Morphology and morphometry of the Gran Sasso (Central Italy) surface karst

Abstract

Surface karst is believed to influence the qualitative and quantitative characteristics of the waters that feed aquifers. This paper focuses on the morphometry of some surface karst features and on their relations with groundwater resource conservation and management. The study area is in the Gran Sasso karst massif of central Italy. It is a typical example of a Mediterranean hydrogeological system, consisting of Meso-Cenozoic carbonate and marly-silicoclastic sedimentary units that were involved in the Apennine thrust belt (lower Pliocene-upper Miocene). Since the upper Pliocene, extensional fault systems cut off the massif, leading to the formation of large tectonic basins (filled with Pleistocene continental deposits) and of areas with a typical basin-and-range morphology. The karst process (probably occurring after the upper Pliocene) gave rise to both large-scale (karst plains and dolines) and medium-scale (karrenfelder) surface features. Morphometry data on 141 karst plains, 382 dolines and 58 karrenfelder were collected by means of field surveys. The orientation and length of 682 macro-faults and 2,196 lineaments were recorded via aerial photos and geological maps. The study showed the occurrence of highly weathered red paleosols (Luvisol and Acrisol) as relict or buried soils of Pleistocene age, and of fairly differentiated soils of more recent age (Phaeozem). Statistical analyses indicated that the karst surface features are not fortuitous, but related to the tectonic fabric of the massif and to its geomorphopedological evolution. The Gran Sasso karst landscape is similar to tropical karst landforms (conical towers), and with the small inselbergs and tors found in the crystalline rocks of the tropical belts of Africa and South America (mainly formed by chemical weathering). This may be due to the fact that the relief of Gran Sasso was modelled by chemical processes that began when the soil cover was thicker and very different from the soils of today. The deepening process of the karst surface forms is related mainly to water availability and, subordinately, to carbonate rock type and fracturing. The morphological evolution of the dolines is controlled by the lithology, since their spatial enlargement in coarse-grained carbonate rocks is more significant than in mudstones. The formation of karrens and pinnacles appears to be connected to the opening of individual joints. This finding suggests that the release of tensile forces after orogenetic stress is still active and that it may have been favoured by slope modelling and ice mass retreat in the last 10,000 years. The collected data point to at least two karst stages, each with different surface features: high-altitude features may be attributed to present and recent karst activity, whereas low-elevation ones (inferred from selected buried and relict paleosols) are likely to have developed since the middle Pleistocene and to be scarcely active today.