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Middle Pleistocene lake sediments of the Olorgesailie Formation from the East African Rift Valley of Kenya contain red beds, with colors ranging from moderate orange pink to dark reddish brown (Munsell color chart). Two distinct mechanisms of hematite formation, distinguished by thin section and scanning electron microscope study, have produced the red sediments.
In the first mechanism, hematite was formed in situ by dehydration of limonitic minerals. The limonitic minerals formed in soils of the source area and were transported to the depositional site mainly by adhering to clay particles. The elevated pH of the depositional environment accelerated the rate of hematite formation, producing ultrafine red coating on the clays. Red sediment formed by this mechanism occurs both as an undisturbed bed and as a reworked intraclast conglomerate.
In the second mechanism, hematite is precipitated from ground water, possibly at elevated temperatures produced by the interaction of basaltic magma with water-saturated sediment. The hematite occurs in three forms: as cement within diatomaceous clay, as replaced or stained plant material, and within siliceous sinters. The sinters are red, have a pumice-like texture, and include diatoms that show effects of dissolution. These characteristics indicate formation of the sinters by boiling of near-surface water-saturated sediment. Red beds formed by this mechanism are redeposited and each of the three hematite forms dominates in specific localities.
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