The stratigraphy of the Amuri limestone group, east Marlborough, New Zealand.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
An extensive study of the stratigraphy of the Amuri Limestone Group (Upper Cretaceous Upper Eocene) and the enclosing units in East Marlborough has been undertaken. The study includes regional correlation of detailed measured sections in conjunction with lithofacies descriptions, micropaleontological age determinations, petrographic examination, and geochemical analysis. A revised New Zealand Paleogene time scale has been compiled to take into account recent major revisions of international Cenozoic geochronology. The Amuri Limestone Group (c.660m maximum thickness) incorporates 6 formations: Mead Hill Formation (mid Haumurian - lower Waipawan); Teredo Limestone (mid Waipawan late Mangaorapan); Lower Limestone (mid Waipawan – mid Mangaorapan); Lower Marl (upper Waipawan - lower Heretaungan); Middle Limestone (lower Mangaorapan - lower Bortonian); Upper Marl (upper Porangan - upper Runangan). The Mead Hill Formation is diachronous and conformable on the Upper Iwitahi Group which includes the Woolshed Formation (lower - upper Haumurian) and the overlying Claverley Sandstone (upper Haumurian). The Mead Hill Formation contains the Flaxbourne Limestone Member (mid Haumurian) and Lower Chert Member (late Haumurian). The Lower Limestone contains the Upper Chert Member (mid Waipawan). The Fells Greensand Member (mid Bortonian) and Grass Seed Volcanics Member (upper Bortonian) are both intercalated within the Middle Limestone and Upper Marl. With the exception of post-unconformity sandy facies, the Amuri Limestone consists of dcm-bedded, light greenish grey, well indurated, foraminiferal biomicritic calcilutites and poorly indurated, smectite-rich marls. Macrofossils are extremely rare. Cretaceous sequences are characterized by a poorly developed Planolites - Teichichnus ichnoassemblage; Paleogene facies are dominated by a Zoophycos - Planolites ichnoassemblage. Pelagic limestone deposition was initiated within a central NW-trending trough and spread outwards onto the adjacent near-horizontal platform. Subsidence of the trough is inferred to have been maintained by reactivation of basement faults. Water depths on the platform are likely to have been relatively shallow (inner shelf) during the Late Cretaceous but much deeper (outer shelf - bathyal) during the Paleocene and Eocene. Basin morphology was the major control on lateral facies variations. Platform sediments are characteristically more thinly bedded, and the thickness of individual Formations is correspondingly attenuated, in comparison with trough facies. Chert and dolomite are restricted to the lower parts of the trough facies. Basin-wide unconformities are recognized in the late Haumurian, mid Waipawan (sub-Teredo Limestone unconformity), mid Bortonian, and mid Whaingaroan. Although these breaks are disconformable in platform areas, they regionally account for large amounts of differential erosion. Submarine erosion, hardground formation, development of a Thallasinoides-dominated ichnofauna, glauconitization, phosphatization, and accumulation of a thin sandy facies are typical of unconformities outside the trough. Within the trough, the Haumurian and Waipawan breaks in deposition are represented by paraconformities or coevally deposited siliceous, pyritic mudstones. The subfeldsarenitic Claverley Sandstone was intra-basinally derived from submarine erosion and reworking of the underlying Woolshed Formation. The detrital sand fraction of the Teredo Limestone was derived from reworking of the locally exhumed Claverley Sandstone, and from remobilization at depth and submarine extrusion of that unit. An extra-basinal source (possibly reworked quartzose coal measures) for the redeposited supermature quartzarenitic Fells Greensand is likely. Pulses of (compressional?) tectonic activity immediately preceded and possibly continued during unconformity development. These tectonic events may provide an independent estimate of the timing of some of the major (Late Cretaceous - Cenozoic) plate tectonic events affecting the New Zealand region. The amount of dextral movement on two of the major Marlborough Faults has been estimated from offsets in lithofacies and isopach patterns. 5-10km of transcurrent movement is recognized on the northern branch of the Hope Fault; 10-15km of right-lateral slip has occurred on the Kekerengu Fault.