Evidence for isovolumetric replacement in some Terra Rossa profiles of northern Jordan

Resumen

We investigated the
rock-soil transition zones of three different Terra Rossa profiles
near the ancient site of Abila, in northern Jordan, in order to
demonstrate the possibility of a metasomatic origin for these soils
using a scanning electron microscope (SEM) equipped with an
energy-dispersive system (EDS). The field interpretation suggested
that the first Terra Rossa might partially have formed by
isovolumetric replacement, the second appeared to represent mainly
the residue of the underlying limestone, and the third seemed to be
derived from colluvial material. However, all investigated rock-soil
transition zones showed microfossils that had partially been
replaced by clay, which could be geochemically confirmed using EDS.
The replacement matrix was free of detrital clay and revealed a
jelly-like, amorphous structure. Large phyllosilicates, some rare
feldspars, and heavy minerals that occurred throughout the
investigated rock-soil transition zones are probably remains of
sandy interlayers in the weathered limestone. We conclude that
isovolumetric replacement contributes to Terra Rossa formation in
northern Jordan although it is not yet possible to quantify the
extent of its contribution or the sources that supply the elements
triggering the pressure-driven growth of clay minerals