Internal structure of a trough blowout, determined from migrated ground-penetrating radar profiles

Internal structure of a trough blowout, determinedfrom migrated ground-penetrating radar pro®les

ADRIAN NEAL and CLIVE L. ROBERTSSchool of Applied Sciences, University of Wolverhampton, Wulfruna Street, Wolverhampton WV1 1SB,UK (E-mail: [email protected])

ABSTRACT

Ground-penetrating radar (GPR) was used to investigate the relationship

between the geomorphological development of a large aeolian trough blowout

and the stratigraphy and internal sedimentary structure of its associated

deposits. Although analogous, many of the data-processing techniques

routinely applied in seismic re¯ection are very rarely applied in GPR

studies. In this study, a simple migration program was used that signi®cantly

enhanced the quality of GPR images from a large trough blowout at Raven

Meols on the Sefton coast, northwest England. These improvements aided

subsequent data interpretation, which was achieved through application of the

principles of radar stratigraphy. GPR shows the pre-blowout dunes to have a

complex internal structure that suggests they were formed in the presence of at

least a partial vegetation cover. Subsequent to stabilization of these dunes a

thin soil developed. This dune soil forms an important radar sequence

boundary and delineates a complex topography beneath the depositional lobe

of the blowout. The internal structure of the depositional lobe of the blowout

does not conform to a model of simple radial foreset deposition, as derived

from contemporary process studies reported in the literature. Instead, the

pattern of deposition has been extensively modi®ed by the antecedent dune

topography and by varying spatial and temporal exposure to important sand-

transporting winds that is partly controlled by interactions between the

regional wind pattern and local dune morphology. Trough blowout deposits in

coastal aeolian sedimentary sequences are likely to be recognized by the

presence of laterally continuous packets of relatively high-angle cross-strata,

which often display a spatially-variable radial dip pattern that is only very

poorly or partially developed. In addition, a soil, or other surface representing a

signi®cant hiatus in dune deposition, is likely to underlie the blowout

deposits, the topography of which will show a clear relationship to the dip

and orientation of the overlying cross-strata.

Keywords Beach, coastal dune, ground-penetrating radar, northwest Eng-land, sedimentary structure, Sefton coast.

INTRODUCTION

Data regarding the stratigraphy and internalsedimentary structure of contemporary aeoliandunes are valuable in helping formulate modelsof dune formation and migration, both at thepresent and in the geological past. Until recently,

obtaining such information has proven extremelyproblematic. Field exposures are generally lim-ited and cutting trenches into unconsolidated,non-cohesive sands is commonly plagued withinsurmountable dif®culties. Consequently, only alimited number of studies have been undertakenin contemporary desert settings (McKee, 1966,

Sedimentology (2001) 48, 791±810

Ó 2001 International Association of Sedimentologists 791

1979; Tsoar, 1982; Rubin & Hunter, 1985) orcoastal settings (McBride & Hayes, 1962; Land,1964; Bigarella et al., 1969; Goldsmith, 1973;Hesp, 1988; Carter & Wilson, 1990; Byrne &McCann, 1993; Ruz & Allard, 1995). Theseinvestigations have generally relied upon oppor-tunistic analysis of ®eld exposures or uponunusual site characteristics that enable trenching.Such an approach, combined with a relativepaucity of data regarding the medium- to long-term morphodynamics of the dunes studied, hasrestricted attempts to relate the internal structureof a dune to its known geomorphological evolu-tion. This is particularly well illustrated in thecase of coastal dune blowouts. Numerous studieshave been carried out with respect to the mor-phodynamics of these features [see reviews inCarter et al. (1990) and Hesp & Hyde (1996)], yetvery little is known about the stratigraphy andinternal sedimentary structure of the deposits thatresult from their development.

During the past decade, the development ofdigital ground-penetrating radar (GPR) systemshas begun to allow the rapid acquisition ofcontinuous, high-resolution data regarding thestratigraphy and internal structure of unconsoli-dated, sand-and-gravel-dominated, sedimentarydeposits. GPR is a non-invasive geophysicaltechnique based on the propagation and re¯ectionof high-frequency electromagnetic (radar) waves.The principles behind GPR are well explained inboth the geological and archaeological literature(Davis & Annan, 1989; Conyers & Goodman, 1997;Reynolds, 1997; Neal & Roberts, 2000). Funda-mental to the successful deployment of GPR inmany sedimentological studies, and in aeolianresearch in particular, has been the fact that insands both sedimentary structures and the watertable can generate signi®cant primary re¯ections(Baker, 1991; van Dam & Schlager, 2000).

Despite the obvious potential of the GPRtechnique, the number of studies undertaken inaeolian settings is limited. However, importantsedimentological information has been obtainedfrom desert dunes (Schenk et al., 1993; Bristowet al., 1996, 2000a; Harari, 1996), coastal dunes(Harari, 1996; van Heteren & van de Plassche,1997; van Heteren et al., 1998; van Overmeeren,1998; Neal & Roberts, 2000; Bristow et al., 2000b)and coversands (stabilized accumulations ofaeolian sand with few well-developed duneforms; van Dam & Schlager, 2000). This has beenachieved despite a lack of advanced data pro-cessing, such as migration. Migration is routinelyapplied to data from the analogous seismic

re¯ection technique in order to remove the effectscaused by the curved nature of the wavefront ofthe radiated energy. These effects include: (1)diffractions generated by point re¯ectors andstrongly curved re¯ectors, obscuring primaryre¯ections; (2) distortions caused by undulatingre¯ectors; and (3) up-dip movement of re¯ectionpoints generated by dipping re¯ectors (Robinson& CËoruh, 1988; Kearey & Brooks, 1991). Conse-quently, re¯ections on migrated seismic or radarpro®les should give a clearer and more realisticpicture of both the form and orientation of there¯ectors that generated them. However, despitethese obvious advantages, migration is not rou-tinely applied to radar data used in sedimen-tological studies.

