The nature of calcareous deposits along pan margins in eastern central Namibia

Abstract

In a region along the western margin of the Kalahari in eastern Namibia and western Botswana, many pan basins have mainly calcareous deposits along part of their margins. These are typically lined by low vertical scarps. In Namibia, these pans are mainly located in dry river beds. The petrographical study of these deposits demonstrates that they consist of lacustrine sediments that have to a varying extent been affected by early-diagenetic processes and by the formation of late-diagenetic features.The original composition of the deposits ranges from highly calcareous sediments, typically with ostracod, diatom and charophyte remains, to entirely non-calcareous diatomites. The deposits generally show an upward increase in total carbonate content, which is mainly a synsedimentary feature. The early-diagenetic processes that affected the deposits include the formation of orthic siliceous nodules. At a later stage, secondary calcite enrichment occurred, leaving only the silica-impregnated sections unaffected. This enrichment partly accounts for the upward increase in total carbonate content in some profiles and often resulted in the development of a highly calcareous surface horizon. Sepiolite and amorphous silica that are part of the groundmass of the deposits may also partly have formed at this stage. Late-diagenetic features include various forms of secondary calcite and silica.This study of pan basins in eastern central Namibia indicates that a lacustrine rather than purely pedogenic origin should also be considered for calcareous deposits that commonly occur along pan margins in other parts of southern Africa.

Freshwater Limestone In An Arid Rift Basin: A Goldilocks Effect

“…It seems that conditions for the development of reasonably thick (0.5-1 m) freshwater limestones in arid to hyperarid setting have to be just right, thus we use the term ''Goldilocks Effect.'' In actual fact, semiarid climate conditions may be optimal for continental carbonate accumulation, as noted from central Namibia (Mees 2002) and in Cretaceous through Holocene limestone settings in Spain (Platt 1989;Ford and Pedley 1996;Alonso-Zarza et al 2006;Bustillo and Alonso-Zarza 2007;Alonso-Zarza et al 2012) Deepwater lakes indicate an overall positive water balance, but shallow lacustrine-palustrine facies imply a transitional state, where positive to negative water balances fluctuate across a narrow gradient and change readily from one state to another (Katz 1990;Ashley et al 2013).…”

Gorongosa by the sea: First Miocene fossil sites from the Urema Rift, central Mozambique, and their coastal paleoenvironmental and paleoecological contexts

“…Mudracks, indicating surface exposure, had previously been noted, and the beds have been interpreted as paleo-pan deposits (abandoned oxbows, small coastal-plain lakes; Pickford, 2013). Lamination is common and is typical of pedogenic calcretes/silcretes, although lacustrine/palustrine processes are commonly involved along pan margins (Mees, 2002). Meandering rivers and oxbow lakes typically characterize low-gradient coastal plains (e.g., Miall, 2006), and, together with oval pans, are also common landscape features of the Cheringoma Coast today (Tinley, 1977).…”

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

“…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]. 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.…”