Preliminary carbon and oxygen isotope data on carbonate sequences from the Pădurea Craiului Mountains, Romania. Paleoenvironmental significance

Isotope stratigraphy is a useful tool for stratigraphic correlation, especially for strata deposited during major perturbations of the carbon and oxygen cycles that affected the marine, terrestrial and atmospheric reservoirs. The Pădurea Craiului Massif (north-westernextremity of the Apuseni Mountains, Romania) had a long-lasting evolution with a pre-Hercynian start, being mainly shaped during alpine orogenesis. Most of its formations belong to the Bihor tectonic unit. The basement is made up of crystalline schists of the mesometamorphic Somes Series. Sedimentation started during Permian with detritic deposits interbedded with rhyolites. The overlying Triassic deposits are unconformable and include detritic formations (Lower Triassic) and massive layers of carbonate rocks (Middle Triassic). The almost complete lack of the Upper Triassic is due to the uplift of the region during the Cimmerian tectonic phase. The Lower Jurrasic deposits include the detritic formation (Hettangian–Lower Sinemurian), the limestone formation (Upper Sinemurian–Pliensbachian) and the marl formation with ammonites and belemnites (Toarcian). The Middle Jurassic consists mainly of marls. The Upper Jurassic formations are massive (over 100 m thick) and are made up exclusively of limestones. During the Upper Tithonian and Lower Cretaceous the limestone deposits have been uplifted which resulted in a paleo-karst surface that hosts discontinuous bauxite deposits. Lower Cretaceous sedimentation started with the deposition of fresh-water limestones (Hauterivian) followed by successive layers of marine limestones (Barremian), marls (Aptian), marine limestones (Aptian), glauconitic sandstone (Aptian- Albian) and ended with a package of red detritic deposits (Albian-Cenomanian?). After the intra-Turonian thrust movements, the Senonian sediments have a post-tectonic character and outcrop in several isolated area. Subsequent positive epirogenetic movements and two main phases of magmatic activity (Upper Cretaceous–Paleocene and Badenian–Pliocene) completed the morphogenesis of the Piatra Craiului Mountains.
   The δ¹³C_PDB and δ¹⁸O_PDB values of bulk carbonate samples from the Mesozoic formations of the Pădurea Craiului Mountains vary from -2.27‰ to +2.97‰ and from - 8.07‰ to -2.91‰, respectively. All isotope compositions correspond to seawater carbonates. An increasing trend of the δ¹⁸O values with the age of the formations is to be noted. The highest values δ¹⁸O (-4.44‰ to -2.91‰) correspond to the Cretaceous formations. The Triassic limestones display the lowest δ¹⁸O value (-8.07‰) and this could be a consequence of the global oxygen isotope excursion recorded at the Permian/Triassic boundary. Such light δ¹⁸O values are consistent with an Early Triassic warm, depleted of oxygen and stagnant ocean. The lowest δ¹³C value (-2.27‰) was obtained for the Upper Jurassic limestone formation. As these limestones have been uplifted and karstified under lateritic conditions during the late Jurassic and early Cretaceous, the ¹³C-depleted value probably resulted from an admixture of primary and diagenetic carbonate. The lower value could also be a remnant of the carbon isotope negative shift documented worldwide and caused by the Early Jurassic (Toarcian) oceanic anoxic event. All Cretaceous limestone formations display positive δ¹⁸C values (1.78‰ to 2.97‰). Positive carbon-isotope excursions are generally compatible with times of low atmospheric carbon dioxide content. The Barremian marine limestones (Blid formation) display slightly lower δ¹³C and δ¹⁸O values relative to the Albian-Aptian limestones. Paired carbon and oxygen isotope determinations provide a possibility of interpreting not only changes in the global carbon and oxygen cycle through time, but as a discrimination criterion in problems of stratigraphic correlations as well. Paleotemperature estimations based on calcium bicarbonate and seawater isotopic fractionations give temperatures as high as 26ºC for the Triassic limestones and between 22–23ºC for the Jurrasic – Lower Cretaceous formations. These results are similar to the isotope temperature records of other carbonatic sequences of equivalent age. A larger number of data and high-resolution carbon and oxygen-isotope stratigraphy are required to develop a better understanding of changes in paleoenvironmental conditions.
   The present isotope study is part of an ongoing project funded by the Romanian National University Research Council (PN II, Programme: IDEAS, contract ID-95).