Deep-water foreland basin deposits of the Cerro Toro Formation,
Magallanes basin, Chile: architectural elements of a sinuous
basin axial channel belt
STEPHEN M. HUBBARD*, BRIAN W. ROMANS and STEPHAN A. GRAHAM
*Department of Geoscience, University of Calgary, Calgary, Alberta, Canada T2N 1N4 (E-mail:
steve.hubbard@ucalgary.ca)
Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, USA
Associate Editor: John Reijmer
ABSTRACT
Coarse-grained deep-water strata of the Cerro Toro Formation in the Cordillera
Manuel Sen˜oret, southern Chile, represent the deposits of a major channel belt (4
to 8 km wide by >100 km long) that occupied the foredeep of the Magallanes
basin during the Late Cretaceous. Channel belt deposits comprise a ca 400 m
thick conglomeratic interval (informally named the ‘Lago Sofia Member’)
encased in bathyal fine-grained units. Facies of the Lago Sofia Member include
sandy matrix conglomerate (that show evidence of traction-dominated
deposition and sedimentation from turbulent gravity flows), muddy matrix
conglomerate (graded units interpreted as coarse-grained slurry-flow deposits)
and massive sandstone beds (high-density turbidity current deposits).
Interbedded sandstone and mudstone intervals are present locally, interpreted
as inner leve´e deposits. The channel belt was characterized by a low sinuousity
planform architecture, as inferred from outcrop mapping and extensive
palaeocurrent measurements. Laterally adjacent to the Lago Sofia Member are
interbedded mudstone and sandstone facies derived from gravity flows that
spilled over the channel belt margin. A leve´e interpretation for these fine-
grained units is based on several observations, which include: (i) palaeocurrent
measurements that indicate flows diverged (50 to 100) once they spilled over
the confining channel margin; (ii) sandstone beds progressively thin, away from
the channel belt margin; (iii) evidence that the eroded channel base was not very
well indurated, including a stepped margin and injection of coarse-grained
channel material into surrounding fine-grained units; and (iv) the presence of
sedimentary features common to leve´es, including slumped units inferring
depositional slopes dipping away from the channel margin, lenticular sandstone
beds thinning distally from the channel margin, soft sediment deformation and
climbing ripples. The tectonic setting and foredeep architecture influenced
deposition in the axial channel belt. A significant downstream constriction of
the channel belt is reflected by a transition from more tabular units to an internal
architecture dominated by lenticular beds associated with a substantially
increased degree of scour. Differential propagation of the fold-thrust belt from
the west is speculated to have had a major control on basin, and subsequently
channel, width. The confining influence of the basin slopes that paralleled the
channel belt, as well as the likelihood that numerous conduits fed into the basin
along the length of the active fold-thrust belt to the west, suggest that proximal–
distal relationships observed from large channels in passive margin settings are
not necessarily applicable to axial channels in elongate basins.
Sedimentology (2008) doi: 10.1111/j.1365-3091.2007.00948.x
 2008 The Authors. Journal compilation  2008 International Association of Sedimentologists 1

Keywords Architectural elements, Cerro Toro Formation, confined channel–
leve´e complex, deep-water stratigraphy, foreland basin, gravity flow deposits.
INTRODUCTION
The study of uplifted foreland basin fill has been
particularly influential in the development of
classical deep-water facies models, from the bed
scale (Kuenen & Magliorini, 1950; Bouma, 1962),
through the depositional system scale (Mutti &
Ricci Lucchi, 1972; Mutti, 1985) to the basin scale
(Dzulynski et al., 1959; Ricci Lucchi, 1985; Mutti
et al., 2003). More recent studies of deep-water
foreland basin strata, driven in part by the need
for reservoir analogues in the oil and gas industry,
have focused on the effects of basin confinement
on sediment distribution and depositional system
development. Examination of outcropping depos-
its from intraslope minibasins (Shultz & Hubbard,
2005), piggy-back basins (Alexander et al., 1990;
Ricci Lucchi, 1990; Sinclair, 2000; Zelilidis,
2003) and from foredeep settings where sea floor
topography and/or basin morphology imparted an
influence on sediment dispersal (Crabaugh &
Steel, 2004; Pickering & Corregidor, 2005; Hodg-
son et al., 2006), provides insight into the distri-
bution and character of reservoir facies in many
hydrocarbon provinces (e.g. West Africa, Gulf of
Mexico).
Channels represent an important depositional
element in the fill of many deep-water foreland
basins (Cronin et al., 1998; McCaffrey et al., 2002;
Grecula et al., 2003; Shultz et al., 2005); however,
large-scale sinuous channel systems (>2 to 3 km
wide) associated with thick successions of con-
structional overbank sediments have rarely been
interpreted in outcropping foreland basin depos-
its (cf. Mutti et al., 2003). An exception is strata
from the Upper Cretaceous Cerro Toro Formation
of the Magallanes foreland basin that crop out in
the Cordillera Manuel Sen˜oret north of Puerto
Natales, Chile (Fig. 1). These strata have not been
studied in detail since the 1960s (Scott, 1966)
and, as such, they offer an opportunity to evaluate
a large channel–leve´e complex characterized by
the excellent outcrop quality typical of uplifted
foreland basin fill (three-dimensional exposure at
the scale of the depositional system). In the
present study the architectural elements associ-
ated with the coarse-grained facies that accumu-
lated in the axis of the Magallanes basin foredeep
are documented, and a depositional model to
account for the observed sediment distribution
and sedimentary characteristics is developed.
Effects on depositional system behaviour from
confining margins of the narrow axis of the
Magallanes basin are notable; a downstream
constriction (possibly coupled with a change in
slope) of the channel system is interpreted from
the transition of dominantly planar beds to units
characterized by extensive scour, a high degree of
lenticularity and the presence of large-scale bed-
forms and barforms along the length of the
depositional system. Numerous architectural as-
pects of the Cerro Toro Formation outcrops are
analogous to those from strata in various hydro-
carbon-bearing provinces where oil and gas are
being sought (Kolla et al., 2001; Abreu et al.,
2003; Samuel et al., 2003; De Ruig & Hubbard,
2006).
BACKGROUND GEOLOGY
The Magallanes foreland basin is an elongate,
north- to south-oriented trough located adjacent
to the Patagonian Andes, the crustal loading of
which was responsible for its genesis (Wilson,
1991). The basin has an oceanic back-arc heritage
(Rocas Verdes basin) that was first initiated in the
region during the latest Jurassic to Early Creta-
ceous by rifting associated with the break up of
Gondwana (Dalziel, 1981; Wilson, 1983, 1991).
Strata of the Jurassic Tobı´fera Formation, charac-
terized by volcaniclastic sedimentary units and
rhyolitic volcanic rocks, as well as thin-bedded
shallow marine sandstone and mudstone of the
Zapata Formation, record deposition in the back-
arc setting (Wilson, 1991; Fildani & Hessler,
2005); oceanic crust, formed in association with
the development of this basin, is preserved in the
Sarmiento ophiolite complex that is present in
the interior portion of the adjacent fold-thrust belt
(Dalziel et al., 1974; Dalziel, 1981). Initiation of
the Andean orogeny and associated fold-thrust
belt development spawned the transition from a
back-arc to foreland basin setting (Dalziel, 1986;
Wilson, 1991). Sedimentologically, this transition
was recorded by deposition of turbiditic strata of
the Punta Barrosa Formation (Fig. 1C; Wilson,
1991; Fildani & Hessler, 2005). Deep-water con-
ditions persisted in the Magallanes foreland basin
for a period of approximately 20 Myr, through
deposition of the Cerro Toro and Tres Pasos
Formations (Fig. 1C; Natland et al., 1974; Wilson,
2 S. M. Hubbard et al.
 2008 The Authors. Journal compilation  2008 International Association of Sedimentologists, Sedimentology

1991; Fildani et al., 2003). Upward shallowing
during the Late Cretaceous and Tertiary is
recorded in deposits of the Dorotea Formation
(Katz, 1963). These strata were incorporated in
the thrust-belt during the Cenozoic, and the
excellent outcrop exposures reflect the combina-
tion of the geometry of open folds and Late
Cenozoic glaciation.
Cerro Toro Formation: palaeogeographic
setting and previous studies
Sediments of the Cerro Toro Formation accumu-
lated in a narrow foredeep constrained by the
Andean thrust-front to the west, and the South
American craton to the east (Ramos, 1989; Wil-
son, 1991). Upper Cretaceous shallow-marine and
non-marine strata equivalent to the Cerro Toro
and Tres Pasos Formations have been identified
60 to 90 km north of the study area in Argentina;
a southward prograding delta is inferred to have
supplied sediment into the axial trough of the
deep-marine basin (Macellari et al., 1989). To the
south, the basin extends for at least hundreds of
kilometres, as evidenced by deep-water conglo-
merate beds roughly equivalent to those of the
Cerro Toro Formation on Tierra del Fuego (Dott
et al., 1982).
Coarse-grained sediments were focused within
an immense channel belt that was present along
the axis of the Magallanes basin foredeep (Fig. 2).
Gravel and sand originating from the actively
uplifting Andean fold-thrust belt to the west
(Cecioni, 1957; Zeil, 1958) were probably
A B
C
Fig. 1. Study area overview.
(A) Landsat image of the Ultima
Esperanza District in southern
Chile. Red areas denote locations
where Cerro Toro Formation
conglomerate (Lago Sofia Member)
is exposed. Inset map shows
location of the study area in
southern South America.
(B) Detailed map of the Cordillera
Manuel Sen˜oret, including the
locations where stratigraphic
sections were measured (black
circles). Shaded grey areas
correspond to topographic contours
(CI = 250 m); the peaks of Cerro
Castillo, Cerro Campana, and Cerro
Mocho are between 1000 and
1100 m in elevation. (C) Strati-
graphy of the Magallanes foreland
basin, with the detailed strati-
graphic architecture of the middle
part of the Cerro Toro Formation
shown on the right.
Sinuous deep-water foreland basin axial channel, Chile 3
 2008 The Authors. Journal compilation  2008 International Association of Sedimentologists, Sedimentology

