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shallow-marine clastics. Carbonate deposition con-

tinued into the Early Ordovician on the Iranian ter-

races (Figure 4).

During the Late Ordovician and Early Silurian, the

central and western parts of the Arabian Peninsula

were covered by Saharan glaciers that advanced from

the south pole, which was then located in African

Gondwana. During this period, nondeposition, ero-

sion, or marginal marine conditions prevailed in east-

ern and northern Arabia. Deglaciation in the Early

Silurian led to a sharp sea-level rise, and the Palaeo-

Tethys Ocean transgressed the Arabian and adjoining

plates, depositing a thick, widespread, organic-rich

shale directly over the glaciogenic and periglacial

rocks of Arabia.

There is a general absence of Devonian deposits

over the north-eastern Arabian shelf region with

the exception of parts of north-eastern Iraq and

locally in Oman, and the carbonate deposition re-

flects a return to lower latitudes. In the Late Devonian

to Early Carboniferous the onset of south-west-

directed subduction along the former passive margin

initiated a phase of back-arc rifting and volcanism.

Lower Carboniferous carbonates occur in northern

Iraq, but elsewhere the Carboniferous is largely miss-

ing, reflecting regional emergence, nondeposition, or

erosion.

Following the Hercynian orogeny in Late Carbon-

iferous to Late Permian times the Central Arabian

Arch developed as a nondepositional uplift, and the

Rub Al Khali became a large nonmarine intracra-

tonic basin. In south and south-east Arabia uplifted

areas developed in the Hadhramaut–Huqf and in the

vicinity of the present Oman Mountains.

Glaciation occurred in Oman, southern Saudi

Arabia, and Yemen, and periglacial and fluviatile con-

ditions existed in central Arabia. In Oman and Yemen

tillites rest directly on a glacially striated Precambrian

basement.

During the deposition of the Permo-Triassic

sequence, back-arc rifting continued at the northern

end of the Arabian Plate, and the new north-east

passive margin was transgressed by a shallow

Permian sea from which was deposited carbonates

and evaporates, thickness variations of which

indicate syndepositional tectonic activity. Over

Arabia the thickness is almost uniform, ranging be-

tween 300 and 600 m, but thickens dramatically to

more than 1200 m east- and southwards. The main

depocentres for the Late Permian carbonates trend

approximately north-west–south-east, parallel to the

axes of the opening Neo-Tethyan and Hawasina

oceans in Oman. There is a general thickening to-

wards the north-east Arabian Gulf and Gulf of

Oman and Iran.

A major period of Late Triassic uplift and erosion

affected the southern part of the Arabian Gulf and led

to the progradation of continental clastic sediment

across the southern Arabian Gulf region.

In the Early Jurassic progressive back-arc rifting in

the eastern Mediterranean led to the development of a

new northern passive margin. During the Jurassic era

rift basins in Syria and south-east Yemen were active,

and intrashelf basins in the south-western Arabian

Gulf, eastern Saudi Arabia, and southern Iraq–

Kuwait were well developed. These intrashelf basins

formed the main source and reservoir rocks for the

large reserves of oil in Arabia.

The Neo-Tethys spreading ridge continued migrat-

ing north-eastwards and progressively subducted

under Eurasia. This Early Cretaceous sedimentation

is dominated by a carbonate sequence related to

major flooding of the Arabian Peninsula.

The onset of Late Cretaceous thrusting in the

Oman Mountains marks a distinctive change in

the pattern of the basin subsidence, and represents

the main phase of thrust tectonics in south-east

Arabia. The Late Cretaceous thrusting during the

closure of the Neo-Tethys is directly related to the

change in plate translation (from a south-west to

north-east direction) in response to the opening of

the South Atlantic Ocean.

