LANDFORM EVOLUTION IN THE COASTAL REGION
OF TARAPACA PROVINCE, CHILE
par C. MORTIMER * et N. SARlt **
RESUMEN
Se considera .que la plataforma
continental y
el alto acantilado costero del Norte Chileno han
sido formados durante el retroceso Pliocénico.
Subsecuentemente la tectonica local produjo una
emergencia del litoral, permitiendo asi la formación de terrazas marinas al pié del cliff.
El fallamiento en
la Costa, se produjo
tucion de las formas
del aluvio más joven
La reconstrucción de
mediante diagramas,
un pediplano.
bloques de la Cordillera de
con anterioridad a la constilitorales y a la depositación
de la Pampa del Tamarugal.
la topografta pre-fallamiento,
revela que correspondta a
La red de .drenaje a traves la Cordillera Costera,
se implantó cuando el alu vio, que empezo a rebasar
desde la Pampa, alcanzó el acantilado retrocedente.
Virtualmente no existe desarrollo de cauces fuera
de los cañones y se piensa que, el drenaje hacia los
numerosas cuencas cerradas de las montañas
costeras, es por percolación subterranea.
La Cordillera de la Costa se levanto al fin del
Paleogeno; a causa de la baja tasa de erosión,
la cantidad de sedimentos depositados en el área
de la fosa Chileno-Peruana, durante el Neogeno,
se ha restringido a los derivados del retroceso
costero.
ABSTRACT
The continental shelf and high cliff of northernmost Chile are considered to have been formed
during Pliocene coastal recession. Subsequentlocal
tectonically induced emergenceof the littoral permitted marine terracing at the cliff foot.
Block-faulting of the Coastal Cordillera occurred
before the formation of the littoral landforms and
before the deposition of the younger alluvium of
the Pampa del Tamarugal. The pre-faulting
topography can be. diagramatically
reconstructed
to reveal a pediplain.
Drainage across the Coastal Cordillera was established when overspilling alluvium from the Pampa
met the receeding cliff.
Channel development is virtually non-existant
away from the canyons, and drainage towards the
--
--
many enclosed basins of the coastal mountains
is thought to be by sub-surface seepage. The Coastal
Cordillera was uplifted at the close of the Paleogene, and, because of the low erosion rates, the
amount of sediment deposited in the Perú-Chile
Trench area during the N eogene has been restricted
to that derived {rom coastalrecessión.
1. INTRODUCTION
Throughout the greater part of northern Chile
exist well defined physiographic
and tectonic
provinces which are, from east! to west; the
Andean
block, the Pampa
del Tamarugal
(central tectonic depression), the Coastal Cordillera and, offshore of a locally terraced littoral;
the Perft-Chile Trench.
This paper is based on a detailed study of that
part of the Coastal Cordillera east of the town
of Iquique lying between 19°30t and 20030t lato S.
(SARlé, 1971). The area can be regarded as typicál
of the coastal region ot the Province of Tarapacá
which has been studied in less detail between
Arica (180 30' lato S.) and the Río Loa (210
30' lato S.) (Figs. 1 and 2).
This is one of the most raid regions in the
world, Iquique has an annual average rainfall
of 1.9 mm. (Almeyda, 1949) and vegetation is
accordingly completely absent from most of
the area. It only occurs in the deep river valleys
which carry exotic water from the Andes, and
on high summits near the coast where condensation from the coastal fog allows a few shrubs
to survive.
Geological mapping (THOMAS1970 a, b, SALAS
and others, 1966; SILVA, in prep,) has revealed
that J urassic and Cretaceous sediments and
volcanic rock's which have been intruded by
various elements of the Andean Batholith constitlite the predominating bedrock of the region.
Orogenic movements occured in the Cretaceous,
and since then the area has been continuously
above sea leve!. A sequence of Tertiary continen(*) Bickerton, Wetherby, Yorkshire, England.
(**) Compañia Minera Exotica, Calama, Chile.
--
-
-
T.XXI
C. MORTIMBR
59"
71.
163
ET N. SARIC : LANDFORM EVOLUTION IN THE COASTAL REGION
58.
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KILOM ETERS
FIG. 1. - Physiog1'aphy and d1'ainageof n01'lhe1'moslChile.
FIG. 1. - Physiog1'aphie el d1'ainage de l'eXI1'~me n01'd du Chili.
'f! SC
-'
I
164
T.XXI
C. MORTIMER ET N. SARIC : LANDFORM EVOLUTION IN THE COASTAL REGION
ral sediments mantles the topography, whereas
on the coastal terraces there are thin deposits
of Plio-Pleistocene marine sediments.
