Journal of South American Earth Sciences 16 (2004) 633–648
www.elsevier.com/locate/jsames
Origin and chronology of Pleistocene marine terraces of Isla de la Plata
and of flat, gently dipping surfaces of the southern coast
of Cabo San Lorenzo (Manabı̀, Ecuador)
Gino Cantalamessa*, Claudio Di Celma
Dipartimento di Scienze della Terra, Università degli Studi di Camerino, Via Gentile III da Varano, 1-62032 Camerino (MC), Italy
Received 31 July 2002; accepted 31 December 2003
Abstract
Using stratigraphic and geomorphological studies, we investigate and propose a model for the genesis of Quaternary staircase
morphologies that occur at two different places along the central coast of Ecuador. In the study area, located landward of where the Carnegie
Ridge impinges on the Ecuador trench, ridge subduction favored spatially nonuniform uplift rates that, combined with Pleistocene
glacioeustatic sea-level fluctuations, led to punctuated forced regression. In turn, depending on the amount of sediment supply and the
average amplitude of uplift rates, this led to the development of a cyclothemic shallow-marine sedimentary succession or of a flight of marine
terraces.
Isla de la Plata displays a sequence of four marine terraces that each mark a period of marine encroachment during uplift. Their age
determination, based on inferences drawn about the amount of long-term tectonic uplift and heights of the eustatic peaks, has quantified
vertical tectonic activity and correlated marine terraces with glacioeustatic variations in sea level. During the last 500 ka, the
chronostratigraphy of the marine terraces of Isla de la Plata suggests a constant uplift rate of ca. 0.4 m/ka.
Along the coast, on the basis of its relation to the late Pliocene and Middle Pleistocene units of the Canoa Formation, the highest surface
present east of Cabo San Lorenzo is inferred as early Pleistocene in age and not, as recently suggested, early Pliocene. Quaternary uplift has
been relatively slower with respect to Isla de la Plata, and synsedimentary uplift decreased from north (ca. 0.3 m/ka) to south (0.1 m/ka). This
trend explains the angular and composite progressive unconformities that generated two, southward-dipping flat surfaces. Deposition of
sediments associated with these landforms was accompanied by glacioeustatic sea-level fluctuation and tectonism that influenced the internal
arrangement of the depositional sequences. The amount of uplift between successive highstands of sea level was not sufficient to create single
marine terraces and instead resulted in repeated terrace reoccupation and the creation of a relatively thick, vertically stacked succession of
shallow-marine and sometimes estuarine and coastal-plain sediments.
From this study emerges the importance of local factors in the depositional response to forced regression, including the fundamental role
played by the magnitude of uplift rates in generating detached or attached depositional sequences.
q 2004 Elsevier Ltd. All rights reserved.
Keywords: Carnegie Ridge; Coastal deformation; Cyclothems; Forced regressive sequence set; Sea-level changes; Tectonism
1. Introduction
The term ‘marine terrace’ is used here to define an
uplifted landform and associated deposits, separated from
each other by steep scarps, that represent relict sea cliffs
formed by a single sea-level oscillation. Flights of
abandoned Pleistocene marine terraces stranded above
* Corresponding author. Tel.: þ 39-0737-402624; fax: þ 39-0737402644.
E-mail addresses: gino.cantalamessa@unicam.it (G. Cantalamessa),
claudio.dicelma@unicam.it (C. Di Celma).
0895-9811/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jsames.2003.12.007
modern sea level occur at several places along the Pacific
coastline of South America, where they are known as
Tablazos (Broggi, 1946). Because terraces are marine in
origin and because the history of Quaternary sea-level
fluctuations, as inferred from oxygen-isotope records in
deep sea cores, shows that interglacial periods have been
characterized by sea-levels peaks similar to that of the
present day (Shackleton et al., 1995), their present
elevations above modern sea-level can only be the result
of long-term, relatively slow, tectonic uplift of
the continental margin. These geomorphological features
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G. Cantalamessa, C. Di Celma / Journal of South American Earth Sciences 16 (2004) 633–648
therefore are valuable markers for assessing recent geological uplift (Carobene and Dai Pra, 1990; Leonard and
Wehmiller, 1992; Ota et al., 1995; Polenz and Kelsey, 1999;
Pinter et al., 2001).
Along the Pacific coast of South America, the highest
marine terraces occur where the plate margin lies above
downgoing aseismic oceanic ridges, where uplift is the
greatest and most rapid (Aubrey et al., 1988; Goy et al.,
1992; Hartley and Jolley, 1995). In such tectonically active
settings, uplifted marine terraces provide useful markers for
assessing the presence, rates, and pattern of coastal
deformation. Models to assess coastal deformation associated with the subduction of aseismic ridges have been
proposed by Macharé and Ortlieb (1992) and Hsu (1992).
In this paper, we focus on studies that assess the role
played by tectonic and eustatic events in the origin and
chronology of marine terraces and flat, gently dipping
surfaces that occur, respectively, on Isla de la Plata and
along the mainland coastline facing the island, south of
Cabo San Lorenzo (Fig. 1). Our aim is to better understand
the processes that control variability in depositional style
during eustatic sea-level oscillation, which leads to the
formation of marine terraces or cyclothemic sedimentary
successions (McMurray and Gawthorpe, 2000). Our
research combines aerial photo interpretations (scale
1:20,000 for the Isla de la Plata, 1:60,000 for the
coastal area) and field surveys made during the past
decade and is part of a larger research project studying
the Plio –Pleistocene deposits present at various localities
along the Ecuador coast (Bianucci et al., 1993a,b, 1997a;
Bisconti et al., 2001; Ragaini et al., 2002). In particular, it
forms part of a stratigraphic study of the Plio– Pleistocene
succession that crops out along the cliff south of Cabo San
Lorenzo between 18090 S and of 18150 S (Bianucci et al.,
1997b; Landini et al., 2002a,b; Di Celma, 2001; Di Celma
et al., 2002).
