Carbon and oxygen-isotope stratigraphy of the Early Cretaceous carbonate platform of Padurea Craiului (Apuseni Mountains, Romania): A chemostratigraphic correlation and paleoenvironmental tool

Stable C and O isotope records were obtained from carbonate rocks spanning the Hauterivian to Cenomanian interval collected in several sections from the carbonate platform of Padurea Craiului (Apuseni Mountains, Romania). In the absence of some key biostratigraphic marker species, stable isotopes were applied as a tool for stratigraphic correlation and dating. The composite delta C-13 and delta O-18 curves for the Early Cretaceous shows variable conditions with large positive and negative excursions and provide information on past environmental changes. The Hauterivian and the Barremian limestones (Blid Formation) display lower delta C-13 values (-2.8 parts per thousand to +2.9 parts per thousand) relative to the Aptian-Albian deposits (-2.6 parts per thousand to +5.4 parts per thousand) (Ecleja, Valea Magurii and Varciorog Formations). The red detrital formation (Albian-Cenomanian) is characterized by a highly variable distribution of the delta C-13 values (-3.5 parts per thousand to +3.9 parts per thousand). Based on the similarities between the C-isotope curve established in Padurea Craiului and from other sections in the Tethyan and the Pacific regions, two major oceanic anoxic events characterized by delta C-13 positive excursions were clearly recognized. The first is the OAE1a event (Early Aptian) in the upper part of the Ecleja Formation and the Valea Magurii Formation. The second is the OAE1b event (Late Aptian-Albian) in the upper part of the Varciorog Formation and in the Subpiatra. Member. The position of the Aptian/Albian boundary is estimated to be at the upper part of the Varciorog Formation, immediately after the beginning of the delta C-13 positive excursion. The delta C-13 data show major negative excursions during the Barremian (Blid Formation), Early Aptian (Ecleja Formation), and Late Aptian (Varciorog Formation). The O isotope variation pattern (-10.2 parts per thousand to -2.1 parts per thousand) is consistent with progressively warming temperatures during the Early Barremian followed by a cooling period. A subsequent warming period culminated in the Early Aptian. A significant cooling phase corresponds to the Late Aptian and Early Albian and the climate cooled again during the Late Albian and into the Early Cenomanian stage. The data provide a better understanding of the Early Cretaceous sedimentation cycles in Padurea Craiului and create a more reliable framework for regional correlations. (c) 2012 Elsevier Ltd. All rights reserved.