Salt tectonics in pull-apart basins with application to the Dead Sea Basin

The Dead Sea Basin displays a broad range of salt-related structures that developed in a sinistral strike-slip tectonic environment: en échelon salt ridges, large salt diapirs, transverse oblique normal faults, salt walls and rollovers. Laboratory experiments are used to investigate the mechanics of salt tectonics in pull-apart systems. The results show that in an elongated pull-apart basin the basin fill, although decoupled from the underlying basement by a salt layer, remains frictionally coupled to the boundary. The basin fill, therefore, undergoes a strike-slip shear couple that simultaneously generates en échelon fold trains and oblique normal faults, trending mutually perpendicular. According to the orientation of basin boundaries, sedimentary cover deformation can be dominantly contractional or extensional, at the extremities of pull-apart basins forming either folds and thrusts or normal faults, respectively. These guidelines, applied to the analysis of the Dead Sea Basin, show that the various salt-related structures form a coherent set in the frame of a sinistral strike-slip shearing deformation of the sedimentary basin fill.     

Halite oolites and ripples in the Dead Sea, Israel

Clastic features in recent halite deposits are observed along the beaches of an artificially dammed part of the Dead Sea. These features include halite oolites (termed halolites in this paper) and ripples. Halite precipitates initially either at the brine surface or on the floor. It is suggested that moderate increase of wave agitation shifts the balance towards brine-surface crystallization, and keeps the growing halite grains in constant motion. In this way rippled structures are formed. A further increase of wave energy leads to the growth of coated halite grains. The accumulation of the various halite grains along the beach, to form soft rippled floor and oolitic beach ridge is brought about during shoreward winds. During calm periods the bulk of the halite crystallizes directly on the floor. It develops into a hard crust which assumes the morphology of the substrate, including the ripple forms.     

Biological marker characteristics of oils and asphalts from carbonate source rocks in a rapidly subsiding graben,Dead Sea,Israel

A detailed GC/MS study of biological marker compounds in the saturated and aromatic hydrocarbon fractions of oils and asphalts from the Dead Sea area, Israel, provided decisive information to the solution of a long-lasting controversy by showing that the asphalts are products of early generation in an immature stage from the same type of carbonate source rock which generated more mature oils. The asphalts are not biodegraded residues of the oils.Oils from six different wells, and asphalts from wells, outcrops, and a floating block from the Dead Sea all have very similar sterane and triterpane patterns. They all lack rearranged steranes (diasteranes) indicating a carbonate source matrix and compare reasonably well with a sample of Upper Cretaceous bituminous chalk from Nebi Musa. The main difference between the oils and the asphalts is a significantly higher triaromatic to mono- plus triaromatic steroid hydrocarbon ratio in the former. This is explained as a result of rapid subsidence and heating of their source rock close to the deep parts of the Dead Sea graben. The oils thus were generated in the more deeply buried source rock blocks under the graben fill, whereas the asphalts either originate from an immature source rock section closer to the graben rims or represent an earlier phase of generation and expulsion.This study also provides general information on the evolution of biological markers in carbonate source rocks. Low-activation-energy processes, like isomerisation of steranes, appear to occur much faster at low temperatures than in shales. The high sulfur content and less cross-linking of the biogenic organic matter into a complex kerogen structure are suggested to be responsible for this. Care should be taken when using only sterane isomerisation to assess the maturity of hydrocarbons from carbonate rocks and of carbonate-derived oils.     

The occurrence of ultra-deep heavy oils in the Tabei Uplift of the Tarim Basin,NW China

Deeply buried heavy oils from the Tabei Uplift of the Tarim Basin have been investigated for their source origin, charge and accumulation time, biodegradation, mixing and thermal cracking using biomarkers, carbon isotopic compositions of individual alkanes, fluid inclusion homogenization temperatures and authigenic illite K–Ar radiometric ages. Oil-source correlation suggests that these oils mainly originated from Middle–Upper Ordovician source rocks. Burial history, coupled with fluid inclusion temperatures and K–Ar radiometric ages, suggests that these oils were generated and accumulated in the Late Permian. Biodegradation is the main control on the formation of these heavy oils when they were elevated to shallow depths during the late Hercynian orogeny. A pronounced unresolved complex mixture (UCM) in the gas chromatograms together with the presence of both 25-norhopanes and demethylated tricyclic terpanes in the oils are obvious evidence of biodegradation. The mixing of biodegraded oil with non-biodegraded oil components was indicated by the coexistence of n-alkanes with demethylated terpanes. Such mixing is most likely from the same phase of generation, but with accumulation at slightly different burial depths, as evidenced by overall similar oil maturities regardless of biodegradation level and/or amount of n-alkanes. Although these Ordovician carbonate reservoirs are currently buried to over 6000 m with reservoir temperatures above 160 °C, no significant secondary hydrocarbon generation from source rocks or thermal cracking of reservoired heavy oil occur in the study area. This is because the deep burial occurred only within the last 5 Ma of the Neogene, and there has not been enough heating time for additional reactions within the Middle–Upper Ordovician source rocks and reservoired heavy oils.     

