Solid Earth, 10, 1451–1468, 2019 https://doi.org/10.5194/se-10-1451-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. Sinkholes and uvalas in evaporite karst: spatio-temporal development with links to base-level fall on the eastern shore of the Dead Sea Robert A. Watson1,a, Eoghan P. Holohan1, Djamil Al-Halbouni2, Leila Saberi3, Ali Sawarieh4, Damien Closson5, Hussam Alrshdan4, Najib Abou Karaki6,b, Christian Siebert7, Thomas R. Walter2, and Torsten Dahm2 1UCD School of Earth Sciences, University College Dublin, Dublin, Ireland 2Helmholtz Centre Potsdam (GFZ), Section 2.1, Potsdam, Germany 3University of Minnesota, Department of Earth Sciences, Minneapolis, USA 4Ministry of Energy and Mineral Resources, Amman, Jordan 5Geographic Information Management, Leuven, Belgium 6Department of Environmental and Applied Geology, University of Jordan, Amman, 11942, Jordan 7Helmholtz Centre for Environmental Research – UFZ, T. Lieser Str. 4, 06120 Halle, Germany anow at: Department of Earth Sciences, University of Graz, 8010 Graz, Austria bvisiting scientist at: Environmental Engineering Department, Al-Hussein bin Talal University, Ma’an, Jordan Correspondence: Robert A. Watson ([email protected]) and Eoghan P. Holohan ([email protected]) Received: 30 September 2018 – Discussion started: 11 October 2018 Revised: 24 July 2019 – Accepted: 5 August 2019 – Published: 30 August 2019 Abstract. Enclosed topographic depressions are character- saturated and undersaturated groundwater, as induced by the istic of karst landscapes on Earth. The developmental rela- 35 m fall in the Dead Sea water level since 1967. More gen- tionship between depression types, such as sinkholes (do- erally, our observations indicate that uvalas and the sink- lines) and uvalas, has been the subject of debate, mainly be- hole populations within them, although morphometrically cause the long developmental timescales in classical lime- distinct, can develop near-synchronously by subsidence in stone karst settings impede direct observation. Here we char- response to subsurface erosion. acterize the morphometric properties and spatio-temporal de- velopment of ∼ 1150 sinkholes and five uvalas formed from ∼ 1980 to 2017 in an evaporite karst setting along the east- ern coast of the hypersaline Dead Sea (at Ghor Al-Haditha, 1 Introduction Jordan). The development of sinkhole populations and in- dividual uvalas is intertwined in terms of onset, evolution Sinkholes and uvalas are types of enclosed topographic de- and cessation. The sinkholes commonly develop in clusters, pressions and they are characteristic of karst regions on within which they may coalesce to form compound or nested Earth. In such regions, which make up ∼ 15 % of the Earth’s sinkholes. In general, however, the uvalas are not defined by dry land surface (BGR et al., 2017; Chen et al., 2017), the coalescence of sinkholes. Although each uvala usually en- bedrock or surficial material is composed of carbonate, sul- closes several clusters of sinkholes, it develops as a larger- fate or halide minerals that are prone to dissolution by wa- scale, gentler and structurally distinct depression. The loca- ter. In limestone karst areas, sinkholes (also termed dolines) tion of new sinkholes and uvalas shows a marked shoreline- and uvalas are distinct with regard to scale, morphometry and parallel migration with time, followed by a marked shoreline- their inter-relationship (Cali´ c,´ 2011). Uvalas typically occur perpendicular (i.e. seaward) growth with time. These obser- on a larger scale than sinkholes; uvala diameters in lime- vations are consistent with theoretical predictions of karstifi- stone karst are typically 1000–5000 m (Cali´ c,´ 2011), whereas cation controlled by a laterally migrating interface between sinkhole diameters in limestone karst are sub-kilometre in Published by Copernicus Publications on behalf of the European Geosciences Union. 1452 R. A. Watson et al.: Sinkholes and uvalas in evaporite karst size and typically in the range of 10–200 m (Bondesan et The rapid development of sinkholes and uvala-like depres- al., 1992). Morphometrically, uvalas commonly have gentler sions in evaporite karst over the last 35 years at the Dead Sea slopes than dolines and they have more complex shapes, both provides a unique opportunity to shed new light on the ge- in plan view and in three-dimensions (Cali´ c,´ 2011). A single ometric and genetic relationships between them. The sink- uvala typically includes numerous sinkholes within it. holes have been the focus of most previous works, with over Historically, there has been considerable debate over the 6500 mapped on the western shore as of 2017 (Abelson et process(es) of uvala formation and over the genetic relation- al., 2017; Arkin and Gilat, 2000; Avni et al., 2016; Taqieddin ship between uvalas and sinkholes. In limestone karst areas, et al., 2000; Yechieli et al., 2006). Together with the uvala- where uvalas have been first and most extensively described, like depressions, they represent a substantial geohazard