Distribution patterns of gypsum and kalistrontite in a dry lake basin of the southwestern Kalahari (Omongwa pan, Namibia)

Abstract

The deposits of the Omongwa pan, southwestern Kalahari, Namibia, are partly gypsiferous and locally contain small amounts of kalistrontite. The distribution patterns of gypsum (CaSO 4 .2H 2 O) and kalistrontite (K 2 Sr(SO 4 ) 2 ) are mainly determined by groundwater depth and by the lithological composition of the deposits. The latter determines the hydraulic conductivity for saturated and unsaturated flow and therefore controls the depth of the interval within the deposits where evaporation and mineral precipitation can take place. Other factors that affect the distribution of the evaporite minerals are the patterns of groundwater flow within the basin and the occurrence of a redistribution of salts during short flooding stages.Gypsum occurs as crystals of four distinct morphological types, which each developed in different conditions: prismatic crystals formed both as subaqueous precipitates and as crystals that developed within the sediment matrix of a brine-saturated surface layer; sub/euhedral tabular crystals formed within brine-filled macropores; hemi-bipyramidal crystals formed in a nearly permanently brine-saturated part of a subsurface horizon (phreatic zone), as the product of recrystallization of an older gypsum occurrence; and lenticular crystals formed by intrasediment growth in the vadose zone. The development of these morphological types of gypsum crystals is marked by differences in the differential inhibition of growth of the various crystal forms. The further growth of lenticular crystals, which are most strongly affected by differential growth inhibition, is still characterized by a difference in degree of inhibition between the {111} and {103} forms, expressed as a change in orientation of the plane of flattening.

Analyses of Recent Sediment Surface Dynamic of a Namibian Kalahari Salt Pan Based on Multitemporal Landsat and Hyperspectral Hyperion Data

“…2017, 9, 170 4 of 24 sand-sized detrital grains with varying contents in halite (>30%), gypsum (5%-40%) and calcite (10%-20%) mixed with the clay and mica minerals sepiolite, muscovite, and smectite. The lower units have evaporite-filled cracks in the upper part (vadose zone) and generally increase in calcite content with depth until the calcareous mudstone bedrock is met in up to 3.5 m below the surface [27]. It is further noted that the mineralogy of the pan is sensitive to changes of the hydrological conditions.…”

“…State-of-the-art methodological tools in optical remote sensing for change detection analyses based on multitemporal Landsat data and for surface spectral mapping based on hyperspectral imagery are used in order to deepen our knowledge and understanding of the surface processes in the Omongwa pan. The pan evolved in calcareous deposits of Kalahari Group which are exposed in the paleo river valley and in interdune space of shallow dunes [27,33,34]. Along these paleo river courses, Early Kalahari deposits were cemented by pedogenic and groundwater calcretes driven by re-occurring changes in moisture and sedimentary regime during the Tertiary until the Late Pleistocene [35].…”

“…This study combines both the potential of hyperspectral EO-1 Hyperion satellite data for the identification and mapping of major salt crust types and the potential of freely available 30 years Landsat time-series archives evaluating the temporal stability of the pan surface, linked with accurate elevation information from LIDAR data. The Omongwa salt pan in Namibian Kalahari, one of the largest and most studied salt pan in its region, is known for episodic flooding events and ongoing formation of evaporates minerals at the surface [27,28]. The Omongwa salt pan is taken as the object of investigation in this paper, as representing other extended saline pan bodies in arid regions of the world that could be similarly studied with newly available remote sensing data.…”

“…Prominent outcrops of calcareous material appear along the pan margins [36], e.g., ~1 m high scarp at the northeastern margin of Omongwa pan. Detailed mineralogical information of the pan deposits are known from a diagonal sampling survey from the SW to NE of Omongwa pan conducted by Mees [27] in 1991 and 1992. The upper lithological unit (top ~50 cm) of the pan deposits is mainly sand-sized detrital grains with varying contents in halite (>30%), gypsum (5%-40%) and calcite (10%-20%) mixed with the clay and mica minerals sepiolite, muscovite, and smectite.…”

“…The upper lithological unit (top ~50 cm) of the pan deposits is mainly sand-sized detrital grains with varying contents in halite (>30%), gypsum (5%-40%) and calcite (10%-20%) mixed with the clay and mica minerals sepiolite, muscovite, and smectite. The lower units have The pan evolved in calcareous deposits of Kalahari Group which are exposed in the paleo river valley and in interdune space of shallow dunes [27,33,34]. Along these paleo river courses, Early Kalahari deposits were cemented by pedogenic and groundwater calcretes driven by re-occurring changes in moisture and sedimentary regime during the Tertiary until the Late Pleistocene [35].…”