2. Geological setting
The study area is located on the overriding north Andean
block (NAB) microplate, just landward of where the
Carnegie Ridge—a 400-km wide, 2-km high aseismic
bathymetric high generated by the eastward migration of
the Nazca plate above the Galapagos hot spot—is subducted
in the Ecuador trench (Lonsdale, 1978). The Ecuadorian
coastal block of oceanic substratum is the southernmost
leading area of the NAB microplate (Kellog and Vega,
1995). It is separated from the Nazca plate to the west by the
Ecuador trench and from the South American plate to the
east by a system of NE-trending, right-lateral, strike-slip
faults and N-trending thrust faults (Dolores-Guayaquil
megashear [DGM]), whose transition into the east Andean
frontal fault system takes place in and to the north of the
inter-Andean valley (Trenkamp et al., 2002). The southern
extremity of the DGM is commonly thought to intersect
Fig. 1. Location map of the Ecuadorian coast showing the study area (box) and its position with respect to where the Carnegie Ridge impinges on the Ecuador
trench (Lonsdale and Klitgord, 1978; Baldock, 1982). The Nazca– South America convergence vector from Trenkamp et al. (2002).
G. Cantalamessa, C. Di Celma / Journal of South American Earth Sciences 16 (2004) 633–648
the Ecuador trench in the region of the Gulf of Guayaquil. A
global position system measurement by Trenkamp et al.
(2002) shows that, as a result of the low oblique orientation
of the Nazca plate movement vectors relative to the Ecuador
trench, in the overriding NAB, the normal components of
eastward subduction are accommodated by margin-normal
shortening, whereas tangential components are accommodated by north-eastward, dextral strike-slip displacement
along the DGM. Because of the northward motion of the
NAB along the DGM, the Carnegie Ridge also sweeps
slowly southward along the Ecuador margin and uplifts the
overriding forearc basement (Gutscher et al., 1999).
Well-preserved raised marine terraces on Isla de la Plata
and uplifted, high-frequency depositional sequences (Van
Wagoner et al., 1988, 1990) of late Pliocene and Pleistocene
age along the mainland coastal stretch facing the island (Di
Celma, 2001; Di Celma et al., 2002) testify that this region
experienced recent vertical movements. The rapid uplift of
southwestern Ecuador was induced by the eastward
subduction of the aseismic Carnegie Ridge beneath this
part of the South America. Gutscher et al. (1999), on the
basis of trench-parallel bathymetric/topographic profiles
interpreted to record a basement uplift signal, suggest that
the Carnegie Ridge extends at least 110 km east from the
trench and that its subduction started at least 2 Ma BP.
In contrast to Chile and Peru (for reviews, see Radtke,
1987; Paskoff, 1977 [Peru]; De Vries, 1988 [Chile]), the
origin of planation surfaces, marine terraces, and associated
deposits along the central southern Ecuadorian coast has
received little recent attention in scientific literature but
rather is represented by conflicting and outdated studies by
Sheppard (1927, 1930, 1937) and Marchant (1961). With
the exception of Coltorti and Ollier (1999, 2000), Pedoja
et al. (2001, 2003), and Ficcarelli et al. (2003), who attempt
to locate these landforms in the general pattern of
Ecuadorian evolution, recent work (e.g. Bristow and
Hoffstetter, 1977; Iriondo, 1994) generally has been limited
to reporting cited studies and has not furthered knowledge
about the evolutionary history of the area during the last
million years. Thus important details such as the age and
mode of landform development, remain unknown.
3. Quaternary shorelines on the Isla de la Plata
3.1. Description
Isla de la Plata (018160 S, 818040 W) is situated in the
coastal stretch opposite the southern side of Cabo San
Lorenzo, approximately 20 km east of the Ecuador trench
and 25 km west from mainland Ecuador (Fig. 2). According
to Baldock (1982), a Cretaceous basaltic complex (Piñon
Formation), which constitutes the basement for Ecuador
west of the Dolores-Guayaquil suture, crops out on it.
Sheppard (1927) was the first to report an array of marine
terraces on the island, sited at approximately 100, 500,
635
and 740 ft (30, 150, and 225 m) above present-day sea level.
Notwithstanding their different elevations, Sheppard (1930)
correlated the three terraces on Isla de la Plata with three
landforms of different altitudes in the Cabo San Lorenzo and
Santa Elena peninsula areas and attributed them to three
different uplift phases. Savoyat (1971) rejected this
interpretation and argued that only one marine terrace
occurs on Isla de la Plata, 160 m above the sea level. We
present evidence for the existence of four terraced landforms
rather than the three identified hitherto (Fig. 2), which
represents the first record of middle and late Pleistocene sealevel fluctuations in Ecuador.
Each terrace, numbered from the oldest (Plata-1) to the
youngest (Plata-4), consists of a seaward-dipping wave-cut
platform carved on the basalts of the Piñon Formation and is
separated by steep scarps that represent relict sea cliffs. The
inner edge (shoreline angle of Kern, 1977; strandline of
Pillans, 1983) is the intersection of the abrasion platform
and the rising cliff face and indicates the location of the
shoreline at the time of the maximum ingression of the sea
onto the landmass (i.e. the highest position reached by sea
level during a given fluctuation). The available topographic
maps of the area, which have a contour interval of 20 m, are
of little help in determining the elevation of the inner edges,
so independent altimetric data were needed. The level of the
inner edges has been established by repeated series of
measurements with a Thommen altimeter, which provides
their elevations with a precision of 5 m.
