versión On-line ISSN 1851-4979
Lat. Am. j. sedimentol. basin anal. vol.18 no.2 La Plata ago./dic. 2011
Findings of intraformational striated pavements in the late carboniferous glacial deposits of the Andean Precordillera, Argentina
Pablo J. Alonso Muruaga 1-5, Carlos O. Limarino 1-5, Luis A. Spalletti 2-5, Ferrán Colombo Piñol 3, Pablo Juárez 4
1 Dto. de Geología, Facultad de Ciencias Exactas y Naturales (Universidad de Buenos Aires), Pabellón 2, Ciudad Universitaria,1428 Buenos Aires, Argentina.
2 Centro de Investigaciones Geológicas, calle 1 # 644, La Plata, Argentina.
3 Departamento Estratigrafia, Paleontología i Geociencies Marines, Facultat Geologia, Unversitat de Barcelona, C/Martí i Franqués s/n, E-08028, Barcelona, España.
4 Facultad de Ciencias Naturales e Instituto Miguel Lillo, Universidad Nacional de Tucumán, 4000 San Miguel de Tucumán, Argentina.
Received January 26, 2012 - Accepted August 28, 2012
One of the most important glacial events in the history of the Earth took place during the Late Paleozoic when large areas of the Gondwana Supercontinent were covered by ice-masses (Crowell, 1978; Hambrey and Harland, 1981; Isbell et al., 2003a,b; Rocha Campos et al., 2008; Fielding et al., 2008; López Gamundí, 2010). Though this glacial event was initially considered a large and unique episode spanning a great part of the Carboniferous and Permian in Gondwana, new geological evidence suggests that the glacial period was not continuous, but rather punctuated by several interglacial events. In this way, López Gamundí (1997) and later Isbell et al. (2003b) divided the Late Paleozoic Ice Age (LPIA) into four major glacial intervals corresponding to the Early Carboniferous (LPIA 1), Late Carboniferous-Early Permian (LPIA 2), Early Permian (LPIA 3) and Late Permian (LPIA 4). This division adequately describes the record of the glacial deposits in Gondwana and allows for the proposal of a general stratigraphy for the glacial deposits demonstrating the existence of low-frequency climatic changes.
In this paper we describe two subglacial basal contact surfaces found in the lower part of the Guandacol Formation (Frenguelli, 1944; Cuerda, 1965) in the Central Precordillera (northwestern Argentina). The Guandacol Formation is well known for containing glacial deposits, including diamictites and resedimented diamictites at the lower third of the unit (Limarino et al., 2006; Marenssi et al., 2002; Marenssi et al., 2005). According to paleontological (palynological) information, and some radiometric dating, the age of the glacial deposits is Late Visean-Early Bashkirian and therefore fits into the LPIA 2 interval of Isbell et al. (2003b).
The presence of intraformational glacial pavements in the Guandacol Formation allows for a more precise history of the LPIA 2 glacial interval suggesting that interglacial periods not only separated low-frequency climatic changes (LPIA intervals) but also were likely present within individual LPIA periods (high-frequency climatic changes). In addition, palynological data and radiometric evidence is discussed in order to be more precise about the age of the intraformational pavements.
PAVEMENT DISTRIBUTION AND DESCRIPTION
Intraformational pavements of the Guandacol Formation were recorded in two localities in the Central Precordillera of Argentina (Fig. 1a): Huaco anticline and Loma de Los Piojos areas.
Figure 1: a) Late Paleozoic paleogeography of the Paganzo Basin showing the location of the Huaco and Loma de Los Piojos areas (modified after Salfity and Gorustovich, 1983). b) Geologic map of the Huaco Anticline area. c) Geologic map of Loma de Los Piojos area. Stars show the location of intraformational striated pavements.
