The upper beds of Coniacian to early Maastrichtian Fika Shales are exposed within the Nigerian sector of Chad Basin to form the Shoreline environment hosting the mudflat that determines the rate of formation of detrital gypsum forms. The sequence of rocks are; Mudstone on top, Grey Shales at intermediate depth and Blue-Black Shales at deeper levels. Two main types of detrital gypsum forms are recognized based on depth of circulation of ground water and available structures within the Mudstone and Shales; (1) Disaggregated detrital gypsum (euhedral to subhedral hemipyramidal gypsum hosted by Mudstone and Grey Shales) and (2) Aggregated detrital gypsum (euhedral nucleation of vein-like gypsum hosted mainly by weathered Blue-Black Shales). The origin of detrital gypsum forms are inferred to be from the reworking and dissolution of thicker primary gypsum beds at depth due to circulation of meteoric water and groundwater and subsequent redeposition within available dessication and deformation structures in Shales and Mudstone when saturation is attained due to evaporitic pumping.
We recognized four facies association; the first form of Mudstone and gypsum-bearing Mudstone is a typical saline mudflat association. The second consist of disseminations of disaggregated subhedral detrital gypsum form around weathered primary gypsum beds within Mudstone; this is interpreted as sheetflow deposit due to reworking of mudflat facies during periods of heavy rainfall. The third consist of Grey Shales hosting well developed disaggregated euhedral-subhedral pyramidal detrital gypsum formed within the fissile structures; this is interpreted as precipitation of saturated sulfate fluid within closed pockets of fissile structures due to evaporitic pumping. The fourth consist mainly of weathered and deformed Blue-Black Shales hosting discontinuous concordant and discordant vein-like nucleation of the aggregated anhedral detrital gypsum; this is interpreted as nucleation of anhedral detrital gypsum within open fissile structures, karst structures and opened deformational structures due to rapid dissolution of primary gypsum beds and subsequent redepositon as a results of evaporitic pumping. These facies association allow the characterization of the Mudflat facies of the Shoreline environment of the Chad Basin that fall within the Nigeria’s sector of Chad Basin and similar environments in the Nigeria’s sedimentary basins.
Table of Contents
1. INTRODUCTION
2. GEOLOGIC SETTING
2.1 Chad Basin
3. METHODOLOGY
3.1 Field Work
4. Field Studies
5. Detrital gypsum forms
5.1 Disaggregated Forms
5.2 Aggregated Forms
6. SEDIMENTARY MODEL
7. DISCUSSION
8. CONCLUSIONS
9. REFERENCES
Research Objectives and Key Topics
The primary objective of this research is to analyze the formation mechanisms of detrital gypsum within the Nigerian sector of the Chad Basin, specifically focusing on how diagenetic processes, evaporative pumping, and structural deformation in the Fika Shales influence the characteristics and distribution of these gypsum deposits.
- Identification and classification of detrital gypsum forms (disaggregated vs. aggregated).
- Sedimentological characterization of the mudflat shoreline environment in the Fika Shales.
- Evaluation of the genetic relationship between primary gypsum beds and secondary detrital forms.
- Assessment of the impact of evaporative pumping and groundwater circulation on gypsum dissolution and re-precipitation.
- Analysis of the structural controls (fissility, dessication cracks) on gypsum nucleation.
Excerpt from the Book
b. Aggregated Forms;
These gypsum forms are anhedral and small. They are found to nucleate in dessication cracks at shallow mudstone levels and at depth they occupy or crystallized within deformational structures within Grey Shales and Blue-Black Shales to form small horizontal and inclined discontinues vein fillings especially in areas of ground water circulation (Fig. 7). Deformation of Shales at depth could be due to prolonged evaporative pumping or movement of gypsum beds under high overburden pressure or differential offloading of overburden sediments. This deformation enhances the circulation of water which led to rehydration of anhydrite to gypsum and partial dissolution of the anhydrite and gypsum due to the dissolved gases in groundwater (SO2 and CO2) and other organic compounds in the Shales. Sometimes rapid dissolution of thick primary gypsum beds could create gypsum Karst at depth which could be occupied by the aggregated gypsum forms as secondary formations. This depends on the thickness of the primary gypsum bed. This nucleation of anhedral gypsum forms could form beds that are concordant or discordant to the host Shale depending on the orientation of the deformational structure (Fig. 7). These gypsum forms were mostly found surrounding Selenite Gypsum forms within the deformed Shales host. The Selenite Gypsum Forms always have irregular eroded surfaces (Maria et al., 1994; Haruna and Orazulike, 2002). This tiny crystals of detrital gypsum found on the Selenite slide goes into extinction at different times with the actual Selenite crystals as the microscope stage is rotated under cross polarized light.
