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Ameghiniana

versión impresa ISSN 0002-7014

Ameghiniana vol.47 no.1 Buenos Aires ene./mar. 2010

 

ARTÍCULOS ORIGINALES

Paleoenvironment of the Auca Mahuevo and Los Barreales sauropod nesting-sites (Late Cretaceous, Neuquén Province, Argentina)

Alberto C. Garrido1

1Dirección Actual: Museo Provincial de Ciencias Naturales "Prof. Dr. Juan A. Olsacher". Dirección Provincial de Minería. Etcheluz y Ejército Argentino C.P. 8340. Zapala, Provincia del Neuquén. República Argentina. algene@copelnet.com.ar

 


Abstract. Outcrops of Anacleto Formation at Auca Mahuevo nesting site (Neuquén Province, Argentina) can be divided into two main sections. The lower section (51 m), exhibits a succession of point bar, levee, crevasse channel and crevasse splay deposits, typifing a fine-grained, mixed load, meandering fluvial system. Twenty five km toward south (Los Barreales locality) these deposits become progressively to distal floodplain facies. Egg-layers are exposed along this 25 km, however a higher concentration of egg-clutches occur into paleosols and abandoned channel deposits developed at ancient levee and meander belt areas. Lithofacial evidences allow attributing for this lower section semiarid climatic condition, with marked alternations of wet and dry seasons. Exceptional muddy sheet-floods burying the eggs and nests with a thick bed of mud, facilitating their preservation. The upper section (17 m), is composed by finer sediments of sloughs and lacustrine deposits developed over a poorly drained floodplain, under conditions of major aridity. South toward Los Barreales, evaporitic deposits was formed in saline-lakes. The tectonic and volcanic processes that affected the basin from the middle to upper Campanian were responsible for the variations in the environmental conditions during the latest phase of the Anacleto Formation deposition. Possibly, these changes provoked the abandonment of this area as nesting-ground.

Resumen. Paleoambiente de los sitios de nidificación de saurópodos de Auca Mahuevo y Los Barreales (Cretácico Superior, provincia del Neuquén, Argentina). Los afloramientos de la Formación Anacleto en el sitio de nidificación de Auca Mahuevo (Provincia del Neuquén, Argentina), puede ser dividida en dos secciones. La sección inferior (51 m), exhibe una sucesión de depósitos de barra de punta, albardón, canales de rotura de albardón y lóbulos de explayamiento; caracterizando un sistema fluvial meandriforme de carga mixta. Veinticinco km hacia el sur (localidad de Los Barreales), estos depósitos pasan progresivamente a facies de planicie de inundación distal. Los niveles fosilíferos expuestos a lo largo de estos 25 km, muestran una mayor concentración de huevos fósiles en los depósitos de paleosuelos y canales abandonados desarrollados en antiguas áreas de albardón y fajas de meandros. Las evidencias litofaciales permiten atribuir para esta sección inferior condiciones climáticas de semiaridéz, con marcada alternancia de estaciones secas y húmedas. Flujos mantiformes excepcionales de carga fangosa soterraron rápidamente nidos y huevos con una gruesa capa de arcilla, facilitando su preservación. La sección superior (17 m), está compuesta por depósitos de barreales y lacustres desarrollados sobre una planicie de inundación pobremente drenada, bajo condiciones de mayor aridez. Hacia el área de Los Barreales, depósitos evaporíticos fueron formados en barreales salinos. Los procesos tectónicos y volcánicos que afectaron la cuenca a partir del Campaniano medio-superior, fueron responsables de las variaciones en las condiciones paleoambientales observadas durante la última fase de depositación de la Formación Anacleto. Posiblemente, estos cambios provocaron el abandono de esta área como sitio de nidificación.

Key words. Anacleto Formation; Upper Cretaceous; Neuquén Basin; Paleoenvironment; Dinosaur nesting-sites.

