Ž .Geomorphology 28 1999 1–22

The impact of Quaternary sea-level and climatic change oncoastal alluvial fans in the Cabo de Gata ranges, southeast Spain

Adrian M. Harvey a,), Pablo G. Silva b, Anne E. Mather c, Jose L. Goy d,Martin Stokes e, Cari Zazo f

a Department of Geography, UniÕersity of LiÕerpool, P.O. Box 147, LiÕerpool L69 3BX, UKb Departamento Geologia, UniÕersidad de Salamanca, E.U. Politecnica de AÕila, 05003 AÕila, Spain

c Department of Geographical Sciences, UniÕersity of Plymouth, Drake Circus, Plymouth PL4 8AA, UKd Departamento Geologia, UniÕersidad de Salamanca, Fac. Ciencias, 37008 Salamanca, Spain

e Department of Geological Sciences, UniÕersity of Plymouth, Drake Circus, Plymouth PL4 8AA, UKf ( )Departamento Geologia, Museo Nacionale CC. Naturales CSIC , CrJose Gutierrez Abascal, 2.28006 Madrid, Spain

Received 10 February 1998; received in revised form 16 September 1998; accepted 23 September 1998

Abstract

Conventionally, a fall in base level is seen as stimulating incision into the distal zones of alluvial fans. In the Cabo deGata ranges of southeast Spain evidence exists to the contrary. Two sets of Quaternary coastal alluvial fans demonstrate theinteraction between climatically-driven variations in the supply of sediment and eustatically-driven changes in base level.The fans are supplied from Miocene volcanic terrain within which no evidence can be found for major tectonic deformationduring the period of fan development. The evolution of the east-coast fans has been affected by variations in sediment supplyand changes in sea level. The west-coast fans were buffered from the effects of changes in sea level by coastal barriers.Three phases of past sedimentation can be identified on the fans. These can be differentiated on the basis of field

Ž .observations of soil profiles particularly colour of the B horizons and accumulation of CaCO , and laboratory analyses of3Ž .sequential iron oxide extractions and magnetic mineral properties. The two earlier major sedimentation phases were

Žcoincident with global glacials )ca. 135 ka and ca. 85–10 ka, based on the stratigraphy and uraniumrthorium dating of.the coastal sediments . High sea levels during the intervening interglacial and during the Holocene caused erosion of the

distal zones of the east-coast fans which led to channel incision into the fan surfaces. On the west-coast fans no such incisionŽ .occurred, simply proximal incision by small fanhead trenches. The youngest relatively minor phase of fan sedimentation

has occurred during the Holocene. These contrasting contexts have produced differing styles of fans, with telescopic fanmorphology on the east-coast and stacked morphology on the west-coast fans. The differences are reflected in the fanprofiles, with steeper gradients dominating the east-coast fans, and extensive lower gradient distal surfaces on the west-coastfans. Fan morphometry, based on analysis of the residuals from drainage area to fan area and gradient regressions, alsodifferentiates between the fan contexts. The fan building phases appear to be controlled proximally by climatically-drivenpulses of sediment supplied to the fans. These occurred during global glacials coincident with low sea levels, and caused fanprogradation onto the exposed foreshore. The intervening global interglacials were times of little fan sedimentation, and on

) Corresponding author. E-mail: amharvey@liverpool.ac.uk

0169-555Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0169-555X 98 00100-7

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the east coast, where high sea levels were able to erode the fan toes, deep through-fan dissection ensued. q 1999 ElsevierScience B.V. All rights reserved.

Keywords: alluvial fans; base level; mineral magnetics; southeast Spain

1. Introduction

Debate occurs over the relative roles of tectonics,base level, and climate in influencing the evolution

Žof alluvial fans Frostick and Reid, 1989; Blair and.McPherson, 1994; Harvey, 1997 . Tectonics may be

responsible for creating the gross relief conducive tofan formation, and for continuing uplift of mountainsource areas, thus sustaining high rates of sedimentsupply. Tectonics may also influence fan gradientsand cause changes in base level, which may lead toincision into fan surfaces. Base-level changes, inaddition to those associated with tectonics, may berelated to an axial drainage to which the fan systemis tributary, or for fans adjacent to coastal areas, tochanges in sea or lake levels. Conventionally, theassumption is that low base levels cause dissectionof the distal zones of alluvial fans.

Climatic factors influence the supply of water andsediment from mountain source areas. Whether fansaggrade or are dissected is controlled by whetherstream power of the channel feeding the fan exceedsthe critical power, ie. that required to transport the

Ž .sediment supplied Bull, 1979 , which depends onthe volumes and rates of water and sediment sup-

Ž .plied Harvey, 1997 . Climatic change can causechanges in these rates, therefore changes in fandynamics. Fan aggradation during the Quaternary, inmany dry regions has been interpreted as a responseto increases in sediment supply during colder or

Ž .wetter climatic conditions Bull, 1991; Dorn, 1994 .The purpose of this paper is to examine how the

interaction between climatically-driven changes ofsediment supply and eustatically-induced changes ofbase level influence the geomorphology of alluvialfans, using evidence from Quaternary coastal alluvialfans in the Cabo de Gata ranges of southeast Spain.The geomorphic characteristics of the fans are de-scribed first, then, using evidence provided by soildevelopment, the evolution during the late Quater-

nary is established and related to the sequences ofclimatic and base-level change.

2. The Cabo De Gata fans

Ž .The Cabo de Gata peninsula Fig. 1 comprisesŽ .Upper Miocene Tortonian andesitic–dacitic vol-

Ž .canic rocks Weijermars, 1991 . Three groups offans occur on the peninsula, on the east- and west-coasts, and inland. Those on the east coast slopedirectly to the Mediterranean Sea; those on the westcoast are buffered from the sea by the Salinas la-goon, and the inland fans have no relationship to

Ž .marine base levels Fig. 1 .Tectonic uplift since the Miocene created the

gross relief of the Cabo de Gata ranges. Over theperiod of fan formation, during the latter part of theQuaternary, tectonic deformation has been small.

