DATAZIONE DI UN LIVELLO CINERITICO DEL PLEISTOCENE MEDIO: RELAZIONI CON SEDIMENTAZIONE, SOLLEVAMENTO E TERRAZZI MARINI NELL’AREA CROSIA-CALOPEZZATI IN CALABRIA (ITALIA)

Abstract

Bigazzi G. & Carobene L., Fission-track dating of Middle Pleistocene ash layer: relationships to sedimentation, tectonic
uplift and marine terraces in the Crosia-Calopezzati area (Calabria, Italy). (IT ISSN 0394-3356, 2004).
In the Crosia-Calopezzati area (Ionian coast of Calabria), east of the Trionto River, crops out a Pleistocene marine succession which is
transgressive over Miocene – Pliocene (?) substratum (Fig. 1; Fig. 2). These sediments deposited during a lowering phase of the Ionian
margin, which caused the formation of important sedimentary basins in Calabria and Basilicata. A significant stage of knowledge of the geology of these regions during lower and middle Pleistocene times had been already attained through studies that were performed on these basins.
The uplift of the study area determined the formation of four orders of marine terraces (Fig. 5) already described in a previous article
(Carobene, 2003). Discovery and analysis of an ash layer interstratified with the Pleistocene clayey substratum (Fig. 3) provided the
opportunity of inferring some considerations on the sedimentation rate and, consequently, on the onset of the sedimentation of the
transgressive body.
It was also possible to deduce some constraints on the beginning of its emersion as well as on the age of the marine terraces and on
the uplift rate of the study area. The main results of this work can be summarized as follows (Fig. 9):
1) Age determination of the pyroclastic layer. The ash layer, referred to as “Calopezzati ash”, looks poorly cemented, porous and white
colored. Dating of a population of glass shards separated from the Calopezzati ash was performed using the fission-track method at
the C.N.R. Institute of Geosciences and Earth Resources of Pisa (Fig. 4). An age of 450,000 a ± 10 % was determined (Table 1). Taking
into account the close analogies found out with the pyroclastic layer named “Parmenide ash” recognized in the nearby Cutro basin, in
the Crotone peninsula (Massari et alii, 2001), the deposition age of the Calopezzati ash might be 420,000 a.
2) Sedimentation rate. The position of the studied ash inside the sedimentary body allowed to compute a thickness of the overhanging
clay cover of around 100 m and to deduce that sedimentation stopped at the end of MIS 9 (280,000 a ago) (Fig. 8). Therefore, during
the considered time span the average sedimentation rate was around 0.6 mm/a.
3) Coastal area lowering (subsidence) and Pleistocene sedimentation. Based upon the mean sedimentation rate (see point 2 above)
and taking into account the computed thickness of the sedimentary prism (around 400-500 m), we argue that sedimentation started
during late lower Pleistocene – early middle Pleistocene times.
4) Onset of the tectonic uplift. The onset of the uplift of the area is about coeval with the ash deposition, therefore an age of around
450,000 a can be deduced for the beginning of the uplift phase (Fig. 6 - A).
5) Emersion of the sedimentary prism. Obviously emersion is diachronic. It took place with evidence in correspondence with the eustatic lowering following the interglacial peak of MIS 11, and it is testified by the sediments located at higher elevation (277 m, Fig. 5); the sedimentary top does not coincide with the most recent sediments.
6) Tectonic uplift and marine terracing. The interglacial high level subsequent the first emersion is attributed to the MIS 9. It originated
a wide terrace (I order terrace) nowadays to a great extent remodelled, with an inner margin at an elevation of 210 m (Fig. 5). An algal
limestone sample yielded a Th/U ≥ 300.000 a (Carobene, 2003). We computed an average uplift rate of 0.62 mm/a. We assume that
the clayey sedimentation stopped with the eustatic lowering (Fig. 8 – A e B). The following interglacial eustatic high level (MIS 7) determined the formation of the II order terrace which nowadays has an inner margin of 105÷120 m (Chapter 4); the corresponding average uplift rate is 0.56 mm/a (Tab. 2). The eustatic peak corresponding to MIS 5.5 originated the III order terrace, with an inner margin at present at around 65÷70 m; the average uplift rate is 0.50 mm/a (Tab. 2). The last eustatic high of stage 5 (MIS 5.1) produced the IV order terrace, which is of less importance than the previous ones as regards surface width and sediments thickness. The present elevation of its inner margin, 25÷30 m, allowed us to compute an average uplift rate of 0.46 mm/a.
The chronological succession of the four orders of terraces and of the sedimentary top elevations (Fig. 5 and Tab. 2) proves an almost
steady course of the uplift rate (Fig. 6 – B), slightly decreasing with time (from 0.69 down to 0.46 mm/a). Taking into account the uncertainties related to the attribution of the age to the palaeo-eustatic peaks and to the estimate of their original elevation, we have reckoned the variability interval of the computed uplift rate values reported above (Fig. 7).