This article was downloaded by: [83.37.2.197]On: 22 October 2014, At: 21:21Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Marine Georesources &GeotechnologyPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/umgt20

Characterization of marine aggregatesoff Waikiki, O'ahu, Hawai'iCharles L. Morgan a , James H. Barry Jr. b & Michael J.Cruickshank ca University of Hawai'i Marine, Minerals Technology Center ,2540 Dole St.,Holmes 246, Honolulu, HI, 96822, USA E-mail:b Sea Engineering, Inc. , Waimanalo, Hawai'i, USAc University of Hawai'i Marine, Minerals Technology Center ,Honolulu, Hawai'i, USAPublished online: 23 Dec 2008.

To cite this article: Charles L. Morgan , James H. Barry Jr. & Michael J. Cruickshank (1998)Characterization of marine aggregates off Waikiki, O'ahu, Hawai'i, Marine Georesources &Geotechnology, 16:1, 75-94, DOI: 10.1080/10641199809379958

To link to this article: http://dx.doi.org/10.1080/10641199809379958

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the“Content”) contained in the publications on our platform. However, Taylor & Francis,our agents, and our licensors make no representations or warranties whatsoever as tothe accuracy, completeness, or suitability for any purpose of the Content. Any opinionsand views expressed in this publication are the opinions and views of the authors,and are not the views of or endorsed by Taylor & Francis. The accuracy of the Contentshould not be relied upon and should be independently verified with primary sourcesof information. Taylor and Francis shall not be liable for any losses, actions, claims,proceedings, demands, costs, expenses, damages, and other liabilities whatsoeveror howsoever caused arising directly or indirectly in connection with, in relation to orarising out of the use of the Content.

This article may be used for research, teaching, and private study purposes. Anysubstantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,systematic supply, or distribution in any form to anyone is expressly forbidden. Terms

& Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Dow

nloa

ded

by [

83.3

7.2.

197]

at 2

1:21

22

Oct

ober

201

4

Characterization of Marine Aggregates offWaikiki, O'ahu, Hawai'i

CHARLES L. MORGANUniversity of Hawai'i Marine Minerals Technology CenterHonolulu, Hawai'i, USA

JAMES H. BARRY, JR.Sea Engineering, Inc.Waimanalo, Hawai'i, USA

MICHAEL J. CRUICKSHANKUniversity of Hawai'i Marine Minerals Technology CenterHonolulu, Hawai'i, USA

Researchers at the University of Hawai'i at Manoa have been working for the pastseveral years to develop the necessary techniques for finding and quantitativelycharacterizing offshore unconsolidated carbonate deposits with potential for beachnourishment and use in construction aggregates for tropical island communities.This article examines particular results of this research, with special attention givento the area offshore from Waikiki Beach. Acoustic surveying, water-jet probing tomeasure the thickness of unconsolidated material and three different samplingmethods were used in this study. Two separate seismic systems were used for thesubbottom profiling survey, a Datasonics Bubble-Pulser® system and a broad-band,frequency-modulated ("chirp") prototype system.

The following conclusions were reached. (1) Many different types of sedimentunderlie tropical island carbonate sand deposits and serve as refusing horizons to jetprobing. Examples include consolidated or unconsolidated reef debris, beach rock,cemented sand, and various types of conglomerates formed from rhodoliths (coral-line algae) or reef detritus. (2) Massive coral growth over clastic deposits is not acommon offshore feature in this area, though it does occur in some areas off theReef Runway. (3) Matrices of the coral Porites compressa, in-filled with sand, mayhave acoustic properties similar to those of the sand bodies. Such deposits may bedifficult to distinguish from unconsolidated deposits from seismic records alone. (4)Significant new prospects for offshore aggregates were found in the insular shelfoffshore from Southern O'ahu. A total of 5,100,000 m3 were mapped off Waikiki.The Makua Shelf deposits in this area presently appear to be the best prospects forcommercial development.

Keywords marine aggregates, Waikiki sand, offshore sand and gravel

Researchers at the University of Hawai'i at Manoa have been working for the pastseveral years to develop the necessary techniques for finding and quantitativelycharacterizing marine carbonate sand deposits with potential for beach nourish-

Received 25 June 1997; accepted 13 November 1997.Address correspondence to Charles L. Morgan, University of Hawai'i Marine Minerals

Technology Center, 2540 Dole St., Holmes 246, Honolulu, HI 96822, USA. E-mail:SauChai@aol.com

75

Marine Georesources and Geotechnology, 16:75-94,1998Copyright © 1998 Taylor & Francis

1064-119X/98 $12.00 + .00

Dow

nloa

ded

by [

83.3

7.2.

197]

at 2

1:21

22

Oct

ober

201

4

76 C. L. Morgan et al.

ment and use in construction aggregates for tropical island communities. This workhas included the acquisition of high-resolution seismic reflection profiles, samples,and hydraulic probing for ground-truth verification in three areas in Hawai'i(Figure 1). A report on this survey was given at the 1991 meeting of the MarineTechnology Society (MTS'91) and published in the proceedings (Barry, 1991; Noda,1991). A complete description of this work is presented by Barry (1995).

This article examines particular results of this research, with special attentiongiven to the area offshore from Waikiki Beach (Figure 2). Certain characteristics ofthe deposits determined for the Waikiki site have geological and economic implica-tions for the sedimentary environments offshore from many tropical island environ-ments. The lessons learned from ground-truth verification of seismic reflectionprofiling indicate significant limitations in the interpretation of such acoustic data,which should be acknowledged in future work in other offshore sedimentarysystems.

Previous Work

The insular shelf of southern O'ahu dips at an angle of about 5° from sea level tothe shelf edge at approximately 90-110 m water depths. The slope then dropssteeply (approximately 15°) to a broad, deep shelf at 500 m (Gregory & Kroenke,1982). The geologic history of the insular shelf off O'ahu is dominated by theepisodic rise and fall of sea level, with the consequent migration of depositionalenvironments and the formation of a geological record of carbonate facies. Theprimary sedimentary processes on the insular shelf are reef accretion, erosion, anddeposition of unconsolidated sediments. Generally, high sea level stands corre-

Areas examined in the study

Figure 1. Hawaii sand project survey areas.

