Diagenetic silcrete and formation of silcrete ventifacts and aeolian gold placers in central otago, New Zealand

Abstract

A diagenetic silcrete horizon from c. 1 to >6 m thick is commonly present within fluvial quartz sandstones and pebble conglomerates of Eocene Hogburn Formation and early-middle Miocene Dunstan Formation in Central Otago. The silcrete was formed by precipitation of crystalline quartz-overgrowth cement from groundwater focused within permeable, moderately well sorted or well-sorted fluvial sands and silts that were deposited on floodplains that broadened ahead of marine and lacustrine transgressions. The silcrete is hosted near the base of these sandstone and siltstone beds in both formations, and extends downward into the conglomeratic, incised-channel strata that locally underlie them. Downward advance of the cementation front was inhibited by reduced porosity arising from the higher clay content of the basal sandy or conglomeratic beds. Late Miocene-Recent uplift has exposed the highly resistant silcrete horizons at the range margins. In situ silcrete undergoes cantilevering and tensile failure as the underlying weakly cemented sediments or kaolinitised schist that support it are eroded. Resultant large slabs and boulders of silcrete are locally preserved on range slopes and on the surface of fans in the adjacent basins. Aeolian erosion and formation of silcrete ventifacts, stone pavements, and aeolian gold placers has occurred locally and episodically in Central Otago in the rain-shadow east of the Southern Alps since they began to rise in the late Miocene. Many of these aeolian features are relict, but they are actively forming near some range fronts exposed to the prevailing westerly wind. Sand eroded from quartz-rich fluvial sediments exposed at these range fronts is reworked backwards and forwards across the range front in a cyclic process: basinward by mass movement and in ephemeral streams, and rangeward by the westerly winds. Ventifacts are less common on leeward slopes facing away from the westerly wind, because the supply of aeolian sediment is limited, and colluvial, fluvial, and aeolian processes all transport potential aeolian sediment in the same direction without cyclic reworking. © 2005 Taylor & Francis Group, LLC.