Angela Bertinelli

Magnetostratigraphy, biostratigraphy, and chemostratigraphy of the Pignola-Abriola section: New constraints for the Norian-Rhaetian boundary

A detailed magnetostratigraphic investigation of the Pignola-Abriola section of Norian to Rhaetian age permits the identification of 22 magnetic polarity reversals grouped in 10 magnetozones. We correlate the magnetostratigraphy of the Pignola-Abriola section with the Newark astrochronological polarity time scale (APTS). In total, 19 correlation options were tested, and only one (option 7) yielded a statistically significant correlation that was consistent with the available information on the stratigraphic age of the Newark APTS. After some adjustments to minimize erratic variations in sediment accumulation rates, a final correlation (option 7.1) was used to generate an age model of sedimentation for the Pignola-Abriola section. The Pignola-Abriola section has been correlated with Rhaetian sections from the literature, notably the current global boundary stratotype section and point candidate for the base of the Rhaetian at Steinbergkogel, Austria, where the Norian-Rhaetian boundary is proposed to be placed at a stratigraphic level containing the first appearance datum (FAD) of conodont Misikella posthernsteini , traced on the Newark APTS to ca. 209–210 Ma. Issues regarding the taxonomy of M. posthernsteini , a species characterized by transitional forms with its ancestor Misikella hernsteini , lead us to propose the alternative option of placing the Norian-Rhaetian boundary at a prominent negative δ 13 C org spike observed in the Pignola-Abriola section at meter 44.5, 50 cm below the level containing the FAD of M. posthernsteini sensu stricto and close to the base of radiolarian Proparvicingula moniliformis zone. This level has been magnetostratigraphically correlated to Newark magnetozone E20r.2r at ca. 205.7 Ma. Assuming an age of ca. 201.3 Ma for the Triassic-Jurassic boundary, the Rhaetian Stage would have a duration of ~4.4 m.y.

HETTANGIAN AMMONITES AND RADIOLARIANS IN THE MT. CAMICIA (GRAN SASSO, CENTRAL APENNINES)

The Vallone di Vradda stratigraphic section is situated in the eastern part of the Gran Sasso range and shows the transition from Late Triassic euxinic facies to Early Liassic open pelagic facies. This paper describes Middle and Late Hettangian ammonite and radiolarian assemblages found in the upper part of the succession. The existence of an anoxic event below the Middle Hettangian beds is noted. Assemblages of small-sized Middle Hettangian ammonites suggest some taxonomic innovation within the early Lytoceratina: gen. n. (Pleuroacanthitidae) and Analytoceras n. sp. indet. Radiolarians found together with ammonites improve the knowledge of Hettangian radiolarian assemblages of the western Tethys.

The Norian-Rhaetian interval in two sections of the Lagonegro area: the transition from carbonate to siliceous deposition

New biostratigraphic data from two stratigraphic sections (Mt. St. Enoc and Mt. Volturino), of the Lagonegro Basin (Southern Apennines, Italy), define the age of the sedimentary events and delineate the sedimentary evolution for the Norian-Rhaetian time interval.The upper part of the Calcari con Selce Fm in the Mt. St. Enoc section is characterized by an alternation of cherty limestones rich in organic matter and thin silicified calcarenites with black cherty layers. The upper part of the Calcari con Selce Fm of the Mt. Volturino section exhibits instead intermediate characters between the Calcari con Selce and Scisti Silicei Fms, with cherty limestones, shales and radiolaritic beds. The biostratigraphic data based on conodonts and radiolarians allowed a good correlation among the Norian-Rhaetian successions throughout the Lagonegro Basin the two sections represent two different environments of the Lagonegro Basin. In fact, radiolarites (Scisti Silicei Fm) are present from the Misikella hernsteini-Parvigondolella andrusovi conodont Zone (late Sevatian) in the Mt. Volturino section while they appeared after the Misikella ultima conodont Zone (uppermost Rhaetian) in the Mt. St. Enoc section. This proves that euxinic carbonate facies and radiolaritic facies were deposited in the same time interval and in the same basin. On the basis of sedimentological analysis and with the support of biostratigraphy, the non-coeval beginning of biosiliceous sedimentation is discussed in order to point out the control factors: different paleobatimetry, upwelling areas or volcanic activity. Our preliminary results show that the Lagonegro Basin during the Norian-Rhaetian interval was probably characterized by an articulated physiography and that volcanism should have had an important role in changing pelagic sedimentation.

MIDDLE AND LATE HETTANGIAN RADIOLARIANS FROM THE MT. CAMICIA SUCCESSION (GRAN SASSO, CENTRAL APENNINES - ITALY)

Well-preserved middle and late Hettangian radiolarians have been discovered S-E of Mt. Camicia (Central Apennines) in limestone beds that contain also Hettangian ammonites. These beds are part of an Upper Triassic to Lower Jurassic carbonate succession, which crops out in the eastern part of the Gran Sasso range. This succession includes euxinic deposits (bituminous dolostone) and other pelagic sediments (mudstone and calcarenites) which developed in a basin rimmed by a carbonate platform. In the same time interval, the well-known formations of Dolomia Principale and Calcare Massiccio were forming on this platform. In this paper we examine the radiolarian assemblages collected in three levels of the ammonite bearing beds, and we define two new genera ( Squinabolia and Turritus ) and eight new radiolarian species ( Anaticapitula parvireticulata , Anaticapitula triangularis, Farcus aquilensis, Farcus leonseveroi , Parahsuum vraddense , Squinabolia multispinata, Turritus venturii, Zhamoidellum sphaericus ), and we define twenty four other species, for a total of thirty eight identified species. The ammonites in the lowest level indicate a middle Hettangian age, those from the two upper levels a late Hettangian. In the Western Tethys successions bearing Hettangian radiolarians are rare and have been found together with ammonites only in the Mt. Camicia section described in this paper. This section permits to correlate the radiolarian assemblages with the ammonite zonation.