Stratigraphic correlation and provenance study of exposed Eocene - Oligocene sedimentary sequences in southern Nigeria using high-resolution heavy minerals and garnet geochemical analyses

Abstract

Provenance and stratigraphic correlations across the exposed Eocene-Miocene sedimentary sequences of southern Nigeria that exhibit similar lithologies and are biostratigraphically-barren have been carried out. Heavy mineral assemblages in the 63–250 μm sand fractions of 43 sediment samples across the sedimentary sequences were assessed via wet-sieving, centrifugation, and polarizing petrologic microscopic analyses. Opaque, apatite, garnet, kyanite, zircon, rutile, staurolite, tourmaline, and epidote heavy mineral species were identified and point-counted. Electron microprobe analysis was carried out on the garnet-heavy mineral species using a CAMEBAX Cameca electron microprobe with a link four-wavelength energy-dispersive mode spectrometer analyser. This study revealed the existence of three heavy mineral associations and three lithostratigraphic formations. Heavy mineral association A is characterized by assemblages of kyanite, zircon, and rutile grains. The occurrence of kyanite in this association indicates derivation from schist and gneissic rocks. Zircon grains indicate sialic to intermediate igneous rocks derivatives that has sustained several periods of recycling, and redeposition. The occurrence of rutile indicates metamafic and metapelitic rocks with other sources being minor. Heavy mineral association B is characterized by assemblages of staurolite, opaque materials, garnet, and epidote. The presence of opaque grains indicates a shorter distance of transport possibly from proximity. The occurrence of staurolite grains indicates metamorphic and magmatic rock derivatives from schists, slates, and gneisses. Garnets are predominantly derived from high-grade metamorphic rock (granulite-facies) metasediments and charnockites and lower grade (amphibolite-facies) metasedimentary rocks and granitoids. An epidote is a metamorphosed igneous rock derived from the adjoining radix. Heavy mineral association C is characterized by assemblages of tourmaline and apatite. Tourmaline grains indicate pneumatolytic rocks, pegmatite, schist, gneisses, and marble source rocks. Apatite indicates derivation from local outcrops exposed during the Benue Trough uplift. The occurrence of apatite and its assemblages with tourmaline indicate felsic igneous rocks with ultramafic compositions, metamorphic rocks with low to high grades and derivatives of diverse protolith compositions. The exposed Eocene-Miocene sedimentary deposits that exhibit similar lithologies and biodata are relatively lacking. They were differentiated into three different lithostratigraphic formations and correlated. These analyses were performed using heavy mineral characterization, optical property, garnet geochemical composition, and inference supported by multivariate statistical analyses. Variations in the numerical values of unstable heavy minerals reveal dissolution and overgrowth through acidic groundwater, changes in climatic conditions, and relative eustatic sea-level changes. This study was able to derive information and demonstrate, differentiate, and validated the significance of resolving provenance and stratigraphic uncertainties using a heavy mineral and garnet geochemical-based approach. These findings were supported by multivariate statistical analyses. This methodology will help to facilitate the prediction and correlation of hydrocarbon reservoirs and source rocks in sedimentary basins globally.