Paper No. 345-5
Presentation Time: 2:05 PM
BEDFORMS INDICATIVE OF SUPERCRITICAL FLOW IN STEEP, SANDY SUBMARINE FANS AND FAN DELTAS: AINSA, EBRO AND TABERNAS BASINS, SPAIN
The deposits of migrating, aggrading bedforms indicative of supercritical sediment gravity flow conditions (supercritical dunes, antidunes, and cyclic steps) have been recognized in submarine fan and fan delta successions in the Ainsa, Ebro, and Tabernas basins in Spain. These outcrops preserve strata that were deposited in tectonically-active settings, with accommodation strongly influenced by active structures and sediment supply fed by nearby high-relief source areas. These basins preserve examples of connected, time-equivalent depositional systems that can be traced, in a source-to-sink manner, from fluvial/alluvial to marginal-marine/deltaic to slope channel and finally deepwater lobe settings. Morphodynamic successions (sets of genetically-related strata that record the evolution of flow conditions deduced from these units) in mouth-bar, delta front, clinoform gully, and perched and terminal lobe depositional settings contain lamina, lamina set, and bed geometry and morphology that are interpreted to be the result of bedform configurations formed under stable and evolving supercritical sediment gravity flow conditions. These features include decimeter-scale sigmoid-like, humpback trough crossbedding, decimeter- to meter-scale asymptotic and downlapping upstream-dipping foresets, brinkpoint and topset preservation, grain and outsized clast imbrication, and associated scour and fluidization structures. Sigmoid-like humpback crossbedding is interpreted to have formed by the migration of dunes. Upstream-dipping asymptotic foreset strata are interpreted to have formed by upstream migrating, long-wavelength antidunes, while upstream-dipping, downlapping foreset strata are interpreted to have formed by upstream-moving traction carpets encroaching into the scour pit of an upstream-migrating cyclic step. Brinkpoint and topset strata preservation is related to bedform climb angle, and the ratio of apparent downstream migration to bed aggradation rate. Grain and clast fabric, including fluidization features, reflect local flow conditions on the lee and foreset sides of cross lamina and bed sets. These features have been replicated in flume experiments in order to document the flow conditions responsible for their formation and bed phase.