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

Received 20 December 2015 Received in revised form 26 April 2016 Accepted 5 May 2016 Available online 8 May 2016 Keywords: Elemental geochemistry Rare earth elements Chemical index of alteration Redox conditions UPPER Triassic 1. Introduction The breakup of the supercontinent Pangea started with rifting in modern eastern North America (Schlische et al., 2003), southern Iberia (Martin-Rojas et al., 2009), and western Morocco and the Atlas Moun- tains region in northwestern Africa (Dewey et al., 1973; Le Roy and Piqué, 2001; Schettino and Turco, 2009, 2011). In the western Tethyan region, this event was marked by the opening of multiple rift basins comprising the Ionian Ocean (Fig. 1). The kinematics of this rifting event resulted in the formation of a number of independent microplates (including the microplate called Adria) and pull-apart basins between Africa and Eurasia (Schettino and Turco, 2011). This first phase of rifting in the Ionian Ocean terminated at the Triassic/Jurassic boundary, when ⁎ Correspondingauthor. ⁎⁎ Correspondence to: T.J. Algeo, Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013, USA. E-mail addresses: manuela.casacci.87@gmail.com (M. Casacci), Thomas.Algeo@uc.edu (T.J. Algeo). http://dx.doi.org/10.1016/j.palaeo.2016.05.007 0031-0182/© 2016 Elsevier B.V. All rights reserved. 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 con- ditions. The basin was characterized by low to moderate productivity and mainly oxic deep waters, with no sys- tematic 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 devel- opment 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.