Late Quaternary shedding of shallow‐marine carbonate along a tropical mixed siliciclastic–carbonate shelf: Great Barrier Reef, Australia

Abstract

The north‐east Australian margin is the largest modern example of a tropical mixed siliciclastic/carbonate depositional system, with an outer shelf hosting the Great Barrier Reef (GBR) and an inner shelf dominated by fluvially sourced siliciclastic sediment wedges. The long‐term interplay between these sediment components and sea level is recorded in the Queensland Trough, a 1–2 km deep N–S elongate basin situated between the GBR platform and the Queensland Plateau. In this paper, 154 samples from 45 surface grabs and six well‐dated piston cores were analysed for total carbonate content, carbonate mineralogy and Sr concentration to establish spatial and temporal patterns of carbonate accumulation in the Queensland Trough over the last 300 kyr. Surface carbonate contents are lowest on the inner‐shelf (<5%) and in the trough axis (<60%) because of siliciclastic dilution. Carbonate on the shelf is mostly Sr‐rich aragonite and high‐Mg calcite (HMC), whereas that in the basin is mostly low‐Mg calcite. Once normalized to remove the effects of siliciclastic dilution, surface Sr‐rich aragonite and HMC abundances decrease linearly to background levels ≈ 100 km seaward of the shelf edge. Core samples show that, over time, normalized aragonite and Sr abundances are greatest during periods of shelf flooding and lowest when sea level drops below the shelf edge. This is consistent with changes in the production of coral and calcareous algae, and the shedding of their debris from the shelf. Interestingly, normalized HMC concentrations on the slope peak during periods of major transgression, perhaps because of maximum off‐shelf transport from inter‐reef areas or intermediate water dissolution. After accounting for siliciclastic dilution, there are strong similarities in both spatial and temporal patterns of carbonate minerals between slopes and basins of the north‐east Australian margin and those of pure carbonate margins such as the Bahamas. A limited set of basic processes, including the formation and breakdown of carbonate on the shelf, the transport of carbonate off the shelf and eustatic sea level, probably controls carbonate accumulation in slope and basin settings of tropical environments, irrespective of proximal siliciclastic sediment sources.

Diagenetic sensitivity of paleoenvironmental proxies: A rock magnetic study of Australian continental margin sediments

“…The ferrimagnetic assemblage of the high‐magnetization zones 1 and 2 is similar to that of the subsurface sediments in their high biogenic magnetite content, the IRM acquisition parameters of the detrital magnetite fraction and the high G/H ratio (Figure 5), indicating that sediments of these zones have not reached an advanced stage of reductive dissolution. Increased sedimentation rates during the early transgressions [ Peerdeman and Davies , 1993; Dunbar and Dickens , 2003] likely shorten the time spent by ferrimagnetic grains in the zone of active iron reduction. A partial dissolution of SD biogenic magnetite likely resulted in the production of the SP fraction.…”

The Modern Mixed Carbonate-Siliciclastic Abrolhos Shelf: Implications For A Mixed Depositional Model

“…For example, areas of southwest Florida, South Florida, Northeast Australia, and the Gulf of Papua illustrate mixed carbonatesiliciclastic systems deposited during regression Dunbar and Dickens 2003;McNeill et al 2004;Dickens et al 2006). However, siliciclastic sedimentation in these environments has proven more effective, or is more prevalent, which is opposite to what is observed for the northern Abrolhos shelf.…”

“…For example, the Great Barrier Reef region had siliciclastic sedimentation dominating during lowstand and transgression (Harris et al 1990;Larcombe and Carter 1998;Dunbar et al 2000;Dunbar and Dickens 2003). On the western Florida coast, the siliciclastic sedimentation also dominated during lowstand and transgressive periods from the Oligocene until the present, while the carbonate facies resided in the more offshore areas during highstand Hine et al 2009).…”

A Late Quaternary record of highstand shedding from an isolated carbonate platform (Juan de Nova, southern Indian Ocean)

“…This phenomenon (highstand shedding of carbonate platforms; Schlager, Reijmer, & Droxler, ) has been documented in both modern‐day settings (Andresen, Reijmer, & Droxler, ; Betzler, Lüdmann, Hübscher, & Fürstenau, ; Droxler, Alley, Howard, Poore, & Burckle, ; Droxler, Haddad, Mucciarone, & Cullen, ; Droxler & Jorry, ; Hine, Wilber, Bane, Neumann, & Lorenson, ; Jorry, Droxler, & Francis, ; Lantzsch, Roth, Reijmer, & Kinkel, ; Paul et al., ; Reijmer, Palmieri, & Groen, ; Rendle‐Bühring & Reijmer, ) and in the geological record (Everts, ; Reijmer, Ten Kate, Sprenger, & Schlager, ; Vecsei & Sanders, ). Past studies (Dunbar & Dickens, ; Page & Dickens, ), however, indicate that mixed systems may be more complicated than the simple highstand shedding model proposed originally by Droxler and Schlager (). Because the record of highstand shedding in the modern oceans is generally limited to carbonate platforms and margins in only a few well‐studied localities, new information is needed to better understand the carbonate response to sea‐level change at a global scale.…”