The Upper Paleozoic pull-a-part Mulargia-Escalaplano basin (S Sardinia, italy): relationships between tectonics and sedimentation

Costamagna Luca Giacomo

2021-01-01

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Abstract

The Mulargia-Escalaplano late to post-Variscan molassic basin in central Sardinia preserves a 300 m thick succession of continental deposits. Alluvial fan-(deltas?), braided and sinuous stream to lacustrine-palustrine?/playa sediments were laid down under wet climates evolving to dryer conditions. Sediments are organized in two superposed main depositional cycles punctuated by recurring volcano-tectonic spikes. The lower cycle is related to the limnic Rio Su Luda Fm (Late Pennsylvanian to Early Permian), while the upper cycle, subdivided into two sub-cycles, is related to the red bed Mulargia Fm (Early to Middle? Permian). The transition between the two stratigraphic units is marked by coarse deposits showing a gradual color change from dark grey to red. If there is any significant stratigraphic gap between them is still unclear yet. Nonetheless, the Mulargia Fm reworked the Rio su Luda Fm, as demonstrated by the embedded pebbles. The deepening of the source of the pebbles testifies an ongoing unroofing process reaching in times the lowest tectonic Variscan units of this part of the chain. Evidence of synsedimentary tectonics as small faults, slumpings, and debris flow are scattered especially in the NW part of the basin. In the limnic cycle and in each of the red bed sub-cycles the depositional energy decreases gradually upwards and SE-ward until the onset of the following tectonic phase. Thus cycles and sub-cycles represent erosive responses to volcanotectonic climax producing, in the end, the smoothing of the surrounding relieves. High-energy passing to low-energy continental environments develop gradually in times and show a vertical and lateral evolution. Based on analogies with coeval and fossil similar examples, the investigated depositional basin is a pull-apart one related to a main NNW/SSE listric fault: the sedimentary facies and the stratigraphy are organized accordingly, with both coarsest deposits and finest and deepest deposits located close to the master fault. The basin widens progressively in times and its depocenter shifts SE-wards. The sedimentological features show this strong directional control, with a SE-directed sediment flow. Sedimentary structures, as crossbedding and imbrications, support this SE flow direction. The bed shape of the coarsest lithologies changes southeastward from tabular to lenticular, as well as their architectural organization, showing growing evidence of lateral accretion in the same direction. Thus the fluvial style evolves southeastward from braided towards sinuous channel patterns. Compositional and textural maturity of the siliciclastic deposits grows southeastward likewise. Carbonate deposits of low-energy, containing subordinate evaporites, grow also SE-ward. Stratigraphical, environmental, and evolutional correlations between the presently separated NW and SE part of the basin have been reconstructed and evidenced by considering the leading role of the tectonics. The Mulargia-Escalaplano basin outcrops show a well-exposed example of the tectono-sedimentary evolution of a late to post—Variscan sedimentary pull-a-part basin, successfully comparable with its analogs of SW Europe, as the Autun, Lodeve (France), and Saar-Nahe ones (Germany).