Tropical/subtropical Upper Paleocene±Lower Eocene ¯uvial deposits ineastern central Patagonia, Chile (46845 0S)

M. SuaÂreza,*, R. de la Cruza, A. Troncosob

aServicio Nacional de GeologõÂa y MinerõÂa, Avenida Santa MarõÂa 0104, Santiago, ChilebUniversidad de Talca, Talca, Chile

Received 31 July 1999; accepted 31 January 2000

Abstract

A succession of quartz-rich ¯uvial sandstones and siltstones derived from a mainly rhyolitic source and minor metamorphic rocks, located

to the west, represent the ®rst Upper Paleocene±Early Eocene deposits described in Chilean eastern central Patagonian Cordillera (46845 0S).

This unit, exposed 25 km south of Chile Chico, south of lago General Carrera, is here de®ned as the Ligorio MaÂrquez Formation. It overlies

with an angular unconformity Lower Cretaceous shallow marine sedimentary rocks (Cerro Colorado Formation) and subaerial tuffs that have

yielded K±Ar dates of 128, 125 and 123 Ma (Flamencos Tuffs, of the Divisadero Group). The Ligorio MaÂrquez Formation includes ¯ora

indicative of a tropical/subtropical climate, and its deposition took place during the initial part of the Late Paleocene±Early Eocene Cenozoic

optimum. The underlying Lower Cretaceous units exhibit folding and faulting, implying a pre-Paleocene±Lower Eocene contractional

tectonism. Overlying Oligocene±Miocene marine and continental facies in the same area exhibit thrusts and normal faults indicative of post-

Lower Miocene contractional tectonism. q 2000 Elsevier Science Ltd. All rights reserved.

ResuÂmen

Una sucesioÂn de areniscas y limolitas ¯uviales, ricas en cuarzo y derivadas de una regioÂn alzada al oeste compuesta principalmente por

riolitas y subordinadamente rocas metamoÂr®cas, representa los primeros depoÂsitos del Paleoceno Superior±Eoceno Inferior identi®cados en

la Cordillera PatagoÂnica central de Chile (46845 0S). Esta unidad se de®ne en este trabajo como FormacioÂn Ligorio MaÂrquez, y a¯ora

tõÂpicamente 25 km al sur de Chile Chico, al sur del lago General Carrera. Sobreyace en discordancia angular y de erosioÂn a capas

sedimentarias del CretaÂcico Inferior, depositadas en un mar somero (FormacioÂn Cerro Colorado), y a tobas subaeÂreas que han dado valores

K±Ar de 128, 125 y 123 Ma (Tobas Flamencos, del Grupo Divisadero). La FormacioÂn Ligorio MaÂrquez incluye una ¯ora caracterõÂstica de un

clima tropical/subtropical, y su depositacioÂn habrõÂa ocurrido durante la parte inicial del oÂptimo climaÂtico del Paleoceno Superior-Eoceno

Inferior. Las capas del CretaÂcico Inferior infrayacentes estaÂn plegadas y falladas, lo que implica un tectonismo pre-Paleoceno Superior±

Eoceno Inferior, mientras que, en la misma zona, capas marinas y continentales del Oligo±Mioceno exhiben fallas inversas y normales, que

senÄalan un tectonismo compresivo post-Mioceno Inferior. q 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Upper Paleocene±Early Eocene deposits; Contractional tectonism; Paleoclimate

1. Introduction

The Tertiary evolution of the central Patagonian Cordil-

lera has been inferred to have been greatly in¯uenced by the

subduction of an Eocene spreading ridge separating the

Aluk and Farallon Plates and, during the last 14±15 Ma,

by the subduction of the Chile Ridge (Cande and Leslie,

1986; Cande et al., 1987; Pardo-Casas and Molnar, 1987;

Ramos, 1989; Ramos and Kay, 1992; Gorring et al., 1997).

