Frumkin et al. 1636 Geological Society of America Bulletin, v. 129, no. 11/12 ABSTRACT We analyzed maze caves and the associated hydrogeology in the northern Negev–Judean Desert in Israel to provide insight on fluid migration and porosity development, with relevance to groundwater and petroleum res- ervoirs on the Arabian Platform flanks. The caves occur specifically in the arid region of the southern Levant, with no equivalent in the moister climate areas further to the north. The karstified bedrock consists of Up- per Cretaceous epicontinental carbonates. Caves were formed mainly above deep faults associated with the Syrian arc fold system. Hypogenic flow is shown to have formed the maze caves particularly under the confine- ment of thick chalk and marl cap rock. Spe- leogenesis occurred during the Oligocene– early Miocene when the Afro-Arabian dome was rising and became erosionally truncated. Calcite deposits depleted in 18 O point to a connection between the caves and recharge over far-field Nubian Sandstone outcrops, north of the Precambrian basement outcrops on the eastern side of the Red Sea. During the early–middle Miocene, the Dead Sea rift began dissecting the region, forming a deep endorheic depression at the eastern margin of the study area and disconnecting the far- field groundwater flow. This was followed by subsiding groundwater levels and associated dewatering of the caves. Fault escarpments and canyon downcutting then dissected the caves, forming the present entrances. The caves are currently mostly dry, with scarce speleothem occurrences. Gypsum crusts with δ 34 S SO 4 values lower than other sulfate depos- its point to bacterial sulfur reduction, hydro- gen sulfide, and sulfuric acid being involved in the speleogenesis. GSA Bulletin; November/December 2017; v. 129; no. 11/12; p. 1636–1659; https://doi.org/10.1130/B31694.1; 13 figures; 1 table; published online 6 July 2017. † amos.frumkin@mail.huji.ac.il Hypogenic karst at the Arabian platform margins: Implications for far-field groundwater systems Amos Frumkin † , Boaz Langford, Sorin Lisker, and Alon Amrani Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel INTRODUCTION Caves are used here as indicators to re- construct the paleogeography and evolution of the northwestern margins of the Arabian platform (Fig. 1) during the mid-Tertiary, and to shed light on the relationships among lime- stone porosity, regional groundwater flow, and tectonic structure. Major erosional events, which create trunca- tion surfaces and depositional unconformities, form important parts of the geological record. However, these events still remain enigmatic due to the lack of chronological constraints (Osborne, 2013; Polyak and Hill, 2013). In the southern Levant, a regional truncation surface (Picard, 1943; Avni et al., 2012) represents such an event where the related formation pro- cesses are relatively unknown. This erosional surface, truncating carbonate strata, is coupled with hitherto undocumented ancient caves. We investigated the karstic caves to elucidate the de- velopment of limestone macroporosity and sub- surface processes associated with the regional truncation surface. Unlike most other karst regions, the Negev and Judean Deserts have been mostly under arid conditions for millions of years (Amit et al., 2006, 2011; Vaks et al., 2013; Matmon et al., 2014). Present karst denudation rates of flat-lying bedrock outcrops along interfluves range from ∼20 mm k.y. -1 in the Mediterranean–semiarid border zone to 1–3 mm k.y. -1 in the hyperarid zone at the Dead Sea Escarpment (Ryb et al., 2014). This indicates good preservation of the previous truncation surface at the interfluves, with only a few tens of meters of denudation since regional truncation. The denudation rarely affects the caves, which are mostly preserved under a thick cap rock. Unlike “common” epigenic caves (Dreybrodt et al., 2005; Ford and Williams, 2007; Palmer, 2007; Slabe and Prelovšek, 2013), hypogenic caves are generated by upwelling water in which reactivity has been enhanced deep beneath the surface, independent of surface or soil CO 2 or other near-surface acid sources (Klimchouk, 2007, 2013; Audra and Palmer, 2015). Such caves may allow a reconstruction of landscape evolution and paleohydrogeology over long pe- riods and large basins (Polyak and Hill, 2013; Piccini et al., 2015). To achieve this, insight into regional cave characteristics and the associated hydrogeology is required (Klimchouk, 2015). Hypogenic speleogenesis has been suggested as a process that formed a small cave in the Negev (Issar and Gross, 1969). Following a major cave survey (Langford and Frumkin, 2013; Frumkin, 2015), the desert areas of southeastern Israel, namely, the Negev and Judean Deserts (Fig. 1), stand out as the most important hypogenic cave province in the southern Levant (including Israel, Jordan, Syria, and Lebanon). Fewer similar caves are known in the much larger nearby regions (e.g., Frumkin and Gvirtzman, 2006; Kempe et al., 2006). Cave formation requires actively circulat- ing groundwater. However, the Negev-Judean Deserts are the driest regions in the southern Levant; therefore, the paleogeographic condi- tions that gave rise to the abundance of caves should be understood. The long-term aridity of the region has prevented overprinting of hypo- genic caves by epigenic processes, thus allow- ing easier identification and understanding of hypogenic features. Geological Background Located on the leeward flank of the Judean Mountains (Fig. 1), the Judean Desert is a rain- shadow desert, while the Negev is a part of the Saharan-Arabian Desert belt. The climate varies between semiarid to hyperarid, with cool win- ters and hot dry summers. Mean annual precipi- tation varies from 400 mm in the northwestern semiarid zone to 30 mm at the southern Negev. Annual potential evapotranspiration varies from 1600 mm in the northwest to 2600 mm on the south (Shachar, 2011). Over 80% of the annual For permission to copy, contact editing@geosociety.org © 2017 Geological Society of America