Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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Flow Processes, Sedimentation, And Stratigraphic Evolution Of The Submarine Channel-Lobe Transition Zone


Ancient and modern distributive submarine-fan depositional systems are common in California, as well as on continental margins around the world. 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 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 a slope channel, through the CLTZ, to the attached basin-floor lobe. At the southern end of the transect, outcrops of the slope channel are up to 10 m thick, and they contain relatively coarse-grained, clast-rich, structureless, often amalgamated sandstones above 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 a basinward direction, due to the updip onlapping of oldest basin-floor bedsets of the lobe. Scouring, amalgamation surfaces, and erosional relief at the base of the element generally decrease in a downdip direction toward the basin floor. The base-of-slope depositional angle is ∼2∞, and the lobe deposited on the proximal basin floor near the northern end of the transect is ∼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 basin-floor 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 confined (channel) domain and the unconfined (lobe and CLTZ) domain; these processes result in the loss of the coarsest grain-size fraction along the transect. Results and interpretations presented herein, which can be applied to turbidite reservoirs in California and elsewhere, are integrated in order to discuss the sequential evolution of a channel-lobe element.