The distribution of dimethylcarbazoles in oils from the Pearl River Mouth Basin, South China Sea

C2-carbazole isomers have been investigated in crude oils from the Hui-Liu Structure Ridge (HLSR) in the Pearl River Mouth Basin (PRMB), South China Sea. The NH shielded isomer, as well as the NH partially shielded isomers, was detected in high abundance and the NH exposed isomers in lower abundance. A small-enrichment trend of 1,8-dimethylcarbazole (DMC) was observed in crude oils along the western part of HLSR (WPHLSR), which may indicate little effect of migration on the C2-carbazole distributions. Two strikingly different distribution patterns of NH partially shielded isomers were observed in the reservoirs along the WPHLSR: one with a preference of 1,3- and 1,6-DMCs and the other with a preference of 1,4- and 1,5-DMCs. All of the oils occurring in the Upper reservoirs have a preference of 1,3- and 1,6-DMCs, whereas those trapped in the Lower reservoirs show a preference of 1,4- and 1,5-DMCs, which may indicate there are two petroleum migration systems in the WPHLSR.     

S-wave velocity images of the Dead Sea Basin provided by ambient seismic noise

Based on passive seismic interferometry applied to ambient seismic noise recordings between station pairs belonging to a small-scale array, we have obtained shear wave velocity images of the uppermost materials that make up the Dead Sea Basin. We extracted empirical Green’s functions from cross-correlations of long-term recordings of continuous data, and measured inter-station Rayleigh wave group velocities from the daily correlation functions for positive and negative correlation time lags in the 0.1–0.5 Hz bandwidth. A tomographic inversion of the travel times estimated for each frequency is performed, allowing the laterally varying 3-D surface wave velocity structure below the array to be retrieved. Subsequently, the velocity-frequency curves are inverted to obtain S-wave velocity images of the study area as horizontal depth sections and longitude- and latitude-depth sections. The results, which are consistent with other previous ones, provide clear images of the local seismic velocity structure of the basin. Low shear velocities are dominant at shallow depths above 3.5 km, but even so a spit of land with a depth that does not exceed 4 km is identified as a salt diapir separating the low velocities associated with sedimentary infill on both sides of the Lisan Peninsula. The lack of low speeds at the sampling depth of 11.5 km implies that there are no sediments and therefore that the basement is near 10–11 km depth, but gradually decreasing from south to north. The results also highlight the bowl-shaped basin with poorly consolidated sedimentary materials accumulated in the central part of the basin. The structure of the western margin of the basin evidences a certain asymmetry both whether it is compared to the eastern margin and it is observed in north–south direction. Infill materials down to ∼8 km depth are observed in the hollow of the basin, unlike what happens in the north and south where they are spread beyond the western Dead Sea shore.     

高升油田莲花油层主要砂体稠油中的酸性化合物

酸性化合物是辽河稠油的主要化学组成部分。对辽河盆地高升油田莲花油层沙三段４个主要砂体（４、５、６、７砂体）稠油样品的分子地球化学研究表明,其酸性化合在型包括饱和的直链脂肪酸、单甲基支链（异构－和反异构）脂肪酸、类型戊二稀酸、五环三萜类羧酸、甾烷类羧酸、单环芳构有机酸和酚、多环羧酸和少量的氮流化合物。饱和烃和酸性倾合物在各砂体中的存在和分布特征对比分析的结果表明,有机质的分布在各砂体中的变化不能简单     

The Northern end of the Dead Sea Basin: Geometry from reflection seismic evidence

Recently released reflection seismic lines from the Eastern side of the Jordan River north of the Dead Sea were interpreted by using borehole data and incorporated with the previously published seismic lines of the eastern side of the Jordan River. For the first time, the lines from the eastern side of the Jordan River were combined with the published reflection seismic lines from the western side of the Jordan River. In the complete cross sections, the inner deep basin is strongly asymmetric toward the Jericho Fault supporting the interpretation of this segment of the fault as the long-lived and presently active part of the Dead Sea Transform. There is no indication for a shift of the depocenter toward a hypothetical eastern major fault with time, as recently suggested. Rather, the north-eastern margin of the deep basin takes the form of a large flexure, modestly faulted. In the N–S-section along its depocenter, the floor of the basin at its northern end appears to deepen continuously by roughly 0.5 km over 10 km distance, without evidence of a transverse fault. The asymmetric and gently-dipping shape of the basin can be explained by models in which the basin is located outside the area of overlap between en-echelon strike-slip faults.     

