The Precordillera terrane of western
Argentina
Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the List of South American countries by area, second-largest ...
is a large mountain range located southeast of the main
Andes
The Andes, Andes Mountains or Andean Mountains (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range is long, wide (widest between 18°S – 20°S ...
mountain range. The evolution of the
Precordillera
Precordillera is a Spanish geographical term for hills and mountains lying before a greater range, foothills. The term is derived from ''cordillera'' (mountain range)—literally "pre-mountain range"—and applied usually to the Andes.
Some place ...
is noted for its unique formation history compared to the region nearby. The
Cambrian
The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...
-
Ordovian sedimentology
Sedimentology encompasses the study of modern sediments such as sand, silt, and clay, and the processes that result in their formation (erosion and weathering), transport, deposition and diagenesis. Sedimentologists apply their understanding of m ...
in the Precordillera terrane has its source neither from old Andes nor nearby
country rock
Country rock is a genre of music which fuses rock and country. It was developed by rock musicians who began to record country-flavored records in the late 1960s and early 1970s. These musicians recorded rock records using country themes, vocal ...
, but shares similar characteristics with the
Grenville orogeny
The Grenville orogeny was a long-lived Mesoproterozoic mountain-building event associated with the assembly of the supercontinent Rodinia. Its record is a prominent orogenic belt which spans a significant portion of the North American continent, f ...
of eastern North America. This indicates a rift-drift history of the Precordillera in the early
Paleozoic
The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon.
The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838
by combining the Greek words ''palaiós'' (, "old") and ...
. The Precordillera is a moving micro-continent which started from the southeast part of the ancient continent
Laurentia
Laurentia or the North American Craton is a large continental craton that forms the ancient geological core of North America. Many times in its past, Laurentia has been a separate continent, as it is now in the form of North America, althoug ...
(current location:
North American plate
The North American Plate is a tectonic plate covering most of North America, Cuba, the Bahamas, extreme northeastern Asia, and parts of Iceland and the Azores. With an area of , it is the Earth's second largest tectonic plate, behind the Paci ...
). The separation of the Precordillera (also named
Cuyania
The Precordillera Terrane or Cuyania was an ancient microcontinent or terrane whose history affected many of the older rocks of Cuyo in Argentina. It was separated by oceanic crust from the Chilenia terrane which accreted into it at ~420-390 Ma ...
) started around the early Cambrian. The mass collided with
Gondwana
Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final sta ...
(the ancient supercontinent in the southern hemisphere) around Late
Ordovician
The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya.
T ...
period. Different models and thinking of rift-drift process and the time of occurrence have been proposed.
This page focuses on the evidence of drifting found in the
stratigraphical
Stratigraphy is a branch of geology concerned with the study of rock layers ( strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks.
Stratigraphy has three related subfields: lithostrat ...
record of the Precordillera, as well as exhibiting models of how the Precordillera drifted to Gondwana.
Location
Precordillera
Precordillera is a Spanish geographical term for hills and mountains lying before a greater range, foothills. The term is derived from ''cordillera'' (mountain range)—literally "pre-mountain range"—and applied usually to the Andes.
Some place ...
is a
Spanish
Spanish might refer to:
* Items from or related to Spain:
**Spaniards are a nation and ethnic group indigenous to Spain
**Spanish language, spoken in Spain and many Latin American countries
**Spanish cuisine
Other places
* Spanish, Ontario, Can ...
geographical term for hills and mountains lying before a greater range. The term is derived from ''cordillera'' (mountain range)—literally "pre-mountain range"—and applied usually to the Andes.
The Precordillera is in western Argentina and can be traced by the thrust-fold belt. It is about 200 km wide, and stretches 800 km from latitude 29°S to 33°S in western Argentina, with its south end in
Mendoza, through
San Juan San Juan, Spanish for Saint John, may refer to:
Places Argentina
* San Juan Province, Argentina
* San Juan, Argentina, the capital of that province
* San Juan, Salta, a village in Iruya, Salta Province
* San Juan (Buenos Aires Underground), ...
