THE KALKSCHIEFERZONE (UPPER MERIDE LIMESTONE LADINIAN) NEAR MERIDE (CANTON TICINO, SOUTHERN SWITZERLAND) AND THE EVOLUTION OF A MIDDLE TRIASSIC INTRAPLATFORM BASIN

The Kalkschieferzone (uppermost Meride Limestone; Late Ladinian) in th e area of Monte San Giorgio near Meride (Canton Ticino Switzerland) an d Ca del Frate (Varese, Italy) is known for irs excellent preservation of numerous small actinopterygian fishes and rare aquatic reptiles. D uring small-scale excavations in the Gaggiolo valley near Meride, some actinopterygians, crustaceans and plants have been recovered in the m iddle Kalkschieferzone. Preliminary studies suggest a seasonally contr olled sedimentation in a shallow basin or lagoon with fresh-water infl uence in the wet season. A high productivity in the surface water and stable density stratification resulted in anoxic bottom water with acc umulation of organic matter. Carbonate laminae were deposited under in creased salinity in the dry season. Mud-cracked stromatolites and hali te crystal casts indicate the presence of evaporitic conditions. The K alkschieferzone represents a late stage evolution of an intraplatform basin, beginning with open marine influence in Late Anisian lime (Gren zbitumenzone) and increasing restriction by growing carbonate platform s during Early Ladinian (Lower Meride Limestone). In Late Ladinian tim e, the basin was filled progressively by carbonate and siliciclastic m ud (Upper Meride Limestone). The shallow lagoon of the Kalkschieferzon e with strong seasonal variation of salinity and water level finely wa s buried by an increasing input of fine siliciclastic material, reachi ng its maximum with the locally evaporitic Pizzella Marls (Raibl Beds) in the Carnian.