Integrated biostratigraphic and palaeoenvironmental analyses of foraminiferal, ostracod and calcareous nannofossil assemblages are here presented for the late Messinian/early Zanclean succession of the Narzole borehole (Albese area, Piedmont, Northwestern Italy). The Narzole core is made out of about 20 m of marine sediments and is stored in the collections of the Torino University, deposited at the Torino Regional Museum of Natural Sciences and until now represents the only documentation of the Messinian/Zanclean boundary (M/Z) in the Albese subsurface. The uppermost Messinian "Lago-Mare" deposits yield reworked marine microfossils and a brackish ostracod assemblage representative of the Loxocorniculina djafarovi Zone, indicating their deposition in oligo-mesohaline shallow waters and the influx of Paratethyan faunas. The boundary between the post-evaporitic Messinian sediments and the overlying early Zanclean Argille Azzurre Fm. (AA) is marked by a 0.50 m thick barren arenitic layer, dark brown to black in its middle part, well correlatable to the black level recognized at the M/Z boundary in the nearby Moncucco quarry outcrop (Torino Hill). The Early Zanclean succession yields abundant microfossils, which document the MPl1 (Sphaeroidinellopsis acme) and the MPl2 foraminiferal zones, and the MNN12 calcareous nannofossil zone. Many bioevents recognized at Moncucco and at the Mediterranean scale are recorded in the studied succession: one sinistral coiling shift of Neogloboquadrina acostaensis, the Globorotalia scitula common occurrence (CO), the re-immigration of Siphonina reticulata, the Globorotalia margaritae first common occurrence (FCO), the first influx of North Atlantic Deep Water (NADW) forms as Cibicidoides robertsonianus. Henryhowella asperrima and Oblitacythereis mediterranea firstly occurred in the MPl1 zone, like in the Moncucco section, while in southern Italy and Mediterranean Pliocene sections they occurred at the base of MPl 2 biozone. Foraminiferal and ostracod assemblages document an epibathyal basin just from the early Zanclean. Fluctuations in water temperature and productivity are suggested by quantitative changes of warm water, oligotrophic surficial taxa (Globigerinoides) versus intermediate water, eutrophic and phytoplankton grazers (N. acostaensis). A deepening of the basin during the MPl1 is suggested by the increasing diversity of benthic foraminifers and of deep bathyal ostracods. All palaeobiological data suggest open marine circulation patterns in this sector of the Northwestern Italy during the MPl1-MPl2 Early Pliocene zones. This palaeoenvironmental interpretation is corroborated by the very high P/(P+B) ratio, the presence of mesopelagic planktonic foraminifers as Sphaeroidinellopsis spp., common to frequent deep cosmopolitan benthic foraminifers (Cibicidoides pseudoungerianus, Sphaeroidina bulloides, Uvigerina peregrina etc.), common bathyal ostracods (Argilloecia acuminata, A. kissamovensis, O. mediterranea, Paijenborchella iocosa, P. dimorpha, Xestoleberis prognata etc.), the rare discoasterids and ceratholiths (Ceratholithus acutus, Amaurolithus primus, A. delicatus), typical tropical open marine taxa, usually very rare in the Mediterranean Zanclean.
Il presente lavoro espone i risultati dello studio micropaleontologico integrato (foraminiferi, ostracodi e nannofossili calcarei) condotto sui medesimi campioni della carota prelevata presso Narzole (Alba, Piemonte, Italia Nord-occidentale) nel 1975, nella quale Sturani ha descritto e illustrato il livello arenitico nero al limite Messiniano/Zancleano e in cui sono conservati circa 20 m di sedimenti marini. Il materiale di Narzole è catalogato nelle collezioni dell'Università di Torino, depositate presso il Museo Regionale di Scienze Naturali di Torino e rappresenta tuttora l'unica documentazione del limite Messiniano/Zancleano (M/Z) nel sottosuolo dell'Albese. I sedimenti messiniani della facies di "Lago-Mare" contengono microfossili marini rimaneggiati e ostracodi salmastri indicativi della zona a Loxocorniculina djafarovi, che documentano condizioni di acque basse oligo-mesoaline e l'influsso di faune della Paratetide. Il limite tra i depositi post-evaporitici messiniani e le sovrastanti Argille Azzurre (AA) dello Zancleano inferiore è evidenziato da un livello arenitico da scuro a nero di 0.50 m di spessore, simile a quello presente al limite M/Z a Moncucco. Le associazioni a microfossili delle AA documentano le zone MPl1 (Sphaeroidinellopsis acme) e MPl2 a foraminiferi planctonici e la zona MNN12 a nannofossili calcarei. Molti bioeventi noti in area Mediterranea e segnalati a Moncucco sono stati riconosciuti anche nella successione di Narzole: un cambiamento della direzione di avvolgimento di Neogloboquadrina acostaensis, i successivi influssi/re-immigrazione di Globorotalia scitula e di Siphonina reticulata, la First Common Occurrence (FCO) di Globorotalia margaritae, i primi influssi di forme profonde, tipiche