In light of the above statements, the aims of thispaper are as follows:

1 to present the results of GPR surveys from alarge trough blowout at Raven Meols on theSefton coast, northwest England;

2 to brie¯y compare unmigrated and migratedradar re¯ection pro®les in order to demonstratethe practical advantages of simple migration;

3 to interpret the migrated re¯ection pro®lessedimentologically, aided by ground-truthingfrom auger holes and ®eld exposures;

4 to relate the sedimentological informationobtained from the interpreted re¯ection pro®lesto the geomorphological development of theblowout, as revealed by aerial photographs andother evidence.

THE STUDY SITE

Blowouts can be de®ned as erosional depressionsor hollows, commonly with adjoining sand accu-mulations, formed primarily by aeolian processesacting on pre-existing, at least partially vegetated,windblown deposits or other unconsolidatedsand-dominated sediments (Carter et al., 1990;Gares & Nordstrom, 1995; Hesp & Hyde, 1996). Alarge, active trough blowout is located at RavenMeols in the southwestern part of the Seftonbarrier system, northwest England (Fig. 1). TheSefton dunes comprise a complex sequence ofactive, recently active and relict deposits, extend-ing back in age to the mid-Holocene (Pye, 1990;Neal, 1993; Pye & Neal, 1993; Pye et al., 1995).Both the beachface and dune deposits consist ofpredominantly ®ne-grained and well-sortedquartz-rich sands (Pye, 1991; Neal, 1993; Pye &Neal, 1993). Wind velocity and directional fre-

792 A. Neal and C. L. Roberts

Ó 2001 International Association of Sedimentologists, Sedimentology, 48, 791±810

quency data indicate that winds capable of sandmovement blow predominantly from the north-west, west and southwest (Pye, 1990; Pye & Neal,1994).

The Raven Meols blowout lies 175 m from thecurrent shoreline, being separated from it by aseries of parallel foredune ridges (Fig. 2). Theblowout consists of two trough-shaped depres-sions, the larger, more northerly of these (blowoutA1, the subject of this study) aligned approxi-mately southwest to northeast and the smallersoutherly trough (blowout A2) aligned west-northwest to east-southeast (Fig. 2).

Blowout A1 shows many of the geomorpholog-ical characteristics associated with trough blow-outs (Cooper, 1958; Carter et al., 1990; Hesp &

Hyde, 1996). The feature is approximately 300 mlong, in places over 110 m wide and is closed atits southwestern end (Figs 2 and 3). The blowoutopens to the northeast, with a well-de®ned windgap allowing access to a large depositional lobe.The lobe is oriented approximately west to east.Carter et al. (1990) indicate that a fundamentaldistinction can be made between `open' and`closed' blowouts, with the former having well-developed wind gaps into and/or out of thehollow.

The southern margin of the depositional lobe isa complex mix of vegetated and unvegetatedcomponents, with a progressive change fromvegetated rim dunes in the west to unvegetatedavalanche slopes, beyond a well-de®ned brink, inthe extreme east. By contrast, the northern lee-slope is typically unvegetated. Northern lee-slopeangles are highly variable (c. 5±33°), but mayreach the angle of repose for ®ne sand. Theeastern end of the depositional lobe is currentlyextending into a small stand of pines, burying thetrees (Fig. 2). These were planted between 1905and 1910 (Gresswell, 1953). Avalanche slopes arecommon at this eastern margin.

GROUND-PENETRATINGRADAR SURVEYS

Two GPR surveys were performed using a Sensorsand Software PulseEKKOä 100 GPR system.Initial trials indicated that 100 MHz antennaeprovided the best compromise between resolu-tion and depth of penetration [the `range-resolu-tion trade-off' of Davis & Annan (1989)]. They alsoindicated that individual traces should be stacked64 times and that a time window of 350 ns and astep-size of 0á2 m were most appropriate foreffective imaging. In the ®rst survey (August1997), eight radar re¯ection pro®les were collec-ted to broadly characterize the stratigraphy andinternal structure of the de¯ation plain, ramp anddepositional lobe (Fig. 3). The sidewalls and rimdunes were too steep to allow pro®ling and laterdata migration. A second survey was subse-quently performed (July 2000) in order to providemore detailed, pseudo-three-dimensional, infor-mation regarding the stratigraphy and internalstructure of the depositional lobe. This surveyconsisted of 21 intersecting transect lines (A±U,Fig. 3).

During Survey 1, common mid-point (CMP)pro®ling was carried out at two locations (CMP1and CMP2, Fig. 3) in order to estimate radar wave

Fig. 1. Location of Raven Meols blowout on the Seftoncoast, northwest England. The sur®cial geology of theSefton coastal area is also shown. Modi®ed from Pyeet al. (1995).