, to
La Rioja
La Rioja () is an autonomous community and province in Spain, in the north of the Iberian Peninsula. Its capital is Logroño. Other cities and towns in the province include Calahorra, Arnedo, Alfaro, Haro, Santo Domingo de la Calzada, an ...
in the north. The northern and southern boundaries are not precisely defined.
In terms of geological regions, the northeastern boundary connects to the western Pampeanas and the
Famatina
Famatina is a town in the province of La Rioja, Argentina. It has 6,371 inhabitants as per the , and is the only municipality in the Famatina Department. Located in fertile valley between Sierra de Famatina and Sierra de Velasco Famatina's econ ...
ranges, and the eastern margin connects to the
frontal Cordillera
The Frontal Cordillera ( es, Cordillera Frontal) is a mountain range in western Argentina that forms part of the Andes. It extends in a north–south direction in the provinces of La Rioja, San Juan and Mendoza. To the west of the Frontal Cordil ...
then the
Chilenia
Chilenia was an ancient microcontinent or terrane whose history affected many of the older rocks of central Chile and western Argentina. It was once separated by oceanic crust from the Cuyania terrane to which it accreted at ~420-390 Ma when Cuya ...
Terrace.
Evidence of rifting
Stratigraphic records
The Early-Cambrian to Ordovician deposits of Precordillera are composed of 3000 m thickness of
marine carbonate sections with multiple phases of
grain size variation. The Precambrian Precordillera basement is
Grenvillian-type and the Cambrian-Ordovician layer is uniquely found in Precordillera compared with surrounding Andes and Sub-Andes belt.
The sedimentary deposit in Precordillera is recognized as "typifying low-latitude stable
passive-margin continental terrace deposits".
That means Precordillera was transformed from thermal continental margin subsiding to
passive margin
A passive margin is the transition between oceanic and continental lithosphere that is not an active plate margin. A passive margin forms by sedimentation above an ancient rift, now marked by transitional lithosphere. Continental rifting cre ...
. The facies types of Cambrian-Ordovician carbonate deposit in Precordillera matches the Southern
Appalachian platform, suggesting the origin of Precordillera from Laurentia.
The Precordillera formation ranging from Early Cambrian to Late Ordovician would be introduced as follows:
Cerro Totora Formation (Early Cambrian)
The Cerro Totora Formation with a thickness of 340 m contains red marine
sandstone
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicat ...
and
siltstone
Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, ...
at the lower section. At upper section, the red
evaporites
An evaporite () is a water-soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocean ...
are interbedded with carbonate sandstone and siltstone. At the top of the formation,
quartz arenites present indicate the upper boundary of the Cerro Totora Formation.
The evaporites and red sedimentary rock indicate the transition from
syn-rift
In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics.
Typical rift features are a central linear Fault (geology), downfaulted depression, called a graben, or more commonly ...
development to the ending of rifting. The upper formation comprises Early Cambrian
olenellid trilobite
Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the ...
s which indicates the region was a normal marine environment.
La Laja Formation (Early to Mid Cambrian)
The 525 m thick La Laja formation is a thick layer of
progradational carbonate complex. The base is composed of a fine grained lime
mudstone
Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from '' shale'' by its lack of fissility (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology.' ...
which contains fossils of olenellid trilobites and
phosphatic
In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid .
The phosphate or orthophosphate ion is derived from phospho ...
brachiopod
Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, w ...
s.
Higher in the layer, the carbonate contain larger grains, so that the carbonates are described as
oolitic limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...
or
grainstone
Under the Dunham classification (Dunham, 1962) system of limestones, a grainstone is defined as a grain-supported carbonate rock that contains less than 1% mud-grade material. This definition has recently been clarified as ''a carbonate-dominated ...
. Overall the sediment shows a couple of depositional conditions – from fine grain mudstone forming in an open marine environment to increasing grain size of grainstone,
oolitic limestone in a shallow marine environment.
Zonda and La Flecha Formation (Middle to Late Cambrian)
The Zonda (200-300 m thick) and La Flecha Formation (400-700 m thick) starts from the continuous layers of La Laja Formation limestone to
peritidal dolomite Dolomite may refer to:
*Dolomite (mineral), a carbonate mineral
*Dolomite (rock), also known as dolostone, a sedimentary carbonate rock
*Dolomite, Alabama, United States, an unincorporated community
*Dolomite, California, United States, an unincor ...