delle "North Atlantic Deep Water" (NADW), quali Cibicidoides robertsonianus. Henryhowella asperrima e Oblitacythereis mediterranea compaiono per la prima volta durante la zona MPl1, come a Moncucco, mentre in Italia meridionale e in Mediterraneo la loro comparsa è segnalata alla base della zona MPl2. I dati micropaleontologici documentano fondali epibatiali già a partire dalla base dello Zancleano. Fluttuazioni climatiche e della produttività sono suggerite dalle variazioni quantitative dei taxa planctonici di acque superficiali, calde e oligotrofiche (Globigerinoides) e delle forme di acque intermedie eutrofiche (N. acostaensis). L'aumento della diversità specifica dei foraminiferi bentonici e degli ostracodi suggerisce un ulteriore approfondimento del bacino e buone comunicazioni con il Mediterraneo durante le zone MPl1 e MPl2. Questa interpretazione è basata sull'elevato rapporto P/(P+B), sulla presenza di foraminiferi planctonici mesopelagici (Sphaeroidinellopsis spp.), sull'associazione a foraminiferi bentonici formata da taxa profondi cosmopoliti (Cibicidoides pseudoungerianus, Sphaeroidina bulloides, Uvigerina peregrina ecc.), sui comuni ostracodi batiali (Argilloecia acuminata, A. kissamovensis, O. mediterranea, Paijenborchella iocosa, Xestoleberis prognata ecc.) e sulla presenza di rari discoasteridi e ceratolitidi (Ceratholithus acutus, Amaurolithus primus, A. delicatus), tipiche forme tropicali di mare aperto, generalmente molto rari nello Zancleano dell'area Mediterranea.
Micropalaeontological analyses of the Narzole core: biostratigraphy and palaeoenvironment of the late Messinian and early Zanclean of Piedmont (Northwestern Italy)
Violanti D;
2009
Abstract
Integrated biostratigraphic and palaeoenvironmental analyses of foraminiferal, ostracod and calcareous nannofossil assemblages are here presented for the late Messinian/early Zanclean succession of the Narzole borehole (Albese area, Piedmont, Northwestern Italy). The Narzole core is made out of about 20 m of marine sediments and is stored in the collections of the Torino University, deposited at the Torino Regional Museum of Natural Sciences and until now represents the only documentation of the Messinian/Zanclean boundary (M/Z) in the Albese subsurface. The uppermost Messinian "Lago-Mare" deposits yield reworked marine microfossils and a brackish ostracod assemblage representative of the Loxocorniculina djafarovi Zone, indicating their deposition in oligo-mesohaline shallow waters and the influx of Paratethyan faunas. The boundary between the post-evaporitic Messinian sediments and the overlying early Zanclean Argille Azzurre Fm. (AA) is marked by a 0.50 m thick barren arenitic layer, dark brown to black in its middle part, well correlatable to the black level recognized at the M/Z boundary in the nearby Moncucco quarry outcrop (Torino Hill). The Early Zanclean succession yields abundant microfossils, which document the MPl1 (Sphaeroidinellopsis acme) and the MPl2 foraminiferal zones, and the MNN12 calcareous nannofossil zone. Many bioevents recognized at Moncucco and at the Mediterranean scale are recorded in the studied succession: one sinistral coiling shift of Neogloboquadrina acostaensis, the Globorotalia scitula common occurrence (CO), the re-immigration of Siphonina reticulata, the Globorotalia margaritae first common occurrence (FCO), the first influx of North Atlantic Deep Water (NADW) forms as Cibicidoides robertsonianus. Henryhowella asperrima and Oblitacythereis mediterranea firstly occurred in the MPl1 zone, like in the Moncucco section, while in southern Italy and Mediterranean Pliocene sections they occurred at the base of MPl 2 biozone. Foraminiferal and ostracod assemblages document an epibathyal basin just from the early Zanclean. Fluctuations in water temperature and productivity are suggested by quantitative changes of warm water, oligotrophic surficial taxa (Globigerinoides) versus intermediate water, eutrophic and phytoplankton grazers (N. acostaensis). A deepening of the basin during the MPl1 is suggested by the increasing diversity of benthic foraminifers and of deep bathyal ostracods. All palaeobiological data suggest open marine circulation patterns in this sector of the Northwestern Italy during the MPl1-MPl2 Early Pliocene zones. This palaeoenvironmental interpretation is corroborated by the very high P/(P+B) ratio, the presence of mesopelagic planktonic foraminifers as Sphaeroidinellopsis spp., common to frequent deep cosmopolitan benthic foraminifers (Cibicidoides pseudoungerianus, Sphaeroidina bulloides, Uvigerina peregrina etc.), common bathyal ostracods (Argilloecia acuminata, A. kissamovensis, O. mediterranea, Paijenborchella iocosa, P. dimorpha, Xestoleberis prognata etc.), the rare discoasterids and ceratholiths (Ceratholithus acutus, Amaurolithus primus, A. delicatus), typical tropical open marine taxa, usually very rare in the Mediterranean Zanclean.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