Significant and widespread breaks in sedimenta-

tion occurred across the Arabian Gulf region in Late

Cenomanian and Turonian times. These stratigraphic

breaks correspond to major tectonic events in eastern

Arabia. In the Late Cretaceous the obduction of a

series of ophiolites along the Neo-Tethys margin led

to reactivation of some of the basement features in

Arabia and localized basin inversions in Syria.

The end of the Cretaceous was marked by a

regional unconformity which resulted in the Late

Maastrichtian and Danian sediments being absent

over most of the Arabian Plate. During the Early

Cenozoic subduction of the Neo-Tethys beneath the

Sanandaj–Sirjan Terrane along the northern margin

of the Neo-Tethys caused the ocean basin to close, a

process assisted by the initiation of rifting and

opening of the Red Sea. The Late Paleocene–Eocene

consists of predominantly shallow marine carbonates

and evaporites. The onset of collision between the

Arabian and Eurasian continents, which commenced

in the Late Eocene, initiated the Zagros orogeny by

suturing the Arabian and Eurasian plates. The colli-

sion created the Zagros foreland basin on the outer

edge of the north-eastern Arabian shelf margin during

the final closure of the Neo-Tethys.

Coeval with the Late Alpine Orogeny in Europe, the

Neogene was a time of maximum compression be-

tween Arabia and Asia. During this period, the Arabian

ARABIA AND THE GULF 151

Plate began to separate from Africa, the Gulf of Aden

opened, and the Dead Sea transform fault acted as a

complex sinistral strike-slip fault.

Economic Geology

The Precambrian basement rocks of Arabia contain a

range of mineral deposits, notably gold, anciently

mined in the Yemen. Important phosphate deposits

occur in a belt that extends from Syria, through

Jordan, and along the Palaeo-Tethyan shoreline into

Egypt and across the Sahara to the Atlantic Ocean.

These are found in Late Cretaceous and Eocene lime-

stone (see Sedimentary Rocks: Phosphates). A range

of evaporite minerals occurs in the Cambrian

Hormuz salt, and in modern sabkhas of the Gulf

coast, where magnesite occurs in significant quan-

tities (see Sedimentary Rocks: Evaporites).

It is, of course, for its petroleum reserves that

Arabia and the Gulf are best known. This basin is

the largest petroleum province in the world. Various

figures have been calculated for its reserves, but it is

generally agreed (figures vary in time and with

author) that it contains some 700  10

BOE (barrels

of oil equivalent). This is about 40% of the world’s

petroleum reserves (see Petroleum Geology: Re-

serves). Petroleum is produced from reservoirs

throughout the stratigraphic column. As a generaliza-

tions deep gas comes from Palaeozoic sandstones,

and oil from Mesozoic carbonates. Petroleum source

beds range from Infracambrian to Cretaceous. Pet-

roleum entrapment is largely stratigraphic, occur-

ring in truncated or onlapped sandstone, and in

reefal or shoal carbonate, within which diagenesis in

general, and dolomitization in particular, has often

played an important part in controlling reservoir

quality.

Further Reading

Al-Hajri S and Owens B (2000) Subsurface palynostratigra-

phy of the Palaeozoic of Saudi Arabia, joint study

between Saudi Aramco and CIMP, in stratigraphic

palynology of the paleozoic of Saudi Arabia, Special

Publication 1. GeoArabia.

Al-Jallal IA (1994) The Khuff Formation, its regional reser-

voir potential in Saudi Arabia and other Gulf countries;

depositional and stratigraphic approach, GEO 1994,

Middle East Petroleum Geosciences Conference, vol. 1,

pp. 103–119. Bahrain: Gulf Petrolink.

Al-Laboun A (1993) Lexicon of the Paleozoic and Lower

Mesozoic of Saudi Arabia. Riyadh, Saudi Arabia:

Al-Hudhud Publishers.

Alshahran AS and Kendall CGStC (1986) Paleozoic to

Early Mesozoic facies, depositional setting and hydrocar-

bon habitat in the Middle East: an overview and some

play concepts. American Association Petroleum Geolo-

gists Bulletin 70: 977–1002.

