This implies that the Awatere Fault was not active as a strike-slip fault in the Tongaporutuan. Depths of deposition shallowed during the latest Tongaporutuan due to a glacio-eustatic sea level fall. In the Kapitean a blue grey conglomerate lithofacies was deposited in a terrestrial environment in the southwest of the middle Awatere Valley. The conglomerate was deposited by alluvial fans derived from mountains to the northwest. The Awatere and/or Fuchsia Faults were probably active as range front faults during this time, uplifting basement on their northwest sides. In the lower Awatere Valley marine deposition continued through this interval, showing that a northeastward tilt had been imparted to the basin by this time. Deposition continued through the Pliocene in the lower Awatere Valley, but there was no further deposition southwest of there, in the Medway and middle Awatere Valleys. Since the Early Pliocene the Awatere Basin has been dissected and deformed by strike-slip movement on the Awatere Fault, by regional block rotations, and by uplift and tilting. Immediately southeast of the Awatere Fault strata have been folded by the Awatere anticline, a northeast trending fold with a plunge that increases northeastward.