Onland evidence for these processes has been provided by

the Tertiary sedimentary basins, the geochemistry of ¯ood

basalts, of young near-trench plutons, and the tectonic

evolution of the region (Forsythe and Nelson, 1985;

Forsythe et al., 1986; Ramos, 1989; Ramos and Kay,

1992; Flint et al., 1994; Nelson et al., 1994; Bourgois et

al., 1996; Ray, 1996; Gorring et al., 1997; Demant et al.,

1998; Lagabrielle et al., 1999). One of the main problems

for any geologic reconstruction has been the poor time

information on the sedimentary successions that were

mostly deposited in a continental environment. This has

precluded the construction of well-constrained tectonic

models for the evolution of the region.

The timing of Tertiary sedimentary units is an important

Journal of South American Earth Sciences 13 (2000) 527±536

0895-9811/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved.

PII: S0895-9811(00)00042-0

www.elsevier.nl/locate/jsames

* Corresponding author. Tel.: 156-2-7375050; fax: 156-2-7776802.

E-mail address: msuarez@sernageomin.cl (M. SuaÂrez).

element for the geologic reconstruction of the evolution of

the Patagonian Cordillera. The previously known Tertiary

record of the eastern part of the AyseÂn Region, south of

Lago General Carrera (46830 0S), in southern Chile (Table

1), had as the oldest unit ¯uvial deposits de®ned as the San

Jose Formation (Flint et al., 1994), underlying marine beds

of the Guadal Formation, originally assigned to the Oligo-

cene±Lower Miocene by Niemeyer et al. (1984), timing

recently supported by a thorough study on invertebrate

fossils that indicated an Upper Oligocene±Lower Miocene

age (Frassinetti and Covacevich, 2000). Therefore, the San

Jose Formation must be prior to the Upper Oligocene±

Lower Miocene.

In this note, the ®rst ¯uvial deposits with a paleo¯ora of

Upper Paleocene±Lower Eocene age to be identi®ed in the

AyseÂn Region, in Chile (46830 0±46845 0S), are described

from south of Chile Chico and their paleoclimatic and

tectonic implications discussed. Approximately 70 km to

the west this unit underlies the San Jose Formation, which

has a different, more volcaniclastic lithology, and according

to preliminary work, a different paleo¯ora.

2. Geological setting

In the area south of Lago General Carrera (Lago Buenos

Aires in Argentina) there are four main areas of exposures of

Tertiary sedimentary rocks, which have different strati-

graphic relationships (Table 1): (1) Mina Ligorio MaÂrquez,

located 25 km to the south of the village of Chile Chico

(Fig. 1), to which this work will be referred. (2) The San

Jose to Rio Furioso area is a NS trending double monocline

composed of the most complete Tertiary sedimentary

succession in Chilean central Patagonian Cordillera, includ-

ing approximately 1000 m of Upper Paleocene±Lower

Eocene to Middle Miocene strata. This is equivalent to the

Cosmelli Basin of Flint et al. (1994). The strata comprises a

succession of continental deposits, including the Upper

Paleocene±Lower Eocene Ligorio MaÂrquez Formation,

overlain by the probably Middle Eocene±Upper Oligocene

San Jose Formation. Overlying marine deposits of Upper

Oligocene±Lower Miocene age (Guadal Formation) (Frasi-

netti and Covacevich, 2000), in turn underlie continental

deposits (Galera Formation) (Niemeyer et al., 1984) that

have been correlated with the Santa Cruz Formation of

Argentina, of late Early Miocene±early Middle Miocene

age (Marshall et al., 1986; Marshall and Salinas, 1990).

South of Lago Buenos Aires at latitude 46830 0S, in Argen-

tina, successions of comparable stratigraphy have been

M. SuaÂrez et al. / Journal of South American Earth Sciences 13 (2000) 527±536528

Table 1

Lithostratigraphic scheme of the area south of Chile Chico, eastern central

Patagonian Cordillera (46830 0±46850 0S)

Fig. 1. Location map.

given somewhat different ages (see Camacho et al., 1998).