Ž .Since Tyrhennian II time Isotope stage 5 , littlelateral movement has occurred on the NE–SW Car-boneras fault, the major strike–slip fault whichbounds the inland margin of the peninsula severalkilometres to the NW of the Cabo de Gata rangesŽ .Bousquet, 1979; Weijermars, 1991 , merely a few

Ž .metres generally less than 3 m of local verticaldisplacement. Tyrhennian II coastal sediments haveundergone only minor deformation, less than in the

Žarea to the north of the Carboneras fault Goy and.Zazo, 1986; Goy et al., 1986a; Zazo et al., 1993 .

Unlike fan sequences elsewhere in southeast SpainŽ .Somoza et al., 1989; Silva et al., 1992 , the depositsof the Cabo de Gata fans themselves show no sign oftectonic disturbance.

Previous work in other parts of southeast SpainŽ .Harvey, 1984, 1987a, 1990 has suggested that cli-matic changes caused major switches in fan dynam-ics between aggradational and dissectional regimes,with the main aggradational phases occurring duringPleistocene global glacials. The modern climate issemi-arid Mediterranean, with mild winters, hot

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Fig. 1. Location map, showing fans of the Cabo de Gata ranges; Study fans—L: La Isleta, C: Cala Carbon, S: Salina, M: Michelin.

( )A.M. HarÕey et al.rGeomorphology 28 1999 1–224

summers, and mean annual precipitation of ca. 200mm, much of which falls as high intensity storms in

Ž .autumn Neumann, 1960; Geiger, 1970 . Pleistoceneclimates are thought also to have been dry, with the

Ž . Ž . Ž .Fig. 2. Photographs illustrating general fan morphology: A Cala Carbon east coast , looking upfan; note the deep through-trench: BŽ .Salina fan west coast , looking downfan: note distal surfaces toe out into lagoon. Qf1, Qf3 and Qf5 fan surface age groups, Qc1 dissected

Ž .colluvial hillslope grading to Qf1 stage : see text.

( )A.M. HarÕey et al.rGeomorphology 28 1999 1–22 5

Žglobal glacials too cold for tree growth Amor and.Florschutz, 1964; Zazo et al., 1996 , but with intense

Žseasonal storms Butzer, 1964; Sabelberg, 1977;.Rhodenburg and Sabelberg, 1980 .

Base level to the east-coast fans is provided bythe level of the Mediterranean Sea, with base-levelchange following Quaternary sea-level change. Highsea levels occurred during Tyrhennian II time and

Ž .again during the Holocene Isotope stages 5 and 1 ,with low sea levels occurring during global glacialsprior to Tyrhennian II and between Tyrhennian II

Žand the Holocene Overjero and Zazo, 1971; Zazo et.al., 1981; Goy and Zazo, 1986 . Currently, some of

the east-coast fans are eroded at the base by coastalcliffs, below which there are fragments of Quater-nary coastal sediments, including what appear to beTyrhennian cemented raised beach sediments. Thewest-coast fans are buffered from the sea by theSalinas lagoon, which is bounded by a barrier beachcomplex. This feature includes Tyrhennian II beach

Ž .sediments Zazo, unpublished data , implying thatthese fans were similarly buffered at that time. Theinland fans are not connected to Mediterranean Seabase levels. Base-level changes, therefore, have hada differential impact between the three fan groups.

The gross morphology reflects these differences.ŽMost of the east-coast fans are telescopic fans Bow-

.man, 1978 with younger segments set into oldersegments. The toes are eroded by coastal cliffs and

Žthe fans are dissected by through-fan trenches Fig..2A . Most of the west-coast fans are stacked fans,

Ž .with extensive younger distal surfaces Fig. 2B . Theproximal zones show minor fanhead trenching. Theinland fans resemble the west-coast fans, but manyare confined rather than mountain-front fans.

3. Methodology

The location and extent of 20 discrete fans withinŽ .the Cabo de Gata ranges have been mapped Fig. 1

from air photographs onto basemaps at a scale of1:25,000. Morphometric properties of the fans andcatchment areas have been derived from these maps.

Four fans, two each on the east- and west-coasts,were selected for detailed study. Each was mappedin the field, and axial fan surface and channel pro-files surveyed. Within these fans, three age-relatedgroups of sedimentary units have been recognised on

the basis of morphostratigraphic relationships. Ex-posed sections have been examined and the con-stituent sediments described. Exposures of coastalsediments have also been examined, and the strati-graphic relationships to the fan deposits assessed, ashave the relationships to other neighbouring coastalsediments for which UrTh and 14C dates are known.

A preliminary investigation of soil developmenton the three groups of sedimentary units has beencarried out to assess the extent to which this mightcorroborate the morphostratigraphic evidence for theage-related grouping of the fan surfaces. Field obser-vations were made of the characteristics of soilprofiles, including depth, B horizon colour, andCaCO accumulation. Soil samples were collected3

from B horizons at 26 sites for laboratory analysis ofiron oxide chemistry and mineral magnetic proper-ties. In the laboratory, homogenised sub-samples ofca. 20 g of soil, were taken from the bulk samples.The wet sequential extraction method was used todetermine the content of pedogenic iron oxides. The

Žpyrophosphate extraction procedure McKeague,.1967 was used to determine the organically com-

Ž .plexed Fe iron, the oxalate extraction procedurepŽ .McKeague and Day, 1966 to determine the

Ž .paracrystalline iron oxides Fe , and the dithioniteoŽcitrate–bicarbonate method Mehra and Jackson,

.1960 to determine the total secondary or free ironŽ .oxides Fe . Mineral magnetic measurements car-d

ried out included low frequency magnetic suscepti-Ž .bility x , frequency dependent susceptibilitylf

Ž .Fd% , saturated isothermal remanent magnetisationŽ . Ž .SIRM at 1 tesla T , anhysteretic remanent mag-

Ž .netisation x , high isothermal remanent magneti-armŽ .sation HIRM , and the application of forward fields

Ž . Žto give Soft IRMq20 mT and Hard IRMq300. ŽmT values, and backfields y20, y30, y40, y50,

.y100 and y300 mT , normalised with SIRM, fol-lowing the standard procedures outlined by Thomp-

Ž .son and Oldfield 1986 . From these primary data,ratios between parameters were derived, again ac-cording to the procedures outlined by Thompson and

Ž .Oldfield 1986 .

4. Fan morphometry

Morphometric data for 20 fans and catchmentareas have been derived. In addition to subdivision

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into east- and west-coast and inland fans, they can befurther subdivided on the basis of gross fan morphol-

Žogy into dissected fans dissected in relation to coastal.erosion or axial drainage incision , distally aggrading

fans, and fans whose geometry is confined either byŽ .adjacent larger fans or by hillslopes Table 1 .