Huaco Anticline Area
In this area, Carboniferous rocks rest uncoformably on Ordovician limestones of the San Juan Formation and all the Paleozoic deposits are folded into a N-S oriented anticline (Fig. 1b). The best exposures of the Guandacol Formation follow both limbs of the anticline, and are also near the axis of the fold where the beds rest subhorizontally over Ordovician limestones. Following Limarino et al. (2002) the Guandacol Formation can be subdivided in two sections. The lower section (Fig. 2) comprises mudstones, sandstones, scarce conglomerates and several beds of diamictites located near the base of the unit (Limarino et al., 2002). Moreover, striated pavements (Fig. 2) carved into Ordovician limestones (San Juan Formation) were described by López Gamundí and Martínez (2000) and Marenssi et al. (2005). This section has been interpreted as being deposited within glacial and fjord environments (Pazos, 2000; López Gamundí and Martínez, 2000; Marenssi et al., 2005).
The upper section of the Guandacol Formation is mainly composed of fine-grained sandstones, shales and mudstones deposited during a postglacial marine transgression that flooded glacial valleys forming fjord environments (Limarino et al., 2002).
Intraformational Striated Pavement. The pavement identified in the Huaco area consists of a striated surface of about 6 m² exposed subhorizontally in the outcrops between Agua Hedionda river and National Road 40 (the exact location is S 30° 8' 23.4"; W 68° 33' 14.2"). The pavement is developed over a massive bed of pebbly muddy sandstone (Fig. 2) that laterally grades into siltstones with isolated clasts of coarse-grained sandstones. The striated surface is overlain by massive siltstones and claystones, which contain clasts of pebble, cobble, and boulder sizes. The surface is undulated resembling the whaleback landform with ridges and grooves parallel to the axis of undulation (Fig. 3a). Ridges and furrows are continuous, approximately 1.5 m long, locally associated with parallel to subparallel millimetric striations (Fig. 3b). The striations show a mean azimuth of 305º-125º but no clear evidence of sense of ice flow was indentified.
Figure 3: a) Cuesta de Huaco intraformational striated pavement (note the whaleback morphology). b) Detail of the linear structures in the surface. c) Loma de Los Piojos intraformational striated pavement (red arrow points to slipped clast mould). d) Detail of the slipped clast mould.
Loma de los Piojos Area
In the Loma de Los Piojos area, the Guandacol Formatión lies in sharp contact over Devonian sandstones and mudstones of the Talacasto Formation (Fig. 1c). The strike of the outcrops is almost north- south, dipping to the west at 15º-25º. The Guandacol Formation in this area (Fig. 2) is mainly composed of mudstones, diamictitic lenses, thin-bedded diamictites, thin beds of fine-grained sandstones, clast- to matrix-supported pebble conglomerates and poorly sorted, coarse-grained sandstones. The beds locally show abundant soft-sediment deformation. Dropstones, in shale beds, are also present. A basal striated pavement was carved into Devonian sandstones (Talacasto Formation) and covered by the basal deposits of the Guandacol Formation (López Gamundí and Martínez, 2000).
Intraformational Striated Pavement. The intraformational striated pavement was developed over a medium- to coarse-grained sandstone oriented N 16º E and dipping 50º N W. It consists of isolated striated surfaces, some of which reach a maximum area of 9 m² (Fig. 3c). They are all part of a single pavement that can be followed along the strike for at least 70 meters. Some surfaces show very low amplitude undulations, with striations oriented oblique and subparallel to the axis of the main undulations. Paleoice flow direction data was obtained, from the orientations of each of the striae from the inclined bed. The restored striations indicate an azimuth of 290º-110º. Although no clear indicators of ice flow direction could be inferred, a slipped clast mould (Fig. 3d) present in the striated surface suggests paleoice flow towards the west.
AGE OF THE GLACIAL EVENT
The glacial event that affected the whole of Gondwana during the Late Paleozoic is probably the most extended glacial era recorded during the Phanerozoic. Initially, the glacial event was described as a long-lived ice age of several tens of millions of years (60 m.y.-80 m.y.) punctuated by brief interglacial periods. However, the growing up knowledge of the Late Paleozoic glaciation evidenced a much more complex history of the glacial era (Isbell et al. 2003a,b; Limarino et al., 2006; Fielding et al., 2008). Isbell et al. (2003a) recognized three major glacial episodes during the latest Devonian-early Mississippian, Late Mississippian-Early Pennsylvanian and latest Pennsylvanian-early Cisuralian. A later work by Fielding et al. (2008) added three glacial periods during the middle Cisuralian and Guadalupian, but they show lesser areal extent along Gondwana and are mainly recorded in Australia.
