Fan beheading and drainage d ,nn khk iversion as evidence of a 3200-2800 BP earthquake event in the...

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Geomorphology 74 (

Fan beheading and drainage diversion as evidence of a 3200-2800

BP earthquake event in the Esmeraldas-Tumaco seismic zone:

A case study for the effects of great subduction earthquakes

J.F. Dumont a,*, E. Santana b, F. Valdez c, J.P. Tihay d, P. Usselmann e,

D. Iturralde f, E. Navarette g

a IRD, UMR 6526, Observatoire Oceanographique, 06235 Villefranche sur mer, Franceb INOCAR, Base Naval Sur, Av. de la Marina, Guayaquil, Ecuador

c IRD, UR 092, Instituto Nacional de Patrimonio Cultural , Quito, Ecuadord UPPA, Pau, France

e CNRS-UMR 6012, Maison de la geographie, 34090 Montpellier, Francef Universidad de Guayaquil, Guayaquil, Ecuador

g ESPOL, Campus La Prosperina, Guayaquil, Ecuador

Received 7 October 2004; received in revised form 25 June 2005; accepted 27 July 2005

Available online 22 September 2005

Abstract

The San Lorenzo area belongs to the Esmeraldas–Tumaco seismic zone where some of the strongest earthquakes of South

America occurred during the 20th century. This paper provides evidence for a succession of geomorphic changes characterized

by the disruption of the Quaternary drainage network and the reshaping of the Cayapas–Santiago estuary. The rise of the La

Boca uplift bordered by the La Boca and San Lorenzo faults is responsible for the southward diversion of the Palabi, Tululbi,

Bogota and Carolina rivers toward the Santiago and Cayapas rivers. The increase of the discharge directed to the Cayapas River

generated the change of the channel pattern downstream from the confluence, and the avulsion of a new estuary through the

coastal plain. According to the dating of beach ridges the avulsion occurred in the period 3200–2800 BP. This period

corresponds also to a faster accretion of the beach ridge margin, interpreted as a response to a small uplift of the shore. The

coherency of the three morphologic evidences—diversion of drainage network, avulsion and increase of coastal accretion—

suggest a unique morphotectonic event, in relation with the activity of the Esmaraldas–Tumaco seismic zone. The opening of a

direct communication through the mangrove margin may have brought favorable conditions for the development of the La

Tolita archaeological site after 3000 BP.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Active tectonics; Drainage diversion; Drainage pattern; Beach ridges; Ecuador

0169-555X/$ - s

doi:10.1016/j.ge

* Correspondin

E-mail addre

2006) 100–123

ee front matter D 2005 Elsevier B.V. All rights reserved.

omorph.2005.07.011

g author.

ss: [email protected] (J.F. Dumont).

J.F. Dumont et al. / Geomorphology 74 (2006) 100–123 101

1. Introduction

Active tectonics of coastal areas involves various

types of natural hazard, earthquake, tsunamis and

modification of coastal morphology. Paleoseismology

is based on several kinds of evidences, among them

the deformation of land surface due to fault motion

(McCalpin, 1996; Morner, 2003). Active tectonics

involves a wider approach, in order to detect active

deformation and document possible paleo-earthquake

events (Keller and Pinter, 2002). The North Andean

coast has produced four great earthquakes with mag-

nitude Mw N7.7 during the last century (Fig. 1A)

(Beck et al., 1998; Gutscher et al., 1999; Herd et al.,

1942 (Mw 7.8)

1958 (Mw 7.7)

1979 (Mw 8.2)

1906 (Mw 8.8)

Esmeraldas-Tumaco seismic zone

CarnegieRidge

Sub

duct

ion

trenc

h

Gulf ofGuayaquil

- 100

0m

-2000 m

Coa

stal

Nazca Plate

0°

80°W

Plateconvergence

+

+

1942

1942

190

1

NazcaPlate

CarribeanPlate 10°N

0°

GUAYAQUIL

Gua

yaqu

il- C

arac

as

Meg

ashe

ar

70°W80°W

NorthAndean

Block

CARACAS

Tala

raA

rc South AmericanPlate

Fig.1A

*

BFig. 1. Geodynamic sketch of the Ecuadorian active margin. Rupture zones

from Collot et al., 2004.

1981), the largest being the 1906 Tumaco earthquake

(Mw 8.8), associated with a rupture zone of 500 km

(Collot et al., 2004; Herd et al., 1981). Co-seismic

deformations, either uplift (Barrientos and Plafker,

1992; Ortlieb et al., 1996), or subsidence (Gonzalez

et al., 2002; Herd et al., 1981), are reported in asso-

ciation with subduction earthquakes along the Pacific

coast of South America. These co-seismic and inter-

seismic deformations depend on the distance to the

seismic zone (Barrientos, 1996; Barrientos and Plaf-

ker, 1992; Carver and McCalpin, 1996; Fitch and

Scholz, 1971). A null elevation change axis frequently

located near the coastline separates areas of alternat-

ing uplift and subsidence during seismic and inter-

And

ean

Cor

dille

ra

Esmeraldas

Tumaco

Cor

dille

raFig. 2

100 km

Guaya

quil

-

Carac

asM

egas

hera

Plateconvergence

Quito

Carchi

Colombia

Ecuador

1979

1958

1958

6

906

Guayaquil

Borbó

nBas

in*

A

and magnitudes of the 1906, 1942, 1958 and 1979 earthquakes come

J.F. Dumont et al. / Geomorphology 74 (2006) 100–123102

seismic deformations (Barrientos, 1996; Fitch and

Scholz, 1971). Up to 8 m uplift and 6 m subsidence

have been observed in relation to the 1946 magnitude

8.2 Nankaido earthquake in Japan, and surface defor-

mation has been continuing 3 years after the earth-

quake (Fitch and Scholz, 1971). Herd et al. (1981)

report co-seismic subsidence of up to 1.6 m in relation

to the 1979 magnitude 7.8 Tumaco earthquake.

This study supports the hypothesis that co-seismic

deformation over a subduction zone may also produce

change of the drainage network and/or adjustment of

the drainage pattern that can be useful for paleoseis-

mology of extreme events. In flat areas and wet

climate, light vertical motion of the surface morphol-

ogy can generate striking changes of the drainage

pattern. This approach has been previously used to

study active surface deformation in wetland and allu-

vial plains of the Amazonian foreland basins

(Dumont, 1991, 1993; Dumont and Fournier, 1994;

Schumm et al., 2000). There, low range vertical defor-

mations resulted in river diversion and channel pattern

change along small rivers, and avulsion of the main

streams. Similar climate and drainage conditions exist

in the Esmeraldas–Tumaco coastal margin of North-

ern Ecuador and Colombia.

2. Geological background

The Nazca and South American plates converge

along the South American active margin at a rate of 7–

8 cm/yr with a nearly W–E trend (De Mets et al.,

1990; Kellogg and Vega, 1995). From the Gulf of

Guayaquil (south) to the Ecuador–Colombian border

(north) the obliquity of the subduction zone with the

plate motion increases up to 30–50 degrees (Fig. 1A)

(Ego et al., 1996) . This oblique subduction favors the

northeastward escape of the North Andean block at a

rate of about 1 cm/yr along the Guayaquil–Caracas

megashear (Dumont et al., 2005; Ego et al., 1996)

(Fig. 1B). The northward motion of the Andean Block

goes along with a roughly N–S trending extension in

the Ecuadorian coastal margin, involving NE–SW

dextral and NW–SE sinestral transtension faults

(Alvarado, 1998; Deniaud, 2000; Dumont et al.,

1997, 2005; Santana and Dumont, 2002).

The San Lorenzo area is located landward of the

Esmeraldas–Tumaco seismic zone (Fig. 1A) (Collot

et al., 2002), defined in reference to the great 1906

earthquake, one of the strongest earthquake regis-

tered on earth (Mw 8.8). The rupture zone of the

1906 earthquake has been reactivated by 3 strong

earthquakes in 1942 (Mw 7.9), 1958 (Mw 7.8) and

1979 (Mw 7.9) (Collot et al., 2004; Herd et al.,

1981; Kelleher, 1972; Swenson and Beck, 1996),

and strong earthquakes also probably occurred in

the same area in 1868 and 1875 (Scheu, 1911).

