The transgression of an erg – sedimentation and reworking/ soft ...

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The transgression of an erg – sedimentation and reworking/
soft-sediment deformation of aeolian facies:the Cretaceous
Troncoso Member,Neuque´n Basin,Argentina
ANNA STRO
¨
MBA
¨
CK
1,2
,JOHN A.HOWELL
1,3
& GONZALO D.VEIGA
1,4
1
Stratigraphy Group,Department of Earth Sciences,University of Liverpool,
Brownlow Street,Liverpool L69 3GP,UK
2
Present address:Statoil ASA,Forus Vest,4035 Stavanger,Norway
(e-mail:acst@statoil.com)
3
Present address:Centre for Integrated Petroleum Research,University of Bergen,
Allegaten 41,Bergen N-5007,Norway
4
Centro de Investigaciones Geolo
´
gicas,Calle 1#644.B1900TAC La Plata,Argentina
Abstract:The Cretaceous Troncoso Inferior Member of the Huitrı
´
n Formation comprises
fluvial and aeolian facies that forma drying-upwards succession within the Neuque
´
n Basin.
The basal fluvial sandstones were deposited as braided river deposits and lie unconformably
on top of either the deep-marine Agrio Formation or,locally,the shallow-marine Chorreado
Member (Huitrı
´
n Formation.).In places,the fluvial sandstones are interbedded with
remnants of aeolian deposits recording an arid environment and ephemeral flows.In the
study area the upper section is predominantly aeolian and was controlled by northerly
winds with both linear and transverse dune types being deposited.The depositional
system was rapidly flooded and dune topography (relief ranging between 2 and 35 m)
was preserved on its top surface.In addition to dune topography,the Troncoso dunes also
show evidence of reworking and in situ soft-sediment deformation related to the flooding.
The principal aim of this paper is to document the soft-sediment deformation and
preservation of topography associated with the flooding of the dune field.
Within the soft sediment deformed and reworked sediments at the top of the Troncoso
Inferior Member spatial and temporal relationships indicate that they formed in a specific
sequence.Initially,water-escape processes created convolutedly folded and dish structures
that were concentrated in areas of slightly higher preserved dune topography.Secondly,the
convolutedly folded and dish facies were eroded and reworked by wave undercutting and
migrating three-dimensional dunes in a shallow-marine environment.This subaqueous
reworking resulted in an interbedded massive and cross-stratified unit.With further deepen-
ing of the water,the topography became stabilized and the uppermost part of the interval
(0.1–0.3 m) was reworked by waves across most of the basin.In the topographic lows
between dunes,liquefaction-induced sediment gravity flows deposited massive–flat-
laminated facies.
The reworked and soft-sediment deformed aeolian dune topography is overlain by the
evaporites of the Troncoso Superior Member.The distribution of flood-related facies and
the amount of preserved dune topography (2–35 m) indicates that the transgression must
have been rapid but of low energy.
The aimof this study is to address geometric and
process aspects of reworked and soft-sediment
deformed facies in a drowned aeolian system.
The Late Aptian Troncoso Inferior Member of
the Neuque
´
n Basin (Argentina) includes aeolian
deposits towards its top,which are overlain by
carbonates and evaporites.The excellent out-
crops east of Chos Malal (Fig.1) provide an
ideal opportunity to study flooded dune
systems.Depositional super-surfaces (sensu
Kocurek &Havholm1993) created by the flood-
ing of dune systems have been recognized pre-
viously in both outcrop and subsurface studies
(e.g.Vincelette & Chittum 1981;Eschner &
Kocurek 1986,1988;Mountney et al.1999).Pre-
served dune topography is also seen in other
From:V
EIGA
,G.D.,S
PALLETTI
,L.A.,H
OWELL
,J.A.&S
CHWARZ
,E.(eds) 2005.The Neuque´n Basin,Argentina:ACase
Study in Sequence Stratigraphy and Basin Dynamics.Geological Society,London,Special Publications,252,163–183.
0305-8719/05/$15.00#The Geological Society of London 2005.
systems such as the Permian Rotliegend–
Weissliegend system (UK,southern North Sea)
(Glennie & Buller 1983;Howell & Mountney
1997;Stro
¨
mba
¨
ck & Howell 2002),the Jurassic
Entrada Sandstone (SW USA) (Benan &
Kocurek 2000) and the Page Sandstone (SW
USA) (Blakey et al.1996).
Within the Troncoso the architecture of the
aeolian and fluvial sequence was studied in
order to understand the depositional system that
were active immediately prior to the transgres-
sion.This is summarized within this paper and
addressed more fully in an accompanying paper
(Veiga et al.2005).The main focus of this
study is the preserved dune topography beneath
the flooding surface,and the distribution of
reworked and soft-sediment deformed facies.
The study area is situated in the NWpart of the
Neuque´n Basin in Argentina (Fig.1).This basin
contains large amounts of Argentina’s hydro-
carbon reserves,and the Troncoso is an important
reservoir interval (Fig.1) (Uliana & Legarreta
1993).Uliana et al.(1975),Legarreta (1985)
and Legarreta & Uliana (1991) have previously
studied the Huitrı
´
n Formation.Veiga et al.
(2005) provide a detailed study of the entire Tron-
coso Inferior Member;however,this is the first
detailed study of the upper soft-sediment
deformed and reworked part of the system.
Basin history and regional stratigraphy
The Neuque
´
n Basin originated during the Late
Triassic due to the extensional collapse of an
Upper Palaeozoic thickened crust (Franzese &
Spalletti 2001).It started as a series of NW–
SE-trending rifts in an intra-arc setting on the
South American foreland.During the Early
Jurassic,regional thermal subsidence (Legarreta
& Gulisano 1989;Legarreta & Uliana 1991)
related to post-extensional cooling of the
extended lithosphere (Uliana & Legarreta 1993)
resulted in the development of a wide marine
embayment on the SWmargin of Gondwana.
During the Jurassic and Cretaceous the
Neuque´n Basin was connected to the Pacific by
a narrow seaway through openings in the mag-
matic arc to the west (Uliana & Legarreta
1993).The sedimentation in the basin was con-
trolled both by eustatic sea-level changes in the
main Pacific and by tectonic uplift of the arc.
The basin was periodically isolated from the
open ocean to the west by eustatic sea-level
falls.The sediments that filled the basin were
sourced predominantly from the SE and pro-
graded towards the NW (Legarreta & Uliana
1991).During the Early Aptian a major fall in
sea level resulted in the deposition of the conti-
nental deposits of the Huitrı
´
n Formation on top
of the marine deposits of the Agrio Formation
(Veiga et al.2005).Towards the centre of the
basin,the Huitrı
´
n Formation includes ephemeral
fluvial deposits,a saline mud flat assemblage and
evaporites (Legarreta 1985).Towards the basin
margins fluvial and aeolian sediments were
deposited.Flooding in the Late Aptian sub-
merged the aeolian/fluvial facies and a closed
hydrological regime resulted in the deposition
Zapala
Chos Malal
Buta Ranquil
Malargüe
Neuquén
0 100 km
35
72
72 66
40
40
CHILE
Argentina
Chile
N
Pampa de Tril
20 km
Chos Malal
N
e
u
q
u
i
é
n
R
v
e
r
N
Curaco
40
40
9
7
4
3
7
2
6
5
1
5 km
0.5 km
Fig.1.Location of the study area within the Neuque´n Basin.Modified after Uliana &Legarreta (1993).The numbers
marked on the location maps to the right indicate the logs in Figure 3.
A.STRO
¨
MBA
¨
CK ET AL.164
of anhydrite.Towards the margins of the basin
the anhydrites are nodular with an algal matrix,
whilst towards the basin centre there is
‘varved’ anhydrite and bedded halite (Legarreta
& Uliana 1991).