Dow

nloa

ded

by [

83.3

7.2.

197]

at 2

1:21

22

Oct

ober

201

4

Marine Aggregates off Waikiki, O'ahu, Hawai'i 77

21°15'N

Figure 2. Waikiki area seismic survey lines.

spond to episodes of reef building, while low stands are indicated by weatheringand erosion of the exposed carbonate complexes. Depositional environments shiftwith the moving shoreline. Shallow-water, high-energy environments may formsedimentary units consisting of conglomerates and coarse sands from wave-abradedreef detritus, and in deeper water these may characteristically grade into sedimen-tary units consisting only of fine sands.

The term "reef rock (or its equivalent, carbonate boundstone) is used here todescribe massive reef limestone units that are formed by a matrix of in situcoral-algal reef growth. Subaerial exposures of reef rock are prevalent on theisland of O'ahu, and include, for example, the reef limestone unit of the WaimanaloFormation (Stearns, 1978). Sherman (1992) has described exposures of this unit atKa'ena Point as the Waimanalo framestone lithofacies. Sherman determined theWaimanalo framestone to be composed of approximately 68% coral, 23% corallinealgae, less than 2% in-situ mollusks, and 7% skeletal sand. The corals weredominated by Porites lobata, Leptastrea purpurea, and rare occurrences of Pocillo-pora meandrina. All of these corals are common to the shallow, high-energy reefenvironments of modern Hawai'i.

Stearns and Chamberlain (1967), and Resig (1969) identified at least eightcycles of marine transgressions and regressions during Pleistocene (and possiblylate Pliocene) time. Stearns (1978) compiled extensive descriptions of the variouspaleo-shorelines of the Hawaiian islands, both submerged and emerged. Thedetailed history of eustatic sea level changes proposed by Stearns and otherassociated evidence in the Hawaiian geological record are complex and not thesubject of this report. A simplified model, based on the work of Stearns, ispresented here and used to interpret major features of the seismic records,

Dow

nloa

ded

by [

83.3

7.2.

197]

at 2

1:21

22

Oct

ober

201

4

78 C. L. Morgan et al.

discussed below. One particular marine terrace, which Stearns named the Makuashelf, is an important feature in the Waikiki area. The Makua shelf occurs in waterdepths of about 70 m and has been observed on many seismic reflection profilesaround the island of O'ahu (Campbell et al., 1972). It is extensively documented inour survey work. Stearns suggests that it was formed during the Waipio low stand,approximately 500,000 years ago, which may have reached a level of about 100 mbelow current sea level, although this is not certain. The great extent and width ofthe - 70-m shelf suggest a long pause of the sea at this level.

The Waipio low was followed by an interglacial transgressive period thatculminated in the Waimanalo low stand (about 8 m above sea level) approximately125,000 years ago. A series of minor regressions and transgressions followed. Theseinclude the Kawela low, the Barbers Point, and the Lahaina Roads low-standshorelines, and the Leahi, Kaneohe, and Makai Range high stands. It is possiblethat much of the limestone reef surface and related sedimentary units that areseen offshore today date from this time.

The Mamala low stand of the sea (—100 m) preceded the last major transgres-sion to present sea level. It was probably during this low stand that some sectionsof the Makua Shelf were eroded. Deep alluvial channels were cut through the reefsand sediments of Waikiki to depths of about 60 m (Ferrall, 1976). According toFerrall, these channels exhibit the youthful characteristics of being steep-sided,relatively straight, and oriented perpendicular to the coastline. The sediments atthe bottom of the channels are indicative of relatively high deposition energy, andinclude boulder-sized alluvium. Submarine equivalents of these channels, particu-larly the Halekulani sand channel, are well documented in this report.

An alternative model to explain many of O'ahu's paleo-shorelines has recentlybeen offered by Fletcher and Sherman (1994). Based on correlations with reefaccretion histories from Barbados and Papua New Guinea, and using the eustaticsea level curve constructed by Shackleton (1987), they have proposed that many ofthe paleo-shorelines observed on O'ahu can be correlated to paleo-climatologicalevents that occurred during the last major transgression, from 18,000 years ago tothe present. Episodic pauses in the rate of sea level rise during this transgressionwere responsible for carving notches in carbonate escarpments around the islandthat were subsequently inundated and preserved as the rise resumed. In theirmodel, many of the submerged shorelines named by Stearns are collected into twocomplexes, the Penguin Bank shoreline complex at —48 to —64 m, and theKaneohe shoreline complex at -24 to -30 m.

The Fletcher-Sherman model presents a simpler chronology for many paleo-shoreline features than the model presented by Stearns, and in effect proposes thatmany of the geological structures observed are much younger than previouslythought.

The great O'ahu shield-building volcanic eruptions had ceased by 2.2 millionyears before the present (2.2 Ma). However, smaller eruptions of the HonoluluVolcanic Series occurred from middle to late Pleistocene time (Macdonald et al.,1983). While flows of basalt and tuffs from this series are interlayered with coastalplain sediments and carbonate reefs in subaerial exposures (Ferrall, 1976), noevidence from seismic records of basalt flows interlayered with carbonate reefsoffshore has been presented for our survey areas in the literature, nor was anyevidence of found in the survey work reported here. However, layers of ash frompyroclastic eruptions may indeed be interbedded with offshore carbonate sedi-

Dow

nloa

ded

by [

83.3

7.2.

197]

at 2

1:21

22

Oct

ober

201

4

Marine Aggregates off Waikiki, O'ahu, Hawai'i 79

ments, and have been observed overlying exposed carbonate reef facies, at depthsof approximately 20-25 m offshore of Diamond Head, Kahala, and Ewa Beach(Fletcher & Sherman, personal communication). Depths to the volcanic shieldbasement from the insular shelf of south O'ahu are on the order of 400-500 m(Gregory & Kroenke, 1982).

