Geology of the Kaikoura Area - GNS Science

CRETACEOUS TO PLIOCENEFollowing amalgamation of the basement terranes and afterthe mid-Cretaceous cessation of convergent margintectonics along the Gondwanaland margin, much of theNew Zealand area was uplifted and then eroded to a lowlyinglandscape. This landscape was subject to extensionas the Tasman Sea opened and the New Zealand continentseparated from Gondwanaland in the late Early Cretaceous.In the west, there was extensive pluton intrusion followedby rifting and the development of sedimentary basins(Bishop & Buchanan 1995; King & Thrasher 1996).Opening of the Tasman Sea ceased in the Paleocene andthe New Zealand area entered a period of prolonged tectonicstability coupled with subsidence and, in the east,widespread marine sedimentation. In the Kaikoura map areathese conditions persisted until the Miocene, when acompressional tectonic regime was initiated as theboundary between the Australian and Pacific platespropagated through New Zealand. Rapid uplift andconsequent erosion provided a vast amount of sedimentthat filled developing basins adjacent to major strike-slipfaults.With the exception of a small area of Late Cretaceousconglomerate in the Moutere Depression, all of the LateCretaceous to Middle Eocene rocks in the Kaikoura maparea occur southeast of the Alpine Fault. These rocks cropout in two geographically distinct areas, easternMarlborough and northern Canterbury. Despite manygeological similarities, the two areas have acquired twoseparate systems of stratigraphic nomenclature (Fig. 28)although some rationalisation has been attempted (Field,Browne & others 1989; Field, Uruski & others 1997). Withinboth areas there are many formalised formations andmembers that have been defined on key stratigraphicsections but are difficult to map out areally. The followingdescriptions refer to many of these stratigraphic units butonly the bold named units are distinguished on the map.Where formations are thin, in areas that are geologicallycomplex, and offshore, the rocks are mapped asundifferentiated Late Cretaceous (lK), Paleocene-Eocene(PE), Oligocene (O) or Miocene (M).Cretaceous sedimentary and volcanic rocksCretaceous rocks are locally well preserved inMarlborough. In the middle Awatere valley and thenorthern Clarence valley, a late Early Cretaceoussuccession is preserved that has elsewhere presumablybeen eroded or was never deposited.The earliest confirmed post-Pahau deposition is that ofthe Champagne Formation (Kcc; Ritchie and Bradshaw1985; Crampton & others 1998) of the Coverham Group(Kc; Lensen 1978). The Champagne Formation is exposedin a narrow, 10 km long band in the middle Clarence valley,from Ouse Stream (Fig. 29) to Dee Stream and in WharekiriStream (too small to be shown on the map). It consists ofhighly deformed sandstone, mudstone and minorconglomerate, typically with boudinage of sandstone beds,faults, asymmetric folds, remobilisation of mud from foldhinges and soft-sediment deformation (Crampton & others1998). The Champagne Formation unconformably overliesPahau terrane and its base is late Early Cretaceous(Crampton & others 2004).Basement versus coverDespite a wealth of previous work, the stratigraphic location of the top of the Torlesse composite terrane in NewZealand is difficult to pinpoint. For example, in the East Cape region, the contact between Early Cretaceous(Korangan) Torlesse Waioeka terrane and overlying Early Cretaceous “cover” rocks is a fault, a high angleunconformity or series of unconformities, an olistostrome breccia, or a channelled contact (Laird & Bradshaw1996; Mazengarb & Speden 2000). In the Wairarapa region, Early Cretaceous (Motuan) Pahaoa Group isplaced within the Torlesse composite terrane and the oldest overlying “cover” rocks are also of Motuan age (Lee& Begg 2002). Radiometric dating of zircons from both below and above the unconformity yields ages of c. 100Ma (Laird & Bradshaw 2004) suggesting derivation from the same or similar-aged source rocks or recycling ofthe zircons.In Marlborough the contact cannot always be defined on the basis of metamorphic grade, degree of indurationor by changes in clastic composition (Montague 1981; Powell 1985). Fossils are rarely preserved in the Pahauterrane and these rocks are comparatively poorly dated. Lensen (1962) included latest Early Cretaceous(Motuan) rocks within the Torlesse composite terrane and Field, Uruski & others (1997) state that a regionalunconformity exists between older and younger, more fossiliferous rocks, but that rocks both above and belowthe unconformity are Motuan in age. This implies that the “regional unconformity” was intra-Motuan and of veryshort duration in Marlborough, even though bedding discordance across the unconformity can be marked.Crampton & others (2004) show no significant stratigraphic break during the Motuan at Coverham, in theClarence valley, but they note the presence of older late Early Cretaceous unconformities (intra-Urutawan).A mid-Cretaceous unconformity is widespread throughout New Zealand. The diachronous nature of theunconformity surface (Laird & Bradshaw 2004) in Marlborough probably reflects ongoing deposition in localised,syntectonic basins adjacent to areas of uplift and erosion. However, there are many locations where a clearerstratigraphic break is apparent, i.e. where the Pahau terrane is overlain by significantly younger Cretaceous orCenozoic rocks.29