spatial and temporal variability of titanomagnetite placer deposits on a predominantly black sand...

(2007) 45–59www.elsevier.com/locate/margeo

Marine Geology 236

Spatial and temporal variability of titanomagnetite placerdeposits on a predominantly black sand beach

K.R. Bryan ⁎, A. Robinson, R.M. Briggs

Department of Earth and Ocean Sciences, University of Waikato, Private Bag 3105, Hamilton, New Zealand

Received 19 December 2005; received in revised form 14 September 2006; accepted 25 September 2006

Abstract

Titanomagnetite forms rich placer deposits along the northwest coast of New Zealand. These deposits were sampled along 3 shore-normal transects spaced over the southern 2 km of a dissipative high-energy beach on the west coast in 5 field campaigns covering oneyear. The percentage of opaque minerals (mainly titanomagnetite) was extremely high in the upper 30 m of the beach face, extendingseaward where these opaque minerals were gradually replaced with variable amounts of lighter augite, hornblende and plagioclase.The pattern appeared to be divided into two regions, a lower seaward and an upper landward region, separated by a point where eithermarine dominated over aeolian processes or where swash dominated over breaking processes. In the seaward region, the percentage ofopaques increased and particle size fined landward as undertow removed the lighter larger particles seaward. In the landward region,the percentage of opaques and particle size were more constant, or even showed the reverse pattern as wind transported the lightermaterial shoreward, or swash asymmetry transported the heavier material seaward. The similarity of settling velocities over the wholebeach face suggests that sorting by size rather than weight plays a dominant role in separating the mineral assemblages. Considerablevariations existed between transects. This could be explained by the spatial changes in surfzone waves and currents that wereassociated with proximity to the southern headland and various rip current channels that characterised this dissipative site.Surprisingly, the percentage of opaques decreasedwhen thewave conditions of the day of samplingweremore energetic. In contrast tomany other placer deposits, these deposits are abundant on the beach face, forming an armouring layer during lower wave energyconditions. During higher wave conditions, the surface layer erodes allowing lighter augite, plagioclase and hornblende to be releasedfrom the sediments below.© 2006 Elsevier B.V. All rights reserved.

Keywords: New Zealand; Muriwai Beach; beach placer deposits; black sands; titanomagnetite; cross-shore and longshore variations

1. Introduction

Beach placer deposits are accumulations of heavyminerals on the upper regions of beaches. They form by

⁎ Corresponding author. Tel.: +64 7 838 4466x7123; fax: +64 7 8560115.

E-mail address: [email protected] (K.R. Bryan).

0025-3227/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.margeo.2006.09.023

mechanical concentration of resistant heavy mineralparticles of high specific gravity by the action of waves,currents, and winds. Studies on placers have ranged fromlarge scale mapping exercises aimed at studying eco-nomically viable deposits (e.g. Roy, 1999; Hou et al.,2003; Dillenburg et al., 2004; Paine, 2005) to moreprocess-based studies aimed at understanding the role ofparticle density in transporting sediment (e.g. Komar andWang, 1984; Peterson et al., 1986; Hughes et al., 2000).

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http://dx.doi.org/10.1016/j.margeo.2006.09.023

46 K.R. Bryan et al. / Marine Geology 236 (2007) 45–59

Moreover, the rapidly-increasing ability to model moreand more complex sediment boundary-layer processeshas renewed interest in obtaining detailed validation dataon the role of mineralogy in determining short time scalevariations in bed composition (Koomans and de Meijer,2004).

Placers are commonly concentrated during transportfrom more remote sources in which the original per-centages are lower. For example,Mount Taranaki in NewZealand provides a constant supply of the heavy min-erals titanomagnetite and ilmenite which are sorted bythe northward littoral drift system along the northwestcoast (Nicholson and Fyfe, 1958; Kear, 1964; Schofield,1970; Kear, 1979). In locations with ample sedimentsupply, sediments will be buried before sorting canoccur. As a consequence, placer deposits are generallyconsidered to be an indication that the beach is in anerosive state (Frihy, 1994). Even when supply is limited,overly energetic conditions will erode the sediment,including the placer (Komar and Wang, 1984). Thedifficulty lies in determining the optimal hydraulic con-ditions for sorting. In principle, differentiation by den-sity, weight, size or shape (or all of these) is madebetween grains during all stages of entrainment, trans-port, deposition and burial. Settling equivalence andtransport sorting differentiate on the basis of settlingvelocity during deposition and transport respectively,whereas entrainment sorting differentiates on the basis ofsize during entrainment. Shear sorting differentiatesparticles vertically during burial by the action of dis-persive pressures inside the bed.

Despite the theoretical understanding of sorting pro-cesses and how these might play a role in determiningspatial and temporal patterns in placer deposits, there isstill relatively poor understanding of which mechanismactually controls sorting on natural beaches. Thedifficulty lies in designing field data collection pro-grammes that cover large spatial and temporal scales, yetprovide sufficient detail needed to draw links betweenthe surface sediments and surfzone processes. Existingprocess studies are generally limited to spatial variationsalong one or two transects with little attention totemporal variations (e.g. Komar and Wang, 1984;Komar et al., 1989; Hughes et al., 2000).

