Cesar Goso, Hector de Santa Ana
Administracion Nacional de Combustibles, Alcohol y Portland
Montevideo
This article attempts to present the geological characteristics and tectonic and sedimentary evolution of Uruguayan basins and the extent to which they have been explored.
Uruguay, is on the Atlantic coast of South America. The country covers about 318,000 sq km, including offshore and onshore territories corresponding to more than 65% of the various sedimentary basins.
Four basins underlie the country: the Norte basin, the Santa Lucia basin, the offshore Punta del Este basin, and the offshore-onshore Pelotas-Merin basin (Fig. 1).
The Norte basin is a Paleozoic basin while the others are Mesozoic basins. Each basin has been explored to a different extent.
NORTE BASIN
This gondwanic and cratonic basin constitutes the southeastern portion of the Parana basin. It is located in northern Uruguay and covers about 90,000 sq km. Maximum sedimentary thickness reaches 3,800 m, including basaltic and associated intrusive rocks.
The different formation processes set forth (thermal, flexural, and rifting) have not satisfactorily explained until now the genesis of this basin.
The sedimentation was controlled in a couple of directions following the trend of embasement weakness. The prevailing alignments during the Paleozoic period were in northwest-southeast direction. During the Mesozoic these fault trends were reactivated, and an important system in the east-west direction that controlled the sedimentation during the Juro-Cretaceous was superimposed.
The earliest sedimentary deposits correspond to the Lower Devonian sequence (Emsian) including a stacking pattern related to a transgressive-regressive cycle beginning with braided fluvial systems, continuing with coastal and marine environments, and finalizing with tidal deposits.
From the Lower Permian up to the Middle Triassic a series of lowstand systems tracts where resedimentation facies took place, and fluvial-deltaic and coastal systems with a retrogradational stacked pattern were arranged above the previous systems. A continuous sea level rise is verified, extensive coastal plains and brief marine systems developed until the Kazanian.
Transitional to continental systems develop in connection with the last phase resulting in a progradational arrangement, culminating with the total continentalization of the basin during the Triassic-Middle Jurassic. A desert system of important extension occurred, followed by igneous activity. This igneous activity was represented by thick basalt outflows and vein intrusions (dikes and laccoliths).
Fig. 2 shows the sedimentary filling stacking pattern and its lithostratigraphic nomenclature.
The exploration of this basin has included gravimetric surveys, 1,650 km of seismic profiles, and the drilling of 10 deep wells that although dry have allowed the definition of several play types (Table 1).
MESOZOIC BASINS
From the Upper Jurassic and related to the breakup of the Gondwana continent and to the African and South American plates separation, different sedimentary basins were generated.
Some of them are found in the continent as the Santa Lucia and Merin basins, while others were developed offshore, as the Punta del Este and Pelotas basins.
The main characteristics of these three sedimentary Mesozoic basins are described below:
SANTA LUCIA BASIN
This basin develops in the south of the country with a surface area of about 6,500 sq km on a cratonic area with an east-northeast/west-southwest direction.
It is an aborted rift with a transversal asymmetric profile, formed by a normal fault system that defines grabens and horsts, with a maximum sedimentary thickness of 2,600 m.
The tecto-sedimentary evolution until the Albian developed through the rift phase was presenting an important thickness in which a low magmatic activity (basalts) and sediments corresponding to alluvial, braided fluvial fans systems can be identified. Interdigitating aeolian and lacustrine deposits were developed toward the central sectors of the basin.
Later from the Maastrichtian until the Paleocene deposits corresponding to the post-rift phase not exceeding 100 m in thickness are present, developing braided fluvial systems near little lacustrine bodies. In Fig. 2 the lithostratigraphic nomenclature defined for this basin is shown.
Exploration occurred in this basin in the 1950s and 1970s, including gravimetric, magnetometric, and seismic surveys and the drilling of 13 deep wells that were dry and showed low hydrocarbon traces. This information allowed establishment of the play types shown in Table 1.
PUNTA DEL ESTE BASIN
The Punta del Este offshore basin covers about 15,000 sq km.
The embasement weakness zone that controlled the rifting has a predominant northwest-southeast direction with a conjugate trend of normal and listric faults creating internal grabens and horsts.
The tectonic and sedimentary evolution includes a rift phase, an interior sag phase (post-rift phase) and final passive-margin phase, involving distinct basin geometry, style of faulting, structural deformation, and stratigraphic succession with more than 4,000 m of thickness.
The rift phase evolution involves continental sedimentation synchronous with volcanism and fault-related subsidence, thus allowing the accumulation of thick packages of nonmarine lithofacies interbedded with extrusive volcanics and volcaniclastics, since the Upper Jurassic until the Aptian.
The sag phase ranges from Middle Cretaceous to Paleocene. This period is characterized by low to moderate rates of sediment influx and subsidence. These deposits can be divided into two intervals: a lower sequence of nonmarine sediments and an upper interval of shoreface to marine sediments. Following these different sequences in the basin, the Tertiary stratigraphy can be divided into at least three major transgressive-regressive packages, due to a strong control by eustatic sea level changes and regional tectonism.
Exploration here has included about 6,500 line km of marine seismic and two deep wells. The lithostratigraphic nomenclature and play types are shown (Fig. 2, Table 1).
PELOTAS-MERIN BASIN
This basin is located in the eastern sector of the country and is characterized by presenting an onshore portion (Merin) and an offshore portion (Pelotas), with dissimilar tecto-sedimentary evolutions.
In Uruguay it has an extension of about 10,000 sq km and stretches into southern Brazil, where prevailing alignments are in a northeasterly direction.
In the onshore sector of the basin the magmatic activity is very important. In the synrift phase strong basaltic flows occurred with intercalated sedimentary packages.
The offshore portion of the basin presents a different evolution; sedimentary events until the Neocomian related to this phase were developed, giving rise to alluvial fan deposits and braided systems, with lacustrine bodies interdigitating. From the Albian and until the Miocene, in the post-rift and drift phases, followed an alternation of continental, marine, and transitional systems that filled this basin.
Exploration of this basin included seismic surveys in the offshore portion and the drilling of four onshore wells that allowed establishment of the play concepts shown in Table 1. The sedimentary stack is shown in Fig. 2.
BIBLIOGRAPHY
de Santa Ana, H., and Veroslavsky, C., Cronoestratigrafia de las secuencias paleozoicas de la Cuenca Norte Uruguaya (borde sureste de la Cuenca de Parana), First Chronostratigraphy of the Parana Basin symposium, Sao Paolo, Brazil, 1993, pp. 41-43.
de Santa Ana, H., Veroslavsky, G., and Gonzalez, S., Geologia de los sedimentos cretacicos continentales del Uruguay, First Meeting on Continental Sedimentation of the Mesozoic Brasilian Basins, Sao Leopoldo, Brazil, 1993, pp. 25-28.
Stoakes, F.A., Campbell, C.V., Cass, R., and Ucha, N., Seismic stratigraphic analysis of the Punta del Este basin, offshore Uruguay, South America, AAPG Bull., Vol. 75, No. 2, 1991, pp. 219-240.
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