Sedimentological implications of durability and physical taphonomic processes on a fringing reef, Lizard Island, Australia

Abstract

This thesis aims to evaluate the way in which the physical durability of the skeletal remains of reef organisms is manifested within reef sediment deposits. To achieve this aim a series of models are developed, through comprehensive laboratory experiments, and are applied to the sedimentary system of Coconut Reef, a fringing reef on Lizard Island, Australia. The sedimentological characteristics of the Coconut Reef system are established through the analysis of 99 surficial samples, collected from the reef flat and beach, along with cores extracted from the adjacent coastal plain. The Coconut Reef deposit is shown to be comprised largely of sand-sized foraminifera and coral dominated sediment. Analysis of the wave record establishes the cross-reef gradient of wave energy, which is shown to be strongly controlled by tidal elevation and distance from reef edge. Propagating waves are found to generate near-bed currents which are able to mobilise reef-flat sediments. A systematic and repeatable set of experiments were undertaken using tumbling barrels to establish the durability and taphonomic alteration of six common reef sediment constituents. Durability is quantified by calculating the half-life of weight loss as a result of abrasion. Results show coral to be the most durable constituent followed by molluscs, foraminifera and Halimeda. Taphonomic alteration is assessed using high resolution digital imagery of experimental material, acquired throughout tumbling experiments. Two models are developed from tumbling experiments; both models simulate the abrasion of material within tumbling barrels. The first model examines the morphometric changes of a single sediment constituent, (Baculogypsina sphaerulata). Application of this model allowed for interpretation of the degrees of time-averaging and transport pathways of reef-flat B. sphaerulata assemblages. Results indicate the principal flow of tests is offshore and to the north. The second model simulates the maturation of a deposit comprised of four key sediment constituents. Outputs of the model reveal a shift from foraminifera dominated to coral dominated sediment as a function of maturity. When ground-truthed within the Coconut Reef system the model indicates nearshore sediments are highly mature, with central and outer reef flats dominated by 'fresh', low-maturity sediment. The application of both models show physical processes are of overriding importance on Coconut Reef. The physical destruction of reef material is significant feature of coral reef sediment systems. The approaches and models developed in this study shed light on the role destructive physical processes play within sedimentary systems, further adding to the suite of tools available to aid in the interpretation of carbonate deposits.