Peculiar karstic features in the Upper Triassic sulphate evaporites from the Secchia Valley (Northern Apennines, Italy).

In the Secchia Valley spectacular sulphate outcrops (Burano Fm., Upper Triassic) are composed mostly of gypsum, anhydrite is a minor constituent.The apparent relative dissolution rates of gypsum and anhydrite rocks are variable and result in peculiar karstic features, which have not been reported in other evaporite deposits. Such differential solution phenomenon can be observed both on the fine scale, along outcrop surfaces, and also at the large scale, controlling drainage patterns of rock masses. These peculiar solution features are the gypsum and anhydrite protrusions and the hypogean bends (“anse ipogee”).Gypsum and anhydrite protrusions. Although anhydrite is less soluble than gypsum, anhydrite-bearing rocks show variable behaviours and may be affected by dissolution to a greater extent than the gypsum rocks. Fine-grained anhydrite layers and lenses, enclosed in gypsum rocks, protrude from the surrounding gypsum, whereas the coarse-grained anhydrite rocks are more depressed. This effect is due to the presence of gypsified rims on the coarse-grained anhydrite crystals. When running water flushes the rocks, gypsum in the rims dissolves faster than anhydrite. The partially gypsified coarse-grained anhydrite is more deeply attacked because the anhydrite crystals fall away from the rock surface or are carried away by the water as gypsum rims are removed. In layers where gypsification of the anhydrite crystals is negligible, and this is the case for the fine-grained variety, the anhydrite rocks are less attacked and form protrusions relative to the gypsum layers.Hypogean bends (“anse ipogee”). The karstic systems run only at a short distance from the surface, where gypsum is dominant. The expected presence of a larger amount of less soluble anhydrite in the core of the outcrops seems to prevent the development of karstic systems cutting across the sulphate masses. The karstic drainage is concentrated along joints parallel to the valley incisions, following release fractures developed as a consequence of the dramatic drop in internal rock pressure due to rapid valley-floor deepening. Because the Secchia Valley is characterized by small streams, almost orthogonally-oriented with respect to the main valley, the karstic systems may seem to flow uphill where they intercept the joints running parallel to the Secchia valley. This peculiar effect is related to the presence of alluvial and mud-slide deposits, which act as barriers and do not allow the direct down slope resurgence of the karstic waters.