Oxygen and carbon stable isotopes in continental and shallow marine deposits of the Salta Basin (Neocomian-Eocene), Northwestern Argentina

SABINO I; ., SIAL A; MARQUILLAS R; DEL PAPA C; FERREIRA V.P; NOVARA M

Punta del Este

Simposio; V South American Symposium on Isotope Geology (V SSAGI); 2006

Carbon and oxygen stable isotopes were analyzed in limestones of the Salta rift (Cretaceous-Paleogene) of northwestern Argentina taking in account the flooding events – marine and lacustrine - occurred in the basin. The organic accumulation rate was high during the main flooding events of the synrift stage and during the accumulation of the marine Yacoraite Formation of the initial postrift stage. The Maíz Gordo and Tunal Formations were deposited in lacustrine environments with less organic accumulation. The Güemes and Juramento Members of the Yacoraite Formation and the base of the Lecho Formation correspond to sedimentary settings with water mixing (marine-continental). The Las Curtiembres Formation and the transition between the Amblayo and Güemes Members of the Yacoraite Formation show strong influence of meteoric diagenesis.   INTRODUCTION The Salta basin (Cretaceous-Eocene) covered a wide region of northwestern Argentina (Fig. 1). The fill of the basin is constituted by the synrift deposits of the Pirgua Subgroup and by the postrift deposits of the Balbuena and Santa Bárbara Subgroups. The strata of the three subgroups together reach a thickness of 5,000 m, composed by redbeds with lacustrine and shallow marine flooding deposits (Salfity and Marquillas, 1994). During the synrift stage occurred two main flooding events that correspond to the lacustrine deposits of the Brealito Member (Cenomanian) of the La Yesera Formation and the Morales Member (Campanian) of the Las Curtiembres Formation (Sabino, 2004). During the initial postrift (Balbuena Subgroup) occurred the marine ingression of the Yacoraite Formation (Maastrichtian-Danian), and subsequent a saline lake was established depositing the Tunal Formation (Danian) (Marquillas et al., 2005). The last main flooding recorded was during the late postrift stage (Santa Bárbara Subgroup) when the Maíz Gordo Formation (Paleoceno-Eoceno) was accumulated in a brackish alkaline lake (del Papa and Salfity, 1999). In this work, the stable isotopes of the carbonate rocks of different stratigraphic levels of the Salta Group were studied (Fig.1). Mainly those that are representative of the flooding events of the basin. We analyzed three units of the Pirgua Subgroup: 1) The lowermost level of the basin, the Brealito Member of the La Yesera Formation in Brealito (Fig. 2), 2) The Las Curtiembres Formation in the Pucará Valley, and 3) The Morales Member of the Las Curtiembres Formation in the quebrada Las Conchas.   Figure 1. Stratigraphic units of the Salta Group.  Unites studied in bold letters.   Figure 2. Location map. 1. Brealito, 2. Pucará, 3. Quebrada Las Conchas, 4. Siete Hermanos, 5. Cabra Corral dam, 6. Quebrada El Chorro, 7. Alemanía, 8. La Viña, 9. Peñas Azules, 10. Garabatal, 11. Chuñapampa.   From the Balbuena Subgroup were analyzed: 1) The limestones at the base of the Lecho Formation in the area of the Cabra Corral dam, and the limestones of the Siete Hermanos Member, also located in the base of the Formation, in Siete Hermanos, 2) The different Members of the Yacoraite Formation, main calcareous level of the Salta Group, in the Cabra Corral dam area and in the quebrada El Chorro (Fig.1), and 3) The Tunal Formation in quebrada El Chorro, Alemanía and La Viña (Fig. 2). From the Santa Bárbara Subgroup was studied the Maíz Gordo Formation, the second calcareous level in importance of the Salta basin. Samples from Peñas Azules, Garabatal and Chuñapampa were selected (Fig. 2).