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 634 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 636 G. Cantalamessa, C. Di Celma / Journal of South American Earth Sciences 16 (2004) 633–648 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. 638 G. Cantalamessa, C. Di Celma / Journal of South American Earth Sciences 16 (2004) 633–648 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 640 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 642 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. References Aalto, K.R., Miller, W. III, 1999. Sedimentology of the Pliocene Upper Onzole Formation, an inner-trench slope succession in northwestern Ecuador. Journal of South American Earth Sciences 12, 69–85. Anadón, P., Cabrera, L., Colombo, F., Marzo, M., Riba, O., 1986. Syntectonic alluvial unconformities in alluvial fan deposits, eastern Ebro Basin margins (NE Spain). 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