ABSTRACT

Endolithic and epilithic lichens are pioneer colonizers of exposed calcrete (caliche) hardpans and fresh rock surfaces throughout the coastal regions of the western Mediterranean.

Petrographic studies reveal that saxicolous (rock substrate) lichens cause characteristic textural and fabric changes of the colonized substrate. As a result of biophysical disintegration, biochemical decomposition and biosynthesis of mineral components, protosoils are formed. Induration of these superficial, biologically weathered rinds, followed by further lichen colonization, leads to the formation of lichen stromatolites.

The effect of saxicolous lichens on exposed, indurated calcrete surfaces has been documented petrographically in terms of textural, fabric and compositional changes with respect to the underlying unaffected part of the calcrete profile. Such documentation provides criteria for the recognition of subaerially exposed substrates and is, thus, of potential importance in helping to delineate former land surfaces in ancient successions.

Laminar calcretes contain organic-rich and organic-poor millimetre laminae, microborings, algal filaments, fungal hyphae, calcite spherulites, spherical structures and decimicron-sized calcite grains arranged in surface parallel layers. Hitherto, these features have been reported by numerous workers but their origin has not been considered. Comparative studies with modern lichens suggest that laminar calcretes examined here are lichen stromatolites, that is, that they owe their origin to successive cycles of organic (lichen) telogenesis, incipient pedogenesis and diagenesis.

Recognition of lichen stromatolites has far reaching implications in terms of palaeoenvironmental reconstructions. Lichen stromatolites demarcate subaerial discontinuity surfaces, thus indicating terrestrial conditions and duplicating palaeotopography.