Gees Bay, North Otago : stratigraphy, paleontology, and geochemistry

Chappell, Debra A. (Debra Ann), 1964-

This study involves detailed geochemical analyses, measured sections, and petrography of a small section on the North Otago coast at Gees Bay about 1 km north of Kakanui. At Gees Bay, a -330 m long section exposes a 5 m thick sequence from the Lower Oligocene (Whaingaroan) Deborah Volcanic Formation, up through the Lower Oligocene (Whaingaroan) upper Ototara Limestone, Upper Oligocene (Waitakian) Otekaike Limestone and Lower Miocene (Otaian) Gee Greensand. Compared to the thick sequence further inland, the Gees Bay section is condensed and complex with the development of a submarine hardground (postWhaingaroan and pre-Waitakian), and numerous phases of phosphate and manganese mineralization (post-Waitakian and pre-Otaian). Detailed illustration of the features of this complex area is shown by numerous sketches and photographs. In the outcrop, there is evidence of a number of very localized events including, on the headland at the northern end of Gees Bay, the development of phosphatic manganese nodules, phosphatized balls in Otekaike Limestone, and phosphatization of the Otekaike Limestone. Vertical and horizontal sections were sampled through the upper unit of the Ototara Limestone and analyzed by X -ray Fluorescence and Electron Microprobe (element mapping) techniques to determine any chemical changes that may have occurred as a result of mineralization, and also the sequence in which mineralization has occurred. Petrography shows that vertically, phosphatization of the limestone decreased down section, with the limestone at the base having similar chemistry to the calculated average composition of least altered limestones in the area. In comparison, the horizontal section showed that the limestone was chemically heterogeneous. A number of mineralization events have occurred in the following sequence: - phosphatization of the Ototara Limestone hardground - manganese nodule formation with subsequent phosphatization - localized phosphatization of the Otekaike Limestone which includes iricorporated octocorals, shell fragments and reworked Kakanui _ Mineral Breccia - formation of phosphate colloform structures infilling depressions in the Ototara and Otekaike Limestones - precipitation of thin, shiny, caramel coloured phosphate layer that coats the Otekaike Limestone with octocorals and the surface of the Ototara Limestone at the contact with the Gee Greensand. Manganese nodules, which occur only at the northern end of Gees Bay were sectioned serially from the core to the surface. Geochemical analyses, element inapping and petrography showed that manganese was precipitated in a grey micritic mud of Otekaike lithology, with subsequent mineralization of sparry calcite and phosphate, implying that the process was occurring in a submarine environment. Late cross-cutting veins are present in the mud and crosscut foraminifera and quartz grains. The data presented in this study indicate the complexity of the processes involved during the formation of this sequence, and this work should provide a basis for more detailed research into areas such as the involvement of micro-organisms in the mineralization processes, why localized features, such as the development of phosphatic manganese nodules occur within the sequence are beyond the scope of this study.

101 leaves : ill. (some col.), maps ; 30 cm. + 1 booklet [8 leaves]

1995Chappell