PER LATUS EXTRAPEDOGENIC ACIDIC WEATHERING OF PYRITIC SEDIMENTARY ROCKS, NEBRASKA, USA

We define per latus soil and weathering processes as those that have a strong lateral component. The recognition of per latus processes compliments the existing concepts of per ascensum and per descensum processes, and also fully resonates with the concept of soil/weathering/groundwater “edge effects” at the edges of major geomorphic features, specifically upland-valley breaks in slope. Pyritic Cretaceous (Carlile and Pierre Shales) and Pennsylvanian (Indian Cave Sandstone) strata exposed along the valley walls of both the Missouri and Republican Rivers in Nebraska have undergone acidic weathering because of the very diffuse discharge of groundwater from outcrop faces and because of the exposure of these reactive strata to atmospheric oxygen and humidity. These phenomena are per latus processes and are also “extrapedogenic” because they occur well below and phenomenologically outside of an extant soil solum. Secondary minerals produced by such weathering include Fe and Al sulfates and sulfate-hydroxides (aluminite, alunite, alunogen, felsöbányaite/basaluminite, copiapite, halotrichite, jarosite, melanterite, rozenite, slavikite), aluminum hydroxides (bayerite, gibbsite), Al phosphate hydroxide (vashegyite) from the weathering of primary marine phosphate nodules (Carlile and Pierre Shales), and kaolinite produced by the weathering of 2:1 layer silicates. Some of these minerals (e.g., alunogen, copiapite) are very labile and only persist for significant lengths of time in protected microenvironments, whereas others are comparatively persistent (e.g., alunite, aluminite), and gibbsite can be considered an end member in a classical terrestrial weathering progression.

The acidic weathering of pyrite-bearing shales and sandstones (particularly shales) at geomorphic edges along stream valleys is linked to mass wasting, which ensures a supply of weatherable bedrock, and also may have a minor positive feedback to the latter set of processes. The environmental consequences of sulfide oxidation and acidic weathering at landscape edges are potentially manifold, and include the release of sulfate ion into ground- and surface waters and the liberation and storage of potentially toxic metals.