AUTHOR

R. T. Beaubouef ExxonMobil ExplorationCo., 233 Benmar, Houston, Texas 77060;rick.t.beaubouef@exxonmobil.com

Rick Beaubouef earned a Ph.D. in geologyfrom the University of Houston in 1992. Sincethen, he has worked as a geologist for Exxonand later, ExxonMobil. He was previously asenior research specialist at ExxonMobil Up-stream Research Co. (formerly Exxon Produc-tion Research Co.) involved in investigationsof a wide range of depositional environmentsand basins using seismic data, well logs, core,and outcrops. However, most of his work hasfocused on the stratigraphy and depositionalfacies of deep-water reservoirs. Since 2002, hehas been a stratigraphy advisor for ExxonMobilGeosciences. In this role, he is responsible forstewardship of technologies related to deep-water reservoir characterization and is involvedin a wide range of exploration, development,production, research, and training activities ona global basis.

ACKNOWLEDGEMENTS

I acknowledge present and former Exxon geol-ogists who participated in this study and con-tributed to our understanding of the CerroToro Formation. These are R. Bissell, M. DeVries,K. Glaser, R. Lindholm, P. McLaughlin, andC. Rossen. However, responsibility for thecontent and conclusions drawn in this paperis mine. I am grateful to my many colleagueswho encouraged me to finally write this paper.In particular, I cite M. Sullivan, C. Rossen,D. Ying, and B. Dixon for inspiration. The reviewsof R. J. Miola, M. S. Ryer, and H. J. White im-proved the paper and are much appreciated.Thanks go to ExxonMobil URC for allowingme to publish this work. Finally, a very specialthanks is extended to my family for putting upwith me while I pulled this off.

Deep-water leveed-channelcomplexes of the CerroToro Formation, UpperCretaceous, southern ChileR. T. Beaubouef

ABSTRACT

The Cerro Toro Formation in the Torres del Paine National Park,

southern Chile, contains a series of deep-water channel complexes

deposited in an elongate Andean foreland basin during the Late Cre-

taceous. This stratigraphic interval represents an essentially contin-

uous depositional record of migrating, leveed-channel complexes.

Collectively, the channel-fill units in the study area form a belt ap-

proximately 5 km (3 mi) wide and several hundred meters thick.

Within the study area, four sets of channel complexes are identified.

This paper focuses on the best exposed of these channel-complex

sets (channel-complex set 3). The channels are filled by bedded con-

glomerate and amalgamated sandstones interpreted to represent the

deposits of high-concentration turbidity currents and debris flows.

Large-scale cross-beds in some of the conglomerates indicate sig-

nificant bed-load transport of gravel- and cobble-forming bars in

the channels. Channel axis to margin facies changes between clast-

supported conglomerate and either (1) thick-bedded sandstone or

(2) matrix-supported conglomerate are observed. Channel-fill facies

lie on erosional surfaces that cut into adjacent interchannel facies.

Beds thin and onlap these surfaces toward the channel margins. Shale

or siltstone drapes of the channel cuts are uncommon and laterally

discontinuous. Bed continuity between channel and adjacent, inter-

channel facies is not observed. The interchannel strata are inter-

preted to represent levee successions that bound the channels. Stra-

tigraphy in the levee units is defined to include (1) basal, sandy

lobe deposits comprised of medium- to thick-bedded turbidites and

(2) overbank facies consisting primarily of packages of fining- and

thinning-upward, fine-grained, thin-bedded turbidites. This vertical

succession is transitional. Distal levee facies include mudstones with

thin-bedded, laterally continuous sandstones. Proximal levee facies

include mudstones with both thin- and thick-bedded sandstones; how-

ever, the thick-bedded sandstones have lower lateral continuity. The

AAPG Bulletin, v. 88, no. 11 (November 2004), pp. 1471 1500 1471

Copyright #2004. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received December 17, 2003; provisional acceptance February 19, 2004; revised manuscriptreceived June 8, 2004; final acceptance June 21, 2004.

proximal levee facies have a higher sandstone percent-

age than the distal levee, but also have greater depo-

sitional and postdepositional complexity, with sand-

filled crevasses, erosional truncation, and slumped beds.

Field observations suggest that these leveed channels

formed in stages that are represented by depositional

and/or erosional events. In chronological order, these

are (1) an initial stage of relatively unconfined, sand-

rich deposition; (2) aggradation of a mud-rich, confin-

ing levee system resulting from overbank deposition

as turbidity flows bypass the area; (3) erosion as the

channel becomes entrenched or as the channel mi-

grates; and (4) filling of the channel-margin relief by

onlap of channel-fill sediments. These stages appear to

have repeated several times during the formation of a

series of channel complexes. In these ways, the Cerro

Toro Formation appears analogous to leveed-channel

systems observed in late Pleistocene submarine fans

and subsurface examples.

INTRODUCTION

Given the costs and challenges of exploring and pro-

ducing deep-water reservoirs, sound stratigraphic, dep-

ositional, and reservoir facies models are essential for

accurate characterization of reservoir targets prior

to drilling. One approach to developing these models

is through the study of outcrops as analogs for sub-

surface systems. An area of recent industry focus has

been on large, confined slope valleys, a depositional en-

vironment previously underemphasized in well-known

depositional models. In fact, the concept of the chan-

nelized slope as a depositional province favorable to the

development of major petroleum reservoirs is relatively

new, and reservoir characteristics are uncertain. Over

the past few years, significant steps have been taken

toward building enhanced stratigraphic models for

slope-channel reservoirs (e.g., Campion et al., 2000;

Pirmez et al., 2000; Kolla et al., 2001; Abreu et al.,

2003; Babonneau et al., 2002; Droz et al., 2003). Leveed

channels have long been recognized as important dep-

ositional elements comprising near-surface, Pleistocene

submarine fans and in the subsurface of basins around

the world (Normark, 1970; Mitchum, 1985; Mutti and

Normark, 1987, 1991). However, the outcrop record

of analogous systems is enigmatic. It has been partic-

ularly challenging to find areas for the study of ancient

leveed channels in outcrop on which different workers

can agree. It is the conclusion of this and previous work

that the Cerro Toro Formation contains excellent out-

crop examples of leveed-channel complexes (Winn

and Dott, 1979; DeVries and Lindholm, 1994; Beaub-

ouef et al., 1996). However, it should be pointed out

that recent workers have disagreed with or questioned

this conclusion (Coleman, 2000; Crane and Lowe,

2001). The objectives of this paper are to (1) provide

documentation of field relationships that are used to

interpret these outcrops as portions of leveed-channel

systems; (2) establish a model for stratigraphic geome-

tries and facies relationships in leveed-channel com-

plexes of the Cerro Toro Formation that may be

applied to subsurface problems in other deep-marine

sequences; and (3) produce a foundation for future

work that may expand, improve, or change our under-

standing of the Cerro Toro Formation.

GEOLOGIC SETTING

The Cerro Toro Formation in southern Chile is a 2000-m

(6600-ft)-thick unit of deep-water clastic sedimen-

tary rocks deposited in the Magallanes basin during

the Late Cretaceous (CenomanianCampanian) (Ce-

cioni, 1957; Katz, 1963). This formation includes an

exceptionally well-exposed conglomeratic interval, re-

ferred to as the Lago Sofia Member (Katz, 1963), that

occurs in outcrops from Torres del Paine National Park

south to Lago Sofia as shown in Figure 1 (Scott, 1966).

These conglomeratic facies are surrounded and encased

by thin-bedded, fine-grained sandstones and mudstones.

The Cerro Toro is underlain by deep-water mudstones

and sandstones of the Punta Barrosa Formation (Albian

Cenomanian) and overlain by the shallowing-upward

clastic succession of the Tres Paso Formation (Santonian

Maastrichtian; Katz, 1963; Scott, 1966; Natland et al.,

1974; Winn and Dott, 1979).

The Magallanes basin developed as an elongate,

north-southoriented foreland basin that formed to

the east of the proto-Andean orogenic belt during the

latest Jurassic to Early Cretaceous (Dalziel et al., 1974;

Wilson and Dalziel, 1984; Biddle et al., 1986). Because

of compression in the adjacent orogenic belt, the basin

began to subside rapidly from shallow-water depths

in the latest Jurassic to 2000-m (6600-ft) water depths

by the Early Cretaceous (Natland et al., 1974; Biddle

et al., 1986). During the Cenomanian and Campanian,

shelf systems fed sediments eastward from a proto-

Andes provenance into the elongate Magallanes basin,

where the sediment transport direction turned south-

ward parallel to the basin axis (Figure 1) (Scott, 1966).

Clasts in the Lago Sofia conglomerates are volcanic,

1472 Deep-Water Leveed-Channel Complexes of the Cerro Toro Formation

plutonic, and metamorphic in origin, indicating deri-

vation from the proto-Andean cordillera (Scott, 1966;

Winn and Dott, 1979).

Within the Torres del Paine National Park, the Lago

Sofia Member makes up a virtually continuous section

in the Cerro Silla syncline east of Lago Pehoe (Figure 2).

