The interaction of geological, geomorphic and pedogenic processes in the genesis of calcrete

Abstract

The term'calcrete' encompasses any secondary carbonate accumulation found within the regolith, irrespective of morphology and degree of induration: Calcretes cover geographically extensive areas of southern Australia as calcareous soils, surficial deposits of carbonate-rich sediment, and calcareous palaeosols. Combined field and laboratory studies of calcretes on the eastern margin of the St Vincent Basin were undertaken to ascertain the mechanisms responsible for calcrete genesis. Geological processes were very important in the development of the Pleistocene carbonate mantle which blankets a variety of substrates in the study area. The unconsolidated carbonate silt in which the calcrete formed contains pellets (10 to 50 pm in diameter) similar to those found in loess. The fine particle size, combined with the regional distribution of the carbonate mantle, indicate that the carbonate silt may have had an aeolian component. During the last glacial maximum when arid climates prevailed in southern Australia and sea level was significantly lowered, conditions were favourable for the formation of carbonate silt. Coastal calcarenites which lacked vegetation and ephemeral carbonate rich lakes, were stranded during the regression and eroded by strong westerly winds. This carbonate was blown inland and mixed with locally reworked acid-insoluble minerals from fluvial/alluvial environments. Deposition of calcareous material occurred in several cycles. Eventually, the deposits formed a blanket of variable thickness over the Pleistocene fluvial landscape. The fluvial nature of the underlying sediments was identified from field observations, granulometric analyses and studies of the surface textures of quartz grains. In the Willunga and Noarlunga Embayments of the St Vincent Basin, the change from fluvial to aeolian depositional environments was gradational and is recorded by compound palaeosols between the Ngaltinga Formation and the carbonate mantle. It has been established that facies variations within the fluvial deposits strongly influenced the form of calcrete. Low rates of fluvial deposition associated with topstratum deposits (Neva Clay Member) allowed the superposition of several carbonate palaeosols and thus the accumulation of thick calcretes. In contrast, where erosion was more rapid associated with channels and, to a lesser extent, crevasse splays of the Snapper Point Sand Member, palaeosols were eroded and rarely preserved. Geomorphic and pedogenic processes had a significant influence on vertical and lateral facies changes in the carbonate mantle. The geomorphic factors which are most important in understanding lateral changes include topography, soil creep, dissolution of hardpans and distance from modern water courses. These factors were identified during mapping of changes in calcrete morphology. The interaction of inorganic and organic pedogenic processes are thought to have controlled the consistent mineralogical changes observed in the carbonate mantle. Typically, low Mg calcite concentrates in nodules, laminae and hardpans at the top of calcrete profiles, declining in abundance with depth as the proportion of calcian dolomite increases. This trend is mimicked by a decline in Ca/Mg ratio (determined from XRF analyses) with depth. It may be explained by the dissolution of both calcite and dolomite from the aeolian deposit, translocation as ions in solution and reprecipitation initially of calcite and then dolomite as the descending solution became progressively Mg enriched. The reprecipitation of calcite may be explained by either the lower solubility of this mineral, or the fact that many of the micro- organisms concentrated in the upper part of the profile biologically control the deposition of calcite within their mucilagenous sheaths and may also influence the inorganic precipitation of micrite by changing pCO2 and moisture levels within the soil. Tubiform calcified filaments of possible fungal and algal origin concentrate in the indurated materials of the calcrete. These filaments, which bind soil macroaggregates and fecal pellets initiated the formation of nodules and are an important component in the coatings on clasts. Two different groups of needle-fibre calcite have been identified. Small microrods are bacterial in origin, whereas larger needle-fibres which may have epitaxial overgrowths to form serrated varieties, grow within mycelial strands associated with plant roots. These calcified micro-organisms, together with rhizoliths and calcareous insect pupal cases, indicate the pedogenic nature of the calcrete. Geological, geomorphic and pedogenic processes interacted during constructional and destructional phases of calcrete genesis. These phases are polycyclic and often coeval. The constructional phase is characterised by the deposition of carbonate silt, followed by the formation of pellets, nodules, platy structure, wedges, rhizoliths, rectangular texture, hardpan and laminae. The macromorphology, mineralogy and micromorphology of each of these calcareous forms helped to identify the mechanisms controlling their formation. During the destructional phase the hardpans were dissolved, brecciated and eroded to liberate clasts which moved downslope and accumulated as colluvial deposits. These deposits were then recemented into rubbly hardpans. Climate and movements in sea level appear to control the timing of major changes from constructional to destructional phases. Thus calcrete genesis is controlled by the polycyclic interaction of climate, geological, geomorphic and pedogenic processes.