Sedimentology and Evolution of the Fluvial-Deltaic System: A Modern Depositional Model Analog from the Red Sea Coastal Region, Saudi Arabia

Siliciclastic sediments represent important lithological unit of the Red Sea coastal plain. Their subsurface equivalents are important targets of groundwater aquifer and hydrocarbon reservoirs in the region. The lithofacies of the modern fluvial deltaic system has several distinct geomorphic units and sub-environments such as alluvial, fluvial, delta plain, aeolian, intertidal, coastal sabkha and eustuarine sediments. This study intends to characterize the lithofacies and the depositional environments and to produce an integrated facies model for this modern fluvial-deltaic system. The study might provide a valuable modern analog to several important subsurface Neogene formations that act as important hydrocarbon reservoirs and groundwater aquifers. The study integrates information and data obtained from landsats, maps and detailed field observation and measurements of facies analysis of the fluvial and deltaic along traveses from the Arabian Shield to the Red Sea coast. The lithofacies sediment analysis revealed four main lithofacies associations namely lithofacies A,B,C ad D. Lithoacies Associations A, which represents the oldest unit is dominated by coarse gravel with minor sands facies. While the lithofacies B is dominated byfine gravel and sand lithofacies, occasionally pebbly, vary from horizontal, planar to massive sands with minor laminated to massive silts and mud facies. The lithofacies in A and B show lateral proximal to distal variation as well as characteristic vertical stacking patterns. The Facies Association A and B indicates a change in fluvial depositional styles from gravelly alluvial fans to gravelly sandy fluvial systems. The lithofacies association C represents the recent fluvial system which consists of minor gravel lag deposits associated maily with various sand lithofacies of planner, horizontal and massive sand associated with massive and limainted sand and mud lithofacies. The lithofacies Association D is dominated with Barchan sand dunes local interfigger with muddy iinterdunes and sand sheets. Lithofacies D occupies rather more distal geomporphic position of the fluvial deltaic system that is adjace to coastal sabkha. The lithofacies associations described here document the evolution and development of the coastal plain sediments through space and time under various autocyclic and allocyclic controls. This included the tectonics and structural development associated with the Red Sea rifting and opening since the Oligocene – Miocene time. Others controls include the evolution of the Arabian shield (provenance) and the coastal plain through space and time as controlled by tectonics, sediment supply, climate and locally by autocyclic environmental This study might be beneficial for understanding the controls and stratigraphic evolution of the Red Sea region and will be of great value for reservoir and aquifer characterization, development and management. This modern analog model can also help in providing geological baseline information that would be beneficial for understanding similar ancient fluvial deltaic sediments. The study might provide guides and leads to understand the subsurface facies, stratigraphic architecture and heterogeneity of any potential groundwater aquifers and hydrocarbon reservoirs.

Distribution and Significances of the Major Oxides in Recent Coastal Sabkha Sediments of the Al-Dafna Plateau, northeast Tobruk, Libya

Twenty sabkha samples were chemically analyzed by XRF technique for determination of their major oxides concentrations. Six cores penetrated both the intertidal and supratidal flat zones developed in six wadis mouths in the coastal stretch of the Al-Dafna plateau, northeast Tobruk city. The significance distribution and concentration of the major oxides are discussed and interpreted. Elements in the raw material of the study area reflect important evidence of geochemical weathering processes that affecting parent rocks as well as conditions of sedimentation, and rate of deposition. Correlation coefficients have been used to illustrate the abundance and distribution of these elements. The distribution of major oxides are follows Si2O > CaO > LOI > Al2O3 > SO3 > MgO > Na2O > K2O. The vertical distribution of major elements is mainly controlled by the abundance and proportions of the clastics, carbonates and evaporite minerals. It was found that silica present in the form of detrital, subrounded to rounded silt- to sand-sized quartz grains, while the content of Al2O3 is associated with terrigenous argillaceous materials. On the other hand, the presence of Fe2O3 is related to the abundance of clastic materials in sabkha deposits. It was found that K2O and Na2O concentrations increase toward the sabkha surfaces.

