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Morphostratigraphy of an ebb-tidal delta system associated with alarge spit in the Piedras Estuary mouth (Huelva Coast,
Southwestern Spain)
J.A. Moralesa, J. Borregoa, I. Jimenezb, J. Monterdea, N. Gilc
aFacultad. de Ciencias Experimentales, Departamento de Geologa, Universidad de Huelva, Huelva, 21819 Spain
bEREBEA. Centro de Calatillas. Carretera de las Islas, s/n. Huelva 21041 SpaincEscuela Politecnica Superior de La Rabida. Departamento de Ingeniera Minera, Mecanica y Energetica, Universidad de Huelva, Huelva,
21819 Spain
Received 24 August 1999; accepted 4 October 2000
Abstract
The Piedras Estuary is one of the most significative estuarine systems on the mesotidal Huelva Coast, in the Northwestern
portion of the Cadix Gulf. The river mouth is presently an estuarine lagoon partially closed by a large spit constructed from an
old barrier island system. This estuary is in an advanced state of infilling and its tidal prism has decreased during the Holocene
causing instability and clogging of old inlets and transforming the barrier island chain into a spit. Sedimentation is controlled by
the interaction of ebb tide currents and the prevailing SW waves. The main sediment supply is provided by an intensive West-to-East longshore current, transporting sand material from Portuguese cliffs and the Guadiana River. Tidal range is mesotidal
(2.0 m) and the mean significant wave height is 0.6 m with an average period of 3.6 s.
A boxcore study allowed five depositional facies to be distinguished in the Piedras Estuary mouth: (1) main ebb channels; (2)
marginal flood channels; (3) ebb-tidal delta lobes; (4) marginal levees; and (5) curved spits. The recent evolution studied in this
area suggests a cyclic evolutionary model for the ebb-tidal delta system. The architectural facies relations shown by the
vibracore/boxcore study confirm that the apical growth of the spit occurred over the innermost of these ebb-tidal deltas.
Consequently the preserved sequence shows the ebb-tidal delta facies under the spit facies. 2001 Elsevier Science B.V.
All rights reserved.
Keywords: Piedras River mouth; Ebb-tidal deltas; Coastal processes; Holocene evolution; Depositional facies architecture; Spain
1. Introduction
Modern coastal fluvio-marine systems are the
product of the interaction of waves, tides and fluvial
supply modified by relative sea-level changes and
climatic setting. As a result of these interactions
estuarine systems can follow different paths of evolu-
tion and infilling (Davis and Clifton, 1987; Nichol and
Boyd, 1993). Ebb-tidal delta systems have receivedconsiderable attention in recent years (Kumor and
Sanders, 1974; Oertel, 1972, 1977; Hayes, 1980;
Aubrey and Gaines, 1982; Fitzgerald, 1984; Sha,
1990 amongst others). These authors focused their
studies on the geomorphological factors which control
their evolution and their sedimentary sequences.
The physiography of ebb-tidal deltas is controlled
by the interaction of waves and tidal currents (Kumor
and Sanders, 1974; Oertel, 1972, 1977; Hayes, 1980;
Marine Geology 172 (2001) 225241
0025-3227/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved.
PII: S0025-3227(0 0)00135-3
www.elsevier.nl/locate/margeo
E-mail address: borrego@uhu.es (J. Borrego).
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Sha, 1990). The action of these dynamic agents on
ebb-delta environments produces a predictable pattern
of evolution, associated with physiographic changes
through time (Oertel, 1977; Aubrey and Gaines, 1982;
Fitzgerald, 1984; Sha, 1990). This geomorphological
evolution generates a characteristic stratigraphic
sequence for these coastal systems and a vertical
facies succession which corresponds to environments
in the deltas (Hayes, 1979; Sha, 1990).
The Piedras Estuary (Fig. 1) is an estuarine lagoon
in an advanced state of sediment infilling. It is
bounded on its marine side by a large spit that was
constructed by a strong littoral drift from the West in a
period of a relatively stable sea level.
The spit that closes off the Piedras River Estuary(named El Rompido or Nueva Umbra spit) has been
studied from different viewpoints by various authors
during the last 15 years. Dabrio et al. (1980) and
Dabrio (1989) studied the sedimentary dynamics of
the spit and suggested a hydrodynamic model to
J.A. Morales et al. / Marine Geology 172 (2001) 225241226
Fig. 1. Regional setting and location of the study area on the Piedras Estuary mouth.
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compared with the other estuaries of the HuelvaCoast. It has an area of about 10 km2 and it is devel-
oped in a NS orientation. During the Holocene rapid
accumulation occurred infilling the most of the inner
estuary (Borrego et al., 1993). The modern estuary is a
narrow channel surrounded by extensive salt marshes.
