Paleoenvironmental implications of stormscoured, ichnofossiliferous mid-tertiary limestones, Waihao district, south Canterbury, New Zealand

Abstract

Tertiary marine transgression culminated in the Waihao district of South Canterbury with deposition of two carbonate formations. The Waikakahi Formation (Whaingaroan-Duntroonian) comprises massive calcareous fine greensand and quartzose biomicrite, devoid of sedimentary structures except for poorly preserved trace fossils. The Arno Limestone (Duntroonian-Waitakian) consists essentially of sorted, bioturbate glauconitic biosparite and shows extensive cross-bedding plus large scour channels in the lower part of the unit; discrete lebensspuren are common and include traces of spatangoid echinoids.Waikakahi sediments accumulated in a distal but shallow shelf environment partially cut off from free marine circulation. Restriction of circulation is inferred to have resulted from a discontinuous line of volcanogenic highs positioned roughly along the present coastline; previous work in the Oamaru region of North Otago supports this inference. A gravelly layer at the base of the Arno Formation reflects opening of this environment to relatively free circulation, which continued during Arno sedimentation. Long wavelength, low amplitude sand waves driven by semipermanent currents moved well-sorted fine bioclastic sediment eastwards across a shallow shelf. Additional bioclastic sediment was contributed by an in situ epifaunal and infaunal community. Whereas the deposits reflect relatively open marine conditions, existence of a shoal or bank to the south-south-east is inferred to have prevented free circulation from that direction; the shoal lay leeward of the remnant volcanogenic high of the Oamaru area. Northward-flowing, storm-generated currents sporadically breached this partial barrier, scoured broad channels in the Arno environment, and locally deposited therein highly glauconitic sediments swept from the shoal. A gradual westward migration of the highest energy zones throughout Arno time is inferred from systematic variation of lebensspuren. Diminution of scour channels and absence of distinctive cross-bedding during deposition of the youngest Arno sediments in the Waihao district suggests that the influence of the southerly bathymetric barrier had diminished and energy levels within the environment had decreased.

A Mössbauer Investigation of Glauconite and its Geological Significance

“…A very low (to negative) sedimentation rate in the genetic environment is considered essential (e.g., Van Andel and Postma, 1954;Cloud, 1955;Burst, 1958b;McRae, 1972), and water depths equivalent to the neritic environment (immediately sublittoral to the upper part of the continental slope) are generally inferred (see McRae, 1972). A periodically agitated low energy environment appears to be necessary (Van Andel and Postma, 1954;McRae, 1972;Odin, 1975), but in many occurrences, current generated structures probably reflect reworking after the glauconite was formed (e.g., Ward and Lewis, 1975;McConchie, 1978;McConchie and Lewis, 1978). A pH of 7 to 8 (Fairbridge, 1967) and slightly reducing conditions (Eh < 0) are generally considered to be the most favorable (Cloud, 1955;Burst, 1958a;Fairbridge, 1967), although the reducing conditions may be confined to the immediate vicinity of the proto-glauconite (Burst, 1958a;Norris, 1964), while the overall conditions remain slightly oxidizing (Van Andel and Postma, 1954).…”

Southwest Pacific Gateways

“…Carter et al, 1996;McCave and Carter, 1997;Thiede et al, 1997;Weaver et al, 1997Weaver et al, , 1998. This work notwithstanding, the lack of cored offshore drill holes means that we remain ignorant of the post-mid-Cenozoic paleoceanography for most of the region, apart from occasional studies from the western shallow water and onland edge of the basin (Ward and Lewis, 1975;Carter, 1985;L. Carter et al, 1996;R.…”

“…Sediments on the eastern New Zealand margin at shelf to upper bathyal depths (50-1000 m) are known to have been strongly affected by currents since at least the late Oligocene (Ward and Lewis, 1975;Carter, 1985;L. Carter et al, 1996).…”

Untitled

“…F12) (Lu and Fulthorpe, 2004), and onshore outcrops of Kekenodon Group greensand and limestone indicate that the current influence extends back to the late Oligocene (Ward and Lewis, 1975). The STF, Subantarctic Front (SAF), and associated currents may have existed close to their present positions relative to New Zealand by the middle or latest Miocene (Carter et al, 2004c;Nelson and Cooke, 2001).…”

“…Seismic interpretation supports a current-related origin by indicating that the paraconformity forms the base of the interval of sediment drift deposition. Indeed, immediately post-Marshall Paraconformity, sedimentation involved sediment drift deposition in shallow (Ward and Lewis, 1975), intermediate Lu et al, 2003), and deep (Shipboard Scientific Party, 1999a;Carter et al, 2004c) water settings. There are indications from Leg 181 drilling that the paraconformity developed in deep (bathyal) water ~1-2 m.y.…”