Spatiotemporal Δ17O variability in the rock record

<p>The  Δ<sup>17</sup>O value of sedimentary sulfate can provide a direct, stable, geological archive of atmospheric-biospheric evolution. Negative Δ<sup>17</sup>O values in gypsum/anhydrite are inherited from the negative Δ<sup>17</sup>O value of atmospheric O<sub>2</sub> which is transferred to sulfate during sulfide weathering. The magnitude of the O<sub>2</sub> Δ<sup>17</sup>O value reflects pCO<sub>2</sub>, pO<sub>2</sub> and gross primary productivity, hence modelling of the geological Δ<sup>17</sup>O record has led to estimates of changing atmospheric composition and primary productivity over Earth history. However, sulfate Δ<sup>17</sup>O values represent a conservative estimate of atmospheric Δ<sup>17</sup>O values as the magnitude of negative Δ<sup>17</sup>O in sulfate can be diluted (or erased) through sulfur cycling. As sulfate is transported away from the site of sulfide oxidation the likelihood of this happening increases.</p><p>Although this effect is acknowledged, the extent to which Δ<sup>17</sup>O values may vary within and between palaeoenvironments, and how evaporite sedimentology may affect stratigraphic interpretations of Δ<sup>17</sup>O values, remains unclear. We present the preliminary results of two case-studies probing the spatiotemporal variability of Δ<sup>17</sup>O values.</p><p>Case-study 1: temporally correlative Tournaisian (Lower Mississippian) evaporites within Carboniferous rift basins of Britain and Ireland were deposited in a range of settings: coastal wetland (Ballagan Fm.); supratidal sabkha on margin of a restricted basin (Ballycultra Fm.); and coastal sabkha on open ocean margin (Middleton Dale Anhydrite Fm.) All three settings plot on a positive slope in d<sup>34</sup>S vs Δ<sup>17</sup>O space with values ranging between δ<sup>34</sup>S ≈ +15 ‰, Δ<sup>17</sup>O ≈ -0.08 ‰ and δ<sup>34</sup>S ≈ +24 ‰, Δ<sup>17</sup>O ≈ -0.2 ‰. We discuss whether this trend (and intraformational trends) represents a spatial variability in sulfate Δ<sup>17</sup>O as controlled by fluctuating fluvial and marine dominance in evaporite depositional environments, or whether this might represent a temporal change in δ<sup>34</sup>S and Δ<sup>17</sup>O.   </p><p>Case study 2: non-marine evaporites of the early Permian Cedar Mesa Sandstone (CMS) Formation in Utah were deposited in continental saline pans in an erg-margin setting that fluctuated through arid and humid cycles. These evaporites record negative Δ<sup>17</sup>O values as low as -270 per meg, however δ<sup>34</sup>S values lie along the marine curve. We interpret the signal preserved in the CMS as recycling of the underlying marine evaporites of the late Carboniferous Paradox Formation which have been uplifted on the basin margin. Hence, we discuss how in non-marine settings the recycling of evaporites can decouple the age of the succession from the age of the atmospheric Δ<sup>17</sup>O signal.</p>

Ground Improvement Using Dynamic Compaction in Sabkha Deposit

The sabkha soil spreads extensively in the Arabian Gulf Coast region. Sabkha is known as a geotechnically problematic soil because of its loose density, soft consistency, high salinity and water content, and occurrence of fine sands and clays. It is generally highly compressible and requires ground improvement for highway and railway construction. The purpose of this study is to provide a guideline for dynamic compaction to improve the bearing capacity of the coastal sabkha deposit. The ground behavior during dynamic compaction was evaluated for various compaction energy conditions using numerical analysis, and field dynamic compaction tests were also performed and compared with the numerical analysis results. It was found that the bearing capacity of sabkha deposit can be effectively improved by dynamic compaction. However, care must be taken to ensure that excessive porewater pressure is sufficiently dissipated during the application of dynamic compaction because the permeability is pretty low due to the high salt content in groundwater in the sabkha area.

