Unità a limiti inconformi utilizzate per la cartografia dei depositi quaternari nei fogli CARG della Sicilia nord-occidentale

Sono riportati i risultati di uno studio sui depositi quaternari del settore settentrionale siciliano compreso tra Trapani e Cefalù. Dettagliati rilevamenti di campagna e analisi stratigrafiche condotti in questa area nell’ambito del Progetto CARG hanno consentito di definire diverse unità a limiti inconformi, del rango di sintema e subsintema, che costituiscono la sintesi regionale degli affioramenti quaternari della Sicilia nord-occidentale. Sono state riconosciute e studiate numerose successioni di terreni di ambiente da continentale a marino costituite da: depositi fluviali, paralici e marini (conglomerati monogenici o poligenici, areniti e/o peliti sciolte o variamente cementate), accumuli eolici (sabbie e arenarie quarzose o carbonatiche, a stratificazione e laminazione incrociata), terreni colluviali (suoli e detriti rimaneggiati con allineamenti di ciottoli), depositi di falda (detriti sciolti o stratificati e cementati), concrezioni calcaree (travertini, croste calcitiche e depositi di grotta) e macereti di frana. I depositi continentali, tipici di ambienti dominati da erosione, affiorano in piccoli e sottili lembi relitti, spesso discontinui e incompleti; i depositi marini e paralici, caratteristici di ambienti a prevalente sedimentazione, si rinvengono in bacini locali e costituiscono affioramenti più estesi e potenti. Il rinvenimento di superfici di discontinuità di importanza regionale che limitano inferiormente e superiormente i corpi rocciosi, ha consentito la proposizione di sette distinti sintemi che ricoprono un periodo di tempo che va dal Pleistocene inferiore all’Olocene. Le discontinuità, marcate a volte da paleosuoli, sono rappresentate da superfici di erosione subacquea (piattaforme di abrasione marina e falesie) o subaerea (superfici, ripe, canali, solchi e cavità prodotti da erosione idrica, processi gravitativi o fenomeni carsici), da superfici di non deposizione o, nel caso del limite superiore, dall’attuale superficie topografica. Le datazioni e le correlazioni di depositi e limiti inconformi sono state rese possibile da alcuni elementi vincolanti quali: a) presenza di forme fossili significative; b) datazioni isotopiche di depositi desunte dalla letteratura geologica; c) continuità fisica di alcune superfici di discontinuità; d) significato paleoambientale o paleoclimatico di depositi e superfici di discontinuità; e) rapporti geometrici fra superfici di discontinuità e/o corpi rocciosi.ABSTRACT: Di Maggio C. et al., Unconformity-bounded stratigraphic units of Quaternary deposits mapped for the CARG Project in Northern and Western Sicily. (IT ISSN 0394-3356, 2009). Quaternary deposits of NW Sicily mapped for the CARG Project (Geological Survey of Italy) during detailed geologic and geomorphologic surveys are made up of: a) fluvial, coastal or marine deposits (mono or polygenic conglomerates, arenites or lutites variously cemented, with plane, inclined or cross-bedded stratification/lamination); b) aeolian deposits (quartzous or carbonate sands/sandstones, with cross-bedded stratification/lamination); c) colluvial terrains (reworked soils and breccias variously cemented, in some case laminated, with stone lines); d) scree slopes (breccias or stratified slope deposits – éboulis ordonné); e) carbonate speleothems (travertines, calcretes and cave deposits); e) landslide debris. Lower and upper boundaries of the Quaternary rocks consist of unconformity surfaces (marked by palaeosols at places), such as: wave cut platforms and cliffs; gullies and rills; erosion surfaces due to sheet erosion or sheet flood; stream channels, fluvial erosion scarps and fluvial erosion/depositional surfaces; separation or depletion surfaces due to mass movement; non depositional surfaces; topographic surfaces. The detection of some unconformity surfaces of regional extent allowed us to define seven main synthems; they have been mapped in several geologic 1:50.000 sheet maps. The Marsala synthem is a Lower Pleistocene 2-80 m thick body of marine/ coastal conglomerates, arenites and lutites, with abundant corals, algae, bryozoa, mollusca (Arctica islandica,Chlamys septemradiata), benthic foraminifera and planktonic foraminifera (Hyalinea baltica, Globorotalia truncatulinoides excelsa); its lower boundary is a marine abrasion surface cut into pre-Quaternary rocks. The Piana di Partinico synthem is made up of 1-5 m thick marine/coastal conglomerates and arenites located on a number of marine terrace surfaces related to sea high stand phases of Middle Pleistocene (Oxygen Isotope Stages – OISs - 17-7); these rocks appear correlatable with 1-3 m thick continental deposits (colluvial terrains and travertines) rich in fossil vertebrates belonging to the early Middle Pleistocene Elephas falconeri Sicilian Faunal Complex; its lower boundary is a wave cut platform carved into Marsala synthem or pre- Quaternary rocks. The Polisano synthem is 1-10 m thick aeolian sandstones and sands with intercalations of breccias talus; the body deposited in coastal, perched or barrier dune, contains rare terrestrial gastropods shells, late Middle Pleistocene in age (OIS 6); its lower boundary is a non-depositional surface at the top of the Piana di Partinico synthem or older rocks. The Barcarello synthem encompasses 1-2 m thick marine/coastal conglomerates and arenites with a rich warm-temperate “Senegalese fauna” including Strombus bubonius; they belong to two orders of marine terrace (OISs 5e and 5c or 5a) and laterally pass into 1-5 m thick, welded colluvial deposits; the latter are composed of both lutitic terrains (reworked soil) and breccias with angular rock fragments (reworked scree and sediment) in a abundant silt or fine-grained clay matrix (reworked soil) containing terrestrial mollusc shells and fossil vertebrates of the Upper Pleistocene Elephas mnaidriensis Sicilian Faunal Complex; their age is correlated with the OIS 5; the lower boundary of synthem is a marine abrasion surface laterally extending to a continental erosion surface; the latter is cutting the Polisano synthem or older rocks. The Imera Settentrionale synthem is made up of Middle – Upper Pleistocene 1-3 m thick fluvial deposits located on river terrace surfaces lying between about 5 and 100 meters above the valley bottom; its lower boundary is a stream erosion surface. The Raffo Rosso synthem is composed of 1-2 m thick coastal dune aeolian sandstones and sands, 1-7 m thick colluvial or gravitational deposits with fossil mammals of the late Upper Pleistocene Pianetti and Castello Sicilian Faunal Complex, 2-10 m thick stratified slope deposits of the last glacial climatic event (OISs 4-2); the lower boundary is a non-depositional surfaces at the top of the Barcarello/Imera Settentrionale synthems or older rocks. The Capo Plaia synthem is made up of coastal to continental deposits of the last glacial climatic event end – Holocene age (OISs 2-1); its lower boundary is formed by variously originated erosion or non-depositional surfaces; the upper boundary is the present day topographic surface. Significant environmental changes during the Quaternary, directly or indirectly due to tectonic movements (block faulting or uplifting) and/or climatic fluctuations in their turn triggering erosion / depositional processes, appear to originate both unconformity surfaces of regional extent and deposits