Coastal bench formation at Hanauma Bay, ,

WILFRED B. BRYAN Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543 ROBERT S. STEPHENS* Hawaii Institute of Geophysics, , Hawaii 96822

ABSTRACT rainfall about 4-8 cm per year. Prevailing winds are from the east- northeast, so that the inner north shore is normally the most sheltered A coastal bench that developed from 1 to 6 m above sea level in part of the bay. basaltic at Hanauma Bay conforms to the upper limit of wetting by Many rock benches have been described from Australia, New wave wash at high tides associated with present sea level; it does not Zealand, and on islands in or around the Pacific basin. Such benches constitute evidence for a recent Holocene highstand on Oahu. Variations are usually ascribed to "cutting" by waves and often are called in bench width and elevation are related to differences both in exposure '' wave-cut platforms.'' Typically they are < 1 to > 3 m above sea level to waves and in exposure to daily heating and drying of the cliff behind and have been cited as evidence for a "2-meter" global eustatic high- the bench. Salt weathering of the sort usually invoked to explain weath- stand. They also have been attributed to storm waves, a mechanism ering effects in deserts is a major factor in the retreat of the cliff and the strongly endorsed by Bartrum (1926) and Edwards (1951). However, consequent formation of the bench. The waves do not "cut" the bench others (for example, Hills, 1949) have noted that coastal benches and but, instead, by daily wetting, protect it from desiccation. The bench platforms are best developed in sheltered water. Johnson (1933) con- forms as a result of the disintegration and retreat of the unprotected cliff. cluded that the supposed "2-meter" bench is the work of the present The same process can satisfactorily explain the formation of Koko sea. His argument was based in part on the observation that bench Bench, presently submerged at — 5 m along the north shore of Hanauma heights commonly vary in direct proportion to their exposure to wave Bay. Use of similar benches as geological indicators of past sea levels height. Others have noted the possible importance of accelerated requires a detailed understanding of the coastal setting and exposure to weathering either within the intertidal zone or just above it. Bartrum waves, and the different responses of specific rock types at and above the (1924, 1935) considered differential weathering to be a factor in es- air-sea interface. tablishing bench height. Wentworth (1938) recognized an analogy between his proposed "water-level weathering" and the corrosion of INTRODUCTION concrete or metal coastal structures at, or just above, high-tide level but was explicit in his view that wave action is the most important The mechanism of formation of coastal rock benches and the factor in bench formation, with water-level weathering playing a sec- relation of benches to sea level have been extensively debated. One ondary role in smoothing an existing bench (Wentworth, 1938, of the best-known examples of such a bench is located in Hanauma p. 29-31). An excellent review of these and other papers can be found Bay on the eastern end of Oahu, Hawaii (Fig. 1). In this paper, we in Trenhaile (1987). present detailed surveyed profiles of the coastal benches along both the north and south shores of Hanauma Bay and describe their relation PREVIOUS WORK to local geologic and climatic factors. The process of formation of the bench is deduced from its relation to variations in structure, lithology, The bench in Hanauma Bay was cited by Wentworth and Palmer microclimate, sea level, and wave action as observed in a variety of (1925, p. 525) as "the best single exposure" of a supposed late Pleis- weather conditions and tidal stages over a six-month period from tocene eustatic bench recognized elsewhere on Oahu and on other February to July, 1989. Hawaiian and Pacific Islands. They believed this bench was cut by Hanauma Bay is the location of a tide gauge station operated for waves of a sea 12 to 15 ft (3.7 to 4.7 m) higher than at present. To date, six months in 1931-1932. Reference bench marks for this station re- the most detailed description is that of Stearns (1935a), who also main on the bench. The normal tidal range is —0.75 m. The bay documented a second, lower bench at Hanauma Bay. His surveyed occupies a compound tuif cone open to the sea toward the southeast; profiles of both the upper and lower benches clearly demonstrate an it is -.4 km wide and —.8 km long (Fig. 2). The depth is about 18 m increase in elevation toward exposed headlands. Stearns (1935a) left between the outer headlands and 4-5 m in front of the coral reef. The open the question of origin and the role of elevated sea level in cutting crest of the living reef is barely awash at low tide. It forms an almost these benches, although he concluded (p. 1481) "the bulk of the continuous barrier across the head of the bay, —100 m from the evidence supports the hypothesis that the upper bench at Hanauma western shore. The shore behind the reef is covered by a coral sand Bay was made by the present sea." Wentworth (1938) published beach, except for a few hundred meters at the northern end of the several photographs and drawings of the Hanauma Bay bench, which back-reef lagoon. The climate along this shore is semiarid, with annual he considered to be a type location for "water-level weathering." Easton (1973) described a submarine bench at —5 m along the northern side of the bay; he named this "Koko Bench." He did not *Present address: Darnes and Moore, 221 Main Street, San Francisco, mention the "lower bench" described by Stearns (1935a), but noted California 94105. that' 'the upper part of the cliff in the intertidal zone below the marginal

Geological Society of America Bulletin, v. 105, p. 377-386, 12 figs., March 1993.

