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Desert Dunefields |
Geological Studies Unit, Indian Statistical Institute, 203 BT Road, Calcutta-700 035, India
The coarse-grained alluvial succession of the Proterozoic Mancheral Quartzite encloses within it a few 2–6 m-thick, typically salmon red, fine-grained, very well-sorted sandstone units. Some 25–400f the fine-grained sandstone units is aeolian — mostly adhesion laminae with subordinate adhesion cross-laminae and wind-ripple strata. The remainder of the sequence is aqueous; either massive or with faintly-developed trough cross-bedding. Based on their grain size, sorting and roundness, the bulk of the aqueous deposits appear to be reworked aeolian deposits. Thick adhesion laminated units within them show a number of superimposed drying-upwards sequences, represented by an upward decrease in laminae spacing within each of the sequences. Individual drying-upwards sequences, 5–15 cm thick, are in places bounded by disconformity surfaces marked by iron crusts.
Stratigraphic relationships with associated fluvial facies indicate that these aeolian sandstones formed in the distal part of the floodplain. An arid climate, vegetation-free landscape, quickly avulsing channel behaviour and rapid basin subsidence favoured development and preservation of these rather thick alluvial plain aeolian deposits. Fluvial dynamics, in contrast to the erg dynamics as interpreted from other interlayered aeolian-fluvial deposits, was the primary control on the depositional features and their internal organization within these aeolian sandstone units.