ŽA conventional morphometric analysis Harvey,.1987b, 1997 has been carried out relating fan area

Ž .and fan gradient to drainage area Fig. 3 , throughthe regression equations:

FspAq , 1Ž .

GsaAyb , 2Ž .

Ž 2 .where A is drainage area, F is fan area km , andG is fan gradient.

For the fan area regression, values of ‘p’ and ‘q’are within the normal range of values found for otherareas and very similar to those found for a large

Žsample of fans from throughout southeast Spain forŽ ..discussion, see the work of Harvey 1997 .

For the fan gradient regression, though the valueof ‘a’ is within the normal range, that for ‘b’ isrelatively high. In comparison with similar studies offans from a range of geographical areas supplied by

Ž .a variety of catchment geologies Harvey, 1992 , thegradient characteristics of the Cabo de Gata fansresemble those of the Methena fans in Greece, fansalso supplied by daciterandesite volcanics. Thismight relate to the weathering characteristics of theselithologies. The fans in both areas are supplied byblocky debris flows from the hillslopes, from whichfluvial channel and sheetflood processes winnowfiner sediments. This might produce steep gradientson proximal fan surfaces or on small fans, andmarkedly less steep gradients on distal surfaces espe-cially on larger fans.

These regression relationships adequately definethe overall response of fan morphometry to drainage

Ž .area. Unlike in some other areas Harvey, 1992 , no

Table 1Topographic characteristics of the 20 Cabo de Gata fans

Dissected Aggrading Confined Total

East-coast 5 2 0 7West-coast 1 3 2 6Inland 0 3 4 7Total 6 8 6 20

Fig. 3. Morphometric relationships between drainage area and fanarea and gradient. Solid symbols relate to dissected fans, largeopen symbols to aggrading fans, small open symbols to confinedfans.

significant improvement occurs to either relationshipwhen multiple regression is used, taking into accountdrainage basin relief or slope characteristics as wellas drainage area.

Apart from the tendency for fan gradients to berelatively steep on the dissected fans, the regression

Ž .plots Fig. 3 do not suggest markedly different fanmorphometries in relation to geographical fan groupor to gross fan morphology. We had previouslyfound, however, that analysis of the residuals fromregressions highlighted the tectonic or topographic

Ž .context of fan groups Silva et al., 1992 . A similaranalysis performed here for the Cabo de Gata fans

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Fig. 4. Residuals from regression analyses shown on Fig. 3. D, Aand C are mean plotting positions of dissected fans, aggrading

Ž .fans, and confined fans, respectively other symbols as in Fig. 3 .

Ž .Fig. 4 suggests some differentiation between thethree fan styles, rather than between the three geo-graphical groups. The dissecting fans tend to besteeper in relation to drainage area, the aggradingfans tend to be larger, and the confined fans tend tobe smaller in relation to drainage areas. The tele-scopic dissected fans are not relatively large, becausecoastal erosion of the fan toes has reduced the areasof the fans.

5. Fan sequences

5.1. Study fans: surface differentiation

ŽFour fans were selected for detailed study see.Fig. 1 for locations , two dissected fans on the east

Ž .coast La Isleta and Cala Carbon , and two aggradingŽ .fans on the west coast Salina and Michelin . Each

fan was mapped in detail in the field, and thecomponent surfaces differentiated on the basis of

Ž .morphostratigraphic relationships Fig. 5 . In addi-tion to the active channels or modern depositionallobes, three main age-related groups of fan segments

Žcould be recognised, which were designated oldest.to youngest Qf1, Qf3 and Qf5.

This differentiation is substantiated by the degreeŽ .of soil development see later and Table 3 character-

istic of each surface. Qf1 surfaces show matureŽcalcic to petrocalcic rhodexeralf Soil Survey Staff,

. Ž1997 soil profile development, of red 2.5–5YR.Munsell hues argillic Bt horizons over Bk or K

horizons, with pedogenic carbonate developmentŽgenerally of stages III–IV terminology after the

Ž .work of Gile et al. 1966 , modified by MachetteŽ ..1985 . In many places where the overlying soil hasbeen stripped, Qf1 surfaces are capped by massiveindurated pedogenic calcretes, similar to those de-

Ž .scribed by Blumel 1986 . Soils on Qf3 surfacesgenerally show less mature development of the pro-

Ž .file, with less red B horizon hues 5–7.5YR andpedogenic carbonate of stages I–II. Stripped Qf3surfaces show much weaker calcrete developmentthan do Qf1 surfaces. Qf5 surfaces generally showlittle soil profile development, with soil colour simi-

Žlar to that of the parent material generally 7.5YR.hues , and little or no pedogenic carbonate accumula-

tion.The east-coast fans, La Isleta and Cala Carbon

Ž .Fig. 5a,b , have extensive Qf1 surfaces, within whichQf3 surfaces are inset. Distal Qf3 surfaces havesubsequently been removed by marine erosion. Qf5surfaces on these fans are restricted entirely to ter-races within the fan trenches.

ŽThe west-coast fans, Salina and Michelin Fig..5c,d , have very limited Qf1 surfaces, on Salina fan

restricted to colluvial surfaces above the fan itself,and on Michelin fan restricted to small segmentsnear the fan apex. The dominant proximal surfaceson these fans are Qf3. These are cut by shallowfanhead trenches which feed into Qf5 distal surfaces.

5.2. Profile relationships

Detailed profiles have been surveyed on each fanŽto characterise fan surface and channel profiles Fig.

.6 . The deep through-trenched profiles of the east-Ž .coast fans La Isleta and Cala Carbon contrast with

Žthe simple profiles of the west-coast fans Salina and.Michelin , where trenching is restricted to shallow

fanhead trenches. Mean gradients calculated forproximal, midfan, and distal zones highlight the

Ž .differences between the fans Table 2 .The overall fan profile of La Isleta is concave, as

might be expected from a fan set back into themountain catchment. The channel profile shows a

( )A.M. HarÕey et al.rGeomorphology 28 1999 1–228

Ž . Ž . Ž . Ž .Fig. 5. Maps of the sample fans, based on field survey: a La Isleta; b Cala Carbon; c Salina; d Michelin. Circled locations relate toŽ . Ž .surveyed profiles see Fig. 6 ; Qf1, Qf3 and Qf5 fan surface age groups, Qc1 dissected colluvial hillslope grading to Qf1 stage : see text.