Seismic measurements in the southern Dead Sea

A seismic refraction profile and several seismic CDP reflection lines were recently occupied in the southwestern part of the Dead Sea. The seismic data, which are of good quality, give a clear picture of the structure of the area. The western flank of the rift comprises a series of step faults, downthrown to the east with a total throw of some 7 km at which depth the Cretaceous base of the post-Cretaceous fill is located. On the east—west lines the base of the fill dips to the east while on the north—south lines this complex dips to the south with a change in direction of dip being evident in the southern portion of this profile. The post-Cretaceous sediments reach a maximum thickness of 7 km but may be even thicker eastward near the main eastern rift fault. These sediments are gently folded, possibly due to differential compaction and are dislocated by small-magnitude adjustment faults. Lateral transition from bedded layers of salt in the graben fill to a diapir is clearly seen.     

The role of hydrogeological base level in the formation of sub-horizontal caves horizons, example from the Dead Sea Basin, Israel

This paper deals with the hydrogeological relationship between base levels of saline lakes and the formation of sub-horizontal caves. The mechanism presented here suggests that many horizontal cave levels in carbonate sequences are created adjacent to the saline lakes shorelines because of the converging of the groundwater flow above the fresh–saline water interface. The main factors that control enhanced carbonate dissolution and cave formation are high groundwater flow velocities in the shallow phreatic zone during a relative long steady state of the water table. High groundwater flow velocities are evident close to the Dead Sea due to the convergent fast flows above the shallow interface adjacent to the shoreline. The same could prevail in the case of previous paleo-lakes that existed in the basin. The synergetic combination of the above preconditions for enhanced cave formation seems to be responsible for the formation of elevation-controlled alignment of paleo-near shore cave levels in the central and southern (Dead Sea) portion of the study area. These are found on the western fault escarpment and basin margin in different stratigraphic horizons of carbonate lithology. Many of the cave levels can be linked to late Quaternary–Holocene lake levels obtained from dated lake sediments within the basin. The most common cave’s elevation was found to be around 200 m below sea level which was the elevation of the Lisan Lake during part of its history. On the other hand, the Hula Basin in the northern part of the Dead Sea Basin was not occupied by saline water bodies since its formation as a base level, and thus the above preconditions for enhanced cave formation did not prevail. Indeed, this is evident by the lack of horizontal cave levels on its western carbonate margins unlike the situation in the south.     

Sinkhole formation and subsidence along the Dead Sea coast,Israel

More than 4,000 sinkholes have formed since the 1980s within a 60-km-long and 1-km-wide strip along the western coast of the Dead Sea (DS) in Israel. Their formation rate accelerated in recent years to >400 sinkholes per year. They cluster mostly in specific sites up to 1,000 m long and 200 m wide, which align parallel to the general direction of the fault systems associated with the DS Rift. The abrupt appearance of the sinkholes reflects changes to the groundwater regime around the shrinking DS. The eastward retreat of the shoreline and the lake-level drop (1 m/year in recent years) cause an eastward and downward migration of the fresh/saline groundwater interface. Consequently, a subsurface salt layer, which was previously enveloped by saline groundwater, is gradually being invaded and submerged by relatively fresh groundwater, and cavities form due to the rapid dissolution of the salt. Collapse of the overlying sediments into these cavities results in sinkholes at the surface. An association between sinkhole sites and land subsidence is revealed by interferometric synthetic aperture radar (InSAR) measurements. On a broad scale (hundreds of meters), subsidence occurs due to compaction of fine-grained sediments as groundwater levels decline along the retreating DS shoreline. At smaller scales (tens of meters), subsidence appears above subsurface cavities in association with the sinkholes, serving in many cases as sinkhole precursors, a few weeks to more than a year before their actual appearance at the surface. This paper overviews the processes of sinkhole formation and their relation to land subsidence.     