The oldest marine terrace on the island (Plata-1) has a
very limited aerial extension and covers the northernmost
part (Fig. 3a). Practically devoid of associated deposits, it
lies at the highest altitude on the island (þ 175 m a.s.l.) and
houses the island’s only lighthouse. According to Sheppard
(1927), the lighthouse is set on a terrace at approximately
740 ft (225 m) above present-day sea level; however, in that
no such altitude exists on the island, Sheppard likely
misestimated the height of the terrace and was referring to a
height of þ 175 m.
At an altitude of þ 160 m, a second terrace (Plata-2) joins
the paleo –sea cliff, which separates it from Plata-1 (Fig. 3a
and b), and drops gradually southeastward to approximately
150 m at its southernmost rim. The extension of this marine
terrace makes it the most conspicuous landform on the
higher areas of the island which suggests it is likely the one
Sheppard (1927) placed at 500 ft (þ 150 m). This terrace is
incised and dissected by a network of deep gullies that flow
mainly north- and southward.
A less prominent step (Plata-3), present exclusively on
the eastern part of the island in the neighborhood of Punta
Escalera (Fig. 3b and c), is approximately 1.5 km long and
300 m wide. Probably unidentified by Sheppard (1927),
Plata-3 has the shoreline angle at an altitude of þ 80 m.
The fourth and youngest of the terraces (Plata-4; Fig. 3b
and c) occupies the entire southern part of the Isla de la
Plata, from Punta Escalera in the east to Punta Machete in
the west. Along the remaining portion of coast, Plata-4 is
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Fig. 2. Geomorphological sketch of Isla de la Plata showing the distribution of the four marine terraces. Inset shows the exact location of Isla de la Plata relative
to the onshore study area.
found as small remnants or a white horizontal line on the sea
cliff at an altitude of ca. þ 55 m (Fig. 3d). This line
represents the shoreline angle given by the intersection of
the wave-cut platform, which no longer exists, and its relict
sea-cliff.
The back-edge of the most recent terrace is the only one
partly covered by alluvial fans deposited during the
intervening period of lower sea level at the foot of
the ancient sea cliff. Where present, it is difficult to locate
the relative strandline in the field. Clapperton (1993, p. 598)
attributes this terrace to substage 5e of the oxygen-isotope
record; the fans that cover it thus represent sedimentary
products of the last 124 ka.
Each of the four marine terraces, formed during sea-level
rise and highstand, was progressively uplifted during a cycle
of sea-level change. They are composed of a wave-cut
platform, which formed during the transgressive stage,
covered by thin veneers of beach deposits (0.3 –2-m thick),
which consist mainly of unsorted sands with basaltic
pebbles and fragmented shells (Fig. 4) and reflect an
exclusively local source (Piñon Formation).
3.2. Chronostratigraphic interpretation of the Isla de la
Plata terrace sequence
Unfortunately geochronological data are not available,
and therefore, the chronology of the terrace sequence is
based exclusively on altimetric, geometric, and geomorphological data. If we presume that, as mentioned
previously, the youngest terrace refers to substage 5e
and that its shoreline angle represents the sea-level peak
of the last interglacial (124 ka), we can calculate the
average uplift rate for the period. Studies of uplifted reef
terraces in New Guinea (Bloom et al., 1974) and Haiti
(Dodge et al., 1983), suggest that, at the peak of the last
interglacial, sea level was 6 m higher than the current
G. Cantalamessa, C. Di Celma / Journal of South American Earth Sciences 16 (2004) 633–648
637
Fig. 3. Panoramic views of the marine terraces of Isla de la Plata. (a) Looking north, terraces related to stages 13 (Plata-1) and 11 (Plata-2). (b) Eastward view
from Punta Machete to Punta Escalera of the southern part of Isla de la Plata, terraces related to stages 11 (Plata-2), 7 (Plata-3), and 5e (Plata-4). (c) Northward
view of the last two marine terraces near Punta Escalera. (d) Small remnants of the last marine terrace located on the northern side of Isla de la Plata, west of
Punta Palo Santo.
level. Therefore, this difference must be subtracted from
the present elevation of the last interglacial terrace before
dividing it by the numerical age. Following this line of
reasoning, Isla de la Plata has been uplifted at an average
rate of ca. 0.4 m/ka during the last 124 ka. This value
represents a net uplift rate, which may not have been
steady.
If we assume that (1) paleo –sea level was close to that of
the present day during the culmination of each marine
transgression, (2) the determined uplift rate remained almost
constant, and (3) therefore, the determined uplift rate is
representative of the long-term uplift rate, then the age of
the earlier terraces can be estimated using their present-day
shoreline angle altitude. Bloom and Yonekura (1985) show
Fig. 4. Close-up views of the last marine terrace of Isla de la Plata and details of its material. (a) Arrows indicate the wave-cut platform (ravinement surface) of
the last marine terrace (Plata-4) carved on the basalt of the Piñon Formation. The overlying deposits are about 0.30–0.50-m thick and composed of (b)
Quaternary shells and angular clasts, (c) rounded cobbles/pebbles, or (d) small, flat clasts of basalt contained in a sandy matrix.
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Table 1
Age and possible correlations with oxygen-isotope stages for Isla de la Plata
terraces
Terrace
order
Shoreline angle
altitude (m)
Numerical
age (ka)
Oxygen-isotope
stage
Age of high sealevel peaka (ka)
5e
7
9
11
13
124
180 –240
303 –339
362 –423
478 –500
Plata-3
Plata-3
55 ^ 5
80 ^ 5
124
200 ^ 12.5
Plata-2
Plata-1
160 ^ 5
175 ^ 5b
400 ^ 12.5
438 ^ 12.5
a
b
Ages of paleo-sea levels taken from Imbrie et al. (1984).
