Abstract
Chatton Formation, is a late Oligocene (Duntroonian (27.3–25.2 Ma) to Waitakian (25.2–21.7 Ma)) unit comprising fossiliferous shallow marine sandstones, sandy siltstones, greensands, grainstones and conglomerates. The formation was studied at thirteen localities in Southland and southwest Otago, from north to south: Thompson’s Farm (Freshford), Muddy Terrace (Freshford), Coalbrook, Wendon Valley School, Wendon Creek, Wendon Valley, Balfour Quarry, Chatton, Charters Road Slump, Copelands Pit, Cosy Dell Pit, Hedgehope Stream and Brydone. Exposures are generally thin with one to two facies exposed making correlations between sections difficult. Thin sections of sandstones and conglomerate lithic clasts show sediment was derived from local sources that include the Caples, Maitai and Murihiku basement terranes and possibly the more distal Rakaia Terrane.
Fossils, especially molluscs, are well preserved and some faunas have a high diversity of species (e.g. n>360 at Cosy Dell) owing to the preservation of aragonitic shells. Molluscan faunules have both life and death components and assemblages appear to be somewhat localised and facies controlled with the exception of families Glycymerididae, Pectinidae, Carditidae, Veneridae, Turritellidae, Calyptraeidae, Naticidae and genera Limopsis, Austrofusus and Amalda which are widespread. Shellbeds represent a wide variety of depositional environments from storm, mass flow or transgressive lag deposits for clast-rich beds to quiet waters with next to no bottom currents for greensands with shells in near-life position. The formation was almost certainly deposited during a transgressive systems tract.
Autecology of fossils with close living relatives revealed that water depth was less than mid sublittoral (100 m) and was often much shallower.
The locally derived sediment, shallow water taxa (including the brackish potamidid Pyrazus) and the observation that many sites have abundant vascular plant debris indicates that emergent land was present in the field area throughout the late Oligocene. Dating of the Chatton Formation using age ranges of foraminifera and molluscs reveals that the formation gets younger in southward and westward directions. Localities are of Duntroonian age in the north and Waitakian in the south and west of the field area. This evidence coupled with dating of the underlying lower Gore Lignite Measures and Pomahaka Formation, indicates that there was a rapid, relatively short-lived transgression of the Chatton Sea forming a shallow seaway during the late Duntroonian. This seaway rapidly regressed in the Waitakian, possibly due to falling global sea levels coupled with rapid influx of sediment caused by the prograding river system that deposited the overlying upper Gore Lignite Measures.