Formation and evolution of grus deposits in the Çine Submassif, Southwest Türkiye: A multi-proxy approach

Abstract

Grus deposits may be a key element in deciphering the evolution of granitic landforms due to their rapid formation, sensitivity to environmental factors, and influence on landscape morphology. This study explores the mineralogical, geochemical and structural characteristics of grus deposits developed on granitoid bedrock through subsurface weathering in a semiarid region, the Çine submassif (SW Türkiye), focusing on how topography, mineral composition and climate affect their formation and spatial variability. Two distinct grus types were identified: thinner (0–20 cm), homogenous, sand-rich deposits on upper slopes, and thicker (40–100 cm), heterogenous, gravel-rich layers in the valley and downslope near bedrock outcrops. The bimodal grain-size distributions among grus types may reflect progressive weathering along topographic gradients, where upper slopes experience limited weathering due to reduced water retention and steeper gradients, leading to the formation of thinner, sand-rich deposits. In contrast, downslopes and valleys allow for prolonged water infiltration and feldspar breakdown, resulting in thicker, gravel-rich deposits with a more advanced weathering profile. Grus consists primarily of quartz, albite, orthoclase and mica, with accessory clay minerals such as kaolinite. Geochemical data indicate a felsic grus composition with limited variations, high Chemical Index of Alteration (CIA) values (59 to 69 %), and a gradual weathering trend towards kaolinite, driven by feldspar weathering and fluctuating water infiltration along slopes.

The Principal Component Analysis (PCA) highlights chemical composition and grain size as key parameters differentiating grus types. The interplay of topography, water dynamics, and local structural and mineralogical characteristics controls both the formation and weathering intensity of grus, with variations in deposit characteristics reflecting the influence of slope position and water infiltration on weathering process. These findings provide new insights into granitoid landform evolution, while further geochronological studies are recommended to clarify the temporal evolution of grus mantles and subsurface weathering features across the region.