Iraqi Bulletin of Geology and Mining

The western part of the Iraqi Western Desert is covered by carbonates, clastics and marl. The main aquifers in the region are within Suffi, Ga'ara, Mulussa, Muhaiwir, Ms'ad, Tayarat, Digma and Akashat formations. However, Nahr Umr, Rutbah and Ratga formations were either not recognized or not announced yet in the involved areas. Ga'ara aquifer is considered the most important one, on a regional scale due to its wide extent and large content of water. Most aquifers are recharged from rainfall and runoff of the intermittent wadis in form of leakage losses through permeable strata, fractures, fissures, joints and cavities into shallow aquifers. It is assumed that a hydraulic connection between aquifers exists to some extent. Leakage of water is either downwards or upwards, depending on piezometric relation of aquifers, lithology, structure and extent of water-bearing horizons.

The available updated hydrogeological information in the Hydrogeological Data Base indicates the presence of a water divide within the studied area, to the west of Rutbah town, with a general North – South trend. This water divide causes the groundwater to flow into opposite directions along its trend; accordingly, the general direction of the groundwater within the eastern part is towards east and northeast mainly, following the discharge zone along the right bank of the Euphrates River. While, at the western part of the studied area, the direction of the groundwater flow is towards the west; beyond the Iraqi borders. Locally, different directions of flow occur throughout the region, depending on the geological setting of water-bearing horizons and the nature of structure and topography. All the available previous studies have not mentioned the presence of this water divide within this part of the Western Desert, and always have, generally suggested that the direction of the groundwater flow is towards the east and northeast.

Salinity of the groundwater increases with depth and also towards discharge zones in general, but it ranges mainly from fresh to slightly brackish within the studied area. Water type varies and includes bicarbonate, chloridic and sulphatic water.

Shari Playa is a closed elongated basin about 20 Km long and (3 – 5) Km wide; located about 150 Km north of Baghdad. It is characterized by the presence of a lake in winter which dries out to form salt playa in summer. The lake water concentrates by evaporation to form salt crust; composed of sodium chloride and sodium sulphate. Since the playa is a structural depression, many springs are developed inside it as a result of faults intersections. The water and brines in the Shari Playa basin indicate three major groups of water: Na+ – Cl– – SO4=; Ca2+ – SO4= and Na+ – Ca2+ – SO4=. Where the second cations and anions concentrations vary in different water sources in Shari Playa basin, other subdivisions may be recognized giving different water types. The supplied water to the depression strongly contributes to the formation of glauberite, gypsum and thenardite in the lake sediments, especially in the central part which is characterized by the presence of organic matter in the sediments. The water and brines are of different hydrochemical characteristics and contributes to the formation of different evaporite minerals or to dissolve them depending on their concentrations. The lake water precipitates gypsum at first, then glauberite forms in the deepest part of the playa lake when the water volume decreases to occupy the central part only. The concentration of the remaining brine increases due to evaporation, where Na+ is high enough to alter gypsum to glauberite [Na2Ca(SO4)2]. Thenardite (Na2SO4) forms in the salt crust when all Ca2+ is consumed from the lake brine before the NaCl precipitation. It also forms within the black mud slurry after ending of glauberite formation, which consumes the Ca2+ in the brine.