Alsharhan AS and Nairn AEM (eds.) (1997) Sedimentary

Basins and Petroleum Geology of the Middle East.

Elsevier.

Alsharhan AS, Nairn AEM, and Mohammed AA (1993)

Late Palaeozoic glacial sediments of the southern Arabian

Peninsula: Their lithofacies and hydrocarbon potential.

Marine and Petroleum Geology 10: 1–78.

Alsharhan AS and Scott RW (2000) Hydrocarbon potential

of Mesozoic carbonate platform-basin systems. In:

Alsharhan AS and Scott RW (eds.) Middle East Models

of Jurassic/Cretaceous Carbonate Systems, Special

Publication 69, pp. 335–358. Society for Sedimentary

Geology.

Beydoun ZR (1991) Arabian Plate hydrocarbon, geology

and potential—A plate tectonic approach. American As-

sociation of Petroleum Geologists, Studies in Geology

33: 77.

Doughty C (1888) Travels in Arabia Deserta (includes

geological map and paper). London: Jonathan Cape.

Glennie KW, Boeuf MGA, Hughes-Clarke MW, et al.

(1974) Geology of the Oman Mountain. Royal Geology

and Mining Society (Netherlands) Transactions 31: 423.

Konert G, Afifi AM, Al-Hajri SA, and Droste HJ (2001)

Paleozoic stratigraphy and hydrocarbon habitat of the

Arabian Plate. GeoArabia 6(3): 407–441.

Murris RJ (1980) Middle East: Stratigraphic evolution and

oil habitat. American Association of Petroleum Geolo-

gists Bulletin 64: 597–618.

Murris RJ (1981) Middle East: Stratigraphic evolution and

oil habitat. Geologie en Mijnbouw 60: 467–480.

Sharland PR, Archer R, Casey DM, et al. (2001) Arabian

Plate sequence stratigraphy, Special Publication 2.

GeoArabia.

Stump TE and Van der Eem JG (1994) Overview of the

stratigraphy, depositional environments and periods of

deformation of the Wajid outcrop belt, southwestern

Saudi Arabia, GEO 1994, Middle East Petroleum

Geosciences Conference, pp. 867–876. Bahrain: Gulf

Petrolink.

Vaslet D, Manivit J, Le Nindre YM, Brosse JM, Fourniquet

J, and Delfour J. (1983). Explanatory notes to the geo-

logical maps of Wadi ar Rayn quadrangle.

Ziegler MA (2001) Late Permian to Holocene paleofacies

evolution of the Arabian Plate and its hydrocarbon

occurrences. GeoArabia 6(3): 445–503.

152 ARABIA AND THE GULF

ARGENTINA

V A Ramos, Universidad de Buenos Aires, Buenos

Aires, Argentina

Introduction

Argentina has a complete stratigraphic record of

the Late Precambrian and Phanerozoic history of the

southern sector of South America, and a large variety

of present and past tectonic settings. As a whole, it

can be divided into an Andean orogenic region and a

stable basement area that is associated with one of

the largest offshore continental platforms of the

world. In order to understand its geology, a brief

description of the present geological setting provides

the key to the comprehensive record of its geological

history. The Andes (see Figure 1) have controlled the

distribution of volcanic and plutonic terrains, the

sedimentary basins, and the structure of the different

fold-and-thrust belts since the early Mesozoic. The

opening of the Weddell Sea and the South Atlantic

Ocean dominated the Mesozoic tectonics with a large

period of extension. The Palaeozoic rocks were the

result of an intricate interaction of sialic terranes or

microcontinents that collided against the western

Gondwana margin: ophiolitic belts, magmatic rocks,

and different basins are testimony of these processes.

Isolated patches of Late Precambrian rocks are wide-

spread throughout most of Argentina, indicating the

complex relationships of cratons, mobile belts, and

shear zones.

