Multi-spectral facies in prevalent carbonate strata of an area of Migiurtinia (northern Somalia): analysis and interpretation

Abstract

Rock expression in a multi-spectral image is referred to as multi-spectral image facies. Thematic Mapper (TM) multi-spectral facies of sedimentary rock bodies outcropping in an overall arid area of northern Somalia were interpreted by: (1) comparison with laboratory reflectance spectra of weathered surfaces of rock samples from the study area; (2) considering the correlation between spectra of weathered and unweathered surfaces; and (3) petrographic analysis. Aerial photo analysis provided stratigraphic and morphological information. The rocks studied comprise mainly carbonate and evaporitic strata. TM facies appeared to be related to rock composition, presence and nature of weathering processes, scale of the lithological variability, stratal pattern in the erosional profile and outcrop morphology. Specific weathering products are, in their turn, apparently related to properties of the host rocks, such as composition and texture. Six prominent 741 (red, green, blue) facies were identified: a red facies, diagnostic of lichen coating, masking carbonate absorptions in TM7; a yellow-green laterally persistent couplet, unchecked in the field; a banded brown and yellow facies, expression of carbonate shelf strata; a purplish-red facies indicating a silcritic duricrust; a yellow facies related to fine crystalline dolostones; a cyan facies due to strong absorption in band 7, diagnostic of hydrous sulphate minerals. The comparison with a classification of the rocks based on reflectance characteristics of unweathered samples facilitated the interpretation of the remote responses and the correlation with lithostratigraphic units previously recognized in the area by different authors.