MULTISPECTRAL FACIES IN PREVALENT CARBONATE STRATA OF AN AREA OF MIGIURTINIA (NORTHERN SOMALIA) - ANALYSIS AND INTERPRETATION

Rock expression in a multi-spectral image is referred to as multispect ral image facies. Thematic Mapper (TM) multi-spectral facies of sedime ntary rock bodies outcropping in an overall arid area of northern Soma lia were interpreted by: (1) comparison with laboratory reflectance sp ectra of weathered surfaces of rock samples from the study area; (2) c onsidering the correlation between spectra of weathered and unweathere d surfaces; and (3) petrographic analysis. Aerial photo analysis provi ded stratigraphic and morphological information. The rocks studied com prise mainly carbonate and evaporitic strata. TM facies appeared to be related to rock composition, presence and nature of weathering proces ses, scale of the lithological variability, stratal pattern in the ero sional profile and outcrop morphology. Specific weathering products ar e, 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, ma sking carbonate absorptions in TM7; a yellow-green laterally persisten t coupler, unchecked in the field; a banded brown and yellow facies, e xpression of carbonate shelf strata; a purplish-red facies indicating a silcritic duricrust; a yellow facies related to fine crystalline dol ostones; 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 correla tion with lithostratigraphic units previously recognized in the area b y different authors.