The 1979 earthquake has been accompanied by a

coastal subsidence of up to 1.6 m, a ground failure

extending up to 60 km inland, and co-seismic uplift

is reported offshore, on the edge of the continental

shelf (Herd et al., 1981).

The coastal margin of central Ecuador is character-

ized by a steady uplift during the Quaternary giving

flight of marine terraces up to an elevation of 300 m

(Pedoja, 2003). This uplift is related to the subduction

of the Carnegie Ridge (Gutscher et al., 1999; Pedoja

et al., 2001). The segment of uplifting coast stops to

the north on the NW–SE trending Yanayaca fault (Fig.

2) (CERESIS, 1985; Pedoja, 2003). From Las Penas

to the Manglar Cap the Bay of Ancon de Sardinas

(Fig. 2) constitutes a wide delta formed between the

outlet of the Cayapas–Santiago and Mataje rivers,

located on the southern and northern borders of the

delta, respectively. The delta area constitutes a 15-km

wide margin of mangrove, beach ridges and tide

channels, bounded sharply inland by a continuous

10–15 m cliff. This cliff also represents the limit of

the mid Holocene post-glacial transgression (Tihay

and Usselmann, 1995).

The Pleistocene deposits of the San Lorenzo area

are represented by the Cachabi (also called Canoa)

and San Tadeo formations (Fig. 3) (Baldock, 1982;

CODIGEM, 1993). The regional extension of these

formations describes a large fan along the lower

slopes of the Andean Cordillera, down to the coast.

The uppermost San Tadeo Formation crops out along

the northern slopes of the fan. It includes pyroclastics,

volcanic conglomerate, laharic and mudflow material,

forming sheets or fluvial terraces entrenched in the

Cachabi Formation (Baldock, 1982). The Cachabi

Formation represents a continental fan made of

sand, mud, tuff, gravel and conglomerate. The mate-

rial is deeply weathered in reddish-yellow clay, with

only ghosts of siliceous pebbles showing the original

facies.

Fig. 2. Geologic sketch of the area of San Lorenzo, localised on Fig. 1. Numbers 1, 2 and 3 represent the different drainage areas described in the

text.


The basement of the Cachabi and San Tadeo for-

mations belongs to the Borbon Formation of the

Borbon Basin, that was active since the Middle

Eocene to the Upper Pliocene (Aalto and Miller,

1999; Deniaud, 2000; Evans and Whittaker, 1982).

The depocenter of the north Borbon Basin is located

below the Ancon de Sardinas Bay, near the estuary of

the Cayapas–Santiago river and the city of San Lor-

Fig. 3. Synthetic stratigraphic section showing the relative position of the different units. No scale.


enzo (Deniaud, 2000). The location of the delta on a

structural low underlines the long term structural his-

tory of this area, as it is generally observed for the

lower valleys and estuaries of large rivers (Potter,

1978). Folding and emergence of the south Borbon

Basin post-dates the late Pliocene (Evans and Whit-

taker, 1982).

3. Methods

We apply a three-step method comprising: 1–the

global analysis of the drainage network, 2–the

detailed analysis of drainage re-routing, apparent

channel pattern anomalies, and avulsion, and 3–the

search for complementary elements, analyzing the

coastal plain and the other evidences of active fault

tectonics from the same area.

The analysis of the regional drainage network con-

stitutes customarily the first step of the search for

evidences of active tectonics (see Schumm et al.,

2000; Keller and Pinter, 2002 for a review on the

question). The first step considers the main trend of

the present drainage network from the Western

Andean Cordillera to the coast on 1 :250,000e topo-

graphic maps (IGM, 1991, 1992a,b), 1 :1000,000e

geologic (CODIGEM, 1993) and 1 :200,000e geomor-

phologic map (PRONAREG-ORSTOM, 1984a,b,c).

This overview gives evidence of a discontinuous net-

work suggesting diversion and re-routing apart from a

previous straight down slope drainage network (Figs.

2 and 3). A drainage network flowing against or very

obliquely to the regional slope may be suspected of

tectonic control (Booth-Rea et al., in press; Jackson et

al., 1996; Keller and Pinter, 2002; Schumm et al.,

2000) as it is observed in some parts of these maps.

However, Bishop (1995) pointed out that most of

drainage diversion–or beheading–and re-routing may

be explained as simple capture, and each case must be

analyzed carefully before stating active tectonics. Pas-

sive drainage capture is generally made by a more

incised or active drainage segment reaching a less

active one upstream. But active tectonics (fault, flex-

ure) can open drainage by-pass toward less active

drainage segments (Bishop, 1995; Bowler and Har-

ford, 1966; Dumont et al., 2005; Jackson et al., 1996).

Comparison with examples previously described in

the literature is important, and the river re-routing in

La Boca described in this paper is comparable to the

classical example of a river blocked and re-directed

against a new or reactivated fault, as described by

Bowler and Harford (1966).


The second step comprises the detailed analysis of

some characteristic points of the drainage diversions

that disrupt the original fan network, at the head of the

fan, in the middle part, and finally in the coastal plain.

Channel slope, channel trend change, as well as dry

valley, are important elements to analyze and interpret

the reshaping of the drainage network. Downstream

the drainage network enters the delta area, showing

combination of anastomosed and meandering pat-

terns. Different channel patterns with the same slope

imply changing hydrologic characteristics such as

discharge and sediment transport (Schumm et al.,

2000), that should be a key for the understanding of

drainage pattern in the coastal plain.

The third step considers complementary elements

that belong to the geographic and structural context.

In the coastal plain we analyze the space–time accre-

tion of the beach ridges in order to date the avulsion

and the opening of the present Cayapas–Santiago

estuary. Also space–time diagram of the beach ridge

accretion will specify the coastal plain evolution.

Finally the evidences of active fault motion are ana-

lyzed in the context of the morphological changes

observed.

4. The drainage network

Between the Carchi area in the Western Andean

Cordillera and the Bay of Ancon de Sardinas (Fig. 2)

the drainage network presents three parts (IGM,

1992a,b). The area 1 (Fig. 2) extends from the Carchi

area (about 2000 m) to Alto Tambo (500 m), and is

drained along by the Mira River, one of the main

rivers of northern Andes. The Mira River valley is

entrenched about 1000 m in the upper slopes of the

Cordillera, and about 200–300 m in the Alto Tambo

area. At Alto Tambo the trend of the Mira River turns

to the north, joining the catchment area of the San

Juan River, and reaching the coast in the Manglar Cap

in Colombia (Fig. 2).

Area 2 (Fig. 2) extends over 40 km downslope

from the Alto Tambo area (500–700 m) to a NE–SW

trending line joining Tululbi to Concepcion (20–50

m). The drainage network of area 2 comprises from

north to south the Mataje, Palabi, Tululbi, Bogota and

Carolina rivers. The trend of the three last rivers

converge upstream in Alto Tambo, close to the Mira

River, and the first two converge at 10 km to the north

(Figs. 2 and 6). This upstream convergence of the

drainage describes a fan network called the El Placer

fan (Winckell and Zebrowski, 1997). In the down-

stream part of area 2 (Fig. 2) the drainage network of

the El Placer fan is sharply re-routed to the SW, the

Palabi, Tululbi, Bogota and Carolina rivers joining

together the Santiago River. Along the NE margin

of the El Placer fan the Mataje River flows straight

down slope, reaching the coast along the structural

border of the Bay of Ancon de Sardinas.

