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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: [email protected] (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

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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

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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.

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