Response of a late Miocene Mediterranean reef platform to high-frequency eustasy

Abstract

The upper Miocene reef complex of Mallorca, a third-order depositional sequence, shows how carbonate deposition responds to high-frequency sea-level cycles. This reef complex displays a fractal architecture of hierarchically stacked high-frequency units from fourth to seventh order. The building blocks are sigmoidal units bounded by unconformities, not parasequences. These sigmodds are stacked in of sigmoids, the sets are stacked in cosets, and these, in turn, are stacked in of cosets. All these accretionary units have characteristics of depositional sequences but differ from the model developed by Exxon and from other carbonate sequence-stratigraphy models. These departures result from changes in carbonate production in relation to sea-level fluctuations, which determined the changes in accommodation in the absence of significant subsidence. All these high-frequency sequences are composed of four systems tracts: (1) a lowstand systems tract formed during the early rise of a sea-level cycle and composed of progradational reef to open-shelf facies thinning toward the basin, (2) an aggrading systems tract formed during sea-level rise and composed of thick lagoon to open-shelf deposits without landward shift of facies, (3) a highstand systems tract formed during the high part of a sea-level cycle and composed of progradational reef to open-shelf facies thinning toward the basin with lagoon beds thin or absent, and (4) an offlapping systems tract formed during sea-level fall and composed of thin downlapping reef facies passing into a condensed open-shelf section, which correlates with an erosion surface on the platform top. Two bounding surfaces can be recognized by lithofacies changes between these systems tracts: the erosion surface on top of the offlapping systems tract and the downlap surface at its base. Both surfaces merge basinward into a condensed section. The sequence boundary, which corresponds to the lowest point of the sea-level cycle, is the erosion surface landward and within the correlative condensed section basinward.