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.