In the outer estuary a spit developed parallel to the
coast in the last 200 years, burying a previous barrier
island system. The apex of this spit constrains the
mouth of the estuary and is associated with a systemof ebb-tidal deltas (Fig. 2).
3.2. Hydrodynamic setting
The Huelva Coast has a mean tidal range of 2.0 m.
As such, it lies on the boundary between a microtidal
and mesotidal tidal coast. This coast is affected by
tidal cycles of different periods, the shortest period
is semidiurnal, while a longer twice-weekly period
alternates spring (mean range is 2.82 m) and neap
(mean range is 1.22) tides. Another yet longer cycle
causes the variations of range between equinox and
solstice tides. This has a six-month period (Morales,
1997). The tidal wave along the coast has an East-to-West displacement, producing low velocity currents.
The tidal wave propagates into the estuary decreasing
its tidal range following a hyposynchronic model
(Borrego and Pendon, 1989), but the displacement
along the channel develops tidal currents stronger
than those observed in the outer coast (Fig. 3A).
Consequently, the external flood current is 0.40 m/s
westward and external ebb is 0.30 m/s eastwardduring a mean spring tide (2 May 1996). During the
same event the inner tidal flood is 0.55 m/s and inner
ebb is 0.64 m/s. The inner ebb tide maintains its iner-tial action after the start of external flood, so the inter-
action between inner and outer tidal currents
generates a three-stage current model (Fig. 3B). The
tidal circulation during the second stage (the transition
from ebb to flood) is the responsible of the recurvedshape of the main ebb channels.
The presence of different tidal cycles creates
several vertical biosedimentary zones in the intertidal
sector. These zones can be considered as sub-environ-
ments which are separated by Critical Tide Levels
(CTLs, sensu Doty, 1949). These CTLs are a func-tion of the duration and frequency of exposure experi-
enced by each elevation point. CTLs are also critical
for the presence of some significant species that are
important as trappers of fine sediment or bioturbation
agents in tide-dominated zones (i.e. vegetals Spartina
maritima appears over MNHW and Zostera noltii
appears under MLW and crustaceans Uca tangerii
appears between MWL and MNHW and Panopeus
sp. appears under MLW). In the Piedras mouth
these statistical levels have been calculated using a
table of theoretical predicted tides published by Insti-
tuto Hidrografico de la Marina for 1996. The topo-graphic values are tied back to the lowest historical
tide at Huelva and are:
Equinox Extreme Spring Low Water level
(EESLW): 0.11 m. Mean Spring Low Water level (MSLW): 0.40 m.
(Exposed no more than six times and 30 min a
month).
Mean Low Water level (MLW): 0.85 m. (It is
J.A. Morales et al. / Marine Geology 172 (2001) 225241228
Fig. 2. Panoramic air photograph of the system in 1994.
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exposed and submerged in all tides, but it has 490 h
per month of submersion and 250 h of exposure).
Mean Neap High Water level (MNHW): 2.30 m.
(Submerged by the 95% of tides but the total time
of submersion does not exceed 200 h per month). Mean Spring High Water level (MSHW): 3.18 m.
(Submerged no more than 10 times and 20 min per
month). Equinox Extreme Spring High Water level
(EESHW): 3.48 m.
The wave regime was described by Morales (1997).
This coastal area is generally affected by low energy
waves, including Atlantic swell waves (48% of time)
and local sea waves (51.75% of time). Prevailing
waves have a mean significant height of 0.40 m and
a period of 4.03 s and come from SW. They are
associated with swell from the Atlantic Ocean (20%
of time). More energetic Southeast waves, mainly
associated with Gibraltar Strait storms, also reach
this coast (8% of time). These waves have a meansignificant height of 3.80 m, but they can be up to
6 m. This wave regime induces a strong West-to-
East littoral drift. The potential longshore sediment
transport values varies between 180,000 m3/yr
(Cuena, 1991) and 300,000 m3/yr (CEEPYC, 1979).
The Piedras River discharge is insignificant since
the construction of two dams in its main channel in
1971, the southernmost of these dams is located just at
the limit of tidal influence (24 km upstream the
J.A. Morales et al. / Marine Geology 172 (2001) 225241 229
Fig. 3. Hydrodynamic measurements for a mean spring tidal cycle: (A) curves of tidal current velocities measured at six stations and tidal height
curve measured at station 6; (B) model of tidal currents circulation on the Piedras Estuary Mouth. The measurement stations are indicated on
the ebb sketch.
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mouth). Before these constructions, the fluvial
discharge from the Piedras River was markedly seaso-
nal, being moderate during winter (around
75 106 m3 /yr) and very low during summer, as
expected in a Mediterranean climate. In addition,
discharge is very variable on an inter-annual scale.