Internal structure of a trough blowout 793


velocities in the subsurface. Saturated and unsat-urated sediments allow the propagation of radarwaves at distinctly different velocities. Conse-quently, CMP1 was surveyed along Pro®le 1a onthe blowout de¯ation plain, where the water tablewas within 0á5 m of the surface, and CMP2 wasperformed along Pro®le 5 on the depositionallobe, where the water table was at a depth of over7 m. During Survey 2, one CMP pro®le wascollected along Pro®le D (CMP3, Fig. 3).

In order to calculate accurate velocities fromCMP surveys, the re¯ections used must behorizontal. This was the case for CMP1, wherethe subsurface re¯ections were all sub-horizontal(Pro®le 1a, Fig. 4). However, almost all re¯ec-tions associated with CMP2 and CMP3 wereshown to have a signi®cant component of dip.The one exception was the water table, whichformed a strong, continuous near-horizontalre¯ection; only this re¯ection was used for thevelocity calculations. Velocities were calculated

using the principles of `normal moveout', in themanner applied to seismic data (Robinson &CËoruh, 1988; Kearey & Brooks, 1991). Velocitiesderived from CMP1 ranged from 0á050 to0á063 m ns±1, with an average of 0á06 m ns±1.Velocities derived from CMP2 ranged from 0á134to 0á138 m ns±1, with an average of 0á135 m ns±1.Velocities derived from CMP3 ranged from 0á132to 0á133 m ns±1, with an average of 0á132 m ns±1.The velocities were used to convert the two-waytravel time of individual re¯ections to depth(Annan & Davis, 1976). Subsequent ground-tru-thing of the water table by hand auger con®rmedthat the velocity derived from CMP2 was appro-priate. The velocity derived from CMP1 iscomparable with others derived for saturatedsands (e.g. van Heteren et al., 1998).

All radar re¯ection pro®les were processed inthe same manner, using version 4á22 of thePulseEKKOä system software. The one exceptionwas the application of a two-dimensional F-K

Fig. 2. Vertical aerial photograph of the study site taken in May 1997. Blowouts A1 and A2 can clearly be identi®ed, ascan a series of parallel foredune ridges (FR) lying to seaward and a stand of pine trees (T) to landward. The location of aformer blowout (B) is also indicated. MIS 1997 Copyright, marketed by Cities RevealedÒ, The GeoInformationÒ Group.



migration program (Sensors and Software, 1996)mid-way through the processing stream, in orderto generate migrated sections. The migrationprogram uses the frequency-wavenumber app-roach of Stolt (1978) and assumes a single,constant velocity. Pro®les 1a and 2 were migratedwith a velocity of 0á06 m ns±1, Pro®les 1b to 8were migrated with a velocity of 0á135 m ns±1 andPro®les A to U were migrated with a velocity of0á132 m ns±1, as derived from the CMP surveys.The main advantage of this particular method ofmigration is that the entire waveform is used,rather than selected elements. The main disad-vantages are that: (1) the survey line topographymust be fairly ¯at; (2) the geometrical spreadingaccounted for is two-dimensional rather thanthree-dimensional; (3) there is no compensation

for attenuation; and (4) only the uppermost layer,as de®ned by radar-wave velocity, can be migra-ted successfully (Sensors & Software, 1996).

Migration resulted in a number of enhance-ments to the re¯ection pro®les. For example,unmigrated radar re¯ection Pro®le 1a shows aseries of near-horizontal, laterally continuous,sub-parallel re¯ections extending to depths ofup to 6 m (Fig. 4a). Migration of this pro®le(Fig. 4b) resulted in: (1) maintenance of thenear-horizontal attitude of the re¯ections, butwith an increase in their continuity and (2) thecollapse of diffractions, giving increased clarity tothe image. Unmigrated Pro®le 6 shows a complexarrangement of sub-horizontal and dippingre¯ections and diffraction hyperbolae (Fig. 5a).Migration of the pro®le (Fig. 5b) resulted in the

Fig. 3. The main geomorphological features of Raven Meols blowout and the location of the radar re¯ection pro®lescollected from GPR Survey 1 (Pro®les 1±8) and GPR Survey 2 (Pro®les A±U). CMP 1, 2 and 3 are the locations ofcommon mid-point pro®les used to obtain velocities for the subsurface radar waves.



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collapse of diffractions, revealing previouslyobscured primary re¯ections. In particular, itrevealed the nature and position of an undulatingre¯ection that de®nes the important radarsequence boundary RM-B. Migration also resultedin an increased continuity to primary re¯ections

in some parts of the pro®le and a change inposition for dipping re¯ections, with an associ-ated general increase in their dip. The changes indip are similar to those that should theoreticallyoccur through migration (Kearey & Brooks, 1991).As the migration used assumes a single, constant

Fig. 5. (a) Unmigrated version of radar re¯ection Pro®le 6, showing the intersection with Pro®le 5 (P5). (b) Migratedversion of radar re¯ection Pro®le 6. See text for an explanation of the changes that have occurred due to migration.(c) Line-drawing interpretation of the migrated radar pro®le showing the position of the water table (WT), radarsequence boundaries RM-A and RM-B, radar facies RM-2 and RM-3 and numerous diffractions beneath the watertable re¯ection that were not removed by the migration program. Such diffractions were seen in GPR Surveys 1 and 2whenever the water table re¯ection occurred at depths greater than approximately 3 m.



velocity, it breaks down when a pro®le has avelocity structure of two or more layers. Conse-quently, the program failed to successfullymigrate the pro®le below the water table.

