Topographic Controls from Deep-Water Sedimentation Patterns - the Casaglia Monte Della Colonna Submarine Slide, Marnoso-Arenacea Formation (Miocene), Northern Italian Apennines
Field studies of the Casaglia Monte della Colonna submarine landslide (Casaglia Slide) of the Marnoso-Arenacea Formation (Miocene) in the Northern Italian Apennines reveal that significant seafloor topography, in excess of 250 meters, can be associated with mass-transport deposits. Three scales of topographic relief were identified above the Casaglia Slide: local, sub-regional, and regional. The extreme spatial continuity of turbidites below the Casaglia Slide suggests that the slide was emplaced onto a basin plain with low gradients above a detachment with ramp and flat geometry. Structures in the ramp zone consist of elongated antiforms and synforms, oriented northwest-southeast and paralleling a main basal ramp. These structures comprised ridges and valleys on the seafloor after emplacement of the slide. Associated local topographic relief is greatest above the ramp zone of the basal detachment and ranges from 12 - 20 meters of relief over distances of 200 - 400 meters. Local relief is lower above the flat zone in the basal detachment (seafloor flat) and ranges from 1 - 5 meters over distances of 300 - 600 meters. The sub-regional topography on the seafloor flat zone is about 40 m of relief over a distance of 3.3 kilometers. The most significant sub-regional topographic relief is observed in the ramp zone where a vertical change of 173 meters over a distance of 1.6 kilometers is related to an antiformal ridge above the ramp zone in the Casaglia Slide. On a regional scale, the topographic relief of the Casaglia Slide changes by about 250 meters over a distance of 9 kilometers. The regional slope of the upper surface of the Casaglia Slide is 1.6 degrees in both the depositional dip and strike directions, a much steeper slope than expected for a basin plain, especially one sloping up in the down depositional dip direction. Within the ramp zone, changes in paleocurrent directions in single beds demonstrate that flows were reoriented by local topography. Bed terminations and the occurrence of dune-scale cross-lamination suggest flow confinement by these structures. Also within the ramp zone, a dramatic decrease in net-to-gross and turbidite bed thickness is observed between two outcrop locations separated by a major antiform. These changes in stratigraphic architecture are attributed to flow stripping and are evidence of mass-transport deposit topography as a major control on sandstone distribution.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009