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Ž .Fig. 6. Surveyed profiles of the sample fans bold numbers relate to locations on Fig. 5 .

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Table 2Characteristic fan surface and channel gradients

Proximal Midfan Distal

La IsletaQf1 surfaces 0.082 0.050 0.046Channel 0.057–0.045 0.054 0.029

Cala CarbonŽ . Ž .Qf1 surfaces 0.047 0.060 s 0.045 n

Channel 0.051 0.040 0.060

SalinaQf3 surfaces 0.070 0.080 –Channel 0.060 0.074 –Qf5 surfaces – 0.080 0.067

MichelinQf3 surfaces 0.029 0.032 –Channel 0.032 0.028 –Qf5 surfaces – 0.030 0.024

Ž .s Midfan surface profile, southern segment.Ž .n Distal fan surface profile, northern segment.

Ž .zone of increased gradient in midfan Table 2 . CalaCarbon channel shows a distal gradient increase,towards modern sea levels. Both channel profilesappear to have been influenced by base-level changes.Both west-coast fans have concave midfan to distalfan profiles and midfan channel gradients less than

Ž .the fan surface gradients Table 2 , as might beexpected from proximally trenched, distally prograd-ing fans. Both west-coast fans show downfan in-creases in fan gradient away from the restricted apex

Žzones towards more open midfan environments Ta-.ble 2 .

5.3. Sediment sequences

The constituent sediments have been examined atexposures on all four fans, though the degree ofexposure is limited on the west-coast fans. Qf1sediments are clearly exposed only in the east-coastfans, where they form the main fan surfaces. At afew proximal sites on La Isleta fan the sedimentscomprise coarse cobble gravels whose propertiesŽe.g., matrix support, poor sorting, random clast

.fabrics indicate deposition by debris-flows. Else-Žwhere the properties clast support, localised scour

.surfaces, bedding, better sorting, imbricated fabricsindicate deposition by fluvial processes in sheet and

channel bodies. In midfan on La Isleta fan, somesections show multiple incision and aggradation

Žphases, all within Qf1 labelled as Qf1a–c on Fig..7A . Each depositional phase is separated by an

erosional horizon forming a trench into the oldersediments. These erosional phases appear to repre-sent major time gaps, because the top of each unit isstrongly cemented with CaCO . Some proximal sec-3

tions show interdigitation of laterally supplied debrisflows and axial fluvial gravels. Distal sections on LaIsleta fan show dominantly sheet gravels with localpalaeosols. Qf1 sediments on Cala Carbon fan showdomination by channelised fluvial flows, with evi-dence for deposition by braided flows includingchannel scours and bar forms. Throughout, the Qf1surfaces on the east-coast fans are capped by mature

Ž .soil profiles see below or strong calcretes.Qf3 sediments on the east-coast fans are either set

into or overlie Qf1 sediments. Except for hillslopeŽ .debris flows Fig. 7B on the margins of the fans,

they are dominantly fluvial gravels, showing scoursurfaces, planar and cross bedding. Sorting tends tobe better than in Qf1 sediments and, especially onCala Carbon fan, sediment sizes tend to be smaller.

On the west-coast fans the main fan surfaces areformed of Qf3 deposits. The limited sections suggestdeposition by matrix-rich fluid flows, in sheets and

Ž .shallow channels Fig. 7C . In the apex zone ofMichelin fan, at ca. 2 m below the Qf3 fan surface,exposed in the base of the fanhead trench, are exten-sive calcretes. These may be groundwater calcretesrelated to groundwater movement from the adjacenthillslope, or pedogenic calcretes formed near the topof an older fan surface, pre-dating the main Qf3sediments, perhaps at the top of now buried Qf1deposits. Qf3 surfaces show soils, with less horizondevelopment, and weaker colours than those on theQf1 surfaces.

Ž .Qf5 sediments form low inset terraces Fig. 7Dwithin the fan trenches of the east-coast and Salinafans, and are well exposed in channel sections. Theyare dominantly of fluvial sands and fine gravels. Onthe west-coast fans they also form the distal fansurfaces, but have little exposure. Soil developmenton these terraces and distal fan surfaces is minimalŽ .see later .

In addition to the fan sediments, other importantQuaternary sediments of the Cabo de Gata include

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Ž .Fig. 7. Photographs of fan sediments: A Qf1 sediments in La Isleta fan, a, b and c relate to successive phases of Qf1 deposition, erosionalŽ . Ž .horizons arrowed, person for scale; B Qf3 debris flows overlying palaeosol P formed on Qf1 sediments, La Isleta fan, note left to right

Ž . Ž .shear fabrics in Qf3 deposits, hammer—30 cm for scale; C Qf3 sheet gravels, Salina fan, notebook—20 cm for scale; D Qf5 terraceŽ . Ž . Žsediments, La Isleta fan, level—12 cm for scale; E dunes at Los Escullos, v: Tortonian volcanics ashes , Qs: Quaternary soil unknown

. Ž . Ž .age, post-Qf1?, pre-dune formed in Quaternary debris flows, buried by Qe Quaternary dunes, La Isleta fan L in background.

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Ž .Fig. 7 continued .

Quaternary coastal sediments. Tyrrhenian II beachsediments, relating to the last global interglacialsea-level high stand have been identified severalkilometres to the west of Cabo de Gata, and dated byUrTh dates to the Tyrhennian high stands between

Žca. 135 and 90 ka Goy et al., 1986a,b; Hillaire-.Marcel et al., 1986 . A continuation of the same

suite of deposits forms the core of the barrier beachcomplex immediately west of Cabo de Gata, which

Žhas also been UrTh dated to the Tyrrhenian Zazo,

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Ž .Fig. 7 continued .

.unpublished data . This indicates that a barrierbeachrlagoon complex existed during the Tyrrhe-nian, buffering the west-coast fans, as the modern

Žfeature does now. Other isolated patches of Tyrr-.henian? cemented beach conglomerates occur in

places around the peninsula, one in a cove to thenorth of Cala Carbon. Mid-Holocene beach sedi-

Ž .ments dated by radiocarbon; Zazo, unpublished dataform most of the modern barrier beach, suggesting

Ž .its presence throughout the modern Holocene highstand.