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Figure 1. Eastern Oahu, Hawaiian Islands, with locations mentioned in text.

rampart commonly is slightly notched or shelved and is covered by a This last conclusion was vigorously challenged by Stearns (1977), on dense growth of the soft alga, sargassum" (Easton, 1973, p. 2277). the basis of a coral fragment dated at 3,500 yr old in beach rock Easton and Olson (1976) presented evidence from dated coral ob- cemented to on the north side of the bay. He believed this tained from drill holes in the reef that sea level reached its present level confirmed a relation between the Hanauma bench and 3,500-yr-old about 3,000 yr ago and has not been significantly higher since then. raised coral reefs on other Pacific islands. He subsequently (Stearns,

BM3 14-1

1000

12-

10-

B H PS 8- O Figure 2. Location map for Hanauma Bay. fc Letters indicate survey stations and reference lo- P « PALEA POINT cations mentioned in the text. o

4-

2-

""—i—i—r~ I —1 8 10 12 14 16 GRID EAST

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m) here. The cliff behind the bench along this section rises vertically 6-10 m and then somewhat less steeply, reaching elevations of 45 to nearly 60 m. Some of the cliff is overhanging and is undercut at about the same level as BM3. A surge channel 3-5 m wide breaks continuity of the bench near the northern end of the coral reef. The surge channel follows the contact between the basaltic tuff and a small lava flow that mantles the original inner flank of the tuff cone. The flow merges with a northeast-striking dike as it crosses the bench and passes into the cliff. Another surge channel is developed along the seaward side of the flow ; beyond this is another small fragment of bench and then another dike. The only major rock fall on the surface of the bench is seaward of the second dike. It is a chaotic jumble of tuff blocks as much as 2 m or more in longest dimension. Beyond the rock fall, the bench attains its broadest and most-impressive development (Fig. 4). The width of the bench here varies from 10 to >15 m and is backed by a near-vertical cliff 10-15 m high. Several large boulders of tuff have fallen from the cliff, but most of the bench is almost free of rock debris Figure 3. View looking southwest; BM3 is on low knoll just behind of any sort. There is no notch indicating undercutting of the cliff. The hammer; note trees on other low knolls beyond and at base of cliff, and outer edge of the bench forms a rampart at least .3 m above the general the undercutting at the cliff base. level of the bench. These features were well illustrated by Wentworth (1938) (Fig. 5). The bench rises, narrows to <2 m, and becomes rougher as it 1978) equated the Hanauma bench with the "2-meter" eustatic level approaches stations C and E. This roughness is due to large cobbles and called this the "Kapapa" stand, after a bench developed on of basalt that stand up in relief and protect the underlying tuff from lithified dune sand on Kapapa Island in Kaneohe Bay. He states that erosion. The elevation increases abruptly (to —4.5 m) as the bench "the bench at Hanauma Bay was made by the 5-foot Kapapa stand of passes around the headland. In the length of the channel, the bench the sea and not by storm waves or biological processes. . . .The bench widens to about 6 m and descends to about 1-m elevation just beyond certainly was not cut by the present sea" (Stearns, 1978, p. 50). the Toilet Bowl. Here, the bench surface is very smooth and nearly The tuff cones making up the craters are among the flat. It is awash with a slurry of olivine sand and water after every youngest of the Honolulu Series vents (Winchell, 1947; Clague and eruption of the Toilet Bowl and probably owes its smoothness to Frey, 1982) that have been dated from 33 to 43 ky B.P. (Gramlich and abrasion by the sand. others, 1971). Wentworth (1926) first attempted to map dips in the tuff On the east side of the surge channel, the bench narrows to a and overlapping relations of the cones. A dike and a small lava flow broad notch but is wide enough to walk on at all tides except in the crop out near the northern end of the coral reef, and another small lava most severe weather. The elevation increases around the exposed flow is in the gully north of the "Toilet Bowl." The tuff and its headland, and beyond reference stations F and G, it rises rapidly to palagonitization were studied by Hay and Iijima (1968). Many small fragments of both fresh and altered basalt are embedded in the tuff, along with conspicuous white fragments of coral. The altered basalt and coral are derived respectively from the underlying Koolau and the Pleistocene age Waimanolo coral reef deposits (Stearns, 1935a, 1935b). On-going mapping of the Koko craters has indicated substantial complexities in the overlapping relationships (G.P.L. Walker, 1989, personal commun.). A final interpretation of eruptive sequences, the location of eruptive centers, and eruptive history has yet to be made, but at present it seems likely that sea level at the time of eruption of the tuff cones was not greatly different than it is now, and that the craters were either breached and open to the sea through- out these eruptions, or became open shortly after eruptions ceased.

DETAILED DESCRIPTION OF THE BENCH

North Bench

A bench can be traced for over 600 m along the north and north- east side of the bay. It begins at the north end of the sand beach behind Figure 4. View of the northern bench, looking east-southeast in the reef and extends northeast and then east past United States Coast mid-morning and at low tide, from the top of the cliff overlooking and Geodetic Survey (USCGS) bench mark BM3, which is located on Hanauma Bay. The basalt dike is at far left, and the entrance to the Toilet a knoll slightly above the general level of the bench (Fig. 3). Beyond Bowl is just beyond the far end of the bench. Note the rock fall just BM3, the bench broadens and becomes very flat as it approaches the beyond the dike, and the unusually wide section of bench in the middle northern end of the coral reef, reaching its lowest elevation (0.8-1.0 distance. The coral reef is visible in the lower left foreground.