Summary of Chapters
1. INTRODUCTION: Provides an overview of gypsum reworking processes in evaporite basins and highlights the current scientific focus on detrital gypsum in the Nigerian sector of the Chad Basin.
2. GEOLOGIC SETTING: Describes the stratigraphic and structural characteristics of the Chad Basin, specifically the Fika Shales as the primary host for gypsum mineralization.
3. METHODOLOGY: Details the field work conducted in various gypsum mining sites and the observational approach used to categorize gypsum specimens.
4. Field Studies: Presents the gathered data on the identified mining sites, including gypsum forms, depth of occurrence, and lithological associations.
5. Detrital gypsum forms: Classifies the two primary types of gypsum found (disaggregated and aggregated) and discusses their specific crystal structures and formation environments.
6. SEDIMENTARY MODEL: Proposes a conceptual model for the paleogeographic and hydrologic conditions that favor the formation of gypsum deposits in the mudflat subenvironment.
7. DISCUSSION: Interprets the findings by linking the diagenetic and tectonic history of the region to the observed gypsum morphologies and distributions.
8. CONCLUSIONS: Summarizes the key findings regarding environmental conditions, formation mechanisms, and the influence of evaporative pumping on gypsum deposition.
9. REFERENCES: Lists the academic literature and geological reports consulted for the study.
Keywords
Chad Basin, Fika Shales, Detrital Gypsum, Evaporative Pumping, Mudflat, Diagenesis, Anhydrite, Sedimentary Model, Saline Mudflat, Shoreline Environment, Disaggregated Gypsum, Aggregated Gypsum, Groundwater Circulation, Mineralization.
Frequently Asked Questions
What is the core subject of this research paper?
The paper investigates the origin and formation mechanisms of detrital gypsum found within the Fika Shales in the Nigerian sector of the Chad Basin.
What are the primary themes discussed in the study?
Central themes include sedimentology, evaporite geochemistry, diagenetic processes, and the structural controls provided by the host rocks on gypsum formation.
What is the primary research question?
The study seeks to understand how specific environmental factors, such as evaporative pumping and groundwater circulation, facilitate the reworking and redeposition of gypsum into various forms.
Which scientific methodology was employed?
The authors utilized field observations, site surveys of nine mining locations, and sedimentological analysis to correlate gypsum forms with specific lithological and structural environments.
What does the main body of the work cover?
The main body covers the geologic setting of the Chad Basin, specific field data from mining sites, classification of gypsum morphology, and a proposed sedimentary model for the formation of these deposits.
Which keywords best characterize this work?
Key terms include Chad Basin, Fika Shales, Detrital Gypsum, Evaporative Pumping, Mudflat, and Sedimentary Model.
How does the depth of the burial affect gypsum morphology according to the study?
The study indicates that gypsum forms change vertically with depth, with thickness and deformation of the gypsum forms increasing as the burial depth increases.
What role does evaporative pumping play in this environment?
Evaporative pumping is identified as the primary force for dehydration, which contributes to the shrinkage of mudstones, creates deep-seated fractures, and drives the circulation of water that leads to the re-precipitation of gypsum.
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- Dr. Ahmed Isah Haruna (Autor:in), A. S. Maigari (Autor:in), M. L. Tahir (Autor:in), Y. D. Mamman (Autor:in), R. B. Gusikit (Autor:in), 2007, Detrital Gypsum Forms in the Nigerian (Southern) Sector of Chad Basin: A Criteria for interpretation in Nigeria’s inland basins, München, GRIN Verlag, https://www.hausarbeiten.de/document/202931
