Paleoslope, sea-level and climate controls on Upper Miocene platform evolution, Las Negras area, southeastern Spain

Abstract

Carbonate platforms in the Las Negras area (southeastern Spain) evolved from onlapping ramps to fringing reef complexes later draped by cyclic shallow marine strata. Although sea-level history and paleoclimate had an effect on platform evolution, substrate topography played a dominant role. The strata are divided into five depositional sequences of Tortonian and Messinian age. From base to top these depositional sequences are labeled as DS1 A (newly dated at 8.5±0.1 Ma) mostly consisting of coarse-grained carbonates and volcanic detritus; DS1Β consisting of fining upward carbonate cycles; DS2 mostly consisting of reef megabreccia clasts and fine-grained carbonates; DS3 mostly consisting of reef and forereef carbonates and some volcanic detritus; and TCC (Terminal Carbonate Complex) dominated by oolitic cyclic carbonates. Ramp strata of DS1A and DS1Β mostly onlap against volcanic basement. DS1A and lower DS1B strata have characteristics that are consistent with deposition in cool water, probably resulting from a temperate climate and possibly coinciding with upwelling. DS1Β strata consist of fining-upward cycles of red algal packstone-grainstone and fine grained wackestone-packstone that suggest repeated variations of water depth from 40-100 m to greater than 100 m. Beds of both facies lap out against basement without any indication of facies change at or approaching the point of onlap. Although sediments likely were generated upslope above the point of onlap in shallow marine water, paleoslope was too steep for those sediments to accumulate permanently. Therefore, deposits of DS1Β represent a type of carbonate ramp in which sediment production was in water depths of approximately 40-100 m or more and any sediments generated in shallower water upslope positions were bypassed downslope until areas of low basement paleoslope were reached. Importantly, the onlap in this type of ramp is not necessarily tracking baselevel. The fining-upward character of DS 1Β cycles may result from an overall high or rising sea level punctuated by higher frequency fluctuations, lower sedimentation rates related to a more temperate climate, or bypass into the distal toe-of-slope setting. The sediments that accumulated during DS 1Β filled in significant basement topography and created a more gently sloping substrate on which later deposits could accumulate. Unlike DS 1A and most of DS 1B, the last phase of DS 1 B, DS2, and DS3 contain significant amounts of chlorozoan facies indicating a shift toward a more subtropical to tropical climate. Normal marine platform deposits of DS2 essentially draped the gentle paleotopography created by DS 1Β deposition, but megabreccias composed mostly of reef clasts indicate instability in upslope areas due to steep paleoslopes and relative sea-level falls. During deposition of the Porites-dominated fringing reef strata of DS3, sea level was at a position near the tops of relatively flat hills created by earlier deposition. Reef aggradation, progradation and downstepping in DS3 created steep forereef slope topography. These reefal platforms were preserved only on substrates of relatively low paleoslope, where earlier deposits had filled in the steep substrate topography. In contrast, steep-sided volcanic hills in the area were sites where DS3 and DS2-equivalent reefs may have been formed but were bypassed downslope because of the steep paleoslope. Finally, during deposition of latest Miocene strata (TCC), sea level was at a position that allowed for accumulation of shallow-water carbonate cycles, but these cycles were only preserved on areas where the substrate, created either by earlier deposition or erosion, was relatively flat.