In the study area,the base of the Troncoso
Member overlies marine deposits of the Chor-
reado Member (Huitrı´n Formation) and the
Agrio Formation (Fig.2).The Troncoso
Member is divided into two intervals:the Tron-
coso Inferior Member,which is composed
primarily of siliciclastic material,and the
Troncoso Superior Member,which is dominated
by evaporites (Veiga &Rossi 1992) (Fig.2).This
study concentrates on the Troncoso Inferior
Member that generally has a lower fluvial and
an upper aeolian portion.Small-scale dune
deposits dominate the lower part of the aeolian
succession,while the upper part is characterized
by larger scale aeolian bedforms (Fig.3).The
thickness of both the fluvial and aeolian deposits
vary considerably:north and west of the study
area no aeolian facies have been observed,
whilst 50 km east of the study area subsurface
data record a thickness of up to 100 m of Tron-
coso Inferior Member deposits (Comeron 1990).
The upper surface of the Troncoso Inferior
Member is irregular and represents preserved
dune topography that varies in height from 2 to
35 m (Fig.3).At the top of the aeolian interval
reworked and/or soft-sediment deformed facies
of varying types and thickness are present.
Within the study area,the evaporites of the
Troncoso Superior Member are mainly white,
laminated anhydrites,in a 5–10 m-thick unit.
Facies associations
The Troncoso Inferior Member within the study
area is composed of the fluvial (FA-F),aeolian
(FA-A),soft-sediment deformed (FA-SSD) and
reworked (FA-R) facies associations.This study
concentrates on the aeolian deposits that were
present prior to the flooding,and the reworked
and soft-sediment deformed facies that formed
when the basin was flooded.Amore in-depth dis-
cussion of the Troncoso facies and their broader
distribution can be found in Veiga et al.(2005).
Fluvial facies association (FA-F)
The Troncoso Member includes a variety of
different fluvial deposits.These are considered
in some detail by Veiga et al.(2005).The follow-
ing brief description refers only to the fluvial
deposits that occur within the study area.They
are discussed because they form an integral
part of the overall drying-upwards succession
that comprises the member.
Within the study area the base of the Troncoso
Member is an erosive unconformity that is over-
lain by conglomeratic and sand-rich,trough
cross-bedded deposits in large cross-bed sets up
to 2 m thick (Fig.4a).Clasts include mudstones
from the underlying Agrio Formation and lime-
stones.Overall,there is an upwards decrease in
cross-bed thickness and mean grain size.The
upper part of the fluvial succession is comprised
largely of reworked aeolian sand,and pebble
clasts are rare–absent.The units are interbedded
with minor silty intervals,small aeolian dune and
aeolian sandsheet deposits.
These fluvial deposits provide evidence of an
ephemeral,braided channel belt.The massive
and cross-bedded sets with conglomeratic base
were formed by erosive bedload processes
within fluvial channels (Miall 1988).On the
floodplains adjacent to the channels,crevasse
Cretaceous
Upper
Lower
Huitrín
Agrio
Mulichinco
Quintuco
Vaca Muerta
Rayoso
Rio Limay
Rio Neuquén
Rio Colorado
Loncoche
Roca
Malargüe
Neuquén
Rayoso
Mendoza
FormationLithology
Group
Troncoso-Inferior
Troncoso-Superior
Evaporites
Reworked/SSD facies
Aeolian dune facies
Fluvial facies
Shallow marine facies
Deep marine facies
(shales and turbidites)
Chorreado Mbr
Agrio Fm
HuitrínFormation
Fig.2.The Cretaceous stratigraphy of the northern
Neuque´n Basin with the studied interval highlighted.
Modified after Gulisano & Gutierrez Pleimling (1995).
THE TRANSGRESSION OF A CRETACEOUS ERG 165
Fig.3.
Logscorrelatedacrossthestudyarea.Lognumbers1–5arefrom
PampadeTril,and6and7fromCuraco.SB,sequenceboun
dary;FS,floodingsurface.
A.STRO
¨
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¨
CK ET AL.166
splays deposited plane-laminated and rippled
sandstones during ephemeral sheet flood events
(Williams 1971).The well-sorted nature of the
fluvial deposits in the upper part of the fluvial
succession may suggest that parts of the fluvial
system were active during aeolian sedimentation
and locally reworked the texturally mature dune
deposits.
Aeolian facies association (FA-A)
Aeolian deposits lie directly above the fluvial
facies in the Troncoso succession (Fig.4b–e).
The basal aeolian sandstones are fine- to
coarse-grained and generally poorly sorted.
They are finely laminated (Fig.4e),although
thicker lenses of coarse-grained material are
seen locally.Individual units are generally not
more than 0.5 m thick and are often of limited
lateral extent (Fig.4d).Higher up in the aeolian
succession,small-scale trough cross-bedded
sets (,1 m) also of limited lateral extent
(metres to tens of metres) occur.These are com-
posed of fine- to coarse-grained sandstones that
are moderately well rounded,and moderate to
well sorted.The foresets dip at 108–308
Fig.4.Troncoso Member facies.(A) Within the study area the base of the Troncoso is represented by massive and
cross-bedded fluvial facies lying directly on the offshore marine deposits of the Agrio Formation.(B) &(C) Large-scale
dune facies.(D) Small-scale dune facies.(E) Aeolian sandsheet association with horizontal wind-ripple laminated
facies.Scale:the lens cap is 5 cm,the hammer is 30 cm and the pole 120 cm (each segment is 10 cm).
THE TRANSGRESSION OF A CRETACEOUS ERG 167
towards the NE (in Pampa de Tril) and ENE (in
Curaco).They are coarser grained towards the
base of the sets where the coarse laminae pinch
out and granule lenses are commonly observed.
Locally (in Pampa de Tril),the small-scale,
cross-bedded sandstones are intercalated with
fluvial deposits.
Higher in the section,the cross-beds become
larger (up to 7 m) and contain fine- to medium-
grained sandstones.The sandstones of these
large cross-sets are pink–grey in colour,and
are generally very well sorted and contain well-
rounded grains (Fig.4b).Individual cross-
laminae are a few centimetres thick and are
separated by fine-grained laminae.The cross-
bedding dips at 158–358 (becoming shallower
at the base of sets) and are directed towards the
NNE in Curaco,but have a more widespread
(northerly) to bimodal directions (towards the
north and east) in the Pampa de Tril.Dune
cross-sets truncate each other (Fig.4c),and the
base of the sets are locally poorly sorted and
coarser grained,and form tabular and finely
laminated intervals that can be up to 0.3 m
thick.These intervals locally contain lenses
of coarse-grained sand and granules,and have
bimodal lamination.This texture is similar to
the tabular sets that are seen at the base of the
aeolian succession but here the deposits are
both thinner and laterally less extensive.
The basal portion of the aeolian deposits was
mainly formed by wind-ripple migration on a
sandsheet or sand flat.Subcritically climbing
wind ripples formed the bimodal lamination at
the basal part of the succession.The coarse
material within the facies probably originates
from the reworking of the underlying fluvial
material.Aeolian sand sheets formed because
there was either insufficient loose fine-grained
sand or because the wind regime was unsuitable
to develop dunes.Kocurek & Nielson (1986)
further suggest that a high water table plays a
major role in the formation of sandsheets as the
water table binds the sediment to the surface
due to capillary water tension,reducing sand
availability (see Kocurek & Lancaster 1999).
Other factors that might also favour sandsheet
formation are surface cementation,periodic
flooding,a significantly coarser grain size and
the presence of vegetation (Kocurek & Nielson
1986).
The small- and large-scale cross-bedded sets
formed by grain-flow/fall processes on the
lee-side of dunes (Hunter 1977).Grain-flow pro-
cesses formed the coarser grained laminae,and
grain-fall processes the finer grained laminae.
