High resolution seismic study of the offshore extension of the Waihemo fault system
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The offshore extension of the Waihemo Fault System, a system of seven southeast trending, northeast dipping, reverse faults, has been studied using high resolution marine seismics, supported by the information from the following sources:
a. onshore stratigraphy (sequences, contacts, thicknesses),
b. onshore faulting,
c. offshore sedimentary sequence (Galleon-1 well),
d. BP Shell Todd deep seismic profiles,
e. aerial observations of structures in shallow water,
f. sampling by underwater divers,
g. underwater videos, and
h. sidescan sonar imagery of the sea floor.
The onshore sedimentary sequence of Haast Schist, Horse Range Greywacke/Schist Conglomerate, Puke-hiwi-tahi Quartz Conglomerate, Herbert Formation, Katiki Formation, Otepopo Greensand, Moeraki Mudstone, Kurinui Formation, Hampden Mudstone, Burnside Mudstone, Concord Greensand, Caversham Sandstone, Goodwood Limestone and overlying terrestrial sediments was divided into seismic groupings of basement, Shag Point Group, Katiki/Herbert Group, Matakaea Group, Concord Greensand, Caversham/Goodwood Group and overlying Pliocene-Recent marine clays/mudstones, and seismic profiles interpreted accordingly.
The Waihemo Fault System, as observed on land, was seen to terminate a short distance offshore. Relatively close to shore, the Waiheino Fault System was seen to consist of the faulted, southeastwardly plunging, Shag Point anticline. The otherwise symmetrical sedimentary sequence across the Shag Point anticline was seen to be truncated by the Waihemo number 2 fault. Southeast of the termination of surface expression of faulting of the Waihemo Fault System, the Shag Point anticline could be seen as a continuous symmetrical sequence cutting progressively older sediments, and back again.
To the south of Shag Point, subparallel to the present coastline and perpendicular to the Waihemo Fault System, the northeast trending Titri Fault System has been inferred. East of the inferred junction of the Waihemo and Titri fault systems, displacement across the Waihemo Fault System is seen to be relatively small, the structure consisting mainly of the faulted Shag Point anticline. It appears that the southeastern side of the Titri Fault System and the northeastern side of the Waihemo Fault System have effectively moved upwards as one, explaining the observed termination of the Waihemo Fault System east of the Titri Fault System. Surface faulting of the Waihemo Fault System ceases less than 8 kilometres from land, evidence of the fault system southeast of that point expressed only by the Shag Point anticline. Surface expression of the Waihemo number 2 fault, which has been mapped as far northwest as the margins of the Maniototo Plains (Bishop, 1979), is also seen to extend further to the southeast than any other Waihemo fault, making it by far the most laterally extensive fault in the Waihemo Fault System. Basement faulting of the Waihemo Fault System, and any overlying folding, terminates with the Waihemo number 7 fault at a distance of only 15 kilometres from land.
The evolution of sedimentary structures identified within the Shag Point area has been explained in terms of Carter's (1988) seismostratigraphic sequences. Seven major, seismically recognisable disconformities have been correlated with postulated eustatic curves (Haq et al., 1987). The author considers, however, that the effects of global sealevel changes have been restricted to modifying sedimentary, evolution within the seismostratigraphic sequences, rather than as a primary influence.
a. onshore stratigraphy (sequences, contacts, thicknesses),
b. onshore faulting,
c. offshore sedimentary sequence (Galleon-1 well),
d. BP Shell Todd deep seismic profiles,
e. aerial observations of structures in shallow water,
f. sampling by underwater divers,
g. underwater videos, and
h. sidescan sonar imagery of the sea floor.
The onshore sedimentary sequence of Haast Schist, Horse Range Greywacke/Schist Conglomerate, Puke-hiwi-tahi Quartz Conglomerate, Herbert Formation, Katiki Formation, Otepopo Greensand, Moeraki Mudstone, Kurinui Formation, Hampden Mudstone, Burnside Mudstone, Concord Greensand, Caversham Sandstone, Goodwood Limestone and overlying terrestrial sediments was divided into seismic groupings of basement, Shag Point Group, Katiki/Herbert Group, Matakaea Group, Concord Greensand, Caversham/Goodwood Group and overlying Pliocene-Recent marine clays/mudstones, and seismic profiles interpreted accordingly.
The Waihemo Fault System, as observed on land, was seen to terminate a short distance offshore. Relatively close to shore, the Waiheino Fault System was seen to consist of the faulted, southeastwardly plunging, Shag Point anticline. The otherwise symmetrical sedimentary sequence across the Shag Point anticline was seen to be truncated by the Waihemo number 2 fault. Southeast of the termination of surface expression of faulting of the Waihemo Fault System, the Shag Point anticline could be seen as a continuous symmetrical sequence cutting progressively older sediments, and back again.
To the south of Shag Point, subparallel to the present coastline and perpendicular to the Waihemo Fault System, the northeast trending Titri Fault System has been inferred. East of the inferred junction of the Waihemo and Titri fault systems, displacement across the Waihemo Fault System is seen to be relatively small, the structure consisting mainly of the faulted Shag Point anticline. It appears that the southeastern side of the Titri Fault System and the northeastern side of the Waihemo Fault System have effectively moved upwards as one, explaining the observed termination of the Waihemo Fault System east of the Titri Fault System. Surface faulting of the Waihemo Fault System ceases less than 8 kilometres from land, evidence of the fault system southeast of that point expressed only by the Shag Point anticline. Surface expression of the Waihemo number 2 fault, which has been mapped as far northwest as the margins of the Maniototo Plains (Bishop, 1979), is also seen to extend further to the southeast than any other Waihemo fault, making it by far the most laterally extensive fault in the Waihemo Fault System. Basement faulting of the Waihemo Fault System, and any overlying folding, terminates with the Waihemo number 7 fault at a distance of only 15 kilometres from land.
The evolution of sedimentary structures identified within the Shag Point area has been explained in terms of Carter's (1988) seismostratigraphic sequences. Seven major, seismically recognisable disconformities have been correlated with postulated eustatic curves (Haq et al., 1987). The author considers, however, that the effects of global sealevel changes have been restricted to modifying sedimentary, evolution within the seismostratigraphic sequences, rather than as a primary influence.
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162 leaves in various foliations (some folded) : ill. (some col.), maps (some col.) ; 30 cm.
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1990Allan
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POLYGON ((170.822827669967126 -45.611923730904053,170.641861819971922 -45.606186312875742,170.650759875774099 -45.392741304135882,170.838193362384033 -45.397158907832768,170.822827669967126 -45.611923730904053))
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Allan, Tim M. H. (Timothy Michael Hope), 1961-, “High resolution seismic study of the offshore extension of the Waihemo fault system ,” Otago Geology Theses, accessed December 7, 2024, https://theses.otagogeology.org.nz/items/show/240.