POSSIBLE ORIGIN OF A CONCAVITY IN THE NA PALI COASTLINE, KAUAI

The present margin of Kauai’s Shield Volcano along it’s north facing Na Pali coast displays a prominent concavity centering at Kalalau Valley, the coast’s largest. Fluted, near vertical, 800-1000 meter-high walls close the back, southern portion, of the amphitheater-shaped valley. A gently-sloping wave-cut terrace extends seaward to a slope break at 1000 meters, which is indicative of the subsidence of the volcanic edifice since the end of shield building, age approximately 5 my. The heads of three prominent submarine channels notch the slope break and line up with major onshore Na Pali valleys. The shield tholeiitic basalt of the Na Pali Member of the Waimea Canyon Basalt displays lithic uniformity along the entire coast. Numerous large sea caves provide evidence of the ongoing intense shoreline erosion, mainly by large ocean waves, commonly in the range of 8-12 m, which pound the coast between October and April each year. Numerous hanging valleys of various heights result from a shoreline retreat that is faster than down cutting within small watersheds.

Kalalau Valley, and its associated offshore submarine canyon, lies directly down dip from the gently-sloping, northwest-trending Alakai Swamp which is fed at its northern end by extreme orographic precipitation in the range of 10-14 m/yr along a prominent north-south trending ridge marked by Mts Waialeale (1570 m) and Kawaikini (1598 m). The highly porous and permeable, columnar-jointed basalt of the Olokele Mbr of the Waimea Canyon Basalt provides for efficient groundwater recharge and movement from the swamp. Almost all of the water entering, and eroding, the valley walls and bottom flow from the steep amphitheater walls of the valley. Waterfalls emanating from the walls mark fault discontinuity surfaces. Kalalau Valley and associated valleys most likely have been eroded at a higher rate than adjacent valleys owing to the down slope movement and the weathering and erosive activity of Alakai Swamp groundwater. The relatively high rate of valley widening and down cutting also produces Na Pali’s prominent concavity. Adjacent Waimea and Wainiha canyons intercept Alakai Swamp groundwater that would otherwise flow to other sections of the Na Pali coastline.