Sintesi preliminare delle caratteristiche tettoniche e sedimentarie dei depositi quaternari della Conca di Sulmona (L'Aquila)

Abstract

The Sulmona Basin is the easternmost Plio-Pleistocene intermontane depression of the central Apennines. Elongated in the NW-SE direction, it is conformable to the main trend of major tectonic structures in the area (Marsica, Morrone, and Maielia range). The Sulmona basin is filled by continental Quaternary deposits. A field survey for facies analysis and the reconstruction of litostratigraphic units and tectonic setting of Quaternary deposits is ongoing. This paper outlines the results obtained to this date. Radiometric K/Ar age determinations on volcanoclastic layers interfingered with continental deposits, together with sedimentoiogic and iithostratigraphic data allow 5 litostratigraphic units to be identified (S. Venanzio, Pratola Peligna, Gagliano, Fiorata and Sulmona Units), Lower Pleistocene alluvial fan deposits form the S. Venanzio Unit, which outcrops in the western part of the basin. These consist of three generations of alluvial fans dipping 25°E: i) the oldest one unconformably overlies the marly limestone of the "Scaglia" formation of Upper Cretaceous age and consists of breccia and very coarse-grained debris flow deposits attributed to a proximal alluvial fan system; ii) an unconformity and paleosols separate the first alluvial fan deposits from the second ones; iii) the youngest alluvial fan deposits are interfingered with the palustrine deposits (calcareous silt, silty-clay with peat seams and occasional lenses of conglomerate and sand bodies) of the Pratola Peligna Unit Results of facies analyses indicate a depositions palustrine and braided plain environment dating to the Lower Pleistocene. The Gagliano and Fiorata Units consist of laminated calcareous silt and silty-clay deposited under deep water lacustrine conditions; thin volcanic layers, made up of ash and small crystal fragments of pyroxene, leucite, subordinate sanidine and occasional 1-cm-wide mica laminse, are interbedded within the lacustrine deposits. Radiometric age determinations give a Middle Pleistocene age for the pyroclastic products. Along the eastern border of the basin (flank of Mt. Morrone) other alluvial fan deposits (Schiappara and Morrone fan deposits) are pres-ent. Lacustrine deposits are covered by braided plain deposits (Sulmona Unit). Fluvial sediments have filled the Sulmona basin during various depositional phases in Middle-Upper Pleistocene times, now forming a still well-preserved flat terrace made up of well-sorted conglomerates and pebbles, with typical cross-bedding structures. Paleodrainage evidence indicates a southeastern provenance toward the Sulmona Plain.The tectonic setting of the Sulmona basin Quaternary deposits highlights a complex deformational history controlled by the Morrone (NW-SE trending) master fault system. Quaternary deposits are displaced by extensionai NW-SE, E-W and NNE-SSW trending fault systems. Geometrical features of the deposits and geophysical data indicate that the basin is an extensionai half-graben basin asymmetrical to NE, filled by a wedge of sediments reaching its maximun thickness (about 500 m) along the master fault.