Area 3 (Fig. 2) covers a 30-km wide lowland

margin (0–50 m), including the San Lorenzo upland

and the coastal plain. The San Lorenzo upland is

characterized by a poor drainage network in the

central part, with the Cayapas–Santiago and Mataje

rivers located at the southern and northern borders of

the area, respectively. The 15-km wide and 50-km

long coastal plain includes mangrove, tidal channels,

salted marshes and beach ridges, and the estuaries of

the Cayapas–Santiago and Mataje rivers. A striped

pattern of beach ridges is observed in the southern

part, disappearing progressively to the north below

mangrove and tidal channels. The Cayapas and San-

tiago rivers drain the Western Andean Cordillera

south of the Mira River catchment area. These rivers

join together before to cross the delta through the

common Cayapas–Santiago estuary. A total of four

large tide channels cross the coastal plain, from

south to north: the Cayapas–Santiago, Boca de

Limones, Bolivar and Mataje channels (Fig. 4). But

only the Cayapas–Santiago and Mataje channels

located, respectively, in the SW and the NE are the

outlets of important rivers. The other ones constitute

the outlet of very small rivers without relation with

the size of the channel (Fig. 4). Near San Lorenzo

the Bolivar channel is connected landward with the

Estero El Salto (Fig. 4), characterized by a typical ria

morphology carved in the San Lorenzo upland, and

interpreted, as it is generally the case for such pat-

tern, either for eustatic or tectonic effect, or both,

suggesting a drowned Holocene valley.

4.1. Reconstruction of the Pleistocene fan drainage

The different segments of the present drainage net-

work are reconstructed on Fig. 5, describing a large fan

with the Mira River as the fan feeder at the head, the El

X3

X4

BA

E

D

C

F

G

I

J

K

MN

P

O

Q

Z

Z'

+

A

B1

B2

Fault

Upland below 20 mUpper Cayapas terrace

Mangrove

Upland over 20 mPleistocene fan

Estimated 2800-3200 BPbeach ridge line

Beach ridges

Limit of recentbeach ridges

pattern

Floodplain area

+ Bank of dead oyster

Nm

0 5 10 15

12 345

67

t,s

A: 658-469 BP *B: 771-539 BP *C: 3471-2983 BP *D: 3251-2854 BP *E: 3260-2830 BP *F: 1707-1299 BP *

G: 2781-2340 BP *H: 460-0 BP +I: 5935-5581 BP *J: 5938-5600 BP *K: 4132-3686 BP *L: 4230-3806 BP *

M: 3200-2780 BP +N: 5050-4860 BP +O: 5450-5130 BP +P: 6880-6660 BP +Q: 3840-3580 BP +

* from Tihay and Usselmann (1995) + This study

Ages from the beach ridges of the coastal plain

H

L

SanLorenzo

Ancón

Valdez

0 5km

Fig. 12

Fig. 11

Fig. 9

Boca de

Limones channelCayapas-Santiago

estuary

EsteroEl Salto

La BocaUplift

COLOMBIA

ECUADOR

Bay of Ancónde Sardinas

La Tolita

Las Peñas

Bolivar

channel

Y2

Y1

X2

X1

B1

ES

LFWS

LF

SLF

N

12

3

4

5

6

7

3σ

2σ 1σ

N

1 3

2

σ σ

σ

Rio Mataje

Mataje Fault



Placer fan as the main part, and the large tidal channels

of the coastal plain as former estuaries. This drainage

network reconstruction is superimposed over the Pleis-

tocene continental deposits of the Cachabi and San

Tadeo formations (CODIGEM, 1993; PRONAREG-

ORSTOM, 1984a,b,c; Winckell and Zebrowski,

1997), and constitutes in our interpretation the last

evidence of the activity of this fan.

The geographic and geologic setting shows that the

El Placer fan is structurally enclosed in a system of

NW–SE trending faults (Figs. 2 and 5). The south-

western border presents a step-like fault arrangement

from Las Penas to Alto Tambo, and the northeastern

border is continuously bounded by the Mataje fault

(Fig. 2). The Mataje fault separates the hilly upland on

the Colombian side from drowned beach ridges and

mangrove on the Ecuador side, evidencing the activity

of the fault during the Holocene. Another NW–SE

trending fault controls the position of the Mira River

upstream from the head of the fan (CODIGEM,

1993).

The present segmentation of the drainage network

suggests the disruption of a large fan network in three

parts: at the head of the fan in Alto Tambo (between

areas 1 and 2, Figs. 2 and 5) and in the middle part

(between areas 2 and 3, Figs. 2 and 5). The Mira River

(the fan feeder) has been re-routed to the north, join-

ing presently the catchment area of the San Juan River

in Colombia. Between areas 2 and 3 the drainage of

the El Placer fan is collected and re-routed to the SW

toward the Santiago River (Figs. 2 and 5). The present

study analyses with more detail the morphology, drai-

nage diversion that occurred at the lower part of the

fan, in the areas 2 and 3.

4.2 Diversion of the Mira River in Alto Tambo

The diversion of the Mira River at the head of the

fan is analyzed on maps (CODIGEM, 1993; IGM,

1992a,b), because local conditions (access facility and

Fig. 4. Morphostructural sketch of the Cayapas–Santiago deltaic area. A: L

the figure. B1: Stereonet of the fault plane data from the San Lorenzo Fau

Wulf net. Letters j1, j2 and j3, respectively, represent the maximum, int

(1979). Arrows represent slickensides. The histogram represents the dev

(azimuth-dip-pitch of the slickenside-motion): 1: N045-75N-60E-norm; 2

norm; 5: N090-80N-82W-norm; 6: N065-80W-80N-norm; 7: N080-85S-8

solutions of subduction earthquakes, for the area of central and northern E

security) did not allow field work in this area. The

northward turn of the Mira River in Alto Tambo, from

NW–SE to N–S, is considered as a diversion because

it drives the fan feeder river outside of the fan, into the

adjacent catchment area of the San Juan River. During

this process the Mira River passes over the structural

border of the fan, represented by the escarpment of the

Mataje fault. We will describe successively the trend

and slopes of the river segments involved, and the

structural fault pattern of the area before to interpret

the process of the diversion.

In Alto Tambo (Fig. 6, point A) the trend of the

Mira River changes from N558WF158 (upstream) to

North F108 (downstream), with a short intermediate

segment trending NE–SW in the turn. The trend of the

Bogota River at the head of the El Placer fan, and the

Mira River upstream from Alto Tambo (La Tigrera)

are lined up, the head of the fan standing about 200 m

higher than the bottom of the Mira River (Fig. 6, point

A). About 10 km downstream from Alto Tambo the

Mira River presents another turn to the NE. (Fig. 6,

point B). The Mira River valley upstream from the

turn is lined up with the head drainage line of the

Mataje River, also located about 200 m higher. Down-

stream from Alto Tambo the Mira River follows the

border of the El Placer fan, between the radial–locally

parallel–drainage network of the fan to the west and

the dendritic drainage network of the Andean slopes

to the east.

The slope of the Mira River valleys is about 1.40%

(1.2%–1.5%) through the Western Cordillera

upstream from Alto Tambo, and drops to 1.2%

(1.1%–1.3%) downstream. However just before the

northward turn the Mira River valley widens, forming

the La Tigrera alluvial plain with a slope lower than

0.6%. This alluvial fill implies a restraining of the

sedimentary transport just before the short NE–SW

trending segment located at the beginning of the bend

(Fig. 6, point A). A similar scheme is repeated down-

stream in point B, before the turn to the NE. At the

etters in squares refer to calibrated radiocarbon dating marked below

lt, located on part A of the figure. Lower hemisphere projection of

ermediate and minimum stress axis calculated by the Carey method

iation between the observed and calculated data. Fault plane data

: N055-85N-60N-norm; 3: N055-85-70N-norm; 4: N040-90-40N-

0W-norm. B2: State of stress obtained from the inversion of focal

cuador, according to Ego et al. (1996).

Cayapas-Santiago

estuary

Cayap

asRive

r

Mira River

San Juan River

Gualpi (Mira)River

Mataje Fault

Old

Santiago

River

San Lorenzo

Borbon

Tumaco

Alto Tambo

La Tola

Las Peñas

Ancón

Mataje

River

Bay of Ancónde Sardinas

BogotaR.

Tululbi R.

Biguaral R.

Palabi R.

Carolina R.

ManglarCap

Yanayaca Fault

0 20 km 1

2

3

Tambo Alto fan

Fan related faults

Observed drainage

Abandoneddrainage

Post fandrainage

Post-fan faults

Fig. 5. Reconstruction of the drainage network of the Pleistocene Alto Tambo fan, combining the drainage areas 1, 2 and 3 of Fig. 2.


head of the El Placer fan the slope of the drainage

lines (Tululbi, Bogota rivers) is higher, about 5%–7%

(Fig. 6, point C).