This study tries to examine the correlation between the hydraulic and geoelectrical parameters in an appropriate way, to construct empirical relationship at sites in Bahr Al-Najaf basin in the middle part of Iraq. It is a first attempt of estimating mathematically the effective porosity, hydraulic conductivity, and transmissivity of a confined and anisotropic aquifer of Middle Dammam formation. This aquifer is distinguished with the presence of fractures and paleo karst features of Carbonate rocks. Sixteen of (VES) Schlumberger array using a maximum current electrode separation of (1000) m are conducted near existed wells and boreholes (with pumping test data). The field (VES) data is interpreted, and values of geoelectrical parameters as layer resistivity and thickness, in addition to Dar Zarrouk parameters are determined. Correlating Dar Zarrouk parameters with hydraulic conductivity and transmissivity (derived from pumping test analyses) at nine positions nearby selected boreholes is made. Then representing the results as several graphical relationships of nonlinear curves with polynomial equations of second order and show very good Reliability (R2). Directly proportional relations between longitudinal conductance with both; the hydraulic conductivity of R2 = 0.893 and the transmissivity of R2 = 0.868 are more conformable for such aquifer of average resistivity (70) Ωm. Mathematically estimated values of hydraulic conductivity, transmissivity and those estimated by pumping test analyses are compared by means of histogram figures. The small error ratios (RMS %) as (5.53) and (6.32) between the average values respectively, verify the validity of this methodology to be applied in other sites of similar characteristics. Average effective porosity (secondary type) and water saturation of Middle Dammam aquifer are geophysically computed by Archie's equation as (22%) and (0.9) respectively. These results are close to the typical porosity of saturated carbonate rocks as (27%), and agree with the prevail knowledge that the studied area is a discharge zone for Al-Shbecha hydrogeological basin.

Gravity and magnetic surveys were carried out to study an area located between, Amara and Qurna Cities, The results of the present survey are merged with those of the previous surveys carried out exIPC; using Oasis Montaj software to produce a unified database, which considered a new addition to update the gravity and magnetic field maps of Iraq.

The most impressive result of the present gravity survey is the negative anomaly (A), found in the middle part of the studied area with NE – SW trend. This anomaly is a completion of the more extended important negative anomaly present in the vicinity of Qalat Saleh – Uzair – Qurna. It is surrounded by high gravity gradient that may be related to fault and/ or contact and may reflect the occurrence of a restricted basin, or it may reflect the negative background of Zagaros root effect. It is worth to mention that most oil fields occur along the high gradient flanks of this anomaly.

The expressive positive gravity anomaly (G1 and G2), are located in the upper middle part of the area with a trend of NW – SE extending beyond the studied area. It may be attributed to uplift in the sedimentary cover corresponding to uplifts in the basement, as indicated in the basement relief map of exIPC. This anomaly may be attributed to the presence of coral reef or anhydrite elongated body present within the sedimentary column.

The magnetic map of the studied area reflects mainly the effect of the basement, but several local anomalies are also detected, some of which are related to local buried iron artifacts, which act as sources of noise. Therefore, some filters were applied to enhance the magnetic picture.

The area of study is (80 × 100) Km in dimensions and lies to the immediate NE of the town of Salman in the Southern Desert. The topography is flat with sporadic sediment – fill depression of which the Salman Depression is the largest and most conspicuous.
The Bouguer map of the area, is characterized by a number of local anomalies having closed contours. These gravity highs and lows have various shapes and dimensions with an average amplitude of one mGal. These anomalies can be grouped into two broad long highs enclosing a zone of gravity lows. These groups are elongated in an E – W direction.

Various workers have shown that solution of calcareous and evaporitic rocks in the Southern Desert by groundwater has been active since the early Miocene times. The subsurface geology of the present area consists of almost horizontal succession of Limestone (the Late Eocene Dammam Formation) underlain by evaporates and limestone of the Late Eocene Rus Formation, which in turn underlain by more than 400 m of limestone of the Paleocene Umm Er Radhuma Formation. Such lithology is highly susceptible to solution by continuously running water.

The aim of the present work is to relate the observed gravity anomalies to possible underground solution hollows and channels with attempts at estimating depth, dimensions and trends of such channels. No tectonic sources for the anomalies are feasible as the entire successions are regular and structurally conformable with no sign of any trend.

Detailed interpretations of the local residual anomalies have indicated the existence of an interconnected E – W trending channel of irregular boundaries occurring at a depth of about 50 m below the surface. It varies in width between 20 and 40 Km and in depth extent up to 60 m.