2. LITI'ORAL LANDFORMS
A clifI which in places is over 1000 m. high
extends along the greater part of the coast of
northern Chile. In Tarapacá Province it is only
broken where infrequent gorges cut through the
coastal mountains. South from Iquique there is
a continuous flight of terraces at the clifI-foot
which are covered by marine sediments. To the
north between Arica and Iquique, terraces are
only locally developed.
BRÜGGEN(1950) proposed that the clifI was
(THOMAS,op. cit.; SILVA,op. cit.). We consider
an actual fault scarp, and that the terraced littoral was a downthrow splinter. Since geological
mapping has shown that the fault do es not exist
(Thomas op. dt.; Silva op. cit.). We consider
that the clifI originated by wave action during
the Pliocene transgressive episode, evidence of
which has been noticed all along the north Chilean coast (Herm, 1969). Coastal subsidence is
thought to have occurred, which permitted the
waves to attack continuously across the developing wave"':cut platform. Evidence of subsidence occurs in the Lluta Valley near Arica, where
continental Miocene sediments and subaerially
deposited ash-flows now lie at least 300 m. below
sea level (KARZULOVIé,
1968). Near to the mouth
of the Rio Loa, warped lacustrine terraces in the
coastal mountains probably indicate downward
tilting to the west of the irnmediate coastal region.
The world-wide tectono-eustatic preglacial sea
level rise proposed by GALLOWAY
(1970) may
have aided the transgression.
If coastal retreat has occurred in the manner
suggested, then one would expect to find evidence
of an extensive submarine abrasion platform
representing the distance of the clifI recession.
Data on the submarine topography irnmediately
west of Tarapacá are not available, but recent
work further south has revealed a continental
shelf of about 10 km. width (SCHOLL
el al., 1969).
This is known to extend, with local variations,
well to the south of Chile, and there is no reason
to believe that the feature dres not extend to
the north. It is here suggested that this continental
shelf is at least in part a complementary feature
to the high clifI, and formed during the coastal recession.
The flight of terraces at the clifI-foot indicates
that there has been a local regression which was
most probably-non uniform and may have involved reingressions. The back of the highest terrace
occurs just south of Iquique at 150 m.a.s.!.,
but the highest terrace level falls both north and
-
-IKmConyon
1::'
Principó)l Summits
.~
COClstol
Es(ormenI
.~~ ,
,,-l.'
Prolilt
__'o\llt
..
KI1..0HEJUS
FIG. 2. - Actual fault scarps in the Iquique region.
FIG. 2. - Escarpements de faille actuels dans la région
d' 1quique.
south from here. At Patillos some 60 km. to the
south it lies at about 100 m.a.s.!., and at Pisagua
it is only 45 m.a.s.!. Between Pisagua and Arica
there are apparently no terrace remnants and
the high clifI is still receeding under wave attack.
The relative fall in sea level of unequal efIect
along the coast was probably therefore caused
by local tectonic movements. Mapping of the
coastal terraces to the south of Iquique has
revealed a great many terraces of difIering altitudes and of short longitudinal extent, and it
has not been possible to distinguish any evidence
T.XXI
C. MORTIMER
165
ET N. SARIC : LANDFORM EVOLUTION IN THE COASTAL REGION
s
N
mt~
1000
900
800
700
GOO
500
B'
B
o
FIG. 3. -
PrQfile E-E' showing local topographic effect of
E- W faults.
which can be said to be theeffect of glacio-eustatic
sea level changes on the littoral.
Where terraces are not present the cliff reaches
an inclination of up to 800, elsewhere terraces
have protected the cliff and degradation since
the sea retreated has reduced the inclination to
nearer 45°.
3. THE TOPOGRAPHY
OF THE COASTAL CORDILLERA
In the detailed study area (Fig. 2Y the coastal
mountain - belt is approximately 40 km. wide,
narrowingto the north. Though the approximate
average height is about 1,000 m.a.s.l., the maximum height of 1761 m. is on Cerro Constancia
(Fig. 2). At the coast the range is terminated
by the steep cliff, but the eastern border of the
Coastal Cordillera is irregular where the mountain slopes finger into flat-Iying alluvium in the
Pampa del Tamarugal. This latter tract is a depositional surface of a11uvium" which slopes gentIy
down from the Andean foothills to meet the barrier
"
of the "C6astal
Cordillera~
---
(Km.
.."