In this paper, we measure the temporal variability ofmineralogy and particle grain size along spatially-sep-arated cross-shore transects on an eroding, dissipativebeach. The objective is to explore the widely-differingtemporal behaviour exhibited by placers on differentparts of a beach exposed to the same general incidentwave and tidal forcing, but different local hydrodynamicconditions caused by proximity to headlands and rip

currents. Local hydrodynamic conditions along eachprofile are inferred quantitatively, using simple model-ling (e.g. Kroon andMasselink, 2002), and qualitatively,using images of wave breaking conditions. This blacksand field site is completely dominated by large, pe-rennial placer deposits, and as such may provide newinsights into placer development not observed at othersites where placers form a lesser proportion of beachsediments (e.g. Komar and Wang, 1984; Hughes et al.,2000).

2. Sorting processes

The simplest mechanism for sorting relies on a dif-ference in settling velocity between the heavy and lightminerals. Thus an assemblage of minerals at any locationshould have similar settling velocities, otherwise calledsettling equivalence by Komar (1989) and Hughes et al.(2000), caused by an inverse relationship between sizeand density. Observations from the Oregon Coast showedsettling velocities that were roughly equivalent over awide range of minerals in a general sense, although nocorrelation existed between cross-shore patterns of placerconcentration and settling velocity (Komar and Wang,1984). They suggested that selective entrainment equiv-alence could be an alternative explanation for sortingwhere larger grains protrude more into the flow and aretherefore more likely to be entrained, leaving the smaller(and coincidentally denser) grains behind as a lag. Thelarge grains form bed roughness elements which provideenclaves between the large grainswhere smaller grains areprotected from the currents (Slingerland, 1977). Subse-quent laboratory experiments showed selective entrain-ment to occur when initial grain distributions werecharacterised by settling equivalence (Li and Komar,1992). Moreover, smaller grains also have larger pivotangles and require greater bottom stress to initiate motion.Entrainment selection can only be important whilesediments remain unsorted, before the placer lag forms.

Once the grains are entrained, transport sorting occursin which grains are transported at different rates de-pending on their respective settling velocities. Grainswith lower settling velocity travel higher in the flow,where the currents are stronger, and therefore will travelfurther (Slingerland and Smith, 1986). Where asymmet-rical waves dominate, landward currents under crests arestronger than seaward currents under troughs, so heavyminerals are more likely to be entrained and transportedlandward, while light minerals are transported in eitherdirection (Komar, 1989). Inundated or fluidised bedsediments can undergo shear sorting or dispersive pres-sure equivalence in which grains in the bed are sorted

47K.R. Bryan et al. / Marine Geology 236 (2007) 45–59

vertically by dispersive pressure caused by the surfaceshearing force (Slingerland and Smith, 1986).

Spatial and temporal variations in placers depend onthe large scale variability of hydraulic sorting conditionsboth along and across the beach. Cross-shore variationsdepend on changes to the waves and currents as thewaves shoal, become skewed and break across theprofile, culminating in swash on the beach face. Ob-servations from dissipative beaches suggest that selec-tive entrainment and transport offshore of light mineralsleave a placer deposit on the upper beach face (KomarandWang, 1984). Conversely, Hughes et al. (2000) showobservations with little horizontal variation in densityand size in the placer deposit, suggesting instead thatshear sorting is the most likely placer-causing process ontheir more reflective beach. In the alongshore, sorting iscontrolled by longshore currents (Frihy and Komar,1991; Frihy and Lotfy, 1994). For example, alongshoreenhancement of placers along the Oregon Coast has beenshown to correlate with changes in alongshore driftpatterns associated with shoreline orientation changescaused by headlands (Peterson et al., 1986). There is alsoa suggestion that placer variations are associated withalongshore patterns of currents and wave heights such ascaused by rip currents and sand bank morphology(Komar, 1989).

3. Field site

Muriwai Beach is a dissipative, mesotidal beachextending some 48 km south from the Kaipara Harbourentrance on the west coast of the North Island of NewZealand (Fig. 1). It is exposed to constant high-energywave conditions from the Tasman Sea and SouthernOcean. The west coast is characterised by significantwave heights of 1–3 m and mean wave periods ranging6–8 s with storm swell heights exceeding 6 m (Gormanet al., 2003). The surfzone comprises an outer dissipativebar and an inner intermediate state bar with a gently-sloping 0.01 beach face (Brander and Short, 2000). Themean spring tidal range is approximately 3 m. Theprevailing winds at Muriwai are onshore, predominantlycoming from the southwest, with average speeds of6.6 m s−1 (Clegg and Johns, 1988). The focus of thisstudy is the southern end of Muriwai Beach, in the2.5 km area between Okiritoto Stream and OtakamiroPoint (Fig. 2).

Muriwai Beach is part of a large scale littoral sed-iment transport system operating along the West Coast,interconnecting beaches to the north and south beginningat Taranaki. Net sediment transport is to the north, as aresult of the dominant southwest wave climate (Gorman

et al., 2003). The sediment moving northward fromTaranaki must navigate past the Mokau, Waikato, andother river mouths draining to the West Coast, and alsothe Aotea, Kawhia, Raglan, and Manukau harbourmouths (Fig. 1), in addition to numerous headlands.Therefore the black sands that characterise MuriwaiBeach are derived dominantly from erosion of the Qua-ternary Taranaki andesites to the south (Fig. 1) withminor contributions from the Waikato River carryingsediments derived from the Taupo Volcanic Zone (Kear,1964; Schofield, 1970, 1975; Kear, 1979; Carter, 1980;Hamill and Ballance, 1985; Laurent, 2000) and almostno contribution from local erosion of the WaitakereRanges (Hamill and Ballance, 1985). Minerals found inWaitakere beach sands include titanomagnetite, augite,rock fragments, plagioclase, hornblende, ilmenite,quartz (Yock, 1973; Hamill and Ballance, 1985, Laurent,2000).

