STRATIGRAPHY OF UPPER MIOCENE OOLITE-MICROBIALITE-CORALGAL REEF SEQUENCES OF THE TERMINAL CARBONATE COMPLEX: SOUTHEAST SPAIN

Abstract

This study documents the stratigraphic characterization of the Terminal Carbonate Complex (TCC) at two locations within the Cabo de Gata area of southeast Spain, La Molata and La Rellana/Ricardillo. The TCC is a distinctive upper Miocene (Messinian) unit consisting of oolite, microbialite (stromatolites and thrombolites), and coralgal reefs deposited in association with high-amplitude cyclic glacioeustacy and evaporitic drawdown of the Mediterranean. The studied locations are approximately 5 km apart and were at or below elevations of late Messinian sea level highstands. Data collected included geologic mapping, 44 measured stratigraphic sections, tracing geometries on photomosaics, petrophysical analysis of 399 core plugs, and petrographic analysis of 87 thin sections. Four cyclic sequences record four relative rises and falls in sea level with amplitudes of 53.6-83.5 m. Sequences commonly have local basal stromatolites overlain by local thrombolite boundstone that is overlain by trough cross-bedded ooid grainstone, which grades upward to volcaniclastic-rich planar bedded ooid grainstone capped by fenestral ooid grainstone. At low elevations, the thrombolite boundstones are thicker and laterally more continuous than at higher elevations. Thrombolites that are stratigraphically high in sequences may be interbedded with trough cross-bedded ooid grainstone. At intermediate substrate elevations, sequences have a build-and-fill architecture, characterized by a relief-building phase followed by a relief-filling phase, with the relatively thin sequences draping paleotopography. Microbialites dominate deposition during the relative sea-level rises and build topographic relief. Oolites dominate deposition during the relative sea-level falls and fill topographic relief. Most of the deposition is during the relative sea-level falls. At higher substrate elevations, close to the highstand position, sequences thicken and yield internal stratigraphic character that is inconsistent with a build-and-fill model. Apparently, the build-and-fill model requires an intermediate substrate elevation and non-optimal carbonate productivity during rapid sea level change. Overall, the sequences progressively show increasing diversity and more normal marine organisms, which may have been caused by decreasing aridity. Lithofacies of the La Molata area evidence more restricted conditions compared to the La Rellana/Ricardillo area lithofacies, likely resulting from La Molata deposits forming in a protected embayment.