Migrated pro®les were used to estimate thereturn centre frequency for the primary re¯ec-tions, using a Fourier transform to calculate anamplitude spectrum for an averaged pro®le.Using this information and the velocity charac-teristics of the pro®le, it was possible to calculatethe average vertical resolution for the re¯ections.The vertical resolution was 0á4 m in the unsatur-ated sands and 0á2 m in the saturated sands.

Topographical corrections were applied to boththe unmigrated and migrated radar re¯ectionpro®les using elevation data relative to ordnancedatum (O.D.). These were collected using aSokkisha Set 4C Electronic Total Station. Theradar re¯ection pro®les were plotted with theirappropriate near-surface velocity, an automaticgain control (AGC) gain with a maximum limitingvalue of 1000 and no vertical or horizontalaveraging. Subsequent to plotting, the elevationscales were amended in order to take into accountthe velocity change at the water table.

Apparent dip data from the intersecting pro®lesof Survey 2 were used to extract true dip data forthe re¯ections associated with the depositionallobe. At intersection points, average apparent dipdirections and amounts were calculated forre¯ections on both pro®les. A trigonometricmethod (Simpson, 1968) was then used to convertthese data into true dip direction and amount.

BLOWOUT RADAR STRATIGRAPHYAND ITS SEDIMENTOLOGICALINTERPRETATION

Prior to interpretation, where the migrationprogram failed to successfully migrate below thewater table, the relevant portions of the unmi-

grated and migrated pro®les were combined.Pro®les were then interpreted using the princi-ples of radar stratigraphy, as ®rst proposed byBeres & Haeni (1991) and Jol & Smith (1991). Thetechnique is based on that of seismic stratigraphy(Mitchum et al., 1977) and relies on the identi®-cation of systematic re¯ection terminations.These terminations de®ne radar sequence bound-aries (Fig. 6). Radar sequence boundaries de®negenetically related packages of re¯ections termedradar sequences. Working within the frameworkof radar sequences, it is then possible to de®nevarious radar facies. These are packages of re¯ec-tions with distinctive con®gurations, continuity,frequency, amplitude, velocity characteristicsand external form. Once the radar stratigraphy hasbeen de®ned, it can be interpreted geologically.

Three main radar sequences, de®ned by tworadar sequence boundaries (RM-A and RM-B) andcharacterized by three radar facies (RM-1, RM-2and RM-3) can be identi®ed on the radar re¯ec-tion pro®les. A description and interpretation ofthe radar stratigraphy is presented in Table 1.

Radar facies RM-1 consists of a series of sub-parallel, laterally continuous re¯ections, inter-preted as representing a series of sub-horizontallystrati®ed sediments extending from at least±0á5 m O.D. (the limit of radar-wave penetration)up to +3á7 to +4á4 m O.D. (Figs 4, 7 and 8). Thesedeposits appear to underlie the whole of thestudy site and their elevation, stratigraphicalcontext and internal structure suggest they are asequence of upper beachface deposits.

The top of RM-1 is marked by radar sequenceboundary RM-A. This surface represents the junc-tion between the underlying beachface sedimentsand the overlying aeolian sediments of radar faciesRM-2. RM-A is best observed 75±100 m alongPro®le 1a (Fig. 4). The elevation of this surfacevaries systematically from +3á7 to +3á9 m O.D. inthe northeast of the study area to +4á2 to + 4á4 mO.D. in the southwest (Figs 4 and 8).

Fig. 6. The basic descriptiveterminology associated with thede®nition of radar sequences andtheir boundaries. Modi®ed fromGawthorpe et al. (1993).



Overlying sequence boundary RM-A is radarfacies RM-2. The re¯ection con®guration suggeststhat RM-2 is composed of complex sets of aeoliancross-strata separated by bounding surfaces(Figs 5 and 8±11). This interpretation is suppor-ted by ®eld evidence from trenches dug into theunit on the southeastern sidewall of the blowout(Figs 3 and 12a). In addition, the trenches andthree auger holes (AH1, Fig. 4 and AH2 and AH3,Fig. 8) indicate that the unit is composed of ®ne-to medium-grained dune sand.

The top of RM-2 is marked by radar sequenceboundary RM-B (Figs 5 and 8±11). Field expo-sures (Figs 3 and 12b) and a hand auger hole(AH3, Fig. 8) con®rm that this surface represents athin, but laterally continuous, humic horizon.Detailed mapping of the elevation of RM-Bbeneath the depositional lobe of the blowoutindicates that it de®nes a complex dune topogra-phy, ranging in elevation from at least +7á5 m to+13á5 m O.D. (Fig. 13). Beneath the western andcentral portions of the lobe, RM-B delineates awell de®ned, but irregular topographical depres-sion bounded by dune ridges (R1 and R2, Fig. 13)

to the north and south. The depression trendsapproximately west to east and opens eastward asthe topography becomes progressively more sub-dued and lower in elevation. The slopes on thenorthern side of dune ridge R1 are relatively steepin the west, but become more gentle and irregularat higher elevations beneath the central portionsof the lobe (P, Fig. 13). The slopes on the southern¯ank of dune ridge R2 generally show the oppositetrend. Beneath the eastern portion of the lobe,RM-B dips gently towards the east.

