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

Ano: 2005 | Volume: 27 | Número: 2

Autores: Roberto Torra

Palavras-chave: estratigrafia; tectônica; rift-system bacia; paleogeografia; Neo- Cretácico-Quaternario; centro e norte do Argentina; regiones limítrofes - Sul America

Resumo Português:

O presente trabalho trata de um estudo de geologia regional,
sedimentologia e tectônica feito nas seqüências marinhas costeiras rasas de idade
neocretàcica a miocênica. Estas seqüências estão coligadas num sistema do tipo
“rift basin” que evoluiu desde o neocretácico. A bacia em estudo encontra-se
localizada no América do Sul, bordejando ao oeste do Escudo Brasileiro.
O sistema regional rift basin da Bacia Chaco Paraná formou-se como
uma típica bacia extensional intracratónica, limitada no oeste pelo antepaís do
orógeno andino, que ainda não tinha sofrido sua principal fase de orogênese e
levantamento regional. As amplas áreas invadidas pelos mares e seus depósitos
associados de seqüências costeiras superficiais cobriram uma ampla pediplanicie
de relevo pouco pronunciado.
A primeira etapa transgressiva começou ao final do Senoniano tardío ou
Maestrichtiano prematuro. Logo aconteceram oi avanços maiores até o Neo-
Mioceno, com retrocessos e avanços sucessivos de menor importancia. Podem
registrar-se pelo menos três etapas de associações de clímax extensionais (crustal
stretchs) e de mar alto associadas com variações eustáticas ocorridas durante o
Senoniano-Maestrichtiano-Paleoceno, o Eoceno e o Mioceno. O primeiro acontecimento
transgressivo aconteceu durante o Neo-Cretàcico, mas seu registro
sedimentológico conhecido é escasso. Porem, seríam necessarias um maior número de perfurações para um reconhecimento mais exato dos depósitos desta
transgreção marinha, bem exposta na região sud-andina (Serras Subandinas) do
noroeste da Argentina e Bolivia.
A segunda e terceira transgressões marinhas aconteceram durante o
Eoceno e o Meso-Neo-Mioceno. Os depósitos deixados por estes mares
transgressivos rasos estão bem documentados tanto nos afloramentos quanto
nos registros de perfurações por causa das suas associações características de
fácies e restos fòsseis.
Os grandes estàgios de quiescência tectônica, pelo menos três, podem
ser inferidos e associados simultâneamente com as grandes variações eustáticas
do nivel do mar relacionadas às idades do Cretácico e Terciário que correspondem
ao Senoniano-Maestrichtiano-Paleoceno, Neo-Eoceno eo Meso-Oligoceno ao Neo-
Mioceno.
O estilo estrutural principal corresponde ao de falhas mestres extensionais
normais de direção norte-sul (tilted and faulted blocks). Mesmo assim, encontram-
se presentes em grande quantidade falhas de direção noroeste - sudeste (strikeslip
faults). Para o margem da bacia do antepaís andino e a provincia de Serras
Pampeanas, predominam as falhas inversas de alto ângulo (upthrust faults), algumas
delas de tipo cisallante (shear fault zones).
Na superficie da região estudada aparece uma quantidade significativa
de cúpulas, agulhas e “necks” vulcânicos e subvulcânicos. Esta atividade
magmática é datada do Plioceno tardío, embora em sua maioria correspondam ao
Pleistoceno e as Quaternário, dada as características morfológicas dos seus edificios
vulcânicos, que não evidenciam nenhum sinal de erosão importante.
As seqüências estudadas não corresponderíam exatamente ao modelo
de PROSSER (1993) descrito para sistemas de tipo extensional rift basins, talvez
devido à grande extensão do área em análise neste trabalho, de caráter regional.
Além disso, as seqüências sedimentars presentes têm uma certa semelhança com a
sucessão de fácies “arenito-pelito-arenito” do modelo proposto por PROSSER
(1993) (sandstone-claystone-sandstone), em especial as que correspondem ao
Eoceno e Mioceno.
Análise paleogeográfica da região indica que as transgressões dos
mares Paranense e Amazônico estiveram conetadas sincronicamente durante parte
Ciência e Natura, UFSM, 27 (2): 25 - 64, 2005 27
do Mioceno, especialmente durante Meso o Neo-Mioceno. Estes extensos mares
também tiveram conesão, um pouco restrita, com o Oceano Pacífico, por meio dos
sistemas de depressões intermontanas das serras do antepaís pré-Andico. Isto
aconteceu antes do levantamento principal do Orógeno Andino ocorrido durante
os últimos 7 M.a. (Plioceno-Neo-Pleistoceno).
Este trabalho apresenta pela primeira vez um modelo de interpretação
regional tectônico-sedimentar sintético, com o fim de realizar posteriormente estudos
de maior detalhe, da ampla Bacia Chaco Paraná. Alem disso, também deveriam
estudar-se com maior rigor os potenciais recursos naturais ali presentes. A vinculação
com a Bacia Amazônica revela significativamente a importância destas seqüências
geológicas e de sua paleogeografia. A dificultade maior desta pesquisa se debe a
escasez de bons afloramentos e abundante cobertura vegetal.

