Paleoceanographic Changes in the Lagonegro Basin (Southern Italy) during the Late Triassic Linked to Oceanic Rifting in the Western Tethyan Region

Abstract:

The Lagonegro Basin was part of the southwestern branch of the western Tethys, an actively spreading young ocean during the Late Triassic (Ciarapica and Passeri, 2002, 2005). The sedimentary environment was a deepening-upward basin, bordered to the north by the Apenninic and Apulian carbonate platforms. Paleoseismic activity is evidenced by frequent debris flows on the basin margins (Passeri et al., 2005). The Lagonegro succession is characterized by Permian to Miocene formations deposited in shallow to deep basinal environments. The Upper Triassic is comprised of deep-marine sediments belonging to the Calcari con Selce (“Cherty Limestone”) Formation of late Ladinian to late Norian-early Rhaetian age and the Scisti Silicei (“Siliceous Shale”) Formation of late Norian-early Rhaetian to Late Jurassic age. The “Transitional Interval” between these two formations is gradational over a 20- to 40-m interval (Miconnet, 1983).

The Transitional Interval was investigated in three sections (Pignola-Abriola, Monte Volturino, and Madonna del Sirino) in the Southern Apennines (southern Italy), representing a proximal-to-distal transect across the Lagonegro Basin. The transition from mainly calcareous to mainly siliceous sedimentation may have been influenced by rapid, post-rift subsidence of the Lagonegro Basin. It also coincided with a shift to warmer or more humid conditions around the Norian/Rhaetian boundary, as reflected in a pronounced increase in the chemical index of alteration (CIA), a weathering proxy (Young and Nesbitt, 1998). Redox proxies indicate mainly oxic conditions in the deep basin, although organic-rich shale beds are present at multiple levels in the otherwise organic-poor succession. The abruptness of the transitions between organic-poor and –rich sediment layers suggests major changes in paleoceanographic conditions, possibly related to switches from lagoonal circulation (linked to a net negative water balance) to estuarine circulation (linked to a net positive water balance). We infer that the Transitional Interval was related primarily to climate-driven hydrographic changes within the Lagonegro Basin.