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Late Triassic Plant Microfossils from the Rancho de Lata Formation, Main Cordillera, Argentina

1992; Micropaleontology Press; Volume: 38; Issue: 3 Linguagem: Inglês

10.2307/1485791

1937-2795

Eduardo G. Ottone, Pamela Álvarez, S. V. Benoit,

Paleontology and Stratigraphy of Fossils

A palynological assemblage consisting of thirty-five species of spores and pollen grains, distributed among twenty-seven genera has been recovered from the lower section of the Rancho de Lata Formation, Main Cordillera of San Juan Province, Argentina. One new genus of monosaccate pollen grains, Variapollenites Ottone, together with seven new species, Anapiculatisporites sandrae Ottone, Anapiculatisporites pamelae Ottone, Variapollenites rhombicus Ottone, Variapollenites curviplicatus Ottone, Variapollenites trisulcus Ottone, Platysaccus olivae Ottone, and Equisetosporites cinctus Ottone, are proposed. The present microfloristic assemblage is of the Ipswich Type, and considered as late Triassic in age. INTRODUCTION The Rancho de Lata Formation crops out in a narrow belt, 250m wide, with an approximately north-south strike, in the western slope of the Espinacito Range, Main Cordillera of San Juan Province, Argentina (text-fig. 1). The palynological sample examined for this report consists of a gray, argillaceous siltstone containing badly preserved remains of Dicroidium sp., Cordaicarpus sp., articulate stems and Coniferopsida seed-scale complex fragments. The sample was collected on the right back of the Rancho de Lata Creek, 170m above the base of the unit, from a lenticular bed, 5m thick (text-fig. 2). P. P. Alvarez and S. V. Benoit, who are responsible for the geological information provided, discovered and collected this material. E. G. Ottone prepared the palynological section of the paper. All figured specimens from this study are lodged in the palynological collection of the Geological Sciences Department, Buenos Aires University. Registered numbers of specimens in this repository are prefixed BAFC-P1. All coordinates cited are mechanical stage readings of the Leitz Orthoplan microscope No. 871008. GEOLOGICAL SETTI'NG The Rancho de Lata Formation rests unconformably on the Permian-Triassic Choiyoi Group. It is in turn, unconformably overlain by Jurassic marine deposits and Quaternary clastic sediments (text-figure 1). The sequence is characterized by the following facies types: a. clastic facies, composed mainly of polymictic fining upward conglomerates, essentially with rhyolithic clasts, and coarse sand matrix. They grade to sabulites, sandstones and tuffs, having a similar average composition as the conglomerates. b. volcaniclastic facies, composed mainly of volcanic breccias usually crossed by calcite veins. Their clasts are also of rhyolithic composition and the matrix is in general altered to chlorite. Tuffs, lithic tuffs and crystalline tuffs are included in the same facies. More detailed geological and biostratigraphical data are available in Alvarez et al. (1991). SYSTEMATIC PALYNOLOGY Anteturma PROXIGERMINANTES Potonie 1970 Turma TRILETES Reinsch emend. Dettmann 1963 Suprasubturma ACAVATITRILETES Dettmann 1963 Subturma AZONOTRILETES Luber emend. Dettmann 1963 Infraturma LAEVIGATI Bennie and Kidston emend. Potonie 1956 Genus Biretisporites Delcourt and Sprumont emend. Delcourt, Dettmann and Hughes 1963 Type species: Biretisporites potoniaei Delcourt and Sprumont 1955. Biretisporites sp. Plate 1, figure 1 Description: Spores radial trilete. Amb subtriangular, with convex sides and rounded angles. Laesura straight, extending to equator, and conspicuously lipped. Lips elevated, constituted by extensions of the proximal exine. Exine smooth, 2-2.5gm thick. Dimensions (6 specimens): Equatorial diameter: 63(66)73gm, lips: 4-7gm high, 3-4gm wide. Comparison: Biretisporites sp. differs from B. spectabilis