PALEOECOLOGY: The presence of ammonites and pelecypods such as inoceramus and presence of glauconites point towards shallow marine environment for the carbonate rocks in the study area.
The limestones of the study area were deposited in an environment of alternating moderate high and low energies. Evidence for this is the presence of biosparite, biomicrites and micrites. Biosparite indicates deposition under vigorous current action where microcrystalline material has been washed away (high energy) while biomicrites and micrite indicates deposition in a low energy environment where current were calm and the microcrystalline oozes did get winnowed out from the shell material. Weller (1960) suggests that high energy result from wave turbulence which keeps the fine sediments in suspension and allows deposition of coarser particles while low energy results from slack in turbulence and eventual settlement of finer clay size particles. This seems to be the case for limestone of the study area.
Folk (1974) attributed the badly broken shells of biomicrite and micrite to the activities of predation organisms which crunch up the shells in search of food and in some cases passes debris through their digestive tracts. This may be the case for biomicrites and micrites beds while fragmentation of shells in biosparites is a result of current action.
DEPOSITIONAL MODEL FOR UNIT A
Limestones of the study area were deposited in an environment where short periods of high energy sedimentation alternates with moderate to low energy sedimentation.
Evidence for high energy phenomena includes massive beds and fragmentation of fossil shell while that of low sedimentation is the occurrence of biomicrite and micrites with thick shelled bivalves (indicative of shallow water origin). The presence of fragments of shells indicates long transport, activities of predation organism and current actions. This probably supports allochtonous origin of the limestone (Umeji, 1985)
PROPOSED ORIGIN OF CARBONATE CONSISTUENTS
The source of the carbonate materials is attributed to biogenic, biochemical and pure chemical factor (Greensmith, 1971)
Considering the amount of skeletal remains and extent of fragmentation, it is not possible that only biogenic aspects of carbonate can supply the amount of carbonate material in the study area.
However, physiochemical factors contributed to considerable changes in the chemical state of the sea water that caused precipitation of carbonates (Alexanderson, 1994).
The evidence that accounted for this is the calcite cement precipitated directly from water.
The abundance of skeletal remains and calcite cement in the limestone units is good evidence that enough biochemical activities existed in deposition of carbonate constituents. However, biogenic and biochemical factors may have co-existed with the biochemical factors in the deposition of the carbonate.
CORRELATION WITH A NAMED FORMATION ON PALEOECOLOGY
The lithologic characteristics, structures and fossil content of this unit make it correlable with the Eze-Aku formation. (Reyment, 1965; Umeji, 1984).
UNIT B. NKALAGU SHALE
This unit is stratigraphically the youngest in the study area. It outcrops as interbeds with the fossiliferous limestone in Nigercem quarry pit of the map area.
The shale beds are gray to dark in colour and vary between 1-5m thickness. It is laminated, fissile and micaceous.
The shale beds generally strike at N40E and dips at 12 NW direction.
Samples of the shale were collected and taken to the laboratory for paleontological analysis using paleontological microscope.
After the paleontological analysis of the shale, it was found that the shale contains the remains of planktonic forams, inoceramic and ammonites. See plate 3.0.
Fayose and de-klasz, 1976; Umeji 1984, noted that impression of ammonites and planktonic forams are locally abundant in the shale of Eze-aku formation.
DEPOSITIONAL MODEL FOR UNIT B.
The dark colour of the shale facies suggests sedimentation in water of poor oxygenation, possibly a restricted, stagnated lagoon or deep offshore marine environment.
The lamination of shale beds probably indicates deposition from suspension in a low energy environment.
The lining and thinning upward of both the shale and limestone interbeds shows deposition in environment with progressively decreasing energy. The presence of a glauconite suggests relatively deep marine environments.
Fayose and de-klasz, (1976) suggest that the predominance of planktonic forams over benthonic ones may support open marine setting.
CORRELATION WITH A NAMED FORMATION.
The lithologic characteristics of this unit also make it correlable with the Eze-Aku shale (Reyment, 1965)