Neogene transitional carbonates: the link between the tropical and the temperate carbonate province
Zusammenfassung der Projektergebnisse
Sedimentological and stratigraphical models of two middle Miocene fault-block-carbonate platforms with transitional carbonate factories in North Sardinia are presented. The Burdigalian Sedini carbonate platform formed at the western margin of the Perfugas subbasin on a fault-bound topographic high. The 10 - 60 m sedimentary succession contains two depositiona! sequences separated by a major erosional unconformity, and several high frequency sequences reflecting the occurrence of higher-order base-level fluctuations. The deposits are massive limestones, bedded limestones and marlstones, which form a carbonate platform with an extension of about 19 km2. Sequence 1 consists of a homoclinal ramp with a warm temperate carbonate factory with abundant coralline algae, frequent larger benthic foraminifers, barnacles, bryozoans, mollusks, and minor corals. Sequence 2 documents a steepening of the carbonate platform slope. In the lower part of this sequence, a belt of submarine dunes separated platform-interior from deeper water bioclasttc deposits. In the upper part of sequence 2, the depositional system consisted of an extensive reef flat with a marked slope break formed by coralline algal bindstones and rhodolithic clinoform beds dipping with up to 27°. The carbonate factory of sequence 2 is rich in reef-building zooxanthellate corals. Along the slope of the carbonate platform, there is a distinct red-algal growth zonation. The clinoform rollover area consists of coralline algal bindstones, which downslope changes into a rhodolith facies where rhodoliths are locally fused by progressive encrustation. Mid-slope rhodoliths are moderately branched, and downslope rhodoliths have fruttcose protuberances, resulting in branching rhodolith growth patterns. There is a sharp change from the rhodolitic facies to the basinal bivalve-dominated facies at the clinoform bottomsets. Red-algal genera identified include Sporolithon sp., Lithophyllum sp., Spongites sp., Hydrolithon sp., Mesophyllum sp., Lithoporella sp., Neogoniolithon sp., and other Mastophoroids and Melobesioids. Genera and subfamilies show a zonation along the clinoforms, allowing palaeobathymetric estimates. The clinoform rollovers formed at a water depth of around 40 m, the bottomsets around 60 m. Results from geometrical reconstruction show that coral reefs in the inner piatform formed in water depths of around 20 m. Therefore, the Sedini carbonate platform is an example of a reefbearing carbonate platform, in which the rim or the platform-interior reefs does not build up to sea level. If outcrop conditions do not allow a detailed view of platform geometries, such geometry may be misinterpreted as the shallow-water rim of a platform. The example of the Sedini carbonate platform also indicates that a mere geometrical reconstruction of platform geometries in subsurface data may lead to misinterpretations, because the edge of the platform does not necessarily indicate sea-level position. The mixed carbonate-siliciclastic succession of the Porto Torres Basin is Burdigalian to Tortonian (?) in age, and shows similarities with the Sedini Limestone Unit with regard to the carbonate factories and facies involved. The succession, however, is characterized by different sedimentary and sequence stratigraphic geometries as a consequence of continuous and strong synsedimentary tectonic imprint. Formation of unconformities was mainly triggered by movements along east-dipping normal faults resulting in block rotation. As a result, the mapped unconformities are expressed differentially throughout the working area depending on their relative position to the rotational pole of the faulted blocks. Thicknesses are greatest in the vicinity of faults, whereas thickness decreases with decreasing distance to the rotational pole.