Deposit Characterization

It has long been thought that Hawai'i's deeper offshore sand resources could bepotentially useful for beach nourishment and construction aggregates, and muchspeculation and research have been undertaken to this end. Moberly and Cham-berlain (1964), in their benchmark study of Hawaiian beaches, published the initialquantification of nearshore sand deposits of selected Hawaiian beaches. Themechanisms of nearshore sediment production, distribution, and transportation forHawaiian reef and beach systems were explored by Inman et al. (1963), Moberly(1968), and Gerritsen (1978). The measurement of the large erosion rates ofHawaiian beaches was systematically carried out for many Hawaiian beaches byChamberlain (1968). Moberly et al. (1965) determined that the primary compo-nents of carbonate sand on Hawaiian beaches were foraminifera, mollusc shellfragments, and coralline algae.

Marine Advisors, Inc. (1968), mapped known sand deposits off the HonoluluHarbor and the Halekulani sand channel off Waikiki. They used divers equippedwith water-jet probes and air-jet probes to identify the base of uncemented sanddeposits. That study led to speculation about possibilities for sand reserve exploita-tion (Casciano & Palmer, 1969). Moberly and Campbell (1969) mapped a sanddeposit in the middle of Kaneohe Bay using seismic refraction techniques. Camp-bell et al. (1970) conducted a 500-J sparker survey off leeward O'ahu, and used thesame system off leeward Maui and Molokai (1971). An EG & G Uniboom® systemwas used at Kahana Bay, Mokuleia, Waimea Bay, and offshore of the Reef Runwayby Coulbourn et al. (1974), and by Moberly et al. (1975) on Penguin Bank.

Large volumes of offshore sand were reported by these surveys: 91 X 106 yd3

(70 X 106 m3) off the Reef Runway, and 373 X 106 yd3 (285 x 106 m3) offleeward O'ahu as a whole (Campbell et al., 1970); 4,036 X 106 yd3 (3,088 X 106

m3) off leeward Molokai and Maui (Campbell et al., 1971), and 350 X 106 yd3

(268 X 106 m3) on Penguin Bank (Moberly et al., 1975).An effort was also made by Ocean Innovators, Inc., to sample the offshore

deposits around O'ahu in several separately funded efforts from 1976 to 1978.Thirteen separate sample locations were chosen around the island. The sampleswere analyzed for their grain size distributions by both HIG (Hawai'i Institute ofGeophysics) and Pacific Concrete and Rock Co., Inc. A total of 279 samples wereanalyzed, and Campbell (1979) and Coulbourn et al. (1988) issued reports on theresults. Dollar (1979) wrote a summary of the cumulative efforts to exploitHawaiian offshore sand deposits to that time. This was followed by two articles byMurdaugh (1979a, 1979b). The first (1979a) included Hawaiian offshore sanddeposits in a resource assessment of hard minerals offshore in the HawaiianArchipelago. The second (1979b) was a feasibility study of mining sand on PenguinBank, based on the reconnaissance mapping of Moberly et al. (1975). Selection ofthe sites for acoustic mapping and sampling was based on these earlier works.

Dow

nloa

ded

by [

83.3

7.2.

197]

at 2

1:21

22

Oct

ober

201

4

80 C. L. Motgan et al.

Methods

Acoustic surveying, water-jet probing to measure the thickness of unconsolidatedmaterial, and three different sampling methods were used in this study. These aredescribed below.

Seismic Stratigraphy

A radiofrequency ranging system with a positioning accuracy of +10 m was usedfor navigation. The acoustic survey lines (Figure 2) form an orthogonal matrixwhose long axis is oriented parallel to the trend of the coastline in waters between20 and 100 m in depth. The 100-m depth contour marks the approximate break inslope of the insular shelf surrounding the island, and thus comprises a naturalsurvey limit. Line spacing of the survey grid was 200 m, which was chosen to ensurediscovery of sand deposits with minimum volumes of 100,000 yd3 (76,444 m3),assuming an average deposit thickness of 2 m.

Two separate seismic systems were used for the subbottom profiling survey, aDatasonics Bubble-Pulser ® system and a broad-band, frequency-modulated("chirp") prototype system developed in Hawai'i by two commercial firms, SeaEngineering, Inc., and Precision Signal, Inc. The Bubble Pulser has a modalfrequency near 350 Hz. Band-pass filters were set at 300 Hz and 1,600 Hz in orderto maximize the high-frequency output, but it was observed that very little signalwas received over 1,000 Hz. The chirp system consists of the topside electronics, anunderwater signal cable, and a towed vehicle. The sonar generates cross-sectionalimages of the seabed and collects digital normal incidence reflection data over thefrequency range from 300 to 10 kHz. The images are constructed by digitizing thereflection data, correlating data with the transmitted waveform using a matchedfilter, and then displaying the backscatter correlation as shades of gray or color ona computer monitor. The Datasonics system was used for the bulk of the mappingwork, and the FM system was used over one well-defined area for comparativepurposes. Because the FM system became available only after most of the surveymapping had been completed, it was not possible to use it more extensively.

Results from a comparison survey line which was run using both systems areincluded here to document the differences between the systems. The comparisonline was selected because it covered the most uniform and thickest sand depositknown in the study area, the Halekulani sand channel. Because the carbonate sandbodies are extremely efficient absorbers of acoustic energy, backscatter returns in athick, relatively uniform sand body constitute a good measure of the net systemnoise. To provide some estimation of the differences between the Bubble Pulserand chirp systems, analogous sections of the seismic profiles, taken from thegraphic plotter records from each survey, were scanned optically using normalizedcontrast control to ensure that the same gray-level range was covered for bothsystems. The sections were selected from the middle of the sand body and arebelieved to come from the same volume of the deposit, within the limits imposedby navigation errors and differences between the system beam forming characteris-tics. The optical scans were then recorded in digital bitmap files (GIF format), andthe gray-levels (8 bit, or 256 levels) were extracted directly from the files for thecomparison.

Reef rock has a distinctive acoustic signature that is readily identifiable on theseismic records, both as a bottom return and as subbottom return using the

Dow

nloa

ded

by [

83.3

7.2.