In particular the marine El Chacay Formation, which could

be correlated with the Guadal Formation, overlies the

Eocene Posadas Basalt and underlies the continental

Miocene Santa Cruz Formation, has been given a Middle

Eocene age (Camacho et al., 1998). It is not clear whether

this age is correct and indicates diachronism of the sedimen-

tary units or re¯ects different age interpretations of the fossil

fauna. Recently, the Ligorio MaÂrquez Formation has been

recognized to underlie the more volcaniclastic sandstones of

the San Jose Formation that, according to preliminary

studies of their paleo¯ora, may have a very different age

and possibly were deposited during a more temperate

climate. (3) In the headland of Rio Chacabuco, on the

north±eastern side of the Chacabuco Valley, quartz-rich

conglomerates and sandstones, interpreted as ¯uvial depos-

its, ovelie tuffs correlated with the Flamencos Tuffs, in turn

overlying the Cerro Colorado Formation, which is underlain

by the IbaÂnÄez Group. The uppermost ¯uvial deposits are

correlated with the Ligorio MaÂrquez Formation based on

its stratigraphic relationship and lithology. (4) South of

Chile Chico, in Chile, and east of a NS fault (Arroyo Las

Horquetas fault of SuaÂrez and de la Cruz, 1996) and south of

Lago Buenos Aires, in Argentina, the Guadal Formation and

the Galera Formation/Rio Zeballos Group of Busteros and

Lapido (1983), crops out on both sides of the border, under-

lying Upper Miocene to Recent basalts on the Argentinean

side. The main difference in the stratigraphy of all the above

units is the absence of ¯ood basalts in the San Jose±Rio

Furioso area and in the Rio Chacabuco headland, the latter

possibly due to erosion.

This work refers to the study of the oldest Tertiary succes-

sion in the Mina Ligorio MaÂrquez area. The oldest expo-

sures in the study area (Figs. 1 and 2), are rhyolitic and

dacitic tuffs and domes of the IbaÂnÄez Group (Table 1).

These rocks were previously included in the Chile Chico

(Niemeyer, 1975; Ray, 1996) and Divisadero formations

(Niemeyer et al., 1984), assigned to the Upper Cretac-

eous-Tertiary (Niemeyer, 1975), late Tertiary (Ray, 1996)

and mid-Cretaceous (Niemeyer et al., 1984). However,

recent stratigraphic, paleontologic and radiometric work

by the authors found that these ignimbrites are of Upper

Jurassic±earliest Cretaceous age, and therefore form part

of the IbaÂnÄez Group of Middle?±Upper Jurassic-Berriasian

age (Niemeyer et al., 1984; Covacevich et al., 1994; SuaÂrez

and de la Cruz, 1996). Samples of these ignimbrites south of

Lago General Carrera, in the hills west and south of Chile

Chico, have yielded late Jurassic±early Cretaceous K±Ar

(biotite) dates of 144 ^ 3 to 149 ^ 4 Ma, and one date of

132 ^ 3 Ma interpreted as a reset age (SuaÂrez and de la

Cruz, 1996).

Strata of the IbaÂnÄez Group underlie clastic sedimentary

rocks of the Cerro Colorado Formation (SuaÂrez and de la

Cruz, 1996). Marine fossils in this formation include Stein-

manella sp. and Ostrea stanton, of early Cretaceous age

(Covacevich, 1996, written communication) and Pterophyl-

lum sp., indicative of a pre-Late Cretaceous age. Palynolo-

gic studies in strata of the Cerro Colorado Formation,

exposed 70 km to the south, on the north-eastern side of

the Chacabuco Valley (Fig. 1), has also given an early

Cretaceous age (Alfaro et al., 1997). Previously, the Cerro

Colorado Formation was included in the ªPrimer Nivel

Marino con Ostreaº and was assigned to the Upper Seno-

nian±Paleocene (Niemeyer et al., 1984). The Cerro Color-

ado Formation has an estimated thickness of 100 m in the

type locality and is characterized by vertical and lateral

facies changes. These include basal quartz sandstones

(locally heavily bioturbated), oyster banks, and shales

deposited in shallow marine and continental environments.