ŽLate Pleistocene dunes assigned to the lateTyrrhenian on the basis of dated sediment sequences

Ž ..to the north of the study area Goy et al., 1986a ,occur in a number of localities on the Cabo de Gatacoast, notably at Los Escullos, just south of La Isleta

Ž .fan. These dunes Fig. 7E rest on an erosionalsurface cut into the underlying Tortonian volcanics,which also truncates Qf1 fan deposits. Locally thedunes bury thin terrestrial debris flows within whicha soil had formed before burial. These deposits mayrepresent a local minor phase of fan activity, perhapspost-Qf1 but prior to the dune phase. The dunesediments extend seaward to below modern sea lev-

els, and inland are buried by Qf3 and Qf5 deposits.The palaeodune surfaces preserve excellent root casts,indicating vegetational stabilisation after deposition.

6. Evidence from soil properties

Previous studies have demonstrated that character-istics of soils can be used to correlate alluvial deposi-

Ž .tional surfaces Birkeland, 1985; Weide, 1985 , andŽto indicate at least relative ages Harden, 1982;

.Harden and Taylor, 1983 . In studies of other dry-Ž .land areas e.g., McFadden and Weldon, 1987 in-

Žcluding Mediterranean regions, e.g., Torrent, 1976;.Torrent et al., 1980 , soil chronosequences, based on

field characteristics, demonstrate increasing rubifica-Ž . Ž .tion of often argillic B horizons Hurst, 1977 , and

the accumulation of pedogenic carbonate horizonsŽ .Gile et al., 1966; Machette, 1985 . Field character-istics can be corroborated by laboratory evidence,particularly from the sequential extractions of soil

Ž .iron oxides Alexander, 1974; Arduino et al., 1986 ,Žand magnetic mineral analysis Thompson and Old-

.field, 1986; White and Walden, 1994, 1997 .

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A previous study of river terraces in a neighbour-Ž .ing part of southeast Spain Harvey et al., 1995

demonstrated an increasing development of the soilprofile with increasing terrace age, and involvedchanges in B horizon soil colour, iron oxide compo-sition, mineral magnetic properties, and subjacent

ŽCaCO accumulation. White and Walden 1994,3.1997 also demonstrated age-related changes in min-

eral magnetic properties of soils on alluvial fans innorth Africa that were related to the formation ofsecondary iron oxides.

6.1. Field eÕidence

On the Cabo de Gata fans a preliminary investiga-tion of soils has been carried out. In the absence ofrecognisable rock pavements on the fan surfaces,because of the long history of human occupation, itwas not possible to use a multiparameter approach

Ž .similar to that used by McFadden et al. 1989 . Ourevidence is limited to the soils themselves. Soilswere examined in the field at exposed sections, andB horizons were sampled at 26 sites for laboratoryanalyses of iron oxide and mineral magnetic proper-

Ž .ties Table 3 . To obtain representation within thedata set across all four fans and all three age-relatedsurface groups, all units were sampled on all fans.On the west-coast fans only small remnants wereinterpreted in the field as Qf1 surfaces. One samplewas taken from the small remnant of what had beeninterpreted as Qf1 adjacent to the apex of Michelinfan and two were taken from the lower part of whatwas interpreted as a Qf1-age colluvial slope aboveSalina fan.

The field evidence suggested increasing profiledevelopment with age, from little sign of pedogene-

Žsis on Qf5 surfaces to mature profiles generally )1. Ž .m in depth on Qf1 surfaces see above . At the 26

Table 3Number of sites sampled for laboratory analysis of soils

Qf1 Qf3 Qf5 Total

La Isleta 2 3 2 7Cala Carbon 3 2 2 7Salina 2 2 1 5Michelin 1 4 2 7Total 8 11 7 26

sampled sites B horizon colour was described usingŽMunsell soil colours interpolating between hues

.where appropriate . A redness index was calculatedfor each soil colour, based on the redness rating of

Ž .Hurst 1977 , where:

redness ratingshue=chromarvalue 3Ž .

using numerical values of 10 for 10R hues down tonumerical values of 2.5 for 7.5YR hues. In thisstudy, as in the river terrace study referred to aboveŽ .Harvey et al., 1995 , a numerical value of 1 wasgiven to 10YR hues rather than the 0 of the HurstŽ .1977 scheme, to increase the sensitivity at thelower end of the scale. Soil carbonate accumulationstage was classified in the field according to the Gile

Ž .et al. 1966 classification scheme modified by Ma-Ž .chette 1985 .

6.2. Laboratory eÕidence

The results of the field observations and the se-quential iron extractions for the 26 samples aresummarised in Table 4, by individual fan and byage-related groups of the fan surfaces. This was doneto ascertain whether minor differences in catchmentandesiterdacite geology might influence age-relateddifferences. CaCO accumulation stages, Munsell3

colours, redness ratings, and Fe values show nodŽ .differences between fans Table 4 . Values for Fep

and Fe were all very low and show no differenceso

either between fans or between age-related groups ofthe fan surfaces. Major differences occur in carbon-ate stage, Munsell colour, redness rating, and Fed

values between the age-related groups. Student’s t-tests show that the differences in redness ratings andFe values between Qf1 and Qf5 surface groups ared

statistically significant at the 0.01 level, and thosebetween Qf1 and Qf3 and between Qf3 and Qf5 aresignificant to at least the 0.05 level.