Subcritically climbing ripples deposited wind-
ripple laminated and,occasionally,bimodally
sorted sandstones at the base of sets and over-
lying second-order bounding surfaces (called
superimposition surfaces:Kocurek 1981) in the
lower part of the succession.The coarser
grained character of the cross-beds at the base
of the succession indicates that it was deposited
in an environment with insufficient fine-grained
sand to build large dunes.Finer grained sand
may have been trapped in adjacent areas,for
example in wet/damp interdune areas,or may
have been winnowed by strong winds and carried
elsewhere.In addition,there may have been a
lack of finer grained sand when the dunes were
being built.The granular material that is locally
observed at the base of aeolian foresets indicates
a time of deflation and winnowing of material
deposited during flash flood events.
Higher up in the succession,the sets become
thicker and the predominance of wind-rippled
strata on bounding surfaces with lack of evidence
for a water-table influence (adhesion structures
etc.) indicates a dry aeolian system (Kocurek &
Havholm 1993).Superimposition surfaces are
absent and,instead,subsets are separated by
widely spaced third-order bounding surfaces
that may indicate changes in wind direction
(Kocurek 1996).
Soft-sediment deformed facies
association (FA-SSD)
This facies association comprises mainly pale-
grey sandstone with the same textural properties
as the large-scale aeolian cross-bedded deposits.
Grain-fall and grain-flow laminae of aeolian
origin are folded to varying degrees of complex-
ity into structures such as convolute laminae,
wavy subparallel bedding,cone-shaped diapirs
and broad synclines (Fig.5c,d).Bed boundaries
within the units are diffuse.The thickness of
units with convolute folding varies from single
cross-beds (cm) to whole cross-sets up to
several metres in thickness.Some examples of
folds are slightly overturned,although no predo-
minant sense of shear was observed.Within
units,the amount of deformation decreases
downwards and the folded strata pass gradually
into the underlying aeolian strata.Ball-and-
pillar-like structures with concave-upwards,
subparallel bedding are locally observed.Inter-
bedded and in association with the convolutedly
folded beds are dish structures.The individual
dishes are 0.1–0.3 m wide and have a concave-
up form (Fig.5e).The dark rusty coloured
dishes are finer grained than the rest of the gen-
erally massive sandstone.The contact between
the soft-sediment deformed material and the
A.STRO
¨
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¨
CK ET AL.168
Fig.5.Reworked and soft-sediment deformed facies association.(A) Massive–flat-laminated facies (section c.7 mhigh)
contain sets with (B) rippled top surfaces,(C) convolute folded facies in the formof a cone-shaped diapir;and (D) more
complex folded pattern,(E) dish facies,with gently curved dishes about 15 cmwide,(F) interbedded massive and cross-
bedded facies with erosive contact to underlying aeolian large-scale dune facies,(G) well-cemented wave-rippled facies
forms the top of the succession and is locally comprised of (H) cross-stratification and lies directly below the Troncoso
Superior Member.Scale:the lens cap is 5 cm,the hammer is 30 cmand the pole 120 cm(each segment is 10 cm).
THE TRANSGRESSION OF A CRETACEOUS ERG 169
underlying aeolian deposits is gradational,and a
thin interval (c.0.5 m) of massive sandstone is
often observed in between the facies associ-
ations.The aeolian deposits that underlie the
soft-sediment deformed sandstones are locally
slightly modified and show structures such as
wavy diffuse subparallel laminae and simple
overturned laminae.
The soft-sediment deformed material is the
result of rapid upwards escape of water and/or
air associated with pressure changes within the
dunes resulting from flooding (Lowe 1975).
The fluid and gas movements folded and
deformed existing laminations.As the dunes
became rapidly saturated,trapped pockets of air
escaped upwards.Loading of water-saturated
sand associated with further flooding resulted in
the upwards migration of water and the saturated
laminations deformed plastically (Lowe 1975).
Further deformation results in loss of most of
the original fabric and the formation of com-
plexly folded strata.Localized massive deposits
result from the total loss of cohesive strength
within the sediment caused either by rapid move-
ments of fluid or seepage of water through the
sediment (Lowe 1975).
The dish structures form when grain-supported
and more compacted sand sinks into less dense
fluidizedzones.The sinkingsandreplaces material
carried up by fluidization.The upwards-migrating
flow carries elutriated fine-grained material that
attaches to the base of the sinking bodies which
form the side of the flow path for the escaping
fluid–sediment mixture (Lowe & LoPiccolo
1974;Rautman & Dott 1977).This process is
most common when the fluidization or lique-
faction of the sediment is unevenly distributed.
Reworked facies association (FA-R)
This facies association has a range of deposi-
tional styles.In the Pampa de Tril area reworked
material is deposited as massive–flat-laminated
sandstones (up to 15 m thick) that have the
same textural properties as the underlying
aeolian dune cross-sets.The base of these depos-
its (i.e.their contact with underlying aeolian
facies) is undulose and sharp (Fig.6).The unit
is grey and vague flat lamination laps onto the
underlying aeolian dune topography (Fig.5a)
and beds are bounded by flat or current-rippled
top surfaces with a thin layer (mm) of very fine
material deposited on top (Fig.5b).The beds
are up to tens of centimetres thick and contain
rare clasts of consolidated sandstone.Locally,
the contact with the underlying aeolian facies
is gradational over 10–20 cm,and comprises
slightly overturned aeolian cross-bedding.
In Curaco,massive beds with similar texture
are observed interbedded with cross-stratified
sandstones (0.4 m thick) at the top of the Tron-
coso Inferior Member succession,overlying
both soft-sediment deformed sandstone and
aeolian deposits (Fig.5f).The scale of the
trough cross-stratified beds in association with
these massive beds is between 3 and 15 cm,
and the palaeocurrent directions,both within
and between sets,are highly variable.Locally,
wave-rippled strata are seen within these sets.
The interbedded intervals have a fairly constant
thickness of between 1 and 3 m,commonly
with a massive base and a low-relief,erosive
basal contact.
At the top of the Troncoso interval lies a very
well cemented rippled unit.This unit occurs
across most of the study area as a fairly
uniform,10–30 cm-thick yellow–rusty coloured
sandstone (Fig.5g).Locally,the surface separ-
ating the sandstone fromthe overlying evaporites
is wave rippled.This uppermost part of the sand-
stone succession is fine- to coarse-grained,fairly
well sorted and well rounded.Wave-rippled
laminae are a few centimetres thick.
The massive character results from transport
and deposition by high-velocity flows with high
fluid content.As the originally dry,aeolian
sand became saturated,the water in the pore
spaces reduced the cohesive strength,and the
mass became liquefied and travelled downslope
as liquefied flows (Lowe 1976).The original
cross-bedding is destroyed and the vague flat
laminations result from shear stresses during
movement (Lowe 1976).The presence of this
type of lamination and the sharp lower boundary
of these deposits provide evidence for sedi-
ment mobilization and transportation rather
than in situ homogenization.The rippled tops
represent the reworking of the bed tops by turbid-
ity currents in the final stages of the flow
(Lowe 1976).Vincelette & Chittum (1981) and
Benan & Kocurek (2000) proposed that similar
massive–rippled facies in reworked aeolian
sandstones in the Jurassic Entrada Sandstone
(SW USA) were deposited by liquefied flows
evolving into turbidites.
In addition to simple saturation and collapse of
the dune flanks,massive sandstones are,in
addition,believed to have formed by collapse
and resedimentation resulting from wave
erosion of the lower parts of the dunes.Such a
process has been invoked to described similar
features from flooded aeolian systems on
the Colorado Plateau (Eschner & Kocurek
1986;Huntoon & Chan 1987).Subaqueous
currents formed the trough cross- and rippled-
stratification seen in association with the massive
A.STRO
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¨
CK ET AL.170
TheTroncoso-Superior-Anhydrite
TheChorreadoMemberTheChorreadoMemberisonly
presentintheeasternparts
ofthe
PampaTriloutcrop,whereiti
s
upto2mthick.
