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1 2006

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Architecture and dynamics of a Triassic/Jurassic carbonate platform-basin transition in the Sciacca area, South Central Sicily: implications for the Neogene accretion of the Maghrebian orogen

Cacciatore M., Di Stefano P.

Volumina Jurassica

2006

Vol. 4, no. 1

81-82

The Sciacca area, in Southwestern Sicily, belongs to the external zone of the Neogene Maghrebian thrust and fold belt. Two different groups of south-verging thrust sheets crop out in this area: the so called "Saccense" units that consist of thick Mesocenozoic neritic and pelagic platform carbonates; the "Sicanian" units made of Permian to Cenozoic deeper-water sediments. The tectonic boundary between Saccense and Sicanian units runs NW-SE from Monte Genuardo to Caltabellotta. This boundary is orthogonal to the main thrust fronts and nearly parallel to the direction of thrust propagation. Thrust-top-basin deposits of Pliocene age obscure the tectonic relationships along this alignment leaving different solutions open for its structural interpretation and for the palinspastic restorations between platform and basin-derived units. Recent sedimentological and stratigraphical studies focused in this zone document the presence of Upper Triassic reef limestones in the Saccense units (Pizzo Telegrafo unit), close to the tectonic alignment. The macro and microfacies analysis reveal a typical Dachstein-type reef composition of these deposits. As consequence they can be interpreted as markers of platform edges facing high-energy, open marine basins. The collected data are thus consistent with the presence of a Late Triassic platform-basin transition close to the present-day tectonic boundary. An intense Jurassic paleotectonic activity along this margin is proved by in situ brecciation and the presence of large polyphase neptunian dykes crosscutting the reef deposits and the overlying Lower Jurassic platform limestones (Inici Formation). The dykes are filled up by Middle to Upper Jurassic Rosso Ammonitico type condensed limestones. Basaltic pillow lavas occur as thick wedges intercalated to the Jurassic pelagic limestones. They indicate repeated episodes of (trans)tensional stresses along the paleomargin. An anomalous thin Cretaceous to Miocene sedimentary cover (Scaglia type calcilutites and glauconitic sandstones) is punctuated by deep erosional truncations and megabreccias. These sediments fill up later generations of neptunian dykes confirming the paleotectonic activity in this zone throughout the Jurassic-Miocene times. In the Sicanian basinal units flanking the tectonic alignment the influence of the platform paleomargin is recorded by extensive Lower Jurassic carbonate aprons interbedded to cherty calcilutites. Upper Triassic reef-derived elements are common constituents of these clastics as recently documented at Campofiorito and Monte Triona. Looking at the present day structural relationships in the studied area we can conclude that the Triassic/Jurassic paleomargin has played an important role as major transpressional dextral escape during the Neogene Sicanian mountain building.