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