This demonstrates the age-related nature of thesecharacteristics of the soils. Furthermore, the dataconfirm the field interpretation of the age-relatedgroups of fan surfaces, and particularly that the mainsurfaces of the west-coast fans are of Qf3 age,whereas those on the east coast are Qf1. In the fewcases where sampling had to be undertaken at de-graded sites, the allocation to age-group based on the

( )A.M. HarÕey et al.rGeomorphology 28 1999 1–22 15

Table 4Mean soil properties, grouped by individual fan and by age-related groups of fan surfaces

n Munsell colour range Redness rating Fe % CaCO staged 3

Range Mean SD Range Mean SD

Grouped by indiÕidual fansLa Isleta 7 2.5–7.5YRr2.5–4r2–6 1.7–15.0 6.2 3.2 0.75–2.30 1.50 0.57 0–IIICala Carbon 7 2.5–7.5YRr4r3–6 2.5–11.3 6.0 3.4 0.61–2.08 1.09 0.55 0–IVSalina 5 2.5–8.5YRr3–4r3–6 1.3–15.0 6.7 5.8 0.72–2.18 1.31 0.53 0–IIIMichelin 7 3–7.5YRr3–4r3–6 2.5–12.0 6.7 3.2 1.12–1.72 1.34 0.21 0–III

Grouped by fan surface age-groupQf1 8 2.5–5YRr2.5–4r3–6 6.0–15.0 10.9 3.6 0.98–2.30 1.74 0.54 II–IVQf3 11 4–7.5YRr3–4r2–6 1.7–8.5 5.3 2.6 0.61–1.72 1.22 0.30 I–IIQf5 7 5–8.5YRr3–4r3–4 1.3–3.8 2.8 0.9 0.72–1.33 0.96 0.28 0

morphological evidence, was confirmed by at leastŽtwo of the three characteristics of the soil colour,

.Fe , CaCO stage .d 3

The mineral magnetic data are summarised in thesame way, by individual fan and by age-relatedgroup of the fan surfaces. One-way analyses ofvariance was applied to both data sets, in an attemptto identify which mineral magnetic variables pickout differences between fans, irrespective of the ageof the fan surface, and which pick out age-related

Ždifferences. The data relating to individual fans Ta-.ble 5 highlight which mineral magnetic properties

might reflect differences in geochemistry of the vol-canic rocks between the feeder catchments. The sig-

nificant differences observed for x , x , HIRM,lf arm

Hard%, the backfield measures, and to a lesser ex-tent SIRM, might reflect differences in the abun-dance of magnetic minerals or the magnetic proper-ties of the minerals being weathered from thebedrock. Interestingly, La Isleta, which is suppliedfrom dacite-dominated rather than an andesite-dominated source rocks, often stands out as differentfrom the other fans. Cala Carbon also stands out asdifferent in some cases, despite its similar andesitedominance to the Salina and Michelin source areas.

Ž .Three measures Fd%, Soft, SIRM show highlyŽstatistically significant age-related differences Table

. Ž .6 . In two cases Fd%, Soft these are in the absence

Table 5Ž .Summary of soil mineral magnetic properties, grouped by individual fans; means standard deviations in brackets

Ž . Ž . Ž . Ž .La Isleta ns7 Cala Carbon ns7 Salina ns5 Michelin ns7 Significant differencesUUŽ . Ž . Ž . Ž . w xx 221 61.1 284 47.4 191 42.8 204 32.4 C–LMSlf

Ž . Ž . Ž . Ž .Fd% 7.40 2.86 4.54 2.21 6.04 2.74 5.93 0.62UŽ . Ž . Ž . Ž . w xSIRM 2308 1015 1232 205 1543 528 2015 498 L–CUUŽ . Ž . Ž . Ž . w xx 926 215 494 267 475 60 650 202 L–SCarm

Ž . Ž . Ž . Ž .x rSIRM 0.484 0.228 0.400 0.199 0.346 0.142 0.317 0.048armUUŽ . Ž . Ž . Ž . w xw xSIRMrx 10.68 3.57 4.32 0.27 7.85 1.31 9.63 0.79 C–SML S–LlfUUŽ . Ž . Ž . Ž . w xw xx rx 4.46 1.33 1.77 0.99 2.56 0.48 3.01 0.55 L–MSC M–Carm lf

UUŽ . Ž . Ž . Ž . w xHIRM 157.2 66.0 53.4 11.3 64.4 14.4 79.6 10.1 L–MSCUUŽ . Ž . Ž . Ž . w xHard% 7.44 2.47 4.42 1.37 5.16 2.83 4.16 1.35 L–MSC

Ž . Ž . Ž . Ž .Soft 863 326 672 129 740 155 836 220

w x w x w x w xBackfield data show significant differences between La Isleta L and Cala Carbon C and Michelin M fans, and between Cala Carbon Cw xand Salina S fans throughout the data range; other differences are significant only in part of the data range.

Units: x , x : 10y8 m3 kgy1 ; Fd, Hard: %; SIRM, 10y8 A m2 kgy1 ; HIRM: 10y5 A m2 kgy1 ; Soft: 10y3 A m2 kgy1 .lf armUU

Data include differences significant at the 0.01 level.UData include differences significant at the 0.05 level.

Ž . w x w x w x w xSignificant differences relate to the fan s highlighted: La Isleta L , Cala Carbon C , Salinas S and Michelin M .

( )A.M. HarÕey et al.rGeomorphology 28 1999 1–2216

Table 6ŽSummary of soil mineral magnetic properties, grouped in relation to age-related groups of fan surfaces; means standard deviations in

.brackets

Ž . Ž . Ž .Qf1 group ns8 Qf3 group ns11 Qf5 group ns7

Ž . Ž . Ž .x 211 67.6 219 44.5 260 58.6lfUU Ž . Ž . Ž .Fd% 7.97 2.83 5.44 1.36 4.53 1.59UU Ž . Ž . Ž .SIRM 1244 396 1780 516 2438 882

Ž . Ž . Ž .x 645 248 631 268 688 331armUU Ž . Ž . Ž .x rSIRM 0.551 0.224 0.334 0.078 0.279 0.064arm

Ž . Ž . Ž .SIRMrx 6.21 1.99 8.88 3.14 9.19 3.66lfŽ . Ž . Ž .x rx 3.31 1.59 3.00 1.30 2.58 1.22arm lfŽ . Ž . Ž .HIRM 72.1 21.3 99.4 63.6 97.6 65.5Ž . Ž . Ž .Hard% 6.43 3.07 5.36 1.88 3.94 1.49

UU Ž . Ž . Ž .Soft 614 160 757 131 1009 237

Backfield data show no significant differences between fan age-groups.Units: x , x : 10y8 m3 kgy1 ; Fd, Hard: %; SIRM, 10y8 Am2 kgy1 ; HIRM: 10y5 A m2 kgy1 ; Soft: 10y3 A m2 kgy1 .lf armUU

Differences significant at the 0.01 level.

of any catchment-related differences, and in the thirdŽ .case SIRM the age-related differences are greater

than the inter-catchment differences. Fd% reflectsthe presence of fine grained pedogenic magnetiteŽ .Dearing et al., 1996 , and it shows a significantincrease with increasing age of the fan surface,clearly reflecting pedogenic processes. The reversetrend is shown by the significant decrease in Softvalues, which probably reflects the decrease incoarser grained magnetite as pedogenic transforma-tions take place. SIRM, which relates to the totalmagnetic properties of the ferrimagnetic and cantedantiferromagnetic minerals present, also shows a sig-nificant decrease through time. This is probably re-lated to the weathering of the primary magneticminerals from the volcanic bedrock, a trend which isparalleled by the overall decrease in x . This occurslf

concurrently with a trend of increasing Hard values,which probably reflects increases in haematite con-

Žtent as oxidation takes place Thompson and Old-.field, 1986 . These last two trends involve consistent

directional changes in the data, but with the limitedsample sizes do not produce statistically significantdifferences between the groups.

