Stratigraphic architecture of a transtensional continental basin In low-latitude semiarid conditions: the permian succession of the Central Orobic Basin (Southern Alps, Italy)

Continental successions in fault-controlled extensional–transtensional basins store a wide range of clastic deposits from various depositional environments, representing a challenging target for hydrocarbon exploration. Recent advances in field-based facies analysis and genetic interpretation of these sediments show that these successions are characterized by great variability in terms of volume, geometry, facies, and stacking patterns. This variability derives from a number of factors that govern the sedimentary processes, such as tectonic setting, basin size and morphology, type and amount of sediments, and climate. The study focuses on a well-exposed Early Permian continental succession (Southern Alps, northern Italy), which was deposited in an extensional–transtensional tectonic regime under a semiarid to arid climate. The study area is characterized by a thick (more than 1000 m) succession of prevailing continental clastics with intercalations of ignimbritic flows and tuffs (Pizzo del Diavolo) Formation resting on up to 800 m of prevailing pyroclastic flows (Cabianca Volcanite). These units are unconformably covered by the Upper Permian alluvial sediments of the Verrucano Lombardo. Sedimentological features reflect a proximal fan system where mass-flow phenomena dominate, a distal fanterminal setting with prevailing sandy sheet-flow processes and a silty floodplain environment, where sheet-flood events dominate. Laterally continuous siltstones and dark shales with carbonate nodules and layers of microbial carbonates in the most depressed part of the basin indicate alternating conditions of shallow lake and desiccated floodplain. Syndepositional tectonic activity is documented in the succession by rapid vertical and lateral facies changes. In this study, we present a data-constrained reconstruction of the depositional architecture of the basin, a description of depositional processes and depositional environments, and a model of facies distribution. Outcrop data, integrated with laboratory analyses, provide a detailed understanding not only of the small-scale elements of the system (sub-seismic scale) but also of the entire basin, focusing on how those elements are interrelated. The studied succession can be considered an interesting outcrop analogue for subsurface extensional–transtensional basins, potentially useful to predict reservoir properties for similar subsurface settings.