transported to the basin via the deltaic system to
the north (Macellari et al., 1989), and a series of
conduits that cut across the western basin slope
(Fig. 2; Crane & Lowe, 2001). Conglomerate and
sandstone of the Cerro Toro Formation presently
crop out in a north- to south-oriented outcrop belt
>100 km in length, extending from the Chile–
Argentina border in the north, to as far south as
Cerro Rotonda, south of Puerto Natales (Fig. 1A).
The Cerro Toro Formation outcrop belt has
been most extensively studied at the Silla Syn-
cline (Fig. 1A), where tectonic folding has re-
sulted in the three-dimensional exposure of
channel strata. The excellent exposure quality,
coupled with the relatively easy accessibility of
the area (located in the Torres del Paine National
Park), initially enticed sedimentologists to the
area (Scott, 1966; Winn & Dott, 1979). The study
of Winn & Dott (1979) established that the
formation represented the deposits of a deep-
water channel–leve´e complex. In the 1990s,
driven by the prospect of producing oil and gas
from analogous channel–leve´e complex deposit
reservoirs, industry scientists and academic
researchers began visiting the Silla Syncline to
map the stratigraphic architecture of the Cerro
Toro Formation (DeVries & Lindholm, 1994;
Beaubouef et al., 1996; Coleman, 2000; Crane &
Lowe, 2001; Beaubouef, 2004; Crane, 2004).
With the focus in the Silla Syncline area, much
of the channellized strata in the formation has
been neglected elsewhere over the last three
decades. Scott (1966) undertook the most exten-
sive study of the formation to date, analysing the
Cerro Toro Formation from Cerro Benitez just
north of Puerto Natales, to north of Laguna Azul
(Fig. 1). Winn & Dott (1977, 1979) revisited some
of the outcrops examined by Scott, but focused
much of their efforts on the Silla Syncline and on
some immense conglomeratic dunes located just
east of Lago Sofia (Fig. 1). A conglomeratic intru-
sion complex present on the southern shore of
Lago Sofia (Fig. 1B), first recognized by Scott
(1966), was the focus of a doctoral study by
Schmitt (1991). Hubbard et al. (2007b) recognized
the analogous nature of these features to large-
scale intrusions that emanate from the margins of
channel bodies in Palaeogene strata of the North
Sea basin. A comprehensive assessment of the
sedimentology and stratigraphic architecture of
the Cerro Toro Formation in the Cordillera
Manuel Sen˜oret (Fig. 1B), the primary goal of
the present study, was not attempted previously.
A conglomeratic member >400 m in thickness
is prevalent in the Cerro Toro Formation at the
Cordillera Manuel Sen˜oret (Fig. 1C; the infor-
mally named Lago Sofia Member of Katz, 1963);
this interval consists of the deposits of the basin
axial channel belt. This member is encased in
thin-bedded bathyal mudstone and sandstone
(1000 to 2000 m water depth; Natland et al.,
1974); in part of channel overbank affinity
(cf. Winn & Dott, 1979; Beaubouef, 2004). The
entire formation has a cumulative thickness of
approximately 2000 m. The conglomeratic Lago
Sofia Member is immediately underlain by a
series of lenticular sandstone beds over a strati-
graphic thickness of <100 m (Fig. 1C). In the
southern part of the study area, three conglomer-
atic channelform bodies are present 150 to 200 m
below the main Lago Sofia Member (Hubbard
et al., 2007b); the largest of these bodies is ca
400 m wide and 75 m thick. At its upper contact,
the Lago Sofia Member grades from conglomerate
through a 100 to 200 m thick section of thick-
bedded to thin-bedded, sand-dominated turbi-
ditic beds, into bathyal mudstone-dominated
deposits (Fig. 1C).
Cenozoic uplift of the Cerro Toro Formation in
the Cordillera Manuel Sen˜oret was associated
with structural deformation of the deposits stud-
Fig. 2. Simplified palaeogeographic and palaeotec-
tonic reconstruction of the Magallanes foreland basin
during deposition of the Cerro Toro Formation (modi-
fied from Wilson, 1991 and Hubbard et al., 2005).
Sedimentation of coarse material, including gravel, was
centralized within a basin axial channel belt that flo-
wed southward for hundreds of kilometres along the
basin foredeep. Sediment fed into this channel system
from conduits to the north, as well as from various
point sources to the west.
4 S. M. Hubbard et al.
 2008 The Authors. Journal compilation  2008 International Association of Sedimentologists, Sedimentology

ied. Over most of the study area, gentle folds and
low-offset faults are typical; their impact on the
stratigraphic architecture of the formation is
readily mapped and, therefore, they do not hinder
the interpretations made in this study. The Lago
Sofia Member was incorporated into thin-skinned
thrust faulting at the western ends of Cerro
Campana and Cerro Ventana (total shortening
observed is at least 1Æ5 km; see Fig. 1B for loca-
tion). This tectonic shortening has been factored
into palaeogeographic interpretations.
ARCHITECTURAL ELEMENTS
Hierarchical architectural element schemes were
originally used to document and characterize
various scales of sedimentological observation
in non-marine deposits (Brookfield, 1977; Ko-
curek, 1981; Miall, 1985); an architectural ele-
ment approach has proven useful for the
description and interpretation of deep-water
strata (Ghosh & Lowe, 1993; Pickering et al.,
1995; Gardner & Borer, 2000; Hickson & Lowe,
2002; Anderson et al., 2006). This study employs
the approach first defined by Ghosh & Lowe
(1993), where first-order architectural elements
are considered to represent individual sedimen-
tary structure divisions within a gravity-flow
deposit (e.g. the plane laminated Tb division in
the Bouma (1962) sequence or the massive S3
division of the Lowe (1982) sequence; Fig. 3).
Second-order elements represent individual grav-
ity flow deposits (sedimentation units), and third-
order elements represent the groupings of similar
sedimentation units (equivalent to lithofacies;
Fig. 3). Regularly recurring groups of genetically
related lithofacies are fourth-order architectural
elements. The bounding surfaces of fourth-order
elements separate distinctive geometrical sedi-
mentary bodies (e.g. channels). The approach of
Ghosh & Lowe (1993) is favoured for this study
because, although the outcrops studied are exten-
sive and well-exposed, the immense scale of the
depositional system and associated sedimenta-
tion units precludes dividing architectural ele-
ments using a strict geometric basis. Because of
the difficulty in identifying large geometric fea-
tures (individual sediment bodies in the order of
up to 8 km wide), it is advantageous to employ a
hierarchical scheme that emphasizes and builds
upon the facies information. Bounding surfaces
and sediment body geometry are documented and
interpreted where identifiable, especially at the
margins of the Lago Sofia Member.
In this paper, five third-order architectural ele-
ment types are described and interpreted, defined
primarily on the basis of their lithology or internal
character: (i) sandy matrix conglomerate (IIIscg);
(ii) muddy matrix conglomerate (IIImcg); (iii)
thick-bedded sandstone (IIIss); (iv) thin-bedded
sandstone and mudstone (IIIsm); and (v) chaotic
units (IIIch) (Table 1). In addition, four fourth-
order architectural elements are recognized, asso-
ciated with deposition in the channel belt thalweg,
and a fifth is recognized in out-of-channel strata
(Table 1). The genetic relationship of the various
fourth-order elements is discussed in context as a
fifth-order architectural element (the complete
succession of channel belt deposits).
Outcrops of deep-water deposits, such as those
in the Magallanes basin, are often utilized as
analogues to deposits imaged seismically in the
sub-surface (Coleman et al., 2000; Fugelli &
Olsen, 2005; Hubbard et al., 2005). It is important
to consider that the fourth and fifth orders of
Fig. 3. Overview of the architectural element scheme utilized in this study. First-order elements represent the
smallest scale of observation (e.g. individual turbidite divisions); the scheme is open-ended upwards, with the
largest order of observation considered in this study represented by that of the entire basin axial channel belt
stratigraphic succession (fifth-order element). Note that the fourth-order elements shown are 50 to 60 m thick.
Sinuous deep-water foreland basin axial channel, Chile 5
 2008 The Authors. Journal compilation  2008 International Association of Sedimentologists, Sedimentology

