A R Fortuin

C J Dabrio

M.D Polo

T B Roep

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GEOGACETA,21, 1997 
Late Messinian coastal barrier and washover fan sedimentation 
in Sorbas (SE Spain) 
Sedimentación en islas barrera yabanicos de washover en el Messiniense Superior de Sorbas (S.E España) 
C.J. Dabrio (*), T.B. Roep (**), M.D.Polo (*) & A.R. Fortuin (**) 
(*) Opto. de Estratigrafía & Instituto de Geología Económica-CSIC, Universidad Complutense, 28040-Madrid (Spain). Email: dabrio@eucmaxsim.ucmes 
(**) Faculty 01 Earth Sciences, Vrije Universiteit, 1081 HV Amsterdam (The Netherlands). Email: roet@geo.vu.nl 
Introduction 
ABSTRACT 
Deposition of washover fans in coastal settings are related to accommodation space, slope of the 
basin margin, and coeval changes of sea level. Some preliminar y models of development and stacking of 
sand bodies are presented 
Key words: Control s of sedimentation, coastal deposits, beach barrie¡; washover fan 
RESUMEN 
El depósito de washover fans en áreas costeras está relacionado con el espacio de acomodación 
disponible, la pendiente del margen de cuenca y los cambios coetáneos de nivel del mar. Se ofrecen 
modelos preliminares del desarrollo y apilamiento de cuerpos arenosos. 
Palabras clave: Controles sedimentarios, depósitos costeros, islas barrera, abanicos de washover. 
Ceo gaceta, 21 (1997), 89-92 
/SSN: 0213683X 
approximate limit 
of marine "Sorbas 
Member" basin 
~ 
Beach-barrier sediments are easily 
recognised in the fossil record by the 
usually well-preserved, gently inclined 
swash lamination. But barrier islands 
inelude a wider variety of sedimentary 
environments, each producing definite 
sedimentary structures and vertical 
sequences of sedimentary facies that 
inelude erosional surfaces. Towards the sea, 
coastal sedimentary units interfinger with 
muddy shelf deposits; landward they end 
abruptly, or change facies in short distances 
to tidal, lagoonal, swash and fluviatile 
deposits producing complex stacking 
pattems of coastal facies. 
Back barrier environments and 
sediments are well known in present day 
examples, but are less precisely considered 
in fossil examples, where they are usually 
referred to as backbarrier or washover-fan 
facies, and supposed to interfinger with 
tidal or fluvial deposits. 
~«J 
[(-.~ 
~<v~ 
~(;9\ 
A detailed knowledge of the 3-D 
arrangement and lateral facies relationships of 
the stacking pattems in coastal deposits are 
essential to approach many geological 
problems such as precise tracing of sea level 
changes, particularly when small scale 
fluctuations ¡Je involved, modelling of sand 
10 km 
Cabo de Gata 
Fig. 1.. Location map. 
D Neogene basins 
I¡'XQI Neogene volcanics 
Fig. 1.- Mapa de localización. 
89 
GEOGACETA,21, 1997 
Stage 1 ;900" was~_"~Ch "dge) _H._I 
m ----m.=.--m--... =~lli\.=s:--.m 
Stage2 
prograded beach ridge 
erosional surface 
,/ Coarsening and 
,/ thickening gravellag 
_---- "p~""I~"d  
a:?:rW~ / "'" 
Vertical scale 
very exaggerated 
Fig. 2.- Sketch of facies relationships resulting from a rise of sea level in heach barrier environ-
ments, with generation of washover fans and erosional surfaces (vertical scale exaggerated). 
Fig. 2.- Esquema de las relaciones de facies producidas por una subida del nivel del mar en un 
ambiente de playa barrera, con generación de abanicos de washover y superficies erosivas. 
bodies acting as migration pathways and 
seals, or having potential interest as reservoir. 
It is well known that the highly-
variable architecture of sedimentary bodies 
deposited in coastal settings is govemed by 
sediment supply, wave-and tidal processes, 
coastal morphology, and accommodation 
space. But these parameters change with 
back barrier 
backshore 
loon swash zone 
time, and erosional surfaces may play a 
prominent role in areas located towards 
land, once a given topographical position 
aboye base level is reached. Besides, lateral 
shift of erosional or depositionalloci very 
often results in destruction of large parts of 
the sediment record. 
We studied washover fans in coastal 
Fig. 3.- Repeated overwash that produces laterally and vertically stacked washover fans. 
