Aragonite dissolution, sedimentation rates and carbon isotopes in deep-water hemipelagites (Livinallongo Formation, Middle Triassic, northern Italy)

Abstract

Hemipelagic nodular limestones are a widespread facies in the Triassic of the Tethys, often considered as being deposited at rather constant sedimentation rates. The aim of this paper is to investigate the sedimentation rate variability in a case-study from the Middle Triassic of the Dolomites, northern Italy, and to suggest possible causes. The nodular cherty limestones of the Livinallongo Fm. and correlated units were studied in four stratigraphic sections and compared to classical successions from the literature. Correlations between sections are based on ammonoid biostratigraphy and tephra stratigraphy. Correlation highlighted conspicuous changes in sedimentation rates through time and between sections, associated with sedimentological evidence of deep-water aragonite dissolution. Deep-water dissolution is believed to have resulted in small hiatuses and in a bias of ammonoid assemblages towards taxa with more resistant shells. Carbonate petrography and geochemistry provide evidence of differential diagenesis of the Livinallongo Fm. Carbonate nodules were lithified at the water–sediment interface, and their C isotope composition is regarded as a proxy of the δ 13C of Middle Triassic bottom seawater. The onset of nodular cherty limestones, occurring contemporaneously at the basin scale, and the coeval carbonate platform drowning events are tentatively explained by the inflow and local upwelling of cool deep water from Panthalassa into the western Tethys.