Bulletin of the Geological Society of America Vol. 71
Total Page：16
File Type：pdf, Size：1020Kb
BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 71. PP. 1279-1294. 2 FIGS. SEPTEMBER 1960 VERTEBRATE-BEARING CONTINENTAL TRIASSIC STRATA IN MENDOZA REGION, ARGENTINA BY ALFRED SHERWOOD ROMER ABSTRACT Rusconi and others have described numerous fishes and amphibians from conti- nental Triassic sediments in the Mendoza region of Argentina, but their stratigraphic position has been poorly understood because of conflicting stratigraphic terminology and incomplete published data on the region. All vertebrate remains appear to be derived from a sedimentary cycle termed the Cacheuta series by a majority of work- ers. The age appears to be Middle Triassic. CONTENTS TEXT Page Las Higueras 1290 Page Age 1291 Introduction and acknowledgments 1279 References cited 1292 Topography 1280 Stratigraphy 1281 Continental sedimentary cycles 1281 ILLUSTRATIONS Estratos de las Cabras 1283 Estratos de Potrerillos 1284 Figure Page Estratos de Cacheuta 1284 1. Topographic sketch map of the precordil- Estratos de Rio Blanco 1285 lera in the Mendoza region, to show posi- Areal geology 1285 tion of localities mentioned in text 1282 Areas considered 1285 2. Sedimentary Triassic exposures in the area Challao region 1286 shown in Figure 1 1283 Mina Atala-Papagallos region 1286 Cerro Bayo-San Isidro 1287 Cerro Cacheuta 1288 TABLE Potrerillos 1289 Paramillos de Uspallata 1289 Table Page Salagasta 1290 1. Stratigraphic equivalencies. 1284 INTRODUCTION AND ACKNOWLEDGMENTS published since 1946. These finds are of great interest to vertebrate paleontologists generally For nearly a century continental sediments of and have potential value for intercontinental Triassic age have been recognized in the precor- stratigraphic correlation. But because of un- dilleran region of Mendoza in the western Argen- familiarity with the complex topographic situa- tine, but, except for a few fish scales described tion of the exposures and the considerable by Geinitz in 1876, vertebrate fossils were long variation in the stratigraphic terminology unknown. In recent years, however, a large employed, foreign workers have found it dif- number of fishes, amphibians, and (to a minor ficult to gain a clear picture of the geologic extent) reptiles have been found in the Triassic position of the faunas concerned. of the area. Cabrera (1944a; 1944b) has de- During 1958, as a member of a collecting scribed a fish and an amphibian, Bordas (1944) expedition sponsored jointly by the Museum a fish, and Minoprio (1954) a therapsid. A of Comparative Zoology1 and the Museo large number of forms, collected with the Argentine de Ciencias Naturales of Buenos co-operation of Prof. Manuel Tellechea, has Aires, I spent 6 weeks studying the Triassic been described by the energetic director of the 1 The expedition was supported in part by grants Mendoza Natural History Museum, Prof. from Life magazine and the National Science Foun- Carlos Rusconi, in a long series of papers dation. 1279 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/71/9/1279/3441867/i0016-7606-71-9-1279.pdf by guest on 27 September 2021 1280 A. S. ROMER—VERTEBRATE-BEARING TRIASSIC STRATA, ARGENTINA deposits of the area. As well as taking part in Cordillera irrigate the potentially fertile soil, the the collection of new materials, I visited, under western portion of this plain is an arid desert, the guidance of Doctors Tellechea and Mino- unsettled except for goatherders in scattered prio, most of the localities from which the "puestos" at infrequent springs. The plain is earlier collections had been made. Some account in great measure covered by alluvial deposits; of certain of the vertebrate localities has been older sediments underlie them but are exposed given by Professor Rusconi and other authors east of the precordilleran region only around in papers noted later. Doctor Minoprio has a few outlying hill and mountain areas (Fig. 1). most recently (1958) discussed the strati- About 25 km west of Mendoza lies an ex- graphic position of the brachyopid amphibians tended precordilleran development with eleva- of the region and has supplied valuable geologic tions reaching more than 3000 m. (Cf. Stappen- data. The present paper supplements and beck 1910, p. 3-14; 1911, p. 276-284.) This summarizes, for those unfamiliar with the begins in the Cacheuta region, about 35 km area, the geological data given by earlier workers southwest of Mendoza, and extends northward, and by Doctor Minoprio. beyond the area of present concern, into San In listing the vertebrates found at the Juan and La Rioja provinces. The southern various localities I have in no case suggested terminus is Cerro Cacheuta, separated from any taxonomic revision of the fauna and have the main mass of the precordillera by a gorge used the nomenclature under which the various through which run the Rio Mendoza, the forms were originally described. transandean railway, and a highway