The syncline is a gently folded, 4-km (2.5-mi)-wide

structure with a north-southoriented axis that plunges

slightly to the north. Other than a thrust fault on the

eastern margin of the syncline, the syncline is cut by

only minor, small displacement faults. Exposures of

these rocks are exceptional; outcrops of coarse-grained

packages can be traced for distances of 510 km (36 mi),

with only scattered areas of cover and fairly continuous

stratigraphic successions of more than 300 m (1000 ft)

thickness can be measured.

PREVIOUS WORK

Most of the previous work done on the stratigraphy

and sedimentology of the Cerro Toro Formation was

published in the period between the late 1950s and

1970s (Cecioni, 1957; Sanders and Cecioni, 1957; Katz,

1963; Cortes, 1964; Scott, 1966; Winn and Dott, 1977,

1979). Following these, the next published record of

work in the Cerro Toro resulted from field studies done

by Exxon in the early 1990s (DeVries and Lindholm,

1994; Beaubouef et al., 1996). More recently, Coleman

(2000) offered a reevaluation of DeVries and Lindholm

(1994), based on his later study of the outcrop. Cur-

rently, a team from Stanford University is executing a

large study of the Cerro Toro Formation (Crane and

Lowe, 2001), as well as the underlying Punta Barrosa

and overlying Tres Pasos (Schultz, 2001). Below is a

brief review of the main results of this previous work

and a discussion of the scope of work on which this

paper is based.

Cecioni (1957) and Katz (1963) described the Cerro

Toro Formation as flysch deposits reflecting a view that

the rocks were deposited in a deep-water basin adja-

cent to an active orogenic belt. However, Cecioni (1957)

limited the lithostratigraphic definition of the Cerro

Toro to the 2000-m (6600-ft)-thick succession of alter-

nating mudstones and fine sandstones. He interpreted

the conglomerates as glacial tillites. Zeil (1957) inter-

preted them to be shallow marine in origin. Katz (1963)

Figure 1. Location of the study area shown in relation to (A) map of southern South America and (B) general geologic map of theCerro Toro Formation in southern Chile. Outcrop distributions and paleocurrent trends are from Scott (1966).

Beaubouef 1473

recognized that the large bodies of conglomeratic units

occur as tongues in Cecionis Cerro Toro Formation

and informally proposed them as the Lago Sofia Mem-

ber of the Cerro Toro Formation. Scott (1966) docu-

mented the paleotransport directions and sediment

pathways for the Cerro Toro, highlighting the Lago

Sofia conglomerates. His work showed consistently

south-oriented paleocurrent directions more than 85 km

(53 mi) of the outcrop belt and pointed out that these

trends were orthogonal to the west-to-east sediment

supply direction into the basin (Figure 1). He inter-

preted the entire Cerro Toro succession to have been

deposited in a deep-water basin. Based on a later study

of foraminiferal assemblages by Natland et al. (1974),

it was determined that the water depth in the Magal-

lanes basin was 10002000 m (33006600 ft) during

the deposition of the Punta Barosa and Cerro Toro for-

mations. This supported the depositional setting envi-

sioned by Scott.

Winn and Dotts work in the 1970s followed that

of Scott (1966) and resulted in a reevaluation of the

mechanisms of sediment transport and deposition.

Although Scotts map of the distribution of the Lago

Sofia Member clearly shows the conglomerates to be

contained in linear belts encased by mudstones and

sandstones of the Cerro Toro, Winn and Dott (1979)

first proposed a leveed-channel model to describe the

stratigraphic relationships between these distinct litho-

facies. The significance of this interpretation was that

their model called for the deposition of the Cerro Toro

Formation through sediment gravity-flow processes, in-

cluding turbidity currents and debris flows, whereas

Scott envisioned reworking of the sandstones and con-

glomerates through bottom currents along the axis of

the basin after initial emplacement. DeVries and Lind-

holm (1994) and Beaubouef et al. (1996) presented

stratigraphic models for the Cerro Toro Formation that

supported the Winn and Dott (1979) leveed-channel

Figure 2. Map of CerroSilla syncline with theTorres del Paine NationalPark. Shown are approx-imate locations of mea-sured sections collectedin 19931994, areas ofthe syncline discussed inthe text, and general lo-cations of lower and upperparts of channel-complexset (CCS) 3. Modifiedfrom DeVries and Lind-holm (1994).

1474 Deep-Water Leveed-Channel Complexes of the Cerro Toro Formation

model. DeVries and Lindholm focused on the fine-

grained facies outside the channel fills. Beaubouef et al.

(1996) presented a general model for leveed-channel

development in the Cerro Toro and some refinements

to the Winn and Dott model.

More recently, Coleman (2000) challenged the in-

terpretations put forth in DeVries and Lindholm (1994)

and offered a contrasting depositional model. Colemans

model suggests that the conglomerate- and sandstone-

filled channels are simple, erosional features cut into

unrelated, finer grained lithofacies surrounding the

channel-fill facies (i.e., the levee and overbank facies

of Winn and Dott, 1979; DeVries and Lindholm, 1994;

Beaubouef et al., 1996). Likewise, the preliminary

results of the Stanford group appear to question the

validity of a channel-levee model for the Cerro Toro

(Crane and Lowe, 2001; W. H. Crane, D. R. Lowe,

2003, personal communication).

The Exxon work in the Cerro Toro Formation was

conducted in 1993 and 1994 in the Torres del Paine

National Park (Figure 2). Initially, the objective of the

work was to investigate the applicability of the Cerro

Toro as an outcrop analog for deep-water leveed chan-

nels, as proposed by Winn and Dott (1979), and to

document the reservoir characteristics of the thin-

bedded turbidites. The work in the 1993 field season

focused on the northwestern portion of the outcrop

and resulted in definition of facies types in the Cerro

Toro Formation, identification of four major channel-

form units, and several measured sections. The second

field study extended the study of the Cerro Toro be-

yond the scope of the 1993 work to include the entire

Cerro Silla syncline (Figure 2). This work included re-

connaissance mapping and detailed measurement of

stratigraphic sections. Reconnaissance work focused

on documentation of lateral facies relationships by walk-

ing out conglomerate, sandstone, and debris-flow pack-

ages. During these two field seasons, approximately

5.5 km (1750 ft; 3.4 mi) of section was measured in a

total of 24 sections in an approximately 40-km2 (15-mi2)

area. During both the reconnaissance and measured sec-

tion work, paleocurrent measurements were made at

suitable bedding planes. In general, reliable paleocur-

rent indicators were difficult to find in the thin-bedded

sandstones in interchannel areas. The trends of channel

margins were deemed as the most reliable indicator of

general paleoflow in the channels. Within the channel

fills, channel-margin orientations and bottom structures

(groove and flute casts) were the primary sources of

paleocurrent information. To a lesser extent, cross-

bedding and clast imbrications were measured. Results

of the second field season included (1) refinement of

the stratigraphic relationships between the conglom-

eratic and thin-bedded units; (2) recognition of higher

order stratigraphic complexity in conglomeratic chan-

nel fills; and (3) documentation of facies variability

in channel complexes. The findings discussed in this

paper are the result of compilation of all field data by

the author but are based primarily on reconnaissance

mapping and measured sections accomplished in 1994.

The illustrations and discussions found herein are taken

from a poster presented by the author to the AAPG

in 1996 (Beaubouef et al., 1996).

LITHOFACIES AND STRATIGRAPHYOF THE STUDY AREA

Two striking aspects of the Cerro Toro Formation are

the extreme range of grain sizes present (cobble to

mud) and the contrast in grain-size attributes between

the channel and interchannel or overbank facies. Addi-

tionally, bedding types are very different between these

facies. Therefore, we can subdivide the lithofacies of

the Cerro Toro into (1) channel-fill facies (the Lago

Sofia Member; Katz, 1963) and (2) interchannel facies,

those deposited outside of and between large channel

complexes. The channel-fill lithofacies are found in

four major south-southeastoriented channel-complex

sets (sensu Sprague et al., 2002) (Figure 3). Channel-

complex sets are comprised of genetically related chan-

nel complexes that are spatially and temporally related,

with similar lithofacies associations and architectures.

The channel-fill lithofacies can be further subdivided as

follows: (1) clast-supported conglomerates with sandy

matrix; (2) matrix-supported conglomerates with muddy

matrix (debris-flow deposits; the diamictites of Winn

and Dott, 1979); (3) and medium- to thick-bedded sand-

stones and pebbly sandstones (Figure 4). Conglomeratic

units exhibit bed thickness on the scale of 13 m

(310 ft) thick in axial areas. Sandstone bedding is on

the scale of meters or less in marginal areas. Individual

channel-complex fills exhibit thickness on the scale of

30 m (100 ft), and channel-complex sets are as much

as 250 m (800 ft) thick (Figure 3). The amalgamated

nature of fill results in a high degree of vertical connec-

tivity between beds, channel fills, and channel complexes.

In general, vertical amalgamation decreases toward

channel margins. The lateral continuity of individual

beds is highly variable because of erosion in channel

axes. The width and continuity of channel-complex

fills are on the scale of 500 m1.5 km (1600 ft1 mi),

Beaubouef 1475

whereas channel-complex sets exhibit widths on the

scale of several kilometers.

