Note illustrative della Carta Geologica d’Italia alla scala 1:50.000, Foglio 641 Augusta

Map Sheet 641 “Augusta” of the Geological Map of Italy 1:50.000 scale, is realized on the base of a convention between the APAT – Geological Survey of Italy (Servizio Geologico d’Italia), the Autonomous Region of Sicily (Regione Siciliana) and Department of Geological Sciences of Catania University.The map sheet is located in the north-east area of the Hyblean Plateau, insouth-eastern Sicily. It lies nearly entirely within the province of Siracusa,and includes the municipal territories of Augusta, Lentini p.p., Carlentini andVillasmundo small towns; only the northernmost part of the map is placed in theterritory of Catania province.The geological map has been investigated and surveyed on a former map at the scale 1:10.000 (CTR–Carta Tecnica Regionale). The field survey has been coordinated by S. Carbone, directed by F. Lentini and performed by G. Sturiale, F. Lentini and S. Carbone.1. -REGIONAL GEOLOGICAL FRAMEWORKThe Hyblean Plateau represents part of the orogenic foreland deformed byextensional and strike slip faults. These faults are associated with extensiveUpper Miocene-Lower Pleistocene volcanic activity and uplift, together with thedevelopment of Plio-Quaternary basins such as Lentini Graben and Catania-Gela Foredeep to the north, both basin being separated by the intervening S. DemetrioHigh. The Catania-Gela Foredeep basin flanks the collapsed margin of the HybleanPlateau to the north and west and runs from Catania to Gela. The thrust front also runs from Catania to Gela, its frontal part being buried by sediments of Early Pleistocene age.The Hyblean Plateau and the adjacent Catania-Gela Foredeep are bordered onthe east by Hyblean-Maltese Escarpment. This fault system extends over a length of about 300 km from the eastern coast of Sicily southward, with a steep slope that drops to more than 3000 m below sea level.Onshore strands of the Hyblean-Maltese Escarpment can be observed along the Ionian coast of SE Sicily and give rise to incipient pull-apart grabens (Augusta and Anapo grabens) of Late Pliocene-Early Pleistocene age. The eastern edge of the Hyblean Plateau is located in one of the most seismically active region of the Mediterranean Area. The seismic history of thisregion is characterized by few earthquakes with high-magnitude. The major activenormal structures are located in the Ionian offshore fault system between Catania and Siracusa and show a NNW-SSE direction and a ENE dip. This coastal area, representing the footwall of the offshore fault system, has been affected during the last 400.000 years by an uniform uplift linked to the recent activity of these structures. The morphological analysis of the several orders of Middle-Upper Pleistocene marine terraces occurring along the coast allowed us to estimate aconstant tectonic uplift rate of about 0.65 mm/y.2. - STRATIGRAPHYThe Hyblean Plateau is composed of a thick, undeformed Mesozoic-Cenozoicshallow-water to basin carbonate sedimentary succession, with intercalatedvolcanic rocks in numerous horizons. Volcanism occurred from the Late Triassic up to Quaternary on the HybleanPlateau. During the Mesozoic, the African continental margin was affected by two main extensional tectonic phases that led to volcanism. In the collisional stage, from the Upper Cretaceous-Paleocene to Middle Miocene, no volcanism occurred in the region. The eruptive activity resumed on the north side of the plateau in the Late Miocene and gradually shifted northward up to the Etneanregion in the Middle Pleistocene. The most recent phase of Hyblean volcanismoccurred during the Pliocene-Lower Pleistocene along the northern margin of the plateau. The volcanic succession records the fluctuation between submarine and subaerial eruptive activity related to sea-level changes and tectonism occurring mainly along NE-SW trending faults. The volcanic products have tholeiitic andalkaline affinity. The youngest Hyblean volcanics are pillow lavas with alkalineaffinity and an age of about 1.4 My. A thick volcanic succession, buried withinthe foredeep Pleistocene sediments on the Catania Plain, is evidenced by a largemagnetic anomaly and sampled by drilling. It proves the extension of the volcanismbeyond the present northern boundary of the plateau. On the whole, Late Miocene-Pleistocene Hyblean volcanism is strictly connected with the neotectonic activityof eastern Sicily. In