The geological map (CODIGEM, 1993) shows that

the El Placer fan is located at the intersection of two

directions of faults trending NE–SW and NW–SE,

respectively (Figs. 2 and 6). Upstream from Alto

Tambo the Mira River valley follows a NW–SE trend-

ing fault, which is parallel to the faults observed on

the borders of the El Placer fan. The NE–SW trending

faults appear on both sides of the Mira River valley,

one segment to the north and two 4-km spaced seg-

ments to the south (Fig. 6). These north and south

segments are about lined up (Fig. 6), the continuous

fault trace crossing the Mira River valley along the

short NE–SW deviation. The NE–SW trending faults

constitute the northeastward extension of the Rio

Canande fault (Alvarado, 1998; Eguez et al., 2003)

Alto TamboEl Placer

Bogotá R.

Tululbi R.

Mira

R.

Mira River

La TigreraM

atajeR

.

San

tiago

Riv

er

Can

andé

Faul

t Zon

e

Carolina R.

Cachabi R.

2000

2000

1000

1000

1000

1000

500

500

500

100

10 km

A

B

C

N

Fig. 6. Morphologic sketch of the head part of the El Placer fan. Thick lines are faults from the geological map (CODIGEM, 1993), and thick

dotted lines are extrapolated faults with high probability, and thin dotted lines subdued possible faults. Letters refer to commentary in the text.


(Fig. 6), suspected of right-lateral Quaternary motion

on the basis of observation on radar images (Alvar-

ado, 1998). The North Canande fault is defined here

as the extension of the Canande fault in the Alto

Tambo area. The NE–SW river segments north of

the Canande fault is interpreted as another segment

of the Canande fault zone.

The Mira River valley presents a right hand offset

of 1.5–2 km at the crossing point with the Canande

fault. The occurrence of the El Tigrera alluvial plain

supports the hypothesis of restraining drainage along

the fault segment, and thus the recent motion of the

fault. This allows speculation that the river offset

corresponds to the fault motion since the entrench-

ment of the Mira River, that is since no more than

approximately the middle Pleistocene. This gives a

rough estimation of a slip rate of 1–2 mm/year.

It is difficult to apply the same analysis to the other

NE–SW trending segment downstream from point B

(Fig. 6), because it is located in an area of relatively

low topography suggesting weak tectonic control and

possibly easier natural diversions and adjustment

(Dumont et al., 2005; Huang, 1993). In those cases

the dextral apparent offset can be far higher than the

tectonic offset along the fault. However the tectonic

offset along the Canande fault and the parallel fault to


the north are not enough to explain the diversion of

the Mira River outside the structural limits of the El

Placer fan. We also need to consider the N–S trend-

ing segments driving the Mira River to Colombia.

The geological map does not show N–S trending

faults in the studied area, but such faults are frequent

in the southwestern part of the Borbon Basin, along

the Esmeraldas River valley (Eguez et al., 2003), and

near the coast (Santana et al., 2001; Witt, 2001). In

particular the two more recent tectonic events

observed in the Pliocene deposits are successively

E–W and N–S extensions (Santana et al., 2001; Witt,

2001), but only the last one affects the Holocene

(Santana and Dumont, 2002) as it will be shown

later in the San Lorenzo area. We interpret here the

NS trending river segments as controlled by faults

formed or reactivated during the early E–W exten-

sion event of the Quaternary, before the Holocene.

NE–SW as well as NW–SE trending faults are able

to have been moved during this event. This exten-

sion tectonic event is able to have reduced the

activity of the fan. The result would be the opening

of new drainage lines toward the north along fault

segments linking new or pre-existing drainage lines.

The entrenchment of the Mira River along the border

structures of the fan occurred during this period. The

more recent and still active N–S extension did not

drastically change this pattern of morphostructures.

This recent event is characterized by transtension

motion, and dextral motion along the NE–SW trend-

ing faults (Santana and Dumont, 2002). It may be

hypothesized that this recent motion is more pre-

cisely responsible for the restraining flow where

the Canande fault cuts the Mira River valley at

point A, resulting in the formation of alluvial fill

like that of La Tigrera.

According to Bishop’s (1995) classification of river

diversion the process observed here involves the pre-

servation of drainage lines and transfer of drainage

areas between adjacent catchments. This is a btop–downQ process, the tectonism at the point of river

capture determining the rearrangement of the catch-

ment area downstream. According to Bishop (1995),

rejuvenation head may appear at or above the elbow

of capture, depending on the original height difference

between the bed of the two rivers involved in the

rearrangement and the change in discharge for the

stream receiving the diverted flow. In the present

case the capture of the Mira River occurs along drai-

nage lines located at the structural border of the fan

with the Andean Cordillera.

Since the diversion occurred the Mira River has

been entrenched about 200 m relative to the head of

the El Place fan. This suggests that the diversion

probably occurred before the Holocene. A rough cal-

culation of a minimum age can be made considering

the entrenchment of the Pastaza River inside its fan on

the other side of the Andes, under similar climate. The

calculation using the 5–6.7 mm/year�1 entrenchment

of the Pastaza River (Bes de Berc et al., 2005) as a

maximum rate gives a minimum of about 40,000

years for the entrenchment of the Mira River, since

the abandonment of the fan drainage network.

More probably, as suggested before, a middle

Pleistocene age can be suggested for the beginning

of the diversion at the head of the fan.

4.3 Southward deflection of fan drainage

The Palabi, Tululbi and Bogota rivers are sharply

re-routed to the southwest against the La Boca upland

in the downslope part of area 2 (Fig. 2). The topogra-

phy of the La Boca area is only 20–30 m over the

bottom of the valleys, but defines a continuous

watershed between the middle and lower parts of the

El Placer fan (areas 2 and 3, Fig. 2). We studied in

particular the turn of the Tululbi and Bogota rivers

near Carondelet (Fig. 7) because of the relatively easy

access and its representativeness as drainage diversion.

North of Carondelet the Tululbi River flows in an

opposite trend relative to the regional slope, as it has

been re-routed against the La Boca upland 5 km to the

north (Fig. 2). The Bogota River reaches the La Boca

upland near Carondelet, flowing inside a 30–40 m

deep and 0.15% sloping valley incised in the Cachabi

Formation. It merges with the Tululbi River at an

elevation of 14 m against the La Boca upland. Down-

stream from the turn, the slope is only 0.03%, with a

meandering pattern at the surface of a wide floodplain

that extends widely to the SE. The border of the La

Boca upland is 15–20 m over this floodplain. This

border is interpreted as a fault that created the mor-

phology that re-routed the Bogota River to the south

together with the Tululbi River. A key element is the

observation of a dry valley across the La Boca upland,

in continuation of the Bogota River upstream from

14 m28m

21 m

Carondelet

La BocaLa

Boca Fault

Zaspi

R.

Bogota R.

Tululbi R.

2 km

Rai

lway

. .

.

.

.

.

.

35m

48m

50m

45m

Slope 0.0

3%

Slope 0.14%

Slope0.15%

La Bocauplift

Fig. 7. Morphologic map of the Carondelet area localized on Fig. 2. See commentary in the text.


Carondelet. A slope of 0.14% is observed across the

upland, comparable to the 0.15% slope of the Bogota

river valley upstream from La Boca. The difference of

elevation between the bottom of the Bogota River

channel and the dry valley at La Boca reach 15 m,

that is interpreted as the vertical relative offset of the

La Boca Fault.

To summarize we interpret the turn of the Bogota

River at La Boca as the result of the motion of the

La Boca Fault, that rises the upland and blocked the

drainage (Fig. 7). A similar situation explains the re-

route of the Tululbi River to the south, along a fault

segment parallel to the La Boca Fault and lined up

with regional morphostructures (Fig. 2). The mor-

phologic pattern at the turning point of the Bogota

River at La Boca suggests that the change has been

relatively sudden, as no intermediate morphology

attests to a progressive or stepped evolution. This

does not mean that the 15-m vertical offset of the

La Boca upland is the result of one tectonic event,

but that one event has been enough to make the

diversion definitive. The Bogota River is entrenched

2–3 m in the floodplain in the Carondelet area,

giving an estimation of the minimum vertical offset

necessary to re-route the river. The morphostructural

and drainage pattern at La Boca is comparable, on a

smaller scale, to the Australian example of the dis-

ruption of the Murray River against the uplift of the

Cadell Block in Australia (Bowler and Harford,

1966).