Following the model proposed by Isbell et al. (2003a) and Fielding et al. (2008) the glacial diamictites considered in this paper correspond to the glacial stage 2 which encompasses from the Late Mississippian to the Early Pennsylvanian. This age is suggested by both paleontological and chronostratigraphic studies. In this regard, the interval of the Guandacol Formation including the intraformational striated pavements yields abundant palynological assemblages in the Cuesta de Huaco area (Césari and Vázquez Nístico, 1988; Pérez Loinaze, 2007; Pérez Loinaze et al., 2010).
The palynofloras were obtained from different stratigraphic levels all which overlie the basal striated pavement and are disposed under and above the intraformational striated pavement (Fig. 2). According to Pérez Loinaze et al. (2010) the palynological assemblages belong to the Subzone A of the Raistrickia densa-Convolutuispora muriornata Biozone which suggests a Serpukhovian age for the glacial diamictites (Fig. 4).
Figure 4: Stratigraphic position and age of the intraformational striated pavement identified in the Guandacol Formation. Note that these glacial deposits are younger than those of the Cortaderas Formation. a) Glacial intervals proposed by Isbell et al. (2003a) and Fielding et al. (2008). b) Schematic stratigraphic sections of Huaco and Del Peñón showing radiometric ages and palynological biozones. c) Location of the localities mentioned in the text.
This age is consistent with new radiometric information obtained by Gulbranson et al. (2010) in the Punta del Agua and Guandacol Formations (Fig. 4). There, a 206Pb/238U 335.99 ± 0.06 Ma age was obtained for andesites unconformably covered by glacial diamictites, suggesting that glacial deposits of the Guandacol Formation are not older than the midlate Visean (Césari et al., 2011). In the Huaco area, the Guandacol Formation yielded an age of 206Pb/238U 318.79 ± 0.10 Ma (Fig. 4) from tuff levels intercalated in postglacial shales free of dropstones and lacking any evidence of glacial processes. This age indicates that during the middle Bashkirian glacial conditions ceased in the studied region. In short, palynological and radiometric ages are consistent and suggest that glacial deposits, including the intraformational pavement considered here, correspond to the glacial stage 2 of Isbell et al. (2003a).
It is interesting to note that glacial deposits considered in this paper are younger than those found in the Cortaderas Formation in the neighboring Río Blanco Basin, which are characterized by palynological assemblages belonging to the Reticulatisporites magnidictyus-Verrucosisporites quasi-gobbettii Biozone of late Visean age (Pérez Loinaze et al., 2010, Fig. 4). These glacial diamictites may correspond to the onset of the glacial stage 2 in western Gondwana separated by an interglacial period from the above considered diamictites of the Guandacol Formation (Fig. 4).
The following conclusions can be drawn from the data presented in this paper:
- The presence of intraformational striated pavements is for the first time stablished in the Guandacol Formation in Cuesta de Huaco and Loma de Los Piojos localities.
- According to radiometric data, the striated pavements are not older than middle late Visean and not younger than middle Bashkirian. Moreover, palynological remains, obtained in the glacial sequence, are consistent with radiometric information and suggest a late Serpukhovian age for the studied horizons. Therefore the striated pavements belong to the LPIA 2 glacial event.
- These striated pavements strongly suggest episodes of advance and retreat of ice during the glacial event in the Paganzo Basin, which was previously suggested by López Gamundí and Martínez (2000) and Marenssi et al. (2002, 2005).
- The age of the striated pavements identified in the Guandacol Formation is similar to the intraformational striated pavement reported by González (1981) from the Hoyada Verde Formation (Calingasta-Uspallata Basin). Whether or not these surfaces mark a major retreat-advance glacial episode in the region should be considered as a possibility. However the exact correlation among Huaco, Loma de Los Piojos and Barreal (Hoyada Verde Formation) based on the identification of subglacial basal contact surfaces will have to be confirmed in the future when more paleontological and radiometric data become available.
J. Menzies and O. López Gamundí are thanked for their constructive reviews of the submitted manuscript. Financial support for this study was provided by a Doctoral grant from the Argentinean research Council (CONICET) and ANPCyT (Agencia Nacional de Promoción Científica y Tecnológica) PICT 0752/7.
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