This terrace has no shoreline angle.
that this method is an excellent predictor of age determinations of Pleistocene terraces and that the assumption of a
uniform average uplift rate and a paleo– sea level equal to
the current level introduces little error and gives good
results with cycles on a time scale of 105 years.
Furthermore, Shackleton (1987) notes that the sea level of
oxygen-isotope stages 1, 5e, 9 and 11 are similar, and
probably higher than those of oxygen-isotope stages 7, 13,
15, 17, and 19, when the sea may not have reached its
present level.
In Table 1, we report the age and respective oxygenisotope stage inferred for each of the marine terraces on
the island. In addition, we show that the ages of the
inner edge of the wave-cut platforms are consistent with
the peak of the respective oxygen-isotope stage in
support of an almost constant uplift rate during the
Middle and Late Pleistocene. Moreover, the terraces on
the island seem to record only interglacial highstands of
the 100 ka, eccentricity-driven sea-level cycles, whereas
intervening interstadials are missing.
Only the most ancient terrace displays an age not
strikingly consistent with that of the oxygen-isotope stage
peaks (stage 13). This inconsistency may be attributed to
the lack of a shoreline angle for this terrace; the altitude
used to calculate its age does not indicate the position of
the shoreline angle at the time of the sea-level peak, but
rather a lower one refers to a previous, more seaward
position of the connection between the platform and the
sea cliff.
The absence of any landform referable to stage 9
could be attributable to that stage’s complete removal
during the deeper retreat of subsequent sea cliffs (stage
7) into the landmass. The factors regulating the extent of
sea-cliff incursion into the landmass as analyzed by
Anderson et al. (1999), include the duration of sea-level
highstand, the far-field wave energy input, and the degree
to which bathymetric drag dissipates wave energy.
Although the assumptions about the terrace chronology
must be confirmed by radiometric ages, they represent the
only working hypothesis that corresponds to the morphostratigraphy observed on Isla de la Plata.
4. The mainland coastal area
4.1. Description
The existence of a flight of three low-relief surfaces in
southwestern Ecuador was observed by Sheppard (1930,
1937), who referred to the sediments associated with them
as ‘Tablazo Formation’ and ascribed their origin to three
Pleistocene uplift phases. According to Baldock (1982),
their age ranges from Early to Late Pleistocene. Marchant
(1961) disagrees with this interpretation and argues that,
though there have been at least three recent sea-level
changes (as proved by wave-cut platforms lacking in
sediments), only one Tablazo deposit exists, faulted at
more than three levels by recent movements. This point of
view is shared by Savoyat (1971).
In the area between Cabo San Lorenzo to the north and
Rio de Caña to the south, three flat morphological features
(Fig. 5) that slope gently southward with progressively
lower angles are distinguished; they are numbered from
oldest (CI) to youngest (CIII).
The highest surface (CI) is preserved exclusively east of
the village of San Lorenzo, near El Aromo. At an altitude of
ca. þ 365 m, only a narrow erosional surface remnant and
associated deposits remain. The intermediate surface (CII)
is dissected by deeply incised streams that have created an
extensive network of gullies. It is widespread along the
coast south of Cabo San Lorenzo and, in the northern part of
the area, is bordered by a distinct fossil sea cliff that has a
shoreline angle at ca. þ 300 m. From there, CII falls steadily
to the south to approximately 125 m a.s.l. (El Mangle),
where it is delimited by a scarp at whose footwall (þ 35 m)
the third surface (CIII) is present. The lowest surface
extends southward up to and beyond Rio de Caña, where it
has an altitude of ca. þ 8 m. At a locality a few hundred
meters south of El Mangle, an exposure along the present
sea cliff shows a paleo– sea cliff cut into the underlying
Canoa Formation sands. The transgressive surface, overlain
by approximately 15 m of sediments associated with CIII,
intersects the relict cliff at ca. 20 m above present-day sea
level. From there, CIII extends as far as 300 m inland; prior
to a southward increase in width, it occurs as a narrow ridge
for approximately 2.5 km parallel to the shoreline. Carbon14 dating of gastropod shells found in this terrace, near the
coastal city of Manta (08560 S, 808390 W) yields a . 32:87 ka
age for the sediments (Kennerley, 1980).
Whereas CI lies unconformably on pre-Pliocene deposits
and is associated with a thin layer of beach sediments, CII
and CIII are associated with a thick succession of
cyclothemic strata that represent parts of the late Pliocene
and Middle –Late Pleistocene sedimentary record (Fig. 6).
This sedimentary succession, which reflects deposition in
offshore, shoreface, and sometimes alluvial environments,
traditionally has been subdivided into two formations: the
Canoa (CnF) and the Tablazo (TF). The cyclothemic nature
of these formations (Fig. 6a) has been interpreted
G. Cantalamessa, C. Di Celma / Journal of South American Earth Sciences 16 (2004) 633–648
639
Fig. 5. Schematic geological and geomorphological map of the Ecuadorian coast south of Cabo San Lorenzo, between 018030 S and 018150 S lat. Numbers and
localities along the coast refer to measured sections shown in Fig. 6.
as the product of glacioeustatic fluctuations of sea level
(Di Celma, 2001; Di Celma et al., 2002). Each cyclothem,
composed of a deepening – shallowing upward facies
association bounded by a ravinement surface amalgamated
with the preexisting sequence boundary, represents depo-
sition landward of the lowstand shoreline. Therefore, it is
equivalent to a depositional sequence generated by a single
oscillation of relative sea level.
