Holocene sedimentary evolution of a mid-ocean atoll lagoon, Maldives, Indian Ocean

Abstract

Based on detailed analyses of cores covering the lagoon of Rasdhoo Atoll, Maldives, six carbonate facies, one soil, and one peat facies have been identified. The abundance of carbonate and rare opaque grains was quantified with a point counter. X-ray diffractometry was used to measure mineralogical composition of samples. The statistical delineation of facies using cluster analysis was based on point count, mineralogical, and textural analyses. In decreasing abundance, the six carbonate facies are classified as mollusk–coral–algal floatstone to rudstone (30 %), mollusk–coral–red algae rudstone (23 %), mollusk–coral–algal wackestone to floatstone (23 %), mollusk–coral wackestone (13 %), mollusk–coral mudstone to wackestone (9 %), and mollusk mudstone (2 %). The carbonate facies represent lagoonal background sedimentation, mostly consisting of fine sediments, and event sedimentation depositing transported coarse-grained reefal components. Fifty-seven carbonate samples and one peat sample were dated radiometrically, covering the Holocene transgression from 10 kyrs BP until today. Comparing the sediment accumulation data of the lagoon with two local sea-level curves, three systems tracts can be identified: (1) a lowstand systems tract characterized by karst and soil deposition >10 kyrs BP, (2) a transgressive systems tract with peat and carbonate separated by hiatus 10–6.5 kyrs BP, and (3) a highstand systems tract dominated by carbonate sedimentation 6.5–0 kyrs BP and further divided into three stages (6.5–3, 3–1, and 1–0 kyrs BP). During the Holocene transgression, sedimentation rates increased continuously to a maximum of 1.4 m/kyr during 3–1 kyrs BP. Modern (1–0 kyrs BP) mean sedimentation rates average 0.6 m/kyr. A simple calculation suggests that two processes (background sedimentation and sand apron progradation) will probably fill up the accommodation space of the lagoon during the Holocene highstand, but these processes will not suffice to fill the larger atoll lagoons of the archipelago.