The alluvial-lacustrine and volcano-sedimentary Quillagua Formation was deposited in a North-South trending closed basin, under arid to hyperarid paleoclimatic conditions. Transverse alluvial fans and a longitudinal, northward flowing fluvial system fed the mostly shallow lacustrine zones. Persistence and large areal extent of the ancient lacustrine zones (i.e. 2,000 km²), diverse water discharge from the alluvial systems and likely differences in areal extent of their catchment areas suggested that hydrochemical changes could develop, both in space and time, in the lacustrine system. Diatom assemblage analyses were carried out in two selected sections to test this possibility. The QT section was related to terminal zones of the axial fluvial system whereas the MO section was closer to short headed alluvial fan distal zones, and further off from the axial system influence.

The inner lacustrine facies in both sections had similar steady salinity conditions. Thus, oligosaline diatom assemblages were dominant, with only minor occurrences of freshwater and eusaline diatoms.

Marginal lacustrine diatom assemblages in both sections also ranged from flesh to eusaline, but these extreme conditions developed more repeatedly than in inner facies. This fact suggests salinity changes due to freshwater local income into the marginal lacustrine areas. Moreover, some differences can be established between the marginal lacustrine diatom records in both sections. Larger water contributions fed from the axial fluvial system in the QT area resulted in more dramatic, extreme salinity changes as recorded by the coeval occurrence of well developed freshwater and eusaline diatom assemblages. On the other side, lesser freshwater contributions from the short headed transverse alluvial fans resulted in a higher dominance of oligosaline diatoms in the related MO marginal lacustrine facies.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah