A carbonate carbon isotope curve from the Aalenian-Bathonian interval is presented
from the Obanya valley, of the Mecsek Mountains, Hungary. This interval is certainly
less well constrained and studied than other Jurassic time slices. The Obanya valley
lies in the eastern part of the Mecsek Mountains, between Obanya and Kisujbanya and
provides exposures of an Aalenian to Lower Cretaceous sequence. It is not strongly
affected by tectonics, as compared to other sections of eastern Mecsek of the same
age. In parts, a rich fossil assemblage has been collected, with Bathonian ammonites
being especially valuable at this locality. The pelagic Middle Jurassic is represented
by the Komlo Calcareous Marl Formation and thin- bedded limestones of the Obanya Limestone
Formation. These are overlain by Upper Jurassic siliceous limestones and radiolarites
of the Fonyaszo Limestone Formation. Our new data indicate a series of carbon isotope
anomalies within the late Aalenian and early-middle Bajocian. In particular, analysis
of the Komlo Calcareous Marl Formation reveals a negative carbon isotope excursion
followed by positive values that occurs near the base of the section (across the Aalenian-Bajocian
boundary). The origin of this carbon-isotope anomaly is interpreted to lie in significant
changes to carbon fluxes potentially stemming from reduced run off, lowering the fertility
of surface waters which in turn leads to lessened primary production and a negative
delta C-13 shift. These data are comparable with carbonate carbon isotope records
from other Tethyan margin sediments. Our integrated biostratigraphy and carbon isotope
stratigraphy enable us to improve stratigraphic correlation and age determination
of the examined strata. Therefore, this study of the Komlo Calcareous Marl Formation
confirms that the existing carbon isotope curves serve as a global standard for Aalenian-Bathonian
delta C-13 variation.
