Distribution and characterisics of calcretes and calcrete-soil profiles on the Northern lower slopes of Uluguru mountains in Morogoro, Tanzania
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Date
1997
Authors
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Publisher
Sokoine University of Agriculture
Abstract
This study was conducted on the area extending northwards from the
footslopes of the Uluguru mountains towards the Ngerengere river. Kihonda transect
was selected for detailed characterization of different calcrete forms and the
associated soils and regolith so as to understand the origin of calcretes, their
characteristics and their influence on soil properties.
Near Ngerengere river, the calcrete layer is considerably thinner (27.5 cm)
than those on the lower slope (76 cm), midslope (135 cm), crest (106 cm) and
neighbouring depression (142 cm) implying the influence of Ngerengere river on
calcrete formation. Less developed calcretes occur near the river while the most
developed calcretes occur at or near the crest.
Nodular, honeycomb and massive calcretes are the major forms of calcretes
occurring on the transect. Their proportions along the transect which reflect their
lateral extension follows the order: nodular (94.9%), honeycomb (87.2%) and
massive (51.3%). A considerable difference in purity exists among different calcrete
forms ranging from 47.6% in nodular calcrete, 75.0% in honeycomb and 97.5% in
massive form. This difference in purity is explained in terms of their difference in
degree of development. A sharp di (Terence in average CaCO3 content ranging from 7.7% in soil
solum to 65.0% in calcrete layer exists. The latter implies a lithological discontinuity
at the soil-calcrete interface. The difference is gradual from 65.0 to 37.7% at the
calcrete-regolith interface. The source of the CaCO3 in the calcretes is therefore the
underlying parent material. The calcretes are mainly constituted of micritic calcite.
Sparite occurs on top of massive calcrete, in fissures, voids and around the relic
minerals implying a multi-phase precipitation of calcite. Minor minerals are quartz,
opaline silica and feldspars.
The studied calcretes were formed during the last glacial period, about 41,000
or more years ago. The gravel and soil layers are considerably younger.
Description
Dissertation
Keywords
Calcrete forms, Calcretes origin, Soil properties