In summary, this preliminary analysis of the mag-netic mineral properties suggests greater complexitythan in other studies of age-related development of

Žsoils White and Walden, 1994; Harvey et al., 1995;.White and Walden, 1997 , perhaps related to the

magnetic properties of the andesiterdacite parent

materials. Three characteristic trends are apparent.First, the highly magnetic properties of the volcanicparent materials produce differences between the

Žfans related to small differences in geology ex-pressed by x , x , HIRM, Hard and the backfieldlf arm

.measures . Second, age-related change can be identi-fied from decreasing values of some magnetic mea-sures over time, related to the loss of the primary

Ž .minerals through weathering e.g., SIRM, Soft .Third, some measures show increases with time,related to the generation of weathering products or to

Ž .pedogenic processes e.g., Fd%, Hard .

6.3. Temporal implications of the eÕidence from soils

The field- and laboratory-based evidence from thesoils demonstrates progressive development of thesoils. By comparison with previous work some sug-gestions can be made as to the time scales involved.Much of the published work on time scale calibra-tion of soil chronosequences in dry regions relates tothe American West. Although a broad similarityexists between the modern climates of southeastSpain and parts of the American West, it is probablethat Pleistocene climates and soil forming regimes

Ždiffered somewhat between the two regions Harvey.et al., 1995 . At best, American work can provide

only approximate indications of age, when related toSpanish soils.

( )A.M. HarÕey et al.rGeomorphology 28 1999 1–22 17

Qf1 soils, with Munsell 2.5–5YR hues, rednessratings of up to 15, and stage III carbonate accumula-tion are similar to soils on the older river terraces of

Žthe neighbouring Sorbas basin Terraces A, B and C:.Harvey et al., 1995 , which are thought to be older

than at least ca. 150 ka. McFadden and HendricksŽ .1985 , working in California, suggest ca. 700 ka forthe development of redness ratings of 15. BirkelandŽ . Ž .1984 and Machette 1985 suggest at least ca. 100ka for the development of stage III carbonate, forsoils in the dry western USA. These properties ac-cord with a probable age for Qf1 greater than 150 ka.

Qf3 soils are thinner, generally show 5YR hues,with redness ratings below 8.5, and have stage I to IIcarbonate morphology. This is similar to some of the

Žyounger terraces of the Sorbas basin Terrace D:.Harvey et al., 1995 , which are thought to relate

Ž .broadly to the last global glacial ca. 70–10 ka .Qf5 soils are thin, with little horizon develop-

ment, 7.5YR hues and redness ratings below 5. Nocarbonate accumulation was observed. These soilsare similar to those on the youngest Sorbas basin

Ž .terraces Terrace E: Harvey et al., 1995 , which haveŽbeen radiocarbon dated to the Holocene Harvey and

. Ž .Wells, 1987 . McFadden and Hendricks 1985 , sug-gest ages of less than 13,000 years for rednessratings less than 1. Given that the parent materials ofthe Cabo de Gata soils have reddish hues in the7.5YR range and associated redness ratings up to 5,this is probably equivalent to normal redness ratingsless than 1.

7. Discussion

7.1. Summary sequence

Putting together the evidence from the four sam-ple fans, including the morphology and the sedi-ments of the fans, coastal sediments, and soils devel-oped on the fan surfaces, the overall geomorphicsequences of east- and west-coast fans can be out-

Ž .lined Fig. 8 .

7.1.1. Qf1 fan depositionMultiple phases of deposition occurred on the

fans, with intervening periods of fanhead trenchingevident on La Isleta fan. These deposits form the

main surfaces of the east-coast fans. Deposition wasat least in part coincident with low sea levels, andthe east-coast fans prograded beyond the presentcoastline. Only small remnants of these sedimentsare present in the west-coast fan catchments; lowerdownfan they appear to have been buried by laterdeposits. Qf1 deposition ceased prior to the high sea

Žlevels of the last interglacial Tyrrhenian II, Isotope.stage 5, ca. 135 ka . Sedimentation was probably

climatically-driven, related to high sediment produc-Žtion during the preceding global glacial Isotope

.stage 6 .

7.1.2. Dissection of Qf1 depositsThe high sea level during Tyrrhenian II times

Ž .Isotope stage 5, 135–110 ka BP caused coastalerosion of the distal zones of the east-coast fans,resulting in foreshortened profiles and distal incision.Tyrrhenian II beach sediments were deposited inisolated locations on the east coast and formed abarrier beach complex on the west coast. The west-coast fans, buffered by the barrier beach and lagooncomplex, experienced no distal erosion. In both areasfanhead trenching occurred, in response to reducedsediment supply from the mountain catchments. At

Žthe end of Tyrrhenian II time late Isotope stage 5,. Ž .ca. 110–90 ka BP? during falling and oscillating?

sea levels, coastal dunes were deposited at Los Es-cullos, over the erosion surface cut into Qf1 fandeposits, and locally buried fan deposits of unknownage.

7.1.3. Qf3 fan depositionOn the east-coast fans, renewed fan deposition

filled fanhead trenches and prograded seawards ontothe former sea floor. On the west coast this phaseforms the main fan surfaces. Precise dating is uncer-tain, but this aggradation phase post-dates theTyrrhenian II beach deposits and the late Tyrrheniandune deposits, but pre-dates early-mid Holocenecoastal deposits, and is therefore late Pleistocene in

Ž .age Isotope stages 4–2 . It probably resulted fromclimatically-driven increased sediment supply fromthe mountain catchments.

