Lateral variations in sediment composition and bedding in Middle Triassic interplatform basins (Buchenstein Formation, southern Alps, Italy)



ABSTRACT Hemipelagic deposits are widespread in Triassic sequences of the Alpine belt and provide important data for stratigraphy and the study of bedding rhythms. The area of the western and central Dolomites of northern Italy escaped strong alpidic deformation and preserves such deposits in their original palaeogeographic setting. The Buchenstein Formation, the object of this study, was deposited in up to 1000 m deep, Middle Triassic interplatform basins, which extend over an area of 500 km2 within the Dolomites. Excellent outcrops and volcaniclastic markers allow a detailed correlation of the formation in both laminated (anoxic) and bioturbated facies down to a bed scale, and show its relationship to coeval carbonate platforms. Correlation of lithostratigraphically well-constrained intervals in the bioturbated facies reveal that lateral thickening and thinning of the deposits depends on the amount of shallow-water debris in the succession and is a function of the distance to coeval carbonate platform sources. In the laminated background deposit, thickness variations in limestones and marls parallel the thickness variations in ash layers, and were caused by local redeposition of sediment on the basin floor. Lateral persistence of laminae indicates that bottom currents were weak in the Buchenstein basin, but were able to redeposit mud in a significant way. In the area around Seceda and Geisler (western Dolomites), a lateral transition from dark-grey laminated to grey bioturbated to red bioturbated facies is observed, which is probably linked to local relief with different oxygenation conditions on the sea floor. A comparison shows that decimetre-scale bedding is preserved in all facies types and that the bedding rhythm is partly the result of different cementation of the sediment during early diagenesis. Correlation of individual layers in the bioturbated facies reveals that beds are thicker and enriched in lime mud in the western part of the basin, and decrease in thickness and contain less micrite towards the east, further away from the main shallow-water areas. Nearslope calciturbidites change gradually from distinct layers into lateral arrays of micrite nodules and bands further out in the basin. These observations point to a platform source of lime mud in the Buchenstein basin.