Palabras clave. Formación Anacleto; Cretácico Superior; Cuenca Neuquina; Paleoambiente; Sitios de nidificación de dinosaurios.


 

Introduction

The Auca Mahuevo sauropod nesting site is located 87 km southeast from Rincón de los Sauces City, Neuquén Province, Patagonia Argentina (figure 1). This fossil field was discovered in 1997, conforming one of the most spectacular dinosaur eggs-bearing places (Chiappe et al., 1998). This site exhibits thousands of egg clutches of the same Megaloolithus Vianey-Liaud, Mallan, Buscail and Montgelard, 1994 parataxon, which based on embryonic remains (bones and fossil tissue) are assigned to titanosaur sauropod dinosaurs (Chiappe et al., 2001). Theropod skeletons (Aucasaurus garridoi Coria, Chiappe and Dingus, 2002), sauropod skeletons, as well as other vertebrate remains have been recovered from the same area (Coria et al., 2002).

Figure 1. Auca Mahuevo and Los Barreales location map / mapa de ubicación de las localidades de Auca Mahuevo y Los Barreales.

Recent explorations have demonstrated that the nesting area extends to the Los Barreales locality, situated 25 km to south from Auca Mahuevo. Thus, the eggs are distributing in four egg-bearing beds exposed along both localities, occuping an area of aproximately 60 km2.
The quality of the egg-bearing succession out-crops in the studied area allowed depicting fine details about their lithologic and sedimentologic characteristics. Based on the analysis of their lithofacies and architectural elements, the purpose of this work is to analyze and discuss the stratigraphic and paleoenvironmental context in which these nesting sites have been preserved.

Geological setting

Egg-layers at Auca Mahuevo and Los Barreales occur within the Anacleto Formation, the uppermost lithostratigraphic  unit  of  the  Neuquén  Group (Cenomanian-Campanian). The Neuquén Group comprises a sequence of continental sediments deposited during the initial foreland stage of the Neuquén Basin (Franzese et al., 2003), conforming three finning-upward fluvial cycles varying in thickness from 500 to 1300 m (Cazau and Uliana, 1973).
Within the study area, the Anacleto Formation contains approximately 70 meters of fine-grained, variegated deposits, composed mainly of reddish-brown mudstones, greenish-gray claystone and micaceous siltstones, and yellowish sandstones (figure 2). The lower contact of this unit is not exposed and the top is overlied by estuarine and shallow marine deposits of the Allen Formation (Malargüe Group) (Ardolino and Franchi, 1996).

Figure 2. Lithostratigraphic profiles of the Anacleto Formation at Auca Mahuevo and Los Barreales localities. (Note: MCF= Collection code of Museo Carmen Funes. PVPH= Paleontología de Vertebrados, Plaza Huincul. PBPH= Paleobotánica, Plaza Huincul. Neuquén province, Argentina) / perfiles litoestratigráficos de la Formación Anacleto en las localidades de Auca Mahuevo y Los Barreales.

The Anacleto Formation age has been estimated by Ardolino and Franchi (1996) between upper Santonian-lower Campanian. Sequential analysis studies conducted by Legarreta and Gulisano (1989) establish ranges of ages for the Anacleto Formation of 83-80 M.y. Dingus et al. (2000) reported the first paleomagnetic research in the Neuquén Group and estimated an age for the Auca Mahuevo deposits between 83.5 and 71.3 Ma. Their paleomagnetic data suggest that the lower portion of the section at Auca Mahuevo, which contains egg-layers 1 to 3, was deposited during an interval of reversed polarity that is tentatively correlated with C33R of the early to middle Campanian.

Sedimentary analysis and interpretation

Based on codes and definitions summarized by Miall (1996), fourteen lithofacies and eight architectural elements are recognized in the Anacleto Formation within the study area. The main features of these facies and sedimentary bodies, are exposed in the tables 1 and 2 repectively.

