标题：Facies and facies architecture and depositional environments of the cretaceous Yolde Formation in the Gongola basin of the upper Benue trough, northeastern Nigeria
摘要：Depositional environments of the Yolde Formation were studied based on the analysis of facies and facies architecture. Five sections within the Gongola Basin were studied and ten lithofocies were recognized based onlithology and sedimentary structures. The sandstone ranges from quartzarenite to subarkose and well to moderately sorted. They comprise of very fine, fine, medium and coarse-grained sandstones. Locally, there is occurrence of thin beds of limestone at the upper most part of the Yolde Formation. The facies architecture at the lower part of this formation is defined by fining upwards cycles interpreted as sequences of fluvial deposits formed from braided river system which consist of succession of massive beds of sandstones fining upwards to thin claystone. The faciesarchitecture in the upper part is defined by:(i) fining upwards cycle of sandstones to claystone interpreted as delta plain distributary channel-overbank facies;(ii) coarsening upwards cycles of mudstones-sandstones interpreted as creavasse-splay deposits in an interdistributary bay fills and;(iii) coarsening upwards cycles of claystones-sandstones or interbedded sandstone and claystones-sandstones are interpreted as delta front sandstones on pro-delta claystones or delta front sandstones on delta slope sandstone and claystones respectively. The dominance of riversandstones over coastal sandstone from bivariate plot relationships and two-sand population plots for the Yolde Formation suggest the upper part of this formation may have formed in a fluvial-dominated delta.
