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Sedimentology. Diagenesis and Ichnology of Cretaceous and Palaeogene Calcretes and Palustrine Carbonates from Uruguay Ana M

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Sedimentology. Diagenesis and Ichnology of Cretaceous and Palaeogene Calcretes and Palustrine Carbonates from Uruguay Ana M Sedimentology. diagenesis and ichnology of Cretaceous and Palaeogene calcretes and palustrine carbonates from Uruguay Ana M. Alonso-Zarza a.*, large F. Genise b, Mariano Verde C • Dept Petro[ogfa y Geoqufmica, Fae cc Geol6gicas, IG£-CSIC, Universidad Complutense, 28040 Madrid, Spain b CONlCET,Division Icn%gia, Museo Argentina de Ciencias Naturales, Angel Gallardo 470,1405 Buenos Aires, Argentina c SNI-ANII Departamento de Evoluci6n de Cuencas, Facultad de Ciencias, Universidad de la Republica, Montevideo, Urnguay ABSTRACT The Cretaceous (Mercedes Formation) and Paleogene (Queguay Formation) deposits cropping out in Wand S Uruguay comprise two terrestrial limestone units that are very rich in trace fossils. The study of these units permits to propose a sedimentological model for palustrine limestones and ca1cretes in which the distribution of different types of trace fossils is considered. The study units include three main types of deposit: lacustrine limestones, palustrine limestones and ca1cretes. The lacustrine limestones are relatively homogeneous and contain gastropods, charophytes and ostracods, but no trace fossils. They were deposited in a relatively more perennial lacustrine environment. The palustrine limestones include four different facies: desiccated Keywords: mudstones, nodular limestones, granular limestones and gravel-sheets. The desiccated mudstones indicate Calcretes a lesser degree of pedogenic modification and the granular limestones a higher degree. The gravel-sheets are Lacustrine carbonates an indication of the reworking of previous limestones deposits during low lakewater levels. Most of the Trace fossils palustrine limestones (except the gravel sheets) contain the same bioclasts as the lacustrine limestones plus a Silicification variety of trace fossils such as Rebuffoichnus sciutto� Fictovichnus gobiensis and different ichnospecies of Chaco-Paranense Basin Celliforma. The ca1cretes are either massive (groundwater) or laminar. The massive ca1cretes are sandy Uruguay limestones made up of a carbonate matrix and cements. The laminar ca1cretes (root mats), which contain alveolar septal structures, occur as centimetre-thick layers and can be seen in all types of deposit. They contain the same trace fossil association as the palustrine limestones; the massive ca1cretes are poorer in such fossils. The distribution of trace fossils in these environments is under strong facies control and provides good evidence of subaerial exposure and semi-arid climates. All the limestones are partially replaced and cemented by opal and quartz, but in all cases the primary structure is preserved. Silicification occurred under groundwater meteoric conditions. Overall, the limestones facies (ca1cretes, palustrine and lacustrine) and their lateral distribution likely reflect the existence of wetland environments in semiarid to sub-humid climates. These climatic conditions were interrupted by a period of increased precipitations, probably the Early Eocene dimatic Optimum. 1. Introduction Islands (Alonso-Zarza and Silva, 2002; Genise and Edwards, 2003 ). In contrast, in many well-described palustrine and calcrete deposits, Insect trace fossils are abundant in palustrine deposits and such as in the Cretaceous Rupelo Formation in the Cameros Basin calcretes. This is particularly true in South America (Genise et al., (Platt, 1989) or in the Tertiary Duero Basin in Spain (Huerta and 2010) e.g., in the Palaeogene palaeosols of the Gran Salitral and Armenteros, 2005), insect trace fossils are not recorded. This may Sarmiento Formations in Argentina (Melchor et al., 2002; Bellosi et al., reflect environmental conditions that were unsuitable for insects, or 2010). Other well-known palustrine and calcrete deposits that more simply that there have been few searches for such trace fossils. contain insect traces include the Pleistocene Bridgewater Formation Although Freytet and Plaziat (1982) give some initial clues on the in Australia (Houston, 1987), the Bembridge Formation of the Isle of distribution of some of these traces in palustrine carbonates and White (Edwards et al., 1998), the Oligocene Brulle Formation of South carbonate palaeosols, most environmental models (e.g., Freytet and Dakota (USA) (Retallack, 1984), and the calcretes of the Canary Verrecchia, 2002; Alonso-Zarza, 2003 ; and Wright, 2007) do not discuss in detail the distribution or typesof trace fossils found in such settings. However, in the last decade, ichnology in continental settings * Corresponding author. has advanced notably, and a recurrent trace fossil association E-mail address: [email protected] (AM. Alonso-Zarza). dominated by insect traces (the Celliforma ichnofacies) seems to be indicative of carbonate palaeosols bearing low plant densities under Navarro, 1991; Veroslavsky and Martlnez, 1996; Martinez et al., 1997, conditions of arid to semiarid environments (Genise et al., 2010). 