Accommodation/sedimentation development and massive early marine cementation: Latemar vs. Concarena (Middle/Upper Triassic, Southern Alps)

Massive early marine cementation (MEC) is a major diagenetic feature of some carbonate platforms. The comparison of two Triassic buildups of similar size and preservation but contrasting degree of MEC (Latemar, Dolomites, and Concarena, Lombardic Alps) allows for the investigation of differential cementation on carbonate platforms. The assessment of the response of platform cementation to changes in accommodation/sedimentation led to the identification of fundamental boundary conditions for MEC: (1) the time interval available, i.e. a low rate of creation in accommodation space and (2) low carbonate production. Other important factors are: (3) margin topography (e.g. walled reefs) and (4) effective fluid flow (e.g. wave energy, open and connected cavities provided by rigid frameworks). The Latemar-mainly aggrading and locally retrograding-lacks MEC due to a high A′/S′ ratio (i.e. creation/destruction of accommodation in time, A′=dA/dt, vs. changes in sediment supply in time, S′=dS/dt) and a low relief of the reefal margin. In contrast, the Concarena-in the beginning slowly and later rapidly prograding-indicates MEC owing to a low A′/S′ ratio and a distinct topography of the reef. Particular features of the Concarena are cement arrangements of lenticular shape and considerable size in the back-reef domain (up to 3 m in diameter and 2 m in height). These arrangements consist of botryoids and isopachous crusts of radiaxial fibrous calcite cements. They represent one of the main components of the platform margin at Concarena. Cathodoluminescence analyses of cements from both platforms quantifies the influence of shallow and deep burial diagenesis and shows that the majority (60-90%) of all cements are of early marine origin.