Post on 08-Mar-2023
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.
References
Alfaro, G., Helle, S., Kelm, U., Palma, S., 1997. Las ocurrencias de carboÂn
en las nacientes del RõÂo Chacabuco, AyseÂn, Chile. Actas VIII Congreso
GeoloÂgico Chileno, Antofagasta, Chile, I, pp. 420±424.
Berry, E.W., 1922. The ¯ora of the ConcepcioÂn-Arauco coal measures of
Chile. John Hopkins University Studies in Geology 4, 73±142.
Berry, E.W., 1938. Tertiary ¯ora from the rio Pichileufu Argentina. Geolo-
gical Society of America Special Papers 12 (149pp.).
Bourgois, J., Martin, H., Lagabrielle, Y., Le Moigne, J., Frutos, J., 1996.
Subduction erosion related to spreading-ridge subduction: Taitao
Peninsula (Chile margin triple junction area). Geology 24, 723±726.
Busteros, A.G., Lapido, O., 1983. Rocas baÂsicas en la vertiente norocci-
dental de la Meseta del Lago Buenos Aires, Provincia de Santa Cruz.
Revista AsociacioÂn GeoloÂgica Argentina 38, 427±436.
Camacho, H., Chiesa, J., Parma, S., 1998. Relaciones estratigra®cas entre
formaciones terciarias en el occidente de la Provincia de Santa Cruz.
Revista de la AsociacioÂn GeoloÂgica Argentina 53, 273±281.
Cande, S.C., Leslie, R.B., 1986. Late Cenozoic tectonics of the southern
Chile Trench. Journal of Geophysical Research 91, 471±496.
Cande, S.C., Leslie, R.R., Parra, J.C., Hobart, M., 1987. Interaction
between the Chile Ridge and Chile Trench: geophysical and geothermal
evidence. Journal of Geophysical Research 92, 495±520.
Covacevich, V., de la Cruz, R., SuaÂrez, M., 1994. Primer hallazgo de fauna
del Berriasiano Inferior (Neocomiano) en la FormacioÂn IbaÂnÄez, RegioÂn
XI, AiseÂn, Actas VII Congreso GeoloÂgico Chileno, ConcepcioÂn, 1, pp.
425±429.
Charrier, R., Linares, E., Niemeyer, H., Skarmeta, J., 1979. K±Ar ages of
basalt ¯ows of the Meseta Buenos Aires in southern Chile and their
relation to the southeast Paci®c triple junction. Geology 7, 436±439.
Demant, A., Belmar, M., HerveÂ, F., Pankhurst, R.J., SuaÂrez, M., 1998.
PeÂtrologie et geochimie des basaltes de Murta: une eÂruption sous-
glaciaire dans les Andes patagoniennes (468 lat. S), Chili. Relation
avec la subduction de la ride du Chili. Comptes Rendus de l'Academie
des Sciences Paris 327, 795±801.
Dingle, R.V., Marenssi, S.A., Lavelle, M., 1998. High latitude Eocene
climate deterioration: evidence from the northern Antarctic Peninsula.
Journal of South American Earth Sciences 11, 571±579.
Engelhardt, H., 1891. Ueber TertiaÈrp¯anzen von Chile, Senckenberg Natur-
wissenschaftlichen Gesselschaft Abhandlungen, Band 16 (4), 629±692.
Ehrmann, W.U., Mackensen, A., 1992. Sedimentological evidence for the
formation of an East Antarctic ice sheet in Eocene/Oligocene time.
Palaeogeography, Palaeoclimatology, Palaeoecology 93, 85±112.
Flint, S.S., Prior, D.J., Agar, S.M., Turner, P., 1994. Stratigraphic and
structural evolution of the Tertiary Cosmelli Basin and its relationship
to the Chile triple junction. Journal of the Geological Society London
151, 251±268 (12 ®gures).
Forsythe, R.D., Nelson, E., 1985. Geological manifestations of ridge colli-
sion: evidence from the Golfo de Penas-Taitao basin, southern Chile.
Tectonics 4, 477±495.
Forsythe, R.D., Nelson, E., Carr, M.J., Kaeding, M.E., HerveÂ, M., Mpodo-
zis, C.M., Sof®a, M.J., Harambour, S., 1986. Pliocene near trench
magmatism in southern Chile: a possible manifestation of ridge colli-
sion. Geology 14, 23±27.
Frassinetti, D., Covacevich, V., 2000. Fauna de invertebrados foÂsiles mari-
nos de la FormacioÂn Guadal en Pampa Castillo, sur del Lago General
Carrera, AiseÂn, Chile. BoletõÂn, Servicio Nacional de GeologõÂa y
MinerõÂa, Chile. (in press).
Gorring, M.L., Kay, S.M., Zeitler, P.K., Ramos, V.A., Rubiolo, D., FernaÂn-
dez, M.I., Panza, J.L., 1997. Neogene Patagonian plateau lavas: Conti-
nental magmas associated with ridge collision at the Triple Junction
(46.58 to 49.58S). Tectonics 16, 1±17.
Lagabrielle, I., Bourgois, J., SuaÂrez, M., de la Cruz, R., Garel, E., Dauteuil,
O., Gutscher, M.A., 1999. From compressional to extensional tectonic
regime at the front of the Patagonian Andes, 468±478S: a response to the
subduction of the Chile Spreading Ridge? Fourth ISAG, Gottingen,
Germany, pp. 411±416.
