The seismic transects were precisely located on the basis of an earlier detailed multibeam bathymetric survey [16] that allowed mapping of the main morphological features of the upper slope of the margin [10]. The two profiles shown in Fig. 2 best summarize the consistent morphosedimentary characteristics of the upper margin, imaged for ∼200 km along the northeastern margin (Fig. 1b).

We interpret the ‘lower glacis’ [10], identified between 300- and 900-m depths, as the abandonment surface at the top of a well-layered, reflective sedimentary sequence (unit 4), which in turn rests upon a group of low-frequency reflectors (Fig. 2a). The top of these reflectors is interpreted as an erosional lateritic land surface (II) (Fig. 2a), based on a comparison with the seismic signatures of the submerged continuation of the regional lateritic surface mapped in the southeastern part of mainland New Caledonia [1,3]. This land surface seals a 0.2-s-thick sedimentary unit (unit 3) that fills up a paleotopography carved into an older sedimentary unit (unit 1) (Fig. 2a). The wavy reflective pattern of unit 1 is interpreted as the signature of coarse, rapidly deposited (deltaic?) clastic sediments [3,18]. Dredging of the escarpment that bounds the glacis downslope suggests that unit 4 is made up of bioclastic limestones and unit 3 of conglomerates (rounded pebbles of basalt we dredged [17]).

The ‘high glacis’, identified between 80- and 450-m depths, is bounded downslope by the retreated escarpment of a master normal fault (Fig. 2). The top of the escarpment coincides with the trace of a subhorizontal lateritic surface (surface I) marking the abandonment of a layered unit (unit 2) made up of marine conglomerates (the dredged sample is made of basalt pebbles taken into a bioclastic carbonate matrix, [17]). Unit 2 is interpreted as recording a late aggradation of unit 1 (Fig. 2).

Variously inclined reflectors of unit 1 in the hanging wall of the master normal fault are suggestive of normal fault-block tilting prior to erosion and subsequent aggradation of unit 3 (Fig. 2a). A number of normal faults with small offsets affect unit 3 and are cut by paleoland surface II. These faults may reactivate earlier normal faults that had previously cut and tilted unit 1; we interpret this faulting to result from minor, possibly synsedimentary, gravity movements (Fig. 2a).

Paleoland surface I underlies a sedimentary ramp, unit 5, whose functional slope coincides with the ‘high glacis’ (Fig. 2a and b). This ramp records a transgressive and/or highstand systems tract, in which several prograding sub-units are distinguished (Fig. 2b). The uppermost slope of the ramp below the barrier shows up to five levels of terraces recording Quaternary paleosea levels (Fig. 2a; [10]). Dredging of unit 5 indicates that it is a carbonate ramp (six dredge samples provided reef limestones, grainstones and micrites, and every dredge sample provided calcareous mud and bioclastic sand [17]). The barrier reef stands at the crest of the carbonate ramp and may be correlated with the latest prograding subunit of that ramp (Fig. 2b).

The lagoon has developed on the rimmed carbonate platform lying behind the ramp (Figs. 2 and 3). Detailed marine charts indicate that the lagoon bathymetry is controlled by longitudinal (i.e., northwest-trending), flat-floored valleys (∼60-m depth) bounding elongate plateaus (∼45-m depth) and by transversal (i.e., northeast-trending) canyons that may attain 90-m depth within the passes of the barrier reef.

Seven seismic profiles were collected within the lagoon. The data show that the flat-floored valleys result from two episodes of river downcutting and subsequent aggradation (units 7 and 8) (Fig. 3). The second incision likely results from the entrenchment of transverse canyons and consecutive capture of the longitudinal valleys by these canyons.

The present results (Fig. 2a), combined with earlier deep-penetration seismic profiles and biostratigraphy of dredged samples collected from the northeastern margin of the NCR further to the southeast [3,8], indicate that unit 3 postdates a major episode of extensional faulting and block tilting that shaped the margin no earlier than the Middle Miocene. Indeed, samples dredged from the noses of tilted blocks are coarse marine conglomerates with mid-Miocene foraminifera-bearing matrixes. (Note: one of the eleven samples had Late Miocene fauna) [3]. Unit 3 is therefore Late Miocene in age at the earliest, and probably Pliocene. Furthermore, the earlier data reinforce our interpretation of unit 1 as being the coarse clastic deltaic sequence of Early to mid-Miocene age, imaged on deep seismic profiles collected along the entire northeastern margin of the NCR [3,18] and preserved on mainland New Caledonia [3,7].

Emplacement of units 1 and 2 (Fig. 2a) was followed by the development of paleoland surface I. This surface was disrupted by Late Miocene extensional tectonics (Fig. 4).

From that time on, the undisturbed proximal continental plateau then became separated from the collapsed part of the Early to mid-Miocene clastic wedge by the escarpment created by the master normal fault (Fig. 4).

Emplacement of unit 3 results from an aggradational episode following a period of aerial dissection of the topography inherited from Late Miocene extension, in relation to a base level found today at a minimum depth of 1200 m (1.6 s, Figs. 2a and 4). After a period of relative stability allowing the emplacement of paleoland surface II, aggradation of unit 4 took place, presumably as a result of relative sea-level rise (Fig. 4).

Onland, a sequence of seven regional, lateritic erosion levels has been mapped by Chevillotte et al. [5]. Paleoland surfaces I and II (Fig. 2a) would correspond to the last two surfaces in their sequence, the preceding surface in their sequence being interpreted as the floor of unit 1 [3].

Relative sea level rise from the Late(?) Pliocene led to the abandonment of unit 4 and has provided accommodation for aggradation/progradation of the carbonate platform on paleoland surface I (Fig. 4). The depth of this paleoland surface (and therefore the thickness of the carbonate platform) continuously decreases from 450 (Fig. 2a) to 250 m (Fig. 2b) towards the northwest over the surveyed area (Fig. 1b). This regional longitudinal slope may result from warping of the margin before or during aggradation of the carbonate platform.

The two incision/aggradation cycles that have affected the floor of the lagoon are likely the result of two ice age-related sea level drops over the last two hundred thousand years (Fig. 4). This is consistent with recent work on the geology of the southwestern part of the New Caledonia lagoon [4,11] and therefore confirms the eustatic origin of these cycles.

The northeastern margin of the New Caledonia ridge has undergone overall long-term subsidence and aggradation after Late Miocene extensional tectonics. Analysis of the samples dredged from the sedimentary units imaged by the NEOMARGES seismic data will allow us to constrain the timing and mode of relative sea-level change record of the margin. Eventually, a comparison with the carbonate platform systems of Northeast Australia [9] will help us better understand the response of passive margins to Late Cenozoic climate variability in the Southwest Pacific.