Two types of lateral facies relationships are

observed from the axes to the margins of the channel

fills: (1) between clast-supported conglomerates and

sandstones and (2) between clast-supported conglom-

erates and matrix-supported conglomerates (Figure 4).

These observations are consistent with those of Winn

and Dott (1979). Graded, inversely graded, and mas-

sive conglomerates dominate axial portions of channel

fills. Large-scale cross-stratification in some of the con-

glomerates are indicative of significant bed-load trans-

port via long-lived currents through the channels. This

facies class can be related to the F3 deposits of Mutti

et al. (1999). Channel-complex sets 1 and 3 exhibit the

first type of axis-to-margin facies change. At channel

margins, the fill is commonly characterized by inter-

bedded and amalgamated Tab sandstones with minor

cross-stratification and pebbly subdivisions, suggest-

ing deposition primarily from the suspended load of

throughgoing flows. This facies class can be related to

the F5F8 deposits of Mutti et al. (1999). Channel-

complex sets 2 and 4 exhibit the second type of facies

changes and apparently require a different depositional

model. Winn and Dott (1979) suggested that these ex-

amples reflect original deposition by debris flows and

later reworking and winnowing by turbidity currents

along the channel axes. This seems like a plausible ex-

planation for these types of facies changes. However,

the presence of flutes at the base of some of the matrix-

supported conglomerates indicate that turbulent flow

was associated with the emplacement of these beds.

Therefore, a more complicated set of transport and dep-

ositional processes may be required to explain these

facies and is beyond the scope of this paper.

Lithofacies in the interchannel areas are primarily

thin-bedded turbidites that are characterized by inter-

bedded Tbc sandstones and silty mudstones, with sub-

ordinate thicker Ta sandstones, muddy debris-flow

deposits, and mass movement facies (Figure 5). This

facies includes the F9 and, to a lesser degree, the F8

deposits of Mutti et al. (1999). Sandstones are commonly

Figure 3. Semischematic geologic section based on measured sections shown in Figure 2. Vertical black lines indicate location ofmeasured sections. Shown are the four leveed-channel complex sets identified in the 19931994 study. The section illustrates the serratenature of the northern margin for channel-complex set 3 as discussed in the text. The northern margin is comprised of several southerlystepping, smaller scale channel margins. The southern complex margin is found in limited outcrops in the western limb of the synclinebut is not as well exposed as that in the north. The complex margins separate channel fill from interchannel lithofacies. The interchannelfacies is here interpreted as levee and overbank strata associated with the development of channels. VE = vertical exaggeration.

1476 Deep-Water Leveed-Channel Complexes of the Cerro Toro Formation

fine grained, but lower medium-grain sizes are ob-

served in some of the thicker beds. Sandstone per-

centages are moderate (as much as 40%) in proximal

and lower portions and low in upper and distal portions.

Proximal interchannel areas are defined as those in close

proximity to channel margins, and distal interchannel

areas are located 24 km (1.22.5 mi) away. Impor-

tantly, sandstone percentages decrease uniformly with

distance from channel margins. However, local chan-

nelized and crevasse-splay sandstones can significantly

increase the amount of sandstone locally. Sandstone

thickness is on a scale of tens of centimeters to 11.5 m

(35 ft) in proximal positions and on the scale of cen-

timeters in distalmost regions. The range of bed thick-

ness decreases with distance away from channel mar-

gins toward the distal interchannel areas. A wide range

of stratal lengths are observed in proximal positions

because of both depositional and postdepositional fac-

tors. Average stratal length increases, and the range

of values decreases with distance from channel margins.

Figure 4. Photographs of channel-fill lithofacies. (A) Clast-supported conglomerate. Matrix is sandstone to sandy mudstone.(B) Interbedded clast-supported conglomerate and sandstone. Conglomerate shows variability of grading. (C) Large-scale flutes atthe base of conglomerate bed. (D) Planar cross-stratification in clast-supported conglomerate. Foresets on inclined bedding planesshow as much as 3 m (10 ft) of relief. (E) Matrix-supported conglomerate. The matrix is a sandy mudstone. (F ) Upper fine- tomedium-grained sandstone, marginal channel fill. Sandstone bed is approximately 0.75 m (2.5 ft) thick. Tab divisions and dishstructures occur near the base.

Beaubouef 1477

In general, within the interchannel facies, thin sand-

stones have the highest lateral continuity; thicker beds

have the lowest.

The photograph in Figure 6 shows the amount

and continuity of outcrop and the three-dimensional

perspectives offered by the exposures. The prominent

ridge-forming resistant units around the rim of the

syncline represent channel-fill units consisting primar-

ily of conglomerates, conglomeratic mudstones, and

sandstones. Three of the four channel-complex sets

are visible from this view along the western limb of

the syncline (see also Figure 3). The oldest, channel-

complex set 1, can be seen on the right-hand side of

Figure 6B, whereas channel-complex set 4 is located

in the northernmost portion of the syncline and is not

visible from this view. Intermediate channel-complex

sets (channel-complex sets 2 and 3) are prominently

exposed along the upper portions of the western limb

of the syncline (left side of Figure 6B) and can be

traced around the syncline to the south and east. Sep-

arating and adjacent to the channel-fill units are thick

sections of thin-bedded sandstones and mudstones (thin

bedded turbidites) that make up the interchannel fa-

cies. These can be seen in the foreground of Figure 6A.

The thin-bedded turbidites are not facies equivalents

to the coarse-grained channel-fill units, but rather are

separated from them by complex channel margins onto

which the channel-fill units lap out. These relation-

ships are most evident for channel-complex set 3, the

best exposed of the channel-levee complex sets.

CHANNEL-COMPLEX SET 3

The approximate locations of the lower and upper

portions of channel-complex set 3 are shown in Figure 2.

The complex set is approximately 250 m (800 ft) thick

in its axis and is approximately 3 km (2 mi) wide. It

is comprised of at least five individual leveed-channel

complexes (perhaps more), and channel-complex fills

are on the order of 3060 m (100200 ft) thick. Based

on limited field control, our best estimate for individ-

ual channel complex widths is 11.5 km (0.61 mi).

The northern (left-hand) margins of these are the best

Figure 5. Photographs of interchannel lithofacies. (A) Fine ripple-laminated sandstone (Tc) and interbedded mudstone. (B)Monotonous, uniformly bedded, thin sandstones and mudstones. Thicker, resistant layers are fine sandstones on the order of severalcentimeters thick. (C) Erosional surface in interchannel facies. Parallel-bedded, thin sandstones and mudstones are seen to bothonlap and drape erosional topography. (D) Synsedimentary, antiformal structure in levee and overbank facies. Thicker bedsprogressively onlap the structure at lower right.

1478 Deep-Water Leveed-Channel Complexes of the Cerro Toro Formation

exposed (Figure 3). These channel complexes are ori-

ented southeast; the lower channel complexes appear

to have a south-southeast trend (160j), whereas theupper channel complexes trend in a more southeasterly

direction (120j). Associated with this swing in paleo-currents, progressively younger channel margins step

southward. This stepping of channel margins results in

a complex interleaving of interchannel and channel-

margin facies along the northern edge of the complex

set. These relationships are interpreted as the strati-

graphic record of a series of migrating, possibly sin-

uous, leveed channels that stacked to form channel-

complex set 3 (Figure 3). Clast-supported conglomer-

ates with subordinate matrix-supported conglomerate

and sandstones dominate lithofacies in the axial channel

fill. Toward the margins, thick-bedded conglomerates

are replaced (to varying degrees) by medium- to thick-

bedded sandstones. The conglomeratic and sandstone

units are seen to be in facies relationships with one

another, as well as separated by erosional contacts. In

Figure 6. Panoramic view of the west limb of the syncline. (A, B) View is to the south and shows the continuity of the outcrop and thethree-dimensional perspectives offered by the exposures. The prominent ridge-forming resistant units around the rim of the synclinerepresent channel-fill units consisting primarily of conglomerates (congl), debris-flow deposits (df ), and sandstones (ss). Betweenchannel-fill units are thick sections of thinner bedded sandstones and mudstones (tbt). chan marg = channel margin. CCS = channel-complex set.

Beaubouef 1479

the next section, four field areas chosen to best illus-

trate the character of channel-complex set 3 along its

northern and eastern edge are discussed.

Lago Pehoe Area; West Limb of the Syncline

Along the west limb of the syncline, east of Lago Pehoe,

the conglomerate-rich portion of channel-complex

set 3 is well exposed (Figures 6B, 7A). In this area,

the conglomerate thickens significantly from north to

south. This thickening results from the stacking of

successive conglomeratic channel fills and is associated

with a series of south-stepping channel margins. The

southerly migration of channel margins limits progres-

sively younger channel fills to more southerly posi-

tions and places thin-bedded, interchannel facies above

older channel fill (Figures 3; 7BD). Amalgamation of

channel axis deposits occurs to the south, where the

conglomerate achieves maximum thickness. From these

observations, the following points can be made. First,

the conglomeratic units seen in this interval do not

represent the fill of a single, large erosional channel as

suggested by Coleman (2000), but rather reflect a

history of episodic deposition in smaller, shifting chan-

nel systems. Second, deposition of the thin-bedded tur-

bidites was also episodic and probably occurred through

overbank deposition adjacent to shifting channels. Third,

interchannel deposition and filling of channels were

not coeval events.

