The Chaco Paraná Basin rift basin system. An approach to the tectonic-stratigraphical evolution from the Late Cretaceous to Quaternary. South America

Abstract

This paper deals with a geologic “regional rift basin system pattern” andits stratigraphical relations with littoral shallow marine sedimentary successionpaleogeography. These successions characterize the large extensional intracratonicChaco Paraná Basin rift system. The basin is located in South America westwardsof the Brazilian Shield. The analyzed rift basin system evolved from the UpperCretaceous (Late Campanian-Senonian-Maastrichtian-early Paleocene) toQuaternary time. The siliciclastic littoral shallow marine successions were depositedfrom Early Campanian-Maastrichtian to Late Miocene during three main successivelittoral shallow marine transgressions of continental extension.These trangressions ocurred on the wide pediplanized terrains of SouthAmerica. These lands exist toward the west of the more positive areas, between theBrazilian Shield and the foreland massifs that were settled in the more westernwardsareas. Laterly, these regional foreland massifs coupled and raised to the AndeanOrogen belt during the last 7 m.y.The extense intracratonic pediplanized low topographic relief areas werethe reservoirs of the siliciclastic littoral shallow marine succession deposits during28 Ciência e Natura, UFSM, 27 (2): 25 - 64, 2005the three successive widespread vast continental littoral shallow marinetransgressions.The first transgressive phenomenon begun at the Latest Campanian-Senonian and/or early Maastrichtian. After this episode, the sedimentarydepositional system continue during Cenozoic until Latest Miocene. Thesesuccessions constitutes a major allostratigraphic unit. The limit with underlyingunits is the regional unconfortmity between the regional volcanic event (Jurassic-Cretacic and interleaved eolianite sandstones) at the base and the undifferentiatedQuaternary sediments (called as the Pampeano and Post-Pampeano Formationssensu lato). Based on many facies analyses there had been checked out differentlevels in the eustatic sea level variations within the allostratigraphic unit.Three major stages of extensional climax were recognized and related tostages of conspicuous eustatical sea level variations. They ocurred on LatestSenonian-Paleocene, Eocene and Miocene.The first transgression occured during the Upper Cretaceous althoughthe sedimentary deposits related to this event are scarce and only a few metersthickness. However, the Upper Cretaceous succession is very well recognized inthe actual pre-andean zone at the north-west of Argentina and Bolivia (the SierrasSubandinas and the meridional faults system just joint to the actual orogen; i.e.Quebrada de Humahuaca outcrops).The second and third extensive regional littoral shallow marinetransgressions occured during Eocene and Middle to Latest Miocene. They arepresent either in well log registers as in most widespread outcrops on the entireSouthamerican continent.The regional analysis led to the deduction of long periods of tectonicquiescence, at least three of them. They may be inferred and synchronously relatedwith eustatic highstand sea level variations that occured during Late Paleocene-Early Eocene, Latest Eocene-Early to Mid Oligocene and Middle-to-Late Oligocene-Early Miocene.The structural style is related with major extensional N-S directedfaultings (regional tilted and faulting blocks). On the other hand, a lot of strike-slipfaults (mainly of regional characteristic) are present crossing the area. They have aclear influence on the accommodation and transfer zones of the rift basin system.Ciência e Natura, UFSM, 27 (2): 25 - 64, 2005 29The strike is north-west to south-east on the border of the basin, to the west, in thecontact with the Pampean Ridges and the narrow-meridionally-extense Sub-Andeanfolded trend (mainly Paleozoic units belonging to the so called Sierras Subandinas).Also, at the west edge of the studied area, there exist many large shear zones andupthrust faults. The strike-slip regional faults dislocated the Pampean and Sub-Andean blocks due to the interaction of crossing regional tilted and fault blocks.For this reason, an en echelon regional block model is characteristic. Incipientcontaminated igneous activities were associated to this cortical weakness zones.Domes, needles and necks of volcanic and sub-volcanic origin appearsat the landscape of the region. A part of the igneous activity were dated on LatestPliocene although mainly correspond to Pleistocene and Holocene. This deductionis obvious because their morphological constitution never was eroded. The volcanicaparatous are morphological unmodified from they extrusion to present days.All the studied successions seems to resemble a long persisting erosive,transportation and depositation similar episodes. This phenomenon is linked to alarge regional (continental) unconformity dated on Late Cretaceous. The entireanalyzed sedimentary succession deposits and their siliciclastic facies associationscorrespond clearly to a “heterolithic facies” which are very common within persistingtide-dominated depositational systems. In fact, this was what happened duringCenozoical times (Torra, 1998b, 2001a). The heterolithic Miocene depositsconstitutes one of the best continental expossed examples.The paleogeographical evidences showed that the Paranense and theAmazonic Seas transgressions had been a littoral shallow marine connection duringlong time on Middle and Late Miocene. During the Late Cretaceous and Eoceneperiods marine connections were also active in the region. This fact is stronglysupported by the tectonic and geomorphological framework of the proto-Southamerican continent, fossil remains as well as similar sedimentary deposits.The Paranense and Amazonic epicontinental seas had been connectedto the Pacific Ocean during the three marine episodes. The connection were formedby narrow inter-mountain valleys, presents in the pre-andean foreland massifs.These events occured previous to the main orogenesis elevation of the AndeanOrogen belt, did occured during the last 7 m.y. (Pliocene-Latest Pleistocene).This paper shows for the first time a syntethic tectonic-sedimentary regi30Ciência e Natura, UFSM, 27 (2): 25 - 64, 2005onal model for the Chaco Paraná Basin rift system which should largelly improveupon later studies. The Chaco Paraná Basin carry many unexamined natural resourceswhich need more regional and local studies for their evaluation. However, the areahas the great problem of a significative vegetation coberture and a few goodoutcrops. The evolution of modern techniques of data acquisition will help toovercome on these difficulties.