The CnF lies unconformably over the Early Miocene
Tosagua Formation and is divided by an extensive angular
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G. Cantalamessa, C. Di Celma / Journal of South American Earth Sciences 16 (2004) 633–648
Fig. 6. Block diagram, based on 39 measured sections showing the stacking pattern of the Plio –Pleistocene depositional sequences and relationships among CI,
CII, and CIII. Arrows indicate uplift and subsidence. (a) and (b) indicate location of photographs shown in Fig. 10. EA, El Aromo; SR, Santa Rosa; EL, El
Mangle; LC, La Cotera; SAU, syntectonic angular unconformity; CnF, Canoa Formation; TF, Tablazo Formation. Except for sections 1 and 2, and sections 15
and 16, which are approximately 1.5 km and 30 m apart, respectively, the spacing between successive measured stratigraphic sections is 200 –250 m.
unconformity into two units (CnFlow and CnFupp) that are
characterized by different deepening – shallowing facies
associations (for a description and interpretation of
sedimentology and sequence stratigraphy of CnFlow, see
Di Celma et al., 2002) and dated as late Pliocene and late
Early Pleistocene, respectively. The TF, whose sediments
are Middle Pleistocene– Holocene, is composed of at least
six cyclothems characterized by a third deepening –
shallowing facies sequence. These cyclothems, along with
those of CnFupp, form a single forced regressive sequence
set (Fig. 6b). The wide ranges of recorded sediment facies
have been grouped into major facies associations, the
principal characteristics of which are summarized in
Table 2. Di Celma (2001) provides a more comprehensive
description of the sedimentary facies and sequence
stratigraphy of the CnFupp and TF.
Due to the slow rates of uplift during sedimentation and
erosion subsequent to each sequence boundary and ravinement surface formation, the shallowing-upward part of the
depositional sequences is completely preserved only in the
last cyclothem (sequence Tb6). Tb6 is composed of a lower
ravinement surface underlying a late transgressive deepening-up shoreface succession, followed by a shallowing-up
shoreface to coastal-plain facies association incised by
broad valleys filled with estuarine sediments and containing
well-preserved, but disarticulated remains of Late Pleisto-
cene –early Holocene continental vertebrate (Cantalamessa
et al., 2001). This succession suggests that fluvial valleys,
carved during low sea levels, where flooded and filled during
the early phase of the Holocene transgression, whereas the
depositional sequence on which they are carved may reflect
the last cycle of sea-level fluctuation (substage 5e).
4.2. Interpretation of the coastal surfaces
At the beginning of the Pleistocene, a slow regional uplift
occurred as a consequence of the early phase of Carnegie
Ridge subduction. During this period, older sediments were
tilted and truncated by the thin, shallow-marine deposits
associated with CI. Roughly circular and concentric
strandlines near the village of El Aromo demonstrate that,
during the late Early Pleistocene present-day Cabo San
Lorenzo was an uplifting island separated from mainland
Ecuador by a narrow, N – S-striking shallow strait (Fig. 7)
(Pedoja et al., 2001). According to our definition of a marine
terrace and stratigraphic evidence, CI and its deposits can be
considered a marine terrace developed, according to its
relationship with late Pliocene and late Early Pleistocene
strata, during the early Pleistocene. Because this surface is
less tilted than the lower unit of the CnF and the lower unit,
in turn, is unconformably overlain by the sequences of
CnFupp that occur at lower elevations and basinward with
Table 2
Sintetic description of the principal facies associations occurring within the Canoa and Tablazo Formations
Facies association
Facies code
Description
Depositional environmenta
Depositional sequence
Systems tract
Clow
Carbonate shell
bed association
Czob
Shoreface to middle shelf
Clow1 to Clow4
TST (compound onlapbacklap shell bedb)
Siliciclastis
siltstone association
Z1
Highly diverse, loosely to densely packed mollusc shell bed. The lower
contact (ravinement surface) is erosive, the upper contact (downlap surface)
is gradual
Relatively monotonous, massive, intensely bioturbated, and poorly
fossiliferous bluish shelf siltstone
Inner to middle shelf
Clow1 to Clow4
HST/RST
Carbonate shell
bed association
Cso
Intensely burrowed, massive, medium-grained yellow sandstone with
sparse fossil remains
Parautochthonous mollusc shell bed with sandy matrix and low to moderate
faunal diversity. The lower contact is gradational, whereas the upper
surface (downlap surface) is sharp. Shell concentration increases upsection
Moderately diverse, loosely to densely packed mollusc shell bed. Shells are
dispersed in bioturbated silty sandstone. Lower and upper boundaries are
gradual and shell concentration increases and decreases upsection with
respect to an interval of maximum condensation (downlap surface)
occurring within it
Massive, intensely bioturbated, medium-grained yellow sandstone devoid
of fossil remains
Barren to poorly fossiliferous, massive, intensely bioturbated silty fine
sandstone
Barren to sparsely fossiliferous, massive, intensely bioturbated fine sandy
siltstone
Sheltered shoreface
Cupp5, Cupp6
TST (onlap shell bedb)
Inner shelf
Cupp7
