Authors:János Haas, Tamás Budai, and Attila Demény
Whitaker, F.F., P.L. Smart 1993: Circulation of saline groundwaters in carbonate platforms: a review and case study from the Bahamas. — In: Horbury, A.D., A.G. Robinson (Eds): Diagenesis and Basin Development. AAPG Studies in Geology, 36, pp. 113
The Garáb Schlier Formation is composed of gray sand, silt, clay and clay marl, deposited in an open marine environment. The aims of this study were to reconstruct its paleoenvironmental features and to propose a hypostratotype of the Garáb Schlier Formation. Eighty-seven samples from the Mv-122 borehole (between 177.0–698.0 m) provided a dataset for detailed qualitative and quantitative analysis. The benthic and planktonic foraminiferal fauna were studied together. The associations indicated three different environments in the section. The lower part of the section records deposition in a normal marine, cool, outer shelf environment. Later on conditions changed to an upper bathyal, normal salinity, cool environment without permanent currents. Assemblages from the upper part of the section suggest that the water depth decreased and an inner shelf, normal salinity sea is inferred, with fluvial influence and open marine connections. The age of the sediment is Late Karpatian (Latest Burdigalian) as indicated by the M4b Planktonic Foraminifera Zone. The middle part of the studied section (337.0–664.0 m) is proposed here as the hypostratotype of the Garáb Schlier Formation.
The diversity of the brachiopods in the Northern Caucasus significantly fluctuated throughout the Paleozoic-Mesozoic. Weak diversifications occurred in the Middle Cambrian, Late Silurian - Early Devonian, and Late Devonian - Early Carboniferous. Since the Late Permian brachiopod assemblages became quite diverse. The maximum number of species was reached in the Rhaetian. The Permian/ Triassic mass extinction and enigmatic Ladinian crisis, on the other hand, led to regional brachiopod demises. In the Jurassic - Early Cretaceous interval the diversity of brachiopods generally decreased. The strongest drops of species numbers occurred in the Toarcian and Berriasian following the Pliensbachian-Toarcian and end-Jurassic global mass extinctions, and in the Kimmeridgian due to the regional salinity crisis. It is evident that some of the regional brachiopod diversifications coincided with the development of rimmed shelves.
Authors:Orsolya Sztanó, Csaba Krézsek, Imre Magyar, Ferenc Wanek, and Györgyi Juhász
A continuous Sarmatian/Pannonian boundary section, which rarely crops out within the Pannonian Basin, was studied near Oarba de Mures/Marosorbó. Alternating beds of clay marl, calcareous marl, siltstone, sandstone and andesitic tuff are present up to a thickness of about 100 m. Traditionally the Sa/Pa boundary was placed at the top of the last significant tuff layer, which has been confirmed by mollusk-bearing strata a few meters above belonging to the "Lymnocardium" praeponticum Zone. This zone is coeval with the Early Pannonian Mecsekia ultima dinoflagellate zone and the C5r magnetic polarity zone. Three ranks of cyclicity connected to sediment gravity flows are present in the outcrops. Coarse silt to sandstone beds were formed by low-density turbidity currents. These individual events represent "dilution cycles" connected to the intensity and abundance of turbidity currents. Turbiditic beds, some 2-5 m-thick series of sandstones, form coarsening/thickening upward cycles of 8-20 m of thickness. This cyclicity may reflect autocyclic lobe switching in deep lacustrine fans. The lowermost 70 m of the succession comprises a major thickening to thinning cycle, while the uppermost part of the sequence seems to represent a longer turbidite-free interval. The last may either reflect climatically-driven allocyclic lake-level variations or impulses of hinterland structural evolution (tectonic activity vs. quiescence). The background sediments show two sorts of seemingly independent rhythmicity: there is marl with variable carbonate content, occasionally forming a few cm of thickening-upward series of calcareous marl, and it also shows various grades of bioturbation. Calcareous marl is often associated with the appearance of fibrous gypsum laminae. The alternating carbonate content of the marl might be generated either by "production cycles" in the photic zone over which the climatic influence is straightforward, or they were formed as the first products of the evaporite succession from hypersaline bottom waters. The lack of bioturbation combined with gypsum may reveal hypersaline and/or dysaeroabic abiotic bottom conditions. These also indicate that turbidity currents had transported not only terrestrial sediments but less saline, O2-bearing water down to the lake floor, interrupting the biota-poor periods. The salinity of the bottom waters in the deepest basin segments might significantly differ from that of the main water mass, and might have been regulated by the composition and amount of ions dissolved from Mid-Miocene salt diapirs cropping out at the lake bottom elsewhere. Accumulation of hypersaline bottom waters may also be facilitated by climatically-determined density stratification of the lake water.