The evolution of holocene reg (gravelly) soils in deserts: An example from the dead sea region

Abstract

Reg soil is an aridisol developed upon a stable alluvial surface composed of sand to coarse gravel. The objectives of the present study were: (1) to examine the initial evolution of Reg soil properties with time; (2) to assess and explain the rates of development of different properties; (3) to establish the suitability of different soil properties for relative-age dating. The time-dependent soil properties examined were: (1) Mechanical weathering of surficial rocks and desert pavement evolution; (2) Chemical and biological weathering of surficial rocks; (3) The thickness of soil profiles and soil horizons; (4) Soil texture in the different horizons; (5) Salts and gypsum distribution and concentration in different soil horizons; (6) Degree of mechanical weathering of gravel in C horizons. Fifteen successive surfaces of the Nahal (= wadi) Ze'elim sequence of alluvial fans were selected for study (Dead Sea region, Israel, under an extremely arid climate). The sequence is of Holocene age — 14,000 years B.P. to recent. The main conclusions are: 1. Reg soils develop rather fast in several 10 3 years since their initiation. However, the various soil properties developed at different rates. 2. The rates of change of three soil properties decrease with time. These are the texture in different horizons (change in added airborne and rain washed dust), salinity within the soil profile (change in added salts and gypsum) and the amount of mechanically shattered rocks. 3. Soil thickness is established rather rapidly. Young and older Holocene soil have practically the same depth. Surficial weathering rates appear to remain fairly constant with time. 4. In coarse gravel the evolution of Reg soils is dependent on added fines and salts of airborne origin and the resultant changing hydrologic regime in the soil profile. The run-off/infiltration ratio changes as more fines and salts are added to the soil profile and as desert pavement develops. 5. Surficial features and fine clay in the soil profile may be used for approximate dating of otherwise undatable Holocene alluvial surfaces. Salts and gypsum content should be cautiously used in a rather general way.