Overlooked sedimentary particles from tropical weathering environments

Two varieties of sedimentary particles that are common in tropical weathering environments have remained largely overlooked by sedimentary petrologists. Alterites are grains that have been so thoroughly altered by chemical weathering processes that identification of the precursor grain is impossible. They consist of secondary weathering products such as clays, zeolites, and iron sesquioxides, as well as incompletely weathered remains of relatively stable mineral phases. Alterites that consist of clay-sand composites allow the transport and deposition of hydraulically dissimilar constituents under a single hydrodynamic regime and may be important in the production of some graywackes. A second class, the pedogenic ferruginous particles, takes on a variety of forms, including laterite fragments, soil pisolites, and oxidized burrow rinds. Although these particles are common in modern fluvial sands in the tropics, wholesale dissolution of amorphous or poorly crystalline iron sesquioxides during burial diagenesis may help to explain the rarity of such particles in ancient sediments. Although their potential for preservation is low, dissolution of ferruginous particles may be an important source of diagenetic iron, which may be redeposited to form ferruginous cements or diagenetic redbeds.