observation in the Ghosh & Lowe (1993) archi-
tectural element scheme represent the smallest
scale of observation most likely to be resolvable in
even the highest resolution conventional indus-
try-standard seismic data sets. The composite
three-dimensional bodies that fourth-order and
fifth-order architectural elements comprise corre-
spond to the scale of ‘depositional elements’
(Mutti & Normark, 1987, 1991), which are com-
monly used to characterize deep-water strata in
modern, high-resolution seismic reflection data
sets (Posamentier & Kolla, 2003; De Ruig &
Hubbard, 2006).
Third-order architectural elements
(lithofacies)
Sandy matrix conglomerate (IIIscg)
Clast-supported conglomerate characterized by a
sandy matrix (IIIscg) represents a significant com-
ponent (ca 50% to 60%) of the Lago Sofia Member
in the study area (Fig. 4A to D). Sedimentary
bodies composed of these units are up to 80 m
thick, and are lenticular along depositional strike
over 1 to 5 km. Individual units of IIIscg are highly
variable, with erosional, planar to undulating
bases (relief up to 10 m locally), and thicknesses
ranging from 10 cm to 10 m. Clasts are well-
rounded and poorly to well-sorted; clasts are as
large as 32 cm in diameter, although maximum
clast size is in the range of 10 to 20 cm for most
beds. Beds <3 m thick are normally graded in
some instances and commonly associated with a
thin overlying sandstone bed < )20 cm thick
(Fig. 4A); inversely graded basal layers < )30 cm
thick (traction carpets) are present locally
(Fig. 4B). Commonly, however, IIIscg beds are
dominated by plane lamination, cross-stratifica-
tion and/or widespread imbrication of clasts
(Fig. 4C and D). Dunes up to 4 m high are present
locally (Winn & Dott, 1977). Often, flame and load
structures characterize bed contacts, particularly
where conglomerate overlies a sandstone bed.
Sole markings, including flute casts, are rarely
exposed. Mudstone rip-up clasts are moderately
abundant and trace fossils are absent.
Sandy matrix conglomerate sedimentation units
were deposited through suspension sedimenta-
tion, or through traction sedimentation beneath
immense high-density turbidity currents (cf.
Lowe, 1982). Variability in bed thickness is most
often related to differential scouring at the base of
turbidity currents. Sandy matrix conglomerate is
commonly associated with muddy matrix con-
glomerate (IIImcg) and lenses of sandstone (IIIss).T
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6 S. M. Hubbard et al.
 2008 The Authors. Journal compilation  2008 International Association of Sedimentologists, Sedimentology

Muddy Matrix Conglomerate (IIImcg)
Conglomeratic units with poorly sorted matrix
characterized by a high mud or argillaceous
component (IIImcg) are an important constituent
(ca 30% to 45%) of the Lago Sofia Member
(Fig. 4E and F). In the Cordillera Manuel Sen˜oret,
these beds typically show clast-supported bases
and matrix-supported tops (Fig. 4F). The transi-
tion from clast-support to matrix-support can be
abrupt, or gradual over the entire bed thickness.
Successions of IIImcg sedimentation units can be
traced for hundreds to thousands of metres
laterally, and are commonly 35 to 80 m thick.
Basal contacts of individual units are flat or
nearly flat (< )5 m relief). Clasts are similar to
those in IIIscg, although they tend to be slightly
larger on average in the muddier deposits
(maximum clast size observed is 45 cm). Along
with an upward decrease in clast percentage,
normal grading of maximum and average clast
size, as well as maximum grain size in the poorly
sorted matrix are observed (cf. Crane, 2004;
Fig. 4. Third-order architectural elements of the Cerro Toro Formation. (A) Normally graded turbiditic conglomerate
sedimentation unit (IIIscg). Divisions on staff are 10 cm long. (B) Inversely graded base of sandy matrix conglomerate
bed (IIIscg). Sandy matrix conglomerate beds (IIIscg) were dominantly deposited by traction, characterized by clast
imbrication. Lens cap for scale is 58 cm in diameter. (C) Plane lamination and (D) large-scale cross-stratification.
Arrow in (C) points to imbricated clasts. Hammer head for scale in (D) is 20 cm long. (E) Rafted sediment block
characterized by overturned internal lamination mixed with muddy matrix-supported conglomerate at the top of a
slurry bed (IIImcg). Staff is 1Æ5 m long. (F) Thick muddy matrix conglomeratic bed characterized by clast-support at
the base and matrix-support upwards (IIImcg; slurry-flow deposit). Person for scale is 170 cm tall.
Sinuous deep-water foreland basin axial channel, Chile 7
 2008 The Authors. Journal compilation  2008 International Association of Sedimentologists, Sedimentology

Hubbard & Shultz, 2008). Large sub-rounded
intrabasinal mudstone clasts (raft blocks up to
5 m in diameter) are characteristic of the upper
parts of the sedimentation units (Fig. 4E), often in
states of partial disaggregation. In rare instances,
thin inversely graded basal layers (traction car-
pets) and flute casts on bed bases are present.
Primary physical sedimentary structures are
absent in IIImcg. However, deeply penetrating
(up to 7 m) sandstone-filled trace fossils (shafts
<1 cm in diameter) are present locally (Glossi-
fungites ichnofacies, including Diplocraterion,
Arenicolites and Skolithos) (Hubbard & Shultz,
2008).
Various researchers have contemplated the
origin of graded muddy matrix conglomeratic
sedimentation units in the Cerro Toro Formation,
as it is apparent that they are not characteristic
deposits of debris flows or turbidity currents
(cf. Middleton & Hampton, 1976; Lowe, 1982). As
the units are characterized by evidence of cohe-
sional and turbulent clast-support, they are
perhaps best classified as slurry flow deposits
(cf. Lowe & Guy, 2000; Crane, 2004). Winn & Dott
(1979) first proposed three possible mechanisms
for the development of these units: (i) competence
of the gravity flow decreased as it evolved,
resulting in the settling of large clasts to a
traction-dominated bed-load (cf. Hampton,
1975); (ii) flows started off turbulently and depos-
ited the basal clast-supported zone, transforming
into flows with more cohesive strength because of
the incorporation of mud; and (iii) turbidity
currents were generated from (and outran) paren-
tal debris flows (cf. Hampton, 1972). Sohn et al.
(2002) studied these units in the region of Torres
del Paine (Fig. 1A) and favoured an origin similar
to the third hypothesis discussed by Winn & Dott
(1979): that debris flows were diluted as they
moved across the basin floor through detachment
and disintegration of an overhanging (hydroplan-
ing) debris flow snout (cf. Mohrig et al., 1998).
Sohn et al. (2002) suggested that the proportion of
IIImcg deposits would decrease down-system as a
result of this transformation; however, this is not
the case as muddy matrix conglomerate deposits
are, in fact, quite abundant down depositional
system from where Sohn et al. (2002) focused
their research (Crane, 2004). Winn & Dott (1979)
discounted their first hypothesis for beds charac-
terized by fluted bases; traction carpets and
normal grading of clasts and matrix material also
support the notion that turbulence was an impor-
tant factor during sedimentation (Crane, 2004).
The abundant mud rafts noted in the upper parts
of these sedimentation units suggest that the
transformation of these gravity flows took place
because of the addition of eroded mud from
channel (or canyon) margins, marginal basin
slopes and from the channel floor (Winn & Dott,
1979; Hubbard et al., 2007b).
Thick-bedded sandstone (IIIss)
Thick-bedded sandstone is an important third-
order architectural element in the Lago Sofia
Member (ca 5% to 10% of interval in places), and
the surrounding fine-grained strata locally
(Fig. 5A to C). Packages of this element 10 to
100 m in thickness are laterally mappable over
distances of < 1 to 2 km. Sedimentation units are
typically identifiable in the field, 25 to 100 cm
thick (maximum 4 m), ranging from highly len-
ticular to planar structures. Mudstone beds
(<4 cm to 1 m thick) are rarely associated with
IIIss within the Lago Sofia Member, as sandstone
units are often amalgamated; mudstone layers are
more common, however, where IIIss is present in
the fine-grained succession that surrounds the
Lago Sofia Member. In some instances, lenticular
beds comprise channelform bodies up to 400 m
wide and 40 m thick in the fine-grained strata
lateral to Lago Sofia conglomerate. Normal grad-
ing of the sandstone beds (medium-grained to
coarse-grained at the base to fine-grained and very
fine-grained towards the top) is common, with
thin inversely graded layers (<5 cm thick) at the
bases of units locally (traction carpets). Sandstone
matrix-supported pebble and granule lags up to
20 cm thick, consisting of disorganized or trac-
tion-structured extrabasinal clasts, and mudstone
rip-up clasts are common. Sedimentation units
are dominantly structureless, locally character-
ized by dish structures or fluid escape pipes
(Fig. 5A), or an upward systematic change in
traction structures (Bouma sequence; Fig. 5B).
Cross-stratification is abundant locally. In some
beds, alternation of Bouma divisions indicates
that surging gravity flows were common. Soft
sediment deformation, flame structures along
intra-sedimentation unit planes and at bed
boundaries, load structures and climbing ripples
are each important locally (Fig. 5C). Sole marks,
including flute casts and tools, are commonly
observed on the bases of overhanging beds. Trace
fossils are rarely present, including Palaeophycus,
Planolites, Ophiomorpha and Teichichnus.
Beds of IIIss were deposited from high-density
and low-density turbidity currents. Structureless
beds represent the suspension fall-out from col-
lapsing high-density flows (S3 divisions of Lowe,
8 S. M. Hubbard et al.
 2008 The Authors. Journal compilation  2008 International Association of Sedimentologists, Sedimentology