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CROSS SECTION OF NORTH BENCH south end of the beach to a platform on the point near the Witch's AFTER WENTWORTH, 1938 Caldron. Small patches of olivine sand are at the base of the cliff at the wider parts of the bench. At the time of our study, only two isolated tuff boulders were on the middle part of the bench. The lowest point H w on the bench (about 0.65 m) is at its most sheltered end, adjacent to S 30.0 fc, the beach (reference station L). The bench varies in width from < 1 m Z to >5 m and, like the north bench, shows local increases in elevation O 20.0 at minor headlands and an abrupt increase in elevation (to ~3 m) at H Î the outermost headland exposed to the open sea (reference station M). S 10.0 The cliff top overhangs the bench, and the cliff beneath is subject to w cavernous weathering, with development of relatively deep shelves and pits. The widest part of the bench is on the elevated platform at the end of the point and along the base of the slope leading up to the platform. Along most of this shoreline, the inner wall of the crater is a dip slope, with bedding in the tuff sloping 30°-40° toward the bay. FEET SOUTH The dip changes abruptly along a fault line that crosses the bench just Figure 5. Typical profile across the north bench, after Wentworth after it widens near its seaward end; beyond the fault, the dip is (1938). The original surface slope of the inner wall of the tuff cone seaward, toward the east-southeast. As on the north bench, the slope projects to the surface of the rampart at the outer edge of the bench. of the final steep increase in elevation at the end of the bench runs Similar relations hold along the south bench. against the grain of the seaward-dipping tuff. The dip slope on the tuff above the bench projects close to the outer edge of the bench, indi- cating that little material has been removed. A prominent outer ram- about 6 m on a platform exposed to the open sea near Palea Point. A part, similar to that along the middle section of the north bench, rises poorly defined continuation of the bench can be traced as far as the 0.3 m or more above bench level (Fig. 6). ridge that forms Palea Point. Farther to the north, discontinuous plat- forms have formed in the tuff. These were mapped and discussed in SURVEY OF BENCH PROFILES detail by Wentworth (1938); his descriptions remain valid. The "lower bench" described by Stearns (1935a) is inconspicu- Survey Methods ous except at the lowest tides, and it is much more discontinuous and variable in elevation than was implied in his descriptions. This bench Bench profiles were mapped using a plane table and telescopic is also subject to burrowing by sea urchins, which give parts of it a alidade with a standard 12-ft surveyors rod; observations were sub- coarsely "sponge-like" texture. We did not study this poorly devel- sequently converted to meters. Resolution of the telescope and cur- oped lower bench in detail, and unless noted otherwise, all subsequent vature of the bench effectively limited the shot length to 300 m or less. discussion applies to the "upper bench." The —650 m along the north bench was thus mapped using both fore- Except for variations in local roughness caused by basalt and and backsights from a series of temporary stations labeled C, D, E, and coral cobbles in the tuff, there is little relation between bench devel- G. Bench mark BM3 was shot from station D, and all other shots were opment and particular lithologic units in the tuff, or between bench subsequently corrected to elevations above or below the level of BM3. slope and the angle of dip of the tuff. Wentworth (1938) remarked that Foresights and backsights between reference stations indicate that the the bench seemed to be smoothest and best developed in those places where the tuff dips at a substantial angle, and our observations support his. Behind the beach at the head of the bay, the tuff is flat or very gently dipping, and the steep inner crater wall facing the bay is not a dip slope but truncates the bedding. Near-horizontal bedding contin- ues seaward to the vicinity of reference station D, where the dip changes abruptly to about 35° south above an erosional unconformity in the tuff. The headland at station E is near the nose of an anticlinal structure in the tuff that plunges gently south; morphologically, this is part of the inner crater rim. The outer crater rim similarly plunges 5°-10° south at about the location of reference station G, and tuff draped over it dips gently both southwest toward the bay and seaward toward Palea Point. On both headlands, the slope of the bench is counter to the dip of the bedding, and the widest and smoothest section of bench is developed on the relatively steeply dipping tuff between stations C and D.

South Bench

The southern bench segment was illustrated by Wentworth (1938) Figure 6. View looking west along the southern bench. Note the but was discussed only briefly by Stearns (1935a). It is much shorter raised rampart and the very narrow walkway between the rampart and and narrower than the north bench, extending about 240 m from the the cliff.

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NORTH BENCH

DIKE l HEADLAND i

HEADLAND i Figure 7. Surveyed profile along the HEADLAND northern bench. The high "spike" indicates I the position of the lava flow; the profile is broken where it crosses the surge channel leading to the Toilet Bowl. BM3