Geological Setting

A large variety of geological provinces exist, from the

active margin related to subduction of the oceanic

Nazca and Antarctic plates beneath South America

in the west, to the extra-Andean stable regions and

large continental platform of the passive Atlantic

margin in the east. The Andean chain in Argentina

has the highest peaks of the western hemisphere; the

Aconcagua, for example, a Miocene volcanic massif,

rises 6965 m above sea-level (a.s.l.) (Figure 2). One of

the main geological features of Argentina is the

segmented nature of the Andes and the associated

foreland regions, as depicted in Figures 3 and 4.

This segmentation is the result of subhorizontal

subduction in the central segment, as represented by

the Pampean flat-slab region in the foreland. This

contrasts with normal subduction in the northern

and southern segments, which have Wadati–Benioff

zones with a dip of about 30



Northern Segment

Four distinctive geological provinces characterize

the northern segment. The first is the Cordillera

Occidental, located along the international border

with Chile; it is an active volcanic belt with large

stratovolcanoes, several of them active, including

the Lascar volcano, and has large ignimbritic fields

of Late Cenozoic age. The second is the Puna, a high

plain at 3750 m elevation; this southern extension of

the Altiplano plateau is mainly composed of Miocene

volcanic rocks and partially cannibalized foreland

basins filled with Palaeogene to Miocene synorogenic

deposits unconformably lying on a low-grade Palaeo-

zoic basement. The third is the Cordillera Oriental;

this thrust belt rises up to 6200 m high and has tec-

tonic slices of Early Palaeozoic sedimentary and

plutonic rocks. The fourth is the Sub-Andean system,

consisting of middle- and low-elevation ranges rang-

ing from 1500 to less than 1000 m high; the Sub-

Andean province includes a series of large folds that

expose a complete sequence of Silurian to Permian

sedimentary rocks thrust on Cenozoic synorogenic

deposits. The active orogenic front is located along

the eastern foothills, 750 km east of the present

oceanic trench. The Sub-Andean belt changes towards

the south into the Santa Ba

rbara System, which is a

range characterized by tectonic inversion of previous

Cretaceous rifts (Figure 3).

Central Segment

The main feature of the central segment is the lack of

active volcanoes along the Andean chain. The central

segment includes the Cordillera Principal, the highest

mountains in the Andes; the mountains are composed

of Miocene uplifted volcanic rocks unconformably

overlying marine Cretaceous and Jurassic carbonates,

sandstones, and shales, interfingered with andesitic

and dacitic lavas and tuffs. These rocks are in tectonic

contact with the Cordillera Frontal, a series of north-

trending mountains composed of Late Palaeozoic

5000- to >6000-m-high volcanics and granitoids that

are overlain by continental Miocene to Pliocene

synorogenic deposits. A wide tectonic valley, the Calin-

gasta–Uspallata tectonic trench, separates the Cordil-

lera Frontal from the Precordillera. The Precordillera

fold-and-thrust belt, a 5000- to 3000-m-high range,

is composed of Early Palaeozoic carbonate platform

ARGENTINA 153

deposits, clastic foreland basin Silurian and Devonian

deposits, and Late Palaeozoic sedimentary rocks, in-

cluding glacial deposits. The broken foreland is char-

acterized by the crystalline basement uplifts of the

Sierras Pampeanas, a block-mountain system formed

during the shallowing of the subducted slab during the

past 12 million to 9 million years. Isolated patches of

Miocene to Pleistocene stratovolcanoes and volcanic

Figure 1 Digital elevation model of Argentina. The extension and importance of the distinct cordilleras can be seen in the different

segments of the Andean chain; note also the presence of plains, volcanic fields, basement uplifts, and large topographic massifs in the

extra-Andean regions. Image courtesy of United States Geological Survey.