An estimate of the recent mean annual freshwater
discharge of the Piedras River was calculated by
Borrego et al. (1995) at less than 11 m3 /s. Currently
a small tributary to the estuary, the Tariquejos creek,
is the main source of fluvial sediment on the fluvial
sector of the estuary (Fig. 1B). It is almost dry during
dry years, but during floods in wet years it can provide
more than 35 m3/s of freshwater. These floods occur
J.A. Morales et al. / Marine Geology 172 (2001) 225241230
Fig. 4. (A) Surficial distribution of sub-environments on the Piedras Estuary Mouth. (B) Topographic profiles where the Critical Tide Levels(Sensu Doty, 1949) position are indicated. Level 0 is the lowest historical tide.
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only once every four to five years, but can introduce a
big amount of sand forming extensive sand bodies on
the marsh of the upper estuary. The tide later slowly
reworks the sand seaward, but part of it can be trappedby cohesive tidal sediments deposited during dry
periods.
4. Present sedimentary environments and facies
Six sedimentary sub-environments and deposi-
tional facies have been distinguished within the
estuarine systems of the Huelva coast. These are:
estuarine channels, lagoons, tidal creeks, tidal flats,
intertidal channel margins and salt marshes (Borrego,
1992; Borrego et al., 1993, 1995; Morales, 1995,1997). Each sub-environment is characterized by a
different vertical and lateral lithofacies association
which defines depositional facies produced by the
interaction between the available sediment and the
prevailing hydrodynamic conditions. Other sub-envir-
onments were described with respect to their mouth
closure features: beaches/spits, aeolian dunes, wash-
over fans, flood- and ebb-tidal deltas and deltaic bar
finger sands (Dabrio, 1989; Morales, 1997). Some of
these depositional facies have a constant relationship
with some of the Critical Tide Levels, the degree of
influence of waves, and flood and ebb currents.
In the Piedras case the beach/frontal spit facieswere well described by Dabrio (1982) as a ridge and
runnel accretion system, but the ebb-tidal delta faciesremains undescribed due to the difficulty of accessing
the intertidal and sub-tidal sub-environments.
Five sub-environments with their respective
depositional facies have been identified in the Piedras
River Estuary mouth (Fig. 4) by examining the
boxcores and observing the surficial sediment and
bedform migration. The sub-environments are similar
to those described elsewhere (e.g. Oertel, 1977;
Hayes, 1980; Imperato et al., 1988; Sha, 1990). The
bedform observations are synthesized in the map ofFig. 5. The identified depositional facies are:
4.1. Inlet-main ebb channel facies
The Piedras system may present one or two of main
ebb channels under the EESLW level as extension of
the main estuarine channel. They typically display a
general NS orientation, but they can also curve to the
SW. Their orientation and morphology is controlled
J.A. Morales et al. / Marine Geology 172 (2001) 225241 231
Fig. 5. Chart showing the surficial distribution of dominant bedforms. Other bedforms may also be present.
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J.A. Morales et al. / Marine Geology 172 (2001) 225241232
Fig. 6. Scheme showing the significant depositional facies observed in trenches and boxcores (B) and their location (A).
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by the ebb tide current during the final moments of the
ebb, when the flow is completely channelized. The
dominant sediment under the EESLW level is medium
to coarse sand with lenses of shell lag accumulations.
The most frequent bedforms are 2D dunes oriented to
the South by the ebb (Fig. 6, BP-5; Fig. 7A). The dimen-
sions of the 2D dunes are: Height 0.40.6 m, Stoss
length 35 m, Lee 0.30.5 m. Subordinate flood
oriented 3D dunes are also present (H 0.030.05 m,
Stoss 0.30.7 m and Lee 0.050.2 m).
4.2. Marginal flood channel facies
Marginal flood channels occur on both edges of the
delta system, located between the EESLW and MLW
levels. The western and eastern channels are some-
what different because of their orientation with
respect to the external flood currents and waves. The
western channel is initiated as an erosional channel
where coarse shell lag deposits constitute the main
sediments (Fig. 6, BP-2; Fig. 7C). Afterwards, it
evolves to become depositional, and is filled mainly
with coarse bioclastic material and very coarse sands.
When the sand is dominant, it forms lunate 3D dunes
(H 0.050.1 m, Stoss 0.30.5 m and Lee 0.1
0.2 m) oriented in the flood tide direction. By contrast,
the eastern marginal channel is developed between the
eastern frontal lobe and a swash platform attached tothe beach. This channel contains medium to fine sand
which forms 2D dunes (H 0.40.5 m, Stoss 3
5 m, Lee 0.40.6 m; Fig. 7D). The runnel sediment
is finer and contains annelid bioturbation (Fig. 7E).