Radar sequence boundary RM-B is overlain byradar facies RM-3 (Figs 5 and 8±11). This radarfacies is interpreted as representing the aeoliandeposits of the depositional lobe of the blowout,with the primary re¯ections de®ning well-devel-oped sets of cross-strata. In addition, a clear third-order bounding surface can be observed at thenortheastern end of Pro®le 1b (185±208 m,Fig. 8), with a similar, but less distinct, surfacealso present in Pro®le 6 (Fig. 5).

True dip data for the re¯ections of RM-3 indicatea distinct spatial pattern in the direction and angleof dip of the cross-strata. These variations in dip

Table 1. Description and interpretation of the Raven Meols blowout radar stratigraphy. The geological interpretationwas aided by the analysis of hand auger hole logs and ®eld exposures.

Description Geological interpretation

RadarFaciesRM-1

Series of sub-horizontal, laterally continuousre¯ections. Typically sub-parallel, butoccasional very low-angle cross-cuttingrelationships are observed. Concordant withRM-A. Often at least partially obscured bya complex series of interfering diffractionsdeveloped below the top of the water table.

Upper beachface sediments displayingsub-horizontal cross-strata.

SequenceBoundaryRM-A

Typically a sub-horizontal, planar re¯ection. Oftenpartially obscured by a complex series ofinterfering diffractions developed below the topof the water table.

De¯ation surface representing the contactbetween the beach deposits of RM-1 andthe overlying aeolian deposits of RM-2.

RadarFaciesRM-2

Series of cross-cutting re¯ections with limitedlateral continuity. Re¯ections have low tomoderate apparent dips and downlap ontoRM-A and toplap onto RM-B.

Aeolian deposits of the dunes in/on whichthe blowout developed, showing acomplex series of aeolian cross-strataseparated by numerous boundingsurfaces.

SequenceBoundaryRM-B

Laterally continuous, undulating re¯ectionwith a complex topography of up to 5á5m.

Thin humic horizon representing theformer vegetated surface of thepre-blowout dunes.

RadarFaciesRM-3

Laterally continuous, sub-parallel, generallymoderate- to high-angle re¯ections that downlaponto RM-B. Occasional low-angle cross-cuttingrelationships between re¯ections. Re¯ectionsdisplay a radial dip pattern in the east of thesurvey area, where dips systematically rangefrom northerly to southeasterly. Elsewhereessentially unidirectional true dips to thenorth and north-northeast are developed.

Aeolian deposits of the depositional lobeof the blowout, displaying occasional thirdorder bounding surfaces. Cross-strataexhibit a radial dip pattern in the easternportion of the lobe, but elsewhere dipconsistently to the north andnorth-northeast.



can be used to help de®ne western, central andeastern portions of the lobe (Fig. 13). In thewestern portion of the lobe, the majority of thecross-strata have a dip direction just east of north,although the most northeasterly cross-strata dipslightly to the west of north. In addition, dip anglesare generally high (typically 26±32°) and conse-quently either approach, or reach, the angle nor-mally associated with dune slip face development[30±33°, Livingstone & Warren (1996)]. The onlyexceptions are the cross-strata in the north thatoverlie a topographical rise associated with RM-Band have lower angles of dip (15±19°).

In the central portion of the lobe, dip directionsand dip angles are more variable (Fig. 13). Themore northerly cross-strata, which overlie themain west-east trending topographical depressionde®ned by RM-B, display dip directions thatrange from just west of north to north-northeast,with the dips becoming more easterly in aneasterly direction. Dip angles in this area rangefrom 16° to 31°. The more southerly cross-strata,which overlie a more elevated `plateau' areaassociated with RM-B (P, Fig. 13), show a widervariation in dip direction (north-northwest tonortheast) and lower dip angles (10±23°).

In the eastern portion of the depositional lobethe cross-strata show a more radial dip pattern,with the underlying topography associated withRM-B being relatively low in elevation anddipping gently to the east (Fig. 13). In the northof this part of the lobe, the cross-strata dip to thenorth and northeast, whereas in the central partthey dip towards either the northeast or east-southeast and in the south they display eithereast-southeasterly or southeasterly dips. Dip an-gles vary considerably (13±33°), reaching anglesindicative of true slip face development only inthe extreme south and east.

BLOWOUT DEVELOPMENT: COMBININGGROUND-PENETRATING RADAR,AERIAL PHOTOGRAPHICAND OTHER EVIDENCE

Establishment of the pre-blowoutdune system

The available evidence indicates that the dunearea in which Raven Meols blowout developed isat least 150 years old. The shoreline prograded

Fig. 7. (a) Migrated radar re¯ection Pro®le 2, showing the intersection with Pro®le 1a (P1a). At 25 m along the radarpro®le an over-migrated area of `ringing' occurs that is believed to be related to the presence of a shallow buriedobject. (b) Line-drawing interpretation of Pro®le 2 showing the position of the water table (WT), radar sequenceboundary RM-A, and radar facies RM-1 and RM-2.