154 ARGENTINA

Figure 3 The different geological provinces of Argentina are controlled by the segmented nature of the Nazca oceanic slab. The

northern and southern segments have a normal 30



dipping subduction zone that contrasts with the Pampean flat slab. Contours

indicate depth (in kilometres) relative to the oceanic slab; triangles are Late Cenozoic volcanoes. The dash-dot line represents the

border between Argentina and Chile. From Ramos, V.A., Cristalliniy, E., Pe

rez, D.J. 2002. The Pampean flat-slab of the Central Andes.

Journal of South American Earth Sciences 15(1): 59–78.

Figure 2 View of Mount Aconcagua, the highest peak in the western hemisphere (6965 m), characterized by Miocene volcanic rocks,

unconformably overlying folded Mesozoic rocks.

ARGENTINA 155

domes, with lavas and ignimbritic flows, spread over

the Precordillera and the western part of the Sierras

Pampeanas as a result of this flattening subduction

process.

Southern Segment

South of the Tupungato stratovolcano, at about



S latitude, active volcanism resumes along the

Cordillera Principal and continues down to the south-

ernmost Andes. The main Cordillera is composed of

a thin to thick-skinned fold-and-thrust belt formed

by Jurassic and Cretaceous marine sediments derived

from the Pacific Ocean. The altitudes, with the only

exception being the active volcanic edifices, do not

exceed 4000–3000 m and are even lower further to

the south. The Cordillera Frontal, the Precordillera,

and the Sierras Pampeanas disappear a few kilo-

metres south of the flat subduction segment. The

Palaeozoic rocks in the foothills are mildly uplifted

in a series of basement blocks, including the San

Rafael Block, yet still preserve the Late Palaeozoic

peneplain. The foreland region has a Late Cenozoic

large volcanic plateau that was formed by a few stra-

tovolcanoes of andesitic to dacitic composition, and

Figure 4 Main orogenic belts in the Andes of Argentina and Chile and adjacent regions. From the Tandilian to the Andean, the

corresponding geological ages are Early Proterozoic, Middle Proterozoic, Late Proterozoic–Early Cambrian, Early Palaeozoic (for

both Chanic and Ocloyic), Late Palaeozoic, Late Mesozoic, and Cenozoic.

156 ARGENTINA

large basaltic fields. This retroarc volcanism, develop-

ed between 35



and 38



S latitudes, is related to the

late Cenozoic steepening of the subduction zone.

Between 36



and 39



S, a large Mesozoic embayment

exposes the Early Cretaceous and Jurassic marine

clastic and carbonate sediments of the Neuque

n re-

troarc basin, which are unconformably overlain by

continental molasse of Late Cretaceous age.

Southernmost Segment

South of the Ayse

n Triple Junction between the Nazca,

Antarctic, and South America plates, at about



S, the Patagonian Cordillera developed along

the border between Argentina and Chile. The main

characteristic of this segment, the continuous elong-

ated batholith along the axis of the Patagonian Cordil-

lera, is exposed for 2000 km and is thrust on Mesozoic

volcanic and sedimentary rocks. The foothills are

characterized by Late Cretaceous molasse deposits

and synorogenic deposits of the Palaeogene and Neo-

gene foreland basins. South of 52



S latitude, an orocl-

inal bend is characteristic of the Fueguian Cordillera

in the southernmost tip of the Andes; it is composed

of Jurassic volcanics and Cretaceous deposits that

were heavily deformed during the Late Cretaceous

closure of the Rocas Verdes marginal basin.

Andean Region

Volcanic Rocks

The Andes of Chile and Argentina are characterized

by thick volcanic sequences developed in the past

26 million to 28 million years. These volcanic rocks

are composed of andesite and dacite lavas interbed-

ded with pyroclastic rocks in stratovolcanoes and

volcanic domes, interfingered with large ignimbritic

fields. The peaks of volcanic activity in the main arc

are represented by thick (up to several kilometres)

packages of highly differentiated rocks that constitute

the Central Volcanic Zone of the Andes, as defined

many years ago in southern Peru, Bolivia, and north-

ern Chile and Argentina. The Ojos del Salado, the

highest active volcano in the world (6800 m a.s.l.), is

an example of a thick pile of andesites, dacites, and

basalts that developed during Late Cenozoic times.

