Geometries of reef advance in response to relative sea-level changes in a Messinian (uppermost Miocene) fringing reef (Cariatiz reef, Sorbas Basin, SE Spain)

The pre-evaporitic Messinian sequence in the Sorbas Basin includes two reef units. At the northern margin of the basin, near Cariatiz, the uppermost unit consists of a fringing reef advancing 1150 m towards the basin centre. Reef-facies distribution and geometries in three N-S sections parallel to reef progradation reveal cyclic relative sea-level changes during reef development. Two orders of cyclicity can be recognised. The lower order (C1 cyclicity) is represented by one cycle and by the beginning of another one that is interrupted in its ascending phase. This cyclicity is modified by higher-frequency cycles (C2 cyclicity), which developed between consecutive wedges of calcarenite beds that onlap an erosive surface on previous deposits and pinch out landwards (inverted wedges). The inverted wedges are thought to represent the lowstand deposits of C2 cycles and no coeval reef growth is recorded. Sea-level rise within C2 cycles results in reef aggradation and aggradation combined with progradation. Reef deposits prograded during the highest sea level, and offlapped during sea-level fall. Relative proportions of aggrading, prograding, and offlapping geometries inside C2 cycles depend on the interference of C1 and C2 cycles. The estimated relative sea-level change in the complete CI cycle is about 100 m. sea-level oscillations in C2 cycles have an amplitude of several tens of metres. Biostratigraphic and magnetostratigraphic data indicate that the Cariatiz reef developed in less than 0.36 Ma. If eustasy was the major factor controlling relative sea-level change, this temporal range, together with the observed amplitudes and relative frequencies, would suggest that C1 and C2 cycles may represent short eccentricity and precession cycles, respectively. However, the estimated sea-level changes are greater than the high-frequency eustatic oscillations reported from Upper Miocene deposits from other areas. Fan-delta siliciclastics locally interfered with reef growth. Main fan-delta activity took place during the high and descending sea-levels of the first C1 cycle but there was no significant clastic influx during the lowstand.