Reworked;
massivetoflatlaminated
Dune (0.8kmwide)
E
~50m
Fluvial;
muddyWidelyspreadunit
ofmuddy
faciesisnotcommonelsewhere
inthestudyarea.
Thefaciesisonlyrarelypresent
andthen
interbeddedwiththin(cm-scale)
sandstones.
Sectionis3.3kmacross
Verticalexaggerationx2
Bimodallylarge-scale
aeolian
c
dipping
rossbedsindicatethatthedunes
inthe
PampaTriloutcropareoflinear
type.
SB
Floodingsurface
Fluvial;
massivetocrossbedded.
This
faciesisthemostcommon
fluvialfacies.
Thefluvialdepositsdominates
the
westernandlowerpartsof
theoutcrop
andwedgesouttowardsthe
east.
Reworked;
wave-rippledThisfaciesisa
thin
(0.1-0.3m)andformsaregionally
extensive
unit.Itismorecommonin
Curaco.
Small-scaleaeoliandunecrossbedding
interbeddedwithfluvialfaciesislocally
seen.
Interdune(~2km)
Dune(0.5kmwide)
W
Fig.4a)
Outcroppattern:
BaseTroncosoSuperior
Massivetoflatlaminated
facies
Aeolianpalaeowinddata,
PampaTriloutcrop,
n=79,
largestpetal:7readings
Scree slope
Fluvialpalaeocurrentdata,
PampaTriloutcrop,
n=244
largestpetal:14readings
Figure4A)
Massivetoflatlaminatedfaciesformed
byliquefiedflows.Thefacies
isthicker
inareasoflowerreliefinthepreserved
dunetopography.
N
N
SE
NW
Thegeneralstratigraphyinthearea
ismarkedbyasharp
unconformitybetweenthemarine
shalesoftheAgrioFmandthe
overlyingTroncososandstones.The
Troncoso-Inferiorformsa
drying-upwardsequencehavinga
dominantlyfluvialandanupper
aeolianfacies.Theaeolianfacies
showsapreservedtopography
of2to35m.Thetopoftheaeolian
sandstonesisreworkedor
soft-sedimentdeformedtovarious
extentbeforeoverlainbythe
evaporitesofTroncoso-Superior
.
Fluvial
AgrioFm
Aeolian
Reworked/SSD
Evaporites
SB
FS
Approx.60 m
Correlation panel continues below
Fig.6.
ThePampadeTriloutcrop(semi-)perpendiculartothepalaeowind
direction.Thelowersectionisthecontinuationofthefirst.The
photographshowspartoftheoutcropand
displaystheextensiveandsharpcontactbetweenthemassive
–flat-laminatedfaciesandtheaeoliandunefacies.Weightedlines
withintheaeoliansectionaresecond-orderbounding
surfaces.
THE TRANSGRESSION OF A CRETACEOUS ERG 171
sandstones in Curaco.The association of massive
and rippled sandstones may favour an interpret-
ation of these beds as density current deposits
(Ta and Tc of Bouma 1962).However,the high
variability in palaeocurrent direction,the
limited thickness and the association with
wave-rippled sandstone indicate that the cross-
bedding was produced by wave-generated cur-
rents locally reworking the dunes (Clifton
1976).Similar processes occur in the upper part
of wave-dominated shoreface systems and indi-
cate that the water depth was less than 10 m
(Elliott 1986).
Wave ripples such as the ones seen in the
topmost part of the reworked succession are
formed by oscillatory processes forced by
waves on the surface of the water.The inter-
action is typically restricted to water depths
that are less than half the wavelength of the
waves (Clifton 1976).The extensive cementation
at the top of the succession may be associated
with the downward percolation of evaporite-
rich fluids from the overlying deposits.
Sedimentary architecture
The Troncoso Inferior Member reaches a
maximum thickness of 50 m and underlies the
evaporites of the Troncoso Superior Member.
Overall,the succession shows a drying-upwards
motif.The basal parts of the Troncoso Inferior
Member are fluvial and the upper parts aeolian.
The contact between the fluvial deposits and
the underlying marine deposits of the Agrio For-
mation and Chorreado Member (Huitrı´n For-
mation) is sharp and locally erosive.It is
interpreted as a sequence boundary (sensu
Posamentier & Vail 1988;Veiga et al.2005).
The top of the Troncoso Inferior Member is a
sharp contact with the overlying evaporites.
This can be classified as both a marine flooding
surface (sensu Van Wagoner et al.1988) and a
depositional super-surface (sensu Kocurek &
Havholm 1993).In the following section the
facies architecture is discussed using data from
four ‘architectural element analysis’ panels
from the two key areas (Pampa de Tril and
Curaco).Special reference is paid to the pre-
served aeolian topography and the soft-sediment
deformed and reworked interval associated with
the flooding of the dune field.
Pampa de Tril (two panels)
Description.The Pampa de Tril outcrop lies in
the northern part of the study area where the
Troncoso Inferior Member attains a vertical
thickness of up to 50 m (Fig.1).The outcrop
stretches 3.5 km SE–NW,perpendicular to the
palaeowind direction (Fig.6),and 3 km SW–
NE,parallel to the palaeowind direction (Fig.7).
In the western parts of the Pampa de Tril outcrop
the continental deposits lie unconformably on
top of ammonite-bearing marine shales of the
Agrio Formation.In the eastern parts of the
SE–NWpanel (Fig.6) the sandstones lie above
a thin (up to 2 m) interval of fine-grained
shallow-marine sandstones of the Chorreado
Member (Figs 2 & 6).
The fluvial package attains a total thickness of
15 m in the NWpart of the outcrop,and pinches
out to the SE where aeolian facies directly
overlie the Chorreado Member.The basal parts
of the Troncoso sandstones in Pampa de Tril
are composed of erosive and massive–cross-
bedded fluvial deposits with abundant rip-up
clasts.Thick,predominantly massive beds (c.
2 m) pass upwards into thinner beds (tens of
centimetres) that are dominantly cross-bedded.
The foreset dips within the basal cross-bedded
sets vary,but are directed dominantly towards
the SE (N1308,n ¼ 244).Horizontal and finely
laminated and small-scale cross-bedded aeolian
facies occur interbedded with fluvial facies as
eroded remnants (western part of the panel in
Figs 6 & 7).Remnant and complete aeolian
dune bedforms have a height up to 2.5 m.
Higher up in the fluvial succession more
varying dip directions are dominantly northerly
directed (n ¼ 56).Planer-laminated and cross-
bedded sets are interbedded with fine-grained
horizons forming stacked packages.In places
accretion co-sets are observed.The fluvial
material becomes better sorted and finer
grained higher up.
Stacked sets of cross-bedded aeolian strata
locally overlie the fluvial deposits in the SW
(below the panel in Fig.6) (log 5 in Fig.3).
These sets display small-scale cross-beds that
have N-trending foreset directions (N0418,
n ¼ 38),and are thin (c.0.5 m) and laterally dis-
continuous.The top of this local deposit is cut at
its top by a thin,cross-bedded and discontinuous
fluvial interval.Across most of the outcrop large-
scale cross-bedded aeolian strata directly overlie
fluvial deposits.The base of the aeolian succes-
sion is sharp and comprises granular size
grains.Two NNE–SSW-trending ridges domi-
nated by large-scale cross-bedded aeolian depos-
its occur in the panel in Figure 5.These are 0.5
and 0.8 km wide and 2 km apart.Between the
ridges occurs a very thin (less than a few
metres) interval of aeolian sandstone.This
upper and dominantly aeolian part of the
section is up to 35 m thick and comprises
cross-bedded sets (up to 7 m thick).The base
A.STRO
¨
MBA
¨
CK ET AL.172
NE
SW
Fluvial facies
10
m
10m
?
?
?
?