in the various mechanisms have been proposed for uvala forma- Dead Sea region and they have already destroyed or damaged tion, such as areally distributed surface dissolution, subsur- several tourism facilities, factories, evaporation pond dykes, face corrosion and mass-wasting, and coalescence of sink- highways, link roads, houses and farmland. holes (see Cali´ c,´ 2011, for discussion). Cvijic´ (1901, 1960) In this paper, we provide a first detailed documentation proposed a direct genetic relationship, whereby sinkholes of the spatio-temporal evolution of both sinkholes and uvala- evolve by coalescence into uvalas and ultimately into a third like depressions on the eastern shore of the Dead Sea, at Ghor type of enclosed karstic depression, a polje, which occurs on Al-Haditha in Jordan. Our aims are to discern the spatio- still-larger scales of several square kilometres to several hun- temporal inter-relationship of these two types of karstic de- dred km2 in area (Kranjc, 2013). Poljes typically have steeper pressions and to examine how their development relates to sides than uvalas, are elongated in plan view and have char- the regional base-level fall caused by the decline of the Dead acteristically wide and flat bottoms. Many authors since have Sea. Our approach combines remote sensing data spanning considered Cvijic’s´ concept of a cyclical karst evolution be- the 50-year duration of base-level fall from 1967 to 2017 tween the three depression forms to be problematic (Lowe with close-range photogrammetric surveys and field observa- and Waltham, 1995; Cali´ c,´ 2011; Sauro, 2012). Nonetheless, tions made during 2014–2017. This approach develops from the concept of uvalas developing by coalescence of sinkholes that presented by Kottmeier et al. (2016) and Al-Halbouni is widespread: the term “uvala” has traditionally been re- et al. (2017) but involves new datasets that are much more garded as synonymous with “compound sinkhole”, “nested extensive in both space and time. By providing a detailed sinkhole” and “valley sink” (cf. Lowe and Waltham, 1995; overview of the evolution of surface subsidence features in Monroe, 1970; Sweeting, 1973). the Ghor Al-Haditha area, our study also complements recent A significant problem for unravelling relationships be- geophysical surveys (Polom et al., 2018) and numerical mod- tween uvalas and sinkholes (dolines) in limestone karst ar- elling (Al-Halbouni et al., 2018, 2019) of subsurface charac- eas is that the landform evolution is controlled by the rela- teristics and processes there. Our results yield the most de- tively slow dissolution kinetics of carbonate minerals. Con- tailed insights to date into the spatio-temporal development sequently, the development of these landform types is not di- of sinkholes and uvala-like depressions in an evaporite karst rectly observable in such areas. Furthermore, the slow rate setting and they provide the clearest yet illustration of the of karstic processes means that limestone areas are suscep- consequences of base-level fall on that development. tible also to long-term geomorphic influences from chang- ing climate and active tectonics. Indeed, the areas in which such uvalas and dolines have been best documented occur 2 Tectonic and hydro-geological framework in tectonically complex settings in which climate has var- ied considerably over time, such that many landforms have The Dead Sea is the hypersaline terminal lake of the Jor- been modified not only by karst processes but also by flu- dan River (Fig. 1a) and it lies within the Dead Sea basin. vial and/or glacial processes (Cali´ c,´ 2011, and references This ∼ 150 km long and ∼ 8–15 km wide basin lies at a left therein). Uvala-like depressions have also been noted in ar- step (or bend) along the left-lateral Dead Sea Transform fault eas of evaporite karst, such as at the margins of the hy- system (Garfunkel and Ben-Avraham, 1996). Maximum tec- persaline Dead Sea (Al-Halbouni et al., 2017; Avni et al., tonic subsidence is ∼ 8:5 km around the Lisan Peninsula, 2016; Baer et al., 2002; Closson, 2005; Frumkin, 2013), the adjacent to our study area (Ten Brink and Flores, 2012). Ebro Valley of northeastern Spain (Gutiérrez et al., 2005), The basin has subsided rapidly from the late Pliocene to the in the central uplands of Turkey (Dogan, 2005; Dogan˘ and present (Ten Brink and Flores, 2012), and during this time Özel, 2005; Waltham, 2015) and within the Arabian Platform has hosted several palaeo-lakes of varying size and longevity of central and eastern Saudi Arabia (Gutiérrez and Cooper, (Bartov et al., 2002; Torfstein et al., 2009). With respect 2013; Youssef et al., 2015). The evaporitic minerals of such to modern global mean sea level (m.s.l.), a high-stand of karst settings have much faster dissolution kinetics but uvala- −162 m m.s.l. was reached at around 25 ka, during the “Lisan like depressions there have not been studied as extensively as Lake” episode. The modern Dead Sea initiated after a major their equivalents in limestone karst.