Dettmann (1963) in being smaller and in possessing less developed lips, and from B. sp. 2 described by Zavattieri (1986) in having a larger size. Genus Calamospora Schopf, Wilson and Bentall 1944 Type species: Calamospora hartungiana Schopf 1944. Calamospora sp. Plate 4, figure 7 Description: Spores radial trilete, amb subcircular. Laesura simple, straight, extending approximately to 1/2 of spore radius. Exine smooth, less than lgm thick, frequently folded. Dimensions (3 specimens): Equatorial diameter: 59(73)87gm. micropaleontology, vol. 38, no. 3, pp. 261-278, plates 1-6, text-figures 1-2, 1992 261 This content downloaded from 157.55.39.83 on Sun, 09 Oct 2016 05:01:48 UTC All use subject to http://about.jstor.org/terms E. G. Ottone et al.: Late Triassic plant microfossils from the Rancho de Lata Formation, Main Cordillera, Argentina TEXT-FIGURE 1 Geological map of the Rancho de Lata Creek Area and location of the fossiliferous site. Comparison: The present specimens are closely comparable with Calamospora tener (Leschik) de Jersey (1962), except for their larger size. Genus Deltoidospora Miner 1935 Type species: Deltoidospora hallii Miner 1935. Deltoidospora minor (Couper) Pocock 1970 Plate 1, figure 6 Dimensions (3 specimens): Equatorial diameter: 52(54)57m, exine ca. 1.5gm thick. Previous records: This species has been widely reported from Mesozoic palynological assemblages. Genus Punctatisporites Ibrahim emend. Potonie and Kremp 1954 Type species: Punctatisporites punctatus Ibrahim 1933. Punctatisporites microtumulosus Playford and Dettmann 1965 Plate 2, figure 4 Dimensions (3 specimens): Equatorial diameter: 53(56)60m, exine ca. 1.7gm thick. Previous records: Originally described from the Rhaeto-Liassic Leigh Creek Coal Measures of South Australia (Playford and Dettmann 1965). Punctatisporites sp. Plate 2, figure 7 Description: Spore radial trilete, amb circular. Laesura simple, straight, short, 1/2-1/3 of spore radius. Exine smooth, ca. 1.51m thick. Dimension (1 specimen): Equatorial diameter: 50Rm. 262 This content downloaded from 157.55.39.83 on Sun, 09 Oct 2016 05:01:48 UTC All use subject to http://about.jstor.org/terms Micropaleontology, vol. 38, no. 3, 1992 Infraturma APICULATI Bennie and Kidston emend. Potonie 1956 Genus Anapiculatisporites Potonie and Kremp 1954 Type species: Anapiculatisporites isselburgensis Potonie and Kremp 1954. Remarks: It is proposed that Anapiculatisporites embraces anisopolar, trilete microspores exhibiting a smooth proximal face and both distal surface and equatorial amb, mainly sculptured by apiculate elements with subordinate grana and verrucae. Anapiculatisporites pamelae Ottone n. sp. Plate 3, figures 3 and 4 Holotype: BAFC-P1 766(2) 56.3/96.2, plate 3, fig, 4. Type locality: Rancho de Lata Creek, western slope of the Espinacito Range, Main Cordillera of San Juan Province, Argentina. Description: Spores radial trilete, amb subcircular to subtriangular. Laesura straight, simple, extending 2/3 of spore radius. Proximal surface smooth, thin, often folded; folds may be parallel to the amb. Distal and equatorial sculpture consisting of coni mostly truncate or rarely pointed, with a few baculae and grana, some of them with confluent bases. Exine between processes laevigate, 2.3-3gum thick. Dimensions (20 specimens): Equatorial diameter: 39(44)50m, sculpture: 0.7-2.3gm high, bases: 0.5-1.3pm in diameter, 0.31.2gm apart. Name derivation: Named after Mrs. Pamela Alvarez. Comparison: Anapiculatisporites pamelae Ottone n. sp. mainly diverges from A. sandrae Ottone n. sp. in having smaller sculptural elements, lacking broadly based cones, and in exhibiting a simple laesura. Anapiculatisporites sandrae Ottone n. sp. Plate 3, figures 1 and 2 Holotype: BAFC-P1 766(15) 28.9/102.4, plate 3, fig. 2. Type locality: Rancho de Lata Creek, western slope of the Espinacito Range, Main Cordillera of San Juan Province, Argentina. L.