4. Sampling profiles and methods

Three profiles were chosen along southern MuriwaiBeach for detailed measurement (“North”, “Middle” and“South”, Fig. 2). To evaluate temporal change along thethree profiles at Muriwai Beach, RTK-GPS surveysusing a local base station were undertaken on 05December 2002, 18 February 2003, 15 May 2003, 28August 2003, and 29 October 2003. The height datumdid not change between surveys and was related to meansea level at the site. Accuracy was b4 cm in the vertical.Additionally, the RTK-GPS was set up on an all-terrainvehicle to survey the groundwater exit point as itmigrated across the beach face during a receding tide.The exit point provided an indication of the percentage oftime the sediments were wet, in addition to providingground-truthing for the subaerial video imagery.

Although there are no local water level measure-ments, tidal data were supplied from the National In-stitute of Water and Atmospheric Research (NIWA)Tidal Model which has been validated using measure-ments from Anawhata, 10 km to the south of Muriwai.Wind and wave data for the offshore region of the studyarea (36°49.54 S, 174°25.29 E) were extracted from theNational Oceanic and Atmospheric AdministrationWavewatch III (NOAA WWIII) model database, pro-viding significant wave height (Hs) and direction, peakperiod, and wind speed and direction. Figs. 3 and 4 showthe significant wave height and direction prior tosampling.

Subaerial video imagery was taken of the beach areaduring most of the study by R. A. Holman as part of theArgus video network. Video footage was averaged over

Fig. 1. Location of the field site on the northwest coast of the North Island of New Zealand.


10 min and rectified using standard techniques. Thesevideo images provide a general impression of thelocation of rip currents and sand banks relative to theprofile locations (Fig. 5). Sand banks are located wherethe light intensity is high in the image due to preferentialwave breaking whereas rip channels are associated withregions of low light intensity intersecting the sand banks.The groundwater exit point was clearly evident in theimages by the change in light reflection between wet anddry sand on the beach face.

A total of 133 surficial sediment samples were col-lected at 10 m intervals between the dune toe and the

spring tide low water line along each profile. A plasticcorer measuring 8 cm in diameter and 10 cm in lengthwas used to collect samples. As such, these surfacesamples represent an average of the top 10 cm, andplacers could be even more enriched at the surface thanreported here. The sediment samples collected from thethree profiles were washed to remove any salts present,then dried at 85 °C for 24 h, following Hughes et al.(2000). 100 g sub-samples were sieved at 1/4ϕ intervals.The relative abundances of mineral species were de-termined using point-counting of grain mounts. Fivehundred grains were counted on each slide; 500 grains

Fig. 2. Map of Muriwai Beach showing the location of the three profile lines.


have been shown to provide a statistical representation ofthe whole population (Bardsley, 1983). The mineralswere identified using standard transmitted and reflectedlight petrographic techniques.

To test the dependence of trends in particle size andmineralogy on variations in the hydrodynamic condi-tions, three probabilities were calculated along eachprofile for the month prior to each of the five fieldexcursions: the probability of inundation, the probabilityof breaking wave influence and the probability of swashinfluence. These provided a better basis for comparisonbetween profiles than cross-shore distance along theprofile, since the cross-shore distance coordinate de-pended on the choice of origin along each profile (which

was the dune toe). Unfortunately there were no surfzonehydrodynamic measurements collected, so these para-meters were inferred from deepwater wave and waterlevel information (e.g. Kroon andMasselink, 2002). Theprobability of inundation, I, was calculated by findingthe fraction of time t that the elevation z at any cross-shore position along the profile was less than the waterlevel η.

I ¼ 1ttotal

Xtzbg ð1Þ

The water level was calculated from the tidal levelηtide as output by the NIWA tidal model with an

Fig. 3. Mean (circles), maximum (stars) and minimum (crosses) ofsignificant wave height. The data have been averaged over 2 months(top panel), 1 week (middle panel) and 1 day (bottom panel) prior tosampling. Data are from the NOAAWavewatch III deepwater hindcast(ftp://polar.ncep.noaa.gov/pub/history/waves, 2003) for the grid cellclosest to Muriwai Beach.

Fig. 4. The average peak wave direction, averaged over the 2 months(stars), 1 week (crosses) and 1 day (circles) prior to sampling. Thedashed line marks the angle of normal wave approach to MuriwaiBeach. Data are from the NOAAWavewatch III deepwater hindcast forthe grid cell closest to Muriwai Beach.


adjustment for wave-induced set-up ηset-up calculatedusing Bowen et al. (1968)

gset−up ¼38g2

ðhb þ gtideÞ1þ 3

8 g2

� �2 ð2Þ

where

hb ¼ Hrms

g; ð3Þ

andHrms is the root mean-square wave height (=Hs /√2).The breaking coefficient, γ, was assumed to be 0.55which is consistent with recent work (e.g. Ruessink et al.,2003). Results were not particularly sensitive to thechoice of γ. The probability of breaking wave influenceBwas calculated by determining the fraction of time overwhich the depth at each cross-shore location was lessthan the depth at which breaking initially occurred, hb