-50 50 150 250 350 450 550 650 METERS EAST OF BM3

precision of the leveling is about ±0.03 m in 300 m. The south bench ness with a wavelength on the order of one to several meters. To was mapped from station K; additional reference stations L and M simplify presentation, the surveys have been projected as straight lines were assigned to this survey to aid discussion but were not occupied. between the most extreme surveyed points (Figs. 7 and 8). The vertical The level of K relative to BM3 was estimated by measuring the height exaggeration emphasizes irregularities in the bench surface. The ba- of both BM3 and K relative to quiet water in the lagoon behind the reef salt dike that crosses the north bench surface is a readily identified at low tide on a calm day. Ripples limited the precision of this method landmark. Also, we have broken the north bench profile where it to about ±0.03 m. These and other local differences in level were crosses the surge channel leading to the Toilet Bowl. The outermost measured using a standard builder's string and level and a weighted bench segment through points F and G increases abruptly to the fiber-glass tape. maximum elevation of the bench as it emerges from the opening of the surge channel and turns east toward Palea Point. The south bench Sea-Level Reference profile is presented in this paper for the first time. Although it is much shorter than the north bench, it has the same relation to sea level and The "official" elevation of BM3 is 5.85 ft (1.77 m), which is based similar local variation in relief. on six months of continuous tide-gauge records at Hanauma Bay, from The most obvious feature of each profile is the increase in ele- December 1931 to May 1932 (USCGS, 1944). In Hawaii, the sea-level vation at both minor and major headlands along both benches. The reference is mean lower low water, and this is the definition used in height of each headland increases with the degree of exposure to ocean this paper. This conforms closely to the upper limit of coral reef waves. This was anticipated from Stearns's earlier description and growth. Seasonal variation in sea level is significant throughout most survey of the north bench, but his more generalized graphic profile of the Pacific, primarily due to change in atmospheric pressure and the showed a gradual and then more rapid monotonic increase from the thermal expansion and contraction of the sea-water column with sea- inner part of the bay to the outermost headland (Stearns, 1935a). A line sonal temperature changes (Wyrtki and Leslie, 1980). In Hawaii, the through the high points of our north bench survey (Fig. 7) would most difference amounts to about 8 cm, with the low in April/May and the closely duplicate Stearns's published profile. The drop in level be- high in September/October. At Hanauma Bay, our measurements tween intermediate high points and the obvious break in the bench at from BM3 and BM2 to water level at low spring tides were both about the Toilet Bowl were not shown by Stearns or mentioned in his 12 cm less than the published bench-mark heights established during description of the north bench. the first half of 1931. This difference implies a sea-level rise of 2 mm per year since 1931. Because our observations were made between OBSERVATIONS OF WAVE DYNAMICS February and July, they are bracketed about the seasonal low, as were the 1931 tide-gauge measurements. Therefore, although the seasonal Wave action on both benches was observed incidentally during range is about the same magnitude as the apparent change since 1931, the surveys, and part of each survey day was devoted to observing and it cannot account for the observed difference. Other factors such as photographing wave action along various parts of the bench system at atmospheric pressure and wind forcing can affect tide levels on any both high and low tide. In addition, we visited the bay for shorter given day, but these effects could not be evaluated with the simple periods to observe and record wave and tidal phenomena at the time methods used in this study. The normal tidal range at Hanauma Bay of maximum or minimum spring tides or during unusual weather is about 0.75 m, with maximum tides to more than 1.0 m (USCGS, conditions; the latter included a typical "Kona" storm blowing from 1944). For discussion and illustration we use 0.8 m as a reference the southwest in February and the passage of Hurricane Delila in July high-tide value. High-tide level on any given day is subject to the same 1989. variable factors as low tide. The north and south benches are completely awash at high spring The surveys were made along the approximate center of each tides even in relatively calm weather; during neap tides, the higher bench. Points were placed on obvious highs and lows to reveal rough- parts of the bench may e scape wetting for a day or two. Normal 15-25

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10

l-J SOUTH BENCH J

w > o m Figure 8. Surveyed profile along the < southern bench. L and M are reference po- Vi HEADLAND sitions to aid discussion; they were not used te 4 - H i as controls during the survey. E-" M W

HEADLAND i

HIGH TIDE

-50 50 100 150 200 250 METERS EAST OF L

knot trade winds may blow for a week or more uninterrupted, bringing walk the length of the benches, typically in ankle-deep water, even at 3- to 5-m waves from the east-northeast. These break heavily on Palea high tide. Point and along the south shore of the bay as far as the Witch's At low tide, considerable standing water remains on the lower Caldron. "Green water" breaks across Palea Point up to at least 8-m parts of the benches, confined behind the ramparts along the outer elevation. Heavy spray reaches more than 15 m to the top of the cliff bench edges. The strand line from the previous high tide can be traced on the outer part of the south shore. Exceptional waves sometimes through a combination of stranded plant debris, reworked sand, and sweep fishermen and hikers off these exposed locations. Typically, we a sharp boundary between wet and dry rock. Seaward of the reef, this would observe one or two such "rogue waves" at times of high tide; line faithfully follows the junction between the bench and the base of they are very effective at wetting the more inaccessible parts of the the cliff. Even at the 6-m level on Palea Point, ponds of salt water benches that may not have been reached by other waves. The main persist for days and may contain active small fish. At lower levels, effect of storms is to change the direction of wave attack; Kona storms many ponds above low-tide level are permanent, as indicated by from the south increase wave intensity on Palea Point and temporarily well-established marine life and algae. The vigorous overwash on the diminish it on the south shore of the bay. These conditions seldom last outermost headlands usually does not preserve a strand line, but for more than two or three days, and at least during our period of darkened wet tuff on the bench surface defines a sharp wetting-limit observation, did not produce waves of larger amplitude than those line along the base of the lighter-colored dry cliff. associated with trade winds. Hurricane Delila passed ~ 160 km south On the part of the north bench behind the reef, waves do not wet of Oahu, produced strong southerly winds for about a day, and created the bench all the way to the cliff base even at high spring tides. The a mild storm surge in the bay that oscillated between normal low and effective wetting level here is about 15 cm lower than BM3. The cliff high tide levels with a period of about 10 min. These conditions were base and notch, where present, are at about the same level as BM3. not serious enough to alter normal beach activity at the park or to It is also of interest that BM3 now projects about 6 cm above the cause park officials to close access to the benches. surrounding tuff, which would have been flush, or nearly so, with the During typical trade wind conditions, waves are refracted around bench mark when it was set in 1931, suggesting that this part of the Palea Point. Part of each wave moves down the surge channel toward bench may be actively eroding. Stunted algarroba trees, however, the Toilet Bowl, and the rest continues along the north bench toward grow on raised knolls about 23 m west and at about the same level as the inner part of the bay. At high tide, and at all levels of tide during BM3; they are included in the description of the placement of BM3, periods of strong surf, the refracted waves are locally focused on each so that they have survived there since 1931 (Fig. 3). successive minor headland along the bench as they move down the It is evident, then, that the upper part of the bench here is not bay. The wetting of both the north and the south benches is largely subject to severe wave action or even to wetting at high tide, and that effected by waves peaking at these headlands and washing over and the relations we observed are typical. Between BM3 and the lava flow, beyond them down the slope of the bench behind, where sea water the bench broadens and flattens and is awash at most high tides to the accumulates in temporary pools. In general, decreasing wave ampli- base of a 10- to 15-cm-high step that extends roughly parallel to, and tude is well matched to the diminishing height of minor headlands about 2 m from, the base of the cliff. along the outer edge of the bench as each attenuating wave moves up the bay. The relatively higher parts of the benches are at places where GEOLOGICAL OBSERVATIONS waves are focused not only by the outline of the shore, but also by projecting ledges or fissures that capture extra water and force it up Widely spaced near-vertical joints in the tuff strike roughly north- to bench level, operating as a kind of hydraulic ram. At high tides, the northeast and east-southeast; many are filled with carbonate. Water benches are almost entirely awash to the base of the cliff. The cliffs are seeping from joints wets the tuff for several inches on either side along not directly attacked by waves, and it is ordinarily perfectly safe to the north side of the surge channel near the Toilet Bowl. The damp tuff