Channel-fill facies commonly lie on erosional sur-

faces that cut into adjacent interchannel facies. Typi-

cally, conglomerates are found in more axial portions of

the channels and are replaced by interbedded sand-

stones and pebbly sandstones near the margins of the

channel complexes (Figure 7F). Beds thin and onlap

the erosional surfaces toward the channel margins,

and bed continuity between channel and interchannel

facies was not observed. Adjacent and to the north of

the margin of the lowermiddle portion of channel-

complex set 3 is a section of interbedded sandstones

and mudstones exhibiting an overall thinning- and

fining-upward character (Figure 7C). This section over-

lies the lower conglomeratic divisions of channel-

complex set 3 (Figure 3). The lower portion of this

unit contains relatively thick Ta,b sandstones inter-

bedded with thin Tc sandstones and mudstones. To the

north, this section laps out against a south-dipping

surface interpreted to represent relict levee topography

(Figure 7D, E). This section is interpreted to represent

the disconformable stacking of overbank strata that

are associated with the southerly migration of channels

in channel-complex set 3 as shown in Figure 3. The

disconformity in the interchannel strata is marked by

the abrupt pinch-out of a 1.5-m (5-ft)-thick sandstone

bed (Figure 7E). These thicker sandstones are common

in the lower portions of the interchannel strata closest

to the channel fill but have limited lateral continuity.

The more thinly bedded units exhibit much greater

lateral continuity. In these settings, sandstone percent-

age can be high but decays rapidly with distance away

from the channel margins. The thick-bedded units are

interpreted to have been deposited during an initial,

sand-rich, distributive stage of channel-complex de-

velopment (discussed below; see Figure 8). In younger

and more distal sections, the interchannel facies is

overall more thinly bedded and contains a smaller per-

centage of sandstone. These more thinly bedded facies

are interpreted to form during an overbanking stage of

channel-complex development (see Figure 8). How-

ever, these intervals can be punctuated by more thickly

bedded, lenticular sandstones (Figure 7G). Sandstone

beds such as these are common in the interchannel

Figure 7. Selected areas in the west limb of the syncline. (A) Basal contact of channel-complex set 3. Clast-supported conglomeratesof channel-complex set 3 overlie matrix-supported conglomerates of channel-complex set 2. (B) Stratigraphic relationships along thenorthern margin of channel-complex set 3. Thin-bedded turbidite facies are seen adjacent to (north) and above the conglomeraticchannel fills of channel-complex set 3. The conglomerate thickens to the south; thickening occurs by the addition of younger units tothe top. (C) Proximal, interchannel facies adjacent to the middle portions of channel-complex set 3. The section shows thinning- andfining-upward packages containing stages 1 and 2 deposits (discussed in the text). (D) Interchannel area adjacent to lower portion ofchannel-complex set 3. Middle and upper portions of the interval lap out against depositional relief in the lower portion. These featuresare interpreted as stratigraphic relationships occurring stacked overbank successions across relict levee topography. (E) Close-up ofoutcrop shown in (D), illustrating thick sandstone bed interpreted as a stage 1 deposit (discussed in the text). (F ) Portion of channelmargin in channel-complex set 3. Inclined erosional surface separates older thin-bedded turbidites from more thickly beddedsandstones and conglomerates. (G) Interchannel strata northeast of channel-complex set 3. This section is stratigraphically youngerand more distal than that shown in (C). Overall, facies are more thinly bedded and contain less sandstone, but more thickly bedded,lenticular sandstones punctuate the section. These are interpreted as crevasse-splay deposits.

1480 Deep-Water Leveed-Channel Complexes of the Cerro Toro Formation

Beaubouef 1481

facies and are interpreted as crevasse-splay deposits.

Thick splay sandstones can significantly increase the

sandstone percentage locally but have limited lateral

continuity.

Lago Sarmiento Chico Area

In the Lago Sarmiento Chico area (Figures 9, 10), the

expression of the eastern margin for the upper portions

of channel-complex set 3 can be examined. Channel-

filling units consist of thickly bedded conglomerates,

conglomeratic debris-flow deposits, and medium- to

thick-bedded sandstones. These are seen in the central

and right-hand (western) portions of Figure 9A. Thinly

bedded, fine-grained turbidites make up the inter-

channel strata adjacent to the channel margin and are

seen in the far left-hand (eastern) side of Figure 9B.

Erosion and onlap onto an inclined margin define the

relationship between the levee units and the channel

fill. However, these facies are not separated by a single,

Figure 8. Schematic diagram illustratinginterpreted stages of channel-complex for-mation. Field observations suggest thatleveed channels formed in four generalstages. In chronological order, these arethe following. (A) An initial stage (stage 1)of relatively unconfined, sand-rich deposi-tion. Deposits include channel fills, channelterminus lobes, and overbank material.(B) Aggradation of a mud-rich confininglevee system resulting from overbank dep-osition (stage 2). Levee formation confinesflows to a central channel. Widespreadoverbanking style of deposition is punctu-ated by crevasse-splay development. Aslevee topography grows, progressivelyfiner material is deposited in the overbankareas, and mass movement facies occur.Minor, secondary channels occur outsideof and parallel to the central channel. Thisstage is accompanied or followed by ero-sion as the channel becomes entrenched ormigrates. (C) Filling of the channel marginrelief by onlap of channel-fill sediments(stage 3). Channel fill occurs through bed-load deposition in the axis and from sus-pension fallout along the margins. Thesestages appear to have repeated severaltimes during channel-complex formation.

1482 Deep-Water Leveed-Channel Complexes of the Cerro Toro Formation

large surface but rather by a complex of numerous

erosional surfaces, which merge along the trend of the

contact to form a composite channel margin. These

relationships are visible near the lakeshore, in the cen-

tral portion of Figure 9, but are particularly well illus-

trated in the Lago Jigsaw area (southeastern portion of

outcrop; east limb of syncline, discussed in the next

section). Also visible in the lower, central portion of

this panorama (at the lakeshore) is a highly chaotic

interval consisting of disrupted, thin-bedded turbidites.

This feature is interpreted as a channel-margin slump,

locally derived from the inner, northeastern wall of the

leveed channel. In this area, the progressive, easterly

thinning and pinch-out of channel-fill strata onto this

composite channel margin are apparent, although the

channel margin is not always well exposed. Addition-

ally, transitions in lithofacies and bedding style occur

along the same trend toward the margin (Figure 10A).

In axial positions in the channel complex, thick, crudely

bedded packages of conglomerate with subordinate

medium- to fine-grained sandstone are the dominant

lithofacies type. As the channel margin is approached,

medium- to thick-bedded, semiamalgamated sand-

stones that thin toward the margin progressively re-

place the conglomerate and ultimately onlap inclined

erosional surfaces. This lithofacies transition can also

be observed to occur vertically, in which conglomeratic

units are replaced upward by sandstones. Such vertical

trends are also cited as evidence of channel migration

through time. This interpretation is supported by the

general change in paleocurrent directions as discussed

previously. Additionally, the channel margin is located

further north (behind the field of view in Figure 9) for

the lower part of the channel complex and steps to the

south-southeast into the central portion of the syncline

for the upper part (visible in Figures 9, 10A). Taken

together, these observations indicate a change in the

orientation and/or position of the channel complexes

through time (Figure 2). The outcrop immediately out-

side (east) of the upper channel complex is characterized

by thin-bedded turbidites that are interpreted as over-

bank facies in a portion of the left-hand (eastern) levee

of channel-complex set 3. This thinly bedded succes-

sion forms a wedge separating the lower and upper

parts of channel-complex set 3. Paleocurrent measure-

ments taken from these strata indicate the predomi-

nance of paleoflow away from, and at high angles to,

the trend of the channel margin.

Figure 10B illustrates the character of the inter-

channel facies exposed along the south shore of Lago

Sarmiento Chico. This facies is characterized mainly

by highly continuous, thin-bedded turbidites, with occa-

sional more thickly bedded (13 ft; 0.31 m), chan-

nelized sandstones. This section is characterized by wavy

bedding, creating the appearance of a series of broad,

low-amplitude folds. The axes of these features trend

at a high angle to the outcrop face and subparallel to

the channel margin. Stratigraphic relationships suggest

that these structures were formed prior to or during

stages of overbank deposition. The topography formed

Figure 8. Continued.

Beaubouef 1483

1484 Deep-Water Leveed-Channel Complexes of the Cerro Toro Formation

during the deformation was draped and progressively

healed by thin-bedded turbidites that postdate the struc-

tures. At present, the origin of these structures are not

well understood, but they may represent (1) syndepo-

sitional folds resulting from large-scale, soft-sediment

deformation in the crest of a levee; (2) syndepositional

folds resulting from basin tectonics; or (3) some type

of large-scale depositional structures such as sediment

waves. Thicker, lenticular sandstones tend to be ponded

within, or near, the troughs of these structures. These

sandstones exhibit limited lateral continuity, are me-

dium to fine grained, and were deposited from both

high- and low-concentration flows (Figure 10CE).

