Palaeogeographical and palaeoenvironmental reconstruction of the Medjerda delta (Tunisia) during the Holocene
Introduction
Since the 19th century, the progradation of the Medjerda (called Bagrada during ancient times) delta and the resulting landscape changes have interested many researchers because of their historical implications for the fate of the city of Utica. Several hypotheses have been advanced regarding the chronology of the progradation of the delta. A first chronological evolution of the infilling of the former Utica gulf (Sinus Uticensis) was deduced from ancient texts by Tissot (1884; 1888). Archaeology combined with morphological field observations then allowed Bernard (1911), Reyniers (1951) and Lézine (1956; 1966; 1968; 1970; 1971) to enrich the corpus of data and propose new hypotheses of evolution. Pimienta (1959) and Jauzein (1971) carried out important geological studies of the delta. Based on photo-interpretation and surface prospecting, geoarchaeological research then provided some clarification and permitted a partial review of the chronological framework (Chelbi et al., 1995; Oueslati et al., 2006; Paskoff, 1994; 1985; Paskoff and Trousset, 1992; Slim et al., 2004). More recently, Delile et al. (2015a; b) used manual coring inside the ancient city of Utica and GIS analysis to clarify this framework. But so far, no stratigraphic data on the scale of the delta has validated these hypotheses of evolution. For this reason, a coring campaign was organized, oriented along two main transects (Fig. 1). The objective was threefold: (1) to understand the maximum marine transgression of the mid-Holocene, (2) to estimate the rate of deltaic progradation and finally, (3) to make the link between the Medjerda delta and its watershed.
Beyond the regional geomorphological and geographical approach, this work sheds new light on the history of the foundation of the city of Utica and the occupation of its territory. Indeed, the most controversial question concerns the chronology of the passage of the river Medjerda into the “northern compartment”, through the corridor formed by the promontories of Utica and Kalaat El Andalous (Fig. 1). This question is crucial both to our understanding of the reason for the establishment of the Phoenicians in this area, and for the location of Utica’s Phoenician and Roman harbours (Pleuger et al., 2019). This problem is also essential for the understanding of the decline of Utica, because the progradation eventually led to the complete isolation of the port city from the sea.
Given the complexity of the progradation process and the difficulty in combining all factors, this paper presents an interdisciplinary study aimed at understanding the Medjerda delta development during the Holocene.
Section snippets
Geological context
Tunisia is located in a dynamic tectonic context, at the convergence between the African and Eurasian tectonic plates (Ben Ayed and Oueslati, 1988). The compression movements toward Africa led to the formation of Pliocene folds and inverse faults, including the anticline of Utica (Coque, 1955; Mejri et al., 2010; Pimienta, 1959). Abrupt contact between Pliocene sandstones and Quaternary potsherd-rich silts attests the Late Pleistocene–Holocene activity of the Utica fault, which might be
Coring
The study of palaeoenvironments and sedimentary processes is carried out through the mechanical extraction of cores (15–20 m deep) to reach the early Holocene levels. The drilling holes were protected by casing to prevent contamination by the stratigraphic units above. This technique is a good compromise for collecting samples in a delta area, avoiding the problems of the water table (Goiran et al., 2014). After recording the stratigraphy of the cores, sediment samples are selected in order to
Results
The stratigraphic logs of the cores are described from bottom to top and the depths are expressed as metres below the current sea level (b.s.l.) or above the current sea level (a.s.l.). Both raw and calibrated radiocarbon dates are given in Table 1; in the discussion below, the calibrated date range at 2σ (95.4% probability) is given. Detailed results about biological indicators, granulometry and mineralogy are given in the Supplementary Materials.
Palaeoenvironmental reconstruction based on bio-indicators and mineralogy
Cores UCN1 (Fig. 3) and UKC1 (Fig. 5) are the cores that have yielded the most complete results. They provide an overall view of the area since they cover the northern compartment of the delta.
Ostracods in unit A of the core UCN1 (Cytheropteron spp; Leptocythere ramose; Loxoconcha gibberosa) are typical of a coastal area and indicate a circumlittoral/infralittoral environment. The presence of broken valves of marine ostracods typical of deeper environments (≃120 m) around 12 m b.s.l. can be
Conclusions
This interdisciplinary study has reconstructed the spatio-temporal pattern of the landscape evolution of the northern compartment of the former Sinus Uticensis in northern Tunisia over the last millennia. It permitted us to understand the maximum marine transgression of the Mid-Holocene and to propose a chronological map of the deltaic progradation, which reached 10 km in 3 millennia. Indeed, our study demonstrates that there was a large and deep bay around the promontory of Utica since the 6th
Acknowledgements
The authors would like to thank the PHC Tournesol and the MISTRALS ENVIMed programs for their logistical and financial support. They also thank the European Research Council Advanced Grant Project Rome’s Mediterranean Ports (102705) for financial support for the coring. They are very grateful to Professor Frédéric Boulvain, François Fontaine, Meriam El Ouahabi and Joël Otten from Department of Geology of the University of Liège for their technical support and helpful comments. The authors also
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