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derived from the much older Koolau basalts beneath the tuff cone is weathered flush with the tuff surface or even below it. Salt blooms appear periodically on the cliffs behind the bench along the north side of the bay. No salt blooms were observed on the overhanging cliff immediately above the south bench, although salt covered the cliffs and slopes of the tuff cone above the Witch's Cal- dron. During these blooms, flakes and scales of salty crust (Fig. 11) peel away and accumulate on ledges and along the base of the cliff on the inner edge of the north bench. Gentle wind gusts are enough to dislodge these flakes; at times there is an almost continual rain of fine flakes and tuff particles. These salt blooms appeared on a major scale three times during our period of observation, although more minor evidence of salt excretion is common on any dry sunny day along the cliffs behind the north bench.

HOLOCENE SEA-LEVEL HISTORY

Coral recovered from drill cores in the Hanauma Bay coral reef (Easton and Olson, 1976) provides the most direct evidence for sea- level variation in the bay during the past 7,000 yr. The reef began to grow during a stillstand about 7,000 yr ago and followed rising sea level. About 3,500 yr ago, it was about 1 m lower than at present. The reef probably was close to its present level 1,500 to 2,000 yr ago. The similarities and differences between this sea-level curve (Fig. 12) and Figure 9. Tuff stands out in bold relief (about 0.15 m) on either side other published curves were discussed by Easton and Olson, who of a wet joint plane in the cliff near the Toilet Bowl. recognized that local tectonic movements could account for the dif- ferences. They concluded that Holocene sea level had never been higher than at present in Hanauma Bay, a conclusion that was con- with its central joint stands out 15-45 cm from the cliff (Fig. 9). In the sistent with observations of reef growth on some other Pacific Islands same area, some tuff beds also drip water; these evidently are small (Newell and Bloom, 1970; Curray and others, 1970). aquifers within the tuff cone. These beds also stand out in relief; one Stearns (1977, 1978) challenged this conclusion on the basis of a overhangs the bench by about 1 m (Fig. 10). The carbonate filling in coral fragment dated at 3,500 yr old that was taken from beach rock the joints stands out in positive relief on the cliff face and on the bench cemented to the north side of the lava flow. He asserted that this surface. Fragments of reef limestone in the tuff also stand out in proved that the bench was cut by a high stand of the sea about 3,500 positive relief both on the cliff face and on the bench surface; it is clear yr ago. The coral was not in growth position, and in his response, that the limestone is more resistant than the tuff. Fresh basalt cobbles Easton (1977) argued that the coral probably was eroded from the reef and bombs in the tuff stand out in positive relief; but altered rock and thrown up by a storm, where it became wedged in a crack next to the basalt. Our own observations confirm that the tuff seems to have

Figure 10. Looking south beneath an overhanging ledge of perpet- ually wet tuff, in the cliff above the bench near the Toilet Bowl. The bench Figure 11. Detail of salt decrepitation on the cliff behind the bench slopes toward the camera, although the tuff dips away from the camera shown in Figure 4. The string is vertical; note intense scaling of the tuff toward the bay. surface.

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CORAL REEF AGE-DEPTH LIMITS Wilhelm, 1970; Chapman, 1980). The role of salt weathering in coastal areas is reviewed in detail by Trenhaile (1987). Johannessen and others (1982) invoked salt weathering to explain strongly differential rates of cliff retreat on the Oregon coast. They stated that cliff retreat is 10-20 times more rapid on south-facing, well-insolated cliffs and that retreat is inhibited where cliffs are kept wet by waterfalls of fresh-water coastal streams. Salt effectively breaks down rock in a number of different ways, including crystallization pressure as salt crystals form in pores or cracks, thermal expansion of confined salt crystals, and expansion during hydration of trapped salt (Winkler and Wilhelm, 1970). Of the common salts, sodium chloride has one of the highest coefficients of thermal expansion (Cooke and Smalley, 1968), so that it is effective on any surface subject to large daily temperature changes. Hydration