Detailed gravity measurements for some of the relatively impressive short wave magnetic anomalies of the westernmost part of the Western Desert indicate that these anomalies are associated with fracture or fault zones present within the sedimentary cover. One of these anomalies is Al-Waleed anomaly zone present at the cross of regionally extended fault zones over nearly circular cone anomaly. The cone anomaly interpreted as igneous chamber present at a depth of about 2.5 Km within the level of Silurian hot shale. Another anomaly zone (East Traibil anomaly) is followed up by gravity measurements for about 20 Km and shows good correlation with the location of the known regional Amij – Samarra Fault Zone. Nehadain Anomaly Zone, which is with arcuate shape, extends for more than 50 Km. The anomaly source is proved to be a gas trap in its Jordanian part. The aim of the work is to introduce such anomalies for farther studies to clarify their economic importance.

The analysis of about 21000 soil and recent sediments samples collected by Iraq Geological Survey in the seventies and eighties of the last century from the Western and Southern deserts and from the Mesopotamia plain analysed for some minor and trace elements (P2O5, K2O, Al2O3, Fe2O3, TiO2, SO3, Cu, Pb, Zn, U, Cr, Ni and V) are statistically treated in this study to extract natural background values for these terrains which represent about 70% of Iraq area. The results are compared with soil analysis in other areas in Iraq of various physiographic, climatic and source rocks conditions, as well as with reported range values for the soil of the world.
Natural (geogenic) background range values are presented in two ways: the first involved all analytical results including natural anomalous values and the second after removing values above the statistically calculated threshold. The concentration range values for the desert and Mesopotamia are comparable to those reported for world soil with some exceptions. Higher upper range values are noticed for Cr, Ni and V, but the median values are within the world range.
The results also indicate significant influence on the background values by various factors including source rocks and pedogenic processes controlling soil development and maturity such as climate, vegetation, and drainage. The distribution of the elements analysed varies between normal and log-normal, higher concentration outlayers are noticed in most trace elements distribution patterns as well as in SO3 distributions.
The distribution of minor elements (TiO2, Al2O3, Fe2O3, K2O and P2O5) is largely controlled by parent rocks. Some trace elements are also related to source rocks, especially U, Cr and Ni. Sulfate is enriched by authigenic processes.
This study clearly emphasizes the impact of various soil-forming processes, parent rocks, physiography and climate on the geogenic background range of the analysed elements. It also suggests that local environmental studies to demonstrate pollution cases should consider comparison with backgrounds of the uncontaminated soil related to the same physiographic terrain instead of making conclusions based on comparison with world averages for soil, shale or Earth crust.

Analysis of major, trace and rare earth elements, in 17 samples, some are composite of 3 samples, collected along a vertical section of Tanjero Formation (Upper Campanian – Maastrichtian) exposed at Dokan district in northeast Iraq, have shown that sharp changes in the geochemical characteristics of the formation occur at certain level in the section. These changes involved a sharp increase in the content of elements (Cr, Ni, Co, Sc and V) that are known to be concentrated in mafic and ultramafic rocks and an obvious decrease in content of elements (Y, Rb, Zr, Hf, Th, ..etc.) known to be relatively concentrated in felsic rocks. These changes reflect sharp changes in the nature of the source rocks supplying clastics to the basin of deposition of the formation. The changes involved the emplacement of island arc volcanics and ophiolites onto the ridge separating the foreland basin to the west and southwest from the Neo-Tethys to the east and northeast. The emplacement of these mafic and ultramafic rocks is undoubtedly related to the tectonic activity associated with the different stages of the collision of the Arabian Plate with Eurasian Plate.

On the bases of these chemical and mineralogical characteristics, it is found necessary to divide the Tanjero Formation into lower and upper parts. It is even suggested that a new formation name should be given for the lower part, in support of previous suggestions by early geologists working on the formation.