Carbon isotopes as a sequence stratigraphic tool: examples from the Lower and Middle Toarcian marly limestones of Portugal
DOI:
https://doi.org/10.21701/bolgeomin.118.1.001Keywords:
Carbon isotopes, marl-limestone alternations, Portugal, stratigraphic sequences, ToarcianAbstract
The Lower and Middle Toarcian series of the Lusitanian Basin (Portugal) are generally characterized by hemipelagic marl-limestone alternations, included in the S. Gião Formation. The succession is informally organized into three 3rd order depositional sequences (ST1 to ST3), each bounded by regional discontinuities. These depositional sequences are characterized in terms of vertical and lateral litho- and biofacies changes. Using accurate ammonite biostratigraphic control, sequence boundaries are assigned to the lowermost polymorphum Zone (SBT1), polymorphum-levisoni Zone boundary (SBT2), intra-bifrons Zone (SBT3) and uppermost bonarelli Zone (SBT4). Maximum flooding intervals are identified in the uppermost polymorphum (ST1), uppermost levisoni (ST2) and uppermost bifrons (ST3) Zones. The uppermost part of levisoni Zone is a 2nd-order maximum transgressive phase. Carbon isotope data from bulk carbonate samples of selected sections in the Lusitanian Basin (Rabaçal, Porto de Mós, Coimbra and Figueira da Foz) reveal this analytical technique as good indicator of 3rd order sequence evolution. Positive δ13C excursions correspond to transgressive (transgressive systems tracts) phases whereas negative trends/excursions are related to periods of regression (lowstand and highstand systems tracts). Furthermore, the global δ13C events recognised in the Early Toarcian sections of several North-European and Tethyan basins are observed in all the studied sections of Lusitanian Basin.
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