(where depth included variations due to tide and set-up).The probability that processes at a particular locationwere influenced by swash S was assessed by followingmethodology outlined in Aarninkhof et al. (2003) whichis based on the Iribarren Number ξo (=β / (H∞ /L∞)

0.5

where β, H∞ and L∞ are the beach slope, deep waterwave height and wavelength respectively). The range ofsea-surface elevation ηswash caused by the swash is givenby

gswash ¼ FffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiR2ig þ R2

ss

qð4Þ

where the sea swell Rss and infragravity Rig swashheights are separately determined by

Rig ¼ 0:65Hs tanhð3:38n0Þ ð5Þand if ξoN0.275

Rss ¼ 0:69n0−0:19ðotherwise Rss ¼ 0:Þ ð6Þ

ηswash migrates up and down the beach with tidal and set-up induced variations to the water level.

An example of these parameters is given in Fig. 6. Theprobability of inundation is constant offshore and thendecreases landward. The probability of breaking in-creases (at least under the moderate energy conditionsshown) landward then decreases inside the averagelocation of the breakpoint. B and S are necessarily lessthan I. The probability of swash influence increaseslandward, then decreases. Various averaging procedureswere trialled (e.g. by averaging over conditions present

ftp://polar.ncep.noaa.gov/pub/history/waves

Fig. 5. Rectified and time-averaged video images of the southern end of Muriwai Beach. Images were taken on December 3rd 2002 (13:00), February15th 2003 (15:00), May 11th (11:00), August 24th (13:00) and October 26th (14:00). (These times were restricted by lighting conditions and imageavailability). The groundwater seepage line measured using RTK-GPS is marked with a black line as a ground truth of the rectification. Profilelocations are marked with heavy black lines (‘S’ = South Profile, ‘M’ = Middle Profile and ‘N’ = North Profile). The black shadow on the Octoberimage is a cloud. The southern portion of the beach is not visible with the camera due to obstruction from the headland. Images are from the ARGUSnetwork, R. A. Holman, Oregon State University. http://cil-www.oce.orst.edu:8080/.

Fig. 6. An example of the cross-shore variation of the probability ofinundation Iweek (crosses), wave breaking influence Bweek (stars),swash influence Sweek (pluses) and when the beach face is dry (circles).This example is from the North profile, averaged over the week prior tosampling, for the sampling period in August.


in the day, week and month prior to when samplingoccurred) but this made no detectable difference to thestatistical analysis.

In order to quantify the effect that various externalprocesses had on the temporal and spatial patterns ingrain size and mineralogy, step-wise multiple linearregression analyses were run using mean particle sizeand percentage of opaques as the dependent variables,and h, Hs, wave direction (θ) and the surfzone pa-rameters (I, S, B) as the independent variables. Hs and θwere averaged over the day, week, and 2-months prior tosampling respectively, to give 6 variables. Only in-dependent parameters which were not substantiallycorrelated amongst themselves were included in eachmodel (for example, Hsweek and Hsmonth were correlated,see Fig. 3, so the one that resulted in a model thatexplained slightly more of the variance was retained).Thiswas necessary since colinearity between independentvariables can result in nonsensical results in multipleregression analysis. The goal of the step-wise regression

exercise is to explain themost amount of variancewith theleast number of variables (this, accordingly, will maxi-mize the F statistic, or the significance of the result). Allreported values are significant with pN0.95.

http://cil-ww.oce.orst.edu:8080/


A parameter indicating the probability that the beachface was dry was also trialled, but did not provide anyimprovement in the regression model. (An example ofthis parameter is plotted in Fig. 6). The variableincorporated a retreating seepage line during the ebbtide following Turner (1995a,b). The hydraulic conduc-tivity was set so that the modelled and observed seepageline (which was measured during surveying) matchedalong the profiles.

5. Results

5.1. Mineralogy

Petrographic analyses showed the sand to be dom-inated by opaque minerals, augite, hornblende, plagio-clase and rock fragments, with minor quantities ofhypersthene, zircon, garnet, quartz, red-brown amphi-bole, blue-green amphibole, volcanic glass, and shellmaterial. Examination of the opaque minerals showedthat titanomagnetite was dominant (∼90%) over ilmen-ite (8–10%), with minor amounts of hematite (2–3%).Titanomagnetite occurred either as homogeneous grainsor with ilmenite exsolution lamellae, and commonlyshowed alteration to maghemite on rims and alongfractures. Some titanomagnetite grains also containedinclusions of apatite, chalcopyrite, and pyrrhotite.

Fig. 7. (Top panel) An example from October, South profile, of the cross-shorsize (line). Minerals are listed clockwise from the top of each pie diagram. (BDashed lines mark the approximate spring tidal range.