Table 1. Lithofacies recognized in the Anacleto Formation at Auca Mahuevo and Los Barreales localities / litofacies reconocidas en la Formación Anacleto en las localidades de Auca Mahuevo y Los Barreales.

Table 2. Architectural elements defined in the Anacleto Formation at Auca Mahuevo and Los Barreales localities / elementos arquitecturales definidos en la Formación Anacleto en las localidades de Auca Mahuevo y Los Barreales.

According to this sedimentological analysis, the deposits of Anacleto Formation at the nesting sites of Auca Mahuevo and Los Barreales, can be divided into two main sections. The 57 m thick lower section constitutes approximately three-quarters of the total sequence, in which overbank deposits conform more than 85% of outcrop thickness. In general, the lateral distribution of lithofacies shows a transitional shift from north to south from overbank to more distal floodplain environments.
At the north area (Auca Mahuevo), the fluvial succession is composed mainly by channel deposits (architectural elements: LA and SB) and proximal overbank  deposits  (architectural  elements:  LV, FF(CH), CR, CS and FF) (figure 3). Lateral accretion deposits are important components in higher sinuosity, fluvial systems (Collinson, 1986); while the predominance of overbank deposits is signed by Miall (1996) as characteristic of rivers bearing a fine-grained sediment load. In this sense, lithofacies association and assemblages of channel architectural elements present in the Anacleto Formation lower section, suggest the development of fine-grained, mixed-load meandering rivers (figure 4).

Figure 3. Schematic cross-sections of the the arquitectural elements associations at Auca Mahuevo (A) and Los Barreales (B) localities (references of codes see table 2) / sección esquemática de la asociación de elementos arquitecturales en las localidades de Auca Mahuevo (A) y Los Barreales (B) (referencias de códigos ver tabla 2).


Figure 4. Paleoenvironmental reconstruction of the egg-bearing lower section of the Anacleto Formation at Auca Mahuevo and Los Barreales localities / reconstrucción paleoambiental de la sección inferior portadora de huevos de la Formación Anacleto en las localidades de Auca Mahuevo y Los Barreales.

At the south area (Los Barreales) finer sediments compose the sequence, mainly characterized by the presence of FF and LS architectural elements (figure 3). In Los Barreales, sandy bodies become progressively to thin, sheet-like, sparse sandy levels. In this sense, a progressive decrease in grain size of the sequence and the development of sheet-like deposits, should mark an increase of distance from the major channel (Bridge 1984). Formation of sheet-like sandy deposits in overbank environments, are favored when channelized flows grade into unconfined sheet floods, as one moves from more proximal overbank environments toward the more distal floodplain as consequence of a diminution in the slope (Tunbridge, 1981). According to Collinson (1986), sand sheet-like bodies interbedded with thick muddy sequences can be ascribed to distal deposits of larger crevasses splays over floodplain areas.
Muddy paleosols (vertisols) and pedoturbated channel deposits constitute the main egg-bearing de-posits. Although the egg-layers 2, 3 and 4 extend across both localities by approximately 25 km (figures 2 and 3), the major concentrations and density of eggs and nests occur in the Auca Mahuevo region, in wich paleosols were developed over ancient levee and meander belt deposits. These facts suggest a preference of close channel environments as nesting sites. In all cases the nests are covered by reddish, structureless mudstones, showing occasionally in the first few centimeters above the base, small wide-spread egg-shell fragments (less than 7 mm size) floating in the muddy matrix. Isolated sub-rounded quartz granules, ranging between 1 to 3 mm in diameter, also are present. In the uppermost part of this layer, mud-cracks and invertebrates traces can be observed. These lines of evidence suggest that the eggs have been covered by muddy sheet splays, possibly originating during seasonal flooding events when highly concentrated suspended load stream floods drained from the channel toward flood plain.
The formation of caliche in paleosols is ascribed by several writers to warm and semiarid climatic conditions (Esteban and Klappa, 1983), while their development is favored when alternating periods of rainfall and drought occur (Jerzykiewicz and Sweet, 1987). In the same sense, the development of vertisols suggest the existence of an alternation of wet and dry seasons (Bridge, 1984).
The 17 m thick upper section exhibit a predominance of lacustrine, pond and slough deposits, reflecting a shift toward more poorly drained flood-plain. In general, FF and LS architectural elements characterized this uppermost part of the Anacleto Formation in both localities. A transition to major aridity toward this part of the sequence is inferred from the presence of sandy levels cemented with gypsum, which are laterally replaced toward south by several thin gypsum layers, indicating the development of small saline, ephemeral lakes. Suggestively, the egg-levels disappear where this shift occurs.