Palynology,sedimentology and palaeoecology of the late Holocene Dead Sea

Palynological and sedimentological studies were performed at two Holocene profiles in erosion gullies (Ze’elim and Ein Feshkha) which dissect the retreating western shore of the Dead Sea. The aim of the project was to analyse possible links between climate, lithology, and vegetation development. The section in Ze’elim shows both lacustrine and fluvial sediments, whereas sedimentation at Ein Feshkha is predominantly lacustrine. The Ze’elim profile, previously used for paleo-lake reconstruction provides an opportunity to compare climate triggered lake levels as paleo-hydrological indicators and vegetation history by use of palynology. The vegetation development in Ze’elim and Ein Feshkha is influenced by both climate and human impact. The pollen record of Ze’elim begins in the Pottery Neolithic, the section of Ein Feshkha in the Late Bronze Age, both records end in the Middle Ages. The Ze’elim section is characterized by sedimentary hiati between the beginning of the Chalcolithic Period until the Middle Bronze Age and within the Late Bronze Age. Settlement periods during the Middle Bronze Age, Iron Age and Hellenistic–Roman–Byzantine Period are indicated by high values of anthropogenic indicators and/or Mediterranean trees. Collapses of agriculture, which can be related to climate effects, are evident during the Late Bronze Age, during the Iron Age and at the end of the Byzantine Period when the lake level curve indicates arid conditions. A comparison of the two pollen records, from different environments, illustrates a more prominent influence of Mediterranean vegetation and cultivated plants in the pollen diagram of Ein Feshkha. The southern Dead Sea region (at the desert fringe) is more vulnerable to regional climate change.     

Radiochemistry of sediments from the southern Dead Sea, Jordan

 A sediment core from the southern Dead Sea was analyzed using gamma spectroscopy as well as ²¹⁰Pb dating in order to ascertain if any radioactive contamination could be detected and to determine the sedimentation rates in the area. Results of this study show no presence of man-made radionuclides in the area. Sedimentation rates were determined to be between 0.25 and 0.4 g/cm²/year. (∼0.5 cm/year), which is in line with what would be expected assuming carbonate layers are annual varves. Received: 31 January 1997 · Accepted: 11 March 1997     

Geochemistry and episodic accumulation of natural gases from the Ledong gas field in the Yinggehai Basin, offshore South China Sea

The Ledong gas field, consisting of three gas pools in a shale diapir structure zone, is the largest gas discovery in the Yinggehai Basin. The gases produced from the Pliocene and Quaternary marine sandstone reservoirs show a considerable variation in chemical composition, with 5.4–88% CH₄, 0–93% CO₂, and 1–23.7% N₂. The CO₂-enriched gases often display heavier methane δ¹³C values than those with low CO₂ contents. The δ¹⁵N values of the gases range from −8 to −2‰, and the N₂ content correlates negatively with the CO₂ content. The high geothermal gradient associated with a relatively great burial depth in this area has led to the generation of hydrocarbon and nitrogen gases from the Lower–Middle Miocene source rocks and the formation of abundant CO₂ from the Tertiary calcareous-shales and pre-Tertiary carbonates. The compositional heterogeneities and stable carbon isotope data of the produced gases indicate that the formation of the LD221 gas field is attributed to three phases of gas migration: initially biogenic gas, followed by thermogenic hydrocarbon gas, and then CO₂-rich gas. The filling processes occurred within a short period approximately from 1.2 to 0.1 Ma based on the results of the kinetics modeling. Geophysical and geochemical data show that the diapiric faults that cut through Miocene sediments act as the main pathways for upward gas migration from the deep overpressured system into the shallow normal pressure reservoirs, and that the deep overpressure is the main driving force for vertical and lateral migration of the gases. This gas migration pattern implies that the transitional pressure zone around the shale diapir structures was on the pathway of upward migrating gases, and is also a favorable place for gas accumulation. The proposed multiple sources and multiple phases of gas migration and accumulation model for the Ledong gas field potentially provide useful information for the future exploration efforts in this area.     

