Geological Characterization of the Bir Djeneb’s Sinkhole (Boukadir, Algeria)

doi:10.21203/rs.3.rs-2594352/v1

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Geological Characterization of the Bir Djeneb’s Sinkhole (Boukadir, Algeria)

https://doi.org/10.21203/rs.3.rs-2594352/v1

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Bir Djeneb is one of the remarkable structures found in the Messinian Kef Formation, a vast outcrop of white Lithothamnium limestone south of the city of Boukadir. This construction remained unknown for many years. Using field research and literature analysis, we examine this phenomenon in this work. According to the most recent classifications of sinkholes, Bir Djeneb is classified as a bedrock collapse sinkhole as a result of the geological characterization. The sedimentary fill of the sinkhole allows us to distinguish between two subsidence episodes. By linking this sinkhole with a major collapse that occurred in 1988 in the midst of Boukadir's Fourth National Road, the dangerous aspect of this sinkhole was brought to light.

Bir Djeneb

Sinkhole

Karst

Messinian

Lithothamnium limestone

The sinkhole known as Bir Djeneb (Devil's hole), located southwest of Boukadir city (Chlef, Algeria), has long been mysterious and has inspired locals to tell fantastic myths and stories. This study's goal was to learn more about Bir Djeneb's karst structure and how it occurred within the vast white limestone formation that occur in south of Boukadir, an area where many cavities can emerge. (Moulana et al., 2019).

About 10–15% of the earth's terrain is made up of well-developed karsts, which are formed when soluble rocks and acidic water mix. The geomorphological model of karstic areas is constituted of several landforms (Polje, ceneote, sinkhole, etc). (Palmer, 2007). In carbonate rocks (limestone, dolomite) and evaportie rocks, the karst phenomenon is highly prevalent (gypsium). Karst landscapes evolve over time, and this change depends on both how fast and how much limestone can be dissolved (Dreybroot, w, 2004).

Bir Djeneb is a sinkhole or a doline; it is one of the most characteristic features of karst landscapes created as the result of bedrock dissolution, and cave roof collapse. Most sinkholes are located where soluble rock is exposed at the surface (Palmer, 2007). It can occur as evolving recent phenomena and old or relict feature testifying ancient phase of karst process

In general, the sinkhole genesis is natural and its formation can take longue time, but the occurrence can be sudden and cataclysmic. Also it can be human induced due to the sensitivity of the Karst terrain and the increasing anthropic impact on the environment (Gutiérrez et al., 2014), one of the typical example of human induced sinkhole in Algerian Sahara is the ONK32 Collapse (Akretche & Okbi, 1996).

In addition, the government has been seriously concerned about potential geohazards related to sinkhole occurrence (Kobal et al., 2015; Tabbagh & Champollion, 2012; Ek et al., 2006; T. Waltham et al., 2005). It has been demonstrated that the cost of responding to a disaster is much higher than the cost of implementing preventative measures (Paraise, 2015). To determine if the chosen solution (filling with concreat) was the suitabale option, it is strongly advised to inspect the Boukadir collapse that occurred on national Road N°4 occurred in 1989.

A group of geologists and speleologists (Club de Spéléologie et Sports de Montagne Béjaîa - CSSMB Association) explored the sinkhole in October, 2021. For the first time, we have reliable data from deep inside this structure, which will be used in the description of Bir Djeneb and understanding this spectacular phenomenon.

By integrating the hydrogeological studies, the geometrical data, and the field observations of Bir Djeneb and its surroundings that the speleologists team provided., a geological cross section across the structure was made in order to classify it according to sinkhole typology (Parise, 2019).

Bir Djeneb is located 5.5 km SWW of Boukadir city (36°01'03.9"N 1°06'35.1"E) in the province of Chlef, northern Algeria. The structure occurs east of both valley of Touchaït and Bir Djeneb (Fig. 1) at altitude of 328 m above the sea level, it is characterized by a circular opening nearly at the center of a circular depression which is 10° inclined toward the Southern-West, where the depression is partly surrounded by a cliff containing several metric cavities.

The karst study required the use of several techniques ( Kobal et al., 2015; Ek et al., 2006 ), to look at the geology of the Bir Djeneb sinkhole's area. The meteorological and hydrogeological environment of the study area was established using data from earlier studies.

The study area is a portion of the North Ouresenis Range, which acts as a limit between the allochtoneous Mesozoic Ouresenis range (substratum) to the southward and the Neogene Lower Cheliff basin to the north (Perrodon, 1957; Van Vleck Anderson, 1936). A partial lithostratigraphical succession of the study area is shown in the outcrops of Kef Bou Rieh (Fig. 2). With a 10 mm thick basal heterogeneous conglomerate and a 150 m thick blue marl formation with some sand marls, a volcanic ash bed marks the upper part of the lithological log.

The upper part of the lithostragraphic log is made of 80 m thick sandy and bioclastic limestone with Lithothamnium. Van Vleck Anderson (1936) grouped the marly formation and the limestone formations into one lithological unite called “Kef Formation”, which can be easily spotted from Sly valley in the East and from the West in Djedouia. Neurdin-Trescartes (1992) observed a slight discordance between the blue marl’s formation and the Lithothamnium limestone formation, and used the volcanic ash layer to establish a regional correlation in the Lower Cheliff basin.

According to the Florida Geological Survey and Kromhout (2017), sinkholes are a highly common occurrence that mostly affects limestone; however, they can also form on other carbonates and evaporite rocks (A. Waltham & Fookes, 2003). Due to the presence of a large exposed limestone formation, the Boukadir region has the potential to develop karst, which will lead to the formation of sinkholes.