Marshall, L.G., Drake, R.E., Curtis, G.H., Butler, R.F., Flanagan, F.M.,
Naeser, C.W., 1986. Geochtonology of type Santacrucian (Middle
Tertiary) land mammal age, Patagonia, Argentina. Journal of Geology
94, 457±492.
Marshall, L.G., Salinas, P., 1990. Stratigraphy of the RõÂo FrõÂas Formation
(Miocene), along the Alto RõÂo Cisnes, AiseÂn, Chile. Revista GeoloÂgica
de Chile 17, 57±87.
Nelson, N., Forsythe, R., Arit, I., 1994. Ridge collision tectonics in terrane
development. Journal of South American Earth Sciences 7, 271±278.
Niemeyer, H., 1975. GeologõÂa de la regioÂn comprendida entre el Lago
General Carrera y el rõÂo Chacabuco, provincia de AyseÂn-Chile.
Memoria de tõÂtulo (unpublished thesis), University of Chile, Santiago,
Chile, 335pp.
Niemeyer, H., Skarmeta, J., Fuenzalida, R., Espinoza, W., 1984. Hojas
PenõÂnsula de Taitao y Puerto AiseÂn, RegioÂn de AiseÂn del General Carlos
IbanÄez del Campo. Carta GeoloÂgica de Chile N8 60±61, Servicio Nacio-
nal de GeologõÂa y MinerõÂa, Santiago, Chile. p. 80.
Pankhurst, R.J., Weaver, S.D., HerveÂ, F., Larrondo, 1999. Mesozoic±Ceno-
zoic evolution of the North Patagonian Batholith in AyseÂn, southern
Chile. Journal Geological Society of London 156, 673±694.
Pardo-Casas, F., Molnar, P., 1987. Relative motion of the Nazca (Farallon) and
South American Plates since late Cretaceous time. Tectonics 6, 233±248.
Petford, N., Cheadle, M., Barreiro, B., 1996. Age and origin of southern
Patagonian ¯ood basalts, Chile Chico region (46845 0S). Third ISAG, St.
Malo, France, pp. 629±632.
Ramos, V.A., 1989. Andean foothills structures in Northern Magallanes
Basin, Argentina. American Association of Petroleum Geologist Bulle-
tin 73, 887±903.
Ramos, V.A., Kay, S.M., 1992. Southern Patagonian plateau basalts and
deformation: backarc testimony of ridge collisions. Tectonophysics
205, 261±282.
Ray, F.M., 1996. Stratigraphical and structural evolution of Tertiary back-
arc basins in southern Chile. Unpublished PhD thesis, University of
Liverpool, England, 208pp.
SuaÂrez, M., de la Cruz, R., 1996. EstratigrafõÂa y tectoÂnica de la zona sureste
del Lago General Carrera (46830 0±478 Lat.S.), Cordillera PatagoÂnica,
Chile. Actas XIII Congreso GeoloÂgico Argentino y III Congreso de
ExploracioÂn de Hidrocarburos, Acta, I, pp. 425±432.
SuaÂrez, M., de la Cruz, R., Bell, C.M., 1996. EstratigrafõÂa de la regioÂn de
Coyhaique (latitud 458±468S), Cordillera PatagoÂnica, Chile. Actas XIII
Congreso GeoloÂgico Argentino y III Congreso de ExploracioÂn de
Hidrocarburos, I, pp. 575±590.
SuaÂrez, M., de la Cruz, R., 1997. CronologõÂa magmaÂtica de AyseÂn Sur,
Chile (458±48830 0 LS). Actas Octavo Congreso GeoloÂgico Chileno,
Antofagasta, Chile, II, pp. 1543±1547.
SuaÂrez, M., de la Cruz, R., 2000. Tectonics in the eastern central Patagonian
Cordillera (45830 0±47830 0S). Journal of the Geological Society,
London 157, 995±1001.
Troncoso, A., 1992. La tafo¯ora terciaria de QuinamaÂvida (VII RegioÂn
Chile). BoletõÂn Museo Nacional Historia Natural, Santiago de Chile
43, 155±178.
Troncoso, A., Romero, E.J., 1998. EvolucioÂn de las comunidades ¯orõÂsticas
en el extremo sur de SudameÂrica durante el CenofõÂtico. In: Fortunato,
R.Y., Bacigalupo, N., (Eds.), Proceedings of the VI Congreso Latinoa-
mericano de BotaÂnica. Monographs in Systematic Botany from the
Missouri Botanical Garden, 68, pp. 149±172.
M. SuaÂrez et al. / Journal of South American Earth Sciences 13 (2000) 527±536 535
Wolfe, J.A., 1971. Tertiary climatic ¯uctuations and methods of analysis of
Tertiary ¯oras. Paleogeography, Palaeclimatology, Palaeoecology 9,
27±57.
Yoshida, A., 1990. Preliminary notes on the palynological remnants from
mina Ligorio MaÂrquez, Chile Chico, Chile. In: Nishida, M. (Ed.). A
Report of the Paleobotanical Survey to Patagonia, Chile, 1989. Faculty
Science, Chiba University, Japan, pp. 12±15.
M. SuaÂrez et al. / Journal of South American Earth Sciences 13 (2000) 527±536536