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Flow Processes, Sedimentation, and Stratigraphic Evolution of the Submarine Channel-Lobe Transition Zone: an Outcrop-Based Study


The submarine channel-lobe transition zone (CLTZ) is the region on a submarine fan that marks the interface between updip channels and downdip lobes. At the scale of an individual architectural element, the CLTZ governs sandstone connectivity between a channel and a lobe, and it serves as the link between locally confined and unconfined conditions for sediment gravity flows that build the submarine fan. Outcrops of the Guaso I turbidite system of Ainsa basin, Spain, include a continuous, ∼4-km-long, paleocurrent-parallel transect through a sandstone unit. This exposure is interpreted as a longitudinal transect from an updip channel, through the CLTZ, to the attached downdip lobe. At the southern (updip) end of the transect, outcrops of the channel are up to 10 m thick, and they contain relatively coarse-grained, clast-rich, structureless, often amalgamated sandstones that overlie an erosive lower contact. There are also planar-laminated and rippled sandstones within the channel. At the interpreted CLTZ, the element thickens by several meters in the basinward direction, due to the updip onlapping of oldest bedsets of the lobe. Scouring, amalgamation surfaces, and erosional relief near the base of the element generally decrease in a downdip direction toward the lobe. At the lobe, the element is up to 16 m thick. It has a non-erosive base, and it contains laterally extensive bedsets. Point counting and grain-size analysis document that sandstones of the CLTZ and the lobe are generally finer-grained and better sorted than channel sandstones. This is interpreted to be due to different combinations of depositional processes operating in the locally confined (channel) domain and the unconfined (lobe and CLTZ) domain. A four-stage, generalized sequential model that describes the stratigraphic architecture and evolution of a channel-lobe element is presented herein.