SEQUENCE STRATIGRAPHY AND THE STABLE ISOTOPE RECORD OF THE ZHADA BASIN, SW TIBET: IMPLICATIONS FOR PALEOELEVATION RESEARCH

The Zhada Basin in southwestern Tibet (~31°N, ~80°E) sits at an elevation of 3.7 to 4.5 km. The basin contains a continuous, undisturbed fluvio-lacustrine record extending from ~9.2–<1 Ma. Continuous sedimentation spanning critical climate transitions in the late Miocene and Pliocene coupled with the basin’s high elevation make this an attractive location to develop a long-term paleoclimate and paleoenvironmental record for the southwestern Tibetan Plateau.

Initial results of C and O isotopic analysis of bulk sediment CO₃ are consistent with predictions based on basin-wide sequence stratigraphy. Sequence stratigraphy and stratal stacking patterns in the Zhada Basin are controlled primarily by two factors: fault-driven tectonic subsidence and the evaporation/precipitation ratio. Carbonate isotopic analyses from multiple sections spanning the >100 km NW-SE extent of the basin confirm that the basin was largely hydrologically open until 5.5–6 Ma. δ¹⁸O and δ¹³C values increase across the stratigraphically lowest transgressive surface reflecting the fault-driven transition from overfilled to underfilled basin conditions. The post-5.5 Ma basin-wide increases in δ¹⁸O and δ¹³C values, and in their correlation coefficients indicate that the basin became hydrologically closed, potentially due to uplift of the Leo Pargil/Qusum mountains which dammed the paleo-river valley and resulted in subsidence of the basin floor. Average δ¹⁸O and δ¹³C values from above the transgressive surface are consistently more positive than stratigraphically lower samples; reflecting the ratio of fluvial input to evaporation.

Comparing the results of stable isotopic analysis of bulk sediment to published gastropod shell data shows that gastropod shells record both more negative and more positive δ¹⁸O values than the bulk sediment analysis. Bulk sediment δ¹⁸O values have narrower ranges than stratigraphically equivalent shell δ¹⁸O values. However, bulk sediment δ¹³C values tend to be more positive than stratigraphically equivalent shell δ¹³C values. These initial results suggest that paleoaltimetry estimates based on CO₃ analysis of bulk sediment in fluvio-lacustrine lithofacies may systematically overestimate the most negative δ¹⁸O values and hence underestimate paleoelevations.