Fig. 5. Third-order architectural elements of the Cerro Toro Formation. (A) Thick sandstone unit (IIIss) characterized
by abundant fluid escape pipes (arrows). Divisions on staff are 10 cm long. (B) Turbidity current deposit (IIIss) with a
thick, massive lower division, a distinctive break (arrow), and an overlying Bouma sequence (dominated by plane
laminations and ripples). Lens cap for scale in (B) and (C) is 58 cm in diameter. (C) Low-density turbidity current
deposit characterized by soft-sediment deformation (IIIss). (D) Thin-bedded turbidites consisting of fine-sandstone
and siltstone beds <5 cm thick (IIIsm) with an overlying scour surface characterised by angular mudstone rip-up
clasts (arrows). (E) Overturned slump deposit (IIIch) associated with interbedded sandstone and mudstone (arrow
points to 1Æ5 m long Jacob staff for scale). (F) Large-scale slump in the conglomeratic Lago Sofia Member. Slumping
was to the right (south); dashed lines highlight laminae within the deposit. Person for scale is 195 cm tall.
Sinuous deep-water foreland basin axial channel, Chile 9
 2008 The Authors. Journal compilation  2008 International Association of Sedimentologists, Sedimentology

1982); traction carpets and cross-stratification are
also consistent with the interpretation that depo-
sition was from high-density turbidity currents
(S2 and S1 divisions of Lowe, 1982). The Bouma
(1962) sequence is indicative of waning, low-
density turbulent flows; beds characterized by
thick structureless intervals, overlain with a 20 to
30 cm thick traction structured layer, record
deposition from the high-density front of a
turbulent flow followed by sedimentation from
the low-density turbidity current tail (Fig. 5B).
Thin-bedded sandstone and mudstone (IIIsm)
Although thin-bedded sandstone and mudstone
is the most abundant third-order architectural
element in the Cerro Toro Formation (Fig. 5D), it
is almost completely absent from the Lago Sofia
Member. The percentage of sandstone beds in this
element is highly variable; the sandstone:mud-
stone ratio is highest (0Æ3 to 0Æ6) proximal to the
Lago Sofia Member, both laterally, as well as
above and below. Sandstone beds are usually
tabular and <20 cm thick (typically 4 to 8 cm),
although some beds are lenticular over tens of
metres, up to 1 m thick, and characterized by
locally erosive bases. Partial to complete Bouma
sequences (ripples and plane lamination) are
characteristic of the normally graded (upper
fine-grained to very fine-grained sandstone and
silt) beds, although these divisions are often
distorted by soft sediment deformation. Climbing
ripples are common to IIIsm in some localities.
Organic detritus and mudstone rip-up clasts are
locally abundant, particularly in lenticular beds.
Flute casts and tool marks are observed on bed
bases. Evidence for biogenic reworking is
moderate to abundant, the trace fossil suite
consisting of Alcyonidiopsis, Arenicolites, Chon-
drites, Gyrolithes, Helminthoida, Helminthopsis,
Ophiomorpha, Palaeophycus, Phycosiphon,
Planolites, Scolicia, Skolithos, Spirophyton,
Thalassinoides and Zoophycos.
Thin-bedded sandstone and mudstone repre-
sents deposits of low-density turbidity currents
and hemipelagic suspension settling, predomi-
nantly in regions outside the main axial channel
belt thalweg in the Cerro Toro Formation. Chaotic
beds with deformed internal architecture (IIIch)
are commonly associated with IIIsm.
Chaotic units (IIIch)
Chaotically bedded, or distorted units (IIIch)
represent a volumetrically small, yet sedimento-
logically significant component of the Cerro Toro
Formation, both in the Lago Sofia Member
(Fig. 5F) and associated with the fine-grained
deposits which encase it (Fig. 5E). IIIch in the
Lago Sofia Member is highly irregular in thick-
ness, up to a maximum of 8 m and the basal
contact is planar. The internal architecture of
units is dominated by distorted and overturned
primary laminations (Fig. 5F). IIIch interbedded
with packages of the fine-grained IIIsm are com-
monly <3 m thick, although the maximum
observed thickness is 10 to 15 m. A distorted
internal architecture, as well as the presence of
overlying lenticular sandstone beds (IIIsm or
IIIss), is characteristic of these units.
Units of IIIch represent remobilized sedimen-
tary layers generated from slumping and sliding
on sloped surfaces (in the Lago Sofia Member and
in surrounding fine-grained deposits), as well as
large-scale rafted sediment blocks (preserved
preferentially in the fine-grained succession asso-
ciated with IIIsm). Slump-generated topography
on the sea floor influenced the distribution of
sediment from subsequent gravity flows, as evi-
denced from the presence of lenticular sandstone
beds on top of slumped units locally (Fig. 5E).
Fourth-order and fifth-order architectural
elements
It is at the scale of fourth-order elements that the
depositional system architecture can start to be
defined, as the lateral and vertical relationships
between the various lithofacies (i.e. third-order
elements) and the geometrical bodies that they
comprise are considered (Fig. 3). The types of
fourth-order architectural elements associated
with deposition in the Lago Sofia conglomerate
(axial channel belt thalweg) include: (IV1) –
relatively planar-bedded packages dominated by
sandy matrix conglomerate (IIIscg with subordi-
nate IIImcg, IIIss and IIIch); (IV2) – relatively
planar-bedded packages dominated by muddy
matrix conglomerate (IIImcg with subordinate
IIIscg and IIIss); (IV3) – randomly stacked con-
glomerate and sandstone (IIImcg, IIIscg and IIIss)
associated with significant scour; and (IV4) –
intervals dominated by thick-bedded sandstone
(IIIss, and lesser IIImcg and IIIscg; Table 1).
Individual genetic sedimentary bodies, most
commonly that of a channel fill in the study area
(Fig. 3), are typically comprised of a single fourth-
order element. One fourth-order element type is
recognized in out-of-channel strata (IV5), charac-
terized by an abundance of IIIsm, and sporadic
occurrences of IIIch and thick accumulations of
IIIss. Through the collective consideration of
10 S. M. Hubbard et al.
 2008 The Authors. Journal compilation  2008 International Association of Sedimentologists, Sedimentology

fourth-order elements, the sedimentary processes
and facies distribution of the axial channel belt
strata as a whole (fifth-order element) are as-
sessed.
Lago Sofia Member conglomerate architectural
elements
IV1, IV2 and IV3 dominate the conglomeratic
succession in the Cordillera Manuel Sen˜oret. In
the northern and central part of the study area,
associations IV1 and IV2, consisting of relatively
planar packages of primarily sandy conglomerate
and primarily muddy conglomerate, respectively
(each 40 to 80 m thick), systematically alternate
through the stratigraphic column (Fig. 6). Evi-
dence of deep scour and extensive erosion is
largely absent, in a region where the Lago Sofia
conglomerate is at least 8Æ5 km wide (in an east–
west orientation, perpendicular to the basin axis;
Fig. 1A).
A B
Fig. 6. Fourth-order architectural elements IV1 and IV2 in the Lago Sofia Member conglomerate. (A) Outcrop photo-
graph of stacked sedimentation units of IIImcg overlain by a thick IIIscg bed (arrow points to person for scale –
160 cm tall). (B) Stratigraphic cross-section through the north–north central part of the study area showing the
alternating packages of dominantly sandy matrix conglomerate (IV1) and muddy matrix conglomerate (IV2) charac-
teristic of the channel belt thalweg in the area. The datum utilized is a horizon characterised by distinctive trace
fossils of the Glossifungites ichnofacies, which can be correlated over much of the channel belt axis. See Fig. 1B for
section locations. Legend at lower left consists of symbols and lithologic fill colours used in each of the measured
sections depicted in the paper.
Sinuous deep-water foreland basin axial channel, Chile 11
 2008 The Authors. Journal compilation  2008 International Association of Sedimentologists, Sedimentology

