Desert Pavements, Vesicular a Horizons and Carbonate Pendants of the Pamir Plateau, Tajikistan

The Pamir Plateau is an arid, high elevation (~4,000 m) mountainous region in eastern Tajikistan. Vesicular A horizons and carbonate pendants form in a variety of geomorphic settings on the Pamir Plateau; desert pavements although less common can be found in incipient form on some fluvial terraces.

Vesicular A horizons and laminated carbonate pendants occur either on sparsely vegetated fluvial terraces, or in nivation depressions. The presence of vesicular material and carbonate pendants in nivation depressions attests to the importance of winter time eolian activity and the abundance of carbonate material in eolian deposits. Carbon isotope ratios (d¹³C_PDB= 0 - +10‰) of both vesicular material and laminated carbonate pendants indicate that plant respired CO₂ is virtually absent in these surficial carbonate deposits. Similarly, carbon isotope ratios of carbonate pendants on fluvial terraces are enriched in ¹³C.

The faulted fluvial terraces at Kara Syy have abundant authigenic carbonate, the uppermost terrace has carbonate pendants throughout the upper 3 m. ¹³C enriched values (d¹³C_PDB= +2 - +7‰) at all depths preclude the presence of significant plant respired CO₂. A more robust relationship between depth and oxygen isotope ratios (d¹⁸O_PDB= -9 - +1‰) indicates that soil water evaporation is high and likely an important factor in formation of authigenic carbonate. Vesicular A (Av) horizons at Kara Syy are highly calcareous and commonly indurated to the point of being lithified. Lithified Av clasts can be found as part of incipient desert pavements overlying younger Av horizons. The lithification of Av horizons is probably a function of both extreme evaporation of soil water and high carbonate content present in both fluvial gravels and eolian material. Age estimates of the Kara Syy terraces based on formation rates of pedogenic carbonate may overestimate the ages of the terraces, and thereby underestimate the rate of fault slip.