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Fig. 9. (a) Migrated radar re¯ection Pro®le 3, showing the position of the intersection with Pro®le 1b (P1b). (b) Line-drawing interpretation of Pro®le 3. (c) Migrated radar re¯ection Pro®le 4, showing the position of the intersection withPro®les 1b and 5 (P1b and P5). (d) Line-drawing interpretation of Pro®le 4. The position of the water table (WT), radarsequence boundaries RM-A and RM-B and radar facies RM-2 and RM-3 are shown on the line-drawing interpretations.



seaward at the study site by approximately 150 mbetween 1845 and 1892 (Pye & Neal, 1994).Ordnance Survey maps and documentary evi-dence (Gresswell, 1953; Pye & Neal, 1994) showthat at the end of the 19th century the shorelinelay only a few tens of metres to the southwest ofthe seaward edge of the present day blowout.Consequently, the beach and dune depositsimaged in Pro®les 1a and 2 (Figs 4 and 7) willhave formed sometime during the period ofshoreline progradation between 1845 and 1892.The sub-horizontal and laterally continuousnature of the strata within the beach sedimentscomprising radar facies RM-1 are indicative of avery low-angle upper beachface during prograda-tion. This is compatible with the current form ofthe upper beachface along rapidly progradingsections of the Sefton shoreline, including thatimmediately to the southwest of Raven Meolsblowout (Pye & Neal, 1994).

Map evidence (Pye & Neal, 1994) indicates thatthe majority of the pre-blowout beach-dune sys-tem to the northeast of the present day blowoutde¯ation plain formed prior to approximately1845. The external form and internal structure ofthe pre-blowout dune deposits, as represented bythe thin humic horizon of RM-B and the cross-strata of RM-2, respectively, are indicative of

dune development in the presence of at least apartial vegetation cover. The complex externalform of the pre-blowout dunes is in agreementwith aerial photograph evidence, particularlyfrom 1945 where a series of irregular dune ridgesare seen directly to the northeast of both blowoutsA1 and A2 (Fig. 14). The northern ¯ank of theridge, directly to the northeast of blowout A1 in1945 and trending west-southwest to east-north-east, currently underlies the southern part of thedepositional lobe (R1, Figs 13 and 14).

Blowout initiation and subsequentdevelopment

The exact causes and timing of blowout initiationat the study site are unclear owing to a lack ofsuitable evidence. The nature of the pre-blowoutdune complex may have helped facilitate blowoutdevelopment, as only a partial vegetation coverwould make the dune surface susceptible tode¯ation. However, there is also some evidencefor a higher incidence of gales in the easternLiverpool Bay area around the turn of the century(Pye & Neal, 1994), with this increase inwindiness probably enhancing the possibility ofblowout formation. Furthermore, disturbance ofvegetation cover and soils may also have been

Fig. 10. (a) Migrated radar re¯ection Pro®le 5, showing the position of the intersections with Pro®les 1b, 4, 6, 7 and 8(P1b, P4, P6, P7 and P8) and the location of Common Mid-Point Survey 2 (CMP2). (b) Line-drawing interpretation ofPro®le 5 showing the position of the water table (WT), radar sequence boundary RM-B and radar facies RM-2 and RM-3.



Fig. 11. (a) Migrated radar re¯ection Pro®le 7, showing the position of the intersection with Pro®le 5 (P5). (b) Line-drawing interpretation of Pro®le 7. (c) Migrated radar re¯ection Pro®le 8, showing the position of the intersectionwith Pro®le 5 (P5). (d) Line-drawing interpretation of Pro®le 8. The position of the water table (WT), radar sequenceboundaries RM-A and RM-B and radar facies RM-2 and RM-3 are shown on the line-drawing interpretations.



aided by the presence of rabbit warrens and byincreased pedestrian pressure. The in¯uence ofdisturbance by both humans and animals in

blowout initiation is well documented (Ritchie,1972; Mather & Ritchie, 1978; Jungerius & van derMeulen, 1988).

Fig. 12. (a) Section in the aeolian unit that can be correlated with radar facies RM-2. Location of the trench isindicated in Fig. 3. Low-angle bounding surfaces (bs) can be seen to separate complex sets of cross-strata. Two othertrenches in the unit showed the same internal structure. The metallic rule is 1 m long. (b) Outcrop of a thin humichorizon (HH) that can be correlated with radar sequence boundary RM-B. The soil is underlain by the pre-blowoutdune deposits that form radar facies RM-2 and is overlain by the deposits of the depositional lobe that form radarfacies RM-3. The scale-bar is 0á1 m long.



By 1945, Raven Meols blowout was alreadywell developed (Fig. 14). The long axis of blow-out A1 was aligned roughly southwest to north-east, an orientation in accordance with theimportant sand-transporting winds from thesouthwest. Blowout A2 had a more saucer-shaped

form at this time. Of the two, only A1 went on todevelop a depositional lobe. Critical in thisdevelopment was the formation of a wind gap atthe downwind end of the blowout. This occurredby 1961, with the wind gap cutting through duneridge R1 that lay directly to the northeast. This

Fig. 13. Elevation contours (in metres O.D.) for the thin humic horizon that delineates the antecedent dune topog-raphy beneath the depositional lobe of the blowout, based on interpretation of data from GPR Survey 2 (Fig. 3). R1and R2 indicate the position of two prominent dune ridges and P indicates a high elevation `plateau' area associatedwith the northeastern ¯ank of R1. Superimposed are average true dip data for cross-strata of the depositional lobe,again based on interpretation of GPR Survey 2 data. The lobe has been subdivided into western, central and easternportions on the basis of the spatial variation in dip for the cross-strata and their resulting relationship to theunderlying pre-blowout dune topography.

