Geology and microfacies analysis of the non-tropical Ototara limestone : paleoenvironment of an Eocene-Oligocene volcanogenic/biogenic succession from North Otago, New Zealand

Berning, Björn.

The Alma Group strata consist of a complex succession of volcanogenic and biogenic rocks that were formed during the Kaiatan and Runangan (Late Eocene) until the end of the early Whaingaroan (Lower Oligocene). These rocks were produced and deposited on the outer shelf and are now exposed along the coast of the Oamaru District in North Otago. Submarine eruptions of volcanoes formed pillow-lava, basalt and thick tuffs that constitute the base of the Alma Group, the Waiareka Volcanics Formation. A high primary productivity, induced by the J upwelling of nutrient-rich water, resulted in the accumulation of diatomite and, furthermore, enhanced the production-rate of the carbonate factory, a species-rich sessile and mobile fauna that established on the volcanic rock bottoms. The depositional system was primarily influenced by the presence or absence of currents: contemporaneously with a fine grained, marly facies of the Ototara Limestone, the Late Eocene Oamaru Diatomite was deposited in the western Oamaru District, in a restricted basin at a depth of about 150 m. Calcarenites and calcirudites of the Ototara Limestone accumulated in the somewhat shallower eastern region on the leeward side of the volcanoes, transported by periodic storm wave action. Due to repeated volcanic eruptions the Late Eocene biogenic sediments are interbedded with tuffaceous material. Iri the earliest Whaingaroan an eruption, occurring near Kakanui, covered the southern Oamaru District with up to 100 m thick tuffs. After the waning of the volcanic activity the upper part of the Ototara Limestone was produced and deposited until the end of the early Whaingaroan. The upper part of the Ototara Limestone was sampled and analysed during this project. 34 thin sections were point counted to determine the relative composition of components, and their microfacies features were examined. The upper Ototara Limestone contains only small amounts of volcanic material and terrigenous clasts are not present, indicating volcanic inactivity and remoteness from land respectively. The biogenic components show a typical composition of temperate water carbonates: bryozoans dominate the assemblage, with lesser amounts of echinoderms, foraminifera and coralline red algae. The classification system for fossil assemblages of temperate carbonates introduced by HAYTON et al. (1995) could be applied to the results. Accordingly, most samples were grouped within the bryomol assemblage, with subordinate echinofor, rhodalgal and rhodechfor assemblages. Bryozoan growth-forms are dominated by the erect rigid delicate-branching type, indicating weak currents in the area of origin. Foraminifera are mainly represented by large forms, such as Asterigerina sp., Amphistegina sp. and Wadella globiformis. Rhodoliths of up to 6 cm in diameter, typical of high energy, shallow water origin, occur frequently in the coarse limestone along the coastline. The common occurrence of these different groups of organisms indicate the presence of a variable seafloor topography with both shallow water, high energy areas as well as low-energy regions. The rhodoliths are rrere interpreted as having originated on the volcanic shoals in shallow water, on the wave-planed edifice of the volcano, while the delicate- branching bryozoans flourished on the rugged hardground on the current-protected, leeward slopes of the volcanoes. During periodically occurring storm events skeletal material of both habitats were transported and deposited together. The occurrence of the foraminifera 'Cycloclypeus' (not clearly identified), Amphistegina, Asterigerina and Wadella, as well as the presence of rhodalgal assemblage, indicate subtropical to warm-temperate sea surface temperatures for at least some periods during the formation of the Lower Oligocene Ototara Limestone. Exposure of the uncemented limestone to meteoric water during early diagenetic history resulted in the dissolution of aragonitic components and absence of moulds after aragonitic fragments. Cement types observed, and indicating a meteoric diagenesis, are scalenohedral spar, syntaxial rim spar, isopachous spar, blocky spar and niicrite. The sediment shows an open to condensed fabric and thus few signs of burial diagenesis. The phosphatised upper boundary of the Ototara Limestone represents the 'Marshall Unconformity', a widely recognised mid Oligocene phase of non-deposition.

124 leaves : col. ill., col. maps ; 30 cm.

2001Berning