Ecosystem distribution profiling of bacteria from a unique hypersaline sediment (sabkha) reveals ecological specialization among communities in the environment

Advances in genomic sequencing technologies resulted in massive microbial diversity data (16S ribosomal gene sequences, rDNA) being generated in every possible environment. However, the majority of microorganisms have never been cultured, and therefore, nor cataloged. This poses a problem for molecular microbial ecologists because a large portion of the marker sequences can not be taxonomically resolved past the phylum taxon level. This tells very little about who or what these microorganisms are doing in relation to their environment. Our study describes an approach to assist in drawing ecological information from a sample when the taxon resolution is poor. We generated 16S rDNA libraries from a hypersaline marine sediment (coastal Sabkha) and saline mangrove soil in Abu Dhabi and then compared the compositional features to a database of 20,470 publicly available microbial community profiles (comprising the entire Earth Microbiome Project, EMP) that were annotated with terms from the Environmental Ontology (EnvO). An accurate taxonomic classification was not possible for 80% of the Sabkha operational taxonomic units (OTUs) beyond phylum level with widely used taxonomy classification tools, but habitat profiling performed on the community revealed strong links to bacterial assemblages of soil and marine origins. To capture the notion of generalist vs. specialist formally, we developed an algorithm to derive empirical probability distributions of OTUs over ecosystems from observed occurrences in the sample database, which then give rise to OTU-specific ecosystem entropies. We observed very low average ecosystem entropy of the Sabkha in contrast to other environmental samples. Based on this concept, the Sabkha community, while of midrange alpha diversity, presented largely specialist characteristics, with most OTUs identified to be unique to the Sabkha habitat. This finding is further corroborated by the observation that the Sabkha sample is unique with respect to the EMP-derived dataset (which contains 74 hypersaline and thousands of marine samples), as a comprehensive UniFrac similarity search did not yield any significant matches. Finally, we show that the ecosystem entropy formalism, which intrinsically accounts for the ability of OTUs to cross ecosystem borders according to a context database, is a novel, informative tool to describe and identify extreme environments in addition to conventional ecological diversity measures.

Ecosystem distribution profiling of bacteria from a unique hypersaline sediment (sabkha) reveals ecological specialization among communities in the environment

Advances in genomic sequencing technologies resulted in massive microbial diversity data (16S ribosomal gene sequences, rDNA) being generated in every possible environment. However, the majority of microorganisms have never been cultured, and therefore, nor cataloged. This poses a problem for molecular microbial ecologists because a large portion of the marker sequences can not be taxonomically resolved past the phylum taxon level. This tells very little about who or what these microorganisms are doing in relation to their environment. Our study describes an approach to assist in drawing ecological information from a sample when the taxon resolution is poor. We generated 16S rDNA libraries from a hypersaline marine sediment (coastal Sabkha) and saline mangrove soil in Abu Dhabi and then compared the compositional features to a database of 20,470 publicly available microbial community profiles (comprising the entire Earth Microbiome Project, EMP) that were annotated with terms from the Environmental Ontology (EnvO). An accurate taxonomic classification was not possible for 80% of the Sabkha operational taxonomic units (OTUs) beyond phylum level with widely used taxonomy classification tools, but habitat profiling performed on the community revealed strong links to bacterial assemblages of soil and marine origins. To capture the notion of generalist vs. specialist formally, we developed an algorithm to derive empirical probability distributions of OTUs over ecosystems from observed occurrences in the sample database, which then give rise to OTU-specific ecosystem entropies. We observed very low average ecosystem entropy of the Sabkha in contrast to other environmental samples. Based on this concept, the Sabkha community, while of midrange alpha diversity, presented largely specialist characteristics, with most OTUs identified to be unique to the Sabkha habitat. This finding is further corroborated by the observation that the Sabkha sample is unique with respect to the EMP-derived dataset (which contains 74 hypersaline and thousands of marine samples), as a comprehensive UniFrac similarity search did not yield any significant matches. Finally, we show that the ecosystem entropy formalism, which intrinsically accounts for the ability of OTUs to cross ecosystem borders according to a context database, is a novel, informative tool to describe and identify extreme environments in addition to conventional ecological diversity measures.