197]

at 2

1:21

22

Oct

ober

201

4

Marine Aggregates offWaikiki, O'ahu, Hawai'i 81

following characteristics: (1) the horizon is highly reflective (has a bright return);(2) there is a lack of continuous horizontal internal reflectors; (3) the presence ofdiffraction tails from bright-scattering point sources such as coral outcrops andpinnacles is widespread; and (4) the seismic signal is rapidly attenuated such thatthis horizon acts as an acoustic basement for the frequency band of the BubblePulser. As a seismic-stratigraphic unit, reef rock is probably similar to what Hineet al. (1981) mapped off Grand Bahama Island as "Pleistocene rock." Depths andthicknesses in units of time were converted to depths in feet using a velocity ofsound in submerged sand of 5,400 ft/sec (1,646 m/s; Moberly & Campbell, 1969).

Ground-Truth Probing and Sampling

Ground-truth surveys began soon after the finish of the seismic surveys andcontinued intermittently as opportunity permitted. The surveys were conducted incollaboration with the Marine Minerals Technology Center by Edward K. Nodaand Associates, Ocean Innovators, Inc., and the University of Washington. Thesurveys included SCUBA diver reconnaissance from a towed sled, diver-operatedjet probes, and both surface and subsurface sampling. Diver-related activities werelimited to a depth of 35 m. Towed-sled reconnaissance enabled mapping ofsand/no sand contacts with limited bottom descriptions. A system of signalsenabled the diver to communicate to persons in the tow vessel who were monitor-ing the navigation. Each of these methods is described briefly below.

Jet Probing

The jet probe consists of a 1-in. pipe connected by hose lines to a hydraulic pump.Water is jetted out the end of the pipe, which is pushed down manually intounconsolidated sand-sized material. Fluidization of the sand enables the probe tobe pushed down by the diver until refusal is met. The nature of the refusal couldoften be determined by feel by the operator. A "hard" refusal indicated a solidbarrier such as solid reef rock or large cobble-sized coral debris. A "crunchy"refusal indicated something less solid, such as a layer of gravel-sized sediment."Soft" refusals were usually due to technical limitations of the probe: The sandbody was thicker than the maximum depth of the probe (about 15 m), or the probewas limited by the length of water hose line that ran to the surface support vessel.Several probes were completed at each station location to ensure reliability.

Jet Sampling

Sand bodies of substantial areal extent and depth as determined by the abovemethods were selected for subsurface sampling using a sampling device invented byOcean Innovators. This "Jet Sampler" used water jets to penetrate into the sandand maintain a fluidized condition surrounding the sampler to prevent seizing. Theprimary jets were reversed at the sampling level and directed through an eductor,which transported a slurry of sand to the surface. Samples were taken at 5-ftintervals with a maximum subbottom depth of 8 m. A total of 77 samples weretaken in this manner.

Dow

nloa

ded

by [

83.3

7.2.

197]

at 2

1:21

22

Oct

ober

201

4

82 C. L. Morgan et al.

Surficial Samples

Some areas beyond workable diver depths were sampled with another specialtysampler developed by Ocean Innovators. This small-tube sampler relies on seabedhydrostatic pressure to fill the sampling chamber, held sealed at 1 atm pressureuntil actuated by contact with the sea floor. Surface samples were also collectedfrom Penguin Bank with this device. For all samples collected, grain-size analysisand petrologic descriptions were completed by Edward K. Noda and Associates(Noda, 1991).

Hard-Rock Corer

Additional ground-truth information was collected with the use of a hard-rockcoring drill developed at the University of Washington (Johnson et al., 1991). Eightholes were drilled off Waikiki and three cores of reef rock recovered. The drillproved incapable of retaining a core of unconsolidated sand, but drill sites in thesand were useful for ground-truth verification of unconsolidated deposits. Maxi-mum penetration of the drill was 10 ft (3 m). Radiometric dating of two of thecores was also completed by other researchers (Grigg, personal communication).

Results

These efforts yielded the following results.

Seismic Profiling

The reef-rock horizons shown on the seismic records are usually unconformablyoverlain by clastic material that may include sand. The clastic deposits in generallack the brightly reflective and point-scattering acoustic characteristics of the reefrock. Moreover, they often contain horizontally continuous internal reflectors.Initial interpretations of the records assumed that the reef rock horizons were thebase of the sand deposits; i.e., all of the overlying sediment was thought to be sand.Thickness estimates of the sand bodies mapped off the Reef Runway in thisfashion are in agreement with previous maps of the sand deposits at the ReefRunway (Campbell et al., 1970), with thicknesses commonly on the order of 60 ft(18.3 m). However, poor correlation between the seismic record interpretations andground-truth surveys made it apparent that a two-layer interpretation of theoffshore geology is too simplistic.

The clastic deposits must be comprised of a range of material with differentlithologies, and while the contact between clastic sediments and reef rock is usuallyclear on the seismic records, the lithologic units within the clastic material itselfare often difficult or impossible to identify. It was originally thought that allhorizons of consolidated sediment or coral debris would show as bright reflectors,similar to reef rock. Instead, the transition between sand deposits and adjoining orunderlying consolidated or otherwise hard sediments is often only a subtle acousticfeature.

A relatively consistent acoustic signature was identified for sand deposits thatincluded (1) a poorly reflective initial surface return, i.e., not "bright," and (2) anacoustically transparent deposit, lacking internal reflectors. This last indicator,

Dow

nloa

ded

by [

83.3

7.2.

197]

at 2

1:21

22

Oct

ober

201

4

Marine Aggregates offWaikiki, O'ahu, Hawai'i 83

however, was not always in evidence. Known sand deposits off Waikiki werejet-prf bed past internal reflectors indicated on the seismic record. Diver surveysalso ' lentified acoustically transparent deposits that are not sand. Stratigraphicinfe -nces should be interpreted within these limitations. The Waikiki alluvialchannel deposits mapped in this way are presented in Figure 3, and a representa-tive acoustic cross section of the terrace deposits is presented in Figure 4.

(a)

Depth in25 Sediments

(m)

i . , . i i , i , , i

0 50 100meters

_ 50

Sand Body

Rubble/Consolidatedi Sediments

(b)

Figure 3. Waikiki channel seismic profile.