Figure5B)
Palaeowind data,
top aeolian unit,
n=79
Largest petal: 9values
Palaeocurrent data,
Thin fluvial unit within
the aeolian facies,
n=18
N
N
Fig.7.
ThePampadeTrilcorrelationpanelparalleltothemainpalaeowind
direction.Theareasonthepanelwithnostratificationmarked
,aswellasthebaseo
fthepanels,are
poorlyexposed.Third-orderboundingsurfacesaredashedlines,
second-orderboundingsurfacesareweightedlines.
THE TRANSGRESSION OF A CRETACEOUS ERG 173
of the large-scale cross-bedded interval contains
granules.Beds dip dominantly towards the north
(N0058,n ¼ 103),but dip directions are locally
(in the NW) bimodal towards the north and SW
(Figs 5 & 6,respectively).The palaeowind
directions at the base of the western ridge are
dominantly westerly directed.In the eastern
ridge,the palaeowind directions are more vari-
able (Fig.6).Third-order bounding surfaces
are very common within both of the ridges
within the large-scale cross-bedded facies
(Fig.7).
At the top of the aeolian interval a variety of
deposits are observed.In the eastern part of the
Pampa de Tril outcrop (Fig.6),reworked depos-
its of massive–flat laminated character have a
measured thickness up to 15 m.The thickness
of the deposits decreases to the west where
the interval is completely absent.Similar
infilling character of the massive–flat-laminated
reworked deposits is seen in the SWparts of the
Pampa de Tril outcrop (Fig.7).Here thinner
(1–2 m thick) and locally developed beds fill
areas of generally lower-dune-topography.The
lower contact with the underlying aeolian
cross-beds is both sharp (erosional) as well as
gradational.The gradational contact is 10–
20 cm thick and comprises slightly overturned
aeolian cross-bedding.The massive–flat-lami-
nated beds in this area extend 50 m laterally
and the flat lamination is a few centimetres
thick (Fig.5b).In the SW parts of the outcrop
(Fig.7),rare,small water-escape structures
(scale of a few centimetres) are seen at the base
of the reworked beds,and rare clasts of lithified
sandstone are observed.
In areas where no massive–flat-laminated
deposits are observed,a 0.1–0.2 m-thick well-
cemented wave-rippled sandstone unit sits at
the top of the succession (logs 3 and 4 in
Fig.3).Locally,there are no signs of reworking
or soft-sediment deformation.
Interpretation.Continental sedimentation began
with fluvial systems that eroded into the under-
lying marine Agrio Formation and Chor-
reado Member of the Huitrı´n Formation.The
erosion incorporated mud and carbonate clasts
into the fluvial deposits.Higher in the fluvial
succession,planar-laminated and small-scale
cross-bedded sets interpreted to have been
formed by sheet floods are interbedded with
aeolian sandsheet and dune deposits.Individual
event beds are stacked into multistorey sets
separated by thin muddy horizons that represent
waning flow and testify to the ephemeral nature
of the depositional events.Evidence for a
more confined,semi-permanent flow is seen
towards the middle of the section where down-
stream and laterally accreting bar complexes
are observed.Local fluvial incision is seen in
the SW parts of the outcrop (Fig.6),where a
thin cross-bedded fluvial bed overlies stacked
sets of small-scale cross-bedded aeolian
sandstone.
The upwards transition from fluvial to aeolian
accumulation implies that the climate became
dryer.The wind reworked the underlying
fluvial deposits into wind-rippled sandsheets
and small aeolian dunes.The initial aeolian
dunes migrated towards the NE.
Successively,dune cross-sets became larger,
and large-scale cross-bedded sets dominate the
upper aeolian section of the Troncoso Inferior
Member.The granular base of this main
aeolian unit indicates a period of deflation of
fluvial material before aeolian sedimentation
started.Aeolian cross-sets (up to 7 m) build up
a total thickness of 35 m.The large-scale cross-
bedded sets are separated by second- and thir-
d-order bounding surfaces (dominantly third
order) that are more widely spaced higher up in
the succession (Fig.3),and shows that the
active wind regime had stabilized by this time.
The bimodal dip pattern of the cross-beds seen
in the western ridge in Figure 6 and the palaeo-
winds measured in the panel in Figure 7 indicate
that the dunes were linear.The linear dunes
carried stacked sets of sinuous elements that
migrated along the dunes,as described from the
modern Namib Sand Sea by Bristow et al.
(2000).The dunes were elongated in a NNE–
SSWdirection and had in outcrop a 2 km wave-
length.The cross-beds at the base of the western
ridge also imply that the dunes were migrating
mainly towards the NWat the onset of sedimen-
tation (Fig.6) (similar to examples by Rubin &
Hunter 1985;Bristow et al.2000),and sub-
sequently developed into more stabilized linear
dunes.The dunes were of more complex type
in the eastern parts of the Pampa de Tril
outcrop.Between the two linear dune ridges in
Figure 5,thin aeolian deposits,probably sand-
sheet deposits and small undeveloped dunes,
occupy the interdune area.
The flooding
When the Troncoso dune field was flooded by the
Troncoso Sea the dunes became saturated with
water and the upper parts were subsequently
liquefied.This induced mass-flow processes
that created a massive–flat-laminated unit
which accumulated within topographically low
areas.The flat lamination developed as traction
carpets in high-density and sandy turbidity
A.STRO
¨
MBA
¨
CK ET AL.174
flows,or by shear stresses on the base of liquefied
flows.Deposition by liquefied flows is favoured
as the massive–flat-laminated deposits are
fairly fine grained and have a rippled top surface.
Water-escape structures due to liquefaction can
be seen locally at the base of these beds.
During late stages in the transgression,and
prior to the deposition of evaporites,local suba-
queous reworking took place and a thin interval
of wave ripples developed locally at the top of
the succession.No wave-rippled interval was
seen on top of massive–flat-laminated facies
and this may indicate that the mass flows took
place after the succession became subaqueously
reworked.The liquefied flows may have incor-
porated the wave-rippled section (seen as
clasts) into the sediment –water mixture that tra-
velled down the dune face.Sections without evi-
dence of reworking,by waves or liquefaction,
might indicate rapid drowning to a depth below
wave base.
Curaco (two panels)
Description.Two panels were produced for the
area around Curaco (Fig.1).One 100 m-long
panel strikes WNW–ESE and is perpendicular
to the dominant palaeowind direction
(A,Fig.8).The second panel is 125 m long
and is orientated towards the north (parallel to
the palaeowind direction) (B,Fig.8).
The basal part of the Curaco succession is 8–
10 m of fluvial deposits that rest unconformably
on the Agrio Formation,similar to the Pampa de
Tril sections.In Curaco,most of the fluvial sand-
stones are cross-bedded and flat laminated,and
are better sorted than those in Pampa de Tril.
The lower sets are trough cross-bedded (tens of
centimetres thick) and have a principal palaeo-
current direction towards the ESE (N1108,
n ¼ 31).Higher in the fluvial interval,current
directions are more variable with a mean
trough direction toward N0408.In this part of
the succession the fluvial deposits are better
sorted and finer grained than in the lower parts.
No mixed fluvial and aeolian deposit and no
finer grained horizons were observed.The top
of the fluvial deposits is well cemented and
forms a laterally extensive peneplain surface
that is overlain by aeolian deposits.
The base of the overlying aeolian succession
comprises 0.5 m of horizontal and finely lami-
nated sandstones.Where this interval is
exposed,it is overlain by stacked sets of 0.5–
1 m-thick cross-bedded aeolian dunes.These
trough to wedge,planar,small-scale cross-
bedded sets are not laterally continuous (a few
metres) and together they form a package up to
4 m thick (Fig.8).Within the basal cross-sets,
foresets are coarse grained and dip towards
the east (N0748,n ¼ 25).The small-scale
cross-sets are separated by first-order bounding
surfaces that have a lateral extent and are
locally overlain by coarse-grained and granular
material.