4.3. Intertidal levee facies
Levees are intertidal sand accumulations with a
triangular form, at elevations between MLW and
MNHW, and spatially located on the main ebb chan-
nel edges. Because of they are ebb dominated bodies
constituting medium to coarse sand. 3D dunes are theprimary sedimentary form and trend to the Southeast
(N 96 to 175E). These bedforms have sinuous ridges
and the following dimensions: H 0.030.1 m,
Stoss 0.62 m and Lee 0.05 0.3 m. Less ener-
getic bedforms such as ripples are developed on the
3D dune stoss during neap tides (Fig. 7F), but these
small bedforms have a poor preservation potential
because they are reworked by the spring tide currents
that form the 3D dunes.
On the seaward edge of the levees, intertidal and
sub-tidal swash bars migrate to the North over the
levees. The bars can climb to higher zones of the
levees, supplying sand to be reworked by the ebb
tidal currents. Sinuous wave sourced ripples are alsopresent in the central zone of the levees, where the ebb
currents have less competence. The ripples are mainly
oriented to the Northeast (N 8 to 25E) but other
directions can be also present because of the refracted
waves. They have the following dimensions:
H 0.010.03 m, Stoss 0.05 0.06 m and Lee
0.020.04 m. Some of these wave ripples can appear
in the 3D dune troughs as interference forms (Fig. 7Gand H). The significance of wave bedforms on the
levees is that the levees are used as swash platforms
during high tide events, when they are covered bywater. A complete accretionary sequence formed by
bedform migration can be observed in Fig. 6 (BP-3,
BP-8, BP-9 and BP-10) and in Fig. 7I and J.
In the lowest intertidal zones (between the MLW
and EESLW levels) bordering less step edges of thelevees, a less energetic regime permits the presence of
finer and bioturbated organic-rich facies, where black
sandy muds are present above the sandy facies depos-
ited in the sub-tidal channels (Fig. 6, BP-4; Fig. 7B).
Some of the intertidal levees are linked to the spits
apex as a swash spit platform. In this case wave-origi-nated forms such as curved swash bars are dominant.
The bars are dominantly composed of well-sortedcoarse bioclastic sands, which migrate to the North
until they attach to the spit apex. The internal structure
is tabular cross stratification which slopes strongly
landward (Fig. 6, BP-1, BP-6 and BP-7; Fig. 7K).
Erosional surfaces are present separating the different
sets. The dynamic of these curved swash bars were
also described by Dabrio and Polo (1987) and Dabrio
and Zazo (1988).
4.4. Frontal delta lobe facies
Frontal lobes are sub-tidal sand bodies located
under the MSLW level in the front of the main ebb
channel. The dominant sediment is fine to medium
sand with moderate sorting. On the inner part of the
lobes, ebb oriented 3D dunes (N140E to N150W)
are present. The 3D dunes have similar size to those
observed in the levees, but they have straight crests.
On the outer sector of the lobes, flood oriented 2D
J.A. Morales et al. / Marine Geology 172 (2001) 225241 233
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J.A. Morales et al. / Marine Geology 172 (2001) 225241234
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dunes (N65W to N15E) and curved swash bars
(N25W to N70E) are the most significant forms
(Fig. 6, BP-11).
4.5. Beach-spit facies
The beach-spit is located in areas not directly influ-
enced by tidal currents and at elevations between the
EESLW and the MSHW levels. Their facies areconstructed by a ridge and runnel process because
they appear in wave-dominated zones. On the seaward
side, fair weather waves induce the formation of sand
bars, which migrate landward onto the spit. Conse-
quently a cross stratification sloped toward the land
is developed. Normally, the bars move over a near-
shore swash trough (Fig. 6, BP-6 and 7) and usuallythis trough develops linguoid ripples with a migrationpattern perpendicular to the bar crest (Fig. 6, BP-7). In
addition, algae can accumulate in the runnel forming
an Organic Accumulation Level (OAL), that
commonly appears under the bar facies in the sedi-
mentary sequence (Fig. 6, BP-6). The migration of the
bars continues up just as far as the MSHW level,
where they stabilize forming a berm. At this juncture
a parallel lamination sloping seawards is formed bythe backwash.
At the apex of the spit the process of bar migration
is the same, but these bars acquire curved form
through wave refraction. Here, they migrate over the
western intertidal levee or swash platform that is
attached to the spit apex (Fig. 7H). This process of
bar accretion is normally more important in this zone
than in the frontal zone, so the spit is in constant apical
elongation. At this location, if a new bar is stabilized
before being attached to the spit apex, then a part of
the intertidal swash platform is isolated from waves.
That allows tidal domination of part of the swash plat-
form. Sediments become finer and more organic and
are bioturbated by crustaceans, bivalves and annelids.The tide forms at this place, a zone similar to a tidal
flat but with a reduced extension. In the absence of
erosion these zones evolve to become small salt
marshes.
The back barrier has a steep slope toward the
estuarine channel. There, only refracted waves act
but tidal reworking also occurs. During periods of
constructive waves a cross stratification sloped
towards the channel is formed (Fig. 7I), but duringspring tides the ebb current reworks a part of the
formed sets creating erosive surfaces. Normally the
erosive surfaces separate cross stratified sets, which
indicates alternating periods of wave construction and
tidal reworking.
