Sedimentation adjacent to atolls and volcano-cored carbonate platforms in the Mozambique Channel (SW Indian Ocean)

Abstract

Recently acquired data from the Iles Eparses (southwestern Indian Ocean) reveal new information about the geomorphology, depositional processes, and sedimentary deposits on the slopes of atolls and atoll-like platforms. The deposits discussed here lie on the deepwater flanks of isolated, inactive volcanos that are capped by shallow, relatively flat carbonate platforms 45-210 km2 in area. Much of the slope geomorphology is controlled by the underlying volcanic edifice. Steep (~25-35°) upper slopes consist of outcrops of volcanic basement, smooth banks, failure scarps, and channels. Sedimentary features seen in the lower slope and proximal basin (2000-3500 m deep) consist of channels, levees, lobes, and mass transport deposits (MTDs). In places, channels terminate 13-18 km from the platform margin, ending in lobes up to 3.5 km across, a feature not often seen in modern carbonates. In the subsurface, MTDs are present near all platforms. Within MTDs, seismic character is variable, often consists of chaotic reflections indicative of sediment gravity flow processes. Subsurface units with organized (retro-or progradational) reflections are interpreted as turbidite lobes or MTDs with compressional features. Core taken within lobes and near the base of slopes reveal decimeter-scale turbidites and debrites composed primarily of graded and massive bioclastic grainstones and packstones with abundant neritic skeletal components, interbedded with hemipelagic aragonitic and clay-rich foraminiferal ooze. Slope depositional processes are therefore primarily gravity-driven and occur at different scales; i.e., bed-scale turbidites and muds may be remobilized and redeposited through slope failure and deposition of large MTDs. Dominant wind direction may also play a role in slope sedimentation: leeward slopes are generally less rugose and show increased sedimentation at the toe of the slope. This study thus provides new insight into depositional systems surrounding atoll-like carbonate platforms, and provides a new analogue for similar deposits in the geologic record. Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. Highlights ► First study to describe the slopes of volcano-cored carbonate platforms in detail. ► Slope sediments composed of muds and platform-derived carbonate sands ► Slopes contain channels, fans, and mass transport deposits. ► Sedimentation is primarily controlled by inherited topography and dominant winds.

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

“…This composition is size-dependent: foraminifers make up an increasingly higher proportion at finer fractions (Figure 4d). This bed and others like it have previously been interpreted as the product of turbiditic events, ultimately originating from the platform top (Counts et al, 2018), but likely incorporating slope material during downslope movement.…”

“…In places, these needles show evidence of dissolution and pitting (Figure b 2 , 1,410 cm depth). Elsewhere, where XRF/XRD data showed aragonite to be less abundant, sediment consisted primarily of platy clay minerals (Figure b 3 , 600 cm depth), identified as silicate clays by EDS analysis (Counts et al., ). While these clay‐rich intervals are found on the sea floor adjacent to the platform, the clastic material likely does not originate from the platform itself, which at present only contains carbonate sediment.…”

“…Further details of slope morphology and sedimentation on Juan de Nova and surrounding areas can be found in Counts et al. ().…”

“…The submarine flanks of the Juan de Nova volcanic edifice are steep (0-47°, with upper slope generally around 25-35°and steeper on the windward side; Counts, Jorry, Leroux, Miramontes, & Jouet, 2018), reflecting the underlying volcanic nature of the platform. The adjacent basin floor ranges between 2,000 and 2,500 m water depth (Figure 1d).…”

“…The marine sediment core that is the focus of this study (MOZ4-CS09) lies on the lower slope of the leeward (northern) side of the platform margin in 1,909 m water depth and on a slope of 4.3°inclination (Figure 1d). Further details of slope morphology and sedimentation on Juan de Nova and surrounding areas can be found in Counts et al (2018).…”