Late Quaternary evolution of the Otago margin

Author:

Osterberg, Erich (Erich Christian)

Year:

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Abstract:

A high-resolution seismic reflection and side-scan sonar survey was conducted over a ~220 km2 region of the Otago margin offshore the Otago Peninsula, including the narrowest (~ 10 km) portion of the continental shelf and the submarine Papanui Canyon. High-resolution seismic reflection data are integrated with multi-channel seismic reflection data, bottom samples, cores, bottom photographs and Remote Operated Vehicle (ROV) video footage collected by previous workers, to determine the timing and regulating processes of late Quaternary Otago margin evolution. Sequence ยท stratigraphic interpretation of the high-resolution seismic data reveals a total of eighteen regionally extensive seismic horizons separating fourteen seismic units, which are further subdivided into four late Quaternary shelf sequences. Sequence boundaries represent subaerial and fluvial erosion during sea-level regressions and lowstands, and are generally amalgamated with a shoreface ravinement surface created during the subsequent eustatic transgression. Deltaic and strandline deposits within the forced regressive systems tract, representing the falling stage of sea level, volumetrically dominate shelf sequences, and show a gradational or seismically undifferentiable boundary with highstand systems tract shore-connected wedges further landward. Forced regressive deltaic deposits include abundant mounded, oblique tangential clinoforms that show progressively lower elevations seaward across the shelf, a seaward-dipping superjacent sequence boundary, and internal truncation surfaces indicative of deposition during higher-order eustatic cycles within a lower-order forced regression. Lowstand shoreward-pinching deltaic and strandline wedges on the outermost shelf were deposited contemporaneously with massive canyon fill facies. Backstepping inner shelf wedges and barriers overlying transgressive incised valley fill facies were deposited during seventh-order (~I k.y.) eustatic cycles within fourth-order (~lOO k.y.) transgressions. Two chronostratigraphic models for the Otago shelf sequence have been proposed through correlation with eustatic curves from benthic foraminiferal ()180 ratios and uplifted coastal terraces. They are evaluated through analysis of the sediment accumulation rates, lowstand shoreline positions, and sequence architecture in each model, and compared to those of temporally constrained shelf sequences elsewhere. The youngest three sequence boundaries in the preferred model correspond to lowstands during oxygen isotope stages 2, 4 and 6, and thus the most recent fourth-order Otago shelf sequence contains two fifth-order (~40 k.y.) sequences separated by a stage 4 sequence boundary. Side-scan sonographs from ~ 120-250 m depth around the rim of Papanui Canyon display two characteristic reflection types. Type A reflections within the canyon head and on the north wall are interpreted as sand or gravel waves produced by canyon-parallel currents. Type B reflections on the north wall and a ~210 m deep inter-canyon terrace represent previously-documented carbonate slabs, ridges and debris associated with fluid expulsion. Papanui Canyon has developed through extensive retrogressive submarine mass movement over the past 500 k.y. or more, evidenced by extensive small-scale slumps, slides and debris flow deposits on the canyon wall. Gravity failure primarily occurs during sea-levellowstands when slope sedimentation rates are high and wave base is lower. . Earthquakes, internal canyon waves and fluid expulsion are also implicated as triggering mechanisms, and potentially induce slope failure during eustatic highstands.

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Thesis description:

1 v. (various pagings) : ill., maps (some folded) ; 30 cm. + 1 computer disk.

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OU geology Identifier:

2001Osterberg

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Location (WKT, WGS84):

POLYGON ((170.689816912706505 -45.603782273102574,171.210400613860259 -45.740776281542402,171.110480283566886 -45.927777923803198,170.570974144855313 -45.778501229669608,170.689816912706505 -45.603782273102574))

Files

http://download.otagogeology.org.nz/temp/Abstracts/2001Osterberg.pdf

Collection

Citation

Osterberg, Erich (Erich Christian), “Late Quaternary evolution of the Otago margin,” Otago Geology Theses, accessed May 22, 2024, https://theses.otagogeology.org.nz/items/show/386.

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