On areas higher than MSHW level, wind is the most
important agent, forming foredunes parallel to the
berm line. Unless the eolian dunes migrate onshore,they can be overwashed by storm waves creating
washover fans that redistribute their sediments, separ-
ating fine sand in the fan lobe from shell lags in thechannel. Usually the wind winnows the fine sand and
only the lags are preserved under new dunes.
5. Facies model
Eleven facies sequences (Fig. 6) were obtained
from trenches or boxcores to describe the nature of
the sediment in the different sub-environments andtheir sedimentary structures. Some of these sequences
allowed us to observe the associations between litho-
facies corresponding to different sub-environments.
Three additional vibracores (Fig. 8) were collected
to corroborate the preserved vertical facies relation-
ships. A facies model (Fig. 9) integrating all these data
has been constructed.
The suggested model (Fig. 9) reflects two different
sectors with distinctive sequences: The first sector is
located on the ebb-delta levees and displays a
sequence where the base is a shelly sandy sediment
with herringbone bedding. A domination of the
oriented laminae is evident in the bedding. These
facies have been interpreted as inlet or main ebb chan-nel facies. On top of these facies, a black or green
organic-rich and bioturbated muddy sand up to 1 m
thick is present. In general, the internal structure is
composed of parallel laminations dipping to the
J.A. Morales et al. / Marine Geology 172 (2001) 225241 235
Fig. 7. Photographs detailing the bedform morphology and facies: (A) linear megarripple trends associated with ripples associated with neap
tidal currents; (B) lunate 3D dunes with wave ripples in their runnels; (C) panoramic view of a lunate 3D dune field; (D) internal structure of a
lunate 3D dune; (E) internal structure of a ripple system; (F) panoramic view of a shallow marginal flood channel; (G) detail of the main ebb
channel margin facies; (H) internal structure of a curved ridge and runnel system at the spit tip; (I) internal structure of the back-spit zone.
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East. These facies have been interpreted as a less
energetic channel margin deposit located on the
eastern flank of the levee. The top of the sequence
corresponds to sandy or shelly sediments, which
show sets of herringbone cross stratification and
lamination mainly oriented parallel to the ebb
sense, but including sets oriented landward due
to swash bar migration. The complete sets slope
to the East. These facies are typical of the intertidal
levees.
The sequence observed in the second sector,
located on the spit (Fig. 9, VP-1), displays only two
of the depositional facies described in the previous
sequences. The sandy shells correspond to the main
ebb channel facies and the black muddy sands corre-
spond to the channel margin deposits. On top of these
J.A. Morales et al. / Marine Geology 172 (2001) 225241236
Fig. 8. Sequences shown by vibracores. Sediment key as per Fig. 6.
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latter sands 0.5 m of shelly sandy facies with herring-
bone bedding are present. These facies are similar tothose observed in the present marginal flood chan-
nels. Parallel laminated or cross-bedded fine sands
are present above the shells, these are interpreted assediments originating from swash bar migration
over a previous swash platform located at the spits
apex.
6. Morphological evolution
6.1. Historical evolution of the spit
The historical development and evolution of the
spit have been studied by different authors who
suggested a variety of interpretations to explain the
morphological changes (Dabrio and Polo, 1987;
Borrego et al., 1993; Zazo et al., 1994; Ojeda and
Vallejo, 1995). These interpretations are discussed
in this chapter. A cartographical study (Fig. 10)
displays the recent historical evolution of this coastal
sector from a small barrier island system separated by
numerous inlets to the development of a littoral spit
linking all the previous islands by clogging of theinlets and drift accumulation. According to the charts
and other previous works (Dabrio and Polo, 1987;
Dabrio and Zazo, 1988), this transition occurredbetween 1862 and 1875, when the spit began extend-
ing to the East. It is interpreted that the change
occurred as a consequence of the progressive reduc-
tion of the tidal prism caused by the sedimentary
infilling in the inner estuary (Dabrio, 1989; Borrego
et al., 1993).
Recently Zazo et al. (1994) have identified older
peat sediments (chronologically dated as 1875 ^ 50
and 1450^ 50 years BP) under the eolian dunes at thecenter of the spit. Consequently they suggested that
the spit elongation began before this age and they
disagree with Borrego et al. (1993) and with their
previous cited papers. For us, the presence of sedi-
ments of this age under old spit bars demonstrates
that the ridge and runnel dynamics started at least
1900 years ago, but do not demonstrated that thesesediments were formed in a spit. We think that they
dated peat sediments formed during the longitudinal
J.A. Morales et al. / Marine Geology 172 (2001) 225241 237
Fig. 9. Idealized cross section showing facies architecture of the system. The relative location of vibracore and boxcore samples are displayed.