Cross-bedded heterolithic sandstones are probably tidal

deposits with the trace fossils Asterosoma (Covacevich,

1996, verbal communication), Gyrochorte sp., Planolites,

Pelecypodichnus sp. Highly bioturbated sandstones contain

occasional marine body fossils.

Subaerial tuffs and tuf®tes, with ignimbrites and

fossil rootlets, of the 140 m thick Flamencos Tuffs,

conformably overlie the Cerro Colorado Formation.

K±Ar (biotite) ages from these ignimbrites have given

dates of 128 ^ 3, 125 ^ 3 and 123 ^ 3 Ma, probably

representing an age near the time of eruption (Barre-

mian±(?)Aptian)(SuaÂrez and de la Cruz, 1996). These

tuffs can be included in the Divisadero Group, repre-

senting the earliest volcanism of this group that usually

have much younger dates of ca.118 and 116 Ma

(SuaÂrez et al., 1996). Twenty ®ve kilometers to the

northwest, Lower Cretaceous marine beds (Apeleg

Formation) reached at least from Hauterivian to

Lower Aptian (SuaÂrez et al., 1996), implying that the

subaerial tuffs of the Flamencos Tuffs represent either

the deposits of a volcanic island or the products of

volcanoes along the coast forming the eastern margin

of the Austral Basin.

Folded strata of the Los Flamencos Tuffs are unconform-

ably overlain by a thick succession of Tertiary sedimentary

rocks and basalts (approximately 800 m thick) that, from

base to top, include (Table 1): (1) Upper Paleocene±

Lower Eocene Ligorio MaÂrquez Formation, (2) Eocene

basalts (Charrier et al., 1979; Petford et al., 1996), (3)

marine Guadal Formation that in the area of San JoseÂ,

50±60 km to the west, have been assigned to the Upper

Oligocene±Lower Miocene (Frassinetti and Covacevich,

2000), (4) Upper Miocene basaltic lavas and plugs that

have given K±Ar whole-rock ages of ca.8 and ca. 5 Ma,

respectively, in agreement with K±Ar dates of Charrier et

al. (1979). Diatremes and basaltic surge deposits, probably

indicating maars and/or tuff rings, have been identi®ed in

the latter.

To the east, in Meseta del Lago Buenos Aires, in Argen-

tina (Fig. 1), Upper Miocene basalts that have given K±Ar

whole-rock dates of ca. 10 Ma, overlie Miocene ¯uvial

deposits of the RõÂo Zeballos Group (Busteros and Lapido,

1983), equivalent to the Galera Formation in Chile. In turn,

M. SuaÂrez et al. / Journal of South American Earth Sciences 13 (2000) 527±536 529

the Upper Miocene basalts underlie a succession of basalts

capped by recent basalts.

3. Fluvial deposits of the Ligorio MaÂrquez Formation

A 60 m thick, subhorizontal succession of quartz-sand-

stones with interbedded shales and some carbonaceous

shales and thin coal horizons, is exposed in the southern

hills to the north of Laguna Los Flamencos, 25 km south

of Chile Chico (latitude 46846 0S). This succession,

previously named Ligorio MaÂrquez Beds (SuaÂrez and de

la Cruz, 1996) is de®ned here as the Ligorio MaÂrquez

Formation. The type locality is at the informally named

Mina Ligorio MaÂrquez, where coal-mining activities have

been unsuccessfully attempted (Figs. 1 and 2). These beds

are subhorizontal and overlie deformed Lower Cretaceous

units of the Cerro Colorado Formation and Flamencos Tuffs

with a subhorizontal angular unconformity (SuaÂrez and de la

Cruz, 1996). They are in turn overlain by basalts with a

gentle erosional unconformity, that 20 km to the north are

in direct contact with underlying IbaÂnÄez Group, separated

M. SuaÂrez et al. / Journal of South American Earth Sciences 13 (2000) 527±536530

Fig. 2. Generalized geologic map south of Chile Chico, eastern central Patagonian Cordillera. Numbers indicate K±Ar dates (SuaÂrez and de la Cruz, 1997, in