2001; and Mart(nez and Veroslavsky, 2004). T6falo and Morris (2009) Thus, these traces can provide data of use in interpreting the and T6fala and Pazos (2010) interpret these limestones as a wide variety palaeoenvironment of the basins in which calcretes and palustrine of carbonates, including calcretes and palustrine limestones. carbonates form. The aims of this work are: 1) to establish the stratigraphic The Cretaceous and Palaeogene of W and S Uruguay are represented framework for these limestones, 2) to determine the processes and by two units formed by continental limestones with an exceptional palaeoenvironments involved in the formation of these carbonates, 3) abundance of trace fossils. These units crop out in Uruguay and to record the distribution of the different typesof trace fossils and the (sparsely) in NW Argentina within the Chaco-Paranense and Santa slight differences in the sedimentary environment that can control the Luda Basins (Fig. 1). There is an agreement, based on the overall occurrence of them, 4) to analyse the silicification processes that these geological setting and the fossil content, that the age of these limestones limestones underwent, and 5) to propose the palaeogeographic ranges from Late Cretaceous to Eocene and that they undoubtedly setting that prevailed during part of the Cretaceous and Palaeogene. represent continental limestones formed in arid to semiarid climates In doing so we will try to show that the traces fossils are a useful tool (Coso and Bossi, 1966; Bossi er aI., 1975; Sprechmann et aI., 1981; Goso­ to understand the sedimentary record of continental carbonates and Aguilar, 1999; Daners and Guerstein, 2004; and Mart(nez and its paleoclimatic constraints. Veroslavsky, 2004). However disaepandes exist in their environmental interpretation. Earlier authors interpreted these limestones to have 2. Geological setting been deposited in lacustrineenvironments ( Lambert, 1939; Serra, 1945; andjones, 1956), whereas more recently they have been interpreted as The carbonates studied in this paper form part of the sedimentary thick groundwater calcretes (Sprechmann et al., 1981; Bassi and record of the Chaco-Paranense and Santa Luda Basins (Fig. 1) that A B -55' c * * 9 10 < Z I- Z W � (!) -320 c:: < MONTEVIDEO _36' o 50 100 -6 ' Fig. 1.Map showing the situation of the study area and the location of the key sectionsj1ocalities. URUGUAY: 1: Sauce Solo; 2: Forestal. 3: Palmar Dam; 4: ANCAPQuarry; 5: Road 26; 6: Quebracho. ARGENTINA: 7: Colon. 8: PN El Palmar; 9: Arroyo Ita; 10: Road 40. Northeastern of Mercedes city. Table 1 overlie the Uruguayan shield, These carbonates were mentioned in Historical review about the nomenclature, Iithostratigraphic treatment and age given to the diaries of Charles Darwin and Priest Damaso Antonio Larrafiaga at the "Calizas del Queguay", the end of the nineteenth century, These rocks crop out mainly in the Authors Nomenclature Age north-western region of Uruguay, in Paysandu, Rio Negro and Soriano Frenguelli (1930) 5ilicified deposits of Quebracho Tertiary Counties, and to a lesser extent near Montevideo, in southern (N) and limestones with Uruguay, Their first detailed description was provided by Frenguelli Strophocheilus fromMigues (5) (1930), who also correlated the deposits from north-western Uruguay Walther (1931) "(alizas siliceas" Mio-Pliocene with those located about 300 km away in southern Uruguay, L1mbert (l940a) "Calizas lacustres de Queguay" Cretaceous-Tertiary- Bulimulus Oligocene Frenguelli (1930) recorded the presence of the gastropods L1mbert (l940b) "(alizas de Queguay" Cretaceous-Tertiary- and Planorbis at Estacion Quebracho and discussed the fossil content, Oligocene including cells from "solitary vespids", The name "Calizas del 5erra (1945) "(alizas de Queguay" Oligocene Queguay" was firstused by Lambert (1940a), }ones (1956) "(alizas de Queguay" Oligocene The stratigraphy of these basins has been widely discussed and Caorsi and Gof'ii "(alizas del Queguay" Oligocene (1958) there is no general agreement on the age or even the lithology of some Goso and Bossi Queguay Formation Paleogene of the units, especially for certain formations, Table 1 summarizes all (1966) the previous stratigraphic work performed on these limestones and Bossi (1966) Queguay Formation Cenozoic reflects the lack of agreement regarding their age and stratigraphic Bossi et aL (1975) Mercedes Formation Cretaceous Mones (1979) Queguay Formation Late Eocene position, 5prechmann et al Queguay Formation, Cretaceous-Paleogene The lowermost sedimentary unit is the Guichon Formation or (1981) Mercedes Formation "Areniscas
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