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The La Laja Formation, Early to Middle Cambrian in age, is represented in the La Laja section by ~440 m 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 and is largely considered a Laurentian terrane attached to Gondwana in Middle Ordovician times. A high-resolution carbon and oxygen isotope analysis allowed the construction of a preliminary curve that aids in stratigraphic correlation and helps understanding environmental aspects. Five members are recognized in the reference section at Quebrada La Laja (Sierra Chica de Zonda, San Juan Province), which represent different facies associations. Subtidal calcareous sandstones and sandy limestones characterize the uppermost 15 m of the El Estero Member. The lower half of the Soldano Member (~85 m thick) exhibits shallowing-upward subtidal cycles with capping oolitic shoals. The upper half of the Soldano Member (62 m thick) is represented by shaly marls and nodular limestones and skeletal- and oolite-rich storm beds attributed to depositional settings ranging from shallow to deep subtidal, with a high percentage of fine terrigenous sediment atop. The Rivadavia Member (~98 m thick) records an abrupt development of shallow subtidal to intertidal, thin-bedded ribbon limestones represented by alternating yellowish dolomitic, fine-grained carbonate silts and dark-gray mudstones, with intraclastic mudstones and brecciated horizons. The overlying Juan Pobre Member (~107 m thick) shows progressive deepening, as recorded by a storm-influenced subtidal facies association. Shallow subtidal limestones associated with herringbone cross-bedded oolitic shoals are common in the uppermost Las Torres Member (~70 m thick). The overlying Zonda Formation is represented by meter-scale, dolomitized microbial-rich peritidal cycles. The d13C record in the La Laja Formation ranges from -2.27 to +1.59 (relative to the VPDB standard). The lower 100 m of the section (El Estero and lower Soldano Members) is characterized by negative d13C values showing a highly variable and saw-tooth pattern ranging between 0 and -2.27, with a unique positive maximum (+1.13) at 55 m from its base. The pattern becomes more stable toward the top of 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, which slowly decreases toward 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 50 m of this member shows values close to 0, as seen in the Zonda Formation. Regarding the d18O curve, most 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. Our d13C curve is consistent, in general trends, with that published by Buggisch et al. (2003) for the upper part of the La Laja Formation exposed in the northern area of the Sierra Chica de Zonda. The high-resolution d13C and d18O curve from the La Laja Formation shows ranges and excursions similar to those documented in Middle Cambrian deposits from other basins, suggesting a global control. Therefore, besides improving potential intra-basin correlations, carbon and oxygen isotope data will allow further comparisons with Cambrian curves elsewhere. A plausible mechanism to produce these positive excursions could be related to high organic bio-productivity and increased burial of Corg (organic carbon) due to high nutrient influx to the ocean associated with a relative sea level fall. Local environmental controls could have, in part, modified the original isotopic signal. Buggisch, W., Keller, M. & Lenhert, O. (2003) Carbon isotope record of the Late Cambrian to Early Ordovician carbonates of the Argentina Precordillera. Palaeogeography, Palaeoclimatology, Palaeoecology, 195, 357-373.

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