ABSTRACT

Soil-free saline substrates near Alexandra have developed from evaporated marine aerosols dominated by NaCl. The salts accumulate on impermeable substrates where there is minimal chemical interaction between water and rock. Most of the known saline sites occur on outcrops of clay-altered schist exposed immediately below the regional Miocene unconformity, and on clay-rich sediments eroded from altered schist. The unconformity zones that host these saline sites occur linearly along structurally controlled depressions. These unconformity zones have been major targets for placer gold mining, and most of the saline sites occur in abandoned mines where impermeable rocks were exposed by historic excavations. Native salt-tolerant plants (halophytes) have colonised parts of the saline substrates, especially on actively-forming clay pans derived from nearby steep mine cuts. Saline sites are ephemeral, as salt is readily removed in water, sediments can bury the surfaces, and incursion of adventive plant species encourages further sedimentation. Erosional disturbance can overcome these effects temporarily, and surface erosion to bare ground along the strike of the structurally controlled schist unconformity zones may also create new saline sites.

摘要

亚历山德拉附近的无土盐碱基质是由以 NaCl 为主的蒸发海洋气溶胶发展而来的。盐类积聚在不透水的基质上，水和岩石之间的化学相互作用最小。大多数已知的盐碱地都出现在暴露于区域中新世不整合面下方的粘土蚀变片岩露头上，以及蚀变片岩侵蚀的富含粘土的沉积物中。承载这些盐碱地的不整合带沿结构受控的凹陷呈线性分布。这些不整合带一直是砂金矿开采的主要目标，大部分盐碱地出现在废弃的矿山中，历史挖掘暴露了不透水的岩石。天然的耐盐植物（盐生植物）已在盐碱地的部分区域定殖，特别是在从附近陡峭的矿山切割中产生的积极形成的粘土盘上。盐碱地是短暂的，因为盐分在水中很容易去除，沉积物可以掩埋表面，外来植物物种的入侵会促进进一步的沉积。侵蚀扰动可以暂时克服这些影响，沿结构控制的片岩不整合带走向裸露地面的地表侵蚀也可能产生新的盐碱地。