.
They show a total of three sequences of shallowing-upward features, one shoaling upward sequence in the Zonda formation and two in the La Flecha Formation. It is described as aggradational carbonate complex because of insignificant lateral sift of deposits. From this point the terrace was undergoing the drifting process, traveling from Laurentia to Gondwana. Also abundant distributions of
thrombolite
Thrombolites (from Ancient Greek θρόμβος ''thrómbos'' meaning " clot" and λῐ́θος ''líthos'' meaning "stone") are clotted accretionary structures formed in shallow water by the trapping, binding, and cementation of sedimentary g ...
s dominate each basal sequence and become less significant in the uppermost intervals.
La Silla Formation (Early Ordovician)
The La Silla Formation (400 m thick) is the start of Ordovician deposits. It shows depositions of subtidal carbonate mudstone and
wackestone which is sandwiched by peritidal carbonates. The formation lacks erosion features as well as terrestrial deposits, indicating an open-marine environment. Models deduced that the Precordillera terrace had been travelling through the
Iapetus Ocean
The Iapetus Ocean (; ) was an ocean that existed in the late Neoproterozoic and early Paleozoic eras of the geologic timescale (between 600 and 400 million years ago). The Iapetus Ocean was situated in the southern hemisphere, between the paleo ...
approaching the Gondwana supercontinent.
San Juan Formation (Mid Ordovician)
The San Juan Formation (330 m thick) continues as a carbonate deposit of
packstone
Under the Dunham classification (Dunham, 1962Dunham, R.J. (1962) Classification of carbonate rocks according to depositional texture. In: Classification of Carbonate Rocks (Ed. W.E. Ham), Am. Assoc. Pet. Geol. Mem., 1, 108–121.) system of limesto ...
,
grainstone
Under the Dunham classification (Dunham, 1962) system of limestones, a grainstone is defined as a grain-supported carbonate rock that contains less than 1% mud-grade material. This definition has recently been clarified as ''a carbonate-dominated ...
,
wackestone, and
mudstone
Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from '' shale'' by its lack of fissility (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology.' ...
. It shows two
transgressive system tracks indicating drowning events, or in other words the
eustatic rise of sea level. The significant reef accumulations can be found in two intervals – the packstone at the boundary above La Silla Formation, and during the sudden change from marine environment to shallow-water grainstone.
As sea level rises, fine and dark mudstone and
shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especiall ...
deposit as an indication of approaching the
subduction zone
Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...
beneath Gondwana.
Late Ordovician unconformity
The sedimentary records no longer continuous on top of the San Juan Formation, and it is marked as an erosional
unconformity
An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...
. On top of the black shale the eastern Precordillera crust received a continental rise, inducing the rapid intensive events shown by the rifting structure on the deposits above the unconformity. The western basin consists of black
graptolitic shale with non-constant thickness which contains silicate clasts, believed to be a kind of rifting-related deposit.
The unconformity symbolizes the increasing tectonic activity, probably the start of rifting events and the collision between Precordillera and Gondwana.
Trilobites
The
olenellid trilobite
Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the ...
s found in early Cambrian sequences are identical to those in the fragments in Laurentia. The similar fossil records have contributed greatly to the hypothesis that Precordillera was linked to Laurentia until Cambrian separation. Precordillera fauna records diverge from that in Laurentia after the Ordovician, and receive an increasing amount of fauna that also appears in Gondwana. This gets to the point that Precordillera is separated from Laurentia by sea-floor expansion, becoming independent continents and approaching Gondwana.
Models of evolution
Regarding
stratigraphical records and fossil evidence, multiple geological models has been published to explain the evolution of the Precordillera contributing to the Pre-Andean history of Gondwana.
Micro-continent model
The micro-continent model was suggested by Thomas and Astini.
