Relative sea-level change in the Mitchells Rocks beds, Wangaloa formation : a combined allostratigraphic, bedform, and grain settling velocity approach
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The cliff-face outcrop at Mitchells Rocks on the South Otago Coast of New Zealand display some 22.5 m (divided into 12 allobeds) of very-well sorted arkosic-arenite sandstone capped by a 1.5 m thick burrow-mottled greensand unit. Field relationships place the Mitchells Rocks Beds in the lower part of the early Paleocene Wangaloa Formation- specifically in the Mitchells Rocks Allomember herein proposed.
Bedforms, present between the erosion surfaces that define the majority of allobeds, plus other criteria such as Ophiomorpha traces;suggest that the strata formed in !Jpper foreshore to _1.ower §_horeface environments. Through the geomorphologic concept of 'effectiveness' and a fades model approach, variation in bedform type has been used to chronicle changes in relative water depth on the~ 'Wangaloan Shelf.
A high precision statistical settling velocity analysis (based the concept of hydraulic equivalence) of 43 sediment samples provided a second and independent relative sea-level curve for the Mitchells Rocks Beds. The rationale behind a detailed settling velocity analysis rests on:
1) that shallow marine sedimentary environments are~ sensitive to sea-level fluctuations;
2) that grain settlingvelocity is a more useful measure of sediment characteristics than grain diameter; and
3) that very-fine sand (that composes the modal grain-size of the Mitchells Rocks sediment) is the most easily erodible grain-size on the Hjulstrom Curve of grain transP9rt characteristi~s.
A similar settling velocity analysis of modern shelf sediments establish' that the standard deviation parameter of the measured spread of grain settling velocities is perhaps the most useful statistic to reveal changing water conditions on the shelf, though the mean settling velocity statistic also appears to work fairly well to discriminate [relative] water depth within each allobed.
Test Sample analyses of the RS~ - the machine used to measure the settling velocity characteristics of both the modern & ancient sediments above - allow errors to be placed on the measured populations. Sensitivity analysis of the RSA shows that a grains' settling velocity is not significantly affected by relatively large changes in water column density and viscosity (designed to simulate cold, moderately muddy saline water) and thus implyJthat changes between measured settling velocity profiles can be directly related to more diagnostic [geohistorical] variables such as the fluid velocity field.
Other techniques employed to clarify other controlling factors in the sedimentary system include: an analysis of shell accumulation types - for net sedimentation rate; petrographic analysis - for sediment supply characteristics; and analysis of fragmented glaucony grains to reveal the degree of sediment reworking.
Reasonably good agreement between the [qualitative] bedform and [quantitative] settling velocity · approa~hes has been obtained to establish a relative sea-level curve capable of forming the Mitchells Rocks Allobeds. Results of this thesis imply that the Mitchells Rocks Beds have formed in response to relatively high frequency /low amplitude oscillations of sea-level superimposed on an overall transgression that itself is probably the result of basement subsidence under passive margin extensional tectonics.
A curve constructed via the addition of four (4) sine curves that maintain the essential periodicities proposed in the Milan:kovitch theory of climatic fluctuation reproduce the essential features of the stratigraphic column at Mitchells Rocks. If the Milankovitch Theory is accepted as the mechanism· responsible for formation of the Mitchells Rocks.Beds then the total time represented by the sandy beds is approximately 200,000 years at an average sedimentation rate of 11 cm /ka.
Bedforms, present between the erosion surfaces that define the majority of allobeds, plus other criteria such as Ophiomorpha traces;suggest that the strata formed in !Jpper foreshore to _1.ower §_horeface environments. Through the geomorphologic concept of 'effectiveness' and a fades model approach, variation in bedform type has been used to chronicle changes in relative water depth on the~ 'Wangaloan Shelf.
A high precision statistical settling velocity analysis (based the concept of hydraulic equivalence) of 43 sediment samples provided a second and independent relative sea-level curve for the Mitchells Rocks Beds. The rationale behind a detailed settling velocity analysis rests on:
1) that shallow marine sedimentary environments are~ sensitive to sea-level fluctuations;
2) that grain settlingvelocity is a more useful measure of sediment characteristics than grain diameter; and
3) that very-fine sand (that composes the modal grain-size of the Mitchells Rocks sediment) is the most easily erodible grain-size on the Hjulstrom Curve of grain transP9rt characteristi~s.
A similar settling velocity analysis of modern shelf sediments establish' that the standard deviation parameter of the measured spread of grain settling velocities is perhaps the most useful statistic to reveal changing water conditions on the shelf, though the mean settling velocity statistic also appears to work fairly well to discriminate [relative] water depth within each allobed.
Test Sample analyses of the RS~ - the machine used to measure the settling velocity characteristics of both the modern & ancient sediments above - allow errors to be placed on the measured populations. Sensitivity analysis of the RSA shows that a grains' settling velocity is not significantly affected by relatively large changes in water column density and viscosity (designed to simulate cold, moderately muddy saline water) and thus implyJthat changes between measured settling velocity profiles can be directly related to more diagnostic [geohistorical] variables such as the fluid velocity field.
Other techniques employed to clarify other controlling factors in the sedimentary system include: an analysis of shell accumulation types - for net sedimentation rate; petrographic analysis - for sediment supply characteristics; and analysis of fragmented glaucony grains to reveal the degree of sediment reworking.
Reasonably good agreement between the [qualitative] bedform and [quantitative] settling velocity · approa~hes has been obtained to establish a relative sea-level curve capable of forming the Mitchells Rocks Allobeds. Results of this thesis imply that the Mitchells Rocks Beds have formed in response to relatively high frequency /low amplitude oscillations of sea-level superimposed on an overall transgression that itself is probably the result of basement subsidence under passive margin extensional tectonics.
A curve constructed via the addition of four (4) sine curves that maintain the essential periodicities proposed in the Milan:kovitch theory of climatic fluctuation reproduce the essential features of the stratigraphic column at Mitchells Rocks. If the Milankovitch Theory is accepted as the mechanism· responsible for formation of the Mitchells Rocks.Beds then the total time represented by the sandy beds is approximately 200,000 years at an average sedimentation rate of 11 cm /ka.
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167 p., [10] leaves of plates : ill. (some col.), maps ; 30 cm.
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1993Smith_RJ
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Citation
Smith, Richard J. (Richard James), “Relative sea-level change in the Mitchells Rocks beds, Wangaloa formation : a combined allostratigraphic, bedform, and grain settling velocity approach,” Otago Geology Theses, accessed February 8, 2025, https://theses.otagogeology.org.nz/items/show/291.