Datazione di un livello cineritico del Pleistocene medio: relazioni con sedimentazione, sollevamento e terrazzi marini nell’area Crosia-Calopezzati in Calabria (Italia)

Nell’area di Crosia-Calopezzati, in destra del fiume Trionto (Calabria ionica), affiora un potente prisma sedimentario pleistocenico che,nelle parti più prossimali, è trasgressivo sui terreni miocenici. Il sollevamento dell’area ha originato la formazione di 4 ordini di terrazzimarini, già descritti in un precedente lavoro; il rinvenimento e la datazione di un livello cineritico nel substrato argilloso pleistocenicoha permesso di ottenere una datazione indiretta degli stessi.Il livello cineritico, per il quale è stata calcolata un’età di 450.000 anni ±10% con il metodo delle tracce di fissione, è stato infatti inquadratostratigraficamente all’interno del corpo sedimentario; ciò ha consentito di trarre anche ulteriori deduzioni e ipotesi, sia cronologicheche quantitative, sul sollevamento e sull’emersione del prisma sedimentario pleistocenico e sulle velocità di sedimentazione.

In the Crosia-Calopezzati area (Ionian coast of Calabria), east of the Trionto River, crops out a Pleistocene marine succession which istransgressive over Miocene – Pliocene (?) substratum (Fig. 1; Fig. 2). These sediments deposited during a lowering phase of the Ionianmargin, which caused the formation of important sedimentary basins in Calabria and Basilicata. A significant stage of knowledge of thegeology of these regions during lower and middle Pleistocene times had been already attained through studies that were performed onthese 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 theopportunity of inferring some considerations on the sedimentation rate and, consequently, on the onset of the sedimentation of thetransgressive 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 onthe 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 whitecolored. Dating of a population of glass shards separated from the Calopezzati ash was performed using the fission-track method atthe C.N.R. Institute of Geosciences and Earth Resources of Pisa (Fig. 4). An age of 450,000 a ± 10 % was determined (Table 1). Takinginto account the close analogies found out with the pyroclastic layer named “Parmenide ash” recognized in the nearby Cutro basin, inthe 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 overhangingclay cover of around 100 m and to deduce that sedimentation stopped at the end of MIS 9 (280,000 a ago) (Fig. 8). Therefore, duringthe 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 startedduring 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 around450,000 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 eustaticlowering following the interglacial peak of MIS 11, and it is testified by the sediments located at higher elevation (277 m, Fig. 5); thesedimentary 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 originateda wide terrace (I order terrace) nowadays to a great extent remodelled, with an inner margin at an elevation of 210 m (Fig. 5). An algallimestone sample yielded a Th/U ≥ 300.000 a (Carobene, 2003). We computed an average uplift rate of 0.62 mm/a. We assume thatthe clayey sedimentation stopped with the eustatic lowering (Fig. 8 – A e B). The following interglacial eustatic high level (MIS 7) determinedthe formation of the II order terrace which nowadays has an inner margin of 105÷120 m (Chapter 4); the corresponding averageuplift 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 atpresent 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 IVorder terrace, which is of less importance than the previous ones as regards surface width and sediments thickness. The present elevationof 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 almoststeady 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 uncertaintiesrelated to the attribution of the age to the palaeo-eustatic peaks and to the estimate of their original elevation, we have reckonedthe variability interval of the computed uplift rate values reported above (Fig. 7).