These highly evolved rocks were the result of large

eruptions through a thick crust (Figure 5); the erup-

tions contaminated the magmas by assimilation and

differentiation through crystal fractionation. The

eruptions alternated in Late Miocene and Pliocene

times with rhyolitic tuffs and ignimbrites, which to-

gether make up large volcanic plateaux. These acidic

plateaux are interpreted as the result of the litho-

spheric and crustal delamination that followed a

period of steepening of the subduction zone after

subhorizontal subduction north of 26



The volcanic rocks south of this latitude record an

opposite trend. Large stratovolcanoes developed dur-

ing most of the Palaeogene along the border with Chile

and they extended into Argentina in Early Neogene

times. The large volcanic fields are younger towards

the east and they gradually migrated to the foreland

along corridors. The volcanic activity decreased in the

Late Miocene and ceased along the main axis be-

tween 8 and 6 million years ago. Isolated high-K vol-

canoes, such as the Sierra del Morro in San Luis, are

located 750 km away from the trench. South of 38



Figure 5 (A) Main geochemical characteristics of the subduction-related volcanic rocks of Argentina, with a continuous calc-alkaline

trend. (B) Contrasting neodymium (Nd) and strontium (Sr) isotopic compositions of the central and southern volcanic rocks. Note the

more primitive composition of the southern volcanic zone rocks, in comparison with rocks from the central volcanic zone.

ARGENTINA 157

and down to 46



S, large isolated patches of vol-

canic rocks represented the main arc along the Andes.

These volcanic rocks, termed the Southern Volcanic

Zone of the Andes, have two distinct segments. The

northern one corresponds to andesites, dacites, and

minor basalts, which erupted through large stratovol-

canoes, such as the Tupungato, Marmolejo, and San

Jose

. The southern segment is represented by basalts,

basandesites, and scarce dacites and rhyolites, which

erupted through several Late Cenozoic volcanoes.

The difference in composition is closely related to

the thickness of the continental crust along the

Andean chain; this varies from over 60 km north of



S down to 42 km at 46



S. South of 46



S there

is a volcanic gap in arc volcanism; this is indicated by

isolated adakite outcrops such as Cerro Pampa and

Puesto Nuevo, which are related to exceptional ocea-

nic slab melting of young oceanic crust prior to the

subduction of an oceanic seismic ridge. Further south,

from 48



Sto52



S, there are five small isolated vol-

canoes formed of basalts and basandesites; these

(including the Lautaro, Aguilera, and Monte Cook

volcanoes) constitute the Austral Volcanic Zone of

the Andes. All of these rocks have an adakitic sig-

nature that indicates small partial melting of the

oceanic slab superimposed onto the asthenospheric

wedge magmas.

Fold-and-Thrust Belts and Their

Synorogenic Deposits

The eastward migration of volcanic activity was

coeval with the deformation, uplift, and development

of synorogenic deposits. The eastern slope of the And-

ean chain records a series of discontinuous foreland

basins, with thick sequences of continental deposits

up to more than 10 000 m thick. The fold-and-thrust

belts vary in style and kinematics from north to south

and were controlled by the segmented nature of the

subduction zones and the previous Palaeozoic and

Mesozoic geological history.

Sub-Andean fold-and-thrust Belt The thin-skinned

sub-Andean Belt, which moved on Late Ordovician,

Silurian, and Devonian shales, has open folds and thru-

sts. The area was covered by marine deposits about

13.5 million years ago, showing that deformation

in the sub-Andean Belt is later than Middle Miocene.

