QUANTITATIVE CONTROLS ON LOCATION AND ARCHITECTURE OF CARBONATE DEPOSITIONAL SEQUENCES - UPPER MIOCENE, CABO-DE-GATA REGION, SE SPAIN

Sequence stratigraphy, pinning-point relative sea-level curves, and ma gnetostratigraphy provide the quantitative data necessary to understan d how rates of sea-level change and different substrate paleoslopes ar e dominant controls on accumulation rate, carbonate depositional seque nce location, and internal architecture. Five third-order (1-10 my) an d fourth-order (0.1-1.0 my) upper Miocene carbonate depositional seque nces (DS1A, DS1B, DS2, DS3, TCC) formed with superimposed higher-frequ ency sea-level cycles in an archipelago setting in SE Spain. Overall, our study indicates when areas of high substrate slope (> 15 degrees) are in shallow water, independent of climate, the location and interna l architecture of carbonate deposits are not directly linked to sea-le vel position but, instead, are controlled by location of gently slopin g substrates and processes of bypass. In contrast, if carbonate sedime nts are generated where substrates of low slope (< 15 degrees in our a rea) are in shallow water, then architecture and location of deposits may be more directly controlled by the relative position of sea level. For these systems, the rates of relative sea-level change are importa nt for determining which systems tracts develop. DS1A-DS1B, cooler-wat er ramps, result from sediment bypass across steep paleoslopes to toes of slopes. Accumulation rates decreased from > 15.6 cm/ky to similar to 2 cm/ky and overall relative sea level rose at rates of 17-21.4 cm/ ky. Higher frequency sea-level rates were about 111 to more than 260 c m/ky, producing onlapping, fining-(deepening-) upward cycles. Decreasi ng accumulation rates resulted from decreasing surface area for shallo w-water sediment production, drowning of shadow-water substrates, and complex sediment dispersal related to the archipelago setting. Typical systems tract and parasequence development should not be expected in ''bypass ramp'' settings; facies of onlapping strata do not track base level and are Likely to be significantly different compared to onlapp ing strata associated with coastal onlap. Basal and upper DS2 reef meg abreccias (indicating the transition from cool to warmer climatic cond itions) were eroded from steep upslope positions and redeposited downs lope onto areas of gentle substrate during rapid sea-level falls (> 22 .7 cm/ky) of short duration. Such rapid sea level falls and presence o f steep slopes are not conducive to formation of forced regressive sys tems tracts composed of downstepping reef clinoforms. The DS3 reefal p latform formed where shallow water coincided with gently sloping subst rates created by earlier deposition. Slow progradation (0.39-1.45 km/m y) is best explained by the lack of an extensive bank top, progressive ly falling sea level, and low productivity resulting from siliciclasti c debris and excess nutrients shed from nearby volcanic islands. Altho ugh DS3 strata were deposited during a third-order relative sea-level cycle, a typical transgressive systems tract is not recognizable, indi cating that the initial relative rise in sea level was too rapid (much greater than 19 cm/ky). Downstepping reefs, forming a forced regressi ve systems tract, were deposited during the relative sea-level fall at the end of DS3, indicating that relatively slow rates of fall (10 cm/ ky or less) over favorable paleoslope conditions are conducive to gene ration of forced regressive systems tracts consisting of downstepping reef clinoforms. The TCC sequence consists of four shallow-water sedim entary cycles that were deposited during a 400 ky to 100 ky time span. Such shallow-water cycles, typical of many platforms, form only where shallow water intersects gently sloping substrates. The relative thic knesses of cycles (< 2 m to 15 m thick), magnitudes of relative sea-le vel fluctuations associated with each cycle (25-30 m), high rates of r elative sea-level fluctuations (minimum of 25-120 cm/ky), and the wide spread distribution of similar TCC cycles in the Mediterranean and els ewhere are supportive of a glacio-eustatic influence. With rates of se a-level change so high, typical systems tracts do not form.