Fig. 14. Sequential geomorphological aerial photographic interpretation for the study site for the period 1945±97.The positions of blowouts A1, A2 and B and dune ridge R1 are indicated on the interpretation from 1945.



breach was followed by rapid lateral growth anddeepening of the erosional hollow, particularlyup to 1975 (Fig. 14). De¯ated sand from thehollow was transported predominantly tothe northeast through the wind gap to form thedepositional lobe or, less signi®cantly, overthe sidewalls to form marginal rim dunes.

The internal sedimentary structure of thedepositional lobe is more complicated thanwould be predicted from models based oncontemporary observations of trough blowoutdevelopment. These suggest a predominance ofrelatively simple, broadly radial deposition andforeset development that is strongly related to theorientation of the blowout trough (Carter et al.,1990; Hesp & Hyde, 1996). Aerial photographsindicate the trough of blowout A1 has beenorientated southwest to northeast throughout itsdevelopment from 1945 to the present day, with adepositional lobe at its downwind, northeasternend (Fig. 14). According to the models of Carteret al. (1990) and Hesp & Hyde (1996) the cross-strata of the depositional lobe of the blowoutshould be radially oriented from northwest tosoutheast, with perhaps the majority dippingbetween north and east. However, the westernand central portions of the lobe are dominated bynortherly or north-northeasterly dipping cross-strata, with a more radial pattern only beingdeveloped in the eastern part of the lobe (Fig. 13).

The lack of correspondence between the ideal-ized and actual internal structure of the deposi-tional lobe appears to be related to the interactionbetween sand movement and deposition by theimportant sand-transporting winds and the pre-existing dune topography on which the lobe isdeposited. Aerial photographs indicate that afterinitial establishment of the depositional lobe,some time between 1945 and 1961 (Fig. 14), itrapidly expanded both northwards and eastwardsas the upwind erosional trough was enlargedthrough erosion. The large topographical depres-sion in the pre-blowout dunes immediately to thenortheast provided accommodation space for thedeposition of the lobe, but also appears to havehelped control its external form and internalstructure.

Aerial photographs show that the sediments inthe southwestern portion of the present day depo-sitional lobe were deposited ®rst, some time duringthe period 1961±75 (Fig. 14). The cross-strata inthis area dip predominantly just east of north,roughly parallel to the relatively steep slopes of theunderlying northern ¯ank of dune ridge R1(Fig. 13). This suggests that these slopes exerted a

strong in¯uence on the nature of sediment depos-ition during the early phases of lobe formation,preventing the development of radially orientedcross-strata. In addition, these slopes also appear tohave helped control the angle of initial depositionon the lobe, as cross-strata dip angles (16±30°) aresimilar to those associated with the underlyingdune topography (c. 18±27°).

Between 1975 and 1989 the depositional lobecontinued to expand rapidly northwards andeastwards, largely ®lling the topographicaldepression into which it was being deposited(Fig. 14). During this time the majority of thesediments associated with the northwestern andcentral portions of the lobe were deposited. Asthe lobe continued to expand in size, eastwardsediment transport is likely to have been aided byincreasing exposure to important sand-transport-ing winds from the west. In addition, thewest±east orientation of the topographical lowinto which the lobe was being deposited is likelyto have helped topographically steer regionallyimportant sand-transporting winds from thesouthwest, west and northwest across the lobein an eastward direction. However, despite clearevidence of both northward and eastward growthof the lobe, the cross-strata formed during thisperiod have a largely northerly dip in the north-western section of the lobe and typically onlyhave north-northeasterly dips in the centralportion of the lobe. Once again, dip directionsare largely parallel or sub-parallel to the northernslopes of underlying dune ridge R1 (Fig. 13), thepresence of this ridge still preventing the devel-opment of a radial dip distribution. In addition,dip angles are generally lower in the central partof the lobe than in the western portion. Thisre¯ects the shallower dip of slopes on the easternportion of ridge R1, the lowest and most variabledips being associated with the relatively hightopographical `plateau' (P, Fig. 13) beneath thesouthern central portion of the lobe.

Between 1989 and 1997 the lobe continuedto expand both further northward and furthereastward, beyond the con®nes of the topograph-ical depression that it had now largely ®lled(Fig. 14). This expansion led to deposition of thesediments forming the eastern and extreme nor-thern portions of the depositional lobe. In theeastern part of the lobe the cross-strata display areasonably well-developed radial dip distribu-tion, with dips systematically varying from northto southeast (Fig. 13). Dip angles are generallyhigher than in the central portion of the lobe,often approaching, and in the extreme east and



southeast reaching, dips indicative of true slipface development. A radial dip pattern appears tohave developed due to the absence of dune ridgeR1, which, as has previously been suggested,prevented such a style of deposition duringearlier phases of lobe development. Instead, thelow elevation and relatively ¯at nature of the pre-blowout dune topography allowed relatively un-restricted development of this part of the depo-sitional lobe. Consequently, its radial dip patterncorresponds more closely to that predicted for thelobe from models based on contemporary studiesof blowout dynamics.

