QUATERNARY SINKHOLES: RECORD OF NATURAL AND HUMAN-INFLUENCED KARST EVOLUTION AND OF ENVIRONMENTAL CONDITIONS

The dissolution of Tertiary evaporites (covered by Quaternary detrital sediments) has been generating the development of extensive fields of sinkholes and it constitutes an important risk in the central Ebro Basin. Karst activity has produced important damages and economic losses through years.

The analysis and comparison of aerial photographs of different years is basic in both the determination of spatial distribution of sinkholes and of changes produced in them through time (60 years in our case). These images represent only a piece of information about the evolutionary stage of sinkholes. In addition, human activities modify the natural evolution of sinkholes. By these reasons, a wider temporal perspective in their study, including periods as long as possible, is necessary.

The study of palaeosinkholes developed through Pleistocene times, and without activity at present, could clarify the future behaviour for the current sinkholes. Their filling represent long periods of time with natural evolution and palaeoenvironmental conditions can be determined from their analysis. The study of sections of palaeosinkholes also permits an access to its internal structure. Their sedimentological, mineralogical and structural features gives information about their genetic environment, mechanisms involved in their development and the evolutionary stages.

Detailed studies have been achieved in old terraces (Early Pleistocene) and medium terraces (Late Pleistocene) of the Ebro River. In both, gravels predominate with less frequent sands and scarce lutite levels (these later mostly in basin-shaped levels). Where paleosinkholes appear, tilted beds, normal and reverse faults are frequent. From the sedimentological study it is concluded that these materials were mainly deposited in a gravel braided river. In some of the studied sections evolution of sinkholes through thousands of years has been reconstructed including evaporite dissolution, collapse, flooding and several subsidence episodes. Clay mineral assemblages in the sinkholes were studied. In old terraces, smectite content is higher than in middle levels, and kaolinite is lower. This fact can suggest either climate warming or change of the source area through time.