Dow

nloa

ded

by [

83.3

7.2.

197]

at 2

1:21

22

Oct

ober

201

4

84 C. L. Morgan et al.

(a)

25

50

75

lgo_

125

• ~ ^ <

(f ^ - *

[ Depth inSediment

. (m)- 25

- 2 5

- 5 0

L 7 5

WaterDepth

(m)

50 100meters Sand Body

Figure 4. Waikiki terrace deposit seismic profile.

Figure 5 shows a comparison of the Bubble Pulser and the chirp system in aprofile across the Halekulani sand channel. Figure 6 presents the results of thecomparison of the two systems using the backscatter values from the sand body inthe Halekulani channel. Figure 7 shows bathymetry, isopachs of sand deposits,traces of the major channels, and the — 73-m marine terrace (Makua Shelf), asinterpreted from the seismic records for the Waikiki survey. Two primary deposittypes can be classified, channel deposits and terrace deposits, based on theirdepositional environment. Prograded shelf deposits and small basin deposits alsoexist more infrequently. A total of 6.7 X 106 yd3 (5.1 X 106 m3) of sand have beenmapped off of Waikiki. Of this, 3.5 X 106 yd3 (2.7 X 106 m3) are from the channeldeposits, and 1.7 X 106 yd3 (1.3 X 106 m3) are from the major terrace deposits.Errors of estimation are probably relatively large in the channel deposits, whencompared with the terrace deposits, because of the more common occurrence ofdiscrepancies between the seismic interpretations and the ground-truth estimates.

Dow

nloa

ded

by [

83.3

7.2.

197]

at 2

1:21

22

Oct

ober

201

4

Marine Aggregates offWaikiki, O'ahu, Hawai'i 85

0 50 100meters

Depth in25 Sediments

(m)

L 50

Figure 5. Comparison profile using chirp system.

Relative Noise Levels

Chirp vs. Bubble Pulser Systems

Chirp zero points, 3,935

Bubl le

L

Chirp

A

50 100 150 200 250 300

Grey Level (0=no signal; 255=maximum)

Figure 6. Gray-level comparison between seismic systems.

Dow

nloa

ded

by [

83.3

7.2.

197]

at 2

1:21

22

Oct

ober

201

4

86 C. L. Morgan et al.

Deposit Thickness (m)

L J J ' 1>3 >6 >9 >I2 >15

Figure 7. Waikiki channel and terrace deposits.

Ground-Truth Sampling

Table 1 summarizes the jet-probe results collected in the Waikiki area. The refusaltypes indicated in this table describe the manner in which the jet probe was refusedfurther penetration at each site. "H" indicates a hard refusal, where the jet-probeoperator noted that the probe was completely stopped. "S" indicates either that thedepth of the probe went to the limit of the device or that no discrete barrier wasnoted by the operator. "C" indicates an intermediate condition where the operatornoted incomplete resistance to further penetration by did not reach an obviouslyhard surface before penetration was terminated.

Table 2 summarizes the collections of sand samples from the Waikiki area.Sample types include material collected from the jet-probe operations in shallowsites near the beaches ("probe wash" samples), a series of large samples collectedfrom within the Halekulani channel ("jet lift" samples), and surficial samples

Dow

nloa

ded

by [

83.3

7.2.

197]

at 2

1:21

22

Oct

ober

201

4

Marine Aggregates offWaikiki, O'ahu, Hawai'i

Table 1Jet-probe results

87

Jet-probestation

AlA2A3A4A5A6DlD2D3D4D5D6D7WAIWA2WA3WB1WB2WB3WD1WD2WEIWE2WF1 'WF2WF3WF4WF5

Waterdepth(m)

27.429.334.827.429.329.327.435.121.324.423.828.732.316.818.3—

16.816.8—

29.030.525.922.029.934.136.631.127.4

Seismicisopach

(m)

11118888

118

141114141123

122323148

118800000

Probedepth(m)

2223341110.322210.3

12108

16050000000

Refusaltype

(see text)

HH/C

HHHH

HH/CH/C

HH/CH/C

SS

H/CHCH

HHH

Position

Lat. 21°N Long. 157°W

16.1316.1316.1016.1516.1116.1315.9715.9516.0015.9716.0015.9815.9716.0916.0716.1016.1916.1816.1916.1516.1216.2016.2016.1716.1316.1316.1616.18

Minutes

50.6550.6650.6650.7150.7050.7050.3150.3350.2950.3050.3250.3350.3350.3250.3250.4050.3550.3550.3250.6450.6850.7050.7050.8050.8050.8150.8050.73

collected from the Makua Shelf. Table 3 summarizes the collection of reef rocksamples using the University of Washington rock drill. As mentioned above, thisdevice was not successful in recovering sand samples but provided unprecedentedrecoveries of reef rock samples. Two of the collection attempts reached fullextension (10 ft, 3.05 m) and indicated soft sediment. Videos of that operationshowed a sandy bottom littered with cobble-sized and smaller coral debris. Verysmall samples were retrieved at most of the sites, probably due to interferencefrom the coral debris. Rock-drill and sample locations from the terrace deposits ofthe Makua Shelf are shown in Figure 8, while the sampling and jet-probing effortsin the Halekulani and Launiu channel deposits are presented in Figure 9. Recentradiometric dating of the consolidated corals collected by the University of Wash-

Dow

nloa

ded

by [

83.3

7.2.

