Lithofacies and diagenesis as controlling factors in the development of solution caves in Dolomite: the example of the Dolomia principale (Trentino - Northern Italy)

Andrea Borsato & Silvia Frisia - Dipartimento di Scienze e della Terra, Università di Milano, Via Mangiagalli 34, 1-20133 Milano. Gruppo Speleologico CAI SAT Lavis

In Trentino region caves mostly develop in the norian Dolomia Principale (Hauptptdolomit) and the liassic Calcari Grigi.

The greatest number of caves formed in the limestones of the Calcari Grigi but the Dolomia Principale (DP) hosts the longest cave systems (Grotta della Bigonda = 18.5 km long; Grotta di Collalto = 5 km long; Grotta del Calgeron = 5 km long).

The preferential karstification of the DP, with respect to other dolomites of the area, is the result of a relatively high dissotution rate, which is function of lithofacies and diagenesis:

1) Lithofacies: the 300 to 1000 m thick DP consists of dolomitized peritidal and subtidal cycles. The well bedded peritidal cycles are characterized by stromatolitic intervals, while subtidal facies are represented by 3 - 5 m thick beds of doloarenites. Both facies show bedding parting porosity determining anastomotic and branchwork patterns in caves systems. On the contrary, intergranular porosity hinders karstification, as it can be observed in the DP massive, vuggy margin facies (Lake Idro area and in the reef facies of Cassian Dolomite (Carnian) and Sciliar Dolomite (Ladinian).

2) Diagenesis: the DP underwent a complex diagenetic history from early shallow subsurface to deep burial. This translates into a variety of textures and microstructural/geochemical heterogeneity, which allow for high dissolution rates, expecially in dislocation-rich, calcian (54-56 mole % Ca, aphanitic dolomites of the peritidal stromatolitic layers. Where the DP was affected by destructive phenomena, which homogenized textures and geochemical properties, karst soution is less intense. This is the case of hydrothermal diagenetic overprint, observed in some structural high areas (Val di Non, Val D'Adige). Similar fabric-destructive processes hindered karstification of other tidal flat dolomites of the Trentino region such as the Dolomia di Val d'Adige (Ladinian - Carnian).