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Tesi Dottorato Università degli Studi di Napoli Federico II Facoltà di Scienze MM. FF. NN. Dipartimento di Scienze della Terra Dottorato in Scienze della Terra – XIX Ciclo Studi geo-paleontologici su livelli ricchi in brachiopodi del Lias superiore del Gran Sasso d’Italia Relatore Dottorando Ch.ma Prof.ssa Dott. Buono Giuseppe Emma Ruggiero Taddei ANNO ACCADEMICO 2006 -2007 A tutti coloro che hanno riempito di emozioni la mia vita, In primis, i miei genitori Antonietta e Rosario e i miei fratelli Salvatore e Tiziana. Introduzione Le successioni carbonatiche fanerozoiche depositatesi in seguito cambiamenti notevoli e generalmente veloci delle condizioni sedimentarie, paleoceanografiche e paleoecologiche sono recentemente oggetto di un considerevole e accurato esame (per esempio: Weissert et al., 1998; Jenkyns et al., 2002, fra molti altri). Prove indipendenti risultanti da studi sedimentologici, paleontologici e geochimici suggeriscono possibili correlazioni tra: a) orizzonti a fosforiti e black shales sincroni; b) bioeventi significativi che interessano la differenziazione e la riduzione di diversità delle associazioni fossili; c) importanti escursioni delle curve isotopiche dell'ossigeno e del carbonio, rappresentanti eventi geologici coevi con imponenti cambiamenti delle condizioni paleoceanografiche globali. (vedi ad esempio: Arthur et al., 1990; Hallam & Wignall, 1999, fra molti altri). Le successioni giurassiche contengono alcuni dei più importanti fra gli eventi geologici di cui sopra (per esempio: Jones & Jenkyns, 2001; Dromart et al., 2003) che sembrano concentrarsi nel Liassico superiore. Durante l’intervallo Pliensbachiano - Toarciano inferiore è possibile osservarne gli sviluppi stratigrafici improvvisi e quasi coevi nelle successioni carbonatiche sia di acque poco profonde che di acque profonde nell’area Tetidea (ad esempio: Jenkyns & Clayton, 1986; Morettini et al., 2002). Le unconformities da annegamento ampiamente diffuse tra il Carixiano medio e il Toarciano iniziale di vari domini peritidali di tetidei (per esempio: Santantonio, 1993; Zempolich, 1993) e la sostituzione del fango carbonatico con marne e argille contenenti livelli a black shales all'interno delle successioni bacinali del Toarciano inferiore (per esempio: Jenkyns, 1988; Jiménez et al., 1996) documentano che la produttività di carbonato nelle acque oceaniche diminuì drammaticamente durante il Liassico superiore. Ciò sembra riflettere significative perturbazioni chimiche e fisiche del sistema idrosfera- atmosfera ad una scala globale (per esempio: Jenkyns et al., 2002). Per quanto riguarda il Toarciano, indagini stratigrafiche integrate suggeriscono che tali perturbazioni sono associate a: 3 a) elevati tassi di carbonio organico sepolto legati ad uno dei principali eventi anossici oceanici (Oceanic Anoxic Event - OAE); b) significativa estinzione di massa associata a differenziazione o riduzione della diversità biotica; c) alte paleotemperature legate ad un episodio di effetto serra (greenhouse); d) trasgressione del livello del mare relativo; (Jenkyns, 1988; Little & Benton, 1995; Hallam & Wignall, 1999; Harries & Little, 1999; Hesselbo et al., 2000; Jones & Jenkyns, 2001; Beerling et al., 2002; Hart et al., 2003). Le perturbazioni ambientali del Toarciano iniziale ebbero i loro effetti anche sui Brachiopodi, infatti, i taxa SPIRIFERINIDA, KONINCKINIDINA e WELLERELLOIDEA si estinsero, mentre alcune famiglie appartenenti alle HEMITHIRIDOIDEA furono interessate da episodi radiativi (Ager, 1987; Williams ed altri. 2002; Manceñido & Owen, 2002; Vörös, 2002). In questo contesto si è deciso di intraprendere uno studio geo-paleontologico su alcuni livelli ricchi in brachiopodi e sulle faune ad essi associati. Le associazioni a brachiopodi studiate provengono dall’area nord-orientale del Massiccio del Gran Sasso d’Italia, ed erano già state segnalate in letteratura (vedi ad es. Cassetti, 1910; Parona, 1908; Morettini, 1950) ma non erano mai state oggetto di uno studio sistematico. Tra i brachiopodi descritti è documentata la presenza della Soaresirhynchia bouchardi (RHYNCHONELLIDAE) che è gia stata oggetto di una pubblicazione (Graziano et al., 2006) nella quale è stata segnalata per la prima volta in Italia. La presenza di tale specie ha consentito di effettuare studi paleontologici multidisciplinari, infatti: a) l’ampia variabilità intraspecifica ha consentito uno studio di carattere statistico; b) l’ampia diffusione areale (Alméras, 1994) ha favorito considerazioni di carattere biostratigrafico e di ricostruire un modello evolutivo e paleobiogeografico del genere Soaresirhynchia; c) il fatto che sia stata indicata come disaster taxon nelle successioni della penisola Iberica (Garcia Joral e Goy, 2000; Fürsich et al., 2001; Ghar, 2005) ha fornito interessanti spunti per l’analisi paleoecologica. Inoltre viene discussa la possibilità di collegare i livelli ricchi in brachiopodi al OAE del Toarciano iniziale e il significato paleoambientale dell'associazione fossile relativa. 