STRUCTURAL CONTROL OF LOCATION AND WATER CHEMISTY OF LAKES AND SPRINGS IN THE OLKHON REGION

Abstract

The location and water chemistry in lakes and springs in the Olkhon region are structurally controlled by Cenozoic faults of two types: i) inherited NE faults that follow the Early Paleozoic syncollisional (late stage) dextral strike-slip faults; ii) NEN pull-apart faults produced by sinistral strike-slip movements at the early stage of Baikal rifting. Water in springs has a generally low salinity (0.2-0.5 g/l) and compositions with invariable cations (Ca-Mg) and variable anions (from HCO3 to SO4 -HCO3). The lakes we studied are fresh-water (up to :. g/l), brackish (1-10 g/1), and saline (14-45 g/1). Fresh-water lakes have mostly Mg-Ca-Na or Mg-Na-Ca bicarbonate chemistry, the composition of brackish lakes is either HCO3-SO4 Na-Mg (Mg-Na) or SO4-HCO3-Cl, Na-Mg (Mg-Na), and that of saline lakes is dominated by SO4 Cl, Mg, and Na. The composition of limnic water is controlled by the chemistry of feeder groundwater and by the grade of water metamorphism. Springs and lakes of different salinities show regular association with the two types of faults: Low-mineralized springs and fresh-water lakes are located within inherited faults, whereas most of brackish and saline lakes are attributed to pull aparts. There are two possible explanations for this regularity. (1) The water chemistry of springs and fresh-water lakes in inherited faults is closer to the meteoric water as the depth of their feeder channels is constrained by complex thrust and strike-slip geometry of older faults with shallow-dipping planes. Pull-apart faults associated with synrift sinistral strike-slip motions are discordant with pre-Cenozoic structures and drain deeper aquifers with compositions (Cl-Na) strongly different from meteoric water. Mixing of various groundwaters and their metamorphism may be responsible for the observed variations in total mineralization and chemistry of lake water. (2) The diversity of water compositions in lakes is rather controlled by metamorphism (most often cryogenic) of the feeder meteoric water, and pull-apart faults maintain long existence of relatively small lakes in which water evolution reaches a high grade.