SEDIMENTOLOGICAL STUDY OF THE TRIASSIC SOLUTION-COLLAPSE BRECCIAS OF THE IONIAN ZONE (NW GREECE)

The Triassic Breccias of the Ionian zone are typical evaporite dissolu tion collapse breccias. Several features indicate the preexistence of evaporites, while alternation of dolomites and evaporites consist a ve ry common association in the subsurface. Brecciation took place in two principal brecciation stages. The first brecciation stage started soo n after deposition, during a period of subaerial exposure due to perio dic seasonal desiccation and small-scale meteoric removal of intrastra tal evaporites. During this stage, the carbonate beds suffered in-situ breakage and carbonate mud infiltrated into fractures. Shortly after, a major brecciation event occurred that affected the still non-well l ithified carbonate fragments, due to progressive dissolution of evapor ites by meteoric water. Carbonate mud continues to be infiltrated in-b etween the:breccia fragments. In the same time, intensive calichificat ion processes were responsible for further brecciation and reworking o f the brecciated carbonate beds locally sediments, testifying a:period of temporary regional emergence (paleosoil). The breccia matrix is ch aracterized by microbreccioid appearance, resulting from internal brec ciation of the coarser clasts. Due to early calichification, the matri x becomes enriched in oxidized clays and by pronounced calichification tends to assimilate the breccia clasts, being gradually transformed i nto a calcrete with floating texture. Clasts microfacies types include phytoclasts with strongly impregnated by Fe-oxides laminae (laminar c alcrete), carbonized plant tissue; lime and dolomitic mudstones with e vidence of former evaporites (dolomite/calcite pseudomorphs after gyps um and/or void-filling anhydrite cement, molds after evaporite nodules , euhedral quartz crystals etc.), carbonate fragments pseudomorphic af ter evaporites, pelsparites/intrasparites, recrystallized dolomites an d dedolomites. The predominance of shallow intertidal to supratidal ca rbonate fragments, indicates that the strata that gave birth to the br eccia, formed in a very shallow, restricted hypersaline, lagoonal sett ing, evolved into sabkha sequences in the frame of a lowstand episode. Sedimentation of dolomite and evaporite is considered that has taken place during arid periods, while meteoric water influx during the wett er intervals. During that lowstand episode, that resulted in a hiatus interval, the breccias have suffered intensive calichification Circula ting pore-fluid brines resulting from evaporation, provoked syngenetic to early diagenetic dolomitization of muds, by increase of molar Mg/C a ratio and provided ions for evaporite nodules / crystal growth. Post -Pliocene to Recent subaerial exposure of the carbonate breccias, led to intensive soil-forming processes, active till today, that accentuat ed the brecciated appearance of the formation. These processes are res ponsible for the formation of porous carbonate breccias, the so-called ''rauhwackes''.