Schematic longitudinal and transversal sections illustrate internal structure (vertical 
scale exaggerated). 
Fig. 3.- Overwash repetido que produce abanicos de washover apilados lateral y verticalmente. 
Las secciones esquemáticas longitudinal y transversal ilustran la estructura interna. 
90 
deposits and related them to accom-
modation space, slope of the basin margin, 
and coeval changes of sea level changes, 
offering a few preliminary models of sand-
body development and stacking. The study 
is based on the upper part (informally called 
Unit I1I) of the late Messinian Sorbas 
Member in southem Spain, because of the 
magnificent outcrop allows detailed three-
dimensionalobservation. 
Geological setting 
Following the fall of sea level that 
favoured the deposition of Messinian 
gypsum in westem Mediterranean basins, 
a minor transgression occurred during 
which the coastal deposits of Sorbas were 
deposited. 
These deposits consist of a stack, up 
to 70m thick in total, of three units with 
roughly coarsening-upward vertical 
trends, informally called 1, 11, and 111, 
formed during temporary seaward 
progradation to the east (Roep et al., 
1979). 
Beach progradation took place in the 
western termination of the relatively 
narrow, east-west oriented embayment 
that formed the basin at that time (Fig. 1). 
We assume that the repeated progra-
dations and outbuilding of the coast that 
generated Units 1, II and In are tied to 
relative changes of sea level in the order 
of tens of meters. By contrast, the vertical 
stacking of lagoon, washover, and barrier 
deposits, algal stromatolites and ero-
sional surfaces in Unit 111 point to minor, 
meter sized, fluctuations of sea level that 
rose hardly enough to cover the 
topographical irregularities of the coastal 
zone and allowed liUle accommodation 
space. 
A large part of the calcarenite Unit III 
observed around Sorbas was generated 
during transgressive events, as landward 
up-stepping washover fans, interfin-
gering with lagoonal muds. Due to 
further transgression, coastal erosion 
removed a part of these washover fans, 
and barriers with active swash zones 
facing the sea (to the east) accumulated 
upon the erosional surface. Subsequent 
sea-Ievel drops forced the coastline to 
jump seaward and barriers were left 
behind as "stranded" ridges, whereas 
renewed transgressions moved new 
girdles of washover fans towards land 
overstepping previous barriers. Pro-
gradation occurred in the most seaward 
part of Unit III. 
Paleomagnetic data (Gautier el al., 
1993; Hilgen et al., 1995) allow us to 
lagoon 
transgressed 
barrier island 
back barrier 
sediment moving 
landwards during 
storms and overwash 
--- ---
------
ggerated vertical scale exa 
, 
coarsening 
and thickening 
upwards sequence 
sigmoidal 
calcarenite unit 
Fig. 4.- Overwash of the whole beach barrier and generation of sigmoidal units of beach 
derived sediments that interfinger with lagoonal mudstone (vertical scale exaggerated). 
Fig. 4.- Overwash de toda la isla barrera y generación de unidades sigmoidales de 
sedimentos derivados de la playa que se interdigitan con las lutitas del lagoon. 
"stranded" barrier transgressed barrier prograding coast 
erosional surface 
(vertical scale exaggerated) 
···---------------------f-------!------:3 
[2-3 [ : 
---------------~-------------------~2 
1-2 3-4 • 
~~..,..:::.....---------------------------~1 
• 
:. -------- ----~4 
f 
small accommodation space large accommodation space 
Fig. 5.- Influence of the available accommodation space in the generation of coastal 
units. 1 to 4 are ideal steps during a fluctuation (rise foIlowed by faIl) of sea level. 
Fig. 5.- Influencia del espacio de acomodación disponible en la generación de unidades 
costeras. I a 4 son pasos ideales durante una fluctuación (subida-descenso) del nivel del mar. 
estimate that many of the smaller jumping 
episodes of Unit III studied here are 
probably related to fluctuations with 
periodicity of about 2,5 Ka_ 
Coastal deposits 
Shelf to basin sediments are mostly 
grey silicielastic micaceous sandstones, 
silt and clay. Beach deposits are 
calcarenites that inelude coated grains, 
and oolites, coexisting with gravels partly 