5.2. General cross-shore trends in mineralogy

The cross-shore pattern of mineral distributionshowed opaque minerals dominating the upper beach,augite dominating the mid-beach, and plagioclase,hornblende and rock fragments mainly near the lowspring tide waterline (Fig. 7). These variations are con-sidered to be related to the respective density of theminerals, which can be split further into higher-densityand lower-density groupings (Komar et al., 1989; Frihyand Komar, 1991; Frihy, 1994; Frihy and Lotfy, 1994;Frihy et al., 1995; Hughes et al., 2000). Generally thehigher-density opaque minerals titanomagnetite andilmenite occurred in association with garnet and zirconand had a finer mean grain size, whereas the lower-density heavy minerals such as hornblende, augite andhypersthene tended to be coarser grained. The leptokur-tic grain size distributions of the light-heavy and heavyfractions resulted in clearly bi-modal distribution whenboth fractions were present (Fig. 8). Moreover, thepercentage of heavy minerals in each sample (%H) wasstrongly inversely correlated (r2 =0.82) with size,indicating opaque grains were smaller. Light-heaviesand light minerals were not so clearly segregated (e.g.along the South profile in May, Fig. 8). Qualitativeobservations also indicated that there was some cross-shore variation in shape, where the titanomagnetite

e distribution of minerals (pie diagrams) and the associated mean grainottom panel) Beach profile from October along the South profile line.

Fig. 8. Cross-shore and alongshore variations in mineralogy and particle size. Each of the 15 panels corresponds to a different profile and time. Thetop portion of each panel shows the cross-shore distribution of mineral abundances grouped as heavies (black bars), light-heavies (gray bar) and lights(white bar). The bottom portion shows the grain size distribution with the modes marked as black bullets. “D” in the cross-shore scale means thesamples are collected at the dune toe.


grains dominating the upper beach face were morespherical relative to the lower-density grains such asaugite.

Although the mineralogy of each size class was notmeasured, the narrow and sometimes bimodal grain sizedistributions allowed estimates of the cross-shore trendsin settling velocities to be made. In cases where thesample consisted mainly of opaque mineral grains (seeFig. 8), the opaque minerals clearly had a very narrowdistribution of particle sizes with a mode of 3.25 ϕ(Dec–May) and 3 ϕ (Aug–Oct). The distribution oflight-heavies (mostly augite and hornblende) is not quiteso leptokurtic. Nevertheless, from results collected whenthere are mainly only light-heavies (e.g. in August alongthe North profile), one can assume that the mode is 2.75

ϕ. Using Gibbs' equations (with the adjustment factor ofKomar for natural sediments), gives a settling velocityrange of 5–5.6 cm/s for opaque minerals (pre-August),5.6–6.2 cm/s (August and later) and a settling velocityrange for the light heavies of 4.6–5.3 cm/s (the lowervalues are for smaller hornblende grains and the largerfor larger augite grains). At least prior to August, there islittle difference between settling velocities of the heavygrains and the light-heavies (given the accuracy ofsieving), and after August the separation is not large.Moreover, a subsample of 10 g from each of the samplestaken along the South profile, was mixed and sieved to0.5 ϕ intervals. Each interval was point-counted and theresults are given in Fig. 9. The settling velocity of morethan 85% of the grains of each mineral type was 5.6–

Fig. 9. Mineralogy of each size fraction of a sample that was takenfrom a mixture of all the samples from each cross-shore locationcollected in May from the South profile. The percent of total weightretained in each sieve is also plotted as a line.

Fig. 10. Beach elevation changes (Δh) as a function of cross-shoredistance over the three months between sampling times.


6.9 cm/s (opaque minerals), 4.2–6.5 cm/s (augite) and4.1–6.2 cm/s (hornblende), and 3.5–6.5 cm/s(plagioclase).

5.3. Forcing and morphological response

Forcing conditions varied during the study whichprovided a good range over which to examine the pro-cesses controlling placer formation. The mean signifi-cant wave height (Hs), averaged over the 2 months priorto sampling, was lowest in summer and autumn(February and May), whereas the mean wave heightaveraged over the week prior to sampling exhibited theinverse behaviour (Fig. 3). Wave height on each sam-pling day increased throughout the study period. Wavesapproached the beach from the southwest at the be-ginning of the study, veering around to a more northerlyapproach toward the end (Fig. 4).

Muriwai Beach experienced significant accretion fol-lowed by erosion during the sampling period. During theautumn (Feb–May) and winter months (May–Aug), thebeach accreted more than 0.7 m. Over the followingspring (Aug–Oct), severe erosion brought the Northprofile down by more than 1 m. The erosion andaccretion patterns appear to mirror the significant waveheight averaged over the 2 months prior to sampling,when the reduced wave height in autumn caused ac-cretion, whereas the spring storms in September andOctober were accompanied by a maximum wave heightof N6 m (Fig. 3) and caused erosion.

There were alongshore differences to erosion trends(Fig. 10). During summer, the South profile eroded

whereas the North profile accreted mildly. In autumn, theMiddle profile accreted most, whereas in winter theSouth profile accreted the most. During the summer andautumn, the waves approached Muriwai Beach from thesouth causing a northward drift which resulted in mosterosion at the South profile, where the sand supply wasblocked by the headland. In August, the wave anglebecame more northerly (Fig. 4), and the wave heightdropped (Fig. 3), which would slacken the alongshoredrift and allow for more accumulation along the Southprofile. Moreover, the transverse bar occurring seawardof the Middle profile migrated southward so that in May,August and October a rip current exited the beach face atthe Middle profile (Fig. 5). Also seaward of the Northprofile, a rip current was established during October. TheSouth profile was only in close proximity to a rip channelin December. These rip currents, and the change in meanmonthly wave angle, account for the observed along-shore differences in erosion and accretion patterns.