It says that Precordillera originates from Laurentia from the south-east of the
Ouachita Embayment. It detached via the
Ouachita Rift in Early Cambrian. After that Precordillera acts as an independent continent traveling through the Iapetus Ocean along the
Alabama-Oklahoma transform fault. Finally the continent collided with the Gondwana continent in middle Ordovician, changing the environment from a
passive margin
A passive margin is the transition between oceanic and continental lithosphere that is not an active plate margin. A passive margin forms by sedimentation above an ancient rift, now marked by transitional lithosphere. Continental rifting cre ...
to extensive
rift
In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics.
Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-grabe ...
ing. This model is strongly supported by the thick carbonate complex as well as matching fauna records.
Continental-collision model
The continent-collision model is an alternative model explaining the evolution of Precordillera.
Dalziel has reconstructed the Paleozoic plate development and proposed a narrow early Paleozoic Iapetus Ocean between Laurentia and Gondwana.
Dalziel named the model as the "Texas Plateau Hypothesis". Texas Plateau is the term describing the Precordillera at the time it was attached to its parental Laurentia, suggesting that Precordillera was always attached to Laurentia until a supercontinental collision between Laurentia and Gondwana shut down the Iapetus Ocean. After the collision, the continent rifted away during late Ordovician, so that Precordillera was no longer attached to Laurentia and stuck to the western Gondwana. This model is also known as "paired rift margin".
Precordillera as not a part of Laurentia
Finney reconsidered the possibility of micro-continent model through
U-Pb zircon dating and discovered that the dating result favours the Gondwana province instead of a drifting model from Laurentia.
He raised multiple ideas, such as reconsidering Western
Sierras Pampeanas
The Sierras Pampeanas (also called Central Sierras or Pampas Sierras) (English: Pampas Mountains) is a geographical region of Argentina.
The Sierras Pampeanas are a chain of mountains that rise sharply from the surrounding pampa region of N ...
as
autochthonous to Gondwana; the rocks between Precordillera and
Famatina
Famatina is a town in the province of La Rioja, Argentina. It has 6,371 inhabitants as per the , and is the only municipality in the Famatina Department. Located in fertile valley between Sierra de Famatina and Sierra de Velasco Famatina's econ ...
are instead a crustal fragment of Gondwana
that cannot be explained by the Laurentian drifting models. He proposed another model saying the Precordillera, or
Cuyania
The Precordillera Terrane or Cuyania was an ancient microcontinent or terrane whose history affected many of the older rocks of Cuyo in Argentina. It was separated by oceanic crust from the Chilenia terrane which accreted into it at ~420-390 Ma ...
was at the Southern margin of Gondwana and started drifting along the transform fault in mid-late Ordovician. Finally it reached the position where it was subducting beneath the Famatinan belt in
Devonian
The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, wh ...
.
Post-collision period
After the collision of Precordillera with Gondwana, Precordillera is dominated by
crustal extension
Extensional tectonics is concerned with the structures formed by, and the tectonic processes associated with, the stretching of a planetary body's crust or lithosphere.
Deformation styles
The types of structure and the geometries formed depend on ...
. Multiple sets of
horsts and
graben
In geology, a graben () is a depressed block of the crust of a planet or moon, bordered by parallel normal faults.
Etymology
''Graben'' is a loan word from German, meaning 'ditch' or 'trench'. The word was first used in the geologic conte ...
s created sudden drowning and shallowing events. Elevated blocks were then eroded and the irregular fragments were collected and deposited at the
wedges or grabens, forming
conglomerate
Conglomerate or conglomeration may refer to:
* Conglomerate (company)
* Conglomerate (geology)
* Conglomerate (mathematics)
In popular culture:
* The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes
** ...
or
breccia
Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix.
The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...
in the graben area.
The fauna records in Precordillera become consistent with Gondwana.
During
Silurian
The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleoz ...
to Devonian periods, increasing
metamorphism
Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of ch ...
or magmatic activities with structural deformation showed the approach of the
Chilenia
Chilenia was an ancient microcontinent or terrane whose history affected many of the older rocks of central Chile and western Argentina. It was once separated by oceanic crust from the Cuyania terrane to which it accreted at ~420-390 Ma when Cuya ...
terrace from the west and Precordillera finally locked into the position with Gondwana.
References
{{Reflist, 30em
Geology of the Andes
Geology of Argentina