Iron and Clay in Tropical Savanna Alluvium, Northern Colombia: A Contribution to The Origin of Red Beds
Along the flanks of Sierra Nevada de Santa Marta, northern Colombia, tropical savanna uplands with reddish to reddish-brown soils yield brown alluvium which contains relatively abundant grains of fresh iron-bearing minerals: magnetite, amphibole, and biotite, with subordinate chlorite and ilmenite. They constitute about 4.5 weight percent of bulk samples and 8.8 weight percent of sand-silt fractions. In older terrace deposits bleached biotite is common, amphibole is rare, and some magnetite has been oxidized to specular hematite. Clay minerals consist of varying amounts of montmorillonite, illite, mixed-layer clay, kaolinite, and chlorite. Amorphous to very disordered aluminosihcate constitutes 10 to 26 percent of the clay fraction. Total iron makes up 3.1 to 7.6 percent of bulk samples which is considerably higher than that in Sonoran Desert alluvium (0.7 to 4.7 percent), and its upper range exceeds that in many ancient red beds. With few exceptions most of the total iron is in the clay fractions and about 75 percent of that is combined in clay minerals. Total iron in clay fractions averages about 6.6 percent: 5.5 to 8.0 percent in alluvium in transit, 4.8 to 9.0 percent in flood-plain sediments, and 3.0 to 5.8 percent in older terrace deposits. Free (extractable) iron in brown pigment constitutes 0.7 to 3.2 percent of clay fractions and averages about 0.75 percent of bulk samples. This is about twice that in bulk samples of Sonoran Desert alluvium (0.38 percent mean) and it is more than that in many ancient red beds (0.67 percent mean). According to these data there is enough free brown hydrated ferric oxide in tropical savanna alluvium to produce hematite pigment in a red bed simply by post-depositional dehydration and crystallization in an oxidizing environment of burial.