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Abstract

Exceptional exposures near the coastal village of Las Negras in southeastern Spain contain undeformed late Miocene carbonate complexes deposited on the slopes of isolated, earlier Neogene volcanic basement highs that were part of an archipelago. Detailed field studies identified five carbonate depositional sequences, locally admixed with volcaniclastic debris, with onlapping and downlapping geometries that reflect a complex history of relative sea-level fluctuations. The first four sequences record an overall evolution from an open-marine platform (ramp) to a fringing reef complex. The open-marine platform (depositional sequences DS1A, DS1B, DS2) is predominantly composed of red algal, bryozoan, mollusk, benthic and planktonic foraminiferal, echinodermal wackestones and packstones. Locally, DS2 contains megabreccia composed predominantly of Tarbellastraea and Porites coral reef blocks. The fourth depositional sequence (DS3) is composed of fringing reef complex strata characterized by Porites reefs. The fifth and youngest depositional sequence (TCC) is composed of normal to restricted marine cyclic carbonates consisting of red algal and mollusk packstones, oolite grainstones, and stromatolites that were deposited upon reflooding of shelf areas after a salinity crisis. The sequence boundaries are characterized by evidence of relative falls in sea level.

These depositional sequences preserve over 200 m of Miocene paleotopography. Such preservation allows for quantification of the relative sealevel curve through the development of "pinning points," which we define as a position of ancient sea level relative to a geologically useful starting elevation. A pinning point curve is constructed by plotting ancient relative elevations of sea level (pinning points) versus relative time. This is accomplished by determining relative elevations of facies deposited at or just below sea level, tracing downslope subaerial exposure on marine rocks, or from an upward transition of marine to nonmarine deposits. Relative elevations for pinning points are determined from preserved paleotopography. Pinning point curves can be used as templates to construct more interpretive relative sea-level curves that incorporate stratal geometries, facies relationships, and stacking patterns. The pinning point curve developed at Las Negras illustrates a complex history with large-scale relative sea-level changes of more than 150 m combined with smaller scale fluctuations. Comparison of relative sea-level histories for equivalent strata from different areas can provide for the quantification of the effect of local, regional, and eustatic variables on carbonate sequence development. The pinning point method illustrated here can be used for carbonate sequences of all ages.

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