Journal of Sedimentary Research, 2007, v. 77, 826–842 Research Article DOI: 10.2110/jsr.2007.079 PALEOENVIRONMENTAL AND CARBON-OXYGEN ISOTOPE RECORD OF MIDDLE CAMBRIAN CARBONATES (LA LAJA FORMATION) IN THE ARGENTINE PRECORDILLERA FERNANDO J. GOMEZ, 1 NEIL OGLE, 2 RICARDO A. ASTINI, 1 AND ROBERT M. KALIN 2 1 Laboratorio de Ana ´ lisis de Cuencas, CICTERRA-CONICET, Facultad de Ciencias Exactas, Fı ´sicas y Naturales, Universidad Nacional de Co ´rdoba, Avenida Ve ´lez Sarsfield 1611, X5016GCA, Co ´rdoba, Argentina 2 Environmental Engineering Research Center, School of Planning, Architecture and Civil Engineering, The Queen’s University of Belfast, Northern Ireland e-mail: fgomez@efn.uncor.edu ABSTRACT: The La Laja Formation (Early to Middle Cambrian) is one of the oldest units exposed at the base of the lower Paleozoic carbonate platform of the Argentina Precordillera. This is a key unit regarding the hypothesis of the Precordillera as a Laurentia-derived allochthonous terrane currently located in the south-central Andes. According to the faunal affinity and stratigraphic development of the thick Cambrian carbonate bank, the Argentine Precordillera would have been attached to Laurentia. The La Laja Formation contrasts with the rest of the overlying units of the Cambro-Ordovician carbonate platform by being partly mixed carbonates–siliciclastics. This dominantly shallow subtidal unit is internally arranged into several Grand Cycles indicating a complex environmental mosaic, probably with local depocenters related to variable subsidence. This unit records the stabilization of the rifted margin of the Precordillera terrane, prior to the broadening of the carbonate sedimentation during the passive-margin drifting stage. A high-resolution d 13 C and d 18 O isotope study, in concert with a detailed paleoenvironmental analysis, was carried out to better understand both environmental and chronostratigraphic evolution of the La Laja Formation. Three d 13 C positive excursions were recorded; the first one at the Glossopleura biozone within the Soldano Member, the second beginning at the base of the Rivadavia Member, and the third during deposition of the Las Torres Member. Comparisons with other Middle Cambrian curves, in the Precordillera and elsewhere (Rocky Mountains and Great Basin, U.S.A., the western Hunan Province in south China, and the Amadeus, Georgina, and Daly basins in Australia) suggest a global control on these excursions. Mechanisms to produce these positive excursions could be related to high bio-productivity and increased burial of C org (organic carbon) produced by high nutrient influx to the ocean associated with a relative sea-level fall. Local environmental controls could have in part altered the original isotopic signal. INTRODUCTION The isotopic signature of diagenetically unaltered marine carbonates can be taken as representative of the fluid (seawater) where the carbonate precipitated. The d 13 C isotopic excursions recorded in marine carbonates result from the combination of global changes in fluxes between different carbon reservoirs of the ocean–atmosphere system, e.g., changes in organic productivity, organic-carbon burial and biological pumping, variations of sedimentation rates and associated accumulation of organic matter, changes in ocean oxygenation state controlling the preservation of organic matter, etc. (Kump 1991; Glumac and Walker 1998; Kump and Arthur 1999; among others). Due to this global control and the presence of secular changes in carbon cycling, chemostratigraphic studies of carbonates have proven to be an invaluable tool where poor bio- stratigraphic control prevents detailed correlation (Kaufman and Knoll 1995; Pelechaty et al. 1996; Cozzi et al. 2004; among others). In spite of this, regional and/or local controls related to basin evolution, e.g., evaporation, freshwater influx, upwelling water, restriction, aging, and mixing of water masses, etc. (Patterson and Walter 1994; Glumac and Walker 1998; Holmden et al. 1998; Panchuk et al. 2005; Panchuk et al. 2006), and/or possible diagenetic alteration (Banner and Hanson 1990) can modify the primary isotopic signature, and this possibility must be considered when using a chemostratigraphic approach for correlation purposes. The Argentine Precordillera in the foothills of the Central Andes is well known for its unusual development of Cambrian and lower Ordovician Laurentian-like passive-margin carbonates. Although various regional studies have shed light on the geological evolution of this allochthonous terrane in South America (Ramos et al. 1986; Astini et al. 1995), little detailed sedimentological work has been published and is not readily accessible in the international literature. In the light of poor bio- stratigraphic control, few reviews and synthesis papers (Astini et al. 1995; Astini 1998; Can ˜ as 1995, 1999; Keller et al. 1998; Keller 1999) have shown the need for enhancing correlation by means of modern high-resolution methods as provided by carbon and oxygen isotope stratigraphy. Although this type of analysis has been applied with success to the Precordillera carbonate record (Buggisch et al. 2003), the interval considered in this paper has not been studied previously. During the early Paleozoic, shallow epicontinental (or epeiric) seas covered vast areas of the continental interior of Laurentia (Pratt and James 1986). According to its faunal affinity (Benedetto 2004 and references therein) and sedimentary development, the Cambrian carbon- ate platform of the Argentine Precordillera would have been developed Copyright E 2007, SEPM (Society for Sedimentary Geology) 1527-1404/07/077-826/$03.00