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Siwalik Paleosols (Miocene, Northern Pakistan): Genesis and Controls on Their Formation
Michael J. Zaleha (*)
Paleosols of the Siwalik Group (upsection, the Chinji, Nagri, and Dhok Pathan Fms) in northern Pakistan are associated with fluvial and lacustrine deposits that accumulated adjacent to the rising Himalayas. There are two types of well-developed paleosols throughout the sequence. (1) Noncalcareous/Bk paleosols are typically composed of a nodular calcite (Bk) horizon overlain by a noncalcareous horizon. Noncalcareous horizons are characterized by the absence of matrix calcite and presence of iron oxide accumulations. Nodular calcite horizons are characterized by abundant, large calcite nodules. (2) Calcareous paleosols are characterized by the presence of matrix calcite and lack of horizonation. The two paleosol types reflect differences in drainage condition , parent material, topography, and water-table elevation. Most noncalcareous horizons formed under relatively well-drained conditions on topographic highs. The lack of matrix calcite in most cases is due to noncalcareous parent material. However, some noncalcareous horizons apparently developed in well-drained topographic lows on noncalcareous and/or subsequently leached calcareous substrates. Nodular calcite horizons formed within the capillary fringe. The source of most of the calcite was groundwater; however, calcite nodule formation was largely dependent upon pedogenesis. Calcareous paleosols formed under relatively poorly drained conditions, probably within the capillary fringe, in topographic lows. Most matrix calcite was inherited from parent material. Both paleosol types formed i areas of little or no deposition. Other poorly developed paleosols represent incipient noncalcareous/Bk and calcareous paleosols, and formed in areas of moderate deposition. No soils formed in areas of high deposition. Cumulative and compound pedogenesis occurred locally. Cumulative pedogenesis was more common during deposition of the upper Nagri and lower Dhok Pathan Fms, consistent with a documented increase in compacted sediment accumulation rates upsection. Features indicative of episodic wetting and drying, including evidence of alternating calcite and iron oxide precipitation, are consistent with previous paleoclimatic interpretations indicating that the regional climate from 15 Ma to 9 Ma was warm, humid, subtropical to tropical, and markedly seasonal, attributable to monsoonal c nditions.
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