Fault movements in the Coastal Cordillera
have produced a topography of isolated internal
drainage formed from a- complex of uplifted and
downsunken blocks (Fig. 2). The margins of the
blocks have a clear topographic expression and
have inclinations of up to 45°. The fault pattern
FIG. 3. -
FAULTS
Profil E-E' montrant les effets des failles E-W
sur la topographie locale.
show east-west and north-south predominating
trends and a11 faults are apparentIy high-angle
and normal. It is interesting to note that although
the north-south striking suite of faults do es not
have any apparent preferred direction of throw,
those faults which strike east-west nearly always
downthrow to the north (Fig. 3). Field observation
led to the conclusion that the east-west fault
system is the younger of the two suites because
of the fresher appearance of the fault scarps.
There is no evidence that the faulting, which
has had such a marked effect on the landscape
of the Coastal Cordillera, has in any way affected
the terraced littoral or its superficial layer of
marine sediments, although fault lines have been
10ca11y etched-out by marine abrasion. It is.
therefore apparent that the cutting of the terraces
took place after the topographica11y expressed
fault movements.
The alluvium of the Pampa del Tamarugal has
partia11y submerged the eastern flank of the
Coastal Cordillera, so that this latter unit continues as a structural block for an unknown distance beneath the Pampa del Tamarugal physiographic province. There is only one fault trace
which can be seen to displace the Pampa surface
(Fig. 2). This indicates that by for the greater
part of the alluvium forming the Pampa surface
was deposited after the faulting.
168
T.XXI
C. MORTIMER ET N. SARIC : LANDFORM EVOLUTION IN THE COASTAL REGION
o
----
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"'''"---.....
~" "
0
",
co Sq!'l Anl~nio
12"'.
CO C~orodo
1
PompQ
..........
K>09
(
i
i
i
"ISO.
50..
'00
oj
_
_
s.1..
"el
TomOn:filOI
...................
A'
Preservedold londscope
YounQsurfaee
CId &ondscopehidden by alluvium
Km
_Foult
FIG. ~. - The Tarapaca Coastal Pediplain (1) reconstructed
from the present topography (2) in section A-A'.
The tectonic basins within the Coastal Cordillera ha ve been partially infilled with alluvium.
Sections through the alluvium occuring in the
cliff-face reveal a completely continental sequence
of interbedded
sandstones and conglomerates
with occasional evaporites and ashf1ows. The
continental sediments reach an observed maximum thickness of over 200 m. at Pisagua. At
this same locallity some 50 m. of the sequence
is seen to be composed of ash-flow tuff which
can only have had its origin near to the volcanic
Andean crest far to the east. The alluvium is
considered to have been locally derived, having its origin in scarp retreat following the block
faulting. In the central and eastern parts of the
Coastal Cordillera the top few metres of the alluvium contain nitrates and other salts.
The landscape which existed prior to the block
faulting and alluviation can be locally reconstructed by diagramatically neutralising the effects
of the faulting and other, later, landform development (Fig. 4). The manner of reconstruction
is clear aiíd 'thé' form' of the oldlandscape
is
well brought out. The exhibited lack of relief
with no evidence of drainage is typical of the old
landscape throughout
the investigated
area,
and the process responsible for the modelling
of at least the latest forms of the old landscape
is considered to have been pedimentation. Accordingly the old landscape has been named the
Tarapaca Coastal Pediplain.
--
FrG. ~. - La pédiplaine cotiere de Tarapaca reconsittuée a
partir de la topographie actuelle. Légende dans l'ordre :
paysage ancien conserpé. surface récente. paysages anciens
marqués par des allupions. faille.
4. DRAINAGE
In the region between 19° 30' and 20° 30' lato
S., the only break in the Coastal Cordillera is
the deep Tiliviche gorge, which cuts across the
Pampa del Tamarugal and the coastal mountains
from a source in the high Andes. Further north
where the Coastal Cordillera is narrower, several
more canyons cut the range, thouth to the south
there is no other similar drainage until the Río
Loa some 130 km. distant (Fig. 1).
CHRISTENSENand others (in press) envisage a
form of fan development from the east crossing
.the Pampa. Rivers are considered to have cut
down across the apex of the fan when the level
of the alluvium within the fan was sufficient to
overtop the range and achieve a marine baselevel, thus establishing such canyons as Tiliviche.
We were unable to find evidence of such fans,
and consider that after the uplift of the Coastal
Cordillera relative to the other physiographic
provinces, the Pampa del Tamarugal began to
infill 'with debris from the Andes. The level of
deposition of the alluvium slowly rose submerging
the eastern flank of the Coastal Cordillera by
alluvium. As the alluvial fill increased in depth
it worked progressively westwards, !!pilling over
into successive tectonic basins and infilling
them at low points of the Coastal Cordillera. At
the same time the coast was retreating towards
the west, so that a marine baselevel was establis-
T.XXI
C. MORTlMER
169
ET N. SARIC : LANDFORM EVOLUTlON IN THE COASTAL REGION
hed when these two processes meto Once the
alluvium from the Pampa overtopped the cliff
there would have been a rapid down-cutting by
the exotic water.
