A new type of bedform produced by backfilling processes in a submarine channel, late Miocene, Tabernas-Sorbas Basin, SE Spain

The late Miocene "Solitary Channel" Tabernas-Sorbas Basin, SE Spain, has been interpreted as a submarine channel fed by sediment gravity flows from the east. In this paper, the channel is reinterpreted as a lower-slope erosional channel fed by sediment gravity flows from the west. The channel shows cobble/pebble lag deposits, including breccias, associated with erosional phases with substantial sediment bypass, and a later infill by episodes of inclined backstepping macroforms (the primary focus in this paper), mainly comprising sands, interpreted here for the first time as channel backfill deposits. These inclined sandy macroforms, typically 2-5 m in height and 30-40 m in length, are described in detail for the first time in this paper, and are interpreted as a new large-scale sedimentary structure. We observe that the seeding process for the inclined sandy macroforms appears to have been in the upstream depression immediately behind ridges on the surface directly overlying cohesive debris-flow deposits. The internal channel architecture is interpreted in terms of fluctuating relative base levels. A purely local tectonic explanation for the inclined sandy macroforms is discounted because within the bed bundles, dips are essentially constant across the intrachannel disconformities. We speculate that the most likely overall change in base level throughout the history of the channel was driven by regional tectonic change. The higher-frequency variations were probably a consequence of fluctuations in sediment supply/caliber from the source area and/or of cycles of eustatic or regional sea-level changes. The channel was abruptly overlain by about 200 m of marls and then a heterolithic sheet-like turbidite system typical of a confined basin-floor setting. This change in depositional style represents a response to a significant over-all decrease in basin-floor gradient, in which there was a differential change in base level, shown by the coeval development of a major angular unconformity farther east (Sorbas area). The channel history is important for sequence-stratigraphic modeling because it demonstrates that a backstepping fill can be caused by an overall tectonic control on the accommodation space (initiation and abandonment). Higher-frequency source-area changes in sediment flux/caliber and/or eustatic sea level probably exert a strong influence on the detailed depositional architecture in the channel (multiple bypass-backfill events).