Keywords

1 Introduction

This study records the sedimentary facies development of Holocene coastal deposits adjacent to Farwah Spit in NW Libya (Fig. 1). No previous work has interpreted the sedimentological features of the Holocene Farwah coastal system on the southern margin of the Mediterranean Sea. This contribution examines macroscopic and microscopic features within these tidal flat sediments. Supratidal-subtidal lithofacies and associated sedimentary structures are described. These are interpreted to record coastal depositional systems (salt marshes, tidal flats, lagoon, flood delta, and spit), and a model is presented.

Fig. 1
figure 1

a Location map of the study area, b interpretation of the depositional environment of the Farwah coastal system, and c a model of the peritidal-shallow lagoonal depositional facies, similar to the hypothetical profile proposed after Lakhdar et al. (2020) for the southeastern coast of Tunisia

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2 Methods

The different lithofacies were described and mapped locally using field observations. The areal extent was interpreted by utilizing Google Earth images (Fig. 1). To examine the tidal changes through time, several visits were made during spring and neap tides. Facies belts were mapped based on satellite images covering the study area. Representative samples were collected from both surface and subaqueous depositional sites. Both macro-structure and micro-texture examinations of samples were conducted during field and laboratory microscopic studies. Several 30–50 cm excavations were dug using a shovel to observe near-surface sedimentary structures within the tidal flat sediments. The TOC of mud deposited within the Farwah lagoon was determined using Rock–Eval pyrolysis.

3 Results

The Holocene sediments comprise bioclastic sands, organic-rich muds, rippled bioclastic sand, and massive to cross-bedded sand, unconformably overlying marginal marine bioclastic and bioturbated sandy deposit of Late Pleistocene age. The lithofacies record the transgression of marginal shallow marine tidal facies. Seaward, the facies change laterally to lagoonal grey muds, rich in organic matter and locally bioturbated. Landward, the facies is covered with thin salt crust with many microbially induced sedimentary structures and/or yellowish sands of aeolian origin. These are interpreted to have been deposited within a lagoon, protected from the sea by a sand spit. Landward, the sand, clay, and evaporite sediments record a supratidal flat with salt marshes. A model for the coastal depositional system is proposed (Fig. 1).

Satellite imagery has been interpreted to extend the field observations, and the recent coastal systems can be observed that also reveal the present-day provenance of the barrier sediments along the Northern Africa continent (i.e., erosion of the Libyan Farwah and Ras Ejdeir headlands that form an essential source for the El Bibane barrier in Tunisia).

4 Discussion

The Holocene Farwah coastal sequence was formed during the last glacio-eustatic cycle, and the exposed prograding surfaces mark the last highstand systems tract (HST) development over the southern Mediterranean Sea that post-dated the last glacial maximum. The marine flooding surface, which separates the transgressive systems tract (TST) from the overlying HST, occurs within submerged strata and has no exposure in the region.

Correlation with the adjacent Tunisian Boujmel Sabkha and Al Bibane lagoon (up to 6.8 ka BP) (Lakhdar et al., 2006, 2020), influenced by similar climatic events, sea-level changes (Hansen et al., 2013) and Mediterranean Sea influence (i.e., Adriatic Sea, SE Italy), reveals that the Farwah facies are prograding spit. The spit and its associated tidal flat and lagoonal facies belts developed within the HST followed postglacial sea-level rise after 22.1 ± 4.3 ka (Shakun & Carlson, 2010). The prograding spit consists almost entirely of a regressive succession emplaced during postglacial sea-level transgression between the latest Pleistocene (14 ka BP) and early Holocene (8 ka BP). Lateral facies changes within the spit allow for identification and lateral tracing of six stacked, prograding successions, which may resemble stacked sixth-order cycles.

5 Conclusions

The Holocene sedimentary succession of the Farwah coastal system records the short duration (approximately 10 ± 2 ka) progradation of spits and associated lagoonal and tidal flat sequences. Facies and environmental analyses, together with the reconstructed stratigraphic architecture of this succession, allow a sequence-stratigraphic interpretation, with the identification of a sequence boundary on the underlying Pleistocene and places most of the Farwah Formation into the HST. The maximum flooding surface, which separates the underlying TST from the overlying HST, occurs within submerged strata and has no apparent exposure in the region.

Correlation with the adjacent Tunisian Boujmel Sabkha and Al Bibane lagoon suggests coeval deposition influenced by the same climatic event and global sea-level change following the last glacial maximum (~14 to 8 ka).

Our study documents the stratigraphic organization of the Farwah coastal facies. It records the interaction between global eustatic variations and sediment supply, both processes under the control of climatic change. Changes in sediment input, climatic variations, and, more recently, human activities close to the eastern opening (inlet) have played an essential role in the development of the flooded delta during the last century.