SUBSTRATE-SLOPE AND TEMPERATURE CONTROLS ON CARBONATE RAMPS: REVELATIONS FROM UPPER MIOCENE OUTCROPS, SE SPAIN

Abstract

Exposures of Late Miocene Tortonian carbonate ramp strata in the Cabo de Gata region southeastern Spain provide a unique op portunity to evaluate controls on platform development as climate evolved from temperate to subtropical The ramp characteristics were controlled by substrate paleoslope sea level and climate to produce onlapping strata in areas where the substrate paleoslope angle became low The ramp strata divided into two depositional sequences DS I A DS I B developed on earlier Neogene volcanic rocks that had been exposed subaerially The ramp facies preserve atransition from bryozoan rich at the base to red algal rich to chlorozoan rich at the top The clear separa tion of DS I A and DS I B ramp strata from overlying reef strata indicate that they were not simply deeper waterequivalents of reef strata Deposition in shallow normal marine tropical to subtropical waters subjected to runoff related nutrient excess can be ruled out as well Based on data from the immediate setting and widespread distribution of similar roughly time equivalent strata DS I A and DS I B strata have characteristicsthat indicate deposition in cool water probably resulting from a temperate climate and perhaps in conjunction with upwelling The facies changes from the base of DS I A to the top of DS I B indicate achange from cool to warm water perhaps indicating a temperate to subtropical climate warming trend that culminated in reef development in later depositional sequences DS 1 A strata onlap thevolcanic basement for the mostpart and are dominated by bryozoan rich facies and red algal packstone facies Mud poor textures grain abrasion and sedimentary structures point to periods with significant current energy mud rich textures and preservation of unabraded grains point to periods of less energy Distributions of benthic and planktonic foraminifers bryozoans bivalves and red algae indicate outer ramp and inner ramp settings Bryozoan rich facies deposited in nearshore environments points to cool water for DS 1 A A possible upward trend of increasing red algae abundance and decreasing bryozoan abundance towards the contact with DS I B may indicate a general warming trend DS I B is characterized by approximately six fining upward subtidal cycles each consisting of red algal packstone grainstone at the base and fo raminiferal wackestone at the top Both facies locally contain echinoderm mollusc and bryozoan fragments and volcaniclastic grains Red algal fa cies dominate mostof DS I B but significant amounts ofchlorozoan grains occur at the top indicating continued warming from cool to warm The six cycles interpreted to have been deposited in approximately 40 m to over 100 m water depths onlap and locally drape volcanic basement with out any indication of facies changes approaching the point ofonlap Downslope transport and reworking is evidenced by calciturbidites conglom erates containing both carbonate and volcanic clast lithologies local mass wasting soft sediment deformation crossbedding and scoured surfaces Although the repeated fining upward DS I B cyclicity reflects fluctuating relative sea level the locations and onlap geometries of ramp strata do not reflect a direct base level control Instead these strata result from piling up of sediments at the toe of slope after bypass from steep upslope posi tions These sediments filled in much of the steep paleotopography and provided gently sloping substrates important for later Tortonian to Messin ian subtropical tropical reef development 40 Arj39Ar dates from an interbedded volcanic unit and recently acquired paleomagnetic data provide the first high resolution age dating of ramp strata in the area and allow forcalculations of accumulation rates The overall rates at which theramp sediments of DS I A and DS I B accumulated are relatively slow varying from an initial rate of 15 6 cm and generally decreasing to a final rate of about 2 cm These rates are consistent with other accumulation rates measured fortemperate climateshelfcarbonates the general decrease in rate may be related to complex sediment dis persal patterns and decreasing shallow water area available for carbonate production Results from our study provide for anew understanding ofthe important interaction between substrate paleoslope sea level and climate in ramp development The geometric and facies characteristics of this new model for cool water ramp development in which bypass across steep paleo slopes is important can be applied to interpret similar strata ofall ages