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The fact that the former highstand whitish sediments of Plio-Pleistocene and lowstand off-white/red sediments of Late Pleistocene containing low As concentrations in the Bengal basin forms the premise for developing a color tool by the ‘Sustainable Arsenic Mitigation Project’ (SASMIT) to guide the local drillers of Matlab, SE Bangladesh, so that they can target low As aquifers. Groundwater samples collected from the black group of sediments representing a highly reducing environment contains very high concentrations of As, Fe2+ , NH4+, PO43-, HCO3–, and DOC and very low SO42- and Mn2+ concentrations; and vice-versa for the red/offwhite/white group of sediments, thus representing a less reducing condition. A very strong to good correlation between DOC with HCO3–, Astot, Fetot, NH4+, and PO43- and Astot with HCO3–, Fetot, and NH4+ within the black group of samples indicate that, oxidation of organic matter is driving all the redox reactions and thereby releasing As in solution. On the contrary, in the case of red/off-white groups of samples, all the aforementioned correlations are poor to negative, reflecting that water samples collected from these sediments are low in organic content, presumably due to weathering of these sediments during the last glacial lowstand and subsequent flushing. Furthermore, negative correlation between Astot with Mn2+ and SO42- in all groups of samples indicating that mobilization of As is neither related to dissolution of Mn-oxyhydroxides, nor to oxidation of sulfide minerals in the study site. Speciation modeling results show that water samples derived from the back sediments are supersaturated with respect to siderite and vivianite, while near-equilibrium for white and under-saturated for off-white/red groups of samples. Though all the four groups of samples are slightly under-saturated with respect to the mineral phase rhodochrosite, groundwater is supersaturated with respect to hydroxiapatite and MnHPO4. Three aquifers (Af1, Af2, Af3) intervened by two aquitards (At2, At3) have been identified in the study area. The shallow aquifer (Af1) extending up to a depth of about 40 to 50 meters b.g.l. consisting of black sediments is characterized by mainly Ca-Mg-HCO3 water-type and water-level fluctuation of about 3 – 4 meters. The intermediate aquifer (Af2) consisting of red/off-white/white sediments underlying At2 is characterized by primarily Na–Ca–Cl water-type and groundwater fluctuation of about ≈ 5 m, indicating water from this aquifer is used for irrigation also. Since well installation in the low As deep aquifer (Af3) consisted of mainly white sediments is costly, the intermediate aquifer (Af2) is the best option to be explored by the local drillers. The occurrence of the red/off-white sediments are primarily limited to the SE half of the study area because of low preservation potential of the red/off-white LST (Lowstand Systems Tracts) deposits, may be due to raivement erosion and aggradation of TST (Transgressive Systems Tracts) and recent HST (Highstand Systems Tracts) deposits. Depth-specific spatial distribution of lithofacies indicates that installation of wells at a depth of about 70 m b.g.l. anywhere in the SE half of the study area would most likely yield low As groundwater.

Geomorphic features have been one of the major tools for sea-level change studies. The present work shows an example of sea-level change study on karst terrain in the tropics. Sea-level notches as well as flank margin caves were identified in the research area and their elevation measured. The time of formation of the sea-level indicators was constrained by lithology study and dating methods such as facies comparison and U-Th dating. Denudation and uplift were also studied for the same purpose. From this study it can be concluded that sea-level stands within the glacial cycle can cause the formation of flank margin caves and that the position of these sea-level stands can be determined. The research area was estimated to have cumulatively uplifted ~22 m in the past 125 ka years (~0.18 mm/yr) while the surface has been denuded some 8 m in the same span of time (~0.064 mm/yr).

The Sulaiy Formation, which is the oldest unit in the Lower Cretaceous succession, is conformably overlain by the Yamama Formation and it is a challenge to identify the precise age of the two formations using foraminifera and other microfossil assemblages. In the eastern side of Saudi Arabia, the Sulaiy Formation and the base of Yamama Formation are poorly studied. The main objectives of this study is to enhance the understanding of the Sulaiy Formation sequence stratigraphical correlation, regional lateral variations and palaeoenvironmental investigation. Lithological and semi-quantitative micropalaeontological analysis of 1277 thin sections taken from core samples from nine cored wells providing a geographically representative distribution from the Saudi Arabian Gulf. These cores intersected the base of the Yamama Formation and the Sulaiy Formation in the total thickness of cored wells of 843.23 meters (2766.5 feet). On the evidence provided by the foraminifera, the Sulaiy Formation is considered to represent the Berriasian to the lowermost Valanginian. The investigation of the micropalaeontology has provided considerable insights into the biocomponents of Sulaiy and the base of Yamama formations in order to identify their biofacies. These microfossils include rotalid foraminifera, miliolid foraminifera, agglutinated foraminifera, calcareous algae, calcispheres, stromatoporoids, sponge spicules, problematica (e.g. Lithocodium aggregatum), molluscs, corals, echinoderms and ostracods. Systematics of planktic and benthic foraminifera is accomplished using the foraminiferal classification by Loeblich and Tappan (1988) as the main source. The assemblage contains foraminifera that recorded for the first time in the Sulaiy Formation. Other microfossils were identified and recorded to help in the identification of the sedimentary environments. The investigation of the micropalaeontology and the lithofacies analysis have provided evidence the identification of the various lithofacies. About twenty four microfacies were identified on the basis of their bio−component and non-skeletal grains. The lithofacies and the bio−component results have provided the evidence of the sedimentary palaeoenvironmental model namely the Arabian Rimmed Carbonate Platform. This palaeoenvironmental depositional model is characterised by two different platform regimes. They are the Platform Interior and the Platform Exterior each of which have unique sedimentary lithofacies zones that produce different types of lithofacies. Each lithofacies is characterised by special depositional conditions and palaeobathymetry that interact with sea level changes and the accommodation space. The important palaeoenvironments are intertidal, restricted lagoon (subtidal), open marine, deeper open marine, inner shoal, shoal and platform margin. Generating, and testing, a depositional model as a part of formulating a sequence stratigraphical interpretation of a region is a key to understanding its geological development and – ultimately – reservoir potential. The micropalaeontology and sedimentology of the Sulaiy Formation in the subsurface have indicated a succession of clearly defined shallowing−upwards depositional cycles. These typically commence with a deep marine biofacies with wackestones and packstones, capped with a mudstone-wackestone maximum flooding zone and an upper unit of packstone to grainstones containing shallow marine biofacies. The upper part of the Sulaiy Formation is highstand-dominated with common grainstones that host the Lower Ratawi reservoir which is capped by karst that defines the sequence boundary. This karst is identified by its abundant moldic porosity that enhanced the the reservoir quality by increasing its porosities into greater values. Integration of the sedimentology and micropalaeontology has yielded a succession of shoaling−upwards depositional cycles, considered to be 4th order sequences, that are superimposed on a large scale 3rd order system tract shallowing−upwards, highstand-associated sequence of the Sulaiy Formation. The Lower Ratawi Reservoir is located within the latest high-stand portion of a third-order Sulaiy Formation sequence. The reservoir consists of a succession of several sequences, each of which is sub-divided into a lower transgressive systems tract separated from the upper highstand systems tract by a maximum flooding surface (MFS/Z). The last of these depositional cycles terminates in beds of porous and permeable ooid, or ooidal-peloidal, grainstone. The reservoir is sealed by the finer-grained sediments of the Yamama Formation.