particular, the eruptive activity distribution is mainly related to a NE-SW extensional tectonic accommodating the dextral component of the NNE-SSW Scicli-Ragusa-Irminio Line: the Hyblean volcanism stopped when that line vanished in the Lower Pleistocene.The exposed sedimentary rocks on the Hyblean Plateau are mostly of Miocene age. This succession can be divided into an eastern and a western facies association. The western part of the Hyblean Plateau is characterized by well-exposed Upper Oligocene-Miocene limestones (Ragusa Fm.) and marls (Tellaro Fm.)deposited on a carbonate ramp under neritic to pelagic conditions. Pliocene andQuaternary near-shore carbonates passing basinward into clays are widespreadalong the margins of the plateau, at places intercalated with mafic volcanics.The eastern Miocene succession consists of massive to thick-beddedcarbonates (Monti Climiti fm.) and overlying reefal to lagoonal limestones(Monte Carrubba fm.) with intercalated pyroclastic rocks (Carlentini fm.). Thissequence, named Sortino group, rests on Upper Cretaceous to Paleogene shallowwater carbonates (Priolo fm.) and volcanics (Capo Passero volcanics), remnants of an earlier platform carbonate sequence, which was deposited on volcanic seamounts.The lowest exposed horizons in the Hyblean Plateau is of Cretaceous age and information on lower horizons is available only from boreholes. In the study area oldest products consist of Cretaceous volcanics and rudist-bearing limestones over which Oligo-Miocene shallow shelf carbonates belonging to the Monti Climiti Formation transgress. The oldest member of this formation consists of a coarse bioclastic carbonate (Melilli Member) but a thick coralline algal rhodolite sequence (Siracusa Limestone Member) comprises its upper part, which frequently exhibits patch-reefs at the top (calcari a echinodermi e molluschi fm.).The Carlentini formation directly overlies the Siracusa Limestones member.In the western sector of the study area (Carlentini-Agnone area) it is dominantly a volcanic sequence consisting of extrusive volcanic strata produced of phreatomagmatic eruptions, but with two intervening bioherm levels. In the east(Monte Tauro) it is characterized by a massive biohermal carbonate.The Monte Carrubba formation lies directly above the final major volcanoclastic episode of the Carlentini fm. and is dominantly a micritic sequencewith some oolitic grainstones in the east (Monte Tauro). The succession is coveredby Pliocene lavas.Lower Pliocene sediments in the eastern Hyblean region crop out discontinuously in the vicinity of Carlentini, as sands and calcareous breccias with gastropod Strombus coronatus (brecce e sabbie di Valle Cupa) further to the west near Buccheri. In the coastal region west of Augusta (Monte Carrubba-Porrazzito-Carlentini) Pliocene sediments are not developed because of thegeneral uplift of the area at the end of the Miocene.In the northern and northeastern Hyblean Plateau there are extensive outcropsof basic, submarine and subaerial lava flows of Pliocene (Militello in Val diCatania fm.) and Lower Pleistocene (S. Febronia fm.) age. They overlie, or areintercalated with coarse-grained sediments containing a Middle-Upper Pliocenefauna. These sediments are discontinuous and of limited thickness and extent, and their shallow water origin also confirms the persistence of local brief marineingressions in an area in which uplift was the principal trend.The Quaternary sediments, distributed along the Ionian coast, were formed during two sedimentary cycles. The earlier, Lower Pleistocene, is made up of calcarenites and sands with Arctica islandica grading up and laterally into bluish clays with Hyalinea balthica. The sediments of the second cycle, Middle Pleistocene, consist of yellowish biocalcarenites, separated from the preceding cycle by a paleosol.A subsequent younger cycle, ascribed to Late Pleistocene (Tyrrhenian), formed by few meters of biocalcarenites and cobblestones with Strombus bubonius crops out in the Monte Tauro peninsula and Brucoli. They unconformable overlay the older deposits.The Pleistocene succession has been grouped into the “Iblei settentrionali”supersyntheme, including the Lentini, the Augusta and the Monte Tauro synthemes.The Upper Pleistocene-Holocene sediments consist of continental and transitional environment deposits, including primarily inactive alluvial deposits (bn) undergoing pedogenesis or terracing