Sedimentological and stratigraphic signature of the Plio-Pleistocene tectonic events in the Southern Apennines, Italy: The Calvello-Anzi Basin case study

The Plio-Pleistocene Calvello-Anzi Basin (CAB) represents a synorogenic shallow marine to alluvial succession deposited during the late phase of the Apennine Orogenic event. While the stratigraphy and sedimentary cycles are well known, the precise role and timing of the tectonic phases affecting this area are unclear. In this paper we report the major findings from a study aimed at unraveling the signatures of the Plio-Quaternary tectonic events affecting the CAB and this portion of the Southern Apennines by using a field integrated approach characterized by facies analysis, structural surveying, and basin architecture. The CAB is located in the axial portion of the Southern Apennine chain and has a roughly E-W orientation which is at a high angle with respect the main NW-SE striking regional tectonic structures controlling the foreland-directed migration of the compression. In the study area, an approximately 1,200-meter-thick package of Zanclean (early-middle Pliocene) to Santernian (middle Pleistocene) shelf, shallow marine, and alluvial deposits accumulated during the late stages of the Apennine orogenic evolution. Within the CAB deposits we recognize two main depositional motifs which roughly correspond to previously established regional cycles that are genetically linked to two corresponding tectonic phases of the Southern Apennine chain. Our results reveal how these two main tectonic phases played a primary role in controlling the depositional architecture, facies, depocenter distribution, and the basin shape throughout the CAB's lifespan. In the proposed model, during the first depositional stage (Zanclean), the CAB formed as a wedge-top basin, controlled by the subsurface propagation of NW-SE-trending thrusts related to the main NE-SW directed shortening of the Apennines. This stage is recorded in the basal units of the CAB infill by progressive unconformities, the development of syn-tectonic shallow marine sedimentary wedges along a high gradient tectonically controlled depositional surface, and a marked SE migration of the main depocenter. During the second depositional stage (upper Piacenzian to Santernian), the CAB was controlled by a series of roughly E-W-trending normal faults which crosscut the preexisting compressional structural fabric. Syndepositional activity along these faults played a direct role in creating an E-W oriented tectonic depression and controlling the accommodation space for the upper portion of the CAB infill. This second stage is marked by two points of evidence. The first point is revealed by a southwestward migration of the CAB depocenter driven by ∼90-degree rotation from a roughly NW-SE elongated pattern to roughly elongated W-E transverse to the Pliocene regional structural fabric. This depocenter drift also coincides with an abrupt change in depositional regime from shallow marine to continental sedimentation. The second and most important point of evidence is the presence of several basin-bounding extensional fault zones which cut the lower Pliocene basin-fill units. We relate these features to the middle upper Pleistocene transition to along-strike extension, widely reported in the Southern Apennines, which is related to the overall extensional regime that is acting on the chain also at present. Data from this study provide additional insight into the evolution of the Southern Apennine chain, the formation of the Plio-Pleistocene CAB, and the debated role of Quaternary tectonics on the basin's development during the post-collision orogenic phases. Given the proximity of the study area with several oil fields and considering that hydrocarbon traps in Southern Apennines are Plio-Quaternary in age, the constraints provided on the timing and the style of the regional deformation may improve the definitions of hydrocarbon traps in the buried Apulian platform.