And Other Ibexian Conodonts in an Early Paleozoic Carbonate Platform Facies of the Argentine Precordillera
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Paleogeographic significance of Clavohamulus hintzei Miller (Conodonta) and other Ibexian conodonts in an early Paleozoic carbonate platform facies of the Argentine Precordillera Institut für Geologie und Mineralogie, Universität Erlangen-Nürnberg, Schloßgarten 5, O. Lehnert* D-91054 Erlangen, Germany Department of Geography, Geology, and Planning, Southwest Missouri State University, J. F. Miller Springfield, Missouri 65804-0089 J. E. Repetski U.S. Geological Survey, 970 National Center, Reston, Virginia 22092 ABSTRACT REGIONAL GEOLOGY AND only on the eastern flank of the Precordillera STRATIGRAPHY (Buggisch and Astini, 1993). The Hirnantian age Pre-Tremadocian conodonts and trilobites of these deposits is well documented by typical and Tremadocian conodonts are reported The Argentine Precordillera of La Rioja, San brachiopod assemblages (Benedetto, 1985, from the Cambrian and Ordovician La Silla Juan, and Mendoza Provinces formed during An- 1986). Middle and Upper Ordovician strata are Formation in the Cerro La Silla section in dean crustal shortening in the late Tertiary. This lacking in many sections of the central Pre- east-southeast Jáchal, San Juan Province, Ar- “thin-skinned” fold and thrust belt of predomi- cordillera, and the top of the carbonate platform gentina. A shallow marine conodont fauna nantly Paleozoic strata is characterized mainly by sequence is missing due to pre-Silurian erosion. contains elements of Clavohamulus hintzei westward-dipping, north-striking imbricate Silurian and Devonian strata in adjacent areas are Miller, a common species in North America, faults. This structural belt extends more than mostly siliciclastic rocks of predominantly ma- but reported for the first time from the early 400 km between lat 28°45′S and 33°15′S and rine origin, but Carboniferous and younger strata Paleozoic platform carbonates of the western shows an east-west shortening of at least 50% in the region are predominantly of continental Argentine Precordillera. The presence of this (von Gosen, 1992). The intense thrust tectonics origin. species suggests a correlation with the Clavo- were triggered by the eastward subduction of a Previously described conodonts from the Pre- hamulus hintzei conodont subbiozone of the flat segment of the Nazca Plate under South cordilleran carbonate platform deposits were ob- Cordylodus intermedius conodont biozone in America (Jordan et al., 1983). tained chiefly from Arenigian or early Llanvirn- North America, considered Early Ordovician There are no outcrops of Precambrian base- ian strata (e.g., Hünicken, 1971, 1989; Serpagli, (Skullrockian Stage, Ibexian Series) in North ment in the Precordillera, although some xeno- 1974; Sarmiento, 1990; Sarmiento and García America, but by South American and Euro- liths of Grenvillian age occur in Tertiary volcanic López, 1993; Lehnert, 1993; Lehnert and Keller, pean standards, this biozone would be of latest rocks (Abbruzzi et al., 1993; Kay et al., 1996). 1994). Upper Cambrian conodonts have been re- Cambrian age. C. hintzei and associated con- The base of the sedimentary succession in the ported only from olistoliths of slope to outer plat- odonts of the La Silla Formation are typical of eastern and central Precordillera (Ortiz and Zam- form carbonates incorporated into Middle the tropical faunas of the North American brano, 1981) begins with at least 2500 m of Cam- through Upper Ordovician slope deposits (Here- Midcontinent Faunal Province; Late Cam- brian to Lower Ordovician carbonates (Baldis dia, 1985; Heredia and Bordonaro, 1988; Leh- brian trilobites from lower in the formation and Bordonaro, 1984) reflecting shallow marine nert, 1994). These faunules are composed of pan- also are typical North American taxa. The platform deposition that is replaced by slope and demic elements and, therefore, useless for presence of these faunas in the platform car- basinal classic rocks to the west (Fig. 1). These paleogeographic discussions (Lehnert and Keller, bonates is consistent with plate reconstruc- carbonate deposits have a wide distribution in the 1994). tions suggesting that the Precordillera was in eastern and central Precordilleran ranges of La The conodont faunas discussed here from the a tropical or subtropical position close to Lau- Rioja and San Juan Provinces, and their strati- La Silla Formation at its type locality (Figs. 1 and rentia during the late Precambrian and early graphic classification is shown in Figure 2. Car- 2) are the first report of conodonts of this age Paleozoic. These new paleontological data bonate production persisted until the early Llan- from the western Argentine Precordillera. Formal provide one more