Late Quaternary history of Henderson Island, Pitcairn Group

Henderson Island is an emergent limestone island. It rises to 33.5 m above modern sea-level (MSL) from a sea-floor depth of about 3500 m and conforms to the pattern of an elevated atoll, although no field evidence was found pertaining to the pre-atoll volcanic history of the island. The emergence of this coral atoll can be explained by lithospheric flexure processes subsequent to the emplacement of the Pitcairn Island volcano, built by two phases of volcanism (estimated at 855 and 540 kyr by K-Ar dating). U-Th dates obtained from Henderson Island indicate that the majority of the presently visible fossil corals have an age between 404-225 kyr. Henderson Island first became emergent above MSL when sea level dropped subsequent to 380 kyr, as the period 440-380 is thought to have been characterized by sea level at least several metres above MSL in the Central Pacific. As a result Henderson Island would have become subaerially exposed from 380 kyr onwards. The uplift rate of Henderson Island is approximately 0.093-0.10 mm/yr indicating that flexure-induced uplift component above MSL commenced between 360-335 kyr, 500 kyr after the initial, and 190 kyr after the last phase of construction of the Pitcairn volcano. The time-lag following the loading of the Pitcairn volcano and the present continuation of uplift on Henderson support the theory that the lithosphere behaves in a visco-elastic manner. Field relations and U-Th dates indicate three main periods of reef development: (1) A prolific reef-building period (440-380 kyr [Unit 1] and 330-300 kyr [Units 2 & 3]) dominated by large, stout branching coral colonies; (2) A shorter period of reef growth (285-275 kyr [Unit 4]) dominated by well-formed large in situ coral colonies and Tridacna maxima; (3) A less prolific period of reef growth (230-215 kyr [Unit 5]) dominated by platy corals enveloping the previous lithologies below 19.6 m. The absence of dateable material from the last interglacial, oxygen isotope substage 5.5 (= 5e) at Henderson Island is in marked contrast to the Late Pleistocene reefs in the southern Cook Islands where reefs of substage 5.5 age skirt older carbonate complexes.