197]

at 2

1:21

22

Oct

ober

201

4

C. L. Morgan et al.

Table 2Sampler recoveries

Station

WO-1WO-2WP-1WP-2WQ-1WQ-2WQ-3WQ-4WQ-5WQ-630-130-230-3E lE2FlGlG2

Samplertype

Probe washProbe washProbe washProbe washProbe washProbe washProbe washProbe washProbe washProbe washJet liftJet liftJet liftSurficialSurficialSurficialSurficialSurficial

Waterdepth(m)

6548543231

1717175359584842

Mediangrainsize

(mm)

0.35

0.38

0.27

0.27

Color

YellowYellowYellowYellowYellowYellowYellowYellowYellowYellowGrayGrayGrayYellowYellowYellowGrayGray

Lat. 21C

16.0816.0016.1516.2116.2316.3116.3516.4016.3916.4816.2016.1916.1715.2115.2515.1215.0215.03

Position

'N Long. 157°W

Minutes

49.6749.6549.6249.5449.7049.6549.6849.5949.7449.7150.3450.3750.3550.0650.1549.9249.5449.54

Table 3Rock drill recoveries

Station

12345678

Waterdepth(m)

5958752424272440

Inferredsand

depth (m)

2330.50.5003

Corerecovery

(cm)

0000

10717836

0

Position

Lat. 21°N Long. 157°W

15.2315.2315.2115.3615.3815.3616.0415.97

Minutes

49.9549.9550.2650.0350.0950.0950.4350.39

Dow

nloa

ded

by [

83.3

7.2.

197]

at 2

1:21

22

Oct

ober

201

4

Marine Aggregates off Waikiki, O'ahu, Hawai'i 89

Rock Drill Site •Surface Sample 0

Figure 8. Terrace deposit sample sites.

Deposit Thickness (m)

>3 >6 >9

ington rock drill (labeled 5 and 6) indicate maximum ages of greater than 40,000years for the reef rock which comprises the shelf at core penetrations of about 2 m(Richard Grigg, personal communication).

Discussion

There are likely to be many different types of sediment that underlie the sanddeposits and serve as refusing horizons to jet probing. Some examples are consoli-dated or unconsolidated reef debris, beach rock, cemented sand, and various typesof conglomerates formed from rhodoliths (coralline algae), or reef detritus. Sub-aerial deposits from marine transgressions may hold clues to equivalent offshoredeposits. Ferrall (1976) offered interpretations of the subsurface geology of Waikikiand Honolulu based on drilling logs from borings made to gather engineeringdesign criteria for the many large buildings constructed in recent years. Geologicalunits described include most of those listed above. Ferrall identified three reefledges at +5 ft (1.5 m), -15 ft (-4.6 m), and -30 ft (-9.1 m, relative to modernmean sea level) from the Waimanalo high sea stand. Reef ledges were generallyformed by Porites lobata and laminations of Lithothamnia (coralline algae). How-ever the — 30-ft ledge is a clastic feature, predominately a conglomerate formed

Dow

nloa

ded

by [

83.3

7.2.

197]

at 2

1:21

22

Oct

ober

201

4

90 C. L. Morgan et ah

• Rock Drill Sites-f- Jet Probe Sites• Jet Sampler Sites

Figure 9. Channel deposit sample and jet-probe sites.

Deposit Thickness (m)

>3 6 9 12 15

from well-cemented coral debris. A 5-ft layer of Porites compressa (branchingcoral), in-filled with silt, is sandwiched between two layers of the conglomerate.

Ferrall states that this type of conglomerate from reef debris is often misiden-tified as in-situ coral reef growth, and it may also be speculated that the acousticsignature of this unit may be similar to that of reef rock, although the layeringsequence might show as diagnostic internal reflectors. Ferrall finds many cases ofcoral reef growth over clastic layers such as reef conglomerate or lagoonal sandsand silts. However, as best as can be judged from the seismic records, massive coralgrowth over clastic deposits is not a common offshore feature, although it wasfound in some areas off the Reef Runway.

The layer of Porites compressa described by Ferrall is similar to a unit found bydiver survey in a channel deposit off Waikiki. Although the reef rock base of thechannel was clear on the seismic records, and the overlying sediment acousticallytransparent, diver surveys and jet probing showed sand deposits to be of far lessareal and vertical extent than predicted. Interlocked branches of Porites compressaextended for approximately 30 m from the edge of the sand deposit to the edge ofthe channel, and likely underlie the sand deposit. The nature of the coral field wasnot determined. It may have been an in-situ forest, a once-flourishing communitythat was subsequently buried in sand, or perhaps a clastic debris field depositedduring a high-energy wave or storm event. A lacy matrix of this coral, in-filled with

Dow

nloa

ded

by [

83.3

7.2.

197]

at 2

1:21

22

Oct

ober

201

4

Marine Aggregates off Waikiki, O 'ahu, Hawai 'i 91

sand, may have acoustic properties similar to those of the sand bodies themselves,and the two geological units, while distinct under direct observation, may thereforebe difficult to differentiate on the seismic records.

Diver surveys on another alluvial channel off Waikiki found well-roundedcobbles formed from the coral Porites lobata underlying sand on the margins of thesand deposit. Again in this case, the sand body was found to be much reduced inareal and vertical extent than predicted from interpretations of the seismic records,and the cobble layer is not readily distinguishable on the records. It thereforeappears likely, in many cases, that the refusing horizon for the jet probes is amassive layer of coral debris which is not readily discernable by the acousticmethods used.

Internal reflectors that appear in many of the sedimentary units may have avariety of origins. Stratification of coral debris in a Waikiki sand channel wasrecently observed by University of Hawai'i divers after the passage of HurricaneIniki (11 September 1992, H. Krock, personal communication). This was probablydue to suspension and transport of sand-sized particles by vigorous wave activity,leaving behind the larger fragments. Many of the fragments also appeared to havebeen freshly broken. Some of the internal reflections in the profiles may similarlybe related to high-energy, extreme climatic events that contributed to stratificationby size segregation or by sudden changes in sediment deposition. The manyinternal reflectors in the sediments depicted in Figure 3 may be related to theposition of this line section near the axis of a submerged channel. Pyroclasticdeposits of ash from the Honolulu Volcanic Series eruptions may also be the causeof internal reflections in some areas, especially in older sediments. However, itshould be noted that internal reflectors are not well expressed in the comparisonsection of the Halekulani sand channel collected with the chirp system (Figure 5)and essentially comprise much of the "noise" estimated in Figure 6. Also, the jetsampling from the channel did not find well-expressed lithological units in thechannel deposit.

