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Detailed lithostratigraphy and radiolarian occurrences around the Jurassic-Cretaceous boundary in the Bosso Valley section, Central Italy
Angela Bertinelli, Xin Li, Marco Chiari, Atsushi Matsuoka

Building: Muséum d'Histoire Naturelle de Genève
Room: Lobby /Hall/
Date: 2018-12-05 10:53 AM – 10:56 AM
Last modified: 2018-12-02

Abstract


The Bosso Valley section is located in the Umbria-Marche area of the Northern Apennines (Central Italy). The stratigraphic succession which outcrops in this area is made of several lithological units, spanning from late Triassic to Miocene, and is referred mainly to pelagic environments (except for the late Triassic-earliest Jurassic units, related to a shallow water environment). Since the Jurassic, the Umbria-Marche area was characterized mainly by carbonatic sediments, with scarce terrigenous influx, deposited in a basin with topographic differences of the sea floor, due to tectonic activity. Different Jurassic stratigraphic sequences were deposited, and from the late Jurassic (Tithonian) the basinal deposits remain uniform throughout the Northern Apennines. The Maiolica Formation (Tithonian to Barremian-early Aptian) is the first pelagic and quite uniform unit, consisting of white to light gray well bedded limestones, with gray to black thin shale interbeds. Light gray to black nodules and layers of diagenetic chert are abundant. The Maiolica Formation has an average thickness of about 300 meters in the Umbria-Marche area, nevertheless there are considerable variations in thickness and facies into the basinal Maiolica sequences (Alvarez,1989; Cresta et al., 1989). The bioclastic content is composed mainly of calpionellids, calcareous nannofossils, radiolarians, siliceous sponge spicules and rare ammonites. The stratigraphic transition of the Maiolica Formation with the underlying Calcari a Saccocoma ed Aptici (if present) or Calcari Diasprigni formations coincides with a major change in the pelagic environment during the late Tithonian, that caused a synchronous change in sedimentation from siliceous, or clay-rich, to calcareous, confirmed by a drastic radiolarian faunal change (Baumgartner, 1984). The Jurassic-Cretaceous boundary (JKB) is recorded in the lower part of the Maiolica Formation. Radiolarians are good candidates for defining the JKB because they are widespread and can be found both shallow and deep sedimentary facies. Evolutionary lineages of several radiolarian taxa across the JKB are reviewed and discussed by Matsuoka et al. (2018).

The Bosso Valley section outcrops along the Cagli-Pianello road, following the Bosso River. This section is a potential candidate for the GGSP for the base of the Berrisian Stage (JKB) and it was studied for calpionellid biostratigraphy and magnetostratirahy by Housa et al. (2004). The marked increase in abundance of Calpionella alpina at the base of the Calpionella alpina Subzone was accepted as the JKB primary marker in the Bosso Valley section.

In this section the Maiolica Formation yields interesting radiolarian assemblages, previously studied by Kocher (1981), Baumgartner (1984) and Jud (1994), as reported in Chiari et al. (2018). Radiolarians from the Maiolica Formation are often poorly preserved, calcified in the limestone and recrystallized in the chert, but very recent detailed field observations and careful sampling revealed well-preserved radiolarians inside the limestone around the cherty layers or nodules. Three samples below and two samples above the JKB of the Bosso Valley section (according Housa et al., 2004) were processed and new radiolarian biostratigraphic studies were carried out by Li et al. (2018).