The Abakaliki Volcaniclastic Rocks: Field relations from resistivity profiles and mapping
The Abakaliki pyroclastics are the best known volcaniclastic rocks in Nigeria. Resistivity profiling across Echara Unuhu and Amike Aba pyroclastic bodies and geological field descriptions of the rock reveal a relationship between apparent resistivity and the different lithotypes within the pyroclastic mass. Apparent resistivity of 50-210ohm-m indicates a group made up of lava flows. Field geological descriptions shows that these rocks are pumices interstratified with thin dark grey to bluish shales, amygdoloidal lapilli basalts and pillow basalts. Apparent resistivity values of 240-275 ohm m suggests the presence of coarse grained agglomerates, breccias and microbrecias with angular to subangular xenoliths of shally country rocks embedded in a groundmass of plagioclase and pyroxene. Quartz veins and enclaves as well as large leisegang rings (concretions) are associated with this lithotypes. Apparent resistivities of 275-434 ohm-m characterize the very fine grained lithic/welded tuff which is the most resistive litho type. The intimate interbedding of the pyroclastic flows with Albian Abakaliki shales as well as the presence of the shale xenoliths in the rocks indicates a contemporaneous age relationship. An Albian age is therefore suggested for the Abakaliki volcaniclastic events.
Key Words: Pyroclastic, Resistivity Profiling, Geology, Lithotype.