The upper Miocene reef-rimmed platform of Mallorca (Spain): factory structure, growth dynamics and diagenesis

The Llucmajor platform on southern Mallorca Island (Spain) is one of the best-exposed Miocene shallow-water carbonate platforms known in the world. Up to 90 m vertical sea cliffs provide a clean, continuous and detailed 3D view of the facies architecture and continuous cores from fifty-two water wells, affording information to interpret the 3D architecture of this 20 km-wide and only slightly deformed carbonate platform. Forty years of research on the Llucmajor platform provided insights into how the mutual interaction of biological and physical processes control the type of carbonate production and sediment dynamics which, along with changes in sea-level, determine the facies stacking patterns and the distribution of internal heterogeneities. Moreover, the ‘modern character’ of the carbonate-producing biota permits to capture the response of the carbonate system to changing ecological and oceanographic conditions. Although learnings from the Llucmajor platform do not apply to older platforms, it is this process-product type of analysis that can be applied to carbonate systems of all ages. Thus through this field guide, two carbonate factories working out of phase and with a non-linear response to sea-level changes are identified. A shallow-water euphotic (well lit) production (corals, red and green calcareous algae and associated heterozoan biota) predominated during rising sea-level when linear reefal rims grading to broad lagoons expanded and increased production and the sediment shed-off from these reef-lagoon systems built-out thick slopes. A deeper oligophotic (low light) factory expanded during the lowstands of sea-level when the light reached the basin floor; red algae, larger benthic foraminifers and molluscs were the prime producers. The Llucmajor platform grew up during the late Miocene, through the transition from greenhouse to icehouse climate that characterised the late Cenozoic. Its building blocks occur hierarchically stacked in response to the sea-level cyclicity. At the bed scale, the sigmoids consist of a lagoonal bed passing successively into sigmoidal reef core and then into slope clinobed and basinal deposits and bounded by an erosion surface onto the lagoon and reef and its correlative conformity basinward. Sigmoids stack in sets of sigmoids, these in cosets of sigmoids and these last in megasets, all of them fitting the characteristics of a depositional sequence when the genetic processes are analysed. Departures from the sequence model rely on the inter-dependence between depth gradient, the type of carbonate-producing biota and the ranges of sea-level cyclicity. Through the visit to a series of selected outcrops, this field guide offers a comprehensive assessment of the Llucmajor reef-rimmed shelf from the basin to the coral-dominated reefal margin and across the open-lagoon to restricted lagoon that defined the shelf top. The itinerary focusses on numerous topics and at different scales, from the study of discrete rock bodies to km-wide outcrop windows resembling high-resolution seismic profiles. The field guide integrates facies analysis, stratal architecture and distinct diagenetic features such as extensive secondary porosity after the dissolution of aragonitic components and pervasive dolomitization. Impressive sub-vertical collapse structures were produced by karstic dissolution during lowstands of sea-level and gravitational collapse of the roof complete the suite of diagenetic overprint. Altogether, the depositional and diagenetic attributes of the Llucmajor platform reveal a complex and highly heterogeneous carbonate system that is used as outcrop analogue for numerous Miocene carbonate reservoirs across the world.