deri ved from local erosion of the barrier 
during beach retreat. 
Lagoon deposits are yellowish to 
white parallellaminated silt and elay, with 
mud cracks, raindrop imprints, salt 
pseudomorphs, insects, rootlets and plant 
remains and imprints of di verse animals 
(Roep et al., 1979; Roep and Beets, 1977; 
Kenter, pers. como). Thin (l to 3 cm 
thick) layers of fining upward, silici-
elastic sandstone that occur interbedded 
are interpreted as deposited during floods 
from fluvial systems feeding the coastal 
lowlands. 
Sediments ofwashover-fan facies are 
wedge-shaped layers of calcarenite that 
occur stacked vertically and tend to 
diverge from the beach barrier, thinning 
distally until they end between lagoonal 
muds. Sediment forming these layers 
derive from the barrier island with minor 
contribution from the shoreface/shelf. 
GEOGACETA,21, 1997 
Vertically, such successions linked to 
transgression may be thought to have 
thickening upward character, because of 
the transgressive advancing coastline. 
Lithological differences in shelf, 
barrier island, lagoon, and washover-fan 
deposits reveal the diversity of source 
areas and coastal transport: 
(a) Rivers bringing in silicielastics 
with variable grain size. 
(b) In situ precipitation of carbonate 
as oolites, coated grains, stromatolites 
and thrombolites, probably under a warm 
elimate. 
(c) Littoral drift (longshore currents) 
moving sediment into the barrier from 
sources of sediment located to the north 
as deduced from dominant lithology. 
Sequences in washover fan 
Lagoonal and related deposits are 
limited upwards by erosional surfaces 
overlain by layers of beach-rock breccia 
generated during ensuing transgressions, 
which implies that sea moved landward at 
the cost of the washovers. Then occur the 
prograding beach facies marking a new 
regression (Fig. 2). This is repeated 
upwards and, mostly, laterally around 
Sorbas. 
Coastal facies models usually assume 
the well-known picture of girdles of 
washover fans moving towards land and 
shifting laterally with transgressions and 
regressions according to the position of 
the coastline. Landward progradation of 
washover fans generates thickening and 
coarsening upwards sequences, often 
truncated during the ongoing trans-
gression (Fig. 2). 
However, such a simple scheme can 
not be maintained because of the facies 
relationships resulting from landwards 
and seawards shift of sedimentary 
environments induced by sea-level 
fluctuations are further complicated by 
simultaneous displacements alongshore 
of overwash and their related washover 
fans, tidal inlets and deltas, eolian ridges, 
etc. forced by littoral drift, prevailing 
winds, and other causes (Fig. 3). As a 
result, what we see are compensation 
cyeles that resemble the type described by 
Mutti and Sonnino (1981) for submarine 
fan lobes_ 
We found thickening upward, thinning 
upward and irregular sequences, for which 
we infer the following scenario: at places 
were a barrier is pierced during storm 
relatively thick stacks of washover fans 
accumulate, but (owing to their fan shape) 
they thin away from the breech towards 
91 
GEOGACETA,21, 1997 
barrier island 3 
swash zone 3 
barrier island 2 barrier island 1 
swash zone 1 
vertical sea le exaggerated 
Fig. 6.- Influenee of irregularities in the substratum on the loeation of beaeh ridges and 
lagoons: Topographieally-higher areas aet as nucIeation (root) areas for aeeumulation 
of barrier island, whereas lower areas serve as swales that evolve into lagoons. 
Fig. 6.- Influencia de las irregularidades del substrato en la generación de crestas de playa 
y lagoones: las zonas topográficamente más eleVadas actúan como zonas de nucleación de 
islas barrera mientras que las deprimidas son ocupadas por lagoones. 
both land and any other places cIose to the 
coastline where it was not pierced. Later the 
reverse may occur, as the barrier islands is 
broken or surpassed in different places. 
This, coupled with variable rates of sea-
Ievel rise may produce diverse types of 
large-scale cross bedding in the landward-
migrating backbarrier. 
In other cases, generalised overwash of 
the barrier produces coarse-grained sigmoidaI 