TST (backlap shell bedb)
Outer part of inner shelf
Cupp5
TST, HST (compound
backlap–downlap shell bedb)
Lower shoreface
Cupp5, Cupp7
TST
Inner shelf
Cupp5
TST, HST
Inner part of middle shelf
Cupp6, Cupp7
TST, HST
Thin autocththonous or parautochthnous shell beds characterized by very
low diversity and prevailing epifaunal and semi-infaunal specimens. The
lower contact is sharp and erosive (ravinement surface)
Well-sorted, trough cross-bedded shell bed dominated by reworked and
poorly preserved fossil material and pebble size clasts. Its lower boundary is
sharp and erosive (ravinement surface) whereas the upper boundary is
gradual
Thin mollusc shell beds characterized by low diversity, mainly epifaunal,
mollusc species. The lower contact is sharp (local flooding surface) or
gradual (burrowed), whereas the upper (downlap surface) is gradational
Intensely bioturbated, Thalassinoides burrowed fine sandstone with
scattered, mainly in situ, mollusc remains
Well-sorted planar or trough cross-bedded sandstone with sporadic soft
sediment deformation features
Dark, massive mudstone interbedded with minor lenticular sand strata
Shoreface
Tb1, Tb5
TST (onlap shell bed)
Exposed upper shoreface
Tb2, Tb3,
Tb4, Tb6
TST (onlap shell bed)
Inner shelf
Tb2, Tb3, Tb4
TST (backlap shell bed)
Lower shoreface –
offshore transition
Upper shoreface
Tb2, Tb3, Tb4,
Tb5, Tb6
Tb1, Tb6
TST, HST
TST
Estuary-lagoon
Tb2
TST
Estuary-lagoon
Estuary-lagoon
Tb2
Tb6
TST
HST
Cupp
Csb
Csbd
Siliciclastis
sandstone association
S1
Sz2
Tb
Siliciclastis
siltstone association
Zs1
Carbonate shell
bed association
Ts1
Ts2
Ts3
Siliciclastic sandstone
association
S1
S2
Siliciclastic mudstone
association
Ms1
M2
M3
a
Intertidal/shoreface (0–10 m); inner shelf (10–50 m); mid-shelf (50–100 m); outer shelf (100–200 m).
Shell bed classification introduced by Kidwell (1991).
641
b
Massive black mudstone rich in vegetal remains
Massive silty mudstone with large concentrations of continental vertebrate
remains and molluscan shells
G. Cantalamessa, C. Di Celma / Journal of South American Earth Sciences 16 (2004) 633–648
Unit
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G. Cantalamessa, C. Di Celma / Journal of South American Earth Sciences 16 (2004) 633–648
Fig. 7. Palinspastic reconstruction of the middle Pliocene to present-day Cabo San Lorenzo area. Stratigraphic data suggest that development of late Pliocene–
Pleistocene succession in the southern part of Cabo San Lorenzo took place in two stages: (a) during the middle Pliocene, the coastline was located to the west
of the present one, and (b) the late Pliocene collision of the Carnegie Ridge with the Ecuador trench caused extension and marine ingression. A relatively
shallow, rapidly subsiding basin formed (Canoa basin). The late Pliocene basin fill (lower Canoa Formation), exposed between Punta Canoa and El Mangle,
comprises four shelf depositional sequences build up several kilometers seaward with respect to the contemporaneous highstand shorelines, which were
probably located near the coastal range (Di Celma et al., 2002). (c) At the beginning of the Early Pleistocene, basin inversion occurred. Uplift tilted the late
Pliocene fill of the Canoa basin and caused the intervening unconformity. The San Lorenzo Island emerged (CI), and several circular paleo-shorelines were
created. (d) As uplift proceeded, the San Lorenzo Island connected to the mainland and the Manta basin was separated by the Canoa basin. Offshore, Isla de la
Plata emerged. (e) In the context of overall tectonic uplift, within the Canoa basin successive glacioeustatic fluctuations generated poorly preserved, southwestmigrating coastlines and an attached, forced regressive set composed of shallow-marine sequences of the upper Canoa and Tablazo Formations (see Fig. 8).
respect to CI and its sediments, it must be younger than
CnFlow and older than CnFupp. Thus, it is not part of the early
Pliocene planation surface, as proposed by Coltorti and
Ollier (1999, Fig. 2; 2000, Fig. 2). As Pleistocene uplift
proceeded, an E –W-trending isthmus connected the island
to the mainland, which created two relatively sheltered
embayments: the Manta basin to the north and the Canoa
basin to the south (Fig. 7). During Middle –Late Pleistocene
G. Cantalamessa, C. Di Celma / Journal of South American Earth Sciences 16 (2004) 633–648
interglacial highstands, the Canoa basin was a small,
southwesterly facing embayment progressively filled by a
120-m thick, largely undisturbed and exceptionally wellexposed succession of cyclically stacked shallow-marine
strata. Other than the sediments of CI, the early Pleistocene
represents an extensive sedimentary gap in this succession,
marked by a prominent angular unconformity overlain, as
noted, by the late Early Pleistocene unit of the CnFupp.
Middle Pleistocene synsedimentary upwarping of the
northern margin produced a very shallow, dipping, monocline structure that resulted from the progressive southwesterly migration of the basin depocenter and formed a
series of offlapping wedges (high-frequency sequences of
CnFupp and TF) separated by gentle but discernible
discordances. The overall south to north thinning of the
Pleistocene succession and its southward tilt indicate greater
crustal movements in the northern part of the region, with
uplift presumably being more pronounced than in the south.
