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 eros ional channel fed by sediment gravity flows from the west. The channel show s 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 in in height and 3 040 in 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 d irectly overlying cohesive debris-How deposits. The internal channel architecture is interpreted in terms of fluctuating re lative base levels. A purely local tectonic explanation for the inclined sa ndy macroforms is discounted because within the bed bundles, dips are essen tially 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 variat ions were probably a consequence of fluctuations in sediment supply/caliber from the source area and/or of cycles of eustatic or regional sea-level ch anges. The channel was abruptly overlain by about 200 in of marls and then a heterolithic sheet-like turbidite system typical of a confined basin-floo r setting. This change in depositional style represents a response to a sig nificant overall decrease in basin-floor gradient, in which there was a dif ferential change in base level, shown by the coeval development of a major angular unconformity farther east (Sorbas area). The channel history is imp ortant for sequence-stratigraphic modeling because it demonstrates that a b ackstepping fill can be caused by an overall tectonic control on the accomm odation space (initiation and abandonment). Higher-frequency source-area ch anges in sediment flux/caliber and/or eustatic sea level probably exert a s trong influence on the detailed depositional architecture in the channel (m ultiple bypass-backfill events).