The location of this profile corresponds with the western sector of profile 1 at Fig. 4.
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growth of a previous barrier island such as Isla de
Levante (Fig. 10, 1862).
According to the cartographical data (Fig. 10), a
tide-dominated ebb-tidal delta developed in the apex
of the spit when the western inlets disappeared. Weinterpret that the size of this delta would have
increased when full tidal prism of the estuary was
forced to drain through the only available inlet.
Variation of the rate of longitudinal growth is docu-
mented by studying aereal photographs. Since 1956
the longitudinal growth was around 30 m/year, but in
1973 increased to reach 63 m/year and in 1993decreased to have again 30 m/year. This variation
was interpreted to be a consequence of the interaction
of human and natural processes that have modified the
sand input from littoral drift (Borrego et al., 1993;
Ojeda and Vallejo, 1995). Older variations in the
modes of accretion were inferred by Dabrio (1989)
by noting the different berm orientations in the spit.Dabrio interpreted these variations as alternations
between periods of tidal and wave domination.
Recently Zazo et al. (1994) identified that four large
prograding sand bodies are represented in all the
coastal systems in the AtlanticMediterranean link-
age coast. The two most recent of them are recognized
in the Piedras Spit. These prograding bodies are sepa-rated by major gaps or swales. They interpreted that
the progradation of the spit bar systems relates to
J.A. Morales et al. / Marine Geology 172 (2001) 225241238
Fig. 10. Historical evolution of the Piedras Estuary from a barrier island systems to a large spit. Note that cartographic projection systems are
different.
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climate changes. The gaps separating these bodies are
formed as a response to long periods of low atmo-
spheric pressure (of about 100 years duration) thatcauses reduced littoral drift and a domination of
erosional processes.
6.2. Cyclic evolutionary model of the ebb-tidal delta
system
The study of the recent morphological changes in the
ebb-tidal delta system (Fig. 11) shows a cyclic pattern of
evolution, created by the interaction of littoral drift andthe inner and outer tidal currents. This evolutionary
model can be summarized in three stages.
6.2.1. Stage 1 (Fig. 11, 1980)
We consider that the cycle initiates with the opening
of a main ebb channel near the tip of the spit. This new
ebb channel can be developed by the erosion of a
previous marginal flood channel or a washover in the
swash platform. The situation as shown in Fig. 3 B-3
during an extreme spring tide may be the cause of this
erosional event if the main ebb channel is not able to
discharge all the tidal prism. After this event the systemhas two main ebb channels (NS oriented) separated by
an extensive swash platform developed on the levees.
6.2.2. Stage 2 (Fig. 11, 19841987)
The western main ebb channel starts to migrate
eastward. At the same time a new swash platform is
developed at the spit tip, while the swash platform
located between the two ebb channels is reduced in
size. This migration process is favored by the SWcurvature of the main ebb current, which induces an
erosionacumulation pattern similar to those present
in meandering channels. During spring tides, stronger
currents contribute to erode the levee located at east-
ern margin, whereas during the neap tides the waves
can develop swash bars that are only preserved on the
western margin of the channel. Consequently the
migration of the western channel is faster than the east-
ern, due to the difference in orientation.
J.A. Morales et al. / Marine Geology 172 (2001) 225241 239
Fig. 11. Morphological changes in the ebb-tidal delta system since 1977, observed in air photographs.
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6.2.3. Stage 3 (Fig. 11, 1991)
The lateral migration of the western channel into
the eastern one occurs. At this stage the system has
only one main ebb channel and it has a wide swash
platform linked to the spits apex. This situation is
unstable, because this channel caries the full tidal
prism drainage. During an extreme tide, the ebb
current can breach the swash platform beginning a
new cycle (Fig. 11, 1994).
7. Discussion and conclusions
The Piedras estuary mouth is presently configured
as an elongated spit associated with an ebb-tidal delta
system located at its apex. Five sub-environmentshave been distinguished (main ebb channels, marginal
flood channels, levees, ebb-delta lobes and the spit).
These environments are related to some of the Critical
Tide Levels (sensu Doty, 1949) affecting this coast.
Thus, the upper limit of the main ebb channel is the
MLW, levees develop between the MLW and
MNHW, whereas marginal flood channels are located
between MLW and MNHW and a swash or spit plat-
form appears between the MWL and the MNHW;
finally spit bars migrates between MNHW and
ESHW, above this level only eolian dunes are present.At the surface, each of the five sub-environments has a
characteristic assemblage of lithofacies that create
depositional facies typical for each environment.
These are recognizable in the stratigraphic record.
The vibracore record displays a pattern of vertical
accretion in which higher elevation sub-environments
aggrade over environments located at lower tide levels.