Higher up in the aeolian succession large-scale
cross-bedded sets (up to 4 m) are separated by
second-order bounding surfaces,containing sub-
sequent third-order bounding surfaces.These
cross-bedded units make up the upper part of
the aeolian succession that reaches a maximum
thickness of 35 m.In the correlation panel per-
pendicular to the dominant wind direction the
foresets dip unimodally towards the NE
(N0578,n ¼ 479) at high angles (208–308)
(Fig.8).The majority of the dip directions lie
between N0208 and N0608.Some inconsistency
is seen within trough-shaped sets (Fig.8).For
example,one trough showed varying dip direc-
tions from N3308 to N0648.The troughs are up
to 70 m wide and have low curvature.The
bases of the sets are sharp second-order bounding
surfaces that are locally overlain by wind-rippled
and fine-laminated strata.Towards the top of the
succession large-scale cross-bedded aeolian sets
are bounded by planar second-order bounding
surfaces (B,Fig.8).In the correlation panel B,
parallel to the wind direction,subhorizontal
bounding surfaces separate uniformly dipping
lee-face deposits.Numerous third-order bound-
ing surfaces are seen within the larger cross-sets.
In the Curaco sections,the uppermost parts of
the studied succession are different to Pampa de
Tril,and are dominated by soft-sediment
deformed and wave-reworked deposits.The con-
volutedly folded soft-sediment deformed units
are locally interbedded with dish structures
(Fig.8A) and are observed where the aeolian
topography is slightly higher.These intervals
are less than 2 m thick and extend for about
40 m with varying thickness and relative pro-
portion of the two different types of deposits
(Fig.8).Both types of deposits are separated
from the underlying aeolian dune facies by a
thin massive and/or convolutedly folded unit in
the form of simply overturned aeolian laminae.
Laterally,the soft-sediment deformed units die
out or pass into interbedded massive–flat-
laminated and cross-stratified units.The soft-
sediment deformed deposits in the Curaco
succession are almost exclusively overlain by a
massive,a cross-stratified or an interbedded
interval of the reworked massive (to flat-lami-
nated) and cross-stratified facies.
The massive–flat-laminated interval extends
for 100 m with a fairly constant thickness of a
THE TRANSGRESSION OF A CRETACEOUS ERG 175
Outcroppattern:
B
A
S
10 m
Reworked;
Aeolianduneoverlainby
wave-rippledandcross-
stratifiedfacies,section
1.75mhigh.
ca.
N
B
10
m
Palaeowinddata,
Aeolianfacies,
n=479
Largestpetal:65values
Palaeocurrentdata,
Fluvialfacies,
n=31
Largestpetal:4values
15 m
I
I.Reworked;
Well-cementedwave-rippledfaciesthat
arelocallycross-
stratified,drapesthetopography
withaneventhicknessof0.2
m.Thecrossbeddingisdippingindifferent
directions.The
wave-rippledintervaloverliescross-stratified
faciesinterbedded
withwave-rippledandmassiveunits,that
sharplyoverlie
aeolianfacies.
Figure 5 H)
II M
Figure5D)&E)
II.
Theaeolianfaciesisoverlainbyconvolute
foldedfacieswhich
occurtogetherwithdishfacies.T
ogetherthesefaciesare
overlainbymassivefaciesandlaterby
wave-rippledfacies.
Reworked/Soft-sedimentdeformed;
N
N
ENE
WSW
A
M
10
m
10m
II
Reworked/Soft-sediment deformed;
Interbedded cross-stratified and massive
facies is overlying convoluted facies,
sectionca.1 m high.
Reworked;
Cross-stratified facies overlain by
thin coarse sandstone pan,section
ca. 0.7 m high.
Reworked;
Wave-rippledfacies
overlyingcross-stratified
facies,section0.5mhigh.
ca.
Figure 4 B)
Figure 4 D)
Figure5F)
Figure 5 G)
I
25 m
5m3m
Fig.8.
TheCuracoCanyoncorrelationpanels.Aisstrikingperpendicular
tothedominantdipdirection.BisperpendiculartoAandisparallel
tothepalaeowinddirection.Different
typesofsoft-sedimentdeformationandreworkingare
seenatthetopofthesuccessionandmoredetaileddrawingsare
highlighted.
A.STRO
¨
MBA
¨
CK ET AL.176
few tens of centimetres and occurs generally in
association with reworked cross-stratified beds.
Locally,they are interbedded to form intervals
1–3 m thick (Fig.8A).The nature of the
reworked cross-stratified strata varies across
the Curaco area,where it locally forms stacked
sets up to a total thickness of 4 m (Fig.8).In
panel A,Figure 8,cross-stratified unit a few
tens of centimetres thick,forms a laterally con-
tinuous interval at the top of the succession
extending for approximately 100 m laterally.
The 0.1–0.2 m-thick well-cemented wave-
rippled (locally cross-laminated) unit observed
at the top of the section in Pampa de Tril is
also present at Curaco.In Curaco,the wave-
rippled unit covers most of the area with the
exception of areas with relatively low preserved
aeolian topography (below c.25 m).There are
also areas with no signs of reworking or soft-
sediment deformation and the evaporites lie
directly on top of aeolian deposits (panel A,
Fig.8).
Interpretation.A SE-flowing fluvial system
produced the first continental deposits in
Curaco.The lack of muddy intervals between
cross-stratified sets and the lack of interbedded
aeolian deposits indicate that these fluvial
systems were less ephemeral than those at
Pampa de Tril.
A sandsheet was the first aeolian sediment
deposited in Curaco.The earliest dune deposits
were not laterally extensive,which suggests
that the dunes were small and that the system
was undersaturated with sediment.The abun-
dance of scouring and the coarseness of the
initial dunes also favour an interpretation of a
system with a restricted supply of dry and fine
loose sand.Low sand input may reflect a wet
climate (and substrate) or proximity to fluvial
systems (Kocurek & Nielson 1986),both of
which would trap sediment.The thin wind-
rippled and granular horizons between the sets
indicate that the area was periodically flooded.
Wind winnowed the finer grained material
leaving a granular lag.
As the climate became dryer,finer sand was
brought into the area and the dunes became
larger.Dune cross-sets (up to 5 m) built up
dunes at least 35 m high.First- and third-order
bounding surfaces that separate cross-sets
resulted from dune migration and changes in
wind direction,respectively.The foresets dip
with a uniform geometry towards the NE
(N0578),have a fairly steep angle and there is a
low abundance of wind-ripple strata;this evi-
dence implies that the dunes were barchanoid–
transverse ridges.The troughs observed at
the top of the section are perpendicular to the
dominant wind direction and indicate that
the dunes in the area were sinuously crested.
The width of the troughs were at least
60–70 m,which is the preserved trough width
recorded in outcrop.
The reworking and soft-sediment deformation
in Curaco were fairly complex and involved a
series of events.Initial soft-sediment defor-
mation caused by liquefaction and rearrangement
of the original dune cross-bedding created a con-
volutedly folded deposit at the time of liquefac-
tion followed by upwards water escape that
formed the dish structures.
Later subaqueous processes partially eroded
the aeolian deposits and the locally present
soft-sediment deformed deposits.Waves cut into
the underlying,slightly modified,dune topogra-
phy and induced liquefied flows that created
massive sandstones.These massive sandstones
are locally interbedded with a cross-stratified
facies.The cross-stratification resulted from
shallow-marine reworking.The final stage of
reworking was the deposition of a regionally
extensive wave-rippled unit,formed as the
water depth continued to increase.The absence
of the rippled unit in palaeotopographic lows
implies that these areas were below the wave
base level.Given that the maximum preserved
topography is approximately 35 m it can be con-
cluded that the waves were small.This is consist-
ent with the basin being relatively sheltered
(Legarreta & Uliana 1991).