It is interesting to note that the bed profile
of the lower Tiliviche demonstrates three major
breaks of slope (Fig. 5) which are neither structurally nor lithologically controlled, whereas
the river profile above the highest change of
slope is in virtual coincidence with' the level of
the Pampa. The upper profile is considered to
be a section of the river whose erosive state is
still in equilibrium with baselevel on the Pampa
del Tamarugal. The lower changes in the profile
of the river may have formed subsequent to
relative falls in sea level during the regression
after the formation of the cliff.
The other canyons to the north of Tiliviche
are considered to have evolved in a similar manner,
but since the Coastal Cordillera narrows northward, the role of coastal recession in facilitating
establisment of a marine base-level to the drainage may have been greater there.
The tilted lacustrine sediments near to the
Río Loa mouth suggest that the saline Lake
Soledad (BRÜGGEN,op. cii.) was tectonically
tipped over the cliff, in this manner establishing
the lower course of the river. Some of the water
from the lake was displaced westward during
warping into a tectonic depression to the north
of the Loa, which, after evaporation, gave rise
to the salt deposits of Salar Grande.
Stream development away from the vicinity
of the major canyons is barely evident except
immediately adjacent to the coast, and perhaps
developed during a marginally more pluvial
climate at some time in the pasto None of the
valleys reach sea level or anywhere near it,
and are hanging in the cliff topo The rest of the
drainage in the Coastal Cordillera is entirely
internal towards the many basins, and the lack
of channel development within these prompts
the conclusion that any rainfall is removed by
seepage through the alluvium towards the centre
of the depressions.
Evaporation is undoubtedly more than adquate
to prevent standing water accumulation, but
the numerous faults which cross the range
must provide a potentially rapid transportation
route for water to pass through to the coast.
Several seepages of brackish ground water at
or near sea level along the foot of the cliff could
be explained this way.
5. OVERVIEW OF THE LANDSCAPE HISTORY
ONSHORE
The north of Chile has had a history of subaerial erosion since the Cretaceous, consequently
only through the interpretation
of landforms
and their associated alluvial deposits can any
history of the Cenozoic be worked out. There
are no ancient river valleys in the studied region
which could have transmitted
relative changes
of a marine base-level inland, so a hierarchy
of landscapes cannot be seen. That which can
be seen is the tectonically dislocated end result
of a single continuous phase of erosion. Nevertheless, by examining and relating the histories
of the Pampa del Tamarugal and coastal area
to that of the Coastal Cordillera an idea of the
Cenozoic evolution can be obtained.
The coastal mountains have acted as a dam
to easterly derived sediment until late in the
Cenozoic, and still act as a partial barrier (TRICART,
1966). Consequently the greater part of the erosion products derived from the western flanks
of the Andes during Neogene times was deposited
in the longitudinal depression east of the Coastal
Cordillera. Volcanic horizons intercalated in the
sediments on the eastern flanks of the longitudinal
depression have yielded K/Ar ages not older
than Miocene (MORTIMER,F ARRARand SARIé, in
prep; CHRISTENSENand others. op. cit.) and it
can be reasonably concluded that the coastal
mountain belt did not act as a barrier before this
time. It is therefore suggested that the present
Coastal Cordillera in Tarapacá Province was
uplifted in relation to the rest of the Andean
mass in the late Paleogene-Lower Miocene periodo
GALLI-OLlVIER (1967) working on the flanks
of the Andes immediately to the east of our
area, recognised in the Juan de Morales regio n
a faulted Tertiary landscape which he called the
Choja Pediplain. It is very probable that this
landscape can be correlated with the similar
landscape we have recognised in the coastal
mountains.
There exists a strong possibility
that these surfaces can be further correlated
with the Atacama Pediplain which lies between
260 and 290 lato S. This later pediplain was shown
to have ceased to develop at the end of the
Miocene (MORTIMER,1969, CLARK el al., 1967;
SILLlT, MORTIMER and
CLARK, 1968).
.
Faulting dislocated the Tarapacá Pediplain
and there after the landscape took on its modern
appearance after limited scarp retreat had occurred adjacent to the fault blocks, resulting in
basin alluviation.
The arid climate which has prevailed through
much of the Caenozoic (GALLI-OLlVIER,
1969) and
resulting lack of dissection are concieved as the
reasons for the remarkable preservation of both
fault scarps and the coastal cliff.