5. Channel pattern of the Cayapas–Santiago

confluence

Between the confluence of the Cayapas and San-

tiago Rivers and the coast we observe crossed chan-

nels of different patterns, anastomosed and

meandering (Fig. 8). The SPOT image (Fig. 8)

shows the main elements of the drainage network,

numbered 1 to 5. Downstream from the confluence

of the Cayapas and Santiago rivers (point 1) the main

active channel runs presently through points 2, 3 and

4. The segment 1–3 is characterized by an anasto-

D: 3251-2854 BP

G: 2781-2340BP

E: 3260-2830 BP

A

3

4

5 km

Cayapas-Santiago

estuary

Fig. 8. Extract of the SPOT image 638-347 of 22/03/94 showing the Caya

on Fig. 2. Points 1 to 5 refer to the different segments of channel pattern

floods along the upper part of the Cayapas–Santiago channel.

mosed pattern that appears downstream from the con-

fluence between the Cayapas and Santiago Rivers,

and continue as a wide estuary through the outer

margin of the delta downstream from point 3. The

Los Atajos River (segment 2–5) is lined up with the

Cayapas River upstream from point 1. The Los Atajos

River is presently a secondary channel with lesser

discharge and limited navigation capacity with respect

to the Cayapas–Santiago estuary.

A meandering channel pattern appears upstream

from point 1 and between points 2 and 5. The char-

acteristics are similar with a meander wavelength of

A'

1

2

5

Santiago R.

Cay

apas

R.

Los

Ata

jos

R.

pas–Santiago estuary through the deltaic shoreland. See localization

described in the text. A and A’ represent the expected extension of


3–5 km, a sinuosity of about 1.8 (River channel

length/river valley length) and channel width of

150–250 m. The ratio of meander wavelength to

channel width (L/W) allows characterization of a

meandering river, the standard value for an equili-

brated stream in tropical environment ranging

between 8 and 11 (Baker, 1978). The value of the

(L/W) ratio is 15 for the channel segment upstream

from point 1, and 13–17 between points 2 and 5 (not

considered the lowermost part influenced by tides).

These homogeneous values suggest that similar con-

ditions of meander formation characterize the Cayapas

River upstream from point 1 and the Atajos River

along segments 2–5. These values are a few higher

than the standard values of Baker (1978). But accord-

ing to Dury (1970) this difference may be interpreted

as a tendency to an underfit pattern, that is a wave-

length longer than the corresponding channel width,

due to a lower discharge than that of the equilibrated

pattern. However such a difference can also refer to

local climate and sediment transport conditions

(Schumm et al., 2000). These values are close to the

9–15 ratio observed in the Amazon regions of north-

ern Peru (Dumont, 1991). These parameters show that

the Atajos River represents the downstream continua-

tion of the Cayapas River. This continuity of equili-

brated meandering pattern along the lower Cayapas

and the Los Atajos rivers suggests that the anasto-

mosed channel pattern observed downstream from the

confluence of the Cayapas and Santiago rivers is

triggered by an excess discharge or sediment transport

issued from the Santiago River.

The morphology in point 2 suggests a channel

avulsion by a crevasse splay event initiated in the

meander curve. Crevasse splay and avulsion are gen-

erated by flood, and the reconstruction of a new

channel through the coastal plain involves successive

steps analyzed by Smith et al. (1989) and synthesized

in Collinson (1996). At the first stage the splay

expands rapidly into the marshes, forming a large

avulsion belt with a high density of anastomosed

channels. This avulsion belt appears on the SPOT

image and aerial photos as an area of homogeneous

grain and colour (A and AV, Fig. 8). Then the network

of anastomosing channels gradually coalesce into

fewer and larger channels. This is the step observed

on our example, and represented by the channel seg-

ment 2–3 (Fig. 8). Downstream from point 3 the

Cayapas–Santiago estuary presents a similar pattern

to the Boca de Limones and Bolivar tidal channels,

suggesting that the lower Cayapas–Santiago estuary

was also formed under tidal conditions. The last step

of a crevasse splay event is the formation of a new

meandering pattern similar to the one observed

upstream from the avulsion, but this step is not com-

pleted in our case. The difference between our case

and a classical model of avulsion (Collinson, 1996;

Smith et al., 1989) is that the evidence of changing

fluvial pattern does not occur at the initiation point of

the avulsion, but at the confluence between the San-

tiago and Cayapas rivers. This clearly means that the

flood and subsequent avulsion have been generated by

an excess discharge provided by the Santiago River.

Usually a river avulsion ends by the partial or total

abandon of the previous river channel (Collinson,

1996; Smith et al., 1989), giving characteristic under-

fit channel pattern if a reduced amount of water

supply is maintained in the old channel (Dumont,

1996; Dury, 1970). In the present case the Los Atajos

River has not been abandoned and does not present an

underfit pattern. The interpretation emphasizes that

the excess of water supply that generated the avulsion

did not end as is the case for a flood, but has been

definitive, because it is necessary to maintain enough

water supply along the Los Atajos river to preserve

the former channel pattern.

6. Morphologic pattern of the Cayapas–Santiago

delta

The Cayapas–Santiago and Mataje rivers join the

sea through a coastal plain showing an ubiquitous

pattern of estuary and delta (Fig. 4). Previous studies

qualified the Mataje River outlet of estuary, and the

Cayapas–Santiago of fluviodeltaic system (Tihay and

Usselmann, 1995). The Mataje River estuary and the

Bolivar channel are located in the north part of the

delta; they are deep and accessible to sea ships. The

Bolivar Channel gives access to the San Lorenzo

harbor through a 5 to 18 m deep natural tidal channel

(INOCAR, 2002, unpublished data from San Lorenzo

harbor master’s office), except at the outlet to the sea

where a sand ridge limits the low tide draw to 3 m (6–

7 during upper tides). On the contrary the Cayapas–

Santiago River outlet is almost closed at low tides by

O: 5450-5130 BP

1000 m

N: 5050-4860 BP

Fig. 9. Extract of aerial photo localized on Fig. 4, showing partly

submerged beach ridges represented by the linear structures along

the central part of the photo. N and O number refer to calibrated

radiocarbon ages. See commentary in the text.


a sub-emergent sand ridge, and presents in the outer

part the bottle pattern of a classical estuary (Perillo,

1995). According to Galloway (1975) and Haslett

(2000) these characteristics classify the area as a

compound wave and tide dominated delta. For

descriptive purpose the entire margin of beach ridges

and mangrove crossed by several large tide channels

will be called Cayapas–Santiago deltaic coastal plain,

or simply coastal plain. The Cayapas–Santiago estu-

ary will refer only to the channel that crosses the outer

part of the coastal plain, as the inner part belongs to a

fluvial system.

The coastal plain is 15 km wide and extends nearly

50 km north-eastward along the coast up to the

Colombian border. It includes two fluvial estuaries

at the outer parts (Cayapas–Santiago and Mataje riv-

ers) and two large tidal channels between them (Boca

de Limones and Bolivar channels) (Fig. 4). The tide

range in San Lorenzo is of about 3.5 m (mesotidal),

and the ebb–flood cycle is asymmetrical, with the ebb

8%–9% shorter than the flood (INOCAR, 2004),

favoring the extraction of suspended material. The

inner part of the Boca de Limones and Bolivar tidal

channels are sinuous and intricate, the main channels

ending sharply against the cliff bordering the coastal

plain. Small rivers connect the large tide channels to

the hinterland, the Bolivar channel ending in the

Estero El Salto, a typical dendrite shaped ria (Fig. 6).

The Mataje Fault bounds the coastal plain to the

north (Fig. 2), and a cliff to the north-east (Fig. 4, B1).

To the south-east a discontinuous scarp limits the

coastal plain from a 5–10 m high alluvial terrace of

the Cayapas–Santiago River system (Winckell and

Zebrowski, 1997). To the south the coastal plain

narrows progressively ending at Las Penas (Fig. 4).