For example, the spit facies characteristically caps ebb-
tidal delta facies. The depositional facies sequences also
reflect differences between the spit zone and the levee
zones. Whereas the vertical sequence under the levees
presents a facies sucession composed of a main ebbchannel, a main ebb channel margin and a levee, the
sequence observed under the spit comprises a main
ebb channel, capped successivelyby a mainebbchannel
margin, a marginal flood channel, a swash platform, spit
bars and dunes. Levee facies are absent. These differ-
ences can be interpreted as a result of the low preserva-
tion potential of levee facies when cross-cut by the
migrating ebb delta channel. The delta lobe facies are
also absent in both sequences because of the low preser-
vation potential, due to the ease of reworking of this
facies by waves.
A morphological study of the delta since 1977
demonstrates that the evolution of this spit and tidal
deltas coastal system relates to a cyclic accretionmechanism. This mechanism relates to the different
rates of migration between the two ebb channels due
to their distinct orientation. The eastern channel
migrates very slowly if at all, while the western one
migrates rapidly by bank undercutting until it captures
the eastern channel. This cyclic evolution linked with
an active West-to-East littoral drift induces an east-
ward migration of the spit facies association over themarginal flood channel.
Another consequence of the cyclic evolution
pattern is the existence of two distinct morphologicalconfigurations at the spit tip that alternate through
time. The first one consists of a marginal flood chan-
nel right at the spit tip, where the swash platform is
very narrow and steep. The second is a wide swash
platform between the spit and the western main ebbchannel. The migrating bars developed in the first
setting create curved berm-lines very close to one
another, whereas in the second configuration longer
bars strongly elongate the spit, isolating these swash
platforms from waves. These start to function as back-
spit tidal flats. This latter case develops if the channelmouth is deflected to the East for an extended period
and there is sufficient sand. Storm or extreme tides canbreach the swash platform creating a new western
channel.
These observations demonstrate that the different
directions shown by the older spit berm-lines do not
corresponds to the alternating periods of wave- and
tide-domination as interpreted by Dabrio (1989). Nor
is it even necessary for a direct relationship to exist
between the presence of backspit tidal flats (gaps or
swales) and long periods of atmospheric low pressure
as suggested by Zazo et al. (1994). This paper docu-ments the formation of the last of these tidal flats
without any requirement for climatic alteration.
The suggested mechanism of spit accretion is very
similar to the classic mechanisms of inlet migration in
barrier island complexes described by Kumar and
Sanders (1974) or Aubrey and Gaines (1982) and
the resultant sequence is similar to those suggested
by Oertel (1972) and Hayes (1980) and Sha (1990)
as a product of this mechanisms. The difference
J.A. Morales et al. / Marine Geology 172 (2001) 225241240
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between those studies and this case reflects the local
setting of the Piedras system.
Acknowledgements
We acknowledge the financial support of the Junta
de Andaluca (P.A.I. Groups RNM-0183 and RNM-
0276) and Huelva University (Plan Propio de Investi-
gacion, Group of Coastal Geology). We also acknowl-
edge the help of Drs J. Shulmeister and G. Perillo for
their suggestions on earlier versions, that contributed
to substantially improving the manuscript.
References
Aubrey, D.G., Gaines Jr, A.G., 1982. Rapid formation and degrada-
tion of barrier spits in areas with low rates of litoral drift. Mar.
Geol. 49, 257278.
Borrego, J., 1992. Sedimentologia del Estuario del Rio Odiel
(Huelva, S.O. Espana), Ph.D. Thesis Univ. Seville, 315 pp.
Borrego, J., Morales, J.A., Pendon, J.G., 1993. Holocene filling of
an estuarine lagoon along the mesotidal coast of Huelva: the
Piedras River mouth, southwestern Spain. J. Coastal Res. 9,
242254.
Borrego, J., Morales, J.A., Pendon, J.G., 1995. Holocene estuarine
facies along the mesotidal coast of Huelva, southwestern Spain.
In: Flemming, B.W., Bartholoma, A. (Eds.), Tidal Signatures in
Modern and Ancient Sediments. Spec. Publ. Int. Ass. Sedimen-
tol. 24, 151170.
Borrego, J., Pendon, J.G., 1989. Caracterizacion del ciclo mareal en
la desembocadura del Ro Piedras (Huelva). XII Congr. Esp.
Sediment. Bilbao. Commun., vol. 1, pp. 97100.
CEEPYC, Centro de Estudios y Experimentacion de Puertos y
Costas Ramon Iribarren, 1979. Plan de estudio de la dinamica
litoral de la provincia de Huelva. Informe, Direccion General de
Puertos y Costas. Madrid, p. 37.
Cuena, G.J., 1991. Proyecto de regeneracion de las playas de Isla
Cristina. Servicio de Costas, M.O.P.T., 100 pp.