The lithostragraphic setting of Boukadir shows thick impermeable blue marls formation that lies beneath the highly permeable Kef limestone formation. Therefore, the major part the underground water flow will be mainly limited in the Kef limestone formation (Fig. 4). A high rates of dissolution that affect limestone rocks are caused by this underground water flow (Bouchenouk, 2013).

With an average temperature of 19 C and modest precipitation of about 304 mm, the study region has a dry Mediterranean climate (Bouchenouk, 2013) There have been no specific paleoclimate studies conducted in the area, but regional studies of North Africa (Hoffmann et al., 2016) identified three main wet periods between 65 and 61 Ka, 52 and 50 Ka, and 37.5 and 33 Ka during the last glaciation events, which contributed to the development of karst. However, even in dry regions with very little precipitation, sinkholes can form (Shaqour, 1994).

We were able to create a cross section into the sinkhole (Fig. 05) using the morphometric information the speleologists had provided, and this helped us to comprehend the colluvium deposit and the limestone bedrock. Bir Djeneb has a 30 m diameter, although its bottom is significantly narrower (16 m), filled with colluvium, leftover soil, and limestone detritus. A depth estimate of 53.5 meters was made, which is nearly 10 meters deeper than the most recent measurement made by Jean Birebent in 1947 (63 meters) (Birbent, 1947). It's crucial to keep in mind that it is difficult to determine a correlation between the diameter and depth of a sinkhole (Marwan et al., 2022).

Understanding how bedrock and the colluvium deposit interact is essential to figure out how sinkholes form (Gutiérrez & Cooper, 2013). The observation of the sedimentary cover enables us to distinguish between two lithological formations (Fig. 05). At the bottom is the limestone formation (bedrock), which is where karst originally developed. This formation is covered by 20 m of a coarsening up colluvium deposit, which was deposited after the initial cavity collapsed.

Sinkhole occurrence can be related to a variety of factors (Strzakowski, 2019), including natural factors like surface inclination, hydrographic conditions, lithology, and seismic activity as well as anthropoid factors (land development, road network) (Papadopoulou-Vrynioti et al., 2013). A recent study revealed how droughts and climate change affect sinkhole formation (Linares et al., 2017). Due to the hydrogeological and climatic conditions of the Boukadir region, we demonstrated in our case study how susceptible limestone formation is to the development of karst and the emergence of sinkholes. Multiple research investigations focused on the genetic classification of sinkholes, which were categorized into two basic types: subsidence sinkholes and solution sinkholes (Parise, 2019; Gutiérrez et al., 2014; Waltham & Fookes, 2003). The Bir Djeneb sinkhole's geological cross section reveals a characteristic feature that matches to a bedrock collapse subsidence sinkhole.

Our method of investigation based on direct field observation, but it is also necessary to use geoelectrical (SEV) or GPR techniques. Geophysical methods can improve the cross section by looking beyond the sedimentary fill of the sinkhole and demonstrating the spatial relationship with other karstic phenomena nearby, such as Ghar Bou Baara, which is located 2.5 km NNW of Bir Djeneb.

We looked at a related occurrence that appears in the same Lithothamnium limestone formation in order to better comprehend the process that creates the Bir Djeneb sinkhole. El Koua cave (Fig. 7) is found in the El Koua mountain, from which it gets its name, on the west side of Bou Mendjel valley, in the Beni Chougrane Range, 5 kilometers north of Bordj city in Mascara province (35°33.581'N 0°18.700'E). A 300 m tunnel leading to a large room over 30 metres wide and 25 m high with a 5 m diameter opening at the roof. The Koua cave serves as an excellent illustration of the Bir Djeneb's early stages of formation.

In 1988, the fourth national road that passes through Boukadir was closed due to the suddenly occurrence a 60 m diameter sinkhole in the road axe (Moulana et al., 2017, 2018), This event further shows that Bir Djeneb is not only a simple geological structure but also a warning message to locale authority to consider this highly dangerous geohazard in the near future (Parise, 2015b; Parise et al., 2015; Gutierrez et al., 2014), especially in the context of the global warming and the drought episode that hit the Chelif region during last decade, other minor scale sinkhole is reported in Merdja Sidi Abed near the East-West highway.

The expedition of the CSSMB's speleologists was a great opportunity to rediscover the Bir Djeneb structure; it provided a comparison between the measurements of Jean Birebent, who measured a depth of -63 m, and the present depth stated by speleologists, which is -53 m.

The relationship between the bed rock and the sedimentary cover is a key feature to classify the sinkhole, according to our observation; Bir Djeneb is a typical dissolution sinkhole. The age of Bir Djeneb’s genesis is still controversy. Nevertheless, based on the stratigraphic relation between the bed rock and the colluvium deposit, the depression resulted from the collapse event happened during Quaternary, only a radioactive datation at the very bottom of the colluvium deposit can give an accurate age.

The 1988 collapse of Boukadir's 4th national road and other nearby small-scale sinkhole reports served to remind of how dangerous sinkholes could be. The emergence of sinkholes is a serious issue, considering the recent drought cycle that hit the area and significantly drained the water table.

Acknowledgement: We would like to thank all members of CSSMB speleology association for the exploration mission of Bir Djeneb, and also the APC of Boukadir for the logistic support, as well as Susan A. Green for her kind advices.

Conflicts of interest: The authors declare that they have no conflict of interest.

Funding: Not applicable

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No competing interests reported.

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Geological Characterization of the Bir Djeneb’s Sinkhole (Boukadir, Algeria)

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1. Introduction

2. Geographic Setting

3. Methodology And Geological Setting

4. Results

5. Discussion

6. Conclusion

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