The first researchers to examine the formation
in this area interpreted a glacio-fluvial channel
origin for the conglomerate beds (IIIscg) (Cecioni,
1957); indeed, the dominance of traction struc-
tures in sandy conglomerate makes a braided
fluvial channel interpretation understandable. In
fact, the sedimentary processes that shaped these
layers were probably not too far from what
Cecioni (1957) interpreted; given the era in which
the strata were examined, it would have been
difficult to imagine that these processes were
possible at the bottom of an ocean basin 1 to 2 km
deep. The abundance of low-angle cross-lamina-
tion, imbricated clasts indicating a range of
palaeoflow directions (45 to 90 spread com-
monly), erosional incision with low to moderate
levels of relief, and lateral pinch-out and inter-
fingering of conglomerate with sandstone units,
are all typical of braid bars from fluvial systems
(Schlee, 1957; Smith, 1974; Miall, 1992). The
interpretation of braided channel characteristics
or morphology in the deep-sea, or in ancient
deep-sea deposits, is widespread (Hein & Walker,
1982; Beldersen et al., 1984; Piper & Kontopou-
los, 1994; Harrison & Graham, 1999), and is used
to explain the depositional nature of thick pack-
ages of traction-structured IIIscg units in the Cerro
Toro Formation.
Thick layers of IIImcg in IV2 (Fig. 6) are distinct
from the stratified intervals of IIIscg in IV1, and
their formation involved a distinctive set of
sedimentological processes. Alternating periods
where fine-grained material is more, or less,
available to the depositional system are not easily
explained, but are probably important in under-
standing the observed stratigraphic architecture
(Fig. 6). Relative sea-level lowering (tectonically
or eustatically induced) may starve the basin of
coarse-grained material periodically (cf. Posa-
mentier et al., 1991), allowing the build up of an
extensive mud and silt drape that is subsequently
reworked (into muddy conglomeratic flows)
when gravel is re-introduced to the basin. How-
ever, thick intervals of fine-grained concordant
beds are not extensively preserved in the thalweg
of the axial channel belt (Lago Sofia Member).
The lowering of sea-level has also been observed
to destabilize the upper slope, instigating mass-
wasting of fine-grained material onto the basin
floor (Posamentier et al., 1991). Again, large-scale
mass-transport complex deposits are not pre-
served in the Lago Sofia Member to support such
a hypothesis; however, associated chaotic beds
(IIIch) are present in laterally equivalent, out-of-
channel deposits (Fig. 5E). Sporadic tectonic
activity in the Andean catchment area could have
been responsible for variable sedimentary input
into the basin; however, conclusive evidence to
support such a theory has not been observed.
Trace fossils of the Glossifungites ichnofacies
characterize the top of one of the thick, muddy
matrix conglomerate-dominated intervals (surface
identified in Fig. 6). Hubbard & Shultz (2008)
indicate that the amount of time necessary to
develop the burrowed horizon is not insignificant
and that it probably represents an important
discontinuity. Despite this, a cause for the alter-
nation of muddy (IV2) and sandy conglomerate
units (IV1) in the northern part of the study area
remains uncertain, and it is possible that an
external forcing mechanism is not responsible for
the observed stratigraphic stacking pattern (Smith
et al., 2005).
Palaeocurrent measurements indicate that the
channel belt flowed in a southerly direction
(Scott, 1966; Winn & Dott, 1979). Downstream,
in the southern part of the study area at Cerro
Benitez (Fig. 1A), the internal architecture of the
Lago Sofia Member is significantly different than
it is to the north. The widespread, tabular sedi-
mentary packages of IV1 and IV2 are replaced by
more randomly stacked conglomerate (IIIscg and
IIImcg) and sandstone beds (IV3; Fig. 7); scours
are much more prevalent and traction structures
in sandy matrix conglomerate beds are distinc-
tive. Specifically, the large-scale dunes identified
by Winn & Dott (1977) are nearly exclusive to the
southern part of the study area; larger-scale
barforms in the order of 15 m high are also
present (Fig. 7). Coincidently, the width of the
Lago Sofia Member conglomerate outcrop nar-
rows considerably to <5 km (Fig. 1A). The study
interprets that the narrowing of the outcrop belt
directly reflects the narrowing of the channel belt.
The decrease in channel belt width is quite
possibly responsible for the observed changes in
the downstream sedimentologic and stratigraphic
architecture of the channel strata, implying that at
least some of the gravity flows that passed
through the channel belt felt both margins of the
4 to 8 km wide constriction. Alternatively, the
change in channel belt width was associated with
a steepening of the depositional slope. Flow non-
uniformity associated with downstream constric-
tion (and/or slope variation) along the channel
system resulted in an increase in velocity, which
was closely linked to an increase in erosion and
scouring (cf. Gee et al., 2001; Gee & Gawthorpe,
2006). When evaluating Quaternary sediments on
the Monterey Fan, Fildani & Normark (2004)
12 S. M. Hubbard et al.
 2008 The Authors. Journal compilation  2008 International Association of Sedimentologists, Sedimentology

made a similar observation which was attributed
to flow non-uniformity; along the length of a
channel system, slope gradient changes related to
local sea floor topography resulted in the devel-
opment of a series of ponded areas (characterized
by tabular turbiditic sediment) connected by
erosional channel conduits.
Architectural element IV4 is dominated by
thick successions of turbiditic sandstone beds
(IIIss) within the Lago Sofia Member of the Cerro
Toro Formation (Fig. 8). It is most prevalent at the
top of the conglomeratic sequence (Fig. 1C),
indicating an overall waning of deposition in
the axial channel belt. Localized lenses of IV4
<20 m thick are also present in numerous strati-
graphic levels throughout the Lago Sofia Member,
commonly associated with channelform bodies
20 to 300 m in cross-sectional width.
Regional channel belt observations and
interpretations
The downstream decrease in channel belt width
had a profound effect on the internal stratigraphic
architecture of the Lago Sofia Member, as evi-
denced from the change from dominance of IV1
and IV2 in the north to that of IV3 in the south.
Mapping the present outline of channel belt
conglomerate outcrops in the Cordillera Manuel
Sen˜oret shows that sediment distribution is con-
tained within a belt that not only narrowed
southward, but was also characterized by a
sinuous planform (Fig. 9). To evaluate whether
this mapped sediment distribution is not partially
a function of fortuitous Quaternary glacial ero-
sion, a vast number (n > 2500) of palaeocurrent
indicator measurements (from clast imbrications,
flute casts, tool marks, ripples and dune foresets)
were taken in the field area. Including only
measurements from an interval from 100 to
300 m above the base of the thick, Lago Sofia
Member conglomerate, rose diagrams are shown
to document downstream variability in average
palaeocurrent vector from various locations in the
channel belt (Fig. 9). In every case, the mean
palaeoflow vector roughly parallels the sinuous
channel belt margins defined by outcrop mapping
of the Lago Sofia Member, suggesting that the
mapped planform geometry reflects the architec-
ture of the depositional system. Palaeocurrent
measurements and mapping of previous authors
confirm this relationship (Fig. 9), and also allow
for a more regional palaeogeographic reconstruc-
tion of the channel belt. In particular, measure-
ments of Scott (1966) and Crane (2004) in the Silla
Syncline, and Hubbard et al. (2007a) at Sierra del
A
B
C
Fig. 7. Fourth-order architectural element IV3 in the Lago Sofia Member conglomerate. (A) Photograph and (B) line-
drawing trace of the complicated internal architecture of the channel belt in the southern part of the study area
(location is Cerro Benitez; Fig. 1B). Numerous scours and a large-scale barform (identified in part B) are present in
this element, and the stacking of sedimentary units is not ordered (C), in contrast to architectural elements IV1 and
IV2. Note that the legend for features in the measured section is shown in Fig. 6.
Sinuous deep-water foreland basin axial channel, Chile 13
 2008 The Authors. Journal compilation  2008 International Association of Sedimentologists, Sedimentology

Toro suggest that two tributaries fed the axial
channel belt from the NNW (Fig. 9). Although the
data shown from the Silla Syncline and Sierra del
Toro are from an apparently comparable strati-
graphic level (within the coarse-grained succes-
sion) to that plotted from the Cordillera Manuel
Sen˜oret, the correlation of units between loca-
tions is difficult because of extensive glacial
erosion of the formation (particularly in the area
of Lago Toro); the basin-scale channel belt outline
presented in Fig. 9 (where defined by dashed
lines) is therefore speculative.
Fine-grained out-of-channel deposit architec-
tural element
The fourth-order element defined from fine-
grained out-of-channel strata that is present
lateral to the Lago Sofia conglomerate is not only
dominated by thin-bedded turbidites (IIIsm), but
is also characterized by the presence of chaotic
deposits (IIIch), and discrete bodies of thick-
bedded sandstone (IIIss) locally (IV5; Fig. 10).
Similar deposits from the Silla Syncline to the
north (Fig. 1) have been the source of consider-
able debate, specifically related to whether they
originated from flows that spilled over the banks
of the channel system (leve´e deposits), or are
genetically unrelated to the conglomeratic chann-
ellized deposits (Winn & Dott, 1979; DeVries &
Lindholm, 1994; Coleman, 2000; Beaubouef,
2004; Crane, 2004). Detailed observations of the
Cerro Toro Formation at Silla Syncline were not
made in this study.
Numerous characteristics of the thin-bedded
sandstone and shale of IV5 (juxtaposed against
conglomeratic channel deposits) reveal its nature:
do they have an overbank flow origin or not?
Palaeocurrent measurements, lateral sandstone
bed thickness and tabularity, evidence for a
depositional slope, and vertical grain-size and
bed thickness trends can all yield insight into
answering this question (Hickson & Lowe, 2002).
Figures 11 and 12 show outcrop exposures of the
channel margin on the north faces of Cerro Mocho
and Cerro Benitez, respectively (see Fig. 1B for
location), where tests were carried out to deter-
mine the nature of out-of-channel deposits in the
study area.
The channel margin at Cerro Mocho is charac-
terized by a stepped profile over a distance of
>2 km (Fig. 10A). Sandy turbiditic strata (IV4)
onlap the sharply defined margin of the channel-
A B
Fig. 8. Fourth-order architectural element IV4 in the Lago Sofia Member. (A and B) Thick-bedded sandstone is
common within the axial channel belt locally, particularly at the top of the coarse-grained sequence (location is Cerro
Benitez; Fig. 1B). Conglomeratic interbeds are characterised by large-scale dunes up to 4 m high (at ca 25 m in
measured section in part A; indicated by arrow in part B). Note person (circled) for scale (160 cm tall) at lower left of
photograph in part B.
14 S. M. Hubbard et al.
 2008 The Authors. Journal compilation  2008 International Association of Sedimentologists, Sedimentology