Underneath the reworked deposits that form
laterally extensive and fairly thin units (c.2–
3 m in total) the preserved aeolian topography
has low relief.This might be caused either by
the reworking of material from the dune tops,
which may have been transported elsewhere,or
because the dune setting was of fairly constant
height across the area.The latter interpretation
is favoured,as there are still signs of preserved
dune topography.Further evidence is the lack
of marine strata and reworked material in areas
of lower relief within the dune topography.The
top parts of the dunes would have been exten-
sively reworked if the wave energy was high
enough and the reworked material accumulated
in areas of lower relief.However,areas that
lack evidence for reworking and soft-sediment
deformation may have been sheltered from
high-energy wave action.
Depositional model – discussion
The data presented above provide information on
the Troncoso Inferior Member depositional
system prior to the Late Aptian flooding of the
THE TRANSGRESSION OF A CRETACEOUS ERG 177
basin.As well as having important implications
for our understanding of the stratigraphy of the
Neuque´n Basin and the evolution of the Andes,
the data have other,more generic,applications.
Flooded dune systems are important because,
under normal conditions of bedform climb,
only a small proportion of the dune is preserved.
When dune fields are flooded the depositional
system becomes fossilized.This provides a
crucial insight into the relationship between
internal architecture and dune morphology
(Mountney et al.1999).The data presented
above also reveal aspects of the processes and
products that occur when dune systems are
flooded.This has important implications for
understanding other flooded aeolian systems
where there are less data available,such as the
Permian Weissliegend of the southern North
Sea Basin (Glennie & Buller 1983;Stro¨mba¨ck
& Howell 2002).In the following discussion
we shall consider the Troncoso depositional
systemboth prior to,and after,the transgression.
Before the flooding
As the shelf became exposed by sea-level fall in
the Early–Late Aptian a fluvial system came to
occupy the whole study area (see discussion in
Veiga et al.2005).Early rivers were erosive
but they developed into a partly ephemeral
braided system with the main transport direction
towards the north (Fig.9).In Curaco,deposition
started off as an extensive aeolian sandsheet and
small aeolian dunes.The sediment supply was
low,and the building material for the dunes
coarse and reworked from the underlying
fluvial strata.As the climate became more arid,
finer grained material was brought into the
basin and larger dunes could develop.In the
northern parts of the study area (Pampa de Tril)
linear type dunes with heights up to 35 m and
2 km wavelength were created.In Curaco,
smaller barchanoid ridges developed (Fig.9).
The wavelength between the linear dune
ridges seen in Pampa de Tril is similar to
modern examples seen in the Rub’ al Khali
Desert (Saudi Arabia) where the mean wave-
length is 2.18 km,and the SWSahara (Maurita-
nia) where the mean wavelength is 1.93 km
(McKee 1979).The mean width of linear dunes
in these areas is 1.21 and 0.94 km,respectively.
This is wider than the linear dunes seen in
Pampa de Tril where the apparent width is
0.5–0.8 km.The Troncoso dunes in this study
may have been smaller and less developed,but
the bimodal cross-beds support an interpretation
of them as linear dunes.Evidence from Besler
(1975),Rubin & Hunter (1985) and Bristow
et al.(2000) shows that linear dunes deposit
transverse-type strata if these migrate laterally,
i.e.if one wind direction is more dominant than
another.This is similar to the cross-beds
observed at the base of the linear dunes seen in
Pampa de Tril that comprise transverse-type
strata.Although Bristow et al.(2000) note that
linear dunes may deposit abundant trough
cross-bedded strata,in Curaco the lack of
bimodal dip directions,as well as the highly
regular geometry of the dune foresets (parallel
to the palaeowind direction),implies that the
dunes were barchanoid–transverse in this area.
The 35 mheight is similar to many modern trans-
verse and barchanoid ridges (Lancaster 1983;
Kar 1990).The change from transverse to
linear dunes (the distance between Pampa de
Tril and Curaco) occurs over a similar distance
(25 km in distance) to changes of the same
dune types in parts of the modern Namib
Desert (Lancaster 1983).When comparing the
Troncoso and the Namib examples,the linear
A
T THE INITIATION OF DRY PHASE
DRY PHASE
25km
25 km
25km
25 km
N
N
?
?
?
Fig.9.The depositional model of the fluvial and aeolian sedimentation prior to the flooding.The sedimentation in the
eastern parts of the area is constrained only by subsurface data.
A.STRO
¨
MBA
¨
CK ET AL.178
dunes in some parts of the Namib Desert
(adjacent to transverse dunes) are more com-
pound,but the spacing and height of the dunes
are similar to the Troncoso.
The interdune areas observed in Pampa de Tril
are starved of sediment.Evidence like this may
suggest that proximity to the palaeoshoreline
(towards the north) may have influenced the
sedimentary pattern.In Curaco aeolian dune
sedimentation seems to have been more persist-
ent as no extensive interdunes are observed.
The change in bedform type,from transverse to
linear,might have resulted from a relative
decrease in sand budget and an increase in
wind variability downwind of the transverse bed-
forms (similar to the southern North Sea:George
& Berry 1993).George & Berry (1993) refer to
linear dunes as sand-passing bedforms,and
barchans and transverse dunes as sand-moving
bedforms.A varying wind regime would have
reduced the sediment-carrying capacity and
the sand would be trapped in the upwind,and
more developed,dune field (in this study,the
Curaco area).The apparently wetter environment
that existed in the northern parts of the study
area may also cause the decrease in sand.
Factors like these would have stabilized the sub-
strate,and the grain transport would have been
hindered as the grains adhered to the damper
surface.
Aeolian sedimentation ended abruptly in the
Late Aptian when the sea inundated the Troncoso
dune field.The transgression came fromthe NW
through a narrow embayment of the Pacific
realm.The initial flooding drowned the dunes
but water depths were shallow enough to allow
wave reworking in addition to soft-sediment
deformation (see discussion in Veiga et al.
2005).
The outcrops provide excellent evidence for
understanding the internal aeolian architecture
of dunes,and especially the poorly understood
linear dunes.The linear dunes observed in
Pampa de Tril show evidence for bimodally
dipping foresets,but also provides evidence
that the initial sedimentation of these ridges com-
menced with dunes depositing ‘transverse-type’
strata.Unimodally dipping strata is generally
seen as evidence for a lateral sediment transport
direction.This type of evidence from the
rock record suggests that the preservation of
linear dunes may have been misinterpreted
as transverse types in previous studies,as nor-
mally only the lower parts of the dunes are pre-
served.Laterally migrating linear dunes have
also been suggested in other studies,for
example by Rubin & Hunter (1985) and
Bristow et al.(2000).
Post-flooding
A range of different soft-sediment deformation
and reworking processes modified the dune topo-
graphy of the Troncoso dune field at the time of,
and after,the transgression (Fig.10).These
processes are reflected in the facies deposited at
the top of the uppermost aeolian unit.
The dune field was quietly submerged when
the sea level rose in the saline basin centre to
the north of the area.The water soaked into the
dune sand because of its high porosity and the
dunes became unstable as pressure changed
internally.The pore spaces within the aeolian
dune deposits expanded as they filled with
water,and the sand grains subsequently became
dispersed within a sand–water mixture.The
dunes were liquefied and water escape and mass
flows were triggered.
In Curaco,early water- and air-escape pro-
cesses modified the aeolian dune topography
and formed convolutedly folded and dish struc-
tures (Fig.8).Although it is proposed that this
in situ soft-sediment deformation was associated
with the flooding,it is important to consider
whether it may have occurred before the sea
inundated the basin.Soft-sediment deformation
has been observed in many aeolian systems and
is attributed to seismic events,cyclic loading
by storm waves or changes in water-table level.