pressures are developed by salts such as Na2C03 or MgS04 when they are exposed to excess moisture. The dominant salt at Hanauma Bay is NaCl, most of which probably is blown onto the cliffs as a fine mist. Figure 12. Upper and lower limits of dated coral and algae from the Because the tuff was produced by phreatic eruptions, it could have reef in Hanauma Bay, based on data and Figure 4 in Easton and Olson been deposited as salty mud, and some salt may still be present (1976). internally. Our observations suggest that most of the salt is derived from surf spray absorbed on relatively cool, damp days; it diffuses to the surface and "blooms" on hot, dry days. The blooms are specific been especially erodable along the contact with the basalt, resulting in to the tuff and do not appear on the basalt or carbonate cobbles. a surge channel between the basalt and the bench to the west at the We suggest that the formation of the bench at Hanauma Bay is present time. At an early stage in the development of the bench, this the result of strongly differential rates of cliff retreat above and below very likely would have been a narrow cleft favorable for trapping loose the sharp wetting boundary associated with daily high tides. Both the coral and rock fragments. In fact, the beach rock containing the coral bench and the cliff face above it are subject to salt saturation by mist forms a "lazy-J"-shaped deposit on the surface of the flow, with the and spray from breaking waves, but only the cliff face above the level long part of the J parallel to, and at about the level of, the outer part of daily wave wetting may become dehydrated. The bench is protected of the adjacent bench. from dehydration, and hence from salt weathering, by repeated tidal bathing and by the ponds of residual sea water that remain between DISCUSSION AND INTERPRETATION high tides. The widest part of the bench is located beneath the cliff receiving the most intense daily heating, and this also is the place The benches in Hanauma Bay vary in elevation from approxi- where the most intense salt weathering was observed (Figs. 4 and 11). mately high-tide level (0.8 m) at points just behind the fringing reef, to The cliff along the south bench is shaded from the sun at all times, and ~6 m above sea level near Palea Point and to ~3 m above sea level although it displays cavernous "tafoni" weathering, it has widened near the Witch's Caldron. The benches do not rise smoothly to these much more slowly than the bench on the north side of the bay, which outermost high points but rise and then fall again to lower levels across faces the sun. This situation resembles that described by Chapman several intermediate highs. This undulating bench surface rules out (1980), who observed that salt weathering takes place in caves in the tectonic tilting as a possible explanation for the changes in elevation. Arabian desert, although it progresses more slowly there than in places Also, local highs are associated with minor headlands or other irreg- exposed to direct sun. ularities in the bench margins that capture and focus wave wash onto Is salt weathering the only important process in bench formation, the surface of the bench. At all positions, bench height is well equil- or are other factors also at work? We rule out direct attack on the cliff ibrated to wetting level at times of high tide under normal trade-wind by storm waves, as the bench can be walked safely at high tide even conditions. The wetting boundary outside the reef is always sharply during most storms. Abrasion along the base of the cliff by rock debris defined at the junction between bench and cliff. Along most of that part moved by wave swash also seems ineffective as a general process on of the north bench exposed behind the reef, however, the wetting the basis of the rarity of rock "tools" on the bench. Direct evidence boundary is located along a low scarp 2-5 m from the base of the cliff. of abrasion is limited to very local effects, such as the smoothing of Soil and vegetation on the bench surface above this scarp indicate that the bench near the Toilet Bowl, and the deepening of cracks and the it is rarely reached by waves. boring of potholes along cracks that have trapped small rock frag- Salt blooms are common on all cliffs facing the bay, but especially ments. Also, abrasion should tend to produce a notch along the base behind the bench on the north side of the bay where sunlight is intense of the cliff, but such a notch is absent behind the best-developed part from mid-morning to late afternoon. These salt blooms cause exten- of the bench. The general absence of boulders and large blocks of tuff sive flaking and spalling of the surface of the tuff. Parts of the cliff that on the bench surface, and even on the submerged Koko Bench, also remain wet adjacent to joints or permeable aquifer beds in the tuff lag suggests that disintegration rather than undercutting or hydraulic well behind the retreat of the rest of the cliff face, providing direct plucking is the most important factor in cliff formation. Abrasion also evidence that desiccation is an important factor in cliff retreat. Salt should reduce the bench to a surface below sea level, equilibrated to weathering is well known in brick or stone used for construction in the base of effective wave action as a limit. Abrasion also would have urban and seaside locations, and it is a recognized phenomenon in worked longest on the outermost parts of the bench, so it should cause desert and seaside weathering (Cooke and Smalley, 1968; Winkler and the bench to slope from the cliff base toward the bay. The bench is very

