Plio-Quaternary morphotectonic evolution recorded in the clastic cave sediments of Sicó massif (central Portugal)

Carbonate karst covers only approximately 5% of mainland Portugal’s land surface, with the most expressive landscapes associated to Middle Jurassic units and corresponding to the Sicó and Estremadura massifs in the central region. Multiple and complex phases of karstification were recognized, controlled by tectonics and climate. A discontinuous Meso-Cenozoic siliciclastic cover was responsible for organizing a fluvial network, as demonstrated by a set of suspended dry/blind valleys and canyons that cut the carbonate reliefs. This fluviokarst testifies how the fluvial network tries to follow the progressive and differentiate post-Jurassic tectonic uplifts of the massifs, mainly during Plio-Quaternary.

Despite this scientific knowledge, the timing of karst landscape evolution over Plio-Quaternary, including the quantitative constraints on the tectonic uplifts and long-term fluviokarst incision rates, remains poorly documented. A multi-scalar approach was used to combine geomorphological regional studies (massifs and entrenched valleys level) and local analysis of siliciclastic sediments preserved in selected caves/rock-shelters. New data were produced using a combination of intersecting sedimentological, geomorphological, geochemical, and geochronological analyses of karst record (forms and deposits).

Several types of clastic cave sediments, such as filtrates, fluvial, aeolian and marine deposits, were identified and compared with the local post-Jurassic siliciclastic covers through facies analysis. An extensive dating program of the siliciclastic fraction, relying on the ¹⁰Be and ²⁶Al cosmogenic nuclides, yield burial ages ranging from ~1.2 to 3.0 Ma.

Facies architecture of the siliciclastic sediments preserved in some small caves/rock-shelters (at an altitude of 360 m), opened on the structurally controlled western border of the Sicó massif, as well as on the slopes of the nearby Poio Novo fluviokarst valley, allow a genetic correlation with marine and aeolian deposits that, at an altitude of ca. 120 m, materialize a mainly Pliocene coastal plain. The obtained ²⁶Al/ ¹⁰Be data confirm a late Pliocene/early Pleistocene chronology for these karst infilling (from 2.3 ± 0.7 Ma to 1.5 ± 0.3 Ma), allowing to estimate local simple linear tectonic uplifts ranging from 0.08 to 0.2 mm/yr for the lasted ~3.0 Ma, being so in agreement with the more empirical values previously admitted by Soares (2006), Cabral (1995), and Ferreira (1991).

Furthermore, these marine and aeolian clastic cave sediments, previously found and describes for the Sicó massif by Cunha (1990), were already recognized in a similar topographic position on the western edge of the Serra de Candeeiros (Daveau, 1973, 1976), permitting to consider a similar uplift history also for the Estremadura massif.

Finally, in addition to strengthening the knowledge on karst landscape evolution, this study highlights the potential of clastic cave sediments as data archives for understanding local/regional morphotectonics.