form body at its westernmost termination
(Fig. 10B); a series of palaeocurrent measure-
ments were made from laterally equivalent beds
on either side of this margin, and in a thin (<4 m)
fine-grained interval that drapes or laps onto the
channel margin (cf. Grecula et al., 2003; Fig. 11).
Imbricated clasts were measured in conglomerate
units, sole marks on the bases of thick-bedded
and thin-bedded turbidity current deposits on
either side of the channel margin, and ripple
foresets in thin out-of-channel sandstone beds.
Measurements from within the channel indicate
that palaeoflow was to the south-east (mean
vectors of 119 to 161), whereas out-of-channel
flows diverged 56 to 98 to the south-west
(Fig. 11). A similar amount of divergence is
recognized between average palaeocurrent vec-
tors in channel and laterally adjacent out-of-
channel deposits at Cerro Benitez (Fig. 12). This
flow divergence is consistent with leve´e deposi-
tion as observed in modern settings (Piper &
Normark, 1983; Hiscott et al., 1997; Fildani &
Normark, 2004), measured from ancient succes-
sions (Morris & Busby-Spera, 1990; Hickson &
Lowe, 2002), and inferred from ancient overbank
lobe deposits associated with leve´es recognized
in 3D seismic data sets (Posamentier & Kolla,
2003; De Ruig & Hubbard, 2006).
Thinning of sandstone beds distally, away from
a channel margin has been documented from
modern leve´e deposits (Piper & Deptuck, 1997),
and has been interpreted in the Cerro Toro
Formation at the Silla Syncline (Beaubouef,
2004). Sandstone and siltstone beds associated
with out-of-channel deposits at Cerro Mocho are
typically <5 cm in thickness (Figs 5D and 10C)
but, because of the excellent outcrop exposure,
can be traced for tens to >100 m (Fig. 10B). Two
stations at two stratigraphic levels are indicated
on Fig. 10B, where sandstone bed thicknesses
were examined to capture any systematic bed
thickness changes distally, away from the axial
channel belt margin. At both levels, the average
sandstone bed thickness was ca 3 cm directly
adjacent to the channel belt. Approximately
115 m from the channel margin at horizon 1
(Fig. 10B), the average bed thickness was 2Æ2 cm.
At horizon 2, the average bed thickness dropped
to 1Æ7 cm over a lateral distance of ca 55 m
(Fig. 10B). Bed thinning is consistent with a leve´e
interpretation for deposits of IV5 at Cerro Mocho.
As a result of discontinuous exposure of associ-
ated units at Cerro Benitez, laterally tracing out-
of-channel sandstone beds is not possible.
Successions of leve´e strata are characterized by
upward fining and bed-thinning successions
attributed to increasing channel relief over time
associated with leve´e buildup (Manley et al.,
1997). Upward fining is accomplished over
variable stratigraphic thickness, from tens of
metres to hundreds of metres. Frequent up-
channel bifurcations and avulsion events result
in the development of numerous, relatively thin
upward-fining cycles as downstream leve´es are
repeatedly abandoned and reestablished. Down-
stream of a stable reach of the channel where
avulsions are infrequent, subtle and thicker
upward fining cycles (>100 m) are preserved
Fig. 9. Mapped conglomeratic outcrop distribution
(defined by yellow lines) and mean palaeocurrent
measurements within the middle part of the Lago Sofia
Member (outline of map area is shown in Fig. 1A). In
the Cordillera Manuel Sen˜oret study area (from Cerro
Castillo to Cerro Benitez) the sinuous nature of the
channel belt is apparent due to the alignment of mean
palaeocurrent vectors with the outlined distribution of
coarse-grained channel facies; the lack of correlatable
exposures to the north of the study area makes the
interpretation of this relationship more tenuous, but it
is apparent that at least two tributaries fed into the
channel belt in the study area. Rose diagrams indicate
the number and variability of palaeocurrent vectors in a
given location; the arrows indicate the location where
measurements were made and the mean flow direction
is also indicated.
Sinuous deep-water foreland basin axial channel, Chile 15
 2008 The Authors. Journal compilation  2008 International Association of Sedimentologists, Sedimentology

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6.
16 S. M. Hubbard et al.
 2008 The Authors. Journal compilation  2008 International Association of Sedimentologists, Sedimentology

(Manley et al., 1997). Upward-fining and bed-
thinning successions in the Cerro Toro Forma-
tion have not yet been observed (Figs 10C &
12D). The authors interpret that this is due to the
fact that the channel belt was stable, unable to
avulse because of confinement in the foredeep
A
B
C
D
E
Fig. 12. Photomosaic (A) and line drawing trace (B) of the channel margin exposed on the north face of Cerro Benitez
(see Fig. 1B for location), characterised by monoclinally (northward) dipping strata. Palaeoflow was into the plane of
the outcrop. The base of the conglomeratic cliff corresponds to the margin of the channel belt, as evidenced by the
presence of fine-grained strata in underlying beds (measured section locations shown in parts A and B; measured
sections shown in parts D and E). (C) Palinspastically restored margin from parts A and B, with rose diagrams
showing palaeocurrent vectors in channel conglomerate (IV3) and in equivalent overbank units of IV5.
Fig. 11. Close up of channel margin at Cerro Mocho (see Fig. 10A for location) showing the divergent mean palaeo-
current measurement vectors from within the channel belt and in laterally equivalent overbank deposits. A fine-
grained layer is present draping, and locally on lapping the channel margin in this location, as illustrated in the inset
at the lower right.
Sinuous deep-water foreland basin axial channel, Chile 17
 2008 The Authors. Journal compilation  2008 International Association of Sedimentologists, Sedimentology

axis. An upward fining succession of leve´e
deposits in such a setting would be difficult to
discern, as it would be observable over 100 m or
more, representing the initial establishment of
the channel belt leve´e and its slow overall
increase in relief. The extensive outcrop of IV5
at Cerro Mocho may offer the opportunity to
observe such an upward fining succession, but
the potentially subtle trend has yet to be
observed; a more rigorous analysis is necessary
to establish its presence or absence.
Common slumped beds (Fig. 5E) and soft sed-
iment deformation (Fig. 5C) indicate that deposits
of IV5 were deposited on a slope, and are
consistent with a leve´e interpretation for associ-
ated strata (Morris & Busby-Spera, 1990). Slump
fold axes are predominantly oriented north–
south, indicating that mass wasting occurred on
slopes oriented parallel to the channel belt (e.g.
leve´e). Because of the fact that overturned beds
are typically not traceable to their point of
attachment to underlying beds, however, the
slump direction cannot be unequivocally deter-
mined in most cases.
Evidence that the fine-grained deposits of IV5
were not highly consolidated during channel
incision and sedimentation also supports the
notion that channel deposits (IV1 to IV4) were
not emplaced within a deep, erosional conduit. A
stepped channel margin (Figs 10A and 13A),
injection of coarse-grained channel material into
adjacent out-of-channel units (Fig. 13B) and an
undulous channel margin contact (Fig. 5D) indi-
cate that the out-of-channel deposits were (rela-
tively) poorly indurated during the emplacement
of channel fill. These observations, along with
those discussed above, are suggestive that the
deposits of IV5 accumulated on a leve´e. Further-
more, the presence of climbing ripples in deposits
of IV5, indicating high sediment fall-out from
low-density turbidity currents, is common in
leve´e environments where flows become uncon-
fined as they spill over channel banks.
Thin-bedded sandstone and siltstone of IV5 is
notably tabular over at least hundreds of metres,
typical of leve´e sequences. In contrast, beds 20 to
150 cm thick in this architectural element are
coarser (up to medium-grained) and lenticular
(Fig. 13C). These beds can be isolated and lentic-
ular over 10 to 20 m (Fig. 13C), or part of
channelform bodies hundreds of metres wide
and tens of metres thick (Fig. 13D). Isolated beds
are observed in close proximity to the channel
margin, and may represent the fill of focused
overbank flows that eroded and subsequently
filled scours in the area (cf. Hickson & Lowe,
2002). Winn & Dott (1979) and Beaubouef (2004)
interpreted similar beds at the Silla Syncline as
crevasse-splay deposits. The larger channelform
bodies are characterized by a significant fine-
grained drape deposit overlain by on lapping
thick-bedded sandstone units (Fig. 13D and E);
the features are exclusively observed a significant
distance (>1 km) from laterally equivalent axial
channel conglomerate (Fig. 10A). Measurement
of flute casts in these bodies at Cerro Mocho
indicates that palaeoflow was nearly due south,
more similar to the mean flow in the axial
channel belt than that indicated in proximal
out-of-channel deposits of IV5 (Fig. 11); this is
consistent with observations made in distal leve´e
deposits by Hickson & Lowe (2002). It is probable
that these features represent crevasse-splay or
distributary channels (or scours), eroded and
filled by focused overbank flow on the leve´e
(cf. Normark et al., 1979).
DISCUSSION
Channel–leve´e complex: architectural
considerations
Channel belt fill morphology
Of the five major mountain peaks in the Cordil-
lera Manual Sen˜oret, Cerro Castillo, Cerro Mocho
and Cerro Benitez (Fig. 1B) are characterized by
the thickest preserved intervals of the Lago Sofia
Member. The top of the coarse-grained interval is
best observed at Cerro Benitez, and in eastward-
dipping strata preserved at the eastern end of
Cerro Mocho at Ventana Creek (Fig. 1B). The total
thickness of the main Lago Sofia Member is 390 to
440 m. Channel belt fill is highly asymmetric, as
evidenced from the gradual (stepped) thinning of
the Lago Sofia Member from 390 m at its eastern
margin (Ventana Creek; Fig. 6) to its western
pinch-out at Cerro Mocho (Figs 10 & 11). The
width of the channel belt varies, as discussed, but
is in the order of 4 to 8 km (Fig. 9). The relief on
individual steps associated with the margin
(Fig. 10A), gives an indication of the stratigraphic
thickness associated with the fill of an individual
channel belt layer, or fourth-order architectural
element (up to 60 to 100 m typically), and allows
for the estimation of channel aspect ratio. Partic-
ularly high aspect ratios of 40 to 130 for the Cerro
Toro Formation channel belt are consistent with
observations of other major trunk channels or
channel belts associated with coarse-grained fill
18 S. M. Hubbard et al.
 2008 The Authors. Journal compilation  2008 International Association of Sedimentologists, Sedimentology

