Diagenesis of a Late Triassic - Early Jurassic Drowning Succession Overprinted by Late Paleofluid Migration Events

Gyori, Orsolya ¹; Mindszenty, Andrea ¹; Molnár, Ferenc ²
(1)Department of Physical and Applied Geology, Eötvös Loránd University, Budapest, Hungary. (2) Department of Mineralogy, Eötvös Loránd University, Budapest, Hungary.

Late Triassic platform carbonate succession is exposed at Tata, in the Transdanubian Range (Hungary) unconformably covered by Early Jurassic neritic carbonates. From the Early Jurassic on, the succession is continuous in the pelagic facies, up to the Early Cretaceous.

The succession is characterized by neptunian dykes and peculiar dissolution and cementation phenomena. This study is focussed on the diagenetic history of the exposed Mesozoic carbonates and the associated paleofluid migration events. The applied techniques included petrography, fluid inclusion and stable isotope analysis.

Deposition of the Late Triassic Dachstein Limestone was interrupted by high-frequency sea-level falls resulting in a Lofer-cyclic succession. During shallow diagenesis early, non luminescent calcite filled the intergranular pore space of the limestone, showing marine isotope signals (δ13C = 1.85 to 2.67, δ18O = (-1.39) to 0.19).

The most spectacular dissolution-cementation phenomena occur along the unconformable boundary of the Late Triassic platform and the overlying hemipelagic Early Jurassic beds. Dissolution resulted in bed parallel vugs filled by radiaxial fibrous calcite on their walls and red mudstone in the remaining space. Their formation might have been associated either with a paleo-watertable or with a boundary between fluids of different composition. Stable isotope values of the radiaxial calcite clearly show marine origin (δ13C = (1.61) to (3.40), δ18O = (-3.45) to 0.21). Red micritic infill in the vugs and in the neptunian dykes of the Late Triassic limestone is supposed to be identical to the overlying Jurassic lime mud.

The whole section is criss-crossed by several, 1 to 20 cm wide late calcite-veins. Stable isotope values of white-yellow calcite, filling the first generation of veins, suggest late diagenetic fluids associated with burial diagenesis (δ13C = (-8.42) to (-5.51), δ18O = (-0.18) to 1.86). The presence of all-liquid inclusions indicates low temperature burial fluids (T<50°C) about 0 to 1.05 NaCl equ. w% salinity.

Fractures characterized by NE-SW and E-W strike are filled by transparent calcite crystals associated with tabular-habit barite. Stable isotope values of calcite are in the range of (-4.06) to (-3.45) δ13C and (-14.10) to (-6.23) δ18O, suggesting meteoric origin. Low temperature (<50 °C) and low salinity (0 to 0.53 NaCl equ. w%) of their parent fluids imply that they could be related to the subrecent karstwater system.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.