Pedogenic and lithogenic gravels as indicators of soil polygenesis in the Brazilian Cerrado

Abstract

Although particles >2 mm are not considered soil material, gravels composed of resistant secondary minerals can be useful records of past soil environments. We investigated gravels (2–8 mm) from a Plinthic Haplustox in central Brazil in order to assess their composition, fabric and genesis. Gravels were initially grouped into six macromorphological types, and investigated by mineralogical, micromorphological and microprobe analyses. The results suggest that gravels can be classified as pedogenic or lithopedogenic. Pedogenic gravels comprise indurated soil materials and include black magnetic gravels, black non-magnetic gravels and red earthy gravels. Magnetic gravels contained mostly quartz, hematite and magnetite–maghemite and were poor in kaolinite, gibbsite and goethite, whereas non-magnetic and red earthy gravels had the same minerals as in the surrounding soil. Lithopedogenic gravels are fragments of resistant rocks impregnated by iron (Fe), aluminium (Al) and manganese (Mn) oxides, including platy shales, quartzites and coarse quartz. The latter two showed oxide impregnations around internal pores, whereas the shale fragments included considerable mica, which contributed illite and potassium to the soil clays and, thus, the pedogenic gravels. These results point to multiple cycles of formation and incorporation of cemented materials into the soil, as well as their later impregnation by Fe, Mn and Al oxides. Thus, the conventional classification of all such gravels as ‘ironstone’ is questionable, and their potential role as indicators of past and present soil environments is demonstrated, which deserves further investigation.