Palaeogeographical and palaeoenvironmental reconstruction of the Medjerda delta (Tunisia) during the Holocene

Highlights

• The Medjerda delta’s progradation reached 10 km over 3 millennia.

• The reconstruction of the palaeogeography helps to better understand the foundation of Utica and its gradual abandonment.

• A wide bay around Utica since the 6^th mill. BC, progressively sealed by the alluvium of the Medjerda from the 1^st c. BC.

• The results highlight in particular an episode of high-intensity flooding around the 4^th century AD.

• The isolation of Utica from the sea is due to an erosive crisis generated by climatic and probably anthropogenic factors.

Abstract

The progradation of the Medjerda delta has been the subject of many studies since the 19th century. The scale and the rapidity of this phenomenon interested researchers in various fields early on, such as geomorphology, geology, palaeogeography, history, archaeology, or geoarchaeology. Indeed, the delta prograded by around 10 km over 3 millennia. At the time of its foundation supposedly at the end of the 12th century BC, the Phoenician city of Utica was located on a promontory bathed by the sea, but the sediments carried by the Medjerda progressively sealed the bay, leaving the tip of the Utica promontory now 10 km inland. This area is therefore an exception to the general pattern along the Tunisian coast, since as over the same period everywhere else there is a regression of the coastline, owing to a sea level rise of several decimeters. Based on multi-proxy analyses of two coring transects, this paper aims to reconstruct the palaeoenvironments and the palaeogeography of the Medjerda delta’s progradation since the mid-Holocene, some aspects of which are described in ancient sources. The results highlight in particular an episode of high-intensity flooding around the 4th century AD, which is consistent with episodes of high floods and an increase in sedimentation rates recorded in the watershed at the end of the Roman period. The gradual abandonment of the city of Utica can certainly be related to the activity of the Medjerda River, but our results show that it is because of an increase of fluvial sediment contribution in connection with an erosive crisis in the headwaters, and not because of the change of course of the river, which had occurred long before.

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.