Due to the continuous, spatially nonuniform tectonic uplift
of the Cabo San Lorenzo area, in successive depositional
sequences maximum-flooding shorelines occurred at progressively lower elevation and in more southwestward
position, which left poorly preserved paleo-coastlines in CII
(Figs. 6b and 8). Along with changes in the facies motif of
successive sequences, which record a systematic reduction
643
in the fine-grained sediments near the midsequence position
in concert with the reduced distance from the maximum
flooding shoreline, this transition also apparently results
from the taphonomic, sedimentologic, and paleoecologic
characters of midsequence condensed shell beds (Di Celma,
2001). The consequent offlapping stacking pattern exhibited
by the depositional sequences along the uplifted northern
margin of the basin and the vertical connection of
successively younger depositional sequences led to the
formation of an attached, forced regressive sequence set,
which suggests deposition during long-term, tectonically
enhanced, punctuated relative sea-level fall. Northward, the
accommodation available during deposition was progressively reduced by the continued uplift in this direction, and
sequence boundaries converged to generate a composite,
southward-dipping surface (CII in Fig. 6b, envelope A in
Fig. 9a). Because of its composite nature, this surface and its
deposits cannot be defined a marine terrace.
The CIII surface and associated deposits (sequence Tb6;
Fig. 10c and d) represent, respectively, the landform and the
depositional sequence related to the last glacioeustatic
variation of sea level (isotopic substage 5e). Therefore, it is
a marine terrace. The uplift rate calculated for this
depositional sequence at the highest elevation of its
shoreline angle (south of El Mangle) is approximately
Fig. 8. Schematic diagram illustrating the middle–late Pleistocene formation of the forced regressive sequence set exposed along the coast south of Cabo San
Lorenzo and of successive shorelines poorly preserved inland of the studied section. CII is a composite surface resulting from the repeated terrace reoccupation
during successive sea-level oscillations and therefore cannot be considered a marine terrace.
644
G. Cantalamessa, C. Di Celma / Journal of South American Earth Sciences 16 (2004) 633–648
Fig. 9. Genetic model proposed for progressive and syntectonic unconformities modified after Riba (1976) and Anadón et al. (1986). (a) An attached forced
regressive sequence set develops as a consequence of a long-term punctuated sea-level fall. The progressive uplift and rotation of the depositional surface, in
the long term, destroys the available accommodation space in the direction of maximum uplift. (b) A transgressive sequence set records the landward increase
of the accommodation space. (c) A composite syntectonic progressive unconformity and SAU occurs as combination of an attached, forced regressive sequence
set and a transgressive sequence set. In contrast to Riba’s (1976) and Anadon et al.’s (1986) models, which indicate that increase and decrease of uplift rates is
the driving mechanism for the development of angular and composite progressive unconformities, we find that a rotative on- and offlap can be developed in the
presence of constant uplift when the rate of the accommodation space available (eustasy þ uplift) decrease and increase, respectively.
Fig. 10. Outcrop photographs of the Plio–Pleistocene sedimentary succession cropping out along the sea-cliff south of Cabo San Lorenzo. (a) Erosional
unconformity surface separating the late Pliocene unit of the Canoa Formation (CnFlow) from its late Early Pleistocene unit (CnFupp) (north of El Mangle). (b)
Southward view of the Tablazo Formation depositional sequences associated with the CII landform at El Mangle. The boundary between the Canoa Formation
and the first depositional sequence of the Tablazo Formation is highlighted by the white line. (c) Syntectonic angular unconformity (SAU) between the late
Early Pleistocene unit of the Canoa Formation (CnFupp) and the lower part of the last depositional sequence (Late Pleistocene) of the Tablazo Formation (TF).
CII is approximately 125 m high. The CIII surface is 35 m high; because of recent erosion, the associated depositional sequence is only partially preserved. (d)
The same SAU about 5 km south of El Mangle. The last depositional sequence is complete in the transgressive and highstand systems tracts and overlies middle
Pleistocene depositional sequences. The sea cliff is approximately 25 m high.
G. Cantalamessa, C. Di Celma / Journal of South American Earth Sciences 16 (2004) 633–648
0.1 m/ka and decreases southward to the mouth of Rio de
Caña, where the coastal area has been stable or undergone
net subsidence during the last 124 ka. This cyclothem lies
unconformably on the underlying CnFupp and TF and,
northward, onlaps them. Therefore, during the last sea-level
variation, the accommodation space available increased in
this direction, and for the first in these two Pleistocene units,
a depositional sequence exhibits an onlapping rather then
offlapping stacking pattern.
The juxtaposition of sequences arranged in an offlapping
stacking pattern (rotative offlap) with those arranged in an
onlapping stacking pattern (rotative onlap) generated a
syntectonic angular unconformity (SAU) that reflects the
association of angular and composite progressive unconformities (Fig. 9).
Riba (1976) and Anadón et al. (1986) developed an
evolutionary model and introduced a new terminology for
this type of unconformities. According to these authors, a
cumulative wedge system results from uplift processes, the
tilting of one side of a depositional surface, and the
subsidence of the other around a rotation axis. It may
develop as a rotative offlap (Fig. 9a) or a rotative onlap
(Fig. 9b) and record increase or decrease in the uplift rate,
respectively. The combination of a rotative offlap and a
rotative onlap generates a composite syntectonic progressive unconformity in the depocenter and an SAU along the
basin margin (Fig. 9c).
The same arrangement of the depositional sequences can
be explained even in terms of the change in the
accommodation space available, which, in turn, strictly
depends on the rates of glacioeustatic sea-level change and
crustal deformation. In this case, an attached, forced
regressive sequence set (rotative offlap) results from the
progressive decrease of the accommodation space in one
direction, whereas the transgressive sequence set (rotative
onlap) records its increase in the same direction. The
depositional sequence associated with CIII (sequence Tb6)
lies on the SAU and represents the only wedge of the
rotative onlap system directly observable in outcrop.