OFFSHORE
Profiles across the Coastal Cordillera reveal
that thelli sumit altitudes show no tendency to
----
170
C. MORTIMER
-
fall towards the coast. In the concept of the
coastal subsidence during cliff formation the
landscape should therefore have had an earlier
easterly inclination. This suggests that the cliff
in the early part of its evolution, when the coastline was further west, was probably even higher
than it is today.
Sedimentation
in the northern part of the
Perú-Chile Trench has been slight, this is to
be expected if the erosion products of the hinterland were trapped in the longitudinal depression east of the Coastal Cordillera. Sedimentation
in the Perú-Chil~ Trench area in Neogene times
would be virtually restricted to material introduced by the formation of the continental shelf
and coastal cliff. This quantity, for the whole
of Tarapacá Province, is estimated as approximately 3.4 X 103 k3 of removed rock. In addition, a very little debris has been derived through
the canyons.
ALMEYDA
(E.A.), 1949 : Pluviometría de las zonas
del' desierto y las estepas calidas de Chile:
Santiago, Edit. Univ., 162 p.
BRUGGEN
(J.M.) 1950 : Fundamentos de la geología
de Chile: Santiago, Instituto Geografico Militar, 374 p.
CHRISTENSEN
(M.N.), PÉREZ (G.), MONTECINOS
(F.),
and CURTIS (G.H.), in press, Late Cenozoic
vo1canism, deformation, and denudation in
northern Chile: Geol. Soco America Bull.
CLARK(A.H.), MAYER(A.E.S.), MORTIMER
(C.), SILLI(R.H.),
COOKE(R.U.), and
1969, Climate : A primary control of sedimentation in the Perú-Chile Trench: Geol. Soco
America. Bull., v. 80, p. 1849-1852.
GALLOWAY
(R.W.), 1970, Coastal and shelf geomorphology and late Cenozoic sea levels: lour.
Geology, v. 78, p. 603-610.
HERM(D.), 1969, Marines Pliozan und Pleistozan
in Nord-und Mittel - Chile, unter besonderer
Beriicksichtigung der Entwicklung der Mollusken - Faunen : Zitteliana, v. 2, 159 p.
KARZULOVIc
(J.), 1968, Estudio hidrogeológico de
la región de Arica, Provincia de Tarapacá :
Santiago, Univ. de Chile, 129 p.
MORTIMER(C.), 1969, The geomorphological evolution of the southern Atacama Desert, Chile :
Ph. D. thesisj Univ. of .London, 283 p.
MORTIMER(C.), FARRAR(E.), SARIC(N.), in prep.,
Cenozoic K/ Ar ages from the Quebrada Tiliviche Camina transect, north Chilean Andes.
SALAS(R.), KAsT (R.), MONTECINOS
(F.), and SALAS
(1.), 1966, Geologia y recursos minerales del
departamento de Arica: Inst. Tnvest. Geol.,
paca, Chile: Memoria Universidad de Chile,
SARIC(N.), 1971, Evolution Cenozoica de la Cotdillera de la Costa, en la Provincia de Tarapaca, Chile : Memoria Universidad de Chile,
176 p.
SILLITOE(R.H.), MORTIMER(C.), and CLARK(A.H.),
1968, A chronology of landform evolution and
supergene mineral ¡ilteration, southern Atacama Desert, Chile: Inst. Mining and Metallurgy Trans., v. 77, Seco B., p. 165-169.
SILVA(L.1.), in prep., Geología de las Hojas Pisagua y Zapiga, Provincia de Tarapaca, Carta
Geologica de Chile: Santiago, Inst. Invest.
Geologicas.
THOMAS
(A.), 1970 : Cuadrangulos Iquique y Caleta
Molle, Provincia de Tarapaca. Santiago (Instituto de Investigaciones 'Geologicas) Carta Geol.
Chile, 21 y 22.
THOMAS(A.), 1970: Beitrag zur Tektonik Nordchiles. Geol. Rundschau, 59, p. 1013-1027.
TRICART
(J.), 1966 : Un chott dans le désert chilien ;
la pampa del Tamarugal : Rev. de Géom. Dyn.,
v. 16, p. 12-22.
.
REFERENCES
. TOE
T.XXI
ET N. SARIC : LANDFORM EVOLUTION IN THE COASTAL REGION
SNELLING(N.J.),
1967 : Implications of isotopic age of ignimbrite
flows, southern Atacama Desert, Chile: Nature,
v. 215, p. 723-724.
GALLI-OLIVIER (C.), 1967, Pediplain in northern
Chile and the Andean uplift : Science, v. 158,
p. 653-655.
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