Isotope signal of the Middle Jurassic carbonate ramp of Calabozo Formation at Arroyo El Plomo, Mendoza, Argentina.

VALENCIO, S. A.; RAMOS, A.; CAGNONI, M. C.; PANARELLO, H.0.; CABALERI, N.; ARMELLA, C.

Salvador Bahía

Simposio; IV South American Sympusium on Isotope Geology; 2003

<!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0mm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman"; mso-ansi-language:EN-US; mso-fareast-language:EN-US;} @page Section1 {size:612.0pt 792.0pt; margin:70.85pt 30.0mm 70.85pt 30.0mm; mso-header-margin:36.0pt; mso-footer-margin:36.0pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> ABSTRACT A good agreement between the facial/microfacial and the chemical and isotopic data is observed in the reconstruction of the palaeoenvironment during deposition of the carbonatic succession of Calabozo Formation, at El Plomo creek. Although these carbonates undergone more than a postdepositional process, petrographic and geochemical determinations indicate that they did not highly affected these rocks. Oxygen isotope system, the most sensitive indicator of diagenesis, show 18O compositions quite depleted, indicating they are obliterated by later postdepositional events. Despite of this, δ18O still maintain a variation pattern that reflects the environmental conditions prevailing during deposition. A shift to enriched values is observed towards the upper levels of each shallowing-upward cycle, indicating the establishment of more intense evaporative conditions and the progressive restriction in the water circulation, with the consequent loss of 16O. On the other hand, carbon isotope system retained best the primary isotopic characteristics. Limestones deposited in the deepest areas of the basin (lower cycle), are the best preserved and their 13C signature is in concordance with the Callovian unaltered marine carbonates. A grow to isotopically lighter carbon is observed through the shallowing-upward levels of the middle and upper cycles, remarking the input of organically derived carbon and episodes of subaereal exposure. Within the grade of resolution of the Bathonian-Callovian seawater curve, Chemostratigraphic age of Calabozo Formation is similar to the biostratigraphic one. Taking into account the resolution of this section of the curve, we can not affirm if the little variations in the 87Sr/86Sr ratios reflect diagenetic alteration or represent real isotope fluctuations.