Sedimentary Geology, 47 (1986), 191-219

I.P. MARTINI¹, A. RAU² and M. TONGIORGI³

1Department of Land Resource Science, University of Guelph, Guelph, Ont. N1G 2W1 (Canada)
²Consiglio Nazionale delle Ricerche, Centro di Studio per la Geologia Strutturale e Dinamica dell'Appennino, Pisa (Italy)
³Dipartamento di Scienze della Terra, Universito di Pisa, Pisa (Italy)

The anatomy of the sedimentary and volcanic fill of a Middle Triassic continental rift at the western margin of the Tethys is illustrated by the study of a 250 m thick sequence on the Tyrrhenian coasts of the Northern Apennines. This sequence contains two cycles of sedimentation:
   (a) The lower cycle comprises a basal transgressive sequence from continental alluvial fans to shallow marine carbonates, and an upper sequence that correspond to an increasing tectonic activity (sub- marine syntectonic facies assemblage). The lower part of the last assemblage is characterized by a thick (22 m), rapidly deposited sequence of coarse carbonate breccias with minor calcareous sandy turbidites. This is overlain by channelized red beds of submarine fans composed of close interstratification and juxtaposition of carbonate breccias with red clastic matrix, and red sandy quartz pebble conglomerates and silty sands. Interbedded alkaline pillow lava record penecontemporaneous submarine eruptions. The lavas are capped by thin red cherty (diaspri) crusts. The lower cycle of sedimentation ends with a karstic surface on relatively deep marine carbonate layers. This suggests a rapid emersion due to the arrest and reversal of the downsinking of the initial ("aborted" or failed) rift.
   (b) The upper cycle of sedimentation starts with more mature fluvial red beds and changes upward into shallow marine sands and carbonates. This upper cycle represents an early stage of evolution of a new major rift. This second major rift led, in Jurassic times, to the opening of an oceanic area between Europe and western Italy.