Resumo Inglês:

This paper deals with a geologic “regional rift basin system pattern” and
its stratigraphical relations with littoral shallow marine sedimentary succession
paleogeography. These successions characterize the large extensional intracratonic
Chaco Paraná Basin rift system. The basin is located in South America westwards
of the Brazilian Shield. The analyzed rift basin system evolved from the Upper
Cretaceous (Late Campanian-Senonian-Maastrichtian-early Paleocene) to
Quaternary time. The siliciclastic littoral shallow marine successions were deposited
from Early Campanian-Maastrichtian to Late Miocene during three main successive
littoral shallow marine transgressions of continental extension.
These trangressions ocurred on the wide pediplanized terrains of South
America. These lands exist toward the west of the more positive areas, between the
Brazilian Shield and the foreland massifs that were settled in the more westernwards
areas. Laterly, these regional foreland massifs coupled and raised to the Andean
Orogen belt during the last 7 m.y.
The extense intracratonic pediplanized low topographic relief areas were
the reservoirs of the siliciclastic littoral shallow marine succession deposits during the three successive widespread vast continental littoral shallow marine
transgressions.
The first transgressive phenomenon begun at the Latest Campanian-
Senonian and/or early Maastrichtian. After this episode, the sedimentary
depositional system continue during Cenozoic until Latest Miocene. These
successions constitutes a major allostratigraphic unit. The limit with underlying
units is the regional unconfortmity between the regional volcanic event (Jurassic-
Cretacic and interleaved eolianite sandstones) at the base and the undifferentiated
Quaternary sediments (called as the Pampeano and Post-Pampeano Formations
sensu lato). Based on many facies analyses there had been checked out different
levels in the eustatic sea level variations within the allostratigraphic unit.
Three major stages of extensional climax were recognized and related to
stages of conspicuous eustatical sea level variations. They ocurred on Latest
Senonian-Paleocene, Eocene and Miocene.
The first transgression occured during the Upper Cretaceous although
the sedimentary deposits related to this event are scarce and only a few meters
thickness. However, the Upper Cretaceous succession is very well recognized in
the actual pre-andean zone at the north-west of Argentina and Bolivia (the Sierras
Subandinas 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 marine
transgressions occured during Eocene and Middle to Latest Miocene. They are
present either in well log registers as in most widespread outcrops on the entire
Southamerican continent.
The regional analysis led to the deduction of long periods of tectonic
quiescence, at least three of them. They may be inferred and synchronously related
with 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 directed
faultings (regional tilted and faulting blocks). On the other hand, a lot of strike-slip
faults (mainly of regional characteristic) are present crossing the area. They have a
clear influence on the accommodation and transfer zones of the rift basin system.
Ciência e Natura, UFSM, 27 (2): 25 - 64, 2005 29
The strike is north-west to south-east on the border of the basin, to the west, in the
contact with the Pampean Ridges and the narrow-meridionally-extense Sub-Andean
folded 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 and
upthrust 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. Incipient
contaminated igneous activities were associated to this cortical weakness zones.
Domes, needles and necks of volcanic and sub-volcanic origin appears
at the landscape of the region. A part of the igneous activity were dated on Latest
Pliocene although mainly correspond to Pleistocene and Holocene. This deduction
is obvious because their morphological constitution never was eroded. The volcanic
aparatous 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 a
large regional (continental) unconformity dated on Late Cretaceous. The entire
analyzed sedimentary succession deposits and their siliciclastic facies associations
correspond clearly to a “heterolithic facies” which are very common within persisting
tide-dominated depositational systems. In fact, this was what happened during
Cenozoical times (Torra, 1998b, 2001a). The heterolithic Miocene deposits
constitutes one of the best continental expossed examples.
The paleogeographical evidences showed that the Paranense and the
Amazonic Seas transgressions had been a littoral shallow marine connection during
long time on Middle and Late Miocene. During the Late Cretaceous and Eocene
periods marine connections were also active in the region. This fact is strongly
supported 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 connected
to the Pacific Ocean during the three marine episodes. The connection were formed
by narrow inter-mountain valleys, presents in the pre-andean foreland massifs.
These events occured previous to the main orogenesis elevation of the Andean
Orogen belt, did occured during the last 7 m.y. (Pliocene-Latest Pleistocene).
This paper shows for the first time a syntethic tectonic-sedimentary regional model for the Chaco Paraná Basin rift system which should largelly improve
upon later studies. The Chaco Paraná Basin carry many unexamined natural resources
which need more regional and local studies for their evaluation. However, the area
has the great problem of a significative vegetation coberture and a few good
outcrops. The evolution of modern techniques of data acquisition will help to
overcome on these difficulties.