Discussion

Since the later Campanian, the Neuquén Basin operated as a foreland basin as consequence of a pronounced subsidence, which is related to a flexural loading developed east of the subduction zone. The development of this process catalyzed the inversion of the regional slope, starting the first Atlantic transgression and the sedimentation of the Malargüe Group (Uliana and Biddle, 1988). During this time, the eastward migration of the magmatic arc occurred, and consequently, a supply of volcanic ash arrived in the basin from the west (Legarretta and Gulisano, 1989).
According to this scheme, the observed changes in the upper section of the Anacleto Formation can be attributed to these events. Following this line of hypothesis, the mentioned shift toward a poorly drained floodplain with the formation of ponds and extensive shallow-lakes, is ascribed to a stage of transition that marks the change in the regional slope of the basin. In the same sense, the change toward more arid conditions may be due to the restriction of the moisture provided from the west, as a consequence of the development of the magmaticarc and the resulting retraction of the Pacific Ocean. The presence of volcanic ash observed near the top of the Anacleto Formation at the Cerro de los Overos locality (50 km west-northwest from Auca Mahuevo), is indicative of the volcanic activity started during this stage. The fact that the vanishing of the egg-layers is accompanied by environmental changes, suggest the posibility that these events provoked the abandonment of this area as nesting-site.

Conclusions

Outcrops of the Anacleto Formation at Auca Mahuevo and Los Barreales nesting sites can be divided into two main sections. In Auca Mahuevo the egg-bearing lower section comprehend a 51 m thick fluvial sucession corresponding to fine-grained, mixed load, meandering fluvial deposits which becomes progressively toward Los Barreales to distal floodplain facies. Paleoclimatic indicators suggest that this sequence was developed under warm climatic conditions, with marked alternations of wet and dry seasons.
Muddy paleosols and pedoturbated channel deposits constitute the egg-bearing levels. These levels were preserved by a quick burial due to exceptional muddy sheet-floods. A major concentration of egg-clutches in the proximal overbank environment (levee and meander belts deposits) may be indicate sauropod preferences to proximal channel areas as nesting sites.
The 17 m thick upper section is composed by finer sediments and thin evaporitic layers corresponding to sloughs, saline-lakes and lacustrine deposits; marking also the top limit of the eggbeds. Sedimentological features indicate that this last part of the sequence was developed over a poorly drained floodplain under conditions of major aridity.
The processes of change that affected the basin from the middle to the upper Campanian, were responsible for the variations in the paleoenvironmental conditions during the latest phase of deposition for the Anacleto Formation. Possibly, these changes provoked the abandonment of this area as nesting-ground.

Acknowledgments

Special thank to R. Coria (Museo Carmen Funes, Neuquén, Argentina), L. Chiappe (Natural History Museum of Los Angeles County, USA), L. Dingus (Infoquest Foundation, New York, USA), L. Salgado (Universidad del Comahue, Argentina) and A. Ballard (Halliburton Company, USA) for their comments and suggestions on the manuscripts. I also extend my gratitude to the technicians D. Hernández and S. Saldivia (Museo Carmen Funes, Neuquén, Argentina) for their assistance during the field work.

References

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Recibido: 10 de diciembre de 2008.
Aceptado:
20 de diciembre de 2009.