Abstract

Jurassic to lower Tertiary pelagic carbonates in the Umbria-Marches Apennines of northern peninsular Italy are well bedded, and synsedimentary submarine slide deposits are common. Bedding planes were formed by pressure solution during compaction. Where original rhythmic alternations in clay content were present, bedding formed along the clay-rich horizons. Where the original sediments were uniform, stylolitic pseudobedding approximately marks paleohorizontal, even where true bedding departs from paleohorizontal because of the presence of soft-sediment folds. Deformational facies observed in synsedimentary slides are compared to a conceptual model of a bedding-plane glide with a longitudinal sequence of four deformational facies: (1) The slide scar, where section is missing, may be identified if biostratigraphic or magnetostratigraphic zones are missing. (2) The stretched rear of the slide is thinned and shows extensional shear joints and sedimentary boudinage. (3) The transported but undeformed main body of the slide may be identified paleomagnetically if it has been rotated, and the basal shear surface may show deformational structures. (4) The shortened front of the slide commonly shows, on a small scale, the ramp thrusts and folds typical of foreland fold-and-thrust belts. The state of induration of the sediments at the time of movement is shown by their theological behavior. In areas of major subsea relief, slides may have moved far enough for the distal parts to lose their coherence, and all the transitional stages from a slide to a pebbly mudstone can be identified.

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