The foreland basin deposits reach over 6000 metres.

The syngrowth strata of Late Pliocene and Pleistocene

age, as well as Global Positioning System (GPS) data

and earthquake locations, indicate active tectonics in

the thrust front. The Cenozoic orogenic shortening

rate was of the order of 6.7–6.9 mm year

1

, but pro-

bably with periods of higher activity during Late

Miocene and Plio-Quaternary times.

Santa Ba´rbara fold-and-thrust Belt The Santa

rbara Belt is controlled by tectonic inversion of

basement faults, and therefore shortening is less

important than in the Sub-Andean Belt. Fault ver-

gence is towards the west, controlled by the polarity

of Mesozoic extension. Sag and synrift sequences of

Cretaceous and Palaeogene age have marine deposits

of Maastrichtian–Palaeogene age with a Pacific pro-

venance. Most of the synorogenic deposits have been

preserved in intermontaneous basins, up to 3-4 km

thick.

Sierras Pampeanas Belt The Sierras Pampeanas Belt

is a region of uplifted basement in which crystalline

basement of Precambrian–Early Palaeozoic age is

widely exposed. The reverse faults that bound the

basement blocks have a dominant west vergence and

are controlled by previous crustal sutures inherited

from the Early Palaeozoic tectonics. This sector coin-

cides with the subhorizontal subduction segment.

The synorogenic deposits record two different stages.

The older stage was an open-to-the-east large fore-

land basin associated with uplift and shortening in the

main Andes. The second stage, which is characterized

by a broken foreland, has the largest subsidence.

Consequently, basins with more than 10 000 m of

Late Miocene–Pliocene deposits have accumulated

in a continental environment.

Cordillera Principal Belt The Cordillera Principal

Belt comprises (1) a northern sector (30



–32



S lati-

tude) characterized by the tectonic inversion of

basement blocks where Late Palaeozoic–Triassic vol-

canic and plutonic rocks are exposed, (2) a central

sector (32



–33



S latitude) where thin-skinned de-

formations of Jurassic and Early Cretaceous marine

deposits are widely exposed, and (3) a southern sector

(33



–37



S latitude) where inversion tectonics

occurred again. All of these belts were deformed in

Late Cenozoic times, resulting in accumulation in the

foothills of up to 3000-m-thick synorogenic deposits

of continental sequences. The southern sector is

linked to the Neuque

n Embayment, a wide retroarc

basin that developed during Early Mesozoic times

and which covered most of the adjacent extra-

Andean platform.

Patagonia fold-and-thrust Belt The Patagonia Belt

also has two different segments. The northern seg-

ment (37



–46



S latitude) has a mild basement uplift

with only local Cenozoic depocentres, because the

main phase of uplift was produced in Cretaceous

times. South of 46



S latitude, thick foreland

basin sequences constitute the Austral or Magallanes

retroarc Basin. The uplift and subsequent deposition

158 ARGENTINA

of the synorogenic deposits were controlled by

collision of several segments of the Chile active

spreading ridge from south to north in the past

14 million years, a collision that is still taking place

at 46



S latitude.

Fueguian fold-and-thrust Belt The Fueguian Belt

was controlled by the Late Cretaceous closure of the

Rocas Verdes marginal basin, a back-arc basin of Late

Jurassic–Early Cretaceous age. This segment is the

only one that records the metamorphism and obduc-

tion of ophiolitic rocks during the Andean deformat-

ion. The basin development is linked to the formation

of the Austral Basin; as a result of the northern verge-

nce of the Fueguian fold-and-thrust Belt, the synoro-

genic deposits fill a series of east- and west-trending

successor basins.

Stable Platform

The extra-Andean region consists of large Late Ceno-

zoic plains, known as the Pampas, where thin Tertiary

and Quaternary sediments cover the basement. These

sequences preserve the first Atlantic transgressions,

which occurred in Maastrichtian–Paleocene times,

and several others, which were produced during the

different episodes of high sea-level in the South At-

lantic. Most of these sediments are capped by thick

sequences of loessic deposits. The stable platform

is characterized by several geological provinces that

encompass uplifts and basinal areas.

