The Cambrian
System in Northwest Argentina
GUILLERMO ACEÑOLAZA) AND
FLORENCIO
ACEÑOIAZA
The Early Cambrian
The precise Lower boundary of the Cambrian System it is not well known in the provinces of northwestem Argentina. It has been estimated to be included in the highly tectonized, dominantly clastic, thick sequences outcropping in the mentioned region. These rocks crop out with different formational names as Puncoviscana Formation s.l. Jujuy, Salta and Tucumán provinces), Suncho Formation (Catamarca Province) and Aguaditas/Negro Peinado Formations (La Rioja Province).
Lithollogically
they are represented by over 2000 meters of shales, slates, sandstones and
quartzites with small participation of conglomerates, limestones and some
interestratified basic lava and tuffs.
The sequence is highly deformed and fractured, with
a general N-S alignment, fact that does not allows to find a complete
undisturbed type section.
Paleoichnological
studies allowed to recognize different levels with abundant trace fossils,
being the only means of dating the earlier mentioned rocks. Among the
recognizable Vendian trace fossils are: Nereites
saltensis, Tasmanadia and probably Sekwia.Higher levels assigned to the Tommotian are characterized by: Oldhamia
radiata, O. flabellata, 0. antiqua, Diplichnites and Dimorphichnus among
others (Aceñolaza and Durand,1986; Aceñolaza et al., 1999 with references).
Aceñolaza
and Durand (1986) recognized a basin with a SW/NE alignment for the Vendian/
Tommotian. This basin was configured by an expansion of the gondwanan margin
between the Rio de la Plata Craton (SE) and the Arequipa Massif (NT, both of a
Rifean age (aprox 1000 My). This process was nominated as Pampean Cycle
("Ciclo Pampeano") or Pampean Orogeny (sensu Aceñolaza
and Toselli,1981).
Jezek
(1990) believes that this early Paleozoic basin had a smooth floor morphology,
with large low gradient depositional fans developed towards the axis of the
basin. Turbidites have been also recognized in this wide fans.
The sedimentary sequence also records post-tectonic granite intrusions of a Mid-Cambrian age (517 My) with important outcrops in Tastil and La Quesera localities (Salta Province). This orogenic episode gave place to an important deformative process of great magnitude, being followed by an erosive event named as "Tilcaric unconformity" or "Tilcaric phase" (Turner,1960). For Jezek (1990), the deformative process that affected Puncoviscana Formation had multiple phases:
The strongest phase occurred at the end of the Lower Cambrian (F1), a
second one during the Ordovician and lastly by the Andean deformation
(Cenozoic fracturation - F3).
The Mesón Group
After the "Tilcaric" deformative and orogenic phase, a new
basin was developed in Northwestern Argentina extending into Southern
Bolivia. The sedimentation associated to this transgression is characterized
by siliciclastic deposits of over 3000 m of thickness. They follow the
Puncoviscana Formation (with an impressive angular unconformity - see cover
picture of this volume - ). Outcrops of this Formation are widely
distributed in the northem provinces of Jujuy (Puna, Eastern and Subandean
ranges), Salta (idem) and Tucumán (with the southernmost eastern outcrop at
the Sierra del Campo).
Quartzites attributed to this sedimentary event have been also
detected eastwards in perforations in the provinces of Santiago del Estero (Arbol Blanco), Chaco (Charata) and Formosa (Mariano Boedo).
The
different tectonic elements that played the role of frame to this basin were
not only the Rio de La Plata Craton and the Arequipa Craton, but also some
structural highs of ageneral SE/NW alignement ("Calchaquí Dorsal" sensu
Auboin
et al., 1973; or "Pampean Craton" sensu
Bracaccini,
1960).
The
development of this basin is related to the first flooding of the Andean
borden from Bolivia southwards, following a corridor that involved areas
limitated between the Arequipa Massif and the Guaporé Craton (SW margin of
the Brazilian shield).
The
sequence is dominated by sandstones and quartzites with hundreds of meters of
thickness in some main depocenters (Santa Victoria, Salta), changing to some
ten unities in some marginal sectors of the same basin (eg. La Pedrera,
Salta-stop Nr. 1 of the field trip-with 65.50 m sensu Sánchez,1994).
These
rocks were unified by Turner (1960) under the name of Mesón Group, being
represented by three formations clearly differentiates as follows from
bottom to top: Lizoite, Campanario and Chalhuahnayoc formations.
Lizoite
Formation: White, pinkish and grayish quartzites and sandstones characterize
lithologically this unit. This formation starts up after the Tilcarian unconformity with a conglomerate level. They show a
finning upwards
arrangement, and have been interpreted as subtidal sand-bars and channels (Sánchez,1999).
Westwards,
in the Puna Region (in Potrerillos, Salta) Aceñolaza (1973) and Aceñolaza
and Bordonaro (1989) recognized poorly preserved pieces of trilobites (Asaphaiceus
sp)and
trace fossils (Cruziana sp),
but
no further studies were done on them.
The
thicknessof this unit varies between 20 to 1,100 m.
Campanario Formation: Two members have been
differentiated within this formation. a lower and sandier "green member", characterized by medium sandstones and quartzites, and an upper
"dark purple member" with fine sandstones and subordinated
quartzites. The Campanario Formation has been interpreted as intertidal flat
sedimentation sporadically affecfed by storm events (Sánchez,1999).
Abundant trace fossils are recorded within the unit
as well defined "pipe rocks" with Skolithos
various type of
Rusophicus
and
Cruuziana among
the most remarkable ones.
This
Formation shows from 30 to 1,100 meters of thickness.
Chalhuamayoc
Formation: Thickening upwards sequence of white to pinkish quartzites and
subordinated sandstones with fewer interbedded fine conglomerates. Abundant
trace fossils were recognized in the sequence
(Skolithos
very abúndant- typical pipe rock occur - ).
Close
to the upper part of this Formation, in the locality of Azul Pampa Qujuy
Province), a fauna that belongs to the Parabolina
(N.) frequens argentina Biozone was also recognized. This unit is
followed upwards transitionally by the Santa Rosita Formation (Santa Victoria
Group) of a Cambro-Ordovician age.
Santa Victoria Group
With
this denomination is unified a few thousands of meters of shales, sandstones
and quartzites of an Upper Cambrian -Lower Ordovician age outcropping in the
northem Provinces of Jujuy and Salta. We have to mention that in certain
sectors of the basin the stratigraphic relation
The Lower unit of this Santa Victoria Group is named as Santa Rosita
Formation, and is the one that includes the Cambrian-Ordovician boundary.
Lithologically is represented by shales and sandstones with subordinated
quartzites of grayish, greenish and black colors, with a thickness of
References
-See references for field
trip-