Mid-tertiary temperate shelf bioclastic limestones, Southland, New Zealand.


Hyden, Fiona Mary.


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Mid-Tertiary temperate-water shelf carbonates are extensively developed in Southland and provide excellent examples of Bryozoan-dominated bioclastic sediments which can be matched with smilar bryozoan-dominated assemblages accumulating on the Otago shelf today. Tertiary sedimentation in the Southland Plains and adjacent areas can be attributed to a simple transgressive-regressive episode on which was superimposed local tectonic events and regional palaeo-oceanographic and eustatic sea-level changes.
Late Cretaceous-Eocene alluvial, lacustrine and deltaic sediments uncomfortably overlie basement rocks. East of the Oreti River, marginal marine beds grade up into shallow marine mudstones of the Winton Hill Formation (early-mid Oligocene, Lwh-Ld) and highly glauconitic sandy mudstones, algal foraminiferid biomicsparrudites and glauconitic biomicrites (Waimea Member) of the Chatton Formation (late Oligocene; Ld-Lw). At Castle Rock, shallow marine mudstones underly foraminiferid-algal and bryozoan biomicsparites of the Caslte Downs FOrmation (late Oligocene-early Miocene; LD-Po). Laterla facies variations suggest a complex coastal and nearshore physiography; the disposition of coars, basement-derived detritus and algal-rich sediment suggests the presence of locally emergent basement highs.
Evidence of very low sedimentation rates, periods of non-deposition, vigourous nearshore erosion is provided by abundant (oolitic) glauconite and limonite, phosphatised (intra)clasts, and a high degree of bioturbation, five omission surfaces occur at the top of the Chatton Formation at Woody Knoll. Coarse, channelised and sheet-like congrolmerates occur locally above the discontinuity (Sharks Tooth Hill Member, Forest Hill Formation) and tectonic instrability may have caused mass failure of coarse detritus.
The conglomerates coincide with a change in sedimentary regime - from inner neritic calcareous sandntones, algal-foraminiferid biosparites, or bivalve-cirriped biomicsparrudites (locally formaing a submarine hard ground) to mid-outer neritic bryozoan-dominated facies (Woody Knoll Member, FOrest Hill Formation). The absence of corals and green algae, calcareouse ooids and pallets, and chemically precipitated micrite is consistent with temperate rather than troopical palalatitudes.
Bryozoan rudites are the dominant facies within the Forest Hill Formation (late Oligocene-early Miocene; ?Lw··Pl), but fluctuating wind and wave-dominated hydraulic regimes and an irregular sea-floor topography gave rise to a complex mosaic of terrigenous-rich and terrigenous poor bioclastic facies. Twenty-seven traction-emplaced or in situ bioclastic or terrigenous facies and three mass-emplaced facies, plus a number of subfacies, have been recognised. Once a bryozoan-brachiopod and thence bryozoan-dominated assemblage became established through an ecological succession, bryozoans and associated faunas covered considerable areas of the sea-floor during a period of tectonic quiescence. Fragmented bryozoan skeletons, mostly cylindrical branching forms, provided adequate substrate for growth of successive colonies and although in situ colonies were rare, broken colonies have not been transported far from their living site. Nodular, free-living hemispherical and hermit bryozoan colonies are particularly conspicuous.
Eocene-Oligocene sedimentation patterns west of the Oreti River contrasted with that exposed in the Forest Hill - Fernhill district and at Castle Rock. Rapidly subsiding flysch basins developed in the Waiau and Te Anau areas, coinciding with inception of the Alpine Fault oblique continental plate boundary. Smaller N-NNE trending fault-controlled basins were also established, and rapidly filled, east of the Longwoods (Oreti-Aparima basin) and east of the Takitimu Mountains (Braxton basin).
The sequence at Clifden and in the Alton Burn records sedimentation on the margin of the Waiau flysch basin. Basal (Eocene) non-marine and marginal marine carbonaceous mudstones and sandstones with thin lignite horizons are overlain by richly microfossiliferous early-mid Oligocene (Lwh-Ld) deep--water mudstones. These sediments, probably deposited on the basinal slope, correspond to the period of maximum transgression further east; they grade up into late Oligocene sandy mudstones and bryozoan-molluscan-foraminiferid biosparites (Te Karara Formation).
Emplacement of thick conglomerates (Sharks Tooth Hill Member) heralded an abrupt change in sedimentary regime - from off shore mudstcnes to shallower, neritic bryozoan rudites (Woody Knoll Member). The apparently younger age of the Forest Hill Formation at Clifden (Po-Pl) compared to that at Forest Hill (?Lw-Pl) is consistent with westward progradation of the carbonate shelf across the now-filled Oreti-Aparima basin and the partially, if not totally submerged Longwoods basement high. Neritic channels provided a conduit for transporting bioclastic sediments into the still subsiding Waiau basin.
A gradual increase in terrigenous detritus towards the top of the Forest Hill Formation reflects gradual shoaling. At Forest Hill, the limestones are overlain by inner neritic sandy mudstones and carbonaceous facies, and the regressive sequence mirrors that of the transgression. At Clifden, the limestones are overlain by neritic sandstones (mid-Miocene).
Sedimentation rates of the order of 1-2 cm/1000 years are considered likely for the Forest Hill Formation. Such low rates of deposition in a temperate water envirorunent periodically gave rise to CaCO3 undersaturation. Consequent sea-floor dissolution of aragonitic and some high-Mg calcite skeletons was arrested only by early lithification in micritic sediments, or by low permeability (e.g. in terrigenous mud-rich sediments).
The observed diagenetic sequence within ·the Forest Hill and Castle Downs formations can be explained by restricted submarine cementation, or close to the sediment-water interface (soft sediment to a depth of several cms was widespread). With shallow burial (tens of metres), ferroan calcites were widely precipitated (phreatic zone). Evidence of subsequent uplift east of the Longwoods is provided by vadose goethite cements which postdate early ferroan calcite rim cements and predate mesogenetic, pore--filling ferroan sparrites. At Castle Rock, the position of a fossil water table can be delineated on the basis of goethite distribution. A more complex diagenetic sequence occurs in facies associated with a submarine hardground at Forest Hill.. Six distinct cement generations are recognised, including ferromanganese and haematite/goethite phases.
Oligocene-Miocene limestone turbidites of the Waiau and Braxton basins are of comparable lithology to the bioclastic limestones of the Forest Hill and Castle Downs formations, but the turbidites possess quite different diagenetic histories which reflect their different depositional and diagenetic environments. The role of compaction is probably the single most important factor in diagenesis of the limestones. Coarse calcarenite to calcirudite grade bioclastic sediments have been sufficiently compacted to reduce high initial porosities to less than 10%. Dissolution of aragonitic skeletons, interpenetrative grain contacts, and selective intergranular solution of bioclasts within terrigenous clay-rich horizons, provided sufficient carbonate solutions to further reduce this porosity to less than 2%.

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487 p., 44 leaves of plates : ill (some col.) ; 30 cm.


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Hyden, Fiona Mary., “Mid-tertiary temperate shelf bioclastic limestones, Southland, New Zealand.,” Otago Geology Theses, accessed July 23, 2019, http://theses.otagogeology.org.nz/items/show/120.