Cl-37 in the Dead Sea system---preliminary results

This study presents the first set of δ measurements in the Dead Sea environment. δ values for the meromictic (long term stratified) Dead Sea water column prior to its complete overturn in 1979 were −0.47‰ SMOC for the UWM (Upper Water Mass) and +0.55‰ SMOC for the LWM (Lower Water Mass). The δ values for the pre-overturn Dead Sea cannot be explained by the prevailing model on the evolution of the Dead Sea during the last few centuries and require corroboration by more measurements. The 1979 overturn wiped out almost completely the isotopic differences between the UWM and LWM. Even so, Cl isotope data could be used to decipher physical processes related to the overturn such as incomplete homogenization of the deep water mass. Inputs into the lake, comprising freshwaters (springs and the Jordan River) and saline springs gave a range of −0.37‰ to +1.0‰ with the freshwater sources being more enriched in δ . Based on the δ measurements of the End-Brine (the effluent from Dead Sea evaporation ponds) and of recent Dead Sea halite, the Cl isotopic composition of the originating brines have been estimated. They gave a narrow isotopic spread, +0.01‰ and +0.07‰ and fall within the same range with Dead Sea pore water (+0.13‰) and with the post-overturn Dead Sea (−0.03‰ and +0.16‰). Rock salt from Mount Sdom gave a value of −0.59‰ indicating its formation at the last stages of halite deposition from evaporating sea water. The hypersaline En Ashlag spring gave a depleted δ value of −0.32‰, corresponding to a residual brine formed in the very latest stages (including bishofite deposition) of seawater evaporation.     

An early first-century earthquake in the Dead Sea

This article examines a report in the 27th chapter of the Gospel of Matthew in the New Testament that an earthquake was felt in Jerusalem on the day of the crucifixion of Jesus of Nazareth. We have tabulated a varved chronology from a core from Ein Gedi on the western shore of the Dead Sea between deformed sediments due to a widespread earthquake in 31 BC and deformed sediments due to an early first-century earthquake. The early first-century seismic event has been tentatively assigned a date of 31 AD with an accuracy of ±5 years. Plausible candidates include the earthquake reported in the Gospel of Matthew, an earthquake that occurred sometime before or after the crucifixion and was in effect ‘borrowed’ by the author of the Gospel of Matthew, and a local earthquake between 26 and 36 AD that was sufficiently energetic to deform the sediments at Ein Gedi but not energetic enough to produce a still extant and extra-biblical historical record. If the last possibility is true, this would mean that the report of an earthquake in the Gospel of Matthew is a type of allegory.     

Geomorphology of the eastern coast of the Dead Sea, Jordan

This paper aims to investigate the geomorphological characteristics of theeastern coast of the Dead Sea as influenced by structural instability andclimatic changes. Aerial photos and map analyses served to distinguish mainmorphological features and slope classes, whereas geomorphological changesalong the cost, mainly those developed by the declining of the Dead Sealevels were located and examined in the field. The eastern coast of the DeadSea was considered an ultimate outcome of stream work practiced undercontrols of sea-level lowerings. Climatic, hydrological and structuralinstabilities characterizing the study coast put watersheds and wadisdraining into the sea under accelerated erosion that produced huge volumes ofsediments to be deposited in the upper levels of the subsiding former lakesof the Dead Sea. Retrogradation exposed subaqueous deltaic-fluviatileformations, contributing to coastal geomorphological development. Also,present climatic seasonal fluctuations, rejuvenating stream work, continue tosubject past depositional environments to denudation as the stream distanceof transport is increased, and newer subaqueous deltas are being formed.Weathering and mass-wasting processes added to such a development byenhancing the formation of karst, tofoni and pedestal features as well as thehigh erodibility rate of deltaic sediments which endagers constructionalprojects along the study coast.     

Hydrochemistry of aquifers in the southern Dead Sea area,southern Jordan

 The demand for water resources in the area south of the Dead Sea due to continued development, especially at the Arab Potash Company (APC) works necessitates that water quality in the area be monitored and evaluated based on the local geology and hydrogeology. The objective of this paper is to provide information on the past and present status of the main aquifers under exploitation or planned for future development. Two main aquifers are discussed: the Safi water field, presently being operated, and the Dhiraa water field, which is being developed. The aquifer developed in the Safi water field is shallow and fed by the Hasa fault system, which drains a significant portion of the Karak mountains. This aquifer seems to be well replenished within the core, where no obvious long-term degradation in water quality can be identified. However, in the low recharge areas within the distal portions of the alluvial fan, there has been a degradation in water quality with time. The degradation is caused by the dissolution of the Lisan Marl, which is present at the outskirts of the fan system, based on hydrochemistry of water in the wells. The Dhiraa field is a deep (800–950 m) aquifer drilled specifically for the extraction of brackish water present in the Kurnub aquifer. Available data indicate that there are at least three distinct water types within this field. These water types are variable in quality, and there may be potential for mixing of these waters, thus affecting the quality of the freshest waters presently available. Tritium and oxygen isotope analysis indicate that the water is old and possibly nonrenewable. Received: 24 July 1995 · Accepted: 26 September 1995