Coastal bench formation at Hanauma Bay, Oahu, Hawaii WILFRED B. BRYAN Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543 ROBERT S. STEPHENS* Hawaii Institute of Geophysics, Honolulu, 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 tuff 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. 377 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/3/377/3381697/i0016-7606-105-3-377.pdf by guest on 23 September 2021 BRYAN AND STEPHENS EASTERN OAHU, HAWAII 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 basalt 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 378 Geological Society of America Bulletin, March 1993 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/3/377/3381697/i0016-7606-105-3-377.pdf by guest on 23 September 2021 COASTAL BENCH FORMATION, HANAUMA BAY, HAWAII 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.