5.4. Temporal and spatial patterns in grain size andmineralogy

Results of the regression analysis indicated thathydrodynamics could account for a good deal of thevariation in grain size and mineralogy. Forty-eight


percent of a total of 58% of variance in mean particle size(dm) was explained by B and Hsday (F=60) only, with Bbeing by far the most important parameter (explaining31% on its own). The total explainable variance is the r2

obtained using all 10 available parameters in the model.Although this maximises the variance, it does notprovide insight into the importance of each parameter.Particle size became finer with decreasing B anddecreasing daily wave height. The I and B parameterswere correlated (and to a lesser extent S) and thereforecould be substituted for B and provide almost as good amodel, although the S parameters were associated withpositive rather than negative regression slopes. (This wastrue of all cases reported below.) Hsweek explained nearlyas much variance in dm as Hsday. Weekly wave angleadded 1% to the variance, where sediments fined withincreasing wave angle.

Part of the reason that so little of the variance in dmwas explained by regression analysis is that the 3 profilesbehaved very differently to changing forcing conditions.Seventy-nine percent of a total of 85% of variance in dmalong the North profile (Fig. 8, right hand column), wasexplained by Hsday, B and θday (F=52), where thesediments fined with decreasing wave height, decreasingbreaking influence and increasing wave angle. Waveheight was by far the most important control on dm,explaining 52% of variance on its own. Conversely, 57%of a total of 73% of variance in dm could be explained bythe I and Hsday along the Middle profile, where dmdecreased with decreasing I and Hsday. Along the Southprofile near the headland (Fig. 8, left column), 66% of atotal of 75% of variance in particle size was explained byB and Hsweek. Particle size fined with decreasing Band Hs.

Despite the correlation of the fraction of opaqueminerals in each sample (%H) with the inverse of particlesize (r2 =0.82), the factors that best explained thedistribution of mineralogy over space and time weredifferent, depending more on the wave conditions duringthe day or week prior to sampling rather than in themonths prior to sampling. For example, of a maximumexplainable of 41%, 32% of variance in %H wasexplained by B and Hsweek, with %H increasing withdecreasing wave height and decreasing probability ofbreaking.

As with the variations in dm, the mineralogy variedquite differently along each profile. For the Northprofile, 73% of a total of 77% of variance in %H wasexplained by B, Hsday, θday, in order of importance. %Hincreased with decreasing breaking influence and waveheight, and also with increasing wave angle. In theMiddle profile, 43% of a total of 80% of variance in %H

was explained by S, so that %H increased withincreasing probability of swash influence. Variationalong the southernmost profile was best explained by S(r2 =0.59 of a total of 0.72), where %H increased withincreasing swash influence.

6. Discussion

6.1. Sorting mechanisms and the formation of placers

In general, particles became finer and more enrichedin opaque minerals as the probability of breaking in-fluence decreased, so they became finer landward. Thispattern of heavy mineral concentration landward hasbeen shown in numerous studies (e.g. Komar andWang, 1984; Peterson et al., 1986), and has beenattributed to the preferential entrainment of larger, low-density particles which protrude more into the flow.These particles are subsequently transported seawardby surfzone processes such as undertow. The sortingcould also be accomplished by wave (or swash) as-ymmetry since shoreward wave-orbital currents arestronger than seaward currents. Thus the more difficultto entrain heavy minerals would be entrained only onthe shoreward portion of the wave cycle. Given theapproximate settling equivalence of the dense andlight-heavy minerals, it is likely that selective entrain-ment is also important at Muriwai Beach. Moreover,mean grain size of each mineral assemblage does notincrease appreciably seaward as described by Komarand Wang (1984).

The other mechanisms for placer formation require asettling difference between heavy and light grains. Forexample, transport sorting would occur if the opaqueminerals had greater settling velocity so that they weretransported lower in the water column or as bedload. Inthis case heavies would be transported landward by thewave drift currents whereas the light-heavies would betransported seaward by undertow. It is unlikely thatcross-shore patterns in enrichment of opaque minerals atMuriwai are related to variations in settling velocity,since there appeared to be little cross-shore pattern insettling velocity. Hence, sorting mechanisms that dependon differences in settling are unlikely to play a role inplacer formation on Muriwai Beach. This similarity insettling velocities caused by an inverse relationshipbetween grain density and grain size is common to placerdeposits from dissipative beaches (Peterson et al., 1986).Hughes et al. (2000) speculate that high turbulence levels(as might occur on a dissipative beach) enhance thedifference in settling velocities between light and heavyminerals, and is clearly an area that needs more detailed

Fig. 11. Bottom two panels show particle size and percentage ofopaque minerals plotted as a function of probability of inundation(Iday). Symbols correspond to the wave conditions on the day prior(Hsday) to sampling and measurement of the profile. The top panelshows S/B (solid line) and the probability that the beach face is dry(dashed line).


work. Shear sorting can cause migration of heavies tothe surface but it is not considered important whenmaximum grain size is small b0.2 mm (Hughes et al.,2000) such as at Muriwai Beach.

6.2. Cross-shore trends in particle size and mineralogy

The upper foreshore at Muriwai Beach was consis-tently highly enriched in black heavy minerals (titano-magnetite and ilmenite) during the study period.Conversely, the lower beach face varied between heavymineral enrichment, a greenish black enrichment ofmainly augite and hornblende, and sometimes a lightbrown enrichment of plagioclase and rock fragmentgrains. This cross-shore variation of enrichment wasstrongly correlated with the landward decrease in theprobability of inundation or breaking. The inundationand breaking parameters (I and B) were cross-correlatedat cross-shore distances b70 m, so it was impossible todetermine which was more important in concentratingthe placer.

