Late Quaternary barrier and fringing reef development of Bora Bora (Society Islands, south Pacific): First subsurface data from the Darwin‐type barrier‐reef system

Abstract

The universally known subsidence theory of Darwin, based on Bora Bora as a model, was developed without information from the subsurface. To evaluate the influence of environmental factors on reef development, two traverses with three cores, each on the barrier and the fringing reefs of Bora Bora, were drilled and 34 uranium-series dates obtained and subsequently analysed. Sea-level rise and, to a lesser degree, subsidence were crucial for Holocene reef development in that they have created accommodation space and controlled reef architecture. Antecedent topography played a role as well, because the Holocene barrier reef is located on a Pleistocene barrier reef forming a topographic high. The pedestal of the fringing reef was Pleistocene soil and basalt. Barrier and fringing reefs developed contemporaneously during the Holocene. The occurrence of five coralgal assemblages indicates an upcore increase in wave energy. Age-depth plots suggest that barrier and fringing reefs have prograded during the Holocene. The Holocene fringing reef is up to 20 m thick and comprises coralgal and microbial reef sections and abundant unconsolidated sediment. Fringing reef growth started 8780 AE 50 yr BP; accretion rates average 5Á65 m kyr À1 . The barrier reef consists of >30 m thick Holocene coralgal and microbial successions. Holocene barrier-reef growth began 10 030 AE 50 yr BP and accretion rates average 6Á15 m kyr À1 . The underlying Pleistocene reef formed 116 900 AE 1100 yr BP, i.e. during marine isotope stage 5e. Based on Pleistocene age, depth and coralgal palaeobathymetry, the subsidence rate of Bora Bora was estimated to be 0Á05 to 0Á14 m kyr À1 . In addition to subsidence, reef development on shorter timescales like in the late Pleistocene and Holocene has been driven by glacioeustatic sealevel changes causing alternations of periods of flooding and subaerial exposure. Comparisons with other oceanic barrier-reef systems in Tahiti and Mayotte exhibit more differences than similarities.

Holocene and Pleistocene fringing reef growth and the role of accommodation space and exposure to waves and currents (Bora Bora, Society Islands, French Polynesia)

“…In the leeward fringing reef, the landward core Faanui 1 was also characterized by a Pocillopora assemblage in the upper core section. The lower portion (>7·8 kyr bp ) of the core consisted of a deeper water (10 to 20 m) assemblage dominated by laminar Montipora and a basal sandy facies (Gischler et al ., ). Microbialite crusts usually occur in core sections older than approximately 6 kyr bp .…”

“…The retrograding to prograding reef architecture may be interpreted as a direct response to the mid to late Holocene decreasing rate of rise of sea‐level and the coeval decrease in accommodation space. The vertical accretion rate of the windward fringing reef in Bora Bora amounts to 3·12 m kyr −1 on average and is somewhat lower than the reported rate of the leeward fringing reef of 5·65 m kyr −1 (Gischler et al ., ). This observation is similar to the data of Cabioch et al .…”

“…Microbialite crusts usually occur in core sections older than approximately 6 kyr bp . This is in accordance with the findings in the other cores in Bora Bora (Gischler et al ., ) and in Tahiti, and has been explained by changes in environmental parameters such as light and energy, nutrients and alkalinity (Camoin et al ., ; Seard et al ., ; Heindel et al ., ; Riding et al ., ). The youngest, progradational section of the windward Puhia fringing reef is detrital and largely composed of unconsolidated sand and rubble.…”

“…Map of Bora Bora including drill sites of this study (Puhia 1 to 3; Tofari 1 to 3; Ome 1 to 2), and other drill locations of previous studies (Gischler et al ., ).…”

“…The work by Montaggioni () on fringing reefs of Tahiti and Moorea is based on one core from each and revealed largely detrital facies and both keep‐up and catch‐up growth patterns, respectively. Fringing reef development on the leeward side of Bora Bora is characterized by both coralgal–microbial and detrital facies, based on a core traverse (Gischler et al ., ). The former study may be characterized as preliminary and the latter included only the leeward island side.…”