International Geology Review, Vol. 43, 2001, p. XXX–XXX. Copyright © 2001 by V. H. Winston & Son, Inc. All rights reserved. Carbon and Oxygen Isotope Compositions of Some Upper Cretaceous–Paleocene Sequences in Argentina and Chile A. N. SIAL,1 V. P. F ERREIRA, Department of Geology, NEG-LABISE, UFPE, P.O. Box 7852, Recife, PE, Brazil, 50732-970 A. J. TOSELLI, INSUGEO, National University of Tucuman, S.M. Tucuman, Argentina 4000 M. A. PARADA, Department of Geology, University of Chile, P.O. Box 13518, Santiago, Chile F. G. A CENOLAZA, INSUGEO, National University of Tucuman, S.M. Tucuman, Argentina 4000 M. M. PIMENTEL, Geosciences Institute, University of Brasilia, Brasilia, D.F., Brazil 70.910-970 AND R. N. ALONSO Department of Geology, National University of Salta, P.O. Box 362, Salta, Argentina, 4400 Abstract The Cretaceous–Paleocene (K–T) transition has been recorded in sedimentary carbonate rocks in northwestern Argentina and southern Chile. In the Yacoraite Basin, Argentina, this transition has been preserved in a 2 m thick marly layer, at the base of the Tunal Formation, which overlies lacus- trine/marine carbonates of the Yacoraite Formation (Cabra Corral dam). The K–T transition is also preserved at Maimara, where Tertiary sandstones overlie a 50 m thick limestone bed of the Yacoraite Formation. In the Magellan Basin, Chile, glauconitic sandstones with calcitic cement and limestone concretions of the Maastrichtian Punta Rocallosa Formation are overlain by sandstones, claystones, and limestones of the Chorillo Chico Formation. The K–T transition is preserved in the lower portion of the Chorillo Chico Formation. δ13 Carbonates of the Yacoraite Formation display bulk-rock C values from +1 to +2‰ PDB, with δ13 a negative incursion (–4‰ PDB) at the K–T transition. C values in the Tunal Formation marls vary δ13 from –3 to –1‰ PDB. At Rocallosa Point, C values in limestone strata, calcite cement, and lime- stone concretions vary from –4 to –33 ‰ PDB, and the lowest value in the Chorillo Chico Formation apparently marks the K–T transition. The δ18O fluctuations in the Yacoraite and Magellan carbonate rocks suggest a temperature drop at the K–T transition, followed by a temperature rise. High 87Sr/86Sr ratios (0.7140–0.7156) characterize the studied profiles of the Yacoraite Forma- tion, documenting an important 87Sr-enriched source of Sr to the water from which these carbonates precipitated. At the Magellan basin, 87Sr/86Sr ratios are closer to the expected values for the global Late Cretaceous–Paleocene ocean. Introduction the result of a meteoric impact, the consequence of which was the generation of a sun-blocking dust SEVERAL STUDIES HAVE FOCUSED on the Late Creta- cloud, cessation of photosynthesis, and disruption ceous-Paleocene (K–T) transition, a time of very of the food chain, causing prolonged ecological important worldwide massive faunal extinction. modification. Among the several hypotheses proposed, the one by In many basins where the K–T transition has Alvarez et al. (1980) advocates faunal extinction as been investigated using C and O isotopes, important environmental changes have been detected (e.g., 1Corresponding author; e-mail: [email protected]. Magaritz, 1989; Ferreira et al., 1994). Only in few 0020-6814/01/XXX/XXX-19 $10.00 1 2 SIAL ET AL. FIG. 1. Location of the five K–T transition sites examined in this study in Argentina and Chile. Location of the Pernambuco-Paraíba Coastal Basin is also indicated. places in continental South America, however, has Paraiba Coastal Basin (Albertão and Kotsoukos, this transition been demonstrated by stable-isotope 1994; Albertão et al., 1994; Ferreira et al., 1994). studies of carbonate rocks. C and O isotopes are Albertão and Kotsoukos (1994) described a sedi- important sensors for climatic changes, and such mentary section at the Poty quarry, in the Pernam- investigations should contribute to a better under- buco-Paraíba Coastal Basin, where a 1 cm thick standing of this transition in the Southern Hemi- clay layer between the Gramame and Maria Farinha sphere. Suitable targets for study include limestones exhibits distinctive features similar to sedimentary carbonate rocks of the Pernambuco- clays found at the K–T transition globally, such as Paraíba Coastal Basin, northeastern Brazil, the Yac- mass extinction, iridium and total organic C anoma- oraite and Neuquen basins, respectively in north- lies, deviations in the patterns of C and O isotopes, western and west-central Argentina, and the and exotic elements (shocked quartz grains and Navidad (Topocalma Point) and Magellan (Punta microspherules). This is, perhaps, the only locality Arenas) basins of Chile (Fig. 1). The Late Creta- in Brazil that contains the K–T transition, according ceous–Paleocene transition also has been recorded to Albertão and Kotsoukos (1994). Ferreira et al. in the Antarctic Peninsula (Souza et al., 1994), (1996) observed a sharp decrease in δ13C values based on sedimentological, biostratigraphic, and from +2‰ PDB to –5.5‰ PDB in the transition petrologic indicators, as well