Examining the residuals of the regression analysisshowed a distinctly non-linear pattern that cannot becaptured by the linear regression model used. Fig. 11shows a landward decrease in particle size and increasein the fraction of heavy minerals up to an inundationprobability of about 50%, after which the fraction ofheavy minerals decreases landward and the particle sizecoarsens. This is particularly true of samples from higherwave energy conditions. This change in characteristics isalso clearly visible at cross-shore distances ≤20 m inFig. 8. The change in pattern midway up the beach couldbe indicative of a change from marine to aeoliantransport processes. Although the hydraulic conductivityis low so that some of the beach face remains saturated athigh tide, thisWest Coast site is exposed to the prevailingwesterly wind, which can commonly reach over 15 m/s.In addition, the beach face is protected from seawardwinds by the faceted dunes. The large tidal range (∼3 mat spring tides) and low beach slope means that much ofthe beach is exposed for long periods during low tide. Inthe marine part of the profile, particles become finer andheavier as the seaward-moving undertow sorts the lessdense, larger particles seaward. In the aeolian portion ofthe profile, a similar pattern is observed, except thelighter particles are moved landward by the prevailingshoreward wind (presumably onto the dunes). Duringeach of the sampling days, the wind speed was generallyconsistent at about 6 m/s except for the May samplingwhere there was a drop to 4 m/s. Fig. 8 suggests that theconcentration of opaques on the upper beach face isreduced in May. A probability of inundation of 50% also

corresponds to the location where the beach face changesfrom being perpetually saturated, to drying at low tide sothat the probability of drying changes very rapidly from0–50% over b10 m.

The location where the cross-shore trend in particlesize and mineralogy reverses, also corresponds to atransition zone where the importance of breaking de-creases relative to the importance of swash (B /S dropsbelow 2, Fig. 11). In this case, in the seaward regionwhere breaking dominates, offshore currents such as


undertow would transport the coarser, lighter materialseaward, whereas in the landward region where swashdominates, the swash could preferentially transport thelighter, coarser material landward. Butt et al. (2001)show that on fine sand beaches such as Muriwai,infiltration on the uprush reduces the bed shear stress andexfiltration on the downrush decreases the bedshearstress, causing a net transport seaward. On a mixedsediment beach this would sort the harder-to-entrainmaterial seaward. This process could only be importanthigh on the beach face where the sediment is unsaturatedfor a portion of the tidal cycle as observed. This wouldleave a lag deposit at the location of the transition zonemarked by a maximum of density and a minimum ofparticle size in Fig. 11.

6.3. Wave energy and placers

The particle size and the percentage of opaqueminerals did not show any strong dependence onmonthlywave conditions, and did not vary with net beachelevation changes, which were measured at 3-monthlyintervals. The grain properties instead responded to theconditions experienced on the day of sampling, which isnot surprising, since the samples were from the surface10 cm. Unpublished analysis of the top half metre ofsediments at Muriwai shows that the sediment hassignificant bedding, with layers of opaque mineralsreaching N10 cm thick separated by layers of lighter-coloured augite, hornblende and plagioclase.

The decrease in particle size and the accompanyingincrease in percentage of opaque minerals withdecreases in the daily significant wave height that wasobserved here is not what has commonly been reportedin other studies of placers. In most cases, placer depositsformed in the winter, which are times of higher waveenergy in the northern hemisphere. The case issomewhat different here since the heavy mineralassemblages are a perennial feature of this beach, andthere is no distinct time when placer formation can beobserved. The placer is simply expanding seaward to fillmore of the beach profile. There are several reasons whythis could be occurring. The surficial sediments high onthe beach face do not, in general, contain any lightergrains (Fig. 8). Therefore there is no obvious source oflight minerals to be transported seaward and cause theobserved cross-shore trend in grain density. However, asthe surfzone passes over the beach during the tidal cycle,the top layer of sediment (the active layer) is entrained,mixed and redeposited, and then acted upon by theswash as the tide retreats over the beach face.Observations clearly show that the sediments are

deposited in layers of dark and light minerals insidethe beach face. During high wave energy conditions, thewaves are able to mix the sediment to a greater depththan in low wave energy conditions. Thus it is only inhigh wave energy conditions, that the waves can mixsome of the deeper, lighter deposits to the surface, andprovide a source for the light minerals which are thensubsequently entrained and sorted seaward by theswash. Low energy conditions simply rework theexisting surface placer deposit, with undertow currentsperhaps transporting some of the placer seaward. Thisexposed West Coast site nearly always has an activesurfzone; even when the wave height decreases, it is stillover 1 m. Koomans and de Meijer (2004) show inlaboratory experiments that the availability of differentmineral fractions for sediment transport is determinedby the concentrations in the active layer which is of theorder of several centimetres. If the placer deposit ac-cumulates to a depth greater than the active layer, it cutsoff the source of the lighter mineral fractions, effectivelystopping any potential for further sorting.

