Abstract: Carbonate Bypass Ramps Controlled by Substrate Paleoslope, Late Miocene of Southeastern Spain: General Implications for Sequence Stratigraphy

Evan K. Franseen, Robert H. Goldstein

Outcrops of Late Miocene depositional sequences in the Cabo de Gata area of southeastern Spain consist of basal Tortonian carbonate ramps that formed the substrate on which Messinian fringing reef complex and topography-draping strata of the terminal carbonate complex were deposited. Location, facies distribution, and geometry of the Tortonian ramps were largely controlled by paleoslope of the substrate combined with relative sea level position.

Ramp strata onlap onto erosionally sculpted volcanic basement, the upper surface of which extended significantly upslope of the point of onlap. During ramp development sea level was at a high position well above the point of onlap. Strata are characterized by fining upward subtidal cycles (2-15 m thick) consisting of foramalgal packstone-grainstone facies, some with resedimented grains, and fine grained wackestone-packstone facies interpreted to have been generated in about 40 m to 100+ m water depths. These subtidal cycles lap out against volcanic basement without any indication of facies changes approaching the lap out. Thus, shallow-water and deeper-water carbonate sediments must have been generated on steeper volcanic substrate in upslope positions, but were bypassed downslope onto catchment areas of low basement paleoslope. Facies variation likely reflects fluctuating relative sea level but ramp location and geometry reflect accumulation of sediment bypassed from upslope positions onto areas of lower basement paleoslope rather than sea-level control. The decreased substrate paleoslope provided by ramp deposition appears to have provided a preferential substrate for accumulation of later fringing reefs.

This new model for substrate paleoslope control on bypass ramp accumulation is predictable for sequence stratigraphy interpretations and has potential for locating reservoir facies using subsurface data on substrate paleoslope.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France