They consist of Ta,b,c divisions, with rip-up clast hori-

zons, and sometimes exhibit evidence for liquefaction

along their margins. They commonly occur in associ-

ation with erosional surfaces, chaotic bedding, and small

debris-flow deposits and are interbedded with thin-

bedded facies. These associations are interpreted to

represent crevasse-splay origins for these sandstones.

That they are ponded in the folds suggests (1) a tem-

poral, and possibly causal, relationship between forma-

tion of the wavy structures and breaching of the channel

margins and (2) that energetic, high-concentration flows

were diverted from the channel and funneled into struc-

turally controlled topographic lows that developed on

the interchannel seafloor.

The channel-margin area for the upper part of

channel-complex set 3 is also exposed on the north side

of Lago Sarmiento Chico (Figure 10A). The conglom-

erate bed in the western portion of the photograph

(left) is stratigraphically equivalent to that seen in the

lower, central portion of Figure 9. Because of heavy

vegetation, the channel margin is poorly exposed in this

locality, but its position may be inferred on the basis of

stratigraphic positions of channel-fill and overbank

lithofacies types and by projecting its trend from the

south side to the north side of the lake. To the west

(left) of the margin are conglomerates, medium-bedded

sandstones, and debris-flow deposits comprising the

stratigraphy of the marginal channel fill. To the east

(right) are thin-bedded turbidites characteristic of the

interchannel facies. Moving from west to east toward

the margin, a lithofacies transition occurs in the chan-

nel fill. Relatively thick, internally amalgamated

conglomerate gives way to well-bedded, semiamalga-

mated sandstones near the margin. These medium-

bedded sandstones cannot be traced laterally into the

thinly bedded section on the right beyond the margin.

Lago Jigsaw Area

Within the Lago Jigsaw area, the upper, eastern mar-

gin of channel-complex set 3 can be examined in detail

(Figure 11). The channel margin is inclined (19j; in-cluding structural dip) to the west and separates channel-

fill units on the right (west) from interchannel units

on the left (east). The contact between the channel-

fill and interchannel units is erosional, and groove casts

at the base of channel sandstones suggest paleoflow

along the margin to the southeast (130j). The channelmargin here is composite in nature and represents the

merger of three main erosional surfaces along the strike

of the margin (Figure 12A). The progressive easterly

onlap of channel-filling units onto this compound mar-

gin is accompanied by facies changes, rapid thinning

of beds, and erosional truncation of beds. The fine-

grained, thinly bedded turbidites of the interchannel

area are weathered and poorly exposed in the central

portion of this panorama but are well exposed in

the areas immediately adjacent to the channel mar-

gin (Figure 12C). Figure 12A illustrates the easterly

thinning and onlap of channel-fill units along a west-

erly inclined set of surfaces that merge to form the

channel margin. The interchannel facies consists of thin-

bedded, fine-grained turbidites with occasional thick-

bedded, planar-laminated sandstones. By contrast, the

channel fill is characterized by thick-bedded sand-

stones and conglomerates deposited from energetic, high-

concentration sediment flows. Here, the relationships

Figure 9. Panoramic view of the Lago Sarmiento Chico area. (A, B) Photographs taken at a high angle to the axis of the synclinewith a view to the south. This view shows the eastern margin for the upper portions of channel-complex set 3. Channel-filling unitsare thickly bedded conglomerates (congl), conglomeratic debris-flow deposits (df ), and medium- to thick-bedded sandstones (ss).These occupy the central and right-hand portions of the panorama. Thin-bedded turbidites (tbt) make up the area adjacent to thechannel margin (far left). Interchannel and channel-fill units are separated by a complex of numerous erosional surfaces, whichmerge along the trend of the contact to form a composite channel margin. Paleocurrents from the interchannel strata indicate flowaway from, and at high angles to, the trend of the channel margin. The channel margin for the lower part of the channel complex islocated farther north (behind the field of view in this panorama) and steps to the south-southeast into the central portion of thesyncline for the upper part (visible here). In the lower, central portion of the photo (at lake level) is a highly chaotic interval consistingof disrupted thin-bedded turbidites. This is interpreted as a slump deposit derived from the inner, eastern wall of the leveed channel.

Beaubouef 1485

Figure 10. Selected areas around Lago Sarmiento Chico. (A) Channel margin on the north side of Lago Sarmiento Chico. To the left ofthe margin are conglomerates, medium-bedded sandstones, and debris-flow deposits. Lithofacies change from relatively thick,amalgamated conglomerate to well-bedded sandstones near the margin. These sandstones cannot be traced to the right of the dashedline. (B) Interchannel facies exposed along the south shore of Lago Sarmiento Chico. The facies are mainly very continuous, thin-beddedturbidites (TBT) with occasional thicker bedded (0.31-m; 13-ft), channelized sandstones. This section is gently deformed into a seriesof broad, low-amplitude folds. Thicker, lenticular sandstones are ponded in fold axes. Collectively, the postfolding deposits areinterpreted to represent a crevasse-splay complex. (C) Close-up photograph of lenticular sandstone encased in thin-bedded turbidites.This sandstone and others in this section appear to have been ponded in structurally defined lows. (D) Close-up photograph of a thick,medium-grained sandstone bed contained in the crevasse-splay complex seen in (C). The margin of this bed is defined by severalerosional surfaces that can be traced into amalgamation surfaces (A) in the sandstone. (E) Lenticular sandstone beds associated with thefill of erosional topography. An erosional surface separates two packages of thin-bedded turbidites.

1486 Deep-Water Leveed-Channel Complexes of the Cerro Toro Formation

between the conglomerate and thick sandstones are very

clear. Lateral facies changes from conglomerate to sand-

stone occurring along the margin likely reflect gradients

in the sediment concentration, grain size, and energy of

flows from the channel axis toward the margin. The

stratigraphy of this marginal channel fill can be divided

into three main packages (Figure 12A). The lower pack-

age rests above the basal erosional surface and consists

of thickly bedded conglomerate that grades laterally

(eastward) and vertically into fine-grained, massive

sandstone overlain by more thinly bedded Ta,b,c sand-

stone beds and debris-flow deposits. These more thinly

bedded units of the lower package are progressively

truncated to the west by the middle erosional surface

(surface 2) that is overlain by minor conglomerates

and thick-bedded, semiamalgamated sandstones. These,

in turn, are truncated to the east by the upper ero-

sional surface (surface 3), which is overlain by thickly

bedded, highly amalgamated, structureless sandstone.

The three main erosional surfaces merge toward the

east to form a composite channel margin. The pro-

nounced erosional surfaces, vertical changes in stack-

ing patterns and bedding types, and facies changes

toward the channel margin indicate multiple episodes

of channel incision and filling associated with shifting

of the channel axis through time.

The channel margin is also visible on the north side

of the lake (Figure 12B). In this locality, the channel

margin is slightly steeper than its equivalent on the

south side of the lake and exhibits a stair-step geometry

(compare with Figure 12A). The differences observed

along the margin of this channel complex on either side

of the lake attest to the significant variability of such

channel margins over short distances. Shown in the

lower right-hand side of the photograph is a debris-flow

unit exposed near lake level. This debris-flow deposit

lies along the outcrop trend of the channel margin and

exhibits limited aerial extent much like the disrupted

unit described from the Lago Sarmiento Chico area.

This unit was deposited from a slumped portion of the

inner wall of the channel margin prior to the filling of

the channel.

In the far, eastern portion of the outcrop is a chan-

nelized feature containing highly chaotic and disrupted

thin-bedded units (Figure 11B). Bedding on either side

of the feature is coherent but exhibits opposing dips.

This feature is interpreted to represent a flank failure

in a proximal to medial position in the levee (DeVries

and Lindholm, 1994). It is interpreted to have origi-

nated because of slumping or foundering of the levee.

This, in turn, may have led to the development of a

slump scar or channel, which was ultimately filled with

mass-transport deposits derived from the foundered

portions of the levee. Positioned in the upper right side

(west) of this feature is a dislocated, lenticular sand-

stone bed. This sandstone may have originated as a

splay deposit that subsequently slumped into the failed

flank of the levee. The presence of deposits such as these

and other mass-transport facies indicate some degree

of topographic and depositional relief and are consis-

tent with the interpretation of these units as portions

of leveed-channel complexes.

Lago Nordenskjold Area

The Lago Nordenskjold area is in the northwest limb of

the Cerro Silla syncline south of Lago Nordenskjold.

The stratigraphic section shown in Figure 13A is

interpreted to represent the distalmost levee associ-

ated with channel-complex set 3 and is characterized

by relatively thin, interbedded fine sandstones, silt-

stones, and mudstones. This section is located 3.5 km

(2 mi) from the axis of channel-complex set 3, and the

orientation of this panorama is at a high angle to the

overall trend of the channel complexes. Stratigraphically

younger sandstones and conglomerates in channel-

complex set 4 overlie and onlap this section and can be

seen in the upper part of the photograph along the

skyline. The basal and middle portions of the fill of

channel-complex set 3 are visible in the background

(to the southwest) of the photograph. The stratigra-

phy can be divided into two large packages of strata

(labeled as PKGs 1 and 2 in Figure 13A), each of which

are characterized by thinning- and fining-upward suc-

cessions of mainly low-concentration turbidites. These

thinning- and fining-upward packages represent dis-

tinct cycles of interchannel deposition, each showing a

transition from a higher energy, more sand-rich deposi-

tional style to a lower energy mode of sedimentation

during the waning stages of a given cycle.

