Abstract
The base of the Abbotsford Formation is redefined in terms of lithostratigraphic principles. In the Dunedin area its base coincides with the conformity or disconformity between the top of the newly proposed Fernhill Member and its coeval Brighton Limestone Member of the Wangaloa Formation, and the base of the Fairfield Greensand Member of the Abbotsford Formation. This contact separates two heterolithic groups of lithofacies. The first characterizes the Wangaloa Formation, which is typified by pebbly/non-pebbly very fine sandstones, HCS sandstones, conglomerates, flaser/lenticular-bedded sandy mudstones and limestones. The second characterizes the Abbotsford Formation, which is typified by repeated alternation of sand/mud/claystones with variously glauconious conglomerates and glauconious sand/mud/claystones. Other historic formational names, Saddle Hill Siltstone and Steele Greensand, are given member status within the Abbotsford Formation. The term 'Barrons Hill Lens' is abandoned. New members are introduced: Fernhill Member (as above), Boulder Hill Member, Quarries Siltstone, Sunnyvale Sandstone, Abbots Creek Greensand, Waterfall Creek Sandstone, and Blackhead Rd. Glauconious Sandstone. The Green Island Sandstone is retained as a formation and its lower gradational contact with the Abbotsford Formation is drawn at the first incoming of significant amounts of micaceous fine-very fine sand at the base of the M/26 core at Abbotsford. The upper contact of the Green Island Sandstone is a marked disconformity surface.
Tectonic setting, predominance of biogenic sedimentary structures over non-biogenic ones, rare preservation of muddy-clayey flasers and lenticular bedding, and abundance of carbonaceous material and pyrite indicate a reducing, subtidal, estuarine to shallow marine offshore palaeoenvironment for the Abbotsford Formation. The inferred fairweather depositional mechanism is one of gentle tidal currents operating in a microtidal regime. However, episodic storm current deposition was active at specific horizons throughout the Abbotsford Formation. The depositional setting of the Green Island Sandstone is discussed in light of recent exposure of its upper facies at Blackhead Rd. The facies includes well developed bi-directional bedding structures comprising dominant low-angle-avalanche bedding with clay drapes and subordinate reactivated surfaces with opposing foresets. These features suggest a strongly asymmetrical tidal depositional mechanism in a different tidal regime to that of the Abbotsford Formation. The inferred palaeoenvironmental setting is that of an estuary and/or tidal delta sandstone complex.
Facies sequences within the Abbotsford Formation define a cyclicity dominated by a base of variously gritty, variously glauconious, sediments resting on a disconformity or discontinuity surface. These sediments pass gradationally upwards to siliciclastic facies which may show various degrees of coarsening up, or further glauconious facies. The cycles are terminated at all scales by discontinuity surfaces. Three types of facies sequences are defined: coarsening-up, siliciclastic-up and glaucony-up sequences. The cyclic sedimentation patterns further suggest an offshore, rather than lagoonal setting for the Abbotsford Formation. The cyclic facies sequences were produced by a combination of localized storm induced deposition and regional scale transgressive-regressive cycles produced by local or eustatic sea level changes.
Application of sequence stratigraphic principles to these cycles enable their identity as parasequences to be established where the cycles can be shown to have regional extent relative to finer scale cycles of purely local extent The stacking patterns of the parasequences are unable to be established because of a lack of suitable controls: exposure is imperfect, faunal data that could aid in palaeoenvironmental deduction is limited, and dating of unconformities lacks the appropriate resolution. However, certain disconformity and discontinuity surfaces which may/may not bound parasequences are candidates for sequence boundaries. Specific examples are examined and the overall application of the global sea level curve to the Abbotsford Formation is discussed.
Allostratigraphic division of the lithostratigraphic section recognizes new allomembers, and various options are discussed for defining the position of the base and top of the Abbotsford Alloformation .
Glauconious sediments are a feature of Abbotsford Formation and are defined as either glauconious
siliciclastic (< 50 % glaucony) or greensands (> 50 % glaucony). Studies of morphology, crystallography, mineralogy and geochemistry are used to ascertain the origins and likely initial glaucony substrates The evolution of the vermicular morphological variety of glaucony is described from TEM analysis and confirms development from micaceous substrates. This with description of gross externa/internal morphology casts doubt on the proposed evolution of such grains from faecal pellets. A synthesis of the glauconization process is proposed which includes the effects of subsequent processes such as reworking and goethitization. The sedimentological and morphological studies of glaucony-bearing sediments suggest that the abundance of glaucony grains observed in the Abbotsford Formation is the result of very low volumes and slow deposition of siliciclastic detritus, chemically dominated continental weathering, repetitive transgressive events, and the presence of abundant available substrates: biogenic clasts, micas, and probably faecal pellets. The variations in morphology, crystallography and chemistry of glaucony grains displayed by stratigraphically equivalent glaucony-bearing sediments suggest the use of morphological variations to be the best means of stratigraphic distinction. However, the use of associated mineralogical components and XRD of glaucony separates is also instructive.
Mud/claystones of the Abbotsford Formation contain various amounts of smectite, kaolinite, I/S, zeolites (Clinoptilolite) and amorphous SiO2. Together these characteristics could suggest the possibility of a volcanic origin. However, bulk sediment chemical compositions are typical of average shales and are not exceptional with respect to other Abbotsford Formation sediments and heavy minerals are dominated by those attributable to non-volcanic provenance. This suggests that the mud/claystones are not primary or secondary bentonites.