The delta morphology shows a striped pattern of

beach ridges which is very clear and continuous to the

south but becomes discontinuous in the central part

and finally disappears to the North below mangrove

(Fig. 4). Partly drowned beach ridges are observed to

the W–SW of San Lorenzo (Fig. 9), showing coarse

beach sand and shell accumulation interpreted as

upper tidal deposits that just outcrop during low

tide. A C14 dating on shell from the beach ridge

facies gave a calibrated age of 5450–5130 BP. The

progressive drowning of the beach ridges from south-

west to northeast gives evidence of a subsidence area

ending abruptly against the Mataje Fault. There are

other evidences of the subsidence such as the deep

tide channels observed to the north where the San

Lorenzo harbor is settled, and the typical ria pattern of

the Estero el Salto. The subsidence since about 5000–

6000 BP is about equivalent to the tide range, that is

3.5 m (INOCAR, 2004), considering that the sea level

did not change significantly since that time. However,

the subsidence would be higher if the middle Holo-

cene sea level was higher than the present one, as

suggested by Tihay (1989) and Tihay and Usselmann

(1995), and the sea level curve of Pirazzoli (1991) for

tropical areas. The clay, alluvium or volcanic con-

glomerate of the Cachabi and San Tadeo formations

that underly the Holocene transgression of beach sand


is not a favorable material to support the interpretation

of regional subsidence due to compacting, and the

increasing subsidence from south to north also favors

the interpretation of a tectonic subsidence.

The hydrographic map drawn from SPOT image

and aerial photos shows that the coastal plain presents

two longitudinal parts, defined on the basis of tidal

channel sinuosity and connections (dotted Z–ZVline,Fig. 4). The inner part presents the higher density of

tidal channels, and the outer part a more homogeneous

and relatively simple pattern, with all the small tidal

channels originating near the inner limit of the outer

part. Crossing the Z–ZVline the channel pattern of the

Santiago–Cayapas estuary changes significantly.

Through the inner belt the channel is relatively narrow

with a poorly anastomosed pattern, but it suddenly

widens, showing an inflexion or the trend to the west

in the outer part. Also the Los Atajos River, old

segment of the Cayapas river, reaches the Boca de

Limones tidal channel near the Z–ZVlimit (Fig. 1) in

an intricate area of broadening and merging of differ-

ent channels. A careful observation of the aerial photo

and the SPOT image shows that the beach ridges get

straight on the channel border in the inner part of the

deltaic coastal plain, but present a turn toward the

upstream direction in the outer part (Fig. 8). This

observation suggests that the inner part of the channel

cuts through pre-existing beach ridges, when the outer

wide channel was formed as a classical seaward

accretion delta. In other words, the present inner and

narrow channel of the Santiago–Cayapas estuary has

been opened through a pre-existing belt of beach

ridges by the time the shore line was near the Z–ZVlimit (Fig. 4). The later evolution of the coastal margin

built a classical bottle shaped estuary (Perillo, 1995).

7. The pattern of beach ridge accretion

The beach ridges are considered as an opportunity

to date the coastline when the avulsion event occurred

that formed the new Cayapas–Santiago channel (Fig.

2, point 3). We also analyzed the succession of the

beach ridges in order to get possible evidence of

irregularities in this process of accretion in relation

with the avulsion event.

The succession of beach ridges is determined using

19 radiocarbon dating, located on Fig. 10. The dating

points do not fit precisely with straight sections, prin-

cipally in the northern part of the coastal plain, due to

field conditions (mangrove, marshes, tide channels).

However, the lateral continuity of the beach ridge

traces, especially in the south part of the coastal

plain, allows interpolating dating using points that

are not exactly on the section line, assuming that the

lateral extension of a beach ridge may be considered

approximately as a time mark. We used 11 dates of

beach ridges from Tihay and Usselmann (1995),

sampled in the southern part of the littoral zone

(Fig. 4, points A to G and I to L). These dates were

made on shells from the ridges, recovered from 1 to 2

m deep holes. The 8 other dates (Fig. 6, points H and

M to Q) come from this study. Seven dates are

obtained on organic material (small wood fragments

or leaves accumulation) recovered at a depth of 1–2 m

by vibro-coring in the wetlands of ridge slack depos-

its, and one (sample O) comes from shell recovered in

beach ridge deposit recovered during low tide.

We checked the homogeneity of dating for the two

types of material by comparing the age of sample O

on shell from a beach ridge deposit with sample N

from organic matter in slack deposit close to the ridge

(Fig. 9). The ages are 5450–5130 BP for sample O

and 5050–4860 BP for sample N, that is 300–400

years less for the sample located 200 m on the seaside.

This result is coherent considering the 1–2 m y-1

mean rate of beach ridge accretion calculated for

this area (see below). The oldest age (6880–6660

BP, P) comes from a tree trunk found in the innermost

part of the Boca de Limones channel, near the foot of

the littoral cliff. This trunk is only partly emerged

during very low tide. Along the present beaches the

tree trunks that are brought to the shore are preserved

only in the upper part of the beach, near the top of the

beach ridges, or on their backside. The present posi-

tion of the old tree trunk imply a subsidence of at least

the value of the tide range (3.5 m) since 6880–6660

BP. The radiocarbon ages of the inner part of the

coastal plain range between 6880–6660 BP (P) and

4132–3686 BP (K), and those of the outer part

between 460–0 BP (H) and 3260–2830 BP (E). The

D–E and M samples which are laterally distant of

about 20 km along the same stripe of ridges have

similar ages. The dates have been plotted as a function

of their distance from the shoreline (Fig. 10A) or from

the inner cliff (Fig. 10B). Two diagrams are used to

5 100 15 km

1000

0

2000

3000

4000

5000

6000

7000

5 01015 km0

1000

2000

3000

4000

5000

6000

7000

Distance from the cliff

Distance from the shoreline

F

G

D M

KL

ON

IJ

P

Age

in y

ears

Age in years

P

IJ

O

N

KL

A B

F

H

G

D E

M

A

B

X1

X2

X3

X4

X1

X2

X3

X4

Y1

Y2

Y1

Y2

Sho

relin

e

400 yr margin of error

Slowaccretion

Slowaccretion

Fastaccretion

Fastaccretion

Slowaccretion

Slowaccretion

FA

FA

Fig. 10. Diagrams of the beach ridge construction of the littoral margin, using age and positions from Fig. 4, with the same letters. Isolated data

such as Q, and dating problem such as C are not used. The dotted line represents the mean accretion of the littoral margin. The lines X1–X2, X2–

X3 and Y1–Y2 associate data along sections positioned on Fig. 4, and represented respectively by open (X1–X2), grey (X3–X4, and black bar

lines ( Y1–Y2). The length of bar lines represents the margin of error. A: age versus distance measured from the present shoreline, and using all

the data. B: age versus distance measured from the inner cliff. The black point represents the present position of the shoreline along the Y1–Y2

segment.


take into account the fact that the shoreline is not

exactly parallel to the inner cliff, the uncertainty

increasing with the distance; however the two dia-

grams present very similar patterns. The diagrams

give evidence of a mean rate of beach ridge accretion

of 2–2.5 m y�1 since about 6000 BP (dotted line Fig.

10). However a more detailed analysis suggests 3

groups of points, defining two periods of relatively

slow accretion and one period of faster accretion

between them. Sampling position allows definition

of two sections through the inner margin (Fig. 10:

X1–X2 and X3–X4) and one through the outer one

(Fig. 10: Y1–Y2). Across the inner margin the X1–X2

section shows mean rates of beach ridge construction

of 1.75 m yr�1 between 5935–5581 BP (I) and 4230–

3806 BP (L), and the section X3–X4 of 0.7–1 m yr�1

between: 6880–6660 BP (P) and 5050–4860 BP (N).

Data from the outer margin along section Y1–Y2 show

N


first a faster construction at a rate of up to 5 m yr�1

between 3251–2854 BP (D) (or 3200–2780 BP, M)

and 2781–2340 BP (G), decreasing to about 1 m yr�1

during the more recent period. There is no data in the

time range 4200–3200 BP (data L and M, D or E,

Figs. 4 and 10), the pattern of the diagram suggesting

a discontinuity more than a progressive change.

