Sedimentary petrography and geo-mechanical assessment of some Cretaceous sandstone in parts of Southern Benue Trough, south-eastern Nigeria

Abstract

Abstract: An assessment of the petrographic and geo-mechanical properties of some exposed Cretaceous sandstones in parts of south-eastern Nigeria was carried out to determine the relationship between these two intrinsic properties of the sandstones. Routinely, petrographic analyses of rocks are often carried out as an easy procedure used to assess the mineralogical composition and therefore mechanical quality of rocks for construction purposes. Calcareous sandstones from four localities (Ngbo, Akpegu, Ezillo, and Abini) were tested for their petrographic, aggregate degradation (AD), and mechanical properties. The aggregate degradation was determined using the Los Angeles abrasion value (LAAV), aggregate impact value (AIV), and aggregate crushing value (ACV) tests.Petrographic analysis revealed two end-member textural characteristics for the sandstones: grain-supported and matrix- supported, and theyare composed of quartz grains, feldspar (plagioclase), fossil fragments and microcrystalline calcite (micrite). The rocks' ACVs and AIVs ranged from 16.34 to 26.29 % and 15.87 to 22.52 %, respectively, while the LAAVs varied from 38.62 to 53.1 %. The slake durability index is within the range of 91.1 to 96.7 % and the strength values of the studied rocks fall within the moderately strong category (25-50 MPa) except for the Ezillo and Abini sandstones (<25 MPa).The statistical relationship using a regression line between the mineralogical components and mechanical properties shows that the quartz content, micrite, and micrite plus fossil fragments have a significant correlation with strength, LAAV, ACV, and the slake durability index. The strong correlation between aggregate crushing value and micrite content shows that the proportion of micrite observed in a rock is of great importance, especially when subjected to applications where crushing forces is significant.It was also observed that the grain-supported samples exhibited better mechanical properties when compared with those that were matrix-supported.