formation of a basin which extended from northwestern. Argentina to central/northern Bolivia. The Cambrian and. Lower Ordovician evolution is characterized by ...
ACTA UNIVERSITATIS CAROLINAE - GEOLOGICA 1999, 43 (1/2)
Upper Ordovician basin evolution in southern Bolivia Sven EGENHOFF 1), Jorg MALETZ2), Bernd-D. ERDTMANN 1) 1) 2)
Institutfiir Angewandte Geowissenschaften II, TU-Berltn, ACK 14, Ackerstrasse 71-76, D-13355 Berlin, Germany Weserstrasse 172, D-12045 Berlin, Germany
INTRODUCTION Rocks of Ordovician age are widely distributed in Bolivia The Eastern Cordillera, a broad mountain chain which runs from southern Peru to northwestern Argentina consists mainly of Early Palaeozoic siliciclastic sediments. These rocks host many of the economically important mineral deposits. However, the sedimentology and biostratigraphy of the Ordovician sediments have been poorly investigated in large parts of the country. This study focuses on Upper Ordovician deposits in the Altiplano and the Eastern Cordillera of southern Bolivia (Fig. 1). Based on sedimentologica data, a reconstruction of the basin geometry and evolution will be presented. As most of the palaeogeographic reconstructions of the southwestern Gondwana margin are based exclusively on investigations from northern Argentina and Chile (Bahlburg 1990; Bahlburg 1991; Bahlburg and Herve 1997), this study intends to include southern Bolivia in these reconstructions for the Upper Ordovician.
contrast, the eastern slope was characterized by low inclinations and mainly pelitic strata, indicating an asymmetry in basin geometry. Still in the lower Caradocian, a fining-trend in the basin successions of NW-Argentina (Bahlburg 1990) and southern Bolivia (MUller et at. 19%) reflects the
BOLIVIA
GEOLOGICAL SETTING In the Upper Cambrian, extensional movements led to the formation of a basin which extended from northwestern Argentina to central/northern Bolivia. The Cambrian and Lower Ordovician evolution is characterized by broad shelf areas in a rift basin which is floored by continental crust. A switch from an extensional setting to a compressional foreland basin in the Arenigian to Llanvirnian resulted in the formation of a deep marine trough in the west of the lower Ordovician shelf region (for northwest Argentina, see Bahlburg 1990). This basin was consecutively filled until the early Silurian in southern Bolivia (Sempere 1995).
BASIN EVOLUTION During the Caradocian, a north-south running foreland basin controlled sedimentation in the Eastern Cordillera and the Altiplano of southern Bolivia. The slope of the western margin bordered in the west by an active fault was characterized by high inclinations and a coarse-grained submarine fan system shedding sediment towards the east. In the basin centre, the deposits consist of fine-grained sandstones and pelites. Flute cast measurements indicate northward sediment transport along the basin axis. In
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Key _
Upper Ordovioian
_
Middle Ordovician
~?Ja LOiNer Ordovician
,•••• political border Fig. 1. Distribution of Ordovician rocks in southern Bolivia. The Eastern Cordillera consists almost entirely of Ordovician siliciclastic sediments whereas in the Altiplano, Ordovician rocks are restricted to isolated outcrops uplifted by Andean tectonics. Map is modified from Reuther et al. (1994).
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pronounced rise of second-order worldwide sea level. It culminated in the upper Caradocian with the flooding of the shelf, which had been exposed since the Middle Ordovician. Still in the Caradocian, the basin started to close in the south. Compressional movement led to the folding of the Ordovician units, the exposure of the Puna region and erosion. This is "documented" in the angular unconformity at the Salar del Rinc6n locality in northwestern Argentina, marking the important hiatus between the Ordovician and the overlying Llandoverian rocks (Benedetto and Sanchez 1990). In southern Bolivia, in contrast, sedimentation continued throughout the Ashgillian into the lower Silurian. The "Ocloyic" compressional movements which are well documented in the Argentine Puna affected southern Bolivia only marginally. This difference in the tectonosedimentary evolution of the basin along strike can be explained by the counterclockwise rotation of the continental block bordering the basin In the west (" ArequipaAntofallaTerrane" AAT, Bahlburg and Herve 1997) with respect to South AmericaIGondwana. This continental block should therefore have approached Gondwana more quickly in the south than in the north. In Fig. 2, this evolution is shown in two steps in a simplified geometrical model:
If the "Arequipa-Antofalla-Temane" moved counterclockwise around a vertical axis that in this example is located in its northwestern corner, the collision with Gondwana would have produced compressional tectonics at the collision point in the southeast. With ongoing compression, the collision zone moved northwards and the basin progressively closed from the south, but in the north it remained open. North of the deformation zone, strikeslip-movements occur which in the south still had a compressional component, but further north were even slightly extensional. After the rotation, the resulting form of the basin is equivalent to the overall thickness distribution of the Ashgillian sediments in southern Bolivia. This model offers a simple analogue for the history of the southwestern Gondwana margin during the Upper Ordovician and explains, how this basin could be characterized by an orogeny in the south, and at the same time by high sedimentation rates, may be even extensional tectonics (Sempere 1995) in the north. It is further useful to reconstruct the relative movements of the "ArequipaAntofalla-Temane" and to describe the fault bord·ering the basin in the west.
REFERENCES Bahlburg H., 1990: The Ordovician basin in the Puna of NW A-rgentina and N Chile: geodynamic evolution from Back-arc to foreland basin. Geotektonische Forschungen, Borntrager, Stuttgart 75: 1-107.
Bahlbl,lrg H., 1991: The Ordovician bacl-arc to foreland successor basin in the Argentinian-Chilean Puna: tektonosedimentary trends and sea-level changes, in: Macdonald D.I.M. (ed.), Sedimentation, tectonics and eustasy. Special Publication International Association of Sedimentologists 12: 465-484. Bahlburg H., Herve F. 1997: Geodynamic evolution and
tectonostratigraphic terranes of northwestern Argentina and nortern Chile. Geological Society of America Bulletin 109: 869-884.
Benedetto J.L., Sanchez T.M., 1990: Fauna y edad del estratotipo Salar del Rinc6n (Eopa1eozoico, Puna Argentina). Ameghiniana 27: 317-326
Fig. 2. Tectonic evolution of NW-Argentina and SBolivia in the Upper Ordovician, reconstructed from sediment geometries: (A) Lower Caradocian situation with an elongated basin which extends from the northern Puna (Argentina) into central Bolivia. AAT" Arequipa-Antofalla-Terrane" (B) Upper Ashgillian situation; ttce basin had closed in the south due to the counterclockwise rotation of the AAT, but sedimentation continued in southern Bolivia.
Muller 1., Maletz 1., Egenhoff S., Erdtmann B.-D., 1996: Turbiditas Caradocianas-(?)AshgiIIianas inferiores en la'Cordillera Oriental al sur de Bolivia: implicaciones cinematicas. Memoirs del XII Congreso Boliviano Tarija, pp. 747-753 .
Reuther KJ., Scheuber E., Wigger PJ. , 1994: Tectonics of the Southern Central Andes. Springer Verlag Berlin, Heidelberg, 333 pp. Sempere T., 1995: Phanerozoic evolution of Bolivia and adjacent regions, in: Tankard A.J., Suarez Soruco R., Welsink H.J. (eds.), Petroleum basins of South America. Memoir American Association of Petroleum Geologists 62: 207-230.
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