5. Discussion and conclusions
As pointed out by both experimental results and
observations of real situations (Lallemand et al., 1992),
the initial phase of marked subsidence connected with the
subduction of an aseismic ridge often is ascribed to local
tectonic erosion (von Huene and Lallemand, 1990),
followed by an uplift phase once ridge subduction begins.
In the study area, the encroachment and collision of the
aseismic Carnegie Ridge with the Ecuadorian margin
probably resulted in basal tectonic erosion beneath the
upper plate and favored the settling of the trench-normal
extension and extremely rapid subsidence during the late
Pliocene along the Ecuadorian coast (Aalto and Miller,
1999; Di Celma et al., 2002). After the late Pliocene,
645
subsidence progressively decreased as the aseismic Carnegie Ridge subducted beneath the NAB, favored the
subsequent regional uplift that produced the southward
tilting of the CnFlow and allowed the formation of CI.
According to Flint et al. (1991), ridge subduction can
produce forearc uplift –collapse processes that, along the
continental margin, can result in relative sea-level
changes at a frequency equal to that of glacioeustatic
processes. Because these cycles are tectonically driven,
they are not globally extensive or synchronous. By
comparing the marine terraces of Isla de la Plata with
marine oxygen-isotope curves, we can discriminate
between eustatic and tectonic controls on the evolution
of marine terraces. Because Pleistocene eccentricy-driven
cycles (100 ka), which are characterized by a distinct
asymmetry between the rising (average ca. 10 m/ka) and
falling (average ca. 1.5 m/ka) limbs (Gawthorpe et al.,
1994), had rates of sea-level changes always higher than
the rate of uplift at Isla de la Plata and marine terraces
appear to fit well with the oxygen-isotope curve, we
conclude that glacioeustasy may have controlled the
development of the flight of step-like marine platforms
on the island and the partially coeval high-frequency
cycles in the CnFupp TF. Unraveling the different genesis
of Isla de la Plata and coastal landforms has important
implications for the reconstruction of recent vertical
motions and the paleogeographic evolution of this part of
Ecuador.
The flight of four Middle – Late Pleistocene marine
terraces on Isla de la Plata was produced by interaction
of tectonic uplift on the order of 0.4 m/ka and
glacioeustatic variations in sea-level during oxygenisotope stages 13 –5e and testifies to the uplift of this
area at least 500 ka BP. Bull (1984) notes that the
number of terraces depends on the uplift rate and that
rates of 1 –2 m/ka are needed before a full sequence can
be preserved. At lower uplift rates, terraces formed
during substage sea-level peaks will be swamped by
subsequent, higher sea levels. The averaged uplift rate
proposed for Isla de la Plata isolated a previously formed
abrasion platform from the rising sea level during a
subsequent highstand and prevent a terrace reoccupation
phenomenon, common in low uplift rate conditions (Hsu
et al., 1989; Leonard and Wehmiller, 1992; Ortlieb et al.,
1996). However, the proposed uplift rate is insufficient to
record Middle and Late Pleistocene sea-level fluctuations
with the same degree of detail as can be found in areas
with higher uplift rates (Pillans, 1983; Bishop, 1991;
Suggate, 1992).
From the late Early Pleistocene, southward-decreasing
syndepositional uplift rates associated with a series of
successive marine incursions occurred along the coastline,
which allowed the superimposition of successively
younger depositional sequences through repeated marine
reoccupation and the development of a relatively thick
cyclothemic sedimentary succession laid down during
646
G. Cantalamessa, C. Di Celma / Journal of South American Earth Sciences 16 (2004) 633–648
global glacioeustatic sea-level changes. Depositional
sequences arranged as rotative off- and onlaps are
indicated by the SAU that generated the CII and CIII
landforms. The former represents the innermost erosional
remnant of the composite surface that resulted from the
convergence of sequence boundaries (envelope A of
Fig. 10a) whereas the latter is the landform associated
with the marine terrace of the last interglacial and the
only one directly related to one of those present on Isla de
la Plata (Plata-4).
Therefore, local factors such as the magnitude of
uplift rates play an important role in controlling
variability in the depositional response to long-term
forced regression. In addition, different oxygen-isotope
stages are preserved at different elevations as distinct
surfaces and associated deposits only where long-term
uplift rates are high enough to isolate a terrace developed
during a particular high sea interglaciation. Inversely,
composite sedimentary successions originating from
many sea-level oscillations are laid down under conditions of slow net uplift. Therefore, the magnitude of
tectonic uplift, more than other local factors, controls
environmental responses to long-term sea-level fall and
determines the development of an attached or detached
sequence set. Whereas McMurray and Gawthorpe (2000)
argue extensively for the important role played by local
factors, such as sediment supply and physiography, on
the development of detached, forced regressive sequence
sets (marine terraces) and attached, forced regressive
sequence sets (cyclothemic sedimentary succession)
during long-term, tectonically driven forced regression,
they disregarded and underemphasized the role of the
intensity of tectonic uplift. Because the amount of
vertical separation needed to isolate successively younger
depositional sequences depends mainly on this local
factor, it represents a fundamental control on whether
marine terraces or cyclothemic sedimentary successions
are developed.
Acknowledgements
This work was funded by Ministero dell’Università e
della Ricerca Scientifica e Tecnologica (MURST)
1998/2000
and
2000/2002
grants
held
by
G. Cantalamessa (Università degli Studi di Camerino)
and is based in part on the second author’s doctoral
thesis, undertaken at Università degli Studi di Pisa
(1997 – 2001). The authors thank Professor Mauro
Coltorti, Francesco Dramis, and Paolo Roberto Federici
for critically reading the manuscript and useful suggestions. The paper also has benefited from comments and
suggestions by journal reviewers Adrian Hartely and
Arthur L. Bloom.
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