Associated with this transition is a change in the

sandstone/shale ratio, which, on average, decreases ver-

tically, but can be as high as 35% in the lower portion

of stratal package 2 (Figure 13B, C). Similarly, decreas-

ing sandstone trends are observed to occur laterally,

along strike, and are particularly visible in package 2.

Additionally, there is a progressive vertical change

in bedding geometry and style of deposition in these

packages. The lower portions typically contain fine-

grained sandstones representing both low- and high-

concentration turbidites (Ta,b,c). These sandstone beds

are interbedded with thin silty mudstones and have

Beaubouef 1487

1488 Deep-Water Leveed-Channel Complexes of the Cerro Toro Formation

thickness on the order of 15 cm1.5 m (6 in.5 ft), are

continuous over distances of 150250 m (500800 ft),

and are commonly lenticular (Figure 13BD). Thicker

sandstones can be traced laterally into thin siltstone

stringers (Figure 13E). The upper portions are mudstone

dominated and are characterized by low-concentration

turbidites, consisting of thinly interbedded, very fine-

grained Tc sandstones, siltstones, and silty mudstones

(Figure 13F, G). Individual sandstone beds average

4 cm (1.6 in.) in thickness and are typically continuous

over the width of the outcrop. Bedsets of these low-

concentration turbidites commonly exhibit draping ge-

ometries and can be seen to mantle preexisting topo-

graphic relief of 5 m (16.5 ft).

This section is interpreted as the distalmost levee

observed in the area. The two stratigraphic packages

(packages 1 and 2) represent individual cycles of levee

growth. The thinning- and fining-upward trends may

reflect a gradual decrease in the amount and size of

overbanking flows reaching this portion of the levee

flank because of pronounced growth of the proximal

levee and progressive confinement of turbidity flows

to the channel. The repetition of these cycles and, in

particular, the return to active, constructional levee de-

velopment following a period of relative quiescence

may be associated with several factors. First, an increase

in the frequency and/or size of high-concentration flows

passing through the channels would have affected the

amount and grain size of overbank material reaching

the levee flank. Second, the sandier divisions may re-

flect diversion of flows from the channel directly to the

levee flank (i.e., channel avulsion, splay deposits, etc.).

Third, these cycles may have been associated with

changes in the relative position and/or orientation of

the channel through time. Finally, some combination

of these factors may be responsible for these stacking

patterns.

It is not possible to demonstrate the chronostrati-

graphic relationships between these cycles of overbank

deposition and levee building and the record of channel

evolution and filling in channel-complex set 3. How-

ever, we may tentatively correlate stratal package 1 with

the lower or middle divisions of the channel-complex set

seen in the western limb of the syncline. Likewise, we

may correlate stratal package 2 with the upper divi-

sions seen in Lago Sarmiento Chico and Jigsaw areas.

SUMMARY AND DEPOSITIONAL MODEL

Channel-complex set 3 is the best exposed and most

completely documented unit in the study area and is

the focus of this paper. Based on observations of

field relationships, the following interpretations

are made regarding deposition of channel-complex

set 3. Sediment gravity flow reaching this portion

of the Magallanes basin included debris flows and

turbidity currents. However, based on the volumet-

ric abundance of thin-bedded turbidites in the Cerro

Toro Formation vs. the amount of matrix-supported

conglomerate in the Lago Sofia Member, turbidity

currents were most common. The flows were most

likely introduced into the foreland basin through high-

gradient transfer zones in the northwest as a result of

catastrophic flooding and/or collapse of coarse-grained

fluviodeltaic or fan-delta systems (Mutti et al., 1996)

and flowed south-southeast along the axis of the basin

(Scott, 1966). Turbulent flows forming and passing

through these channels were large, highly energetic,

and strongly stratified with the coarsest and most

concentrated portions near the base and the finer, least

concentrated portions of the flow near the top. In the

sense of Mutti et al. (1999), these were highly efficient

flows capable of moving cobble and pebbles over long

distances. The facies change from clast-supported

conglomerate to sandstone reflects a general transition

in flow velocities from channel axis to margin. Within

channel axes, the flows had sufficient capacity to

transport cobble as bed load, forming bars on the floor

of the channels as suggested by Winn and Dott (1979).

These bedding styles indicate relatively sustained flow

through the channels. The coarsest portion of the

suspended load was lost from the flows along the

periphery of the channels, resulting in the deposition of

tabular sandstone beds that thin and lap out against the

channel walls. Additionally, some of the sandstones

Figure 11. Panoramic photograph of the Lago Jigsaw area. (A, B) The Lago Jigsaw area with a view to the south. View shows theupper, eastern margin of channel-complex set 3. The channel margin is inclined to the west and separates channel-fill units on theright from interchannel (levee) units on the left. The channel margin is composite. The progressive onlap of channel-filling units ontothis compound margin is accompanied by facies changes, rapid thinning of beds, and erosional truncation of beds. In the far, easternportion of the outcrop is a channelized feature containing highly chaotic and disrupted fine-grained units. This feature is interpretedto represent a flank failure in a proximal-medial position in the levee.

Beaubouef 1489

1490 Deep-Water Leveed-Channel Complexes of the Cerro Toro Formation

observed in the channel fills may represent suspension

deposition in the distal (downflow) facies equivalents of

gravelly bars instead of their lateral equivalents.

As these flows passed through the channels, the

upper, more dilute portions of the flows were allowed

to escape the channels and deposit the remaining sus-

pended load. As overbank aggradation led to levee

formation, progressively finer material was released

from the channels. Medium sand was the coarsest grain-

size fraction that could be deposited as overbank ma-

terial. However, the bulk of the overbanking flows

contained mud and fine sand. As the overbanking flows

spread away from the channel, their capacity to trans-

port sand decreased. The proximal overbank setting

was an area of rapid-flow transitions as they expanded

and became unconfined after exiting the channel. Li-

thofacies in these areas are consistent with deposition

from both high- and low-concentration turbidity cur-

rents. The grain-size distributions in these flows ranged

from medium sand to mud. Minor debris-flow deposits

were locally derived and mud rich. Additionally, the

proximal overbank appears to have been an area of

accentuated topography and relatively high deposi-

tional gradients. The slumped and mass movement

facies, together with channelized, ponded, and crevasse-

splay sands, are cited as evidence for topographic relief.

By contrast, the distal overbank setting was an area of

relatively steady, low-velocity, unconfined, and blan-

keting flows. Lithofacies in these areas are consistent

with deposition from primarily low-concentration

turbidity currents. The range in grain size in the flows

reaching these settings was from fine sand to mud,

reflecting a general sorting and fining of the flows away

from the proximal overbank settings. The distal over-

bank setting appears to have been an area of subdued

topography and lower depositional gradients, as evi-

denced by draping and concordant bedding and lack of

mass movement facies.

These descriptions are considered generalizations

regarding the depositional processes involved in the

formation of the channel complexes and their fill. How-

ever, stratigraphic relationships suggest a distinct chro-

nology of depositional events. The formation of chan-

nel complexes appears to have occurred in repeated

cycles that include three main stages as shown in Figure 8.

In chronological order, these stages are as follows:

1. An initial stage of relatively unconfined, sand-rich

deposition (Figure 8A). In this early stage, the sys-

tems consist of weakly confined and distributive

networks of channels and depositional lobes. Over-

all, this stage results in the formation of relatively

sand-rich deposits containing channel fills, channel

terminus lobes, and overbank facies.

2. Aggradation of a mud-rich, confining levee system

(Figure 8B). Resulting from overbank deposition

from bypassing flows, rapid aggradation of overbank

deposits forms levees. As a consequence, a central or

primary channel is formed, which serves to confine

subsequent flow. This stage is characterized by a

widespread, blanketing style of deposition in inter-

channel areas, punctuated by crevasse-splay develop-

ment. Continued aggradation results in differential

topography on the levees, and mass movement fa-

cies develop.

3. Filling of the channel-margin relief by onlap of

channel-fill sediments (Figure 8C). During the

channel-filling stages, flow is confined to channel

by leveed margins. Channel-filling occurs through bed

load-dominated sedimentation in channel axes and

suspension deposition along the margins.