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flat, however, and in fact often has its highest level at the outer edge, ering, with the possible exception of number 2, as outlined by Chap- a relation consistent with preservation by wetting, but not with abra- man (1980, p. 128): sion. Finally, the only direct evidence for active lowering of the bench at the present time comes from the bench segment near BM3, which 1. The rock surface is rough, soft, and spongy and can be crumbled easily lies slightly above the normal reach of the waves, so that abrasion by with the fingers. Fine granules, rock powder, thin friable exfoliation scales, and wave wash cannot be the main factor there. thicker plates cling to the surface. Fracturing and swelling of the whole rock face is evident. Can a case be made for cutting of the bench by a higher stand of 2. A zone of especially intense weathering, a meter or less thick, occurs the sea? The Holocene sea-level history at Hanauma Bay is already at the base of many cliffs and pedestal rocks. well defined by the coral-reef study by Easton and Olson (1976), which 3. Small salt crystals may be detected in the rock pores and in small veins, concluded that sea level rose to its present level, or a little below, about and thin sheets of salt may back the exfoliation scales. The surface rock may have a salty taste. 3,000 yr ago. We agree with Easton (1977) that the dated loose coral 4. Beneath the rock face there is usually a loose, white talus of weathered fragment from beachrock on the lava flow is not valid evidence for a detritus, appropriately called "rock meal." This is a mixture of fine granules, former high stand at Hanauma Bay. If one still wishes to attribute the rock powder, and crumbling exfoliation scales derived from the rock surface bench to higher sea level, then how is that level to be decided? Does above. the bench represent low water, high water, wave base, or some other 5. Within rock hollows and caves, where weathering is less intense, the walls are covered thinly with granules and rock powder, but exfoliation scales level? How is the more than six-fold variation in level to be explained? are rather rare. Rock meal forms the floor of many of these openings. Failure to address these questions is reflected in the variety of levels ("12-15 foot," "2-meter," "5-foot," and "1.5 meter") other workers Given the time available for bench formation, is salt weathering have assigned to the sea level presumed to be associated with this a reasonable process? Measurement of thickness of flakes of material bench. The variations in elevation of the bench must be explained also, removed during two of these salt blooms indicates that about 1 mm of and this requires recognition of the importance of wave reach, which material is removed by each event. Four blooms per year, possibly a in turn eliminates the need to postulate higher sea level. In the same conservative estimate, would remove about 4 mm from the cliff each way, differences in bench width can be explained best by significantly year. Allowing 3,500 yr for benching related to present sea level, cliff different rates of cliff retreat, rather than by processes relying pri- retreat would be about 14 m, which is close to the width of the bench marily on some sort of cutting action. in front of the most intensely weathered cliff. This weathering rate is Because the present reef in the bay is equilibrated to present mean about 3 cm in 25 yr and therefore is also consistent with the 6 cm of lower low water, a higher Holocene sea would have to have risen and tuff removed around BM3, placed in 1931. This rate is at the low end fallen so rapidly that coral could not have kept pace; but that would of the range (3-330 mm/yr) defined for cliff retreat in California (Emery not have allowed time for a significant amount of bench formation. If and Kuhn, 1980) for a variety of rock types, but it is similar to rates the sea remained high for a time sufficient to form the bench and only they give for moderately well-consolidated sandstone. recently fell to its present position, then substantial amounts of emer- Bench formation at the wet-dry boundary also can explain the gent reef should remain. In the first case, only a rudimentary bench level of the -5-m Koko Bench at Hanauma Bay. On the basis of their would have been formed in association with the higher stand and study of the reef, Easton and Olson (1976, p. 718) found it hard to would have been destroyed by the more prolonged benching associ- explain Koko Bench, because the reef data indicate a stillstand at -7 ated with present sea level. This again requires the existing bench to to -8 m 5-7,000 yr ago. This required them to suggest that Koko be explained in relation to the present sea. The second case seems Bench may have formed as much as 4 m above the sea level at the time effectively ruled out by lack of an emergent reef. We conclude that if of its formation. In our survey, the present bench above Koko Bench, a Holocene high stand did occur, then it was too brief to have left any between stations D and E in Figure 8, is from 2 to 4 m above present record either in the reef or by benching. mean lower low water. If sea level at "Koko time" was at -8 m, the The bench behind the reef, which now supports some soil and upper limit of coral growth indicated in Figure 12, and if the relative vegetation, equilibrated to a level about 30 cm above the present wetting levels then were similar to what they are now along this same normal wetting level at high tide, and it is backed by a notch at the foot part of the bay, then a bench could have been formed at -4 to -6 m of the cliff. The most likely explanation for this is that wetting formerly at this time. The length of this stillstand is about 1,500 yr; the 6-m width occurred at a higher level when the reef was less developed and of the bench (Easton, 1973, p. 2277) is consistent with that predicted permitted higher waves to break on the edge of the bench, resulting using the same rates inferred for retreat of the modern cliff. in a higher "swash" level. The notch also could reflect the greater We conclude that salt weathering above the level of daily wetting exposure of this part of the bench to trade winds blowing down the by high tides is a major factor in the cliff retreat that has formed the bay, combined with the greater abundance of sand here that could be bench in Hanauma Bay. The bench reflects the daily upper limit of blown against the base of the cliff. The cliff behind this part of the wetting by the present sea. The waves do not "cut" into the cliff, but bench (and behind the whole length of beach) is shaded during the rather preserve the bench relative to the cliff by daily wetting. The afternoon and is correspondingly less subject to salt blooms. Thus, same mechanism may have formed Koko Bench about 5,500-7,000 yr bench formation here may have been influenced relatively more by ago during a stillstand at about -8 m. Although salt weathering seems abrasion, and less by salt weathering. to be a very effective agent of cliff retreat in the predominantly tuf- faceous rocks surrounding Hanauma Bay, the fresh basalt and lime- CONCLUSIONS stone inclusions do not respond to it in the same way. Benching in cliffs formed of these rocks might develop at different levels, at different The coastal bench that developed on basaltic tuff along the shores rates, or not at all. Also, this mechanism has not created a bench at of Hanauma Bay is produced by weathering of the cliff above the level a single level relative to sea level. Thus, "fossil" rock benches must of daily wetting by waves at high tide. During salt blooms, the cliff be used with great caution as indicators of past sea level. Such use behind the bench shows all the characteristics of intensive salt weath- requires understanding of the bench-forming mechanism and its re-