units that interfinger down-wards and 
landwards with lagoonal muds (Fig. 4). 
Sedimentary controls and synthetic 
models 
Transgression after deposition of 
calcarenite unit 11 created a wedge-
shaped accommodation space to the east 
("sea") that changed to a more box-
shaped space towards the west Cland") 
(Fig. 5). As the accommodation space 
available for deposition of unit 111 was 
neatly different in these two places, it 
occurs with distinctive geometry and fa-
cies distribution. Thus, unit III displays a 
complex arrangement of facies, 
associated with extensive deposition of 
washover-fan sand and lagoonal mud in 
back-barrier settings, seaward and 
landward step of coastlines, and progra-
dation in the deeper (eastern) part ofthe 
unit. This is interpreted as the result of a 
jumpy behaviour of the coastline. 
Taking the thickness of Unit III as an 
92 
indicator of water depth and magnitude of 
sea-Ievel f1uctuations involved in the 
changes of coastlines, a water depth of 
ca. 10m was deduced. This is further 
supported by the position that occupy in 
the vertical sequence certain sedimentary 
features indicative of the coastline such 
as bird-foot tracks, swash zones, and 
beach-rock Ievels. 
There are three types of barrier 
deposits and swash zones in unit III (Fig. 
5): (1) "stranded" coastal barriers left 
behind after seaward jump of the 
coastline during regression; (2) swash 
zones connected with erosional parts of 
the coastline at the cost of landward-Iying 
washover sediments; and (3) prograding 
coastal sequences. 
The reason for this complex 
behaviour lies in the limited depositional 
space (accommodation), where minor 
f1uctuations of sea level have great 
influence, and usually result in dramatic 
regressions and transgressions. Slight 
regressions, for example, may cause an 
offshore bar to build up quickly near or 
aboye sea level, after which the coastline 
jumps seaward. Thus, the three types can 
be related laterally to the variable 
accommodation space during a 
f1uctuation of sea level (Fig. 5). 
Besides the available accommodation 
space, there is still one more factor that 
controlled the generation and preser-
vation or destruction of washover fans in 
the Sorbas Member: The bottom (top of 
Unit 11) was not perfectly plane and 
horizontal: Instead, gentle north-south 
directed, synsedimentary topographic 
irregularities, 3 to 4m high, further 
restricted the already Iimited vertical 
space available for sedimentation. 
Interference of these irregularities with 
the oscillating sea level contributed to the 
jumpy trend of the coastline. This effect 
added to the angle of incidence of waves 
under E-W prevailing winds. The origin 
of irregularities is related to gentle 
folding. 
Topographically-higher areas acted as 
nucleation (root) areas for accumulation 
of barrier island, whereas lower areas 
acted as swales that evolved into lagoons 
(Fig. 6). One of the zones preferred for 
lagoon and washover-fan development is 
located just under Sorbas town. 
Acknowledgements 
CJD and MDP received financial 
support from European Union Project 
EV5V-CT94-0445 and Spanish 
DGICYT ProjectAMB95-1 005-CE. It is 
a part of INQUA Shorelines Commission 
and IOCP Project 367. 
References 
Dabrio, c.J. and Polo, M.D., (1995): 
Geogaceta, 18: 75-78. 
Oautier, F., Clauzon, O., Suc, J.P., 
Cravatte, J. andViolanti, D., (1993): CR. 
Acad. Sci. Paris, 318 (11): 1103-1109. 
Hilgen, F.J., Krijgsman, W., Langereis, 
C.O., Lourens, L.J., Santarelli, A., 
and Zachariasse, W.J., (1995): Earth 
Planet. Sci. Lett., 136: 495-510. 
Mutti, E. and Sonnino, M., (1981 ):Abstract 
Volume, 2nd European Regional Meeting 
oi lnternational Association oi 
Sedimentologists, Bologna: 120-123. 
Roep, T.B. and Beets, D.J., (1977): 
Messinian Seminar 3, Málaga (1977): 
Fieldrip Ouidebook 2: 22-36. 
Roep, T.B., Beets, D.J., Dronkert, H. 
and Pagnier, H., (1979): Sedimento 
Geol., 22: 135-163. 


Late Messinian coastal barrier and washover fan sedimentation in Sorbas (SE Spain)

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Late Messinian coastal barrier and washover fan sedimentation in Sorbas (SE Spain)

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