CONCLUSIONS

GPR can provide unique insights into the strati-graphy and internal sedimentary structure ofunconsolidated, sand-and-gravel dominated,sedimentary deposits. Though broadly similar tore¯ection seismic data, processing techniquessuch as migration have not been routinely appliedto GPR data gathered for sedimentological stud-ies, despite the potential advantages. This studyhas demonstrated that a relatively simple migra-tion program can signi®cantly improve the qual-ity and clarity of radar data and consequently aidgeological interpretation. In particular, migrationhelps restore dipping re¯ections to their truepro®le position and apparent dip, removediffractions associated with point re¯ectors orcomplex, undulating re¯ectors and remove `struc-tural' distortions imparted on undulating re¯ec-tions. Despite these bene®ts, migration must beapproached with caution due to the inherentlimitations of all migration programs. However, ifthese limitations are understood and accountedfor in interpretation, there is no reason whymigration should not form part of standarddata-processing routines in GPR-based sedimen-tological studies.

The principles of radar stratigraphy were suc-cessfully used to interpret the radar re¯ectionpro®les acquired from Raven Meols blowout. Theinterpretation allowed unique insights into thehigh-resolution stratigraphy and internal sedi-mentary structure of the deposits of both thepre-blowout beach-dune system and the blow-out. In combination with aerial photographicand map analysis, it allowed a detailed modelfor the development of the study site to beconstructed.

The pre-blowout dunes, in which the blowouttrough is developed and on which the deposi-

tional lobe of the blowout is deposited, show acomplex external morphology de®ned by a thinhumic horizon. This soil can be identi®ed in ®eldexposures and auger holes and forms a distinctradar sequence boundary on GPR pro®les. Thecomplex external form and internal structure ofthe pre-blowout dunes suggests deposition in thepresence of at least a partial vegetation cover. Theoccurrence of only a sparse or limited vegetationcover may have aided subsequent blowout devel-opment, although changes in wind climate anddisturbance by rabbits or humans may also havemade signi®cant contributions.

The internal sedimentary structure of thedepositional lobe of the blowout is seen to departsigni®cantly from a simple model of radial foresetdeposition, as derived from contemporary mor-phodynamic studies reported in the literature.Modi®cation of this simple pattern of depositionoccurred owing to the nature of the underlyingdune topography on which deposition took place,and varying exposure in both time and space to avariety of sand-transporting winds. Modi®cationof the regional wind pattern by local dunetopography is likely to have helped controlspatial and temporal variations in wind exposureacross the lobe. In the northern and centralportions of the lobe a radial dip pattern is notdeveloped. Instead, cross-strata dip at angles andin directions largely controlled by the orientationand dip of slopes on the northern ¯ank of a largedune ridge that underlies the southern part of thelobe. Only in the eastern portion of the deposi-tional lobe do the cross-strata develop a moreradial dip distribution. Here the underlying dunetopography is low in elevation and dips verygently to the east, allowing unrestricted develop-ment of the overlying depositional lobe.

Overall, the study suggests that the internalstructure of the depositional lobe of any troughblowout is likely to be complex. However, generalcriteria for the recognition of large-scale troughblowout deposits in coastal aeolian dune sequenc-es can be proposed. At the resolution of this study,identi®cation would rely primarily on the com-bined presence of: (1) packets of large, relativelyhigh-angle cross-strata (typically 15±30°), that arewell-developed, laterally continuous and some-times reach values associated with true slip facedevelopment; (2) cross-strata that can displayvery-well to very-poorly developed radial dippatterns, even within the deposits of a singledepositional lobe; and (3) a dune soil, or othersurface indicative of a depositional hiatus,directly beneath the lobe deposits that demarcates



a pre-blowout dune topography with a clearrelationship to the dip and orientation of theoverlying cross-strata. In coastal settings, suchstructures would differ signi®cantly from those sofar described for other types of dune deposit(McBride & Hayes, 1962; Land, 1964; Bigarellaet al., 1969; Goldsmith, 1973; Hesp, 1988; Carter &Wilson, 1990; Byrne & McCann, 1993; Ruz &Allard, 1995; Bristow et al., 2000b). However,research in this ®eld is still somewhat limited.Consequently, many more studies will be requiredbefore the diverse range of internal sedimentarystructures present in the deposits of blowouts andother coastal dunes are fully characterized.

ACKNOWLEDGEMENTS

Assistance provided by the Sefton Coastal RangerService, the Sefton Coast Life Project and TonySmith of Sefton Metropolitan Borough Council isgratefully acknowledged. Martin Fenn, RobertArklay and Dr Craig Reuben Smith provided ®eldassistance. Peter Fenning and Andy Brownof Earth Science Systems Ltd are thanked fortheir logistical and technical support. Access toarchive aerial photographs was provided by theJoint Countryside Advisory Service, Maghull. KayLancaster and Karl MacNaughton helped to pre-pare the illustrations. Sytze van Heteren andChris Schenk reviewed an earlier version of thismanuscript and their comments led to substantialimprovements. The research was supported byNERC Geophysical Equipment Pool Loan 555 andby the School of Applied Sciences, University ofWolverhampton.

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Documents

Internal structure of a trough blowout, determined from migrated ground-penetrating radar profiles