Dabrio, C.J., 1989. Playas e Islas barrera-lagoon. In: Arche, A. (Ed.),
Sedimentologa, vol. 1. Serv. Publ. CSIC, Madrid, pp. 349394.
Dabrio, C.J., 1982. Sedimentary structures generated on the fore-
shore by migrating ridge and runnel systems on microtidal andmesotidal coast on S. Spain. Sediment. Geol. 32, 141151.
Dabrio, C.J., Boersma, J.R., Fernandez, J., 1980. Evolucion sedi-
mentaria de la Flecha del Rompido (Huelva). IX Congr. Esp.
Sediment., Salamanca. Commun., vol. 1, pp. 6768.
Dabrio, C.J., Polo, M.D., 1987. Holocene sea-level changes, coastal
dynamics and human impacts in southern Iberian Peninsula. In:
Zazo, C. (Ed.), Late Quaternary Sea-level Changes in Spain.
I.G.C.P.-200/CICYT PR 83-2460. Instituto de Geologa,
Museo Nacional de Ciencias Naturales, pp. 227247.
Dabrio, C.J., Zazo, C., 1988. Riesgos geologicos en zonas litorales.
In: Ayala, F.J., Duran, J.J., Peinado, T. (coords.), Riesgos
Geologicos, Serv. Publ. I.T.G.E., Madrid, pp. 227250.
Davis, R.A., Jr., Clifton, H.E., 1987. Sea-level change and the
preservation potencial of wave-dominated and tide-dominatedcoastal sequences. In: Numendal, D., Pilkey, O.H., Howard, J.D.
(Eds.), Sea Level Fluctuations and Coastal Evolution. Spec.
Publ. Soc. Econom. Palaeontol. Mineral. 41, 167178.
Doty, M.S., 1949. Critical tide factors that are correlated with the
vertical distribution of marine algae and other organisms along
the Pacific Coast. Ecology 27, 315328.
Fitzgerald, D.M., 1984. Interactions between the ebb-tidal delta and
landward shoreline: Price Inlet, South Carolina. J. Sediment.
Petrol. 54, 13031318.
Hayes, M.O., 1979. Barrier island morphology as a function of tidal
and wave regime. In: Leatherman, S.P. (Ed.), Barrier Island.
Academic Press, New York, pp. 1 27.
Hayes, M.O., 1980. General morphology and sediment patterns in
tidal inlets. Sediment. Geol. 26, 139156.Kumar, N., Sanders, J.E., 1974. Inlet sequence: a vertical succession
of sedimentary structures and textures created by the lateral
migration of tidal inlets. Sedimentology 21, 491532.
Instituto Hidrografico de la Marina, 1996. Anuario de mareas 1996,
Ministerio de Defensa. Cadiz, 230 pp.
Imperato, D.P., Sexton, W.J., Hayes, M.O., 1988. Stratigraphy and
sediment charactristics of a mesotidal ebb-tidal delta, North
Edisto Inlet, South Carolina. J. Sediment. Petrol. 58, 950968.
Lanesky, D.E., Logan, B.W., Brown, R.G., Hine, A.C., 1979. A new
approach to portable vibracoring underwater and on land. J.
Sediment. Petrol. 39, 655657.
Morales, J.A., 1995. Sedimentologa del estuario del Ro Guadiana
(S.O. Espana-Portugal), Servicio de Publicaciones, Huelva
University, 321 pp.
Morales, J.A., 1997. Evolution and facies architecture of the meso-
tidal Guadiana River delta (S.W. Spain, Portugal). Mar. Geol.
138, 127148.
Nichol, S.L., Boyd, R., 1993. Morphostratigraphy and facies archi-
tecture of sandy barriers along the eastern shore of Nova Scotia.
Mar. Geol. 114, 5980.
Ojeda, J., Vallejo, I., 1995. La Flecha de el Rompido: ana lisis
morfometrico y modelos de evolucion durante el periodo
19431991. Rev. Soc. Geol. Esp. 8 (3), 229238.
Oertel, G.F., 1972.Sedimenttransporton estuary entrace shoals and the
formation of swash platforms. J. Sediment. Petrol. 42, 858863.
Oertel, G.F., 1977. Geomorphic cycles in ebb deltas and related
patterns of shore erosion and accretion. J. Sediment. Petrol.47, 11211131.
Sha, L.P., 1990. Sedimentological studies of the ebb-tidal deltas
along the West Frisian Islands, the Netherlands. Geol. Ultraiec-
tina, No. 64, 160 pp.
Zazo, C., Goy, J.L., Somoza, L., Dabrio, C.J., Belluomini, G.,
Improta, S., Lario, J., Bardaji, T., Silva, P.G., 1994. Holocene
sequence of sea-level fluctuation in relation to climatic trends in
the AtlanticMediterranean linkage coast. J. Coastal Res. 10,
933945.
J.A. Morales et al. / Marine Geology 172 (2001) 225241 241
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