Depositional environment and carbon/oxygen isotopic composition of the Paleocene carbonates exposed around Kalambaina, Sokoto basin, Nigeria
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Date
2013
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Advance Journal of Physical Sciences
Abstract
The limestone-shale sequence exposed at the Kalambaina quarry belongs to the Paleocene marine sediments deposited in the Sokoto Basin during the second phase of sediment deposition when the Tethys Sea moved southwards through the Sahara. The exposed section measured ca.19.5m in thickness. The lower part of the section consists of limestone-shale interbeds while the upper part is phosphatised shale. The limestone is clayey, consisting of marl and shale components. The beds are generally massive to wavy laminated. Petrographic study reveal that the limestones are mud supported and consist of two main carbonate rock types including bioclastic wackestone and bioclastic packstone. Larger foraminifera, corals, bryozoans, echinoderms, algae and gastropods represent the major bioclasts while lithoslasts are the main non-bioclastic grains of the lithofacies. The associated shales consist of many detrital materials, phosphorites, gypsum and carbonaceous organic matter. Carbon/oxygen isotopic composition vary from –5.29%o to –6.55%o 18O PDB and –1.24%o to –3.40%o PDB 13C in the whole rock compare to the average values of –2.33 18O and +0.05 13C PDB for Tertiary Sea water. The lighter oxygen and carbon fractions in the samples is a reflection of the depletion of heavier isotope of oxygen and carbon consequent to late stage recrystallisation, influx of fresh water and formation of variable carbonate cements during progressive diagenesis. Estimated formational paleo temperatures for the kalambaina limestone vary from 26.80 to 32.85C. The study suggests that carbonates of the Kalambaina formation were deposited in shallow marine, shelf lagoonal setting and have undergone several stages of post depositional changes. The late stage cements have reduced the interparticles and moldic porosities of the carbonate
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Keywords
Kalambina, carbonate, sediments