Yossef H Hatzor

Distinguishing between chalk and shale remains a fundamental challenge in engineering geology. Traditional sedimentological classifications rely on visual or genetic criteria that fail to predict mechanical behavior, creating ambiguity across transitional lithologies such as “marly chalks” and “chalky marls.” To bridge descriptive geology and functional rock mechanics, this study introduces a novel, quantitative framework that redefines the chalk–shale transition based on geomechanical performance rather than arbitrary mineralogical limits. Integrating mineralogical, petrophysical, and mechanical data from the Ghareb Formation (Israel), we identify the clay/carbonate ratio (R=0.2−0.5) as a critical geomechanical boundary. Across this zone, material behavior shifts from a stiff, carbonate-bound matrix to a moisture-sensitive, clay-dominated domain characterized by distinct strength (UCS) degradation, permeability variations, and laboratory testing and preparation challenges. By grounding classification in objective mechanical behavior, this framework unifies the language and nomenclature for formation evaluation across disparate investigation scopes from energy exploration to civil infrastructure. This eliminates scope-dependent mischaracterizations, enhances geotechnical design assumptions, improves laboratory testing protocols, and standardizes site characterization globally.