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Substrate-slope and temperature controls on carbonate ramps; revelations from upper Miocene outcrops, SE Spain


, : Substrate-slope and temperature controls on carbonate ramps; revelations from upper Miocene outcrops, SE Spain. Special Publication - SEPM (Society for Sedimentary Geology) 56(Pages 271-290

Exposures of Late Miocene (Tortonian) carbonate ramp strata in the Cabo de Gata region (southeastern Spain) provide a unique opportunity 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 (DSIA, DSIB), developed on earlier Neogene volcanic rocks that had been exposed subaerially. The ramp facies preserve a transition from bryozoan-rich at the base, to red algal-rich, to chlorozoan-rich at the top. The clear separation of DSIA and DSIB ramp strata from overlying reef strata indicate that they were not simply deeper water equivalents 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, DSIA and DSIB strata have characteristics that indicate deposition in cool water, probably resulting from a temperate climate and perhaps in conjunction with upwelling. The facies changes from the base of DSIA to the top of DSIB indicate a change from cool to warm water, perhaps indicating a temperate to subtropical climate warming trend that culminated in reef development in later depositional sequences. DSIA strata onlap the volcanic basement, for the most part, 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 DSIA. A possible upward trend of increasing red algae abundance and decreasing bryozoan abundance towards the contact with DSIB may indicate a general warming trend. DSIB is characterized by approximately six fining-upward subtidal cycles, each consisting of red algal packstone-grainstone at the base and foraminiferal wackestone at the top. Both facies locally contain echinoderm, mollusc and bryozoan fragments and volcaniclastic grains. Red algal facies dominate most of DSIB, but significant amounts of chlorozoan 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 without any indication of facies changes approaching the point of onlap. Downslope transport and reworking is evidenced by calciturbidites, conglomerates containing both carbonate and volcanic clast lithologies, local mass wasting, soft sediment deformation, crossbedding and scoured surfaces. Although the repeated fining-upward DSIB 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 positions. These sediments filled in much of the steep paleotopography and provided gently sloping substrates important for later (Tortonian to Messinian) subtropical/tropical reef development. (super 40) Ar/ (super 39) Ar 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 for calculations of accumulation rates. The overall rates at which the ramp sediments of DSIA and DSIB accumulated are relatively slow, varying from an initial rate of >15.6 cm/ky and generally decreasing to a final rate of about 2 cm/ky. These rates are consistent with other accumulation rates measured for temperate climate shelf carbonates; the general decrease in rate may be related to complex sediment dispersal patterns and decreasing shallow-water area available for carbonate production. Results from our study provide for a new understanding of the 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 paleoslopes is important, can be applied to interpret similar strata of all ages.

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