Sedimentary Petrography and Geo-mechanical Assessment of some Cretaceous Sandstone in Parts of the Southern Benue Trough, South-Eastern Nigeria
Abstract
An assessment of the petrographicand geo-mechanical properties of someexposed Cretaceous sandstones in parts ofsouth-eastern Nigeria was carried out todetermine the relationship between these twointrinsic properties of the sandstones.Routinely, petrographic analyses of rocks areoften carried out as an easy procedure usedto assess the mineralogical composition andtherefore mechanical quality of rocks forconstruction purposes. Calcareoussandstones from four localities (Ngbo,Akpegu, Ezillo, and Abini) were tested fortheir petrographic, aggregate degradation(AD), and mechanical properties. Theaggregate degradation was determined usingthe Los Angeles abrasion value (LAAV),aggregate impact value (AIV), and aggregatecrushing value (ACV) tests.Petrographicanalysis revealed two end-member texturalcharacteristics for the sandstones: grainsupported and matrix-supported, and theyarecomposed of quartz grains, feldspar(plagioclase), fossil fragments andmicrocrystalline calcite (micrite). The rocks'ACVs and AIVs ranged from 16.34 to 26.29% and 15.87 to 22.52 %, respectively, whilethe LAAVs varied from 38.62 to 53.1 %. Theslake durability index is within the range of91.1 to 96.7 % and the strength values of thestudied rocks fall within the moderatelystrong category (25-50 MPa) except for theEzillo and Abini sandstones (<25 MPa).Thestatistical relationship using a regression linebetween the mineralogical components andmechanical properties shows that the quartzcontent, micrite, and micrite plus fossilfragments have a significant correlation withstrength, LAAV, ACV, and the slake durabilityindex. The strong correlation betweenaggregate crushing value and micrite contentshows that the proportion of micrite observedin a rock is of great importance, especiallywhen subjected to applications wherecrushing forces is significant.It was alsoobserved that the grain-supported samplesexhibited better mechanical properties whencompared with those that were matrixsupported.