Although, in general, the particle size varied only inresponse to changes in the contribution of various min-eral fractions, there appeared to be a general coarseningof the grain size over the whole beach face during thestudy. This is evident in cases where the sample wascompletely composed of heavy minerals (Middle profilein winter, May–October, and North profile in summer,December). In these samples, the particle size coarsenedfrom a mode of 3.25 ϕ to a mode of 3 ϕ. Moreover, therewere only very small amounts of sediment with a 3 ϕmode and a 3.25 ϕ mode present at the beginning andend of the study respectively. This indicates a change inthe supply of sand to the beach system, perhaps as asediment ‘slug’ migrated around the southern headland.This change in supply may have been associated with thehigher mean wave height during the two months prior toboth the August and October sampling periods relative tothe previous mean wave heights (December, Februaryand May). This coarsening of the opaque fractionaccounts for why the particle size variations were betterexplained by variations in the wave height forcing thanwere variations in the mineralogy.

6.4. Alongshore trends in particle size and mineralogy

There was considerable alongshore variation in theresponse of the beach to varying wave conditions. TheSouth profile was protected from the dominant waves bythe headland, and therefore wave height had very littleeffect on mineralogy along this profile. The influence ofswash best accounted for the variation in mineralogy,


again indicating that swash asymmetry may play a role insorting the placer. The North profile was most exposed tothe incident wave conditions, and so mineralogy andparticle size both varied with changing significant waveheight of the day prior to sampling. Sediments from theNorth profile also become more enriched in opaqueminerals as the waves changed from a southerlyapproach to a northerly one. This may be an indicationof a change from a northward to a southward longshorecurrent, which would carry the light-heavies southwardaway from the North profile.

Past work has shown enhancement of heavy mineralsalongshore from a major headland, the distance depend-ing on how far the headland protrudes into the longshoredrift (Peterson et al., 1986). In the examples fromOregon, the placer deposits occur on the up-drift leesideof the headland where the shoreline orientation changesand interrupts the longshore drift. Although this distanceappears to correspond with the Middle profile, whichshows greater placer concentration, there is not a sig-nificant change in the shoreline orientation and the driftis northward rather than southward. It could be that acertain distance is needed for the northward longshorecurrents to strengthen and therefore they reach theirmaximum ability to transport away lighter minerals nearthe Middle profile.

Both the Middle and North profiles also tended tohave more opaque minerals when they were in closeproximity to the exit location of a rip channel. Thisoccurred particularly from May to October for theMiddle profile and in December to October for the Northprofile (Fig. 5). Although the rip current provides astrong seaward flow to carry the light-heavies seaward,wave energy and therefore also swash energy arereduced in areas of rip currents. So proximity to a ripcurrent exit point should have a similar effect on sed-iments to a reduction in the wave climate, explaining theobserved (Fig. 8) increase in opaqueminerals when theseprofiles were near rip-channels.

Although our sampling regime is not detailed enoughto detect it, presumably alongshore variations to thecross-shore location of the groundwater exit point shouldcorrespond to alongshore variations in the divisionbetween aeolian and marine processes, and thus explainsome of the alongshore variation in composition. Spatialvariability in the water table can be caused by alongshorechanges in the underlying geology and stream dischargesonto the beach. Water table variations could also becaused by changes to the hydraulic conductivity, whichin turn are associated with alongshore changes to thegrain size properties, providing a mechanism for afeedback cycle. Moreover, alongshore variations in set-

up associated with wave breaking patterns could elevatethe groundwater table between rip current exit points.

7. Conclusions

Muriwai Beach is characterised by perennial depositsof titanomagnetite and ilmenite along the upper beachface which extend seaward from time to time, displacingsurficial deposits of augite, hornblende and plagioclase.The placer is most enhanced at the cross-shore locationwhere the beach is on average inundated for 50% of thetime, declining upward and downward from these points.This may correspond either to the point separating theinfluence of aeolian and marine processes or the pointseparating swash domination and breaking domination.

The approximate settling equivalence between denseand light grains across the beach face suggests thatselective entrainment is responsible for sorting. Thedenser grains reside in the interstices between the largergrains, and thus require greater bed shear stresses forerosion. This is similar to other dissipative beaches, butin contrast to results from reflective beaches (Hugheset al., 2000), where shear sorting may play a moreimportant role.

In general, sediments become more enriched inopaque minerals right across the beach face duringlow-wave conditions and also near rip current exit pointswhere the wave energy is reduced. This is in contrast toprevious work showing that placers form under higherwave energy conditions. In most beaches, the lighterminerals are in ample supply, with heavier mineralsgenerally making up a very small portion of the particlesize distribution. Muriwai is unusual in that the reverse istrue: sediment is dominated by heavy minerals, and lightminerals are often unavailable due to burial in layersbeneath the beach face.

Acknowledgements

Scholarship funding was awarded to A. Robinson bythe University of Waikato, Broad Memorial Fund, andthe New Zealand Federation of Graduate Women. Fieldassistance was provided by D. Immenga, B. Spendelowand G. Wheatley (CKL Surveyors). Laboratory assis-tance was provided by I. Blair, G. Xu, A. Burgess, and B.Needham. The National Institute of Water and Atmo-spheric Research Ltd provided tidal predictions. Thetime-averaged video images are from R. A. Holman's(Oregon State University) ARGUS network. C. Pilditch,G. Coco, P. McComb, and W. de Lange provided usefuldiscussion on the interpretation of the results. Commentsfrom two anonymous reviewers improved the paper.


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