Paleoenvironmental analysis of a lower Miocene regressive sequence, Upper Tengawai River, South Canterbury, New Zealand.

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Douglas, B. J. (Barry James), 1947-

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Abstract:

The Lower Miocene Bluecliffs Silt and Southburn Sand (505 m thick) form the regressive sequence of sediments exposed at the Upper Tengawai River section, South Canterbury.
The sedimentologic, coarse fraction, microfauna, macrofauna and stratigraphic relationships of these sediments are investigated to provide multi-variable criteria for paleoenvironmental interpretation.
The Bluecliffs Silt is a predominantly massive bedded sandy mud-sandy silt which is divided into lower and upper members. The Upper Bluecliffs Member is readily distinguished from the lower member by the coarser grain size, higher glauconite content, abundant biogenic structures and disparities in the micro and macrofauna. Southburn Sand Exe Creek Member sediments comprise pi-cross-stratified sands, homogenous fine sand beds, and shell beds. The Te Ngawai Member exhibits a diversity of complex interbedded lithologies from which eight lithofacies are recognized and which present good evidence of cyclicity. These illustrate gradations from cross-stratified coarse to medium sands, flaser and alternating sand and mud beds through to intensely bioturbated muds, and lignite seams.
Sediments were analysed according to the empirically based statistical techniques of Passega, Rizzini, Visher and Folk. These techniques have been proposed for the interpretation of mechanisms of transport, and the depositional environment of sediments from grain size data. Under low energy conditions, fine sediment deposition by suspension was dominant in the Lower Bluecliffs Silt environment. The currents increased in intensity locally in the Lower Bluecliffs Silt, and in the Upper Bluecliffs Silt environment they were associated with the progressive shallowing of the sea. Shoreward, Exe Creek Member sediments were comparable with Recent high energy wave agitated littoral - beach and moderate energy sheltered nearshore environments. Te Ngawai Member sediments indicate a diversity of sediment transport mechanisms ranging from high energy, predominantly bedload sedimentation, medium-low energy fluctuating saltation and suspension sedimentation, through to low energy, suspension dominated sedimentation. The widely fluctuating range of current velocities indicated for the Te Ngawai Member typify Recent and ancient tidal-flat estuarine environments.
Coarse fraction analyses reveal sediments of comparable identity with those described from the Recent Central Texas shelf environment - Lower Bluecliffs Silt resembles outer shelf; Upper Bluecliffs Silt, inner shelf; and Exe Creek Member, gulf beach and inlet environments. Variations in coarse fraction composition within the Bluecliffs Silt are interpreted in relation to their geographic and bathymetric location on an inferred topographic profile, constructed from variations in mean grain size.
The ratio of benthonic-planktonic foraminiferids in the Bluecliffs Silt indicates higher concentrations of benthonic foraminiferids on the offshore topographic highs. The benthonic foraminiferids also progressively increase (in the Upper Bluecliffs Silt and Exe Creek Member) towards the shore. The generic distribution of benthic foraminiferids and ostracods from the Lower and Upper Bluecliffs and Exe Creek Member are respectively similar to Recent foraminiferid and ostracod associations in outer shelf, inner shelf and nearshore-littoral environments.
A number of foraminiferids common to forms which adhere to vegetation in Recent environments occur in both the Bluecliffs Silt and Exe Creek Member. They thereby infer that the depth at which these sediments were deposited was no greater than the maximum depth at which photosynthesis occurred.
Nine major benthic macrofaunal assemblages have been recognized, and may be distinguished by differences in taxonomy, trophic type, density, diversity and mode of life, that may be related to inferred geographic position, environmental stability and food resources. Bluecliffs Silt assemblages (six) were defined on the relative numerical abundance of taxa; The similar recurring bathymetric and geographic location of these assemblages on the Bluecliffs sea floor further l substantiates the validity of the topographic profile technique.
Macrofaunal assemblages defined are as follows: (1) Pteromyrtea-Notocorbula Assemblage (2) Limopsis-Notocorbula Assemblage (3) Flabellum-Notocorbula Assemblage ( 4) Limopsis- Notocorbula-Zeacolpus Assemblage (5) Zeacolpus-Austrofusus - SpissateZZa Assemblage (6) - Notocorbula-Zeacolpus-Waiparia Assemblage (7) Glycymeris-Waiparia-Amalda Assemblage (8) Ophiomorpha Assemblage (9) Bartrumia Assemblage.
Macrofaunal assemblages (1~9) indicate a broad variation in depth from moderate shelf depths, progressively shallowing to shallow water-intertidal depths.
Local variations in sea floor relief on the Bluecliffs Shelf may be similar to ridge and swale topography known from Recent shelf environments. Variations in grain size and rates of sedimentation caused by estuaries dissipating sediment by tidal flow onto the shelf, may have been capable of moulding sea floor topography.
Stratigraphic relationships, together with sedimentation structures and grain size data facilitate the detailed interpretation of Southburn Sand lithofacies. Exe Creek Member depositional environments exhibit a transition through a marine subtidal nearshore high energy zone, offshore bar, and intertidal-beach zone. Te Ngawai Member stratigraphy represents a tidal flat-estuarine environment and within which the following depositional environments are recognized: (1) intertidal sand bar (2) a wide diversity of tidal channel types (3) outer low tidal flats (4) outer high mud flats (5) mid tidal iii flats (6) inner low tidal flats (7) inner high mud flats (8) marshland.
The cyclic succession of Te Ngawai Member lithofacies exhibit fining and coarsening upward sequences that reflect the zonation of sediment transport processes across intertidal flats. From these sequences a quantitative measure of paleotidal range of 2.2 - 3.5(+) metres has been calculated for the Southburn Sand depositional environment.

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325 leaves : illus. (part col.), 2 fold. tables in pocket ; 30 cm.

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1975Douglas

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POLYGON ((171.254056964749822 -44.561508676777478,170.487140854458602 -44.545298026708949,170.517738315441193 -44.04872478814336,171.285975715325833 -44.06522099598795,171.254056964749822 -44.561508676777478))

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http://download.otagogeology.org.nz/temp/Abstracts/1975Douglas.pdf

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Douglas, B. J. (Barry James), 1947-, “Paleoenvironmental analysis of a lower Miocene regressive sequence, Upper Tengawai River, South Canterbury, New Zealand.,” Otago Geology Theses, accessed May 22, 2024, https://theses.otagogeology.org.nz/items/show/83.

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