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The replacement by ferroan calcite with preservation of the original structures can be used as a new criterion for identifying skeletons originally composed of high‐magnesian calcite. This applies to bryozoa, rugose corals, echinoderms, many foraminifera, most ostracods, red algae, and serpulids. On the other hand, skeletons originally composed of low‐magnesian calcite were never replaced by ferroan calcite, as shown by belemnites, brachiopods, and most of the pelecypods. Using this criterion, an original low‐magnesian calcite composition is inferred for Tentaculites and some ostracods and foraminifera, whereas a previous high‐magnesian calcite composition is inferred for trilobites, oligostegina and certain ooids. Chemical instability of high‐magnesian calcite is suggested to be the driving force of the replacement by ferroan calcite. In most of the thirty‐seven samples investigated, of Oligocene to Devonian age, the ferrous iron concentration of the interstitial fluid increased during diagenesis, as shown by well established sequences of cement A and B and fissure fill. This offers a relative time scale for diagenetic processes. Ferroan calcites contain up to 6 mol % FeCO3 and up to 5 mol % MgCO3. In this range of concentration, the distribution coefficients for Fe and Mg between calcite and solution at about 25°C are about 1 to 0‐03, respectively, according to experiments. Possible sources of iron are iron oxides and hydroxides as well as clay minerals including glauconite. Though a submarine origin below the sediment surface is conceivable for ferroan calcite, there are serious limiting conditions such as low Eh and, at the same time, lack in sulphate‐reducing bacteria. On the other hand, ferroan ‘dedolomite’, compositional zonality in individual ferroan calcite overgrowths, low δ18C and δ18O values, and low Mg concentrations point more to a meteoric‐phreatic origin of many ferroan calcite occurrences.

ZusammenfassungTiefe Spalten in Flachwasserkalken, welche mit Tiefwassersediment gefiillt sind, wurden verschiedentlich beschrieben, besonders aus der Trias und dem Jura der Tethys und aus dem Mitre1-bis Oberdevon des variscischen Orogens. Mass flows kommen in den gleichen Formationen und Gebieten vor. Auf Hydra (Griechenland) konnte gezeigt wetden, dab als Zwischenglied dieser beiden Erscheinungen sogenannte Internbreecien aug treten, deren Karbonatklasten wie Puzzles aneinanderpassen. Sie weisen auf Hang-oder hangnahe Positionen hin und entstehen sehr wahrscheinlich durch flexurartige Deformationen yon Kalken in geringer Versenkungstiefe. ErdbebenstSBe m6gen mit iiberhydrostatischem Druck an der Brecciierung der schwaeh verfestigten Kalksedimente beteiligt sein. So wird Material fiir mass flows bereitgestellt.Diese Vorg~inge sind mit tektonischen Absenkungen verkniipft; die FlachwasserkalkBreccien enthalten oft eine Matrix yon Sedimenten tieferen Wassers. Solche Breccien und Spalten entstehen in Gebieten und Perioden mit submarinem ,,rifting", spezieH im Oberdevon und in der Trias. Fi,ir die letztere kSnnen die einzelnen Brecciierungs-Ereignisse yon Griechenland bis zu den Alpen verfolgt werden und entsprechen vermutlich auch gr/SBeren Transgressionen in der germanisehen Trias. AbstractDeep fissures in shallow-water limestones filled with deep-water sediments are described from many places especially in the Triassic and Jurassic of the Tethys and in the Middle to Upper Devonian of the Variscian Mountains. Mass flows occur in the same formations and areas. On the island of Hydra (Greece) it was possible to show that both phenomena are linked by internal breccias of well-fitted limestone fragments. They are indicative of slope or near-slope positions and are very probably caused by flexure-like deformations of shallow-burial limestones. Seismic shocks producing overpressure may be instrumental in the disintegration of the semi-lithified limestones thus providing the material for mass flows.These events are associated with a teetonical subsidence as shown by deep-water matrix in the breceiated shallow-water limestones. They occur in areas and periods of submarine rifting, especially in late Devonian and Triassic time. At least four Triassic brecciation events can be traced from Greece to the Alps and correspond presumably to major transgressions in the eratonic basin of middle Europe. R6sum6Des profondes fissures remplies de s6diments d'eau profonde ont 6t6 d6erites, dans un certain hombre de loealit6s, dans des ealeaires marius de faible profondeur, surtout du Trias et du Jurassique de la M6sog6e, ainsi que du D6vonien moyen et sup6rieur de l'orog6ne hercynien. Des 6eoulements en masse (mass flows) se pr6sentent dans les m6mes formations et r6gions.