Seismicity is discounted as a driving mechanism
as there is no evidence of convolution on a
regional scale (the facies were not seen in
Pampa de Tril).Changes in water-table level
may have formed the convolutedly folded and
dish structures,but as these are not seen
elsewhere in the Troncoso system they are
also discounted,unless they were related to the
transgression.Storm waves may have formed
these types of facies,as waves may have been
active both at the onset of the transgression and
after the dunes were drowned below the storm
wave base.
As the contact between the soft-sediment
deformed deposits and the overlying massive
and cross-stratified facies is erosional,the for-
mation of the soft-sediment deformed strata is
interpreted to have been before shallow-marine
reworking (Fig.10).Although marine processes
reworked the dunes,no large volumes of
marine strata were deposited.Only a maximum
of 3 m of cross-stratified,massive and wave-
rippled deposits are seen.More marine sediment
is seen in the Curaco area compared with the
Pampa de Tril and was preserved after deposition
due to an increase in the water depth during the
flooding.This may suggest that the water depth
in the area around Curaco was shallower than
THE TRANSGRESSION OF A CRETACEOUS ERG 179
1.Water-escape;dishandconvolutefoldedfaciesformed
andthe
duneprofilemodified.
2.Wavereworking/undercutting;cross-stratifiedandmassive
toflat
laminatedfaciesformedandtheremnantdunetopography
from
stage1modified.
3.Reworkingbywavesaftersea-levelrise;wave-rippled
facies
formeddowntoacertaindepth.
4.Liquefactionprocessescreatedmassivetoflat-laminatedfacies
byliquefiedflowsinthenorthernpartsofthebasin.This
isbelieved
tohaveoccurredaftermostofthedunesweremodifiedbywaves
or
initiatedbystormwavesindeep-water.
Originaldunetopography
Modifieddunetopographyafterstage1
Waveundercuttingcausesslumping
and
producesmassivefacies
?
Modifieddunetopographyafterstage2
Sealevel
Sealevel
Sealevel
Sealevel
Fig.10.
Proposedmodelforthesequenceofprocessesthatformedthe
flood-relatedfaciesaftertheTroncosoduneswereflooded.
A.STRO
¨
MBA
¨
CK ET AL.180
around Pampa de Tril.The amount of reworking
may also have depended on the time available for
marine processes to redistribute material.
Oscillatory forces reworked the top of the
sandstone succession to form a thin wave-rippled
interval in areas with higher preserved dune
topography.The deposition of the wave-rippled
interval must have occurred in fairly quiet water
as it simply draped the top parts of the preserved
topography.The local absence of a wave-rippled
interval in both Pampa de Tril and Curaco was in
areas of generally lower topography and may
imply that the water depth was too great in those
areas,i.e.the dunes drowned quickly to a depth
below the fair-weather wave base.
The reworked massive–flat-laminated unit is
believed to have been formed by liquefied
flows in an already drowned system.The lique-
fied flows deposited the most voluminous units
of all flood-related processes (up to 15 m
thick).However,it is interesting to consider
whether these flows occurred before or after the
marine reworking.Assuming that the topmost
wave-rippled interval accumulated on all the
dune highs across most of the study area,its
absence may indicate that the liquefied flows
reworked and removed it.A massive–flat-lami-
nated facies has also been observed by Benan
& Kocurek (2000) in the Jurassic Entrada Sand-
stone in NewMexico where this facies filled,and
on-lapped,the underlying dune topography,
similar to the examples seen in the Pampa de
Tril outcrops.Benan & Kocurek (2000) also
interpret this type of facies as having been depos-
ited by mass-flow processes.This geometry
(infilling) was also proposed for the southern
North Sea (UK) where the Weissliegend facies
fills in the preserved topography of the Rotlie-
gend dunes (Stro¨mba¨ck &Howell 2002).In com-
parison,the marine reworked strata are seen as
thin (0.1–3 m thick) laterally extensive units.
High preservation of dune topography implies
that the transgression of the Troncoso dune field
was fairly rapid but of fairly low energy.If the
transgression was slower,there would have
been more time for marine reworking,which
would have permitted less dune preservation
but more voluminous marine deposits.There
would also be evidence of marine reworking
across the whole area.If the transgression was
slow and/or of higher energy no dune topogra-
phy would have been preserved.
Conclusions
The outcrops of the Troncoso Member con-
stitute a great opportunity to study flooded
aeolian systems as a large amount of the dune
topography was preserved after being trans-
gressed.The three-dimensional architecture
allowed study of the preserved dunes together
with the processes that reworked and caused
soft-sediment deformation of them during the
flooding.The reworking and soft-sediment
deformation could further be related to the
amount of dune preservation.
The transgression caused extensive reworking
and soft-sediment deformation of the dunes,
as is demonstrated by the various flood-related
facies at the top of the sandstone succession
(Fig.10).In the south (Curaco),towards the
basin margin,the flooding initiated liquefaction
followed by water escape.Water escape pro-
duced convolutedly folded bedding and dish
structures within slightly higher preserved dune
topography.The modified dune topography in
Curaco was further reworked by waves to form
marine strata.When the waves undercut the
dunes they triggered local liquefied flows that
created massive sandstone.A thin wave-rippled
interval is present across most of the area,but
absent in areas with massive and flat-laminated
facies.This implies that the liquefied flows
may have eroded the wave-rippled deposits or
that these were not deposited at all (i.e.were
deposited below the wave base).Liquefied flow
deposits,in the form of massive–flat-laminated
facies,were seen only in the Pampa de Tril
area.Voluminous reworking of the linear
dunes by liquefied flows deposited massive–
flat-laminated facies that came to occupy lower
areas within the dune topography.The northern
parts of the field area (Pampa de Tril) were
quickly submerged and there would have been
no time to develop a wave-rippled facies.
Some areas show no signs of reworking or soft-
sediment deformation.
The massive and flat-laminated facies depos-
ited by liquefied flows forms the greatest
volume of reworked strata,and may imply that
these deposits have greatest significance when
studying flooded dune systems in the subsurface.
Different types of marine facies were deposited
as fairly thin (c.2–3 m) laterally extensive
units.The flood-related facies do not seem
to influence the amount of dune preservation.
Instead there is a different amount of pre-
served aeolian relief of the strata below the
flood-related deposits.In the areas with more
liquefied flows,the relief is greater compared
with the dunes that were reworked by marine
processes,although this might also be due an
artefact of the dune setting at the time of the
transgression.
The amount of marine strata,as well as the
large amount of preserved topography,indicate
THE TRANSGRESSION OF A CRETACEOUS ERG 181
that the flooding of the Troncoso dune field was
rapid but of fairly low energy.
Below the flood-related units at the top of the
succession,the Troncoso Inferior Member was
deposited as a sand-rich facies in both fluvial
and aeolian environments.The fluvial deposits
are dominated by cross-bedded and plane-lami-
nated sandstones.The geometry of these sand-
stones indicates that they were deposited within
low-sinuosity fluvial channels and as sheet
floods.When the climate became drier large
aeolian dunes started to develop and fluvial depo-
sition was restricted to interdune corridors.
Evidence that an extensive aeolian dune field
began to accumulate after the main fluvial phase
is present across the whole study area.Aeolian
sets were preserved as dunes/draas,at least
35 m high.The sedimentation style changed
downwind (northwards) fromtransverse/barcha-
noid into linear dunes.The different types of
dunes accumulated within 25 km from each
other,with dune scale and geometry similar to
parts of the modern Namib Desert (Lancaster
1983).
We would like to thank ARCOBritish Ltd (BP Amoco) for
funding this research.Further we would like to thank K.W.
Glennie and J.D.Marshall for their constructive feedback
on this paper as a part of a thesis undertaken by
A.Stro¨mba¨ck at the University of Liverpool,UK (‘An
evaluation of the Weissliegend facies of the UK,Southern
North Sea’,2001).R.Blakey,D.Loope and A.Tripaldi are
thanked for the review of the paper in its present form.
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