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Alternating Carbonate and Siliciclastic Beds in a Fault-Controlled Continental Basins (Early Permian, Northern Italy): Sedimentological Control on Facies Organization and Diagenetic Overprint


Sedimentary processes in continental successions deposited in fault-controlled extensional-transtensional basins are controlled by different factors: tectonics, basin size and morphology, depositional mechanisms, sediment supply, climate. These settings, where a wide range of depositional environments is recorded, represent a challenging target for hydrocarbon exploration. The Early Permian continental succession (more than 1000 m thick) of the Southern Alps of Italy was deposited in a transtensional basin, under arid/semiarid climate conditions, where an alluvial fan to floodplain/shallow lacustrine-palustrine system was developed. The distal part of this depositional system is characterized by the alternation of: a) fine-grained sandstones to shales (heterolithic facies association), in very thin to medium-size beds, with plane-parallel laminations and subaerial exposure structures (desiccation cracks, vertebrate tracks, raindrop imprints) and b) carbonatic layers, mainly oncolithic. Plane-parallel layering and high lateral persistence indicate a nearly flat depositional surface, homogenous sedimentation conditions, and scarce erosional processes, suggesting deposition of siliciclastics by sheet-floods during episodes of rare and heavy rainfall on the surrounding reliefs. The continental carbonates are characterized by oncoids up to 7 cm in size and build continuous beds up to 40 cm thick. Carbonate production reflects periods of reduced to absent siliciclastic input, suggesting the alternance of almost exclusive depositional processes, able to shift sedimentation from intrabasinal (carbonates) to extrabasinal (sandstones and pelites, when carbonate production was absent). The bedded siliciclastics are frequently characterized by intercalation of sandstones with carbonate cement, characterized by a brownish color that characterize the whole beds or aligned nodules along a single bed. The origin of these carbonate cement is considered to be related to early diagenetic conditions. These carbonate cements are observed only in the sandstone layers: their occurrence in continuous layers or aligned nodules was probably controlled by the original porosity of these layers and by the presence of an oscillating water table. The result of this evolution is a succession with a sedimentological cyclicity (oncoidal palustrine carbonates and siliciclastic) plus a diagenetic overprint, documented by carbonate cements in sandstone layers.