preparation); AA 0 location of measured column of Fig. 5 (at lower part of ®gure).

by 1±2 m thick quartz sandstones of the basal Cerro Color-

ado Formation. Immediately above the Ligorio MaÂrquez

Formation these basalts have yielded a K±Ar whole-rock

age of 41.6 ^ 1.4 Ma (mid-Eocene; unpublished data of the

authors). This age is in general agreement with the K±Ar

whole rock ages of 57 ^ 1 to 44 ^ 5 Ma (Upper Paleocene±

Eocene) (Charrier et al., 1979; Petford et al., 1996; SuaÂrez

and de la Cruz, in preparation), obtained from samples of

the same unit further north.

Previously, the Ligorio MaÂrquez Formation was included

in the ªPrimer Nivel Marino con Ostreaº together with the

Lower Cretaceous Cerro Colorado Formation, and assigned

to the Upper Cretaceous±Paleocene (Niemeyer et al., 1984).

More recently, the Ligorio MaÂrquez Formation has been

included in the San Jose Formation, together with the

Cerro Colorado Formation by Ray (1996). Lithologically,

however, the quartz-rich Ligorio MaÂrquez Formation is

different from the volcaniclastic San Jose Formation, and,

recently the Ligorio MaÂrquez Formation has been identi®ed

underlying the San Jose Formation in the area of RõÂo

Furioso, on the hills north of the Chacabuco Valley

(47830 0S; Fig. 1), 60 km W of the studied area.

The Ligorio MaÂrquez Formation at the type locality is a

succession of light grey and yellowish sandstones and silt-

stones with occasional thin coal horizons. The sandstones

comprise medium to very coarse-grained beds, locally with

granules and occasional pebbles, some cross-bedded

(Fig. 3), and ®ne-grained sandstones commonly with paral-

lel lamination and carbonaceous laminae. Some beds

contain abundant fossil ¯ora. A few coal beds, between 20

and 75 cm thick, occur in the upper part of the succession.

Fossil rootlets are common in siltstones and grey mudrocks

(Fig. 4), and are occasionally present in the upper parts of

some sandstone beds. Locally, the sandstones incorporate

intraclasts derived from the underlying sandstones and

shales. Some sandstones ®ll paleochannels carved in other

sandstones or in shales with fossil rootlets. A measured

column (Fig. 5), taken approximately 2.5 km SW of the

ªmineº, shows that sandstones dominate in the middle of

the section (13 m thick), with a poorly exposed lower unit

(16 m thick) of alternating shales and sandstones, and an

upper succession (16 m thick) dominated by shales, with

intercalated carbonaceous shales, thin coal horizons, and

cross-bedded sandstones, some probably representing

crevasse splay deposits.

The clasts are mainly composed of quartz fragments,

mostly of volcanic origin, fragments of rhyolite porphyries,

with silici®ed microcrystalline groundmass and occasional

fragments of vitric ash and banded rhyolite and of meta-

morphic rocks. These silicic volcanic source rocks probably

consisted of the IbaÂnÄez Group, the Flamencos Tuffs or other

units of the Divisadero Group.

M. SuaÂrez et al. / Journal of South American Earth Sciences 13 (2000) 527±536 531

Fig. 3. Coset of cross-bedded sandstones. See geologist as scale.

