Initial sedimentation and subsequent diagenesis in the Eastern Southland Lignite Basin, southern New Zealand

Abstract

The Eastern Southland Lignite Basin (ESLB) was initiated in the Late Oligocene at the same time that transpressional deformation began along the Alpine Fault. Initial sedimentation occurred near sea level, with interfingering of marine sediments and non-marine lithic conglomerates and associated lignite measures. Basal conglomerates are immature with rounded and angular clasts, and were locally derived from rising basement rocks on nearby basin margins. Conglomerates in younger lignite measures in the ESLB contain abundant quartz clasts derived from Central Otago schist, 100 km away, as well as some locally derived lithic material. Groundwater movement in the evolving basin caused pervasive clay alteration of lithic conglomerates and the underlying basement down to >10 m below the unconformity, as well as partial alteration of lithic clasts in sandstones. Alteration occurred under chemically reducing conditions and formed kaolinite, ferrous iron-bearing vermiculite-smectite and siderite. Post-depositional uplift in the ESLB has resulted in erosion of more than half of the sediments originally deposited. Erosion of the diagenetically altered rocks rich in clay left only minor residual quartz sand and gravel, with the bulk of the sediment being transported from the basin as clay in suspension or bed-load. In contrast, younger quartz-bearing conglomerates higher in the lignite measures sequence were recycled into the quartz-rich Pliocene conglomerates and younger gravels which now mantle the lignite measures over much of the basin.