4 Capitolo 2 – Precedenti conoscenze In questo capitolo si intende fornire un quadro generale delle precedenti conoscenze su alcuni argomenti che costituiscono il background di tale lavoro di tesi per quanto riguarda gli studi biostratigrafici, paleoecologici e paleobiogeografici. Tale capitolo presenta la seguente strutturazione: 1. Introduzione alla biostratigrafia del Giurassico inferiore 2. Biostratigrafia a Brachiopodi del Lias • Biozonazioni • Correlazioni con altri taxa • Area umbro-marchigiana 3. Paleobiogeografia dei Brachiopodi nel Lias • Paleobioprovince a brachiopodi nel Lias • Paleobiogeografia a brachiopodi dell’area tetidea 4. Early Toarcian Extinction e Oceanic anoxic event 5 2.1 - Introduzione alla biostratigrafia del Giurassico inferiore A grandi linee, la distribuzione degli organismi nel Giurassico inferiore è fortemente influenzata da eventi di estinzione di massa. All’inizio del Giurassico inferiore bassa si registra una fioritura di diversi gruppi che dopo la grande estinzione Permo-Triassica, e un periodo di recovery nel Trias, sono affetti da evidenti radiazioni areali e adattative (Serpagli & Raffi, 1996). Al termine del Lias, si assiste invece ad una nuova fase di regressione nella diversità del biota marino, in connessione all’evento di estinzione del Toarciano inferiore ed alle variazioni paleoambientali che hanno portato a tale evento (vedi paragrafo 2.4), prima di nuove radiazioni evolutive nel Toarciano inferiore e medio. In ogni caso negli ultimi anni il moltiplicarsi degli studi a carattere biostratigrafico, e la possibilità di studi integrati e l’affinamento delle tecniche di ricerca e di analisi (si pensi ad esempio all’uso della statistica) ha consentito un notevole passo in avanti sia nella scelta dei taxa da utilizzare, sia soprattutto nella possibilità di costruire scale biostratigrafiche valide per aree più estese e/o con più taxa. A scala globale sono numerosi i taxa impiegati per la costruzione di scale biostratigrafiche valie per il Lias. Ad esempio, in una recente rivisitazione della biostratigrafia dell’area inglese, Simms et al. (2002) riportano schemi biostratigrafici per i seguenti taxa: Ammoniti, Belemniti, Brachiopodi, Crinoidi, Foraminiferi, Ostracodi, Dinoflagellati, Miospore, Nannofossili calcarei, segnalando inoltre faune con valore biostratigrafico di Bivalvi, Gasteropodi, Scafopodi, Oloturie, Coralli. Ma anche diversi altri taxa sono stati usati per zonazioni biostratigrafiche, tra quelli non citati in precedenza di grande importanza sono i Radiolari. Inoltre notevoli progressi sono stati fatti anche con i taxa continentali, a esempio possono essere citati i tentativi di biozonazione con piante (Wang et al. 2005) e tracce di vertebrati (Thanner et al., 2006). Come indicato in precedenza la volontà di correlare biozonazioni di aree diverse e quelle tra taxa diversi ha costituito una spinta verso la modifica delle prime scale biostratigrafiche con l’inserimento di specie più cosmopolite. Ciò ha ulteriormente favorito anche gli studi paleobiogeografici. Non a caso si è assistito negli ultimi anni alla pubblicazione di dettagliati studi paleobiogeografici a scala globale per numerosi taxa, sempre limitandoci al Lias vanno citati come esempio gli studi condotti sui Bivalvi, 6 Brachiopodi, Ostracodi, Ammoniti (Aberhan 2002, Arias, 2006, Damborena 1996, 2002; Mancenido, 2002; tra molti altri). Comunque, in generale, i taxa che offrono una maggiore risoluzione e una maggior possibilità di utilizzo per la biostratigrafia del Lias rimangono le Ammoniti tra i “macrofossili” e Foraminiferi e Nannofossili calcarei tra i “microfossili”. In questo scenario però anche i brachiopodi già da diversi anni sono usati come fossili guida, grazie alla possibilità di una buona correlazione con le ammoniti alle quali sono spesso associati, e grazie anche alla radiazione evolutiva (Manceñido & Owen 2001), adattativa (Ager 1965, Dulai 2001, Vörös 1986, 2000) e areale, che li interessa nel Lias. Basti pensare ad esempio al record pressoché continuo riscontrato nella penisola Iberica (Goy et al., 1984) e dell’Ungheria (Bakony Mt.; Vörös 1970, 1983, 1993)). 7 2.2 - Biostratigrafia a Brachiopodi del Lias 2.2.1 - Biozonazioni Come indicato in precedenza i brachiopodi sono tra i taxa più diffusi nei mari liassici; ciò ha consentito la costruzione di numerose scale biostratigrafiche basate su tale taxon. Tra tali lavori vanno citati ad esempio quelli condotti per le seguenti aree (vedi tab. 2.1): • Gran Bretagna (Ager,1956; Simms et al 2002) • Francia (Almèras et al.,
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