The abundant fossil roots present mainly in the shales

together with the coal beds indicate a continental

depositional environment, probably a ¯ood plain, adja-

cent to ¯uvial channels represented by the channelized

cross-bedded sandstones. Isolated channel ®ll cross-

bedded sandstones, 80 cm thick, interbedded in the

shales with intercalated coal beds in the upper part of

the section, may represent crevasse splay deposits. The

succession may represent a relatively sinuous ¯uvial

system.

Cross-bedding measurements indicate paleo¯ow

directions from SW to NE, from W to E, form NW

to SE, and more occasionally, from N to S. The over-

all trend is from the west with variability probably

re¯ecting the sinuosity of the ¯uvial system. Some of the

paleocurrents may have been measured in crevasse splay

deposits.

4. Fossil ¯ora and age

The Ligorio MaÂrquez Formation includes an assemblage

M. SuaÂrez et al. / Journal of South American Earth Sciences 13 (2000) 527±536532

Fig. 4. Paleosoil with fossil rootlets.

M. SuaÂrez et al. / Journal of South American Earth Sciences 13 (2000) 527±536 533

Fig. 5. Measured stratigraphic column (see AA 0 for location in Fig. 2)

of fossil leaves collected from the center of the measured

succession (Fig. 5), and identi®ed as: Podocarpacaeae:

Podocarpus inopinatus Florin; Lauraceae: Camphoromoea

speciosa Engelhardt, cf. Cinnamomum sp., cf Goeppertia

ovatifolia Engelhardt, Nectandra proli®ca Berry, Nota-

phoebe ovatifolia Berry,? Ocotea sp.? Persea sp. Phoebe

elliptica Engelhardt, unidenti®ed Lauraceae; Melastomata-

ceae:? Miconia sp.

This tapho¯ora shares 5 out of 6 species identi®ed at

speci®c level with those from ConcepcioÂn-Arauco, Chile

(398S) (Engelhardt, 1891; Berry, 1922) dated as Upper

Paleocene. It also shares 4 out of 6 species with the RõÂo

Pichileufu fossil ¯ora of Argentina (Berry, 1938), Upper

Paleocene±Lower Eocene in age, and 4 out of 6 species

with the QuinamaÂvida tapho¯ora of Chile (Troncoso,

1992; Troncoso and Romero, 1998), considered as Lower

Eocene in age. Excluding these tapho¯oras the single

species elsewhere recorded is Podocarpus inopinatus

found in the Maastrichtian beds of Cerro Guido, in southern

Chile.

An Upper Paleocene±Lower Eocene and most prob-

ably an Upper Paleocene age is suggested for the ¯ora

from the Ligorio MaÂrquez Formation. However, there is

a pending problem because there are basalts with a late

Paleocene K±Ar value of 57 ^ 1 Ma (Charrier et al.,

1979), early Eocene Ar±Ar ages of 51.7 ^ 0.7 and

51.8 ^ 0.9 Ma (Petford et al., 1996), and an Early

Eocene K±Ar date of 53.4 ^ 1.8 Ma (SuaÂrez and de la

Cruz, unpublished data) which may indicate that, if

these values re¯ect eruption ages and if basalts every-

where postdate the Ligorio MaÂrquez Formation, this

formation would be restricted to the Paleocene. This

possible interpretation however, leaves a problem if

Yoshida's (1990) samples of few Nothofagidites pollen

in sedimentary strata from the Mina Ligorio MaÂrquez

locality, were taken from the uppermost levels of the

Ligorio MaÂrquez Formation. If this is correct, it will

imply climate deterioration during deposition of, prob-

ably, the uppermost Ligorio MaÂrquez Formation. The

latter could correspond to the short, cold period occur-

ring in the high southern latitudes during the early Late

Paleocene (Dingle et al., 1998).

The diversity of Lauraceae species in the Ligorio

MaÂrquez assemblage is noteworthy and is best compared

to the Upper Paleocene ConcepcioÂn-Arauco tapho¯ora.