7.1.4. Dissection of Qf3 depositsŽThe early-mid Holocene rise in sea level Isotope

.stage 1, ca. 7 ka BP , caused renewed coastal erosion

()

A.M

.HarÕey

etal.r

Geom

orphology28

19991

–22

18

Fig. 8. Schematic summary sequence of fan aggradation and dissection in response to pulsed sediment supply and sea level fluctuations, illustrating contrasts between Cabo deGata east- and west-coast fans.

( )A.M. HarÕey et al.rGeomorphology 28 1999 1–22 19

of the distal zones of the east-coast fans, againresulting in foreshortened profiles and causing inci-sion into the distal surfaces of these fans. Holocenebeaches were formed and again the barrierbeachrlagoon system protected the west-coast fansfrom toe erosion and distal incision. Again, fanheadtrenching occurred on both sets of fans, especiallythose on the east coast where distal and fanheadtrenches linked.

7.1.5. Qf5 fan depositionMinor phases of aggradation and dissection caused

terrace formation within fanhead trenches, and minoraggradation of distal fan surfaces on the west-coastfans in mid-late Holocene time. These phases may berelated to minor climatic fluctuations or to human-in-duced changes in sediment input.

7.2. Styles of fan deÕelopment and the role of baseleÕel

During the late Quaternary, the Cabo de Gata fanshave switched between two regimes of fan dynamics,

one dominated by aggradation and the other bydissection. The aggradational phases produced verti-cal accretion of the fan surfaces plus distal extensionŽ .Qf1, also Qf3 on the west-coast fans , or trenchfilling in proximal and midfan zones plus distal

Ž .extension Qf3 on the east-coast fans . These phaseswere produced by an excess supply of sediment,

Žcausing critical power to exceed actual power Bull,.1979 . They occurred during global glacials, and

although the precise local climatic conditions areunknown, were probably climatically-driven. Theseperiods also coincided with eustatically low sea lev-els. On the east-coast fans deposition extended sea-ward of the modern coastline. Distal erosion did notoccur because of the large sediment volumes gener-ated and the low offshore gradients. The gradients ofthe modern sea floor off La Isleta and Cala Carbonare only 0.035 and 0.015, respectively, well below

Žthe depositional gradients of the fan surfaces see.Table 2 and below the threshold gradients of ero-

sion. At times of sea-level fall, the sea merely with-drew and allowed the fans to extend seaward, bury-ing the former sea floor.

Ž .Fig. 9. A schematic summary model of the development of the Cabo de Gata fans, distinguishing between east-coast coastal sites, andŽ .west-coast lagoonal sites.

( )A.M. HarÕey et al.rGeomorphology 28 1999 1–2220

During the dissectional phases, decreases in thesupply of sediment causing lowering of the threshold

Ž .of critical power Bull, 1979 allowed erosion toreplace aggradation. Both east- and west-coast fansunderwent fanhead trenching. At the same times,during global interglacials, sea level rose, causingerosion of the toes of the east-coast fans, steepeningof distal profiles, and a wave of distal trenchingwhich subsequently cut back towards midfan loca-tions to intercept the fanhead trenches. On the westcoast, buffered from effects of sea-level change bythe barrier beachrlagoon complex, no such distaltrenching occurred, and the dissectional profiles aresimply of fanhead trenching and minor progradation

Ž .into the lagoon. Holocene aggradation Qf5 repre-sents merely a small scale minor aggradation phase,perhaps related to human activity, within an other-wise dissectional phase.

In this sequence, periods of high sediment supplyŽ .Quaternary global glacials , coincide with periods oflow base levels, and periods of lower sediment sup-

Ž .ply Quaternary global interglacials with high baseŽ .levels Fig. 9 . The contrasting fan morphologies

reflect the interaction between proximal and distalcontrols; between climatically-driven changes in wa-ter and sediment supply to the fans and eustatically-driven changes in base level. The trenched proximalzones on all the fans are evidence of response tosediment controls. The contrasts between east- andwest-coast fans are primarily related to the differen-tial response to base-level change. Dissection of thedistal zones of the east-coast fans was caused, not aswould be conventionally expected by falls in baselevel, but during periods of high base level, bymarine erosion of the fan toes. This caused profileforeshortening and steepening.

8. Conclusions

Three main conclusions may be drawn.Ž .1 Relationships have been identified among tec-

tonic, climatic and base-level factors that influencealluvial fan development in the late Quaternary onthe Cabo de Gata fans. No evidence can be found forthe influence of tectonic factors, other than in creat-ing the gross topography conducive to fan develop-

ment. Major changes in the supply of sediment to thefans occurred in relation to climatic changes, withthe major sediment pulses coincident with globalglacials, and dissection occurring during the inter-vening global interglacials. Base level appears toinfluence the style of fan development, leading tomarked contrasts between the dissected fans on theeast coast and the proximally-trenched but distally-prograding fan morphology of the west-coast fans.

Ž . Ž2 Morphostratigraphic and soil evidence iron.oxide extractions and mineral magnetics confirm

three main groups of fan deposits. Soil character-istics, including profile development, colour of the Bhorizons, redness rating, and carbonate accumulationstages, are in accord with the age-related sequences.Mineral magnetic analyses can be used to differenti-ate between the effects of variations in the volcanicbedrock, the effects of weathering of the primaryminerals and the effects of other pedogenic pro-cesses. The fan sequences can be related stratigraphi-cally to the established late Quaternary coastal se-quence.

Ž .3 Base level is important in fan evolution, butits influence does not accord with the conventionalassociation of dissection with low base levels. Distalfan dissection is associated with toe erosion inducedby high sea levels. These occur at times of limitedsediment input to the fan system, when the fans areunder a dissectional regime, thus enhancing andmodifying spatial patterns of dissection. Low sealevels are associated with fan extension onto theformer sea floor, at times coincident with high sedi-ment input.

Acknowledgements

This research has been partially supported by theBritish CouncilrSpanish Government—Acciones In-tegradas Programme. Diane Spivey helped with someof the field surveying. John Dearing gave help in theinterpretation of the mineral magnetic data. We alsothank the staff of the Graphics section of the Depart-ment of Geography, University of Liverpool forproducing most of the illustrations.

( )A.M. HarÕey et al.rGeomorphology 28 1999 1–22 21

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