The transition between stages 2 and 3 is marked

by erosion as the channel becomes entrenched or mi-

grates. Shifting of the thalweg and erosion by through-

going flows results in the entrenchment of channel

Figure 12. Selected areas around Lago Jigsaw. (A) Closer view of the channel margin on the south shore of the lake (as shown inFigure 11). Note easterly thinning and onlap of channel-fill units along a westerly inclined set of surfaces that merge to form themargin. Channel fill is thick-bedded sandstones and conglomerates. Lateral facies changes from conglomerate to sandstone occurtoward the margin. Paleocurrents from the base of channel-fill units indicate flow to the southeast along the margin. Interchannellithofacies consist of thin-bedded, fine-grained turbidites with minor, thick-bedded, planar-laminated sandstones. (B) Channel marginon the north shore of Lago Jigsaw. View is of the equivalent section in (B). Here, the channel margin is slightly steeper than on thesouth side of the lake and exhibits a stair-step geometry. Shown in the lower right-hand side of the photograph is a debris flow unit atlake level. This unit is interpreted as a slumped portion of the inner wall of the channel margin. (C) Close-up of channel-complex baseat the margin. Here, the basal channel margin is subparallel to bedding in the underlying units. Note the thin-bedded, tabular natureof sandstone beds in the interchannel facies. (D) Measured stratigraphic section collected at the channel margin in (A). Shownschematically are lateral relationships along the channel margin.

Beaubouef 1491

1492 Deep-Water Leveed-Channel Complexes of the Cerro Toro Formation

and degradation of channel margins. Repetition of these

stages, combined with channel shifting or migration,

results in stratigraphic relationships in the complex as

illustrated in Figure 14. In this schematic illustration,

the left margin of the channel complex is charac-

terized by a complicated interleaving of the levee and

overbank strata (deposits of stages 1 and 2) and channel-

fill units (stage 4 deposits). The right margin is a com-

posite erosional scarp that separates channel and in-

terchannel strata. The levee and overbank strata are

comprised of packages of upward and laterally thinning

fine-grained turbidites. The coarsest grained deposits

are found in the channel axes, and their positions

record the changing locations of channel thalwegs

through time. In off-axis positions, these are replaced

by sandstones that onlap the erosional surfaces. The

sandstones are both in facies with and cut by the

conglomerates.

IMPLICATIONS FOR DEEP-WATER RESERVOIRS

Direct comparisons between the rock properties of the

Cerro Toro Formation and most hydrocarbon reser-

voirs cannot be made because of the age, deformation

history, and low-grade metamorphic alteration of the

formation. However, useful sedimentologic and strat-

igraphic analogies can be drawn between the Cerro

Toro outcrops described here and Tertiary deep-water

systems such as those currently being explored and

produced in offshore west Africa (Abreu et al., 2003).

In turn, these analogies may provide some clues about

the characteristics and production performance of

channel-fill and associated overbank reservoirs. Con-

sidering channel-complex set 3 as an analog for such

reservoirs, some generalizations can be made (Figure 14;

Table 1). For the purposes of this discussion, a hypo-

thetical case is considered, in which the Cerro Toro

strata have not been altered by diagenesis and meta-

morphism but exposed to burial histories typical of

most Tertiary reservoirs in 11.5 km (0.61 mi) of the

subsurface. A coordinate system is employed, where xis oriented parallel to the depositional strike of the

channel complexes, y is oriented parallel to the axesof channel complexes, and z is vertical with respect tothe original stratigraphic layering (Figure 14).

Reservoir Characteristics of Channel-Fill Lithofacies

The highest proportion of reservoir grade sediments

are found in the channel-fill lithofacies. Collectively,

these types of reservoirs would likely represent the

focus of field development and production strategies,

because they would contain the highest density of hy-

drocarbon resources and the highest concentration of

coarse-grained, interconnected reservoir elements.

The three-dimensional geometry of individual channel-

fill elements was not determined in this study. How-

ever, based on their sedimentology and internal stra-

tification, many appear to represent channel-parallel

(y-oriented), erosion-based, barlike elements. Theseunits amalgamate along the axes of individual channel

Figure 13. Lago Nordenskjold area. (A) Photo of northeast limb of the syncline, south of Lago Nordenskjold with a view to the east-southeast. This section is located 3.5 km (2.1 mi) to the north of the axis of channel-complex set 3. This is the most distal interchannelsection studied. It is characterized by relatively thin, interbedded fine sandstones, siltstones, and mudstones. Stratigraphically youngersandstones and conglomerates in channel-complex set 4 overlie and onlap this portion of the levee complex. The stratigraphy of theunderlying section can be divided into two large packages of strata (labeled as PKGs 1 and 2). Each shows thinning- and fining-upward successions. Similar thinning and fining trends are observed to occur laterally and are particularly visible in package 2.(B) Lithofacies and bedding types in the lower portion of PKG 2 shown in (A). Resistant beds are fine-grained sandstonesinterbedded with silty mudstones (dark). Thicker sandstones seen here are typically 1015 cm (46 in.) thick. Some of the individualsandstone beds on the right are amalgamated and progressively thin and become nonamalgamated toward the left. These occur inassociation with the general northeasterly thinning in package 2 (shown in A). (C) Close-up of lithofacies type shown in (B). Resistantbeds are fine- to very fine-grained sandstones interbedded with silty mudstones (dark). (D) Approximately 13-cm (5.1-in.) thickcurrent laminated sandstone (Tc) beds such as this are the dominant sandstone types found in the setting. This bed is comprised ofnumerous 12-cm (0.40.8-in.) ripple sets separated by planar-laminated muddy siltstone. The apparent paleocurrent directionfrom ripple foresets is to the left (northeast). (E) Very thin siltstone stringers interbedded with burrowed mudstone. These siltstonesare laterally equivalent to thicker, fine-grained sandstone beds and can be traced to beds seen in (B and C). (F) Thinly interbedded,very fine-grained sandstones, siltstones, and silty mudstones in the upper portion of stratal package 1 (PKG 1 in A). These parallel-bedded thin turbidites are continuous over the width of the outcrop. (G) Close-up of lithofacies shown in (F). The light-colored, veryfine-grained sandstone and siltstone beds exhibit flat bases and grade upward into dark-colored mudstones that are locallyburrowed. The lower sandstone bed exhibits weak current ripples (Tc) and is approximately 5 cm (2 in.) thick.

Beaubouef 1493

complexes that, in turn, cluster to form an elongate

reservoir core or sweet spot parallel to the y-axis.Stacked and amalgamated clast-supported conglom-

erate and well-sorted sandstones with no intervening

fine-grained drapes suggest that good communication

would exist between the channel fills in a channel com-

plex. In these areas, vertical permeability (Kz) wouldbe high, and only minor barriers or baffles to fluid

flow would exist in channel-complex axes. These res-

ervoir discontinuities would result from the apparently

random occurrence of mud-rich, matrix-supported

conglomerate (debrites). Along the x-axis, reservoirquality degrades from the axis toward the channel-

complex margins because of trends of decreasing grain

size, bed thickness, and amalgamation. Additionally,

potential reservoir compartmentalization increases

in the marginal areas because inclined erosional sur-

faces and slumps may interrupt the continuity of these

facies.

Locally, the axes of individual channel complexes

are in contact across erosional surfaces. In these areas,

coarse sandstones and conglomerates are stacked to form

continuous vertical sections, forming conduits for pres-

sure communication and fluid flow between complexes.

Depending on their extent and geometry, these zones of

enhanced amalgamation may allow for connectivity and

Figure 14. Schematic diagram illustrating channel complex after repetition of depositional stages shown in Figure 8. (A) Stratigraphicrelationships similar to those seen in portions of channel-complex set 3 (refer to Figure 8). (B) Line drawing based on (A) showingidealized gamma-ray curves for vertical sections through the interval, with emphasis on aspects of reservoir characteristics.

1494 Deep-Water Leveed-Channel Complexes of the Cerro Toro Formation

pressure equalization between all scales of reservoir ele-

ments in the channel-complex set. Identification and map-

ping of such reservoir connections are extremely impor-

tant for field development strategies and understanding

well performance over the life of a deep-water field.

Reservoir Characteristics of Interchannel Lithofacies

The interchannel lithofacies would represent an

important but secondary reservoir target for explora-

tion and field development strategies. Volumetrically,

Table 1. General Reservoir Characteristics of Channel-Fill and Interchannel Lithofacies in Leveed-Channel Complexes such asChannel Complex Set 3 of the Cerro Toro Formation

