Depositional Sequences and Correlation of Middle to Upper Miocene Reef Complexes, Nijar and Las Negras Areas, Southeastern Spain

Evan K. Franseen, Carol Mankiewicz, Lloyd C. Pray

During the Serravallian through Messinian, reefs fringed topographic highs that rimmed Neogene basins in the western Mediterranean. Detailed studies of middle to upper Miocene carbonate strata in the Nijar and Las Negras areas (southeastern Spain) document the effects of relative sea level change on sedimentation, biotic composition, reef development, and diagenesis. We have identified several previously unrecognized, regionally correlatable depositional sequences and erosional breaks that we interpret resulted from sea level fluctuations. Limited microfossil dating and correlation of depositional successions with third-order eustatic sea level curves suggest that most carbonate deposition occurred during the Serravallian and Tortonian, not Messinian as formerly thought.

Lowermost strata (Serravallian-Tortonian) lack major reef development, although some allochthonous reef-facies blocks occur, consisting mainly of Tarbellastraea and Porites framework. Packstones and wackestones compose these shallow-dipping carbonate sediments, interpreted to have been deposited in a shallow-shelf environment during a net sea level rise.

Uppermost strata (upper Tortonian) predominantly comprise packstones and grainstones of distal to proximal reef-foreslope and reef-talus facies that dip 10°-30°. Reef core, mostly Porites framework, is locally preserved. Field geometries indicate minimum shelf-to-basin relief of 100 m. The fringing reefs developed in a "downstepping" style (prograding basinward in pulses as sea level fell), possibly reflecting more frequent (fourth-order) sea level changes. A regional subaerial erosion surface that truncates reef-complex strata is locally overlain by flat-bedded, shallow, restricted-water carbonates (Messinian).

Results of our studies suggest that detailed field studies coupled with the use of published eustatic sea level curves may provide a powerful tool for correlation of middle to upper Miocene strata not only in Spain, but in the entire western Mediterranean.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.