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

Abstract

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 However, all investigated rock-soil transition zones showed microfossils that had partially been replaced by clay, which could be geochemically confirmed using EDS.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.

Red Mediterranean Soils in Jordan: New insights in their origin, genesis, and role as environmental archives

“…Only at this site could a 'bleached zone' in contact with bedrock be identified (Lucke et al, 2012). This is in agreement with the exceptional increase of soil development intensity towards the bottom of the RMS, which had initially been assumed to be connected with the obvious fluvial deposition of allochthonous material in the upper 100 cm of the profile, but might now be attributed to isovolumetric replacement.…”

“…This process also leads to a release of acids, which explains the association between Terra Rossa and karst. Similar replacement features were described for several Terrae Rossae in Jordan by Lucke et al (2012). Close investigation of the rock-soil contact zones of different limestones with a Scanning Electron Microscope (SEM) equipped with energy-dispersive systems (EDS) made it possible to identify a jelly-like mass of apparently amorphous clay minerals that partially replaced microfossils and calcite plates (Lucke et al, 2012).…”

“…Therefore, it seems likely that basalt weathering did not contribute significantly to soil development near Amrawah, but to some degree near Umm Queis and Umm el-Jimal. In this context, the elevated Ti/Zr-ratios in the Yarmouk Limestone profile could be taken as indication of the presence of volcanic material from the Cretaceous that had been deposited inside the limestone rocks, as suggested earlier by Lucke et al (2012).…”

“…The larger amount of vermiculite could be explained by a more intensive clay mineral neo-formation in the basalt soil. However, the samples of UQ 13 and AB 3 superimpose each other completely, pointing to strong similarities between the clays of the RMS on basalt and the Yarmouk Limestone, which might be explained by Cretaceous volcanic material in the limestone (Lucke et al, 2012). One should note that Palygorskite could only be found in the immediate surface of the basalt soil although it is present in the bedrock.…”

“…Lucke (2008) dated this deposition event tentatively to 6880 BC. At the bottom of the profile, a 'bleached zone' was identified during another study, which is due to the occurrence of replacement processes (Lucke et al, 2012). Two samples on the slightly harder limestone next to the karst pocket were also investigated in order to ascertain whether local variations of the sedimentary rock has contributed to dissimilarities in the soil properties (samples Yar 1 Mitte, Yar Mitte unten, and Yar 5).…”