Modern Lauraceae are mostly distributed in tropical and

subtropical regions of the world, principally in Central

and South America (mainly in Brazil) and southern

Asia. Therefore, this assemblage can be postulated as

re¯ecting tropical to subtropical conditions. This is

supported by the fact that entire margined leaves predo-

minate in the foliar morphology, indicating high mean

annual temperatures, very low mean range of temperatures

and wet conditions (Wolfe, 1971). The few taxa involved

in this analysis suggest caution when considering these

conclusions.

5. Discussion and conclusions

A succession of quartz-rich ¯uvial sandstones and silt-

stones derived from a mainly rhyolitic source with minor

exposures of Paleozoic metamorphic rocks located to the

west, exposed 25 km south of Chile Chico, represent the

®rst Upper Paleocene±Lower Eocene deposits described in

this region. This unit is here de®ned as the Ligorio MaÂrquez

Formation and overlies with an angular unconformity Lower

Cretaceous shallow marine sedimentary rocks (Cerro Color-

ado Formation) and subaerial tuffs that have yielded K±Ar

dates of 128, 125 and 123 Ma (Flamencos Tuffs).

Paleogene temperature ¯uctuations have been documen-

ted from the Antarctic Peninsula and from the southern high

latitude open oceans. Oxygen isotope and ¯oral data from

shallow marine and terrestrial areas of the northern Antarc-

tic Peninsula suggest temperature changes from relatively

warm values during the late Maastrichtian time, through a

short, cold period during the early Late Paleocene, before

rising sharply in the middle to Late Paleocene, when the

Late Paleocene±Early Eocene Cenozoic optimum took

place (Dingle et al., 1998). From Middle Eocene time, the

increasing illite content in oceanic sediments is interpreted

as evidence of physical weathering under progressively

colder conditions (Ehrmann and Mackensen, 1992). The

Ligorio MaÂrquez Formation was deposited during the Late

Paleocene±Early Eocene Cenozoic optimum, with the possi-

bility of having been restricted to the Paleocene. A not yet

con®rmed reference to a mixed ¯ora (Yoshida, 1990) in the

upper strata, may re¯ect the short period of colder tempera-

tures recognized during the Late Paleocene in southern hemi-

sphere high latitudes (see Dingle et al., 1998).

The absence of intercalated pyroclastic beds and of detri-

tus of direct pyroclastic origin, suggests deposition during a

gap in volcanic activity. This agrees with the absence of

Paleocene radiometric dates in the plutonic rocks so far

analysed of the AyseÂn region (448±498S) (see SuaÂrez and

de la Cruz, 1997; Pankhurst et al., 1999). Further work is

needed to understand the tectonic setting of the Ligorio

MaÂrquez basin (possibly a foreland basin, generated by

thrusting, or a hemigraben, later inverted).

The underlying Lower Cretaceous units exhibit folding and

faulting, implying a pre-Upper Paleocene±Lower Eocene

contractional tectonism (SuaÂrez and de la Cruz, 1996).

Although at the type locality the Ligorio MaÂrquez Formation

is subhorizontal, immediately east of these exposures, the

overlying Oligocene±Miocene marine and continental units

(Guadal and Galera formations) are deformed indicating

contractional tectonism, also operative during and/or after

the Miocene. Therefore, there was different timing for contrac-

tional tectonism in the area (SuaÂrez and de la Cruz, 2000).

Acknowledgements

This work was ®nanced by FONDECYT project No

M. SuaÂrez et al. / Journal of South American Earth Sciences 13 (2000) 527±536534

1930246, FONDECYT project No 1960097 and the Servi-

cio Nacional de GeologõÂa y MinerõÂa, Chile, in collaboration

with the Regional Government of the XI Region, Chile. We

appreciate the comments of Michael Bell to the manuscript.

We wish to thank the ®eld assistance given by Leonardo

ZunÄiga and Juan LoÂpez. CONAF gave camping facilities,

which are greatly appreciated.

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