Petrography and geochemistry of turonian eze-aku sandstone Ridges, lower Benue trough, Nigeria implication for provenance and tectonic settings.

Abstract

An integrated petrographic and geochemical study of the Turonian sandstone of Eze-Aku Formation exposed within the southern portion of the Benue trough, was undertaken to infer the sandstone provenance and tectonic setting of the basin. Field observations show that the sandstones, about 30m thick on average, are fairly parallel, linear, northeastsouthwest trending ridges characterized by a coarsening upward sequence. Sandstone facies recognized were bioturbated, cross-stratified and channel sandstones which suggest a shallow shelf environment, generally below wave base. Petrographic studies reveal that these sandstones contain quartz, feldspars, rock fragments and minor amounts of micas. Modal analysis framework grains suggest a subarkosic sandstone. The petrographic characteristics indicate plutonic igneous rocks as the dominant parent rock with minor contribution from metamorphic rocks. Average major elements abundance for this Turonian sandstone are SiO₂, ( 62.7%), TiO₂ (1.23%), Al₂ O₃ (21.99%), Fe₂ O₃ ( 6.46%), MnO (0.02%), MgO (1.42%), CaO (0.63%), Na₂ O (2.12%) and K₂O (1.46%).Tectonic setting discrimination diagrams based on major elements suggest a continental block provenance in a passive continental margin. As indicated by the CIA (chemical index of alteration) and CIW (chemical index of weathering) of the Eze-aku sandstone (average values of 86 and 89 respectively), their source area underwent intense weathering and recycling in a humid climatic condition. The petrography and geochemistry results are consistent with a himid climate and low-relief highlands. Thus suggesting that the sandstone source is probably from Oban and Cameroon massifs which were situated in a humid climatic setting during the Turonian times.

Keywords: Benue Trough; Eze-Aku Formation; Geochemistry; Petrography