A

1000 m

P: 6880-6660 BP

Fig. 11. Extract of aerial photo localised on Fig. 4, showing the San

Lorenzo Fault (A) between the mangrove and tidal channel to the

west and the upland to the east. P refers to the radiocarbon age of a

tree trunk.

8. Evidence of Holocene tectonics: the San Lorenzo

Fault

South-west and north of San Lorenzo a sharp

lineament observed on the SPOT image and aerial

photos defines the limit between the coastal plain

and the San Lorenzo upland (Figs. 4, 11, 12). On

the field this corresponds to a 10–20 m high scarp

visible over more than 10 km. This scarp has been

previously identified and interpreted as an active fault

(Eguez et al., 2003; Santana and Dumont, 2002), for

the sharp limit with the littoral plain, but precise

description and argument for its active motion have

never been presented.

The scarp line cuts the reddish weathered detritus

of the Cachabi Formation. North of San Lorenzo the

lineament splits into two branches (Fig. 4). The main

scarp follows the eastern branch, but disappears pro-

gressively in the upland. North of San Lorenzo the

western segment is a tiny line across mangrove and

wetland (Fig. 12), but becomes sharper to the north,

making again the border of the upland when the

eastern line disappears (Fig. 4).

The usual question with such coinciding fault and

coast line is whether the scarp line represents a fault

line that post-dates and cuts the mid-Holocene trans-

gression, or a beach angle that uses locally a former

fault scarp. We observe here some ubiquitous ele-

ments, suggesting that different events–transgression

and fault–may have occurred more or less simulta-

neously. For example the conformity between the

trend of the inner beach ridges and the scarp suggests

a shore angle of the postglacial sea rise, but also the

line is too continuous and sharp on aerial photos to be

just a 6000-year-old fossil sea cliff (Fig. 11). In parti-

cular, no weakening or deviation of the line appears

near the lows and small valleys across the cliff, as

observed when a shoreline meets estuaries. Finally the

trace of the north-western segment of the San Lorenzo

Faults through the coastal plain of mangrove (Fig. 12)

is considered as a determining element, to interpret

this lineament as a fault motion that necessarily post-

dates the mid-Holocene transgression. We did not find

evidence of beach deposits on the upland side of the

San Lorenzo Faults to attest the post-transgression

motion of the fault scarp. The interpretation suggested

here is that the 20-m scarp and lineament of the San

Lorenzo Fault probably predates the post-glacial

transgression, but has been reactivated later.

N

Fault

1000 m

Fig. 12. Extract of aerial photo localized on Fig. 4, showing the San

Lorenzo Fault through a wetland of mangrove and tidal channels.


The fault plane of the eastern segment of the San

Lorenzo Fault has been observed near San Lorenzo

(point B1, Fig. 4A). It trends N40–608E and dips of

708 to 808 to the north-west, and cuts the brown-

reddish sandy clay of the Cachabi Formation (Fig. 4

B1). Associated fault planes from the same area trend

E–W (Fig. 4 B1, fault planes numbers 5 and 7) and

dip generally to the north, with some associated

planes dipping to the south. The calculation of the

main stress axis with the Carey method (Carey, 1979;

Carey and Mercier, 1987) gives a maximum stress

direction r1 nearly vertical, and r2 and r3 nearly

horizontal trending N808E and N1708E, respectively.The slickensides (Fig. 4 B1) present a relative disper-

sion that is explained by the high dip of the fault

planes. The statistical analysis shows that the angle

between the measured and calculated slickensides is

lower than 58 for most of the fault planes, and less

than 118 for all, and thus the result may be considered

as coherent. The conclusion of the Holocene tectonic

activity is a nearly N–S extension with a transtension

motion along E–W to NE–SW trending faults. The

San Lorenzo faults trend parallel to the faults

observed on the south-eastern border of the La Boca

uplift, and also parallel to the North Canande fault in

Alto Tambo. The motion of all these faults is probably

not synchronous, but they belong to the same family

and are representative of the more recent and probably

active fault activity of the area.

9. Discussion

9.1. A succession of related morphologic events

Three important morphological changes have been

identified: 1–the diversion of the Mira River to the

north at the head of the El Placer fan, 2–the diversion

of the drainage network to the south-west in the lower

part of the fan, and 3–the channel pattern change

along the Cayapas–Santiago river segment. The ana-

lysis of beach ridges accretion in the coastal plain is a

complement of point 3. It is possible to consider all

these morphological changes as independent. How-

ever, there is some coherency and relation from one

change to the other, and the hypothesis supported here

is a link between them. The diversion of the Mira

River in Alto Tambo, at the head of the fan, presents

the same style of diversion than the one observed in

the lower part of the fan in La Boca. It is considered as

an early event inside the same structural scheme of

evolution, but not synchronous of the events that

occurred at the lower part of the fan.

The drainage diversion in La Boca implies the

presence and the motion of the La Boca Fault,

which is sub-parallel to the San Lorenzo Faults. The

La Boca and the San Lorenzo faults border the La

Boca uplift, the regional structure responsible for the

re-routing of the lower part of the El Placer fan

drainage toward the south-west. The north-eastern

segment of the San Lorenzo Faults presents a trans-

tension motion (dextral-normal), and a similar motion

can be easily hypothesized for the La Boca Fault. The

diversion of the Palabi, Tululbi, Bogota and Carolina

rivers to the Southwest against the La Boca uplift, and

their re-routing toward the Santiago River, should

have increased the discharge of this river. However

this increase of the discharge is accompanied by a

decrease of the slope along the diverted river seg-

ment, meaning that the discharge increase cannot be

correlated with an increase of the sedimentary charge

of the Santiago River. This excess discharge is con-

sidered as responsible for the channel pattern change,

from meandering to straight, observed downstream


the confluence between the Santiago and Cayapas

rivers, and the avulsion that created downstream the

new Cayapas–Santiago estuary through the coastal

plain. The dual pattern of the Cayapas–Santiago estu-

ary through the coastal plain supports this interpreta-

tion: the narrow upstream part has the pattern of a

river avulsion channel opened through the pre-exist-

ing coastal margin of beach ridges, and the wide

downstream part has the classical tide-wave domi-

nated pattern of estuary.

We consider that the motion of the La Boca fault

created the favorable situation for the increase of the

discharge, but the precise time correlation between

fault motion and the onset of the avulsion event may

be discussed. An avulsion characterizes a peak of

discharge, that is also generally a peak of precipita-

tion elsewhere in the catchment area. In this area the

highest precipitation occurs generally during El Nino

periods, with two to three times more rain than during

normal periods, and concentration of the rain during

short periods (Perrin et al., 1998). That means that the

avulsion may have occurred during a period of high

precipitation following the re-routing of the drainage

along the La Boca uplift. However special attention to

the pattern of the avulsion leads us to discard a simple

climate induced avulsion: after the avulsion there is

no abandonment or even notable reduction of the

previous channel (the Los Atajos River). This

means that the discharge increase that generated the

avulsion did not really disappear after that event. A

peak of precipitation and flood may have triggered

the avulsion, but the return to normal conditions

would have left an underfit pattern for the Los Atajos

river, as observed for the avulsion events in the

western Amazonian basin (Dumont, 1996). This

means that the discharge increase is definitive, and

that a climate induced flood event is not the explana-

tion. This interpretation is coherent with the fact that

the channel pattern change which is associated with

the avulsion began at the confluence between the

Santiago and Cayapas rivers, and not just at the

avulsion point as is usually the case (Collinson,

1996; Smith et al., 1989).

9.2. Age of the drainage disruption

According to the proposed scenario the age of the

beach ridges at the point where the channel pattern of

the Cayapas–Santiago estuary changes from a fluvial

to an estuary pattern (Fig. 8, point 3) is indicative of

the age of the drainage diversion in La Boca and the

resulting avulsion of the Cayapas–Santiago River.

This age can be determined using the striped pattern

of beach ridges (Figs. 8 and 4). The D sample (3251–

2854 BP) (Fig. 4) is the closest point, the sudden

widening of the channel beginning about 200 m sea-

ward. Also the extrapolation of beach ridge traces on

Fig. 5 crosses the point of channel change at about

3000–3200 BP.