Ferrall (1976) identifies two buried alluvial channels, part of the ancientManoa stream drainage, that course through Waikiki: the Kaiulani channel andthe Launiu channel (Figure 5). Offshore, two channel deposits trend southwestthrough the central portion of survey area. These may represent alternate ordivergent paths of the same water course, and were probably cut during theMamala low sea stand (Stearns, 1978; Ferrall, 1976). They are probably theoffshore equivalent of FerralFs Launiu channel. The easternmost submarine chan-nel, the Halekulani sand channel, was previously mapped by diver surveys withwater jet probes and air jet probes (Marine Advisors, Inc., 1968; Casciano &Palmer, 1969). In close proximity to the northwest the other, a smaller channel, ishere also named the Launiu channel. In the southeast sector of the survey area,the — 73-m marine terrace is well developed, but buried by significant deposits ofsand such that it has no clear bathymetric expression (Figure 5). The terrace ispoorly developed or completely missing in the central area of the survey, butreappears as a less developed feature in the northwest region of the survey area.

Speculation by previous researchers has been that ancient beach sand depositsmay be preserved in the offshore (Moberly et al., 1975). The Makua Shelf de-posits mapped in this report may in part be composed of such ancient beachdeposits near the base of the deposit, but the bulk of the sediments are probablycomposed of material transported offshore and over the edge of the escarpment

Dow

nloa

ded

by [

83.3

7.2.

197]

at 2

1:21

22

Oct

ober

201

4

92 C. L. Morgan et al.

that bounds the terrace. The distinctive yellow color found in some of theterrace-deposit surficial samples suggests that these sediments come directly frommarine sources rather than from terrigenous sources. We suspect that the graycolor of the channel deposits is due to etching and staining, possibly with sulfidesor refractile organic compounds, of the sands exposed to stream outwash andpotentially carrying relatively large concentrations of humic and other organicacids.

A significant effort was made in this study to verify interpretations of seismicreflection records with ground truth, especially in the shallower depths (less than120 ft). Instead of confirming sand deposits, the ground truth revealed importantweaknesses of the seismic reflection method when prospecting for carbonate sand,and has further constrained the location and extent of previously postulated sanddeposits. Nevertheless, significant new prospects have been found. The geology ofthe insular shelf of southern O'ahu was found to be more complex than antici-pated, and calls for additional study. In order to aid further research, threecalibration lines with specific acoustic/geological problems were established offWaikiki. A total of 6.7 x 106 yd3 (5.1 X 106 m3) of sand have been mappedoffshore of Waikiki.

The Makua Shelf (Stearns, 1978) at — 73 m appears to be the single mostimportant widespread geological feature for sand prospecting in this region, actingas a depositional surface for sand that is transported across the insular shelf.Confidence is high for the deposits mapped on this feature, particularly the1.7 x 106 yd3 (1.3 x 106 m3) of terrace deposits mapped offshore of Waikiki.Being far from the beach, and lying beneath an escarpment, these sands areattractive for beach replenishment, as they have been definitely removed from thelittoral system and, based on the few surficial samples collected to date, do notshow the staining and poor sorting characteristics of the channel deposits. Basedon the radiometric dates reported by Grigg (personal communication), the age ofthe substrate for these cut terraces is consistent with Fletcher and Sherman (1994)at less than 40,000 years. The unconsolidated deposits which fill them must beyounger than this and thus cannot be 500,000 years old, as suggested by Stearns(1978). These terraces were probably cut into the consolidated deposits approxi-mately during the Mamala low stand of sea level, about 18,000 years ago, and havebeen filled with the existing deposits since that time. Future coring of thesedeposits can provide additional information about the recent geological history ofthe island and will be a necessary prerequisite for the commercial recovery of thesedeposits.

Conclusions

The following conclusions have been drawn from this study.

1. Many different types of sediment underlie tropical island carbonate sanddeposits and serve as refusing horizons to jet probing. Examples includeconsolidated or unconsolidated reef debris, beach rock, cemented sand, andvarious types of conglomerates formed from rhodoliths (coralline algae), orreef detritus.

2. Massive coral growth over clastic deposits is not a common offshore featurein this area, though it does occur in some areas off the Reef Runway.

Dow

nloa

ded

by [

83.3

7.2.

197]

at 2

1:21

22

Oct

ober

201

4

Marine Aggregates offWaikiki, O'ahu, Hawai'i 93

3. Matrices of the coral Porites compressa, in-filled with sand, may haveacoustic properties similar to those of the sand bodies. Such deposits maybe difficult to distinguish from unconsolidated deposits from seismic recordsalone.

4. Significant new prospects for offshore aggregates were found in the insularshelf offshore from Southern O'ahu. A total of 5,100,000 m3 were mappedoff Waikiki. The Makua Shelf deposits in this area presently appear to bethe best prospects for commercial development.

References

Barry, J. H. 1991. Characterization of deep sand deposits in the tropical'reef environment ofHawai'i. MTS'91 an Ocean Cooperative: Industry, Government and Academia, conferenceproceedings. Washington DC: Marine Technology Society, 579-585.

Barry, J. H. 1995. Characterization of sand deposits on the insular shelf of southern O'ahu,off Western Molokai, and on Penguin Bank. University of Hawai'i School of Ocean andEarth Science and Technology Report 95-2.

Campbell, J. F. 1979. Size analysis of offshore sand. Report for the Marine AffairsCoordinator, Office of the Governor, State of Hawai'i, Task Order 183.

Campbell, J. F., W. T. Coulbourn, and R. Moberly. 1972. Pleistocene drainage patterns inHawai'i as revealed by seismic reflection profiling. Geological Society of AmericaAbstracts with Programs {Cordilleran Section)' 4(3): 135-136.

Campbell, J. F., W. T. Coulbourn, R. Moberly, and B. R. Rosendahl. 1970. Reconnaissancesand inventory: off leeward O'ahu, UNIHI-SEAGRANT-70-02. Honolulu, HI: Univer-sity of Hawai'i Sea Grant Program (also HIG 70-16).

Campbell, J. F., B. R. Rosendahl, W. T. Coulbourn, and R. Moberly. 1971. Reconnaissancesand inventory: off leeward Molokai and Maui, UNIHI-SEAGRANT-TR-71-02. Hon-olulu, HI: University of Hawai'i Sea Grant Program (also HIG 71-17).

