Holocene stratigraphy and sequence architecture of the Blueskin Bay estuary, East Otago

Thomas, David Gregory, 1975-

The Holocene stratigraphy of the Blueskin Bay estuary was investigated using percussion cores. The Blueskin Bay estuary Holocene sequence comprises eight lithofacies arranged into a central estuarine basin consisting of an exposed intertidal flat, a sheltered intertidal flat and two estuarine bay head deltas. The exposed and sheltered intertidal flats are occupied by marine-influenced to paralic sand accumulations consisting of transgressive to highstand open-bay/estuarine deposits. The estuarine bay head deltas are dominated by tidalfluvial highstand point-bar and channel-lag deposits. Radiocarbon ages from in-situ and reworked shell and organics are used to establish the chronology of the Holocene stratigraphy. Holocene marine deposition commenced during the post-glacial transgression (ea. 9-7ka) and was dominated by an open-bay depositional environment in the position of the present central estuarine basin. At the time of the maximum transgression (ea. 6.2 ka) the majority of the Holocene estuarine sediment was in place. Hightsand deposition has been characterised by a period of erosion within the central estuarine basin and the episodic accretion and progradation of the estuarine bay-head deltas and around the bay margins. A database of 18 radiocarbon dates from estuarine sediments of the Blueskin Bay estuary, of which 8 are unpublished, is presented in this study. The elevation data have been reduced to a common datum (Mean Sea Level, MSL) and the sources of error assessed. Using modern lithological and biological relationships relative to present sea-level, radiocarbon dates can be converted into paleosea-level indicators. The upper and lower limits of the paleosea-level dataset provide an envelope representing local relative sea-levels. The envelope is consistent with a culmination of the post-glacial transgression after 6.5 ka BP, followed by a minor regression of -1.4 m from 5.5-5 ka BP, followed by a minor transgression of+ 1.4 m between 5-3.2 ka BP. Twelve radiocarbon dates from estuarine sediments of the Papanui and Hoopers Inlets located on the Otago Peninsula, corrected to a common datum (MSL), are used to constrain a relative sea-level curve and provide a proxy for the relative sea-level curve of the Blueskin Bay estuary. The paleosea-level dataset for the Papanui and Hoopers Inlets indicates a stillstand of +0.2m occurred at ~6 ka BP, followed by a minor regression of -0.7 m between 6 and 3.8 ka BP, followed by a minor transgression of +0.5 m from 3.8 to 3 ka BP. As with the newly proposed Blueskin Bay estuary relative sea-level curve, the last 3 ka BP of the sea-level curve for the Papanui and Hoopers Inlets has been stable. Newly proposed relative and eustatic sea level curves for the Blueskin Bay estuary, Papanui and Hoopers Inlets provide additional reference localities for New Zealand Holocene regional sea-level studies. All available data from the Holocene sediments infilling the estuary of Blueskin Bay and the Papanui and Hoopers Inlets suggest there has been no tectonic uplift or subsidence of the East Otago coast or Otago Peninsula during the mid to late Holocene. The large core and radiocarbon database from the Blueskin Bay estuary allowed an analysis and interpretation of the systems tracts and parasequences developed during the late Holocene. Within this interpreted sequence stratigraphy, the transgressive systems tract (TST) corresponds to the sequence boundary between the basal Holocene/Pliestocene superimposed by a ravinement surface and/or marine erosion surface (MES-1 ). The maximum flooding horizon (MFH), peak eustatic sea-level horizon (PESH), and/or peak relative sea-level horizon (PRSH), defined as isochrons equivalent to the maximum transgression of the shoreline and peak eustatic or relative sea level [within a cycle] respectively (after Larcombe & Carter, 1998), are not necessarily marked by a physical surface/sedimentary boundary within the Blueskin Bay estuary. The majority of the Holocene sediments deposited within the Blueskin Bay estuary correspond to the highstand systems tract (HST) of the post-glacial sea-level cycle. A geochemical study of the Blueskin Bay estuary sediments reveals distinct provenance signatures and some evidence for trace metal contamination proximal to possible pollution point sources.

2 v. : ill. (some col.), maps ; 30 cm.

2000Thomas

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