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lation to the sea level at time of formation, the variation in bench height Easton, W. H., 1973, Submarine bench at 5 m, Oahu, Hawaii: Geological Society of America Bulletin, v. 84, p. 2275-2280. and width that can result from variations in wetting level by waves, the Easton, W. H., 1977, Reply to Discussion of Radiocarbon profile of Hanauma Reef, Oahu, Hawaii: Geological Society of America Bulletin, v. 88, p. 1535-1536. possible effects of local microclimate, and the response of specific Easton, W.H., and Olson, E. A., 1976, Radiocarbon profile of Hanauma Reef, Oahu, Hawaii: Geological rocks to these factors. Society of America Bulletin, v. 87, p. 711-719. Edwards, A. B., 1951, Wave action in shore platform formation: Geological Magazine, v. 88, p. 41-49. Emery, K. 0., and Kuhn, G. G., 1980, Erosion of rock shores at La Jolla, California: Marine Geology, v. 37, p. 197-208. ACKNOWLEDGMENTS Gramlich, J. W., Lewis, V. A., and Naughton, J. J., 1971, Potassium-argon dating of Holocene basalts of the Honolulu Volcanic Series: Geological Society of America Bulletin, v. 82, p. 1399-1404. Hay, R. L., and Iijima, A., 1968, Nature and origin of palagonite tuffs of the Honolulu Group on Oahu, We thank M. Garcia for base maps and the use of plane table Hawaii: Geological Society of America Memoir 116, p. 331-376. Hills, E. S., 1949, Shore platforms: Geological Magazine, v. 86, p. 137-152. equipment, J. Sinton for his interest and encouragement in this study, Johannessen, C. L., Feireisen, J. J., and Wells, A. N., 1982, Weathering of ocean cliffs by salt expansion W. H. Easton for many useful comments on a preliminary version of in a mid-latitude coastal environment: Shore and Beach, v. 50, p. 26-34. Johnson, D. W., 1933, Supposed two-meter bench of the Pacific shores: International Geographic this manuscript, and the reviewers and associate editor for suggested Congress, Comptes Rendus, tome 2, fasc. 1, p. 158-163. Newell, N. D., and Bloom, A. L., 1970, The reef flat and "two-meter eustatic terrace" of some Pacific improvements to the manuscript. The senior author thanks the Hawaii atolls: Geological Society of America Bulletin, v. 81, p. 1881-1894. Institute of Geophysics for partial salary support. Travel and field Steams, H. T., 1935a, Shore benches on the island of Oahu, Hawaii: Geological Society of America Bulletin, v. 46, p. 1467-1482. expenses were supported by Grant No. NA86-AA-D-SG090, Sea Stearns, H. T., 1935b, Pleistocene shore lines on the islands of Oahu and , Hawaii: Geological Society of America Bulletin, v. 46, p. 1927-1956. Grant Project No. R/0-10-PD at Woods Hole Oceanographic Institu- Stearns, H. T., 1977, Discussion of Radiocarbon profile of Hanauma Reef, Oahu, Hawaii: Geological tion. The U.S. Government is authorized to produce and distribute Society of America, v. 88, p. 1535. Steams, H. T., 1978, Quaternary shorelines in the Hawaiian Islands: Bernice P. Bishop Museum Bulletin, reprints for governmental purposes notwithstanding any copyright v. 237, 57 p. Trenhaile, A. S., 1987, The geomorphology of rock coasts: New York, Oxford University Press, 384 p. that may appear hereon. United States Coast and Geodetic Survey, 1944, Tidal bench marks: Pamphlet T-32, Islands, p. 25-35. Wentworth, C. K., 1926, Pyroclastic geology of Oahu: Bernice P. Bishop Museum Bulletin, v. 30, 121 p. REFERENCES CITED Wentworth, C. K., 1938, Marine bench forming processes: Water level weathering: Journal of Geo- morphology, v. 1, p. 6-32. Bartrum, J. A., 1924, The shore platform of the west coast near Auckland: Its storm wave origin: Reports Wentworth, C. K., and Palmer, H. S., 1925, Eustatic bench of islands of the north Pacific: Geological of the Australian Association for the Advancement of Science, v. 22, p. 493-495. Society of America Bulletin, v. 36, p. 521-544. Bartrum, J. A., 1926, "Abnormal" shore platforms: Journal of Geology, v. 34, p. 793-806. Winchell, H., 1947, Honolulu Series, Oahu, Hawaii: Geological Society of America Bulletin, v. 58, Bartrum, J. A., 1935, Shore-platforms: Reports of the Australian Association for the Advancement of p. 1-48. Science, v. 22, p. 135-143. Winkler, E. M., and Wilhelm, E. J., 1970, Salt burst by hydration pressures in architectural stone in urban Chapman, R. W., 1980, Salt weathering by sodium chloride in the Saudi Arabian desert: American atmosphere: Geological Society of America Bulletin, v. 81, p. 567-572. Journal of Science, v. 280, p. 116-129. Wyrtki, K.,and Leslie, W. G., 1980, The mean annual variation of sea level in the Pacific Ocean: Hawaii Clague, D. A., and Frey, F. A., 1982, Petrology and trace element chemistry of the Honolulu Volcanic Institute of Geophysics Publication 80-5, 159 p. Series, Oahu: Implications for the oceanic mantle below Hawaii: Journal of Petrology, v. 23, p. 447-504. Cooke, R. U., and Smalley, I. J., 1968, Salt weathering in deserts: Nature, v. 220, p. 1226-1227. MANUSCRIPT RECEIVED BY THE SOCIETY MARCH 12, 1991 Curray, J. R., Shepard, F. P., and Veeh, H. H., 1970, Late Quaternary sea-level studies in Micronesia: REVISED MANUSCRIPT RECEIVED JUNE 11, 1992 CARMARSEL expedition: Geological Society of America Bulletin, v. 81, p. 1865-1880. MANUSCRIPT ACCEPTED JUNE 21, 1992

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