Dynamics and facies analysis of Mid-Late Holocene reef systems in French Polynesia

Abstract

The detailed reconstruction of reef geometry, composition and evolution during the mid-late Holocene brings valuable information regarding coral reef dynamics and coastal processes during periods of higher sea level and wave energy regimes.This study provides a high-resolution reconstruction of the reef growth history in French Polynesia and their response to low-amplitude, high-frequency relative sea-level (RSL) changes and associated environmental changes over the past 6,000 years. After the stabilisation of sea level at its present position, the newdevelopment of reef systems was initiated by the creation of accommodation space due to a glacio-eustatic sea-level rise from 6.0 to 4.1 kyr BP, and controlled by the antecedent topography of the islands. A single,short-lived sea-level highstand of less than one metre between 4.1 and 3.4 kyr BP is documented preceding a fall in sea level between 3.6 and 1.2 kyr BP. The reported RSL changes are characterized by slow rates ranging from a few tens of millimetres per year up to 2.5 mm/yr and by significant sea-level stability (stillstands) lasting more than a century and up to 250 years, defining a step-like pattern.Regionally, the persistence of stable and optimal depositional environments over the last 6,000 years is demonstrated by the constant overall composition and diversity of reef communities and the almost continuouswidespread development of microatoll fields and reef flat units. The facies distribution as well as the lateral extension and shift of facies belts have been governed by variations in accommodation space, which are controlled by RSL changes and antecedent topography. The recurrent sea-level stillstands that punctuated the mid-late Holocene sea-level history have played a significant role in the development of dense reef frameworks.The widespread development of mid-late Holocene reef deposits in coastal areas, especially reef flat units and storm conglomerate deposits, suggests that they have played a prominent role in the formation and shaping of modern islands and islets, creating significant topographic features that formed the foundations of these islands.