in the rock record (75 to 100, Eschard et al., 2003;
50 to 80, De Ruig & Hubbard, 2006) and in modern
settings (50, Kenyon et al., 1995; ca 100, Klaucke
et al., 1998). The sinuosity of the channel belt in
the study area (Fig. 9) is very low (1Æ06), again
consistent with other major trunk channel sys-
tems (Klaucke et al., 1998; De Ruig & Hubbard,
2006).
Inner leve´es
Inner leve´es, which form within the confines of
large channel or channel belt conduits, have been
A
C
D
B
E
Fig. 13. Characteristics of channel margin and overbank deposits (IV5) at Cerro Mocho. (A) Stepped channel margin
(indicated by arrows); person at right is sitting on thin-bedded units that are in a laterally equivalent position to the
thick-bedded channel sandstone present just above person at left (indicated by arrow on the left). Note 1Æ5 m long
staff for scale. (B) Incisional channel margin contact with arrow pointing to sandstone injection into fine-grained
overbank deposits. Person for scale is 175 cm tall. (C) Lenticular sandstone bed in fine-grained overbank succession
(IV5). Note staff with 10 cm long divisions for scale. (D) Channelform element associated with IV5 characterised by
basal drape and lenticular beds that onlap the margins of the feature. (E) The measured section through this chan-
nelform body shows that fine-grained slumped units characterize the base of the channel, and overlying sandstone
beds are present in amalgamated and non-amalgamated packages. The legend for features in the measured section is
shown in Fig. 6.
Sinuous deep-water foreland basin axial channel, Chile 19
 2008 The Authors. Journal compilation  2008 International Association of Sedimentologists, Sedimentology

demonstrated to be important depositional ele-
ments in many deep-water depositional systems
(Hubscher et al., 1997; Deptuck et al., 2003). The
features record flows associated with underfit
channels; these underfit systems are contained
within much larger conduits, whose original
morphology was shaped by considerably larger
flows. Evidence for the development of inner
leve´es within the axial channel belt is interpreted
locally near the base of the Lago Sofia Member at
Cerro Benitez (Fig. 14). In this locality, a stepped
channel margin profile bounding conglomeratic
units, and interbedded sandstone and mudstone
overbank units (inner leve´e deposits) are pre-
served; subsequent erosion within the channel
belt was significant, as inner leve´e units cannot be
traced laterally for greater than 25 to 50 m. The
overbank sandstone beds are 10 to 50 cm thick on
average, upper fine-grained to lower medium-
grained, are massive with normally graded tops
(horizontal laminations present locally), contain
rare mudstone clasts and moderately abundant
trace fossils (Ophiomorpha and Palaeophycus),
and have sharp or loaded basal contacts. The
mudstone interlayers are associated with thin
silty beds characterized by ripples that indicate
variable flow directions. An important criterion
for inner leve´e deposit recognition is their pres-
ence within the confines of the coarse-grained
channel belt fill; the deposits do not lie directly
adjacent to leve´e deposits associated with the
channel belt as a whole (i.e. the fine-grained
member of the Cerro Toro Formation present
lateral to the conglomeratic Lago Sofia Member).
The coarseness, thickness and internal architec-
ture of sandstone beds suggest that the relief on
inner leve´es was not nearly as developed as that
of the leve´es of IV5 (Fig. 5D). Preservation of these
inner leve´e deposits is not widespread, therefore,
it is not possible to discern whether the features
were rarely formed within the channel belt, or
just associated with poor preservation potential
due to intra-channel belt erosion.
Palaeogeographic reconstruction and the effect
of basin confinement on the depositional
system
It is well-established that the complicated archi-
tecture of foreland basins can impart a significant
influence on the deep-water depositional systems
that form within them (Ricci Lucchi, 1985; Mutti
et al., 1999; Kneller & McCaffrey, 1999; De Ruig &
Hubbard, 2006). Interpreting the narrowing of the
channel belt in the study area (Fig. 9) is not
straightforward given the outcrop data available,
although it seems probable that processes active
along the western basin margin played an impor-
tant role in constricting flow within the deposi-
tional system; the authors speculate that the
differential eastward propagation of the active
Andean fold-thrust belt significantly influenced
channel belt behaviour, consistent with the inter-
pretation by Wilson (1991) that the position of the
western margin of the Magallanes basin was
closely tied to the position of the Andean fold-
thrust belt.
Based on interpretations from the present
study, as well as the studies of recent researchers,
a revised palaeogeographic reconstruction of the
Magallanes basin during the time period when
the middle part of the Lago Sofia Member was
deposited is presented in Fig. 15. To account for
deposition of the Cerro Toro Formation, Winn &
Fig. 14. The stepped channel mar-
gin present at the base of the Lago
Sofia Member on the North Face of
Cerro Benitez, with laterally equiv-
alent inner leve´e deposits in the
foreground at the left. Staff is 1Æ5 m
long. See Fig. 12A for location.
20 S. M. Hubbard et al.
 2008 The Authors. Journal compilation  2008 International Association of Sedimentologists, Sedimentology

Dott (1979) envisioned a north- to south-oriented
elongate submarine fan in the Magallanes basin,
fed from a canyon-channel system that originated
from the fold-thrust belt to the west, taking a
sharp bend towards the south as it encountered
the basin axis. Based on mapping of Upper
Cretaceous non-marine strata to the north of the
study area in Argentina, however, the deep
Magallanes basin is also interpreted to have been
delta-fed, from an axial fluvial depositional sys-
tem (Fig. 15; Macellari et al., 1989). Crane & Lowe
(2001) and Crane (2004) have speculated that the
channellized deposits of the Cerro Toro Forma-
tion at the Silla Syncline represent a conduit that
fed the axial channel belt from the west (Fig. 15).
Based on extensive faulting in strata to the east of
the syncline, the authors considered a piggy-back
setting for this conduit (Crane, 2004). Notably in
the Silla Syncline area, coarse-grained units
(conglomerate and sandstone) are present in a
series of offset channelform sedimentary bodies
separated by continuous mudstone layers (Beau-
bouef, 2004; Crane, 2004). Beaubouef (2004)
recognizes numerous ‘channel complexes’ within
each of the lenticular sedimentary bodies (chan-
nel complex sets). Conglomeratic deposition in
the assumed basin axis proper (cf. Winn & Dott,
1979), is recorded by outcrops present at the
eastern part of Sierra del Toro, and in the
Cordillera Manual Sen˜oret; patchy outcrops to
the north of Sierra del Toro, and a thick succes-
sion of coarse-grained material at Cerro Rotonda,
south of Puerto Natales, are also attributed to
deposition in the basin axial channel belt
(Fig. 1A). The stratal hierarchy used by Beau-
bouef (2004) is not easily defined in the Cordillera
Manuel Sen˜oret because of the amalgamation of
channel complexes throughout the entire Lago
Sofia Member. The fourth-order architectural
elements as defined in this paper are conceptu-
ally equivalent to the channel complexes of
Beaubouef (2004), although generally much wider
(4 to 8 km in the study area vs. 500 m to 1Æ5 km at
Silla Syncline). Although a series of distinct
channel complex sets are recognized at the Silla
Syncline, because of the amalgamation of units at
the Cordillera Manuel Sen˜oret, the entire strati-
graphic succession of coarse-grained units is
delineated as one immense channel complex set
(fifth-order element). Stratigraphic correlations in
the Cerro Toro Formation between the Silla
Syncline and the Cordillera Manuel Sen˜oret are
a focus of a Stanford University group currently
researching at Sierra del Toro (Bernhardt et al.,
2007), and may shed insight into the equivalency
of units (i.e. channellized bodies) across the
region. As discussed, the sinuous planform archi-
tecture of the axial channel belt between Cerro
Castillo and Cerro Benitez is delineated based on
outcrop distribution and palaeocurrent measure-
ments (Fig. 9). A terminal sandy lobe has not yet
been identified down-system of the channel
deposits examined in this study; however, the
Cerro Toro Formation (and equivalent units)
further south have been understudied due in
large part to inaccessibility.
The authors attribute major gravity flow initia-
tion in the basin to delta front failure, on deltas fed
from high-discharge rivers sourced from a moun-
tainous Andean catchment area (cf. Normark &
Piper, 1991). Well-developed traction structures,
including 4 m high dunes, were formed through
sustained gravity flow currents, perhaps originat-
ing from hyperpycnal flow generated in conduits
Fig. 15. Palaeogeographic reconstruction of the Ma-
gallanes basin during deposition of the middle part of
the Lago Sofia Member (not to scale). Various outcrop
and geographic localities are indicated. The axial
channel belt was fed coarse-grained sediment from a
delta to the north, and from conduits sourcing material
directly from an Andean catchment area along the
western margin of the basin. Outcrop observations and
interpretations that have been incorporated into the
diagram from this study (and from the study of Crane,
2004, Beaubouef, 2004 and Hubbard et al., 2007a), in-
clude the presence of two tributaries in the north, the
narrowing of the channel belt towards the south, and
the sinuosity of the channel belt between Cerro Benitez
and Cerro Castillo.
Sinuous deep-water foreland basin axial channel, Chile 21
 2008 The Authors. Journal compilation  2008 International Association of Sedimentologists, Sedimentology