Lithofacies

Net/Gross

(Net Sand/Conglomerate

in Gross Interval) Thickness of Bedding Continuity of Bedding

Interchannel FaciesPrimarily thin-bedded,

fine-grained turbidites

moderate to high in

proximal and lower

portions, low in upper

and distal portions

sandstones on a scale

of centimeters to 1 m (3 ft)

in proximal positions,

on the scale of centimeters

in distalmost portions

wide range of stratal

lengths in proximal

positions caused by

depositional and

postdepositional factors

Interbedded Tbc sandstones

and mudstones, with

subordinate thick, Tasandstones, crevasse-splay

complexes, and mass

movement facies

values decrease

uniformly with

distance from

channel margin

range of bed thickness

decreases with distance

from channel margins

average stratal lengths

increase, and range of

values decrease with

distance from channel

margin

local channelized

and crevasse-splay

sandstones can

significantly increase

N/G locally

interbedded nature of

facies results in poor

vertical connectivity

of sandstones

in general, thin sandstones

have highest lateral

continuity; thicker beds

have the lowest

Channel-Fill FaciesPrimarily clast-supported

conglomerates with

matrix-supported

conglomerates

(debris-flow deposits)

and sandy turbidites

very high in axial

positions because of

intercutting channels

and limited drape facies

conglomerates on

the scale of several

meters thick in axial

areas; sandstones on

scale of 1 m (3 ft) in

marinal areas

continuity of individual

beds is highly variable

because of erosion in

channel axes

Axial fill characterized

by graded, inversely

graded, and massive

conglomerates; marginal

fill characterized by

bedded-amalgamated

Tab sandstones

values decrease

(but still high)

toward channel

margin because of

interbedded nature

of marginal fill

individual channel-fill

packages (bedsets) on

the scale of meters;

channel complexes on

the scale of 100 m (330 ft)

width and continuity of

channel fill packages

(bedsets) on the scale

of hundreds of meters;

channel complexes on

the scale of 1 km (0.6 mi)

amalgamated nature

of fill results in high

degree of vertical

connectivity between

beds, channel fills, and

channel complexes

amalgamated nature of

fill results in high degree

of lateral connectivity

between beds, channels,

and channel complexes

Beaubouef 1495

the interchannel strata are the most significant facies

types associated with these channel complexes. How-

ever, the lower sandstone percentages, finer grain sizes,

and interbedded nature of this facies would make it

a less favorable reservoir target than the channel-fill

facies. Despite this, the interchannel facies deserves con-

sideration because of its genetic association with the

channel fills and volumetric significance.

The reservoir quality of individual sandstone beds

in interchannel settings would be good. Although rela-

tively thin and fine grained, they are well sorted, and

many have high lateral continuity. As such, these beds

would be characterized by high horizontal permeability

(Kx,y). However, because these sandstones are inter-bedded with extensive mudstone intervals, the vertical

permeability (Kz) of interchannel intervals would, ingeneral, be very low. At the scale of the channel com-

plex, interchannel reservoir quality is best in the prox-

imal and lower portions but progressively degrades

both vertically (z-axis) and laterally along the x-axis(away from the channel margins). Although sandstone

percentage can be high in the proximal and lower

interchannel intervals (stage 1 deposits), lateral conti-

nuity of individual thick beds is variable. By contrast,

whereas lateral continuity of beds in the upper levee

sections (stage 2 deposits) is generally good, the per-

centage of sandstone is very low, and individual beds are

very thin. Superimposed on these general stratigraphic

and reservoir trends are occurrences of local debris-flow

deposits, mass movement facies, crevasse-splay com-

plexes, and secondary channels. Muddy debris-flow

deposits, slumps, and mudstone-filled channels will

interrupt the lateral continuity of interchannel intervals

and negatively impact horizontal permeability (Kx,y).However, splay and minor channel-fill sandstones will

provide connections between otherwise nonamalga-

mated sandstone beds. In these cases, the vertical per-

meabilty (Kz) will be locally enhanced and providepathways for fluid flow and pressure communication.

Connectivity between Channel-Fill and Interchannel Facies

The first-order controls on channel and interchannel

connectivity are the stacking pattern arrangements and

facies juxtapositions resulting from the depositional his-

tory of the channel-complex set (Figures 8, 14). The

nature of the channel-fill and interchannel contacts in the

study area varies stratigraphically and geographically. In

places, the margins of the channel complex are charac-

terized by a complicated interleaving of the levee and

channel-fill strata. In other places, the margins form a

composite erosional scarp that separates channel and

interchannel strata. Additionally, the geometry of the

channel margins can change over very short distances

along the length of the channel complexes (Figure 11).

The primary stratigraphic controls on potential pres-

sure communication and fluid mobility between the

channel and interchannel strata are the large differ-

ences in effective permeability of units in contact with

one another across complex channel margins and the

transmissibility of channel-margin zones. Amalgamat-

ed channel-complex axes, with high y-axis continuityand good Kz, will generally form zones of preferentialfluid movement along the trend of the channel-complex

set during production. The interchannel reservoirs

would be depleted at a much lower rate than the

channel-fill units. Thus, the potential for stranded or

bypassed hydrocarbon in these facies is high. Addition-

ally, it should be pointed out that syn- and postdepo-

sitional complexities along the channel margin might

preclude communication locally. Fine-grained channel-

margin slumps (Figures 8, 11) would form baffles to

fluid flow between channel and interchannel strata.

However, production of hydrocarbon residing in

interchannel strata from wells completed in channel-

fill units is possible. Although beds of the channel-fill

units do not extend beyond the channel margins, res-

ervoir connectivity may exist between the channel-

fill and levee strata. This is caused by sand-on-sand

contacts on either side of the erosionally defined chan-

nel margins and lack of drapes. In cases where splay or

minor channel sandstones connect thin, nonamalga-

mated, interchannel sandstones that are also in direct

contact with channel-fill units, large areas may be in

pressure communication. In such cases, it is possible

for fluids in the channel and interchannel strata to be

produced simultaneously.

DISCUSSION

The stratigraphy of the Cerro Toro Formation in the

Cerro Silla syncline comprises an approximately 1-km

(0.62-mi) vertical section, including a series of migrat-

ing leveed-channel complexes within a 40-km2 (15-mi2)

area. Within this section, channel complexes can be

grouped into genetic associations that form four ma-

jor channel complex sets (sensu Sprague et al., 2002).

This analysis supports the general conclusions of Winn

and Dott (1979) but conflicts in important ways with

the model of Coleman (2000). The results of those

1496 Deep-Water Leveed-Channel Complexes of the Cerro Toro Formation

studies are compared and contrasted with those of this

study in Figure 15.AQ17

The Winn and Dott (1979) (Figure 15A) model is

a very similar to that shown in Figure 14 of this paper

in terms of scale and stacking patterns of leveed-channel

complexes. In addition, this work is consistent with that

of Winn and Dott (1979) in terms of the facies changes

occuring in channel fills. However, two differences

exist. First, we observed no units that could be traced

from the levee-overbank areas into the channel-fill units

as shown in their model. Second, the relationships

between the coarse channel-filling units and the fine-

grained levee-overbank strata are shown as facies changes

in their model. We find the two lithofacies associations

to be in erosional contact everywhere. Instead of a

facies change, we find a complex interleaving of chan-

nel fill and overbank strata along serrate edges of chan-

nel complex sets formed by stepped channel margins

(Figures 14, 15C).

The Coleman (2000) model appears to be that of a

simplified backfilled erosional channel cut into sheet

sandstones (Figure 15B). The ubiquitous thin-bedded,

fine-grained turbidities that comprise the largest

volume of the Cerro Toro deposits are not represented

in the model shown in Figure 15B. Applying a dep-

ositional model developed for the Brushy Canyon

Formation (Permian, west Texas), the sheet sandstones

are interpreted to represent the build phase of a build-

cut-fill-spill model (Gardner and Borer, 2000). The

channel surfaces and the conglomeratic fill represent

the cut and fill phases, respectively. From his descrip-

tions, the thin-bedded turbidities represent the spill

phase. In some ways, this model is similar to that

presented in Figure 13, but some important distinctions

should be drawn. First, the conglomeratic units clearly

do not represent the fill of simple erosional channels

as shown in Figure 15B. Collectively, the channel-fill

units represent composite channel-form bodies com-

prised of numerous smaller scale channel elements. We

observed no master scour surfaces that cleanly sep-

arate the channel-fill from interchannel strata. Again,

we find a complex interleaving of channel fill and

overbank strata along the margins of complex sets

(Figure 15C). Second, the Coleman (2000) model places

overbank deposition as the final stage of deposition

(the spill stage) in a sequence of four stages. Strati-

graphic relationships between channel and interchan-

nel strata contradict this interpretation. This work sug-

gests the thinly bedded interchannel strata aggraded

adjacent to the channels prior to channel filling and

following a phase of distributive or weakly confined

sand-rich deposition (Figure 13). This is simply based

on the observations that (a) channel-fill units lap out

against erosional surfaces cut into the fine-grained fa-

cies and (b) the very thinly bedded turbidite sections

overlie the more thickly bedded turbidities of the in-

terchannel areas. In addition, once the channels were

filled and began to spill, we would expect to find coarser

grained deposits in the interchannel areas; not only

thin-bedded turbidities, unless the types of flows reach-

ing these portions of the channel complexes changed

upon their filling in each case. Third, the Coleman

(2000) model does not make a distinction between

medium- to thick-bedded sandstones within the chan-

nel fills vs. those in interchannel areas (Figure 15C).

Finally, the Coleman (2000) model depicts no facies

changes or internal variability within the channel-fill

lithofacies.

The Cerro Torrro Formation appears analogous to

leveed-channel systems observed in Quaternary sub-

marine fans and older, subsurface examples. In partic-

ular, the depositional model presented here (Figure 13)

is very similar to that described for the stratigraphy

and lithofacies of the Amazon Fan (Shipboard Sci-

entific Party, 1995; Pirmez et al., 1997; Pirmez et al.,

2000) and the Zaire Fan (Babonneau et al., 2002; Droz

et al., 2003). The weakly confined sand-rich stage de-

posits of the Cerro Toro (Stage