Radiolarians from the cyclic Messinian diatomites of Falconara (Sicily, Italy)


en Geodiversitas 21 (4) - Pages 593-624

Published on 24 December 1999

The diatomitic sediments in Falconara (6.93 to 6.08 Ma) have a total thickness of c. 27 m, and the sequence is composed of 41 diatomite/claystone couplets. The deposition of these biosiliceous sediments, seemingly modulated by the astronomical precession cycle (21,000 years period), has been suggested to have taken place in a shallow basin with diatomaceous and claystone sediments being deposited during low and high sea level stands, respectively. Polycystine radiolarians show major changes in the assemblage compositions and the total abundances between the different cycles, from being barren in Cycle 2 till > 142,000 radiolarians/g in Cycle 26. Radiolarians are usually not present in the claystone samples, this is also the case with diatoms. However, one claystone (S11) had a significant high number, > 43,000 radiolarians/g sediment, while nine others had a low content of radiolarians. We have in our counts recognised 68 morphotypes. Q-Mode Factor analysis has been used on the raw counting data. By using five factors we were able to explain 83.65% of the cumulative variance, while using nine factors allowed us to explain 96.52% of the variance. These nine factors displayed well-defined peaks that can be used to navigate within the profile and to correlate between sections. Another high resolution stratigraphical tool could be represented by the peculiar faunal composition of each of the diatomites. In our pilot study we selected three diatomites and compared their assemblage composition and radiolarian abundances. Cycle 11 is dominated by Stichocorys delmontensis and Lithomitra lineata, c. 50 and 36% maximum values respectively. In Cycle 21 S. delmontensis and Botryostrobus auritus/australis are most common, with 40 and 30% as maximum values respectively. In both these cycles nassellarians are the dominant group. In Cycle 26 Larcoidea sp.,Porodiscus sp., and Spongotrochus glacialis are the dominant taxa.

Keywords :

Radiolaria, Miocene, Italy, ciclicity