Carbon and Oxygen isotope record of Early to Middle Cambrian carbonates (La Laja Formation) from the Precordillera terrane, Argentine.

GOMEZ, F.J., OGLE, N., ASTINI, R.A., Y KALIN, R.,

Mendoza (Argentina)

Congreso; Gondwana 12 (Geological and Biological Heritage of Gondwana); 2005

The La Laja Formation, Early to Middle Cambrian in age, is represented by 450m of alternating mixed and pure shallow-marine carbonates. This unit is at the base of the thick Cambro-Ordovician passive-margin carbonate bank of the Argentine Precordillera, which is largely considered a Laurentian terrane attached to Gondwana in Middle Ordovician times. High-resolution carbon and oxygen isotope analysis has been used to construct a preliminary curve that aids in stratigraphic correlation and helps to understand environmental aspects. Five members are recognized in the reference section at Quebrada La Laja (Sierra Chica de Zonda, San Juan Province) representing environmentally different associations. Sub-tidal calcareous sandstones and sandy limestones characterize the El Estero Member (~98m) at the base, which exhibits shallowing upward sub-tidal cycles with capping oolitic shoals towards the transition with the Soldano Member. The Soldano Member (~65 m) yields deeper sub-tidal intervals with a high percentage of fine terrigenous sediment represented by shaly marls and nodular limestones, and skeletal- and oolite-rich storm beds. The Rivadavia Member (~103 m) shows an abrupt development of peri-tidal thin-bedded ribbon limestones represented by alternating yellowish dolomitic fine-grained carbonate silts and dark-gray mudstones, with intra-clastic mudstones and brecciated horizons. The overlying Juan Pobre Member (~107.m) shows progressive deepening into a storm-influenced sub-tidal facies association. Shallow sub-tidal limestones associated with herring-bone cross-bedded oolitic shoals are common in the uppermost Las Torres Member (~70m). The overlying Zonda Formation is represented by dolomitized metre-scale microbial-rich peri-tidal cycles. The d13C record in the La Laja Formation ranges from -2.27‰ to +1.59‰ (relative to the VPDB standard). The lower 100m of the El Estero and its transition to the Soldano Member are characterized by negative values. This interval shows a highly variable and serrate pattern ranging between 0 and -2.27 ‰ with a unique positive maximum (+1.13 ‰) at 55m from its base. The pattern becomes more stable in the Soldano Member varying between -1.1 and -0.5 ‰ with a negative excursion (-1.99‰) in the mixed fine-grained deposits of its upper reach. An abrupt positive excursion up to +1.59 ‰ is recorded in the Rivadavia Member, followed by a slow  decrease to negative values across the transition with the Juan Pobre Member, reaching -1.69 ‰ within the mixed fine-grained deposits. The upper 50 m of the Juan Pobre Member range between -1.37 and -0.29 ‰ and are followed by a positive excursion up to 0.78 ‰ in the lower section of the Las Torres Member. The upper 50m of this member show values close to 0 ‰ as seen in the overlying Zonda Formation. Most d18O values range from -7 to -8‰ (VPDB), with the exception of a less negative excursion within the Rivadavia Member and a similar peak across the transition into the Zonda Formation. Although isotopic change seems dependant on environmental change, the oxygen record only co-varies with the d13C record in the Rivadavia Member (r2=0.67). Our preliminary d13C curve contrasts in resolution and general trends with that published by Buggisch et al. (2003) covering the upper part of the unit and sampled in the northern area of the Sierra Chica de Zonda. The high resolution d13C vs. d18O curve from the La Laja Formation shows ranges and excursions similar to those observed in other Early–Middle Cambrian deposits around the world (Brasier & Shukov, 1998; Montañez et al., 2000; Zhu et al., 2004 and Saltzman, 2005). Thus, besides improving potential intra-basin correlations, it will allow further comparisons with Cambrian curves elsewhere.   Brasier and Shukov, 1998, The falling amplitude of carbon isotopic oscillations through the Lower to Middle Cambrian: northern Siberia data: Canadian Journal of Earth Sciences, v. 35, p. 353-373. Montañez, I.P., Osleger, D.A., Banner, J.L., Mack, L.E., and Musgrove, M., 2000, Evolution of the Sr and C isotope composition of Cambrian oceans: GSA Today, v. 10, p. 1-7. Saltzman, M.R., 2005, Phosphorus, nitrogen, and the redox evolution of the Paleozoic oceans: Geology, v. 33, p. 573-576. Zhu, M.Y., Zhang, J.M., Li, G.X., and Yang, A.H., 2004,  Evolution of C isotopes in the Cambrian of China: Implications for Cambrian subdivision and trilobite mass extinctions: Geobios, v. 37, p. 287-301. Buggisch, W., Keller, M., and Lenhert, O., 2003, Carbon isotope record of Late Cambrian and Early Ordovician carbonates of Argentine Precordillera: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 195, p. 357-373.