GRAIN-SIZE, COMPOSITION, POROSITY AND PERMEABILITY CONTRASTS WITHIN CROSS-BEDDED SANDSTONES IN TERTIARY FLUVIAL DEPOSITS, CENTRAL SPAIN

Permeability measured with a portable probe permeameter on outcrops of cross-bedded sandstones ranges between 0.9 and 19 D. The highest perm eability (2-19 D with an average of 8.5 D) occurs in the coarsest grai ned foreset laminae (CFL), intermediate values (2-12 D with an average of 5.3 D) occur in finer grained foreset laminae (FFL) and the lowest values (0.9-10 D with an average of 4.8 D) occur in bottomset layers (BL). In the cross-beds the average grain size ranges from medium grai ned sand in the CFL to fine grained sand in the FFL and BL. In all thr ee subfacies, the average size of the primary pores is approximately 1 phi unit smaller than the average grain size. The abundance of unstabl e carbonate clasts correlates with increasing average grain size, micr itic clasts being most abundant in the CFL. Conversely, quartz content increases with decreasing grain size and is highest in the FFL and BL . Diagenetic destruction of primary porosity by compaction and cementa tion, as well as generation of secondary porosity through dissolution, were controlled by the original mineralogical composition of the sand . Contrasts in grain size determine the primary pore size contrasts an d differences in composition between CFL, FFL and BL. Permeability con trasts reflect variations in average primary pore size rather than dif ferences in total porosity. Probe permeability contrasts between CFL. FFL and BL depend on contrasts in average pore size and contrasts in m ineralogical composition between the subfacies.