Carbonate-to-biosilica transition at the Norian–Rhaetian boundary controlled by rift-related subsidence in the western Tethyan Lagonegro Basin (southern Italy)

Abstract

The Lagonegro Basin, located in the Southern Apennines, Italy, records a transition from carbonate sediments of the Calcari con Selce Fm. (“Cherty Limestone”) to biosiliceous sediments of the Scisti Silicei Fm. (“Siliceous Shale”) around the Norian–Rhaetian boundary (NRB), Upper Triassic. Sedimentologic and chemostratigraphic analysis of three sections (Pignola-Abriola, Monte Volturino, and Madonna del Sirino) representing a proximal-to-distal transect across the basin document marine productivity, redox, and chemical weathering conditions. The basin was characterized by low to moderate productivity and mainly oxic deep waters, with no systematic secular variation throughout the ~ 10-Myr-long study interval. A weathering proxy, the chemical index of alteration (CIA*), shifts toward higher values (from ~ 0.70 to ~ 0.80) in all three study sections, suggesting development of warmer and/or more humid conditions just before the NRB. The lack of evidence for changes in marine productivity or redox conditions and the time-transgressive nature of the carbonate-biosilica transition suggests that the latter was caused not by changes in planktonic communities but, rather, by a rise in the CCD, the most likely cause of which was seafloor subsidence within the actively rifting Ionian Ocean (i.e., the westernmost branch of the Tethys Ocean). The tectonically active character of the Lagonegro Basin is also reflected in evidence of synsedimentary paleoseismic activity, e.g., numerous debris flows around the basin margins. The present study thus documents sedimentologic changes within a marginal basin in a geologically young ocean

The Norian “chaotic carbon interval”: New clues from the δ 13 C org record of the Lagonegro Basin (southern Italy)

“…Repeated thin and well-laminated interbeds of black shales with the shaly interval across and above the NRB suggest a transient period between suboxic and/or anoxic to more oxic conditions (Casacci et al, 2016). The observed sedimentation pattern suggests that the Pignola-Abriola section belongs to the intermediate facies association (Scandone, 1967;Rigo et al, 2005;Giordano et al, 2010;Casacci et al, 2016).…”

“…The Scisti Silicei is characterized by centimeter-thick multicolored cherts, radiolarites, and often siliceous shales (Amodeo, 1999;Bertinelli et al, 2005a;Giordano et al, 2010). This formation represents the beginning of the biosiliceous sedimentation within the Lagonegro Basin, occurring from the uppermost Norian to lower Hettangian, thus the base of the Scisti Silicei is considered diachronous (Bertinelli et al, 2005a;Reggiani et al, 2005;Giordano et al, 2010Giordano et al, , 2011Casacci et al, 2016). The gradual passage between the Calcari con Selce and the Scisti Silicei is represented by the so-called "transitional interval" (Miconnet, 1982;Amodeo and Baumgartner, 1994;Amodeo, 1999;Bertinelli, 2003;Rigo et al, 2005Rigo et al, , 2012aGiordano et al, 2010).…”

“…This part of the section is characterized by a progressive decrease in the relative abundance of carbonates in favor of the cherty and siliceous components (Amodeo, 1999;Bazzucchi et al, 2005;Rigo et al, 2005;Tanner et al, 2006;Giordano et al, 2010). Repeated thin and well-laminated interbeds of black shales with the shaly interval across and above the NRB suggest a transient period between suboxic and/or anoxic to more oxic conditions (Casacci et al, 2016). The observed sedimentation pattern suggests that the Pignola-Abriola section belongs to the intermediate facies association (Scandone, 1967;Rigo et al, 2005;Giordano et al, 2010;Casacci et al, 2016).…”

Sedimentary record of Upper Triassic impact in the Lagonegro Basin, southern Italy: Insights from highly siderophile elements and Re-Os isotope stratigraphy across the Norian/Rhaetian boundary

“…The onset of NRB extinctions was accompanied by negative organic carbon-isotope excursion (δ 13 C org ), suggesting that a combination of climatic and environmental changes impacted the biota at a global scale (Zaffani et al, 2017;Rigo et al, 2020). However, the causes of the NRB extinctions and biotic turnover events are still the subject of active debate (Trotter et al, 2015;Casacci et al, 2016;Rigo et al, 2020). Catastrophic processes such as emplacement of large igneous provinces (LIPs) and extraterrestrial impacts have been discussed to account for the biotic turnover events across the NRB (Sephton et al, 2002;Zaffani et al, 2017;Rigo et al, 2020).…”

Biotic and environmental changes in the Panthalassa Ocean across the Norian (Late Triassic) impact event

“…2) have been examined for redox changes using Mo and U enrichments and Ce/Ce* anomalies. Redox changes in the middle to upper Norian interval have not been detected (Casacci et al 2016). In summary, the Norian impact did not appear to induce redox changes in the Panthalassa and western Tethys oceans, although this interpretation requires further geochemical study at other middle to late Norian sites.…”