ORIGIN OF ASSOCIATED TERRA ROSSA AND KARST BY REPLACEMENT

Authigenic terra rossa clay partly replaces limestone across a 9-cm-wide metasomatic reaction zone several feet below the earth's surface at Bloomington, Indiana. The replacement is pseudomorphic and isovolumetric. The clay partly replaces calcite forming sharp, intracrystalline microstylolites. This tiny hacksaw texture proves Maliva & Siever's fundamental new idea (1988) that a replacement happens because the new mineral grows and, via the induced stress, pressure-dissolves the host (not because the host chemically dissolves first and “pulls” the guest behind – the traditional nonworking consensus). Terra rossas in the Caribbean, Florida, Texas, Kentucky, southern Europe, southern Australia, and elsewhere probably have the same origin, but replacement has never been suspected or sought.

The iron, aluminum, and silicon needed for clay growth are provided probably by dust. (Saharan dust is abundant in the Caribbean and southern Europe.) The dust dissolves at the surface, the solutes leak in, reach the reaction front several feet deep, and precipitate to make the red clay crystals that replace the limestone. Terra rossa is thus a unique laterite – one none of whose major elements comes from its parent limestone, and one that supports wineries.

The volume-adjusted replacement of limestone by clay produces acid. This acid dissolves voids just ahead of the reaction zone. The new porosity propels advection of solutes to the reaction zone. This accelerates further replacement, which in turn produces more acid. Through this feedback – the reactive-infiltration instability, modeled in the 80s – a planar dissolution front should become a set of regularly spaced funnels: This is how karst forms. The terra rossa is thus not an alluvial mud that gets trapped in preexisting karst funnels or sinks, as traditionally held. Terra rossa, as it replaces limestone, carves the very karst that contains it. That is why the two are associated.