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Carbonate-to-biosilica transition at the Norian–Rhaetian boundary controlled by rift-related subsidence in the western Tethyan Lagonegro Basin (southern Italy)
Full text linkReceived 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: [email protected] (M. Casacci), [email protected]
(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
Oceanic rift-driven transition from carbonate to biosilica sedimentation in the western Tethyan Lagonegro Basin (southern Italy) during the Late Triassic
No full textHydrothermal influences in the Late Triassic Lagonegro Basin (southern Italy): evidence of contemporaneous mid-ocean-ridge spreading in the western Tethys
No full textHydrothermal influences in the Late Triassic Lagonegro Basin (southern Italy): evidence of contemporaneous mid-ocean-ridge spreading in the western Tethys
No full textThe south-western branch of the Tethys Ocean was interested, during the Late Triassic, by the opening and spreading of an oceanic basin, the Ionian Ocean. Its north-western termination was characterized by carbonatic/siliceous deposition inside the Lagonegro Basin, which was imposed on transitional crust. Three stratigraphic sections of the Lagonegro Basin sedimentary record, representing a proximal-to-distal transect (e.g., Casacci et al., 2016) from the continental margin,
have been investigated to evaluate the interaction of this basin with the adjacent Ionian Ocean. This ocean was actively spreading since the Middle Triassic (Passeri et al., 2014) but no investigation has been done on the influence of the ridge activity on its peripheral portion situated northward, represented by the pelagic sediments of the Lagonegro Basin. Three major inputs into the sediment were recorded: detrital clay, biogenic silica and carbonates, and most of the samples here studied consist of mixtures of these components. In order to discriminate the chemical signature of the main fractions, bulk and partition geochemical investigations have been carried out in relation of the three main components of the sediments. The rare earth elements (REEs) reflect different pattern for each sediment fraction with the SiO2-bio group showing middle REEs enrichment, suggesting Fe oxyhydroxy influence; the clays group shows a flat pattern typical of shale from siliciclastic input; the carbonate group indicates heavy REEs enrichment and Ce negative anomaly, pointing out seawater influence. Enrichments in hydrothermally derived elements, from the submarine weathering of middle oceanic ridge basalts (MORB), were found in both the clay and carbonate fractions indicating a moderate mixing of the two end-member solutions, hydrothermal fluids and seawater, during the deposition of the Lagonegro Basin sediments. The tectonics of the basin and the different setting for each section left the shallower and more proximal section to the continental margin, without any record of the hydrothermal influence. The results can be considered as a first geochemical evidence of the hydrothermal activity related to the Ionian Ocean ridge, in the Lagonegro Basin deposits
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