developed during different pulses of theglacial-eustatic activity; active alluvial deposits (b), active and inactive palustrine(e), old beaches (d) and longshore bar (g) deposits; eluvium and colluvium (b2) andalso anthropic deposits (urban structures, industrial installation, archaeologicalsites, etc). Widely outcrops of anthropic deposits of urban solid and fill materialswasters, and numerous active quarries are localized around the northwestern sector of the San Demetrio hill.3. - THE TECTONICS OF THE AREA AND OF THE HYBLEAN-MALTESE ESCARPMENTDuring the Late Cretaceous the extrusion of a thick cover of submarine volcanics produced a discontinuous chain of seamounts trending along the present-day Ionian coastline, from Capo Passero to north of Augusta. Upon themwidespread reefs developed, while to the west pelagic sedimentation continuedin the basinal environment established during the Early Mesozoic following thedissection of the Triassic platform (the Ragusa Belt of Patacca et alii, 1979).From the Late Cretaceous to Late Miocene, the eastern and north-eastern areas of the Hyblean Plateau (the Syracuse Belt of Patacca et alii,1979) probably comprising some areas now lying in the near offshore, took on the role of structural highs, whereas the region to the southwest formed a moderately deep subsiding basin.An Early Tertiary tectonic phase caused emergence of some eastern areas andfurther emphasized the paleoenvironmental characters of the two Hyblean sectors.The stratigraphic history points to a period of relative tectonic tranquillityduring Oligocene and Miocene times. During Messinian time the region from Agnone to Syracuse was emergent while to the south, in the Capo Passero area, hemipelagic marls and evaporites deposited.Southeast of Capo Passero, on the Ionian bathyal plain, evaporites have beenrecognized by seismic profiles. Messinian marls have been also recovered alongthe scarp (Cita et alii, 1980) as well as Lower Pliocene marls which can be easily correlated with coeval sediments cropping out in land. Messinian marls and evaporites and Lower Pliocene marls actually crop out intwo areas separated by a throw of about 3000 m, the Capo Passero plain onshoreand the Ionian bathyal plain offshore.A contemporaneous deposition of the Early Pleistocene deposits is unlikelyat these very different elevations. This suggests the occurrence of strong post-Messinian displacements along the Hyblean-Maltese Escarpment.Neotectonic activity in the Hyblean area facing the Hyblean-MalteseEscarpment occurred at three main phases: Late Messinian?-Early Pliocene, LatePliocene–Early Pleistocene, and Middle Pleistocene to Recent.The older tectonic phase, of a compressional character, is detectable in the Hyblean Plateau as well as in many parts of Sicily and Calabria.In the Hyblean Plateau the Early Pliocene tectonic activity was characterizedby a wrench faulting which generated dextral movements along the NE-SW trending fault planes and sinistral movements along NW-SE trending conjugatefault planes (Ghisetti & Vezzani, 1981; Lentini et alii, 1984).The development of NW-SE fault trends along the Ionian coast, further emphasized during the following Early Pleistocene extensional phase, controlled the distribution of the Early Pleistocene sediments and the location of the coastlines.The main results of this second tectonic phase consist of the formation of some coastal horst-graben structures delimited by NNW-SSE and NW-SE faultssubparallel to the modern Pantelleria Graben. and to the neotectonic fault systemfound in the Maltese Islands. The coastal grabens are filled by a thick cover ofsediments, the oldest of which are of Early Pleistocene age. They lie on Miocene or Cretaceous rocks, because Pliocene sediments are absent.The seaward continuation of the NNW-SSE trending faults links with the Hyblean-Maltese fault system.Morphological features such as a submarine valley along the scarp south of Syracuse are probably the topographic expression of one or more of the NW-SE trending faults (C.N.R., 1991).The final phase of faulting, of Middle Pleistocene-Recent? age, is developed within the Hyblean area and is shown by the reactivation of faults originated during earlier phases and affecting the Lower Pleistocene sediments.This tectonic stage can be better observed to the north, along the eastern side of Mount Etna, where Quaternary lava flows are cut by faults parallel to the Hyblean-Maltese Escarpment.