as O isotopes of calcite from the Gramame to the Maria Farinha limestone, cement and organic indicators (biomarkers and with a corresponding increase in δ18O from –6‰ vitrinite). PDB to –1‰ PDB that suggests a significant tem- C and O isotope data for carbonates from the K– perature drop at the transition, as typically observed T transition are available for the Pernambuco- worldwide (e.g., Magaritz, 1989). UPPER CRETACEOUS–PALEOCENE SEQUENCES 3 An important climatic change during the K–T According to Palma (1984), the Yacoraite Forma- transition was recognized by Ashrof and Stinnes- tion exhibits high-energy water facies (terrigeneous beck (1989) on the basis of the fossil record of the and intraclastic carbonate rocks) and quiet-water Gramame and Maria Farinha limestones. The cli- facies (claystones, mudstones, and wackestones). mate during the deposition of the Maastrichtian Gra- Carbonates include limestones, slightly magnesian mame limestone was tropical to subtropical and to dolomitic limestones, calcitic dolomites, and changed into subtropical to temperate during the dolomites. The fossil record encompasses gastro- deposition of the Paleocene Maria Farinha lime- pods, ostracods, pelecypods, and reptiles, as well as stone. wood fragments, karophytas, and stromatolites that Oxygen isotopes studies (Hsu and Wissert, 1980) indicate lacustrine sedimentation. Lack of strati- suggest that during the Late Maastrichtian, temper- graphic discontinuity between the Maastrichtian Yacoraite Formation and the Paleocene Olmedo atures in the South Atlantic Ocean were in the 18– Formation, led Palma (1984) to assume that 25°C range, with a cooling immediately before the the K–T transition was preserved in the Olmedo K–T transition. A gradual temperature decrease to Formation. values as low as 10°C during the Late Maastrichtian was observed by Huber et al. (1995) studying oxy- The deposition of the Salta Group during the gen isotopes in planktonic foraminifera from the Neocomian to Campanian was controlled by a rift environment, but there is no unique interpretation Deep Sea Drilling Project (DSDP) in the southern for the depositional environment of the Yacoraite Atlantic Ocean. sediments. Marquillas et al. (1999) proposed that In this study, we examine the behavior of C, O, the post-rift environment was marked by a deep and Sr isotopes and bulk chemistry (Si, Mg/Ca, Sr, Maastrichtian marine ingression that marked the Fe, and Mn) of sedimentary carbonates from the termination of an arid environment and led to a Yacoraite Formation (Maimara and Cabra Corral, carbonate system that persisted until the early northwestern Argentina), the Topocalma Formation Paleocene. (Topocalma Point and the village of Algarrobo), Dar- A lacustrine depositional environment for the win’s Navidad Basin (170 km southwest of Santi- Yacoraite Formation sediments was recognized by ago), and Magellanes Province in Chilean Patagonia Palma (1984) on the basis of observed dinosaur to evaluate evidence of a possible K–T transition. tracks and plant remains. Marquillas et al. (1999) The new isotopic data will contribute to the knowl- assumed that the Lower Cretaceous lacustrine depo- edge of the geochemistry of the global ocean during sitional environment graded into a marine one dur- the K–T transition event. ing the Maastrichtian, or a fluvial environment with saline mud flats with clastic and carbonate lacus- Geological Setting trine systems until the middle Eocene. Presence of some foraminifera and fish species in the Yacoraite K–T transition in northwestern Argentina Formation implies a marine influence. This is a plausible hypothesis regarding sea level rise during The K–T transition has been preserved in the the Late Cretaceous, and would imply that marine transition of the Maastrichtian Yacoraite Formation faunal elements could have undergone adaptation to to the Paleocene Olmedo Formation (Salta Group), continental conditions (Palma, 1984). Ammonites in northwestern Argentina. This group has an evolu- are found in Bolivia, demonstrating the existence of tionary history from Early Cretaceous to Middle an open sea up to that extension of the basin, and Paleocene and occupies a variety of sedimentary that, to the south, there was a connection with the environments with a cumulative thickness of depos- Atlantic Ocean, reflecting the fact that some units its of ~5,000 m (Salfity and Marquillas, 1994). The contain foraminifera. Yacoraite Formation lime- Salta Group is subdivided into the Balbuena (older) stones in some localities in Argentina contain and the Santa Barbara (younger) subgroups. The marine fish fossils (some were flying fish; e.g., Gas- Yacoraite Formation overlies the Lecho Formation teroclupea branisai at Tres Cruces) and, in Bolivia, and forms a part of the Balbuena Subgroup, and is these fossils were found together with ammonites. overlain by the Tunal Formation (same as the Near the top of the Yacoraite Formation in the sec- Olmedo Formation; Fig. 2) of the Santa Barbara tion at Tonco Valley (dinosaur track locality), a few Subgroup.