Morphology and distribution of dolines on ultramafic rocks 1 from airborne 2 LIDAR data- the case of southern Grande Terre in New Caledonia (SW Pacific)
Résumé
Dolines are closed geomorphological depressions which are surface manifestations of karstic
systems. Usually developed on limestone, they also typify the morphology of the New Caledonian
landscape, particularly on the southern massif of the main island (known as Grande Terre). These
dolines are evidence of subsidence, suffosion and collapse phenomena resulting from dissolution
weathering of peridotites. However, extensive underground drainage systems are still not yet
recognized. Semi-automatic mapping of dolines is carried out on a 148 kmC area of the Massif du
Sud from a high accuracy LIDAR digital elevation model. 8,601 dolines ranging from 1 mC to 2 kmC
are identified and morphologically characterized with precision. The majority of them are small,
shallow and round-shaped, yet more complex shapes are locally observed. Size distribution analysis
allows the setting of a threshold of 20,000 mC above which surface processes rather than chemical
weathering control doline evolution. Doline density analysis reveals high concentrations on flat areas
where ferricrete overlies the complete weathering profile, especially in the case of elevated, rainy
watersheds. Dolines are aligned and elongated along a N 135 A 5° major fracture direction, which is
inherited from the obduction of the Pacific Plate upper mantle in the Late Eocene. Finally, we propose
a pioneering morphometric typology of dolines that provides important clues as to pseudokarstic
activity. We define collapse, bowl-shaped and flat bottom dolines. Collapse and bowl-shaped dolines
are assumed to denote active pseudokarst. They may widen and deepen, or eventually be filled by
sediments. They are distinguished from flat bottom dolines that are partially to completely filled, which
suggests that they are associated with paleo-pseudokarsts. However the groundwater flow paths
associated with the genesis and evolution of dolines must be clarified, and collapse and bowl-shaped
dolines should be hydrologically monitored.