AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Tufa Facies and Their Stable Isotope Record of Lake Paleohydrological Changes, Searles Lake, California
(1) University of Houston, Houston, TX.
Over 500 tufa mounds occur in the dry bed of Searles Lake, southeastern California. The mounds range from minor features to 45 m in height, most are 5 to 12 m high. These mounds formed from spring vents in the Pleistocene lake bottom.
The mounds in the northern and middle areas display complex architectures, which consist of four distinctive tufa facies: (1) porous tufa accumulations (P), including the innermost (P1) and the outermost deposits (P2), which are composed of highly irregular thin, commonly sub-vertical to short horizontal sheets and threads of carbonate, (2) multiple generations of nodular deposits (N) consisting of millimeter size coated grains, (3) dense layered columnar tufa deposits (C), and (4) finely laminated crusts (LC). There is a simple sequence of tufa deposition, from P1 to N or C to LC and lastly P2. The sequence indicates the climatic and hydrologic history of Searles Lake during tufa formation.
The relationship between δ18O and δ13C values in a closed-basin lake like Searles Lake is a function of hydrological change, vapor exchange, biogenic productivity, and lake alkalinity and salinity. Low δ18O and δ13C values in facies P1 (average: -1.48‰ (O), +1.68‰ (C), n = 14) and P2 (average: -2.93‰ (O), 0.99‰ (C), n = 33) are attributed to rapid increases of the lake volume; whereas high δ18O and δ13C values from facies N (average: +1.83‰ (O), +4.14‰ (C), n = 24) and C (average: +0.62‰ (O), +3.98‰ (C), n = 7) are due to the decease of the lake volume and high evaporation and biogenic productivity. Facies LC formed when the mounds were exposed and their relatively high δ18O and δ13C values (average: +1.36‰ (O), +4.43‰ (C), n = 12) are due to evaporation and fast CO2 degassing. Covariance of δ18O and δ13C values occurs in the tufa deposits due to the hydrologically closed environment of the lake during tufa formation. Different facies, however, display variations in the covariant trends due to the insensitivity of δ13C values to lake volume changes under the condition of hyper-alkalinity.