Uplift Areas

Several old mountain systems emerge from the extra-

Andean plains; from north to south, these regions

consist of the Tandilia and Ventania systems and the

Somuncura

and Deseado massifs.

The Tandilia System is a basement uplift produced

during the Cretaceous at the time of opening of the

South Atlantic Ocean, which exposed remnants of an

Archaean basement (Figure 4), deformed by the Early

Proterozoic Tandilian Orogen of Transamazonian

age (2000 Ma). The basement is covered by thin

sequences of quartzites and carbonates of Late Pro-

terozoic–Early Palaeozoic age. The Ventania System,

a thin-skinned fold belt 600 km south of Buenos

Aires, developed as a consequence of the Patagonia

collision against Gondwana during Permian times.

Thick sequences of clastic platform marine deposits,

capped by the Sauce Grande Tillites in the Late Car-

boniferous, were thrust with a north vergence over

the Gondwana margin. These rocks are a western

extension of the Cape Fold Belt of South Africa, and

the correlation between its glacial deposits with

the Dwyka Tillite was one of the first geological

arguments to support the existence of continental

drift in the early twentieth century.

The Somuncura

basement massif is preserved in the

northern sector of Patagonia (Figure 6). A thermally

uplifted region during the development of a transient

hotspot 27 Ma, the Somuncura

Massif is bounded by

two east- and west-trending aulacogenic basins. The

resulting thick alkaline basalts constitute the large

Somuncura

shield volcano. These basalts are associ-

ated with other alkaline plugs and volcanic flows of

Miocene age. This volcanic cover is unconformably

overlying a Late Triassic–Early Jurassic rhyolitic plat-

eau, Late Palaeozoic arc granitoids, and a metamor-

phic basement. The Deseado Massif is another

uplifted basement; it is located in south central Pata-

gonia (Figure 6), where metamorphic rocks of Early

Palaeozoic age are covered by a thick pile of rhyolitic

lavas and ignimbrites of Middle Jurassic age. The

plateau basalts that characterize this sector of Pata-

gonia, which was formed in the past 14 million years,

were controlled by the thin sequences of alkaline

basalts that are related to asthenospheric slab-

windows generated during the Chile seismic ridge

collision.

Basinal Areas

The platform area of the extra-Andean region has

several basins, most of them with an orthogonal

trend to the continental margin. The main basins are

from north to south.

Chaco-Parana´ Basin The 5-km-thick Chaco-Parana

Basin records a complex history. This region first

consisted of Early Palaeozoic redbeds that were part

of an early foreland basin associated with the Pam-

pean deformation in Late Proterozoic–Early Cam-

brian times; this was followed by Silurian and

Devonian marine and continental deposits related to

a reactivation of the deformation in the western

sector. Late Palaeozoic glacial and marine sequences

subsequently formed during a period of generalized

extension. Cretaceous redbeds and tholeiitic basalts

then formed during the development of the Parana

plume, about 130 Ma, and, finally, a thin sequence of

distal foreland deposits of Cenozoic age was associ-

ated with dynamic subsidence of the stable platform

during Andean deformation.

Salado and Colorado Basins The Salado Basin is an

aulacogenic basin that formed during the opening of

the South Atlantic Ocean. Synrift deposits of Late

Jurassic–Early Cretaceous age are covered by marine

Late Cretaceous and Cenozoic deposits. The sedi-

mentary sequence is coeval with the development of

the continental platform in the Atlantic margin. The

ARGENTINA 159

Figure 6 Generalized map of the Early Palaeozoic terranes and cratonic blocks of Argentina.

160 ARGENTINA

Elsevier Encyclopedia of Geology - vol I A-E

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