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SEDIMENTOLOGICAL STUDY OF THE TRIASSIC SOLUTION-COLLAPSE BRECCIAS OF THE IONIAN ZONE (NW GREECE) (1998)
The Triassic Breccias of the Ionian zone are typical evaporite dissolution collapse breccias. Several features indicate the pre-existence of evaporites, while alternation of dolomites and evaporites consist a very cornmon association in the subsurface. Brecciation took place in two principa1 brecciation stages. The first brecciation stage started soon after deposition, during a period of subaerial exposure due to periodic seasonal desiccation and small-scale meteoric remova1 of intrastrata1 evaporites. During this stage, the carbonate beds suffered in-situ breakage and carbonate mud infiltrated into fractures. Shortly after, a major brecciation event occured, that affected the still non-well lithified carbonate fragments, due to progressive dissolution of evaporites by meteoric water. Carbonate mud continues to be infiltrated in-between the breccia fragments. In the same time, intensive ca1ichification processes were responsible for further brecciation and reworking of the brecciated carbonate beds locally sediments, testifyinga period of temporary regional emergence (paleosoil). The breccia matrix is characterized by microbreccioid appearance, resulting from interna1 brecciation of the coarser clasts. Due to early calichification. the matrix becomes enriched in oxidized clays and by pronounced calichification tends to assimilate the breccia clasts, being gradually transformed into a calcrete with floating texture.
Clasts microfacies types include phytoclasts with strongly impregnated by Fe-oxides laminae (laminar calcrete), carbonized plant tissue, lime and dolomitic mudstones with evidence of former evaporites (dolomite/calcite pseudomorphs after gypsum and/or void-filling anhydrite cement, molds after evaporites nodules, euhedral quartz crystals etc.), carbonate fragments pseudomorphic after evaporites, pelsparites / intrasparites, recrystallized dolomites and dedolomites.
The predominance of shallow intertidal to supratidal carbonate fragments, indicates that the strata that gave birth to the breccia, formed in a very shallow, restricted, hypersaline, lagoonal setting, 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 meteïÞc water influx duMg fue wetter intervals.
During that lowstand episode, that resulted in a hiatus interval, the breccias have suffered intensive calichification. Circulating pore-fluid brines resulting from evaporation, provoked syngenetic to early diagenetic dolornitization of muds, by increase of molar Mg/Ca ratio and provided ions for evaporite nodules / crystal growth. Post-Pliocene to Recent subaerial exposure of the carbonate breccias, led to intensive soil-fonning processes, active till today, that accentuated the brecciated appearance of the formation. These processes are responsible for the formation of porous carbonate breccias, the so-called "rauhwackes'".
Carbonates & Evaporites, v. 13, no. 2, 1998, no. 207-218.
Department of Geology, University of Athens. Pánepistimioupoli, 15784 Athens. Greece