dominated by fluvial systems characterized by
high concentration of suspended sediment (Nor-
mark & Piper, 1991; Mulder & Syvitski, 1995), or
by retrogressive slope failure (Hughes Clarke
et al., 1990). Gravelly traction structures, along
with the paucity of sandstone and mudstone
within channel belt deposits in the study area,
indicate that a significant amount of sediment by-
passed the area. Evidence of bypass associated
with finer-grained, turbidity current tail deposits
(Grecula et al., 2003) are rarely observed in the
axial channel belt, although their preservation
potential in this depositional system is intuitively
low. In support of this, a fine-grained drape
overlying the channel belt basal scour is only
preserved when overlain by sandstone beds at the
margin of the channel belt (Fig. 11).
In modern passive margin settings, where large-
scale channel–leve´e complexes have been studied
in great detail (e.g. the Amazon Fan), understand-
ing of the systematic downstream evolution of
these depositional systems has developed based
on a series of key observations. These include: (i)
channel cross-sectional area decreases down-
stream (Flood & Damuth, 1987); (ii) channel slope
decreases nearly uniformly downstream, associ-
ated with a decrease in channel sinuosity (Flood
& Damuth, 1987); and (iii) the leve´e height
decrease downstream corresponds to a coarsening
of leve´e deposits (Hiscott et al., 1997; Manley
et al., 1997). These characteristics from passive
margins differ from those of a confined system
such as the Magallanes basin. Changes in channel
slope may periodically be non-uniform because of
tectonic influences on the sea floor or basin
margins (Fig. 9; De Ruig & Hubbard, 2006). Fur-
thermore, input from conduits along the length of
the channel belt complex (e.g. conglomeratic
deposits of the Cerro Toro Formation at the Silla
Syncline; Crane & Lowe, 2001) may significantly
change the magnitude and composition of gravity
flows that pass through, and subsequently form,
the depositional system at various points along its
length. Systematic downstream changes in chan-
nel behaviour and resultant sedimentary deposit
characteristics, like those observed on the Ama-
zon Fan (Flood & Damuth, 1987; Hiscott et al.,
1997) may not be expected in a confined, tecton-
ically active setting like a foredeep.
Applications of study to hydrocarbon
exploration
Deposits of large-scale deep-water channels in the
sub-surface represent a common hydrocarbon
reservoir target (Reeckmann et al., 2003; Samuel
et al., 2003). Consequently, analogues from high-
resolution seismic data sets (Kolla et al., 2001;
Abreu et al., 2003; Deptuck et al., 2003) and
outcrops (Cronin et al., 1998; McCaffrey et al.,
2002; Beaubouef, 2004) are commonly examined
in order to better understand the stratigraphic
architecture of channellized strata, and ultimately
improve exploration success and exploitation
strategies in similar deposits.
An exceptional sub-surface analogue to the
axial channel belt complex in the Cerro Toro
Formation is present in Oligocene–Miocene strata
of the Austrian Molasse foreland basin (Fig. 16;
Hubbard et al., 2005). The Austrian depositional
system is characterized by a low sinuosity axial
channel belt 3 to 6 km in width (Fig. 16A),
dominated by coarse-grained (sandstone and
conglomerate) fill (De Ruig & Hubbard, 2006);
thick successions of fine-grained units that sur-
round the axial channel belt deposits accumu-
lated, in part, in a leve´e environment (Fig. 16B),
with additional input from the foredeep margins.
An outcrop showing a cross-section of the entire
(5 to 8 km wide) channel belt in the Cerro Toro
Formation is not readily observed; therefore, the
three-dimensionally imaged Austrian channel
offers important insight into the architecture of
the channel belt in the Cerro Toro Formation. The
feature in the sub-surface of Upper Austria is
asymmetric (Fig. 16A) and, as in the Cerro Toro
Formation, the architecture of the Austrian chan-
nel belt and overbank sediment was strongly
influenced by the margins of the narrow foredeep
(De Ruig & Hubbard, 2006; Hubbard et al, 2008).
Various elements of the Cerro Toro Formation are
potentially analogous to reservoir deposits in the
petroliferous Austrian Molasse basin, including
those associated with deposits of the channel
thalweg and margin, and overbank-distributary
channels.
CONCLUSIONS
The Upper Cretaceous Cerro Toro Formation in
the Cordillera Manuel Sen˜oret was deposited in a
4 to 8 km wide deep-water channel belt with
transport southward along the axis of the Magall-
anes basin foredeep. Exceptional outcrop expo-
sures, including those of the channel margin in
several localities, permit an interpretation of the
genetic relationship between channel and out-of-
channel deposits. Five fourth-order architectural
elements characterize the sequence of strata in the
22 S. M. Hubbard et al.
 2008 The Authors. Journal compilation  2008 International Association of Sedimentologists, Sedimentology

Cerro Toro Formation. IV1, relatively tabular
layers of predominantly sandy matrix conglom-
erate 40 to 80 m thick is interbedded with IV2,
similarly tabular and thick layers of mostly
muddy matrix conglomerate; these fourth-order
elements were deposited in the axial channel belt
in the northern part of the study area. More
randomly distributed conglomeratic units with
extensive erosional incision (IV3) characterize the
channel belt fill in the southern reaches of the
study area. Successions of thick-bedded sand-
stone tens of metres in thickness, IV4, are present
locally within the channel belt, most notably at
the top of the conglomeratic member of the Cerro
Toro Formation and in channel bodies at the
margins of the channel belt. Fine-grained leve´e-
overbank units constitute IV5.
A mapped downstream constriction of the
channel system corresponds with a shift from
tabular beds of IV1 and IV2 to more erosive and
lenticular beds of IV3. The axial channel complex
is characterized by a sinuous planform architec-
ture, as delineated from outcrop distribution and
extensive palaeocurrent measurements. This
architecture was affected by the tectonically
influenced western basin margin slope.
The coarse-grained, gravel-dominated channel
belt was associated with constructional overbank
deposits consisting of interbedded sandstone
and mudstone deposits (IV5). Evidence that the
fine-grained deposits accumulated on a leve´e to
the axial channel belt (and that they do not
represent older, genetically unrelated units)
includes: (i) palaeocurrent measurements indi-
cate that overbank flow diverged 50 to 100
from the mean flow direction in the channel belt;
(ii) overbank sandstone beds thin laterally, away
from the contact with coarse-grained channel
belt deposits; (iii) sedimentological observations,
including a stepped channel margin profile and
lateral injection of channel material into adjacent
overbank units, suggests that the fine-grained
out-of-channel beds were not highly indurated
when they were incised by channel belt pro-
cesses; and (iv) presence of slumped units
(associated with leve´e topography), and sand-
stone beds that show evidence of high sediment
fall-out (attributed to flow unconfinement as it
overspills the channel). Channelform bodies
associated with overbank strata represent scours
or distributary channels, formed and filled by
overbank flow.
A systematic downstream change in channel
character has been observed from large-scale
channels from modern passive margins (e.g. the
Amazon fan). It is apparent from observations of
the Cerro Toro Formation in the tectonically
active, confined Magallanes basin setting, that
downstream changes in channel architecture and
deposits are complicated by various factors. Local
influences on channel behaviour, including: (i)
variability in basin width and effects from lateral
margin slopes; and (ii) the presence of multiple
sediment sources or conduits along the length of
Fig. 16. Seismic characterization of Oligocene–Miocene strata from the Molasse basin of Austria (modified from De
Ruig & Hubbard, 2006). (A) Plan view amplitude map showing a sinuous basin axial channel belt (characterized by
bright amplitudes) surrounded by constructional, fine-grained overbank deposits (associated with darker ampli-
tudes). (B) Seismic cross-section of the basin axial channel belt with outlined coarse-grained fill laterally juxtaposed
against dimmer, finer-grained leve´e–overbank deposits (palaeoflow was out of the plane). The channel belt is 3 to
6 km wide and the channel fill succession in part B is ca 250 m thick. Like the channel belt in the Cerro Toro
Formation, the fill is coarse grained (conglomeratic) and characterised by asymmetry. The two outlined boxes in part
B are considered analogous to outcrop panels presented in Figs 12A and 10B.
Sinuous deep-water foreland basin axial channel, Chile 23
 2008 The Authors. Journal compilation  2008 International Association of Sedimentologists, Sedimentology

the channel system, complicate the proximal–
distal relationships that have been documented
from large channel systems in passive margin
settings.
ACKNOWLEDGEMENTS
Funding for this research was provided by Ame-
rada Hess, Anadarko, BP, Chevron, Conoco
Phillips, ENI-AGIP, ExxonMobil, Husky,
Marathon, Nexen, Occidental, Petrobras, Roho¨l-
Aufsuchungs AG, and Shell (affiliate members of
the Stanford Project on Deep-water Depositional
Systems). Discussions in the field with various
geoscientists from these companies have helped
to refine many of the interpretations presented in
this study. Fieldwork was ably assisted by
M. Shultz, T. Erohina, M. Solari, E. Sperling,
J. Covault and L. Cassel. The study benefitted
from insightful discussions with Bill Normark,
Don Lowe, Will Crane and Andrea Fildani.
Reviews of the manuscript by Zoltan Sylvester,
Rick Beaubouef and Bill Morris significantly
improved the clarity and focus of the paper.
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