9.3. Post-seismic deformation of the coastal plain?

Before 4000 BP the rate of beach ridge construc-

tion is slow (0.7–1.7m/yr). After that we lack data in

the time range 4200–3200 BP. A period of fast

accretion (5 m/yr) appears after 4000–3600 BP, and

in the time range 3200–2700/2700–2300 BP (Fig.

10). The transition from the early period of slow

accretion to the period of fast accretion is not docu-

mented, but no progressive change is perceptible.

From 2000 BP to present we observe a progressive

slow down of the accretion rate (about 1 m/yr), but

the present rate (during the last century for example)

is not documented.

The faster beach ridge accretion may be due

either to an excess of sediment supply to the coast,

or to vertical motion of the coastal plain due to co-

or post-seismic deformation. We have observed that

the river diversion against the La Boca uplift corre-

sponds to a lower slope than the previous one, and

thus cannot be correlated with an increase of the

sedimentary supply to the coast. Short-term climatic

variations may be related to variation of the beach

ridge construction but at a higher frequency of var-

iation than the progressive change observed over a

3000-year period. Our data suggests to correlate the

fast beach ridge accretion in the time range 3200–

2300 BP with a co- or post-seismic uplift of the

coastal plain. This is more probably a post-seismic

motion because of the duration of the event and the

progressive variation to the recent period of slow

accretion. In a low sloping shore platform a small

uplift can give account of the formation of beach

ridges some hundreds of meters seaward. The pre-

sence of emerged banks of dead oysters in life

position (Fig. 4) supports this hypothesis.


9.4. Relation with the seismic zone

The seismic activity of the Esmeraldas–Tumaco

area is originated in the subduction zone (Collot et

al., 2004, 2002; Herd et al., 1981). The motion of the

Las Boca and San Lorenzo faults cannot be directly

linked to the seismogenic zone, but only to the accom-

modation of the deformation in the upper crust of the

overriding plate. The comparison of stress tensor in

the subduction zone (Ego et al., 1996) and in the

upper plate in the San Lorenzo fault (Fig. 4) supports

this interpretation. The state of stress obtained from

the inversion of focal solutions from subduction earth-

quakes (Fig. 4 B2) shows a ENE–WSW shortening

(Ego et al., 1996). The comparison with the state of

stress of the San Lorenzo Fault (Fig. 4 B1) suggests a

permutation according to r1Yr2; r2Yr3 and

r3Yr1, that is coherent with an accommodation of

the deformation in the upper plate, favored by the

oblique subduction. This structural relation allows

discarding the effect of gravitational accommodation

in the upper inner edge of the continental shelf,

because the motion should be represented by E–W

or NW–SE trending extension instead of the observed

north–south one.

The seismic activity along the South American

Pacific margin is frequently accompanied by co-seis-

mic deformation (Barrientos, 1996; Barrientos and

Plafker, 1992). In the Esmeraldas-Tumaco seismic

zone similar vertical motion have been mentioned,

but have never been precisely quantified and located

relative to the earthquakes. The available information

from the Tumaco area reports generally co-seismic

subsidence (Herd et al., 1981), but also uplift adjust-

ment that seems to occur after the earthquake, and

reaching 33 mm in 28 years (Tihay and Usselmann,

1998). This data supports the hypothesis of a post

seismic uplift of the shore margin in the central part of

the Cayapas–Santiago delta.

10. Conclusion

The San Lorenzo area represents a good example

of how to provide evidences of active tectonics under

wet tropical regions. The tropical climate is not sui-

table for the preservation and the observation of fault

scarps, especially with low topography and soft mate-

rial. However drainage network is generally dense and

active, and may be a suitable morphological tool. New

faults showing recent or active motion have been

identified on the basis of morphologic analysis, the

North Canande, La Boca and Mataje faults, and pre-

cision has been brought on the Holocene motion of

the San Lorenzo fault. In these cases the morphologic

expression of active tectonics seems enhanced by the

change from shortening to extension tectonics in the

coastal margin that occurred during the Quaternary,

and particularly the recent N–S extension, that is

clearly associated with the disruption of the lower

part of the Pleistocene El Placer fan.

The regional evolution of the drainage network in

the San Lorenzo area describes the disruption of the

drainage network of the Pleistocene El Placer fan,

diverted to the north at the head of the fan and to

the southwest in the central part. This southwestward

diversion increases downstream the discharge of the

Santiago River, and triggered the avulsion of the

Cayapas–Santiago River through a new estuary

opened across the coastal plain of beach ridges. The

avulsion is dated 3200–2800 BP by the beach ridges

located at the point of changing pattern of the estuary,

showing anastomosed pattern for the avulsion channel

in the inner part and the new tide-wave dominated

estuary in the outer part. The avulsion event will be

called bLa Tolita eventQ from the position of the La

Tolita archeological site at the point where the avul-

sion channel reaches the coast.

The disruption of the El Placer fan and the related

morphological changes are clearly related to fault

motion. The diversion to the north of the Mira

River at the head of the fan is related to the motion

of the North Canande Fault and other N–S trending

faults. The motion of the La Boca Fault diverted the

drainage of the central part of the fan toward the

Santiago River, evidencing the La Boca uplift. This

uplift is bordered to the NW by the San Lorenzo

Fault, that makes the border of the coastal plain and

presents evidences of motion since the mid-Holo-

cene. These morphologic-fault relationships are inter-

preted as the effect of one or several strong

earthquakes in the time range 3200–2800 BP. How-

ever the northward diversion of the Mira River pre-

dates the river diversion observed against the La

Boca Fault, suggesting that the tectonic changes

that disrupted the El Placer fan migrated from the


upper part of the western Andean Cordillera to the

foothills and the littoral margin.

Additional and independent elements come from

the analysis of the pattern of beach ridges accretion. A

period of fast beach ridges accretion apparently

accompanied and continued after the 3200–2800 La

Tolita event. The transition to the recent period of

slow accretion is progressive. This progressive change

in beach ridge accretion cannot be related to climate

oscillations or increase of the sediment supply after

the avulsion event, but to small uplift of the coastal

margin in relation to one event or a crisis of seismic

events. The present period documented for the last

century is characterised by a high seismic activity, but

we do not know precisely what is the present rate of

beach ridge accretion in the area. This precision will

help to specify whether the present historic period that

includes the 1906 Tumaco Mw 8.8 earthquake may be

compared or not with the previous 3200–2800 BP

period of fast ridge accretion.

Finally a tentative relation can be made between the

morphologic and cultural evolution of the area. It is

worth noting the particular position of the La Tolita site,

at the point of the coast where the avulsion channel has

been opened (Fig. 4). The pre-Colombian occupation

of the studied area is dominated by the La Tolita

culture, one of the most important coastal occupations

along the South American coast (Bouchard, 1995;

Valdez, 1987). The settlement in La Tolita began

about 3000 BP, and the peak of the development

occurred between 2250 and 2000 BP (Valdez, 1987).

According to Valdez (1987) the location of the La

Tolita site in the Cayapas–Santiago estuary, close to

the Pacific Ocean, is the result of a specific strategy to

get easier access to areas along the littoral, but also

towards the hinterland. In the case of La Tolita the new

drainage network of the Santiago River may have made

the gold bearing area more accessible, thus increasing

the capacity of acquiring one of its major resources.

The seaport that was also now available afforded the

coastline trade along the Pacific and gave the La Tolita

center its cultural reputation and economic hegemony.

Acknowledgements

This study is part of the DEMA3-IRD (Institut de

Recherche pour le Developpement, France) project,

realized in cooperation with the bVariabilidad CosteraQINOCAR (Instituto Oceanografico de la Armada,

Ecuador) project. We are grateful to Edgar Rivas

(INOCAR) and Kervin Chunga (University of Guaya-

quil) for their help during field work, and to the

personal of the San Lorenzo Harbor Master’s Office

for their help in logistic organization of field work.

The final manuscript has benefited from comments

and suggestions from Martin Stokes and Edward A.

Keller, for which they are thanked. We thank Ben

Yates for the final correction of the text. This study

participates in the activity of the IGCP project 495,

Land Ocean Interaction. This is publication 756 of

UMR 6526-Geosciences Azur.

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Fan beheading and drainage d ,nn khk iversion as evidence of a 3200-2800 BP earthquake event in the...

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