from I wish to express the gratitude which I, and Lujan to Potrerillos. The main transandean the other members of our party, feel for the highway, however, ascends to the summit of aid extended to us by so many inhabitants of the precordillera west of Villavicencio, about the charming and friendly city of Mendoza 50 km north of Mendoza, returns to the valley during our work in this area. Prof. Carlos of the Rio Mendoza near Uspallata, and thence Rusconi, in cordial fashion, made freely avail- begins the ascent of the cordillera proper. able for study the many valuable fossils in Various names are applied to portions of the his museum's collections. Prof. Manuel Tel- precordillera. The major summit ridge in lechea, who is most familiar with the sediments Mendoza province is frequently termed the of the area, accompanied us on various occasions Sierra de Uspallata; to the north, in San Juan and, with characteristic enthusiasm, guided province, the principal continuation of the us to areas of Triassic exposures and collection precordillera is the Sierra de Tontal. Near localities. Geologists of the Comision de Energia Mendoza a spur of the precordillera extends Atomica and the Yacimientos Petroliferos eastward from a point north of Estancia San Fiscales, notably Dr. Hector Martinez Cal for Isidro to terminate north of Challao, a short the Mendoza region, were most helpful in distance northwest of Mendoza. From a supplying geological data. Dr. H. J. Harring- point much farther north, another spur, the ton, who has worked extensively in the Men- Sierra de Las Penas and its prolongment, the doza region, furnished helpful criticism while Cerro de las Higueras, extends southeastward this paper was in preparation. We are especially (broken by a canyon for the Rio de Las Penas) grateful to Dr. Jose Luis Minoprio for his to subside into the plain in the region of constant support and aid in our work and to Salagasta, about 35 km north of Mendoza. him and Seiiora Minoprio for the cordial West of Mendoza an alluvial fan about 15 hospitality extended to all the members of the km broad slants upward about 700 m to the party. more precipitous slopes of the mountains. This fan conceals most of the much faulted TOPOGRAPHY Tertiary and older deposits present here. To the south, adjancent to the Rio Mendoza, Mendoza and the adjacent municipalities the precordillera is broken by numerous arroyos of Las Heras and Godoy Cruz lie at the western draining into that river. On its western slope, margin of the vast plain which extends west, in the region adjacent to the transandean gradually rising, about 600 miles from the highway—the bleak upland Paramillos de pampas of the La Plata River region to the Uspallata—the contours are more gentle. foothills of the Andes. Mendoza is about The main ridge of the cordillera (here 700m above sea level. Except where (as in the including Aconcagua, highest mountain of the Mendoza region) rivers descending from the Western Hemisphere) runs roughly parallel Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/71/9/1279/3441867/i0016-7606-71-9-1279.pdf by guest on 27 September 2021 TOPOGRAPHY 1281 to the prccordillera. Toward the south the whole is excellently summarized by Groeber two are not clearly separated, although the and Stipanicic (1952). In some areas the gorge of the Rio Mendoza between Uspallata "Rhaetic" cycle is clearly separable from the and Potrerillos may be taken as the dividing underlying "Paganzo" series by intervening line. Farther north, however, a north-south andesites (the series Choiyoilitense of Groeber) trough, which has an elevation of 2100-2300 and is initiated by a prominent conglomerate. m, lies between the two ranges. This includes, In other localities, however, the andesites at the south, the Valle de Uspallata and the or conglomerate or both may be absent, and Pampa de Yalguaraz and, farther north in considerable confusion of the two cycles may San Juan province, the Llanos del Leoncito exist. In the Mendoza region this is particularly and the lower course of the Rio de los Patos true in the Paramillos de Uspallata. which flows north to join the Rio San Juan. In Mendoza province the Triassic series is present to a considerable extent in the uplands STRATIGRAPHY of the precordillera itself, but certain important areas lie around the eastern and southern Continental Sedimentary Cycles flanks of the precordillera, where the beds are present in a much folded and faulted fashion. Marine Paleozoic sediments, ranging from In great measure the Triassic beds are, even Cambrian to Pennsylvanian, are present in the here, covered by later deposits and are only precordilleran regions, and marine deposits are exposed in specific areas noted in subsequent also found in the Jurassic and Cretaceous sections. They are known to be present, under of the Andean region. The Permian and early Tertiary sediments, far to the south (cf. Mesozoic, however, are represented (apart Trumpy, 1943; Baldwin, 1944, Fig.