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.

Extracto

ABSRACT

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.

INTRODUCTION

The reworking and redeposition of Gypsum has been recognized as significant processes in many evaporite basins Maria et al, (1994). Most of the literatures on these subject matter deals with ancient marine evaporite from Messinian to Mediterranean region (Hardie and Eugster, 1971; Parea and Ricci-Luchi, 1972; Schreiber et al., 1976; Schlager and Bolz, 1977; Vai and Ricci-Luchi, 1977; Rouchy, 1982; Aigner and Bachmann, 1989). There are few detailed work on detrital gypsum in Continental Lacustrine settings (e.g. Chad Basin), although some sedimentological work has been carried out in Chad Basin by Vischer, 1910; Raeburn and Brynmor, 1934; Carter et al. 1963; Barber, 1965; Petters, 1983; Ekweozor and Mukhopadhyay, 1989; Matheis, 1989. The earliest work on evaporite in the north eastern region was done by Vischer (1910) in a geographical account of an early expedition into the areas of the Chad Basin and immediate environments. Carter et al., (1963) reported gypsum occurrence within a sequence of blues black shales, containing few, thin, lava flows. Reyment (1965) confirmed this by reporting the Fika formation as consisting of blue – black shales, occasionally gypsiferous with a thickness exceeding 430 meters. Maglione (1981) also confirmed the present of gypsum mineralization in well drained, well aerated environments within the Chad Basin part of which is the research area.

Recent detailed work on gypsum was carried out by Haruna, 1998; Haruna and Orazulike, 2002 reported the occurrence of five gypsum Forms (Detrital, Balatino-Laminated, Selenite, SatinSpar and Alabaster gypsum forms) in the Fika Shales within the Nigerian sector of Chad Basin. Jonathan (2002) also reported similar gypsum forms from the Fika member around Nafada at the flank of Dumbulwa-Bage high. Most of these works were on economic geology of the gypsum and so the detrital gypsum form only received the benefit of mention and not detailed treatment. Work on detrital gypsum has not received much attention in recent times in the Nigeria’s sedimentary basins. In this paper, we attempted to show how evaporative pull, diagenetic reactions and inversion of meteoric water and also the continuous circulation of ground water led to reworking of the gypsum beds by enhancing the dissolution of gypsum/anhydrite at different depths and subsequent recrystallization of detrital gypsum of different structures within the unique fissile structures and the diagenetic deformational structures of the gypsiferous shales.

GEOLOGIC SETTING

Chad Basin

The Chad Basin is the largest intracratonic area of inland drainage basin in Africa (Raeburn and Brynmor, 1934; Barber, 1965) and occupies an area of about 230,000km2 in the Central Sahara and Southern Sudan (Fig. 1). The eastern and northern boundaries lie partly in the Central Sudan and partly in the Sahara respectively. The Western limit is the water divide which divides the Niger and the Chad Drainage system and the southern limit is the water-shed between the Chad and Benue system (Fig. 2). About one tenth of the basin is situated in the northern part of Nigeria. Within Nigeria sector, the altitude of the basin falls from about 530m at the western margin to about 300m within the lake along a distance of about 240km (Matheis 1989).

The Chad Basin is endoreic (i.e. it does not drain to the outside) (Fig. 2). The Chad basin is separated from Upper Benue by a basement dome (Dumbulwa- Bage high; Zaborski et al, (1997) and Zambuk Ridge Carter et al., 1963; Obaje 2004) (Fig.3). The Chad basin contains Albian-recent stratigraphic sequences among which are the Fika Shales that host gypsum mineralization.