Evolution of the eastern Southland lignite basin

Stein, Jenny-Rebecca Kahurangi.

In 2007 Solid Energy New Zealand Ltd conducted high density drilling of the Croydon and Mataura Coalfields of Eastern Southland. Fourteen fully cored holes were drilled in Croydon, down to 305 m depth and thirty-nine from Mataura, down to 231 m depth. These cores were made available by SENZ Ltd for research at the University of Otago. The Croydon and Mataura Coalfields are areas lacking in quality surface exposure and so the cores were an opportunity to gain an unprecedented insight into the stratigraphy of the Eastern Southland Lignite Basin.
All of the cores collected from Mataura penetrated the regional unconformity at the base of the Tertiary sequence, exposing sandstones, siltstones and minor conglomerates of the Murihiku Terrane. The cores are located between two prominent subterranean basement strike ridges previously identified by Isaac & Lindqvist (1990). Mapping depth to basement within the Mataura cores, relative to modern sealevel, reveals the present day basement relief between these highs to be gently dipping ~ 3° NW between two prominent subterranean highs. Geochemical analysis of alteration zones within the uppermost basement strata suggest a history of likely surface exposure and weathering followed by substratal alteration.
Within a number of the Mataura cores Murihiku-derived lithic conglomerates overlie the basement unconformity. The conglomerates indicate active faulting and erosion at the time of earliest sedimentation within the coalfield, inferred to have been occurring along major faults of the Dunsdale Fault System. The age of this fault system is not well constrained, but the conglomerates in base of the cores indicate the faults were active at least by the Late Oligocene.
Reconstructing of basement relief relative to paleosealevel reveals that at the time of a Late Oligocene marine incursion into the coalfield the basement relief was approximately flat lying between the subterranean basement highs in the Mataura Coalfield. This incursion deposited fossiliferous silts and sands of the Chatton Formation across much of Eastern Southland; these occur in the cores from Mataura as a sequence of silt – sand facies averaging 8 m thick at Mataura, and 21 m thick in one core from Croydon. A new fossil locality within the Croydon Coalfield has yielded a diverse array of remarkably well preserved fossils, including Teredo bored wood, a reef coral and estuarine and rocky shore taxa. The Late Oligocene fauna are consistent with shallow marine origin, proximal to a rocky shore and wooded landmass.
Marine regression resulted in deposition of the extensive Gore Lignite Measures. Ti-in-Quartz analysis of quartz gravels within the upper measures in Croydon are consistent with the gravels being derived from the Haast Schist, but reveals an igneous signature of unknown origin within several quartz grains.

vi, 143 leaves : ill. (some col.), maps ; 30 cm. + 1 CD-ROM (4 3/4 in.) and 3 loose maps.

2009Stein