argument for recent models virnian (Hünicken and Ortega, 1987; Keller et al., subdivision of lithostratigraphic units recognized of the Precordillera as a displaced terrane de- 1993; Keller et al., 1994), when drowning of the within the Cambrian and Ordovician carbonate rived from the Ouachita Embayment at the carbonate platform was followed by deposition platform deposits of the Argentine Precordillera southern margin of Laurentia. of black graptolitic shales (Gualcamayo Forma- proposed by Keller et al. (1994) defined the La tion of Fig. 2, and equivalent units). Late Ordovi- Silla Formation and refined understanding of cian (Hirnantian) deposits that document the well other previously defined lithostratigraphic units. *E-mail: [email protected] known glaciation in Gondwana are preserved The base of the La Silla is marked by the appear- GSA Bulletin; April 1997; v. 109; no. 4; p. 429–443; 8 figures; 3 tables. 429 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/109/4/429/3382668/i0016-7606-109-4-429.pdf by guest on 23 September 2021 LEHNERT ET AL. in the La Silla Formation. The trilobites and con- odonts are mostly of North American affinities and can be correlated easily with biostratigraphic units in the United States. Those units, in turn, can be correlated with units in Europe and Australasia. Well-preserved trilobites were collected by N. E. Vaccari (Universidad Nacional de Córdoba, Argentina) at 14.8 m above the base of the La Silla Formation (horizon A, Fig. 3; Keller et al., 1994; Vaccari, 1995). The trilobites were identi- fied by Vaccari (1994, 1995) as Plethopeltis ob- tusus Rasetti. This trilobite is known in North America to range through the Saukiella serotina Figure 1. Location map of trilobite subbiozone of the Saukia trilobite bio- the Cerro La Silla section, zone, the Eurekia apopsis trilobite biozone, and showing the distribution of the overlying Missisquoia depressa trilobite sub- Cambrian and Ordovician biozone of the Missisquoia trilobite biozone limestones and clastic rocks (Fig. 4). This species and its occurrence were dis- in the western Argentine Pre- cussed recently by Loch, Stitt, and Derby (1993) cordillera (modified from under the name Plethometopus obtusus (Rasetti). Keller and Bordonaro, 1993). Dissolution of 2.5 kg of limestone from horizon A produced no conodonts. The oldest conodonts from the La Silla occur approximately 40 m higher than its trilobite fauna (horizon B on Figure 3). Clavohamulus hintzei Miller, Teridontus nakamurai (Nogami) and Oneotodus aff. O. simplex (Furnish) were found at this level (Table 1). The most significant and abundant species, C. hintzei, has not been re- ported previously from South America, but in North America it is widely distributed and diag- nostic of the Clavohamulus hintzei conodont sub- biozone of the Cordylodus intermedius conodont biozone (Miller, 1988; Ross et al., 1993). T. naka- murai, a long-ranging, cosmopolitan, coniform species is common in shallow marine deposits beginning at the base of the Cordylodus proavus conodont biozone. This species, however, is not ance of carbonate mudstones and wackestones et al. (1994) defined the base of the San Juan for- diagnostic of a particular zone (Miller et al., above crystalline dolomites of the La Flecha For- mally at the lowest occurrence of a diverse and 1982, Text-figs. 5 and 6), because it occurs in mation (Fig. 2). The La Silla is 346.15 m thick at abundant open marine fauna that includes bra- strata older than the C. proavus conodont biozone its type section at Cerro La Silla (Figs. 1, 3; Kel- chiopods, echinoderms, sponges, and trilobites. elsewhere in the world (e.g., Shergold and Nicoll, ler et al., 1994, Fig. 3), where it consists mostly The San Juan Formation contains three types of 1992, Fig. 2). Oneotodus simplex has been found of carbonate mudstones, peloidal and intraclastic reef structures (mounds, sponge-algal mounds, in the Oneota Dolomite in Iowa (Furnish, 1938; grainstones, with intercalated layers of biolami- and stromatoporoid patch reefs). The composi- Ethington and Brand, 1981), in the Gasconade nates and dolomites (Fig. 3). The carbonate mud- tion, faunal content, and sedimentological fea- Dolomite in central Missouri (Repetski et al., stones generally are strongly bioturbated. Cross- tures of these mounds reflect the same tectono- 1993), in the House Limestone in western Utah, bedded oolitic grainstones are rare. Wackestones stratigraphic environment represented by similar and in the McKenzie Hill and Cool Creek For- are peloidal, essentially confined to the upper mounds in the western United States (Toomey mations in Oklahoma (Ethington et al., 1987). part of the section and may contain nautiloids, and Nitecki, 1979). The single coniform specimen from the La Silla gastropods and gypsum (Fig. 3). The La Silla Formation has