Morphology of fore-reef slopes and terraces, Takapoto Atoll (Tuamotu Archipelago, French Polynesia, central Pacific): The tectonic, sea-level and coral-growth control

Abstract

A dataset of 41 bathymetric profiles detected by satelline imagery and echo-sounding to a maximum depth of 38 m along the fore-reef slopes of Takapoto Atoll (Tuamotu Plateau, French Polynesia, central Pacific) were qualitatively and quantitatively analyzed in order to identify the main geomorphological features. In addition, scuba-diving investigations from a selected site resulted in defining coral composition and cover rate. Six distinct fore-reef sectors were morphologically differentiated on the basis of seaward extension and slope gradient. Only the north end, and a restricted area of the mid-south-east side extend seawards over an average distance of 150 m, with a relatively low gradient (23°–38°). Most of the outer-slope areas are narrow (<100 m on average) and steep (average slope equals to 45° to up to 70°). Geomorphic features include spur-and-groove systems, sub-horizontal to gently dipping terraces (23–45 m wide) ending seawards in vertical steps (<10 m high) and drop-offs (>10 m high), outer-reef crest-like reliefs (0.5 m–1.5 m high), pinnacles (2 m–17 m high), coral heads (<2 m high), trenchs and concave scars. The recognized seven terrace levels occur at the respective depths of about −5 m, −10 m, −15 m, −20 m, −24 m, −30.5 m and −38 m. Based on comparison with subsurface chronostratigraphy of drilled reef sequences from some other Pacific atolls, the terraces are interpreted as remnants of reef tracts formed during Pleistocene interglacial, high sea-stands, respectively during MIS substage 5e (about −125 ka), MIS substages 7a (about −195 ka), 7c (about −210 ka), 7e (about −240 ka) and 9e (−330 ka). After depth correction of ground lowering during periods of emergence and subsidence effects, terrace elevations were expected to indicate eustatic sea-level positions at each identified high sea-stand, at approximately 7 m (MIS 5e) and 2.2 m (MIS 7a) above present sea level and 9.5 m (MIS 7c), 11 m (MIS 7e) and 8.6 m (MIS 9e) below present sea level. During the post-glacial sea-level rise, from about 9.5 ky, the studied portions of fore-reef slopes were re-flooded. It was assumed that coralgal deposits in the form of thin veneers, coral heads, outer-reef crest-like reliefs and pinnacles have simply overlapped pre-existing structures. Locally, the fore-reef zones are carved by trenchs, i.e. incipient detachment faults, and scars, i.e. imprints of former landslides. Similarly, some drop-offs may be regarded as fault walls. The prevailing direction of faulting–collapsing surfaces is consistent with the SW–NE orientation of the main axis of the atoll and submarine ridges rising up from the north-western floor of the Tuamotu Plateau. Analysis of the composition and distribution of coral communities to −30 m depth revealed high, downslope-increasing coral coverage (24% to 67% on average) and the sharp replacement of robust-encrusting Pocillopora by sub-massive to plate-like Porites from depths greater than −10 m. Significant inter-regional differences were detected with atolls in west Pacific, probably due to local ecological constraints.