Abstract
Field observations at freshly exposed quarry sites, grain size analysis, mass spectrometry and statistical methods have been used to determine the depositional setting of sandstones from the Oligocene-Miocene Ogwashi-Asaba Formation. Some of the sandstones are ferruginized and indurated; others are friable, cross-bedded, non-ferruginized, white to yellow to reddish brown on weathering, and show top/bottom mantled pebbles. Textural examination indicates that the sandstones range from fine to coarse sands, with graphic mean grain size of -0.3 to 2.13ɸ. Standard deviation ranges from 0.62 to 1.52ɸ and implies moderately sorted to poorly sorted sediments. Textural indices suggest that most of the sediments were deposited under fluvio-deltaic to shallow marine environment by fluvial and beach processes adjacent to a near shore whirlpool agitating turbidity conditions. Major elemental oxides show SiO2 content > 71% with extreme depletion of mobile oxides such as Na2O, CaO and the ferromagnesian minerals through weathering and sedimentary processes.
Highlights
Sandy sedimentary successions are invaluable parts of sedimentary basins earth wide and, these sequences constitute potential aquifers and petroleum reservoirs
The sedimentary deposits of the Niger Delta Basin are well known for their rich aquifers and petroleum potential
Previous work on the Ogwashi-Asaba Formation include a localized hydrogeological assessment of its shallow aquifer horizon [1], the geochemical assessment of the lignites that occur in it by Ogala [2] and Bassey and Eminue [3] who use evidence from petrography, geochemical and sedimentological data obtained from a range of sedimentary rock types in the formation to infer that the sandstones of the formation are texturally, chemically and mineralogically immature
Summary
Sandy sedimentary successions are invaluable parts of sedimentary basins earth wide and, these sequences constitute potential aquifers and petroleum reservoirs. Previous work on the Ogwashi-Asaba Formation include a localized hydrogeological assessment of its shallow aquifer horizon [1], the geochemical assessment of the lignites that occur in it by Ogala [2] and Bassey and Eminue [3] who use evidence from petrography, geochemical and sedimentological data obtained from a range of sedimentary rock types in the formation to infer that the sandstones of the formation are texturally, chemically and mineralogically immature The objective of this present work is to attempt to infer depositional settings of the formation using sedimentological and geochemical attributes of the sandstone lithologic unit. These attributes include various textural characteristics and analyses and interpretation of related statistical indices
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