Casciano, F. M., and R. Q. Palmer. 1969. Potential of offshore sand as an exploitableresource in Hawai'i, SEAGRANT 69-4. Honolulu, HI: University of Hawai'i Sea GrantProgram.

Chamberlain, T. 1968. The littoral sand budget, Hawaiian Islands. Pacific Science 22(2):161-183.

Coulbourn, W. T., J. F. Campbell, P. N. Anderson, P. M. Daugherty, V. A. Greenberg, S. K.Izuka, R. A. Lauritzen, B. O. Tsutsui, and C. Yan. 1988. Sand deposits offshore O'ahu,Hawai'i. Pacific Science 42(3, 4): 267-299.

Coulbourn, W. T., J. F. Campbell, and R. Moberly. 1974. Hawaiian submarine terraces,canyons, and Quaternary history evaluated by seismic reflection profiling. MarineGeology 17: 215-234.

Dollar, S. J. 1979. Sand mining in Hawai'i: research, restrictions, and choices for the future,UNIHI-SEAGRANT-TP-79-01. Honolulu, HI: University of Hawai'i Sea Grant Pro-gram.

Ferrall, C. C. 1976. Subsurface geology of Waikiki, Moiliili, and Kakaako with engineeringapplication. Master's thesis, University of Hawai'i Department of Geology and Geo-physics, Honolulu, HI.

Fletcher, C. H., and C. E. Sherman. 1994. Submerged shorelines on O'ahu, Hawai'i: archiveof episodic transgression during the deglaciation. Journal of Coastal Research, SpecialIssue No. 17.

Gerritsen, F. 1978. Beach and surf parameters in Hawai'i, UNIHI-SEARGRANT-TR-78-02.Honolulu, HI: University of Hawai'i Sea Grant Program.

Gregory, A. E., and L. W. Kroenke. 1982. Reef development on a mid-ocean island:reflection profiling studies of the 500-meter shelf south of O'ahu. American Associationof Petroleum Geologists Bulletin 66(7).

Dow

nloa

ded

by [

83.3

7.2.

197]

at 2

1:21

22

Oct

ober

201

4

94 C. L. Morgan et al.

Hine, A. C, R. J. Wilber, and A. C. Neumann. 1981. Carbonate sand bodies alongcontrasting shallow bank margins facing open seaways in Northern Bahamas. AmericanAssociation of Petroleum Geologists Bulletin 64: 261-290.

Inman, D. L, W. R. Gaymand, and D. C. Cox. 1963. Littoral sedimentary processes onKauai, a subtropical high island. Pacific Science 17: 106-130.

Johnson, P., J. C. Wilson, and R. Strickland. 1991. New hard rock drill successful in deep seatest. Sea Technology, February 1991.

MacDonald, G. A., A. T. Abbot, and F. L. Peterson. 1983. Volcanoes in the sea, the geology ofHawai'i. Honolulu, HI: University of Hawai'i Press.

Marine Advisers, Inc. 1968. Sand survey at Waikiki and at the Honolulu Harbor entrance,O'ahu. Report prepared for the State of Hawai'i, Department of Transportation,Harbors Division, Honolulu, HI.

Moberly, R. 1968. Loss of Hawaiian littoral sand. Journal of Sedimentary Petrology 38(1):17-34.

Moberly, R., D. L. Baver, and A. Morrison. 1965. Source and variation of Hawaiian littoralsand. Journal of Sedimentary Petrology 35(3): 589-598.

Moberly, R., and J. F. Campbell. 1969. Hawaiian shallow marine sand inventory. Part 2. Ahuo Laka sand deposit, Kaneohe Bay, O'ahu, SEAGRANT 69-1. Honolulu, HI: Universityof Hawai'i Sea Grant Program (also HIG-69-10).

Moberly, R., J. F. Campbell, and W. T. Coulbourn. 1975. Offshore and other sand resourcesfor Hawai'i, UNIHI-SEAGRANT-TR-75-03. Honolulu, HI: University of Hawai'i SeaGrant Program (also HIG-75-10).

Moberly, R., and T. Chamberlain. 1964. Hawaiian beach systems, HIG-64-2. Honolulu, HI:Hawai'i Institute of Geophysics, University of Hawai'i at Manoa.

Murdaugh, W. B. 1979a. Preliminary OCS resource assessment, marine hard minerals,Hawaiian archipelago. Final report prepared for U.S. Geological Survey, ConservationDivision, Menlo Park, CA. Contract Order 53009, Requisition 6416-0014.

Murdaugh, W. B. 1979b. Preliminary feasibility assessment, offshore sand mining, PenguinBank, Hawai'i. Final report prepared for the U.S. Geological Survey, ConservationDivision, Menlo Park, CA.

Noda, E. K. 1991. Sand source investigations for Waikiki Beach fill. Consultant reportprepared for the State of Hawai'i Department of Transportation, Harbors Division,Honolulu, HI.

Resig, J. M. 1969. Paleontological investigations of deep borings on the Ewa Plain, O'ahu,Hawai'i, HIG-69-2. Honolulu, HI: Hawai'i Institute of Geophysics, University of Hawai'iat Manoa.

Shackleton, N. J. 1987. Oxygen isotopes, ice volume and sea level. Quaternary ScienceReviews 6: 183-190.

Sherman, C. E. 1992. Depositional, diagenetic, and sea level history of late Pleistocenecarbonates, O'ahu, Hi. Master's thesis, Department of Geology and Geophysics, Uni-versity of Hawai'i at Manoa, Honolulu, HI.

Stearns, H. T. 1978. Quaternary shorelines in the Hawaiian Islands. Bernice P. BishopMuseum Bulletin 237. Honolulu, HI: Bishop Museum Press.

Stearns, H. T., and T. Chamberlain. 1967. Deep cores of O'ahu, Hawai'i and their bearingon the geologic history of the Central Pacific Basin. Pacific Science 21, April.

Dow

nloa

ded

by [

83.3

7.2.

197]

at 2

1:21

22

Oct

ober

201

4