--> Simplicity Versus Complexity in Lobes Deposited by Transitional Sediment-Gravity Flows: Results From 3-D Flume Experiments

AAPG ACE 2018

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Simplicity Versus Complexity in Lobes Deposited by Transitional Sediment-Gravity Flows: Results From 3-D Flume Experiments

Abstract

Although submarine fans have been studied for six decades, 3D depositional heterogeneities in sub-seismic scales are poorly understood due to scarcity of three dimensionality in outcrops. We use 3D flume experiments bridging the gap between subsurface and outcrop dataset. Bathymetry in sub-millimeter resolution, 3D acoustic imaging, and extensive coring of experimental deposits have enabled us to quantitatively analyze the stratigraphy and surface architecture of lobes formed by unconfined sediment-gravity flows. Our study reveals how discrete transport processes result in intricate deposits and deviation from the classic lobate form. Although a complex overall geomorphology the down-flow change in the axial thickness of basal-sand layers can be predicted using an advection-settling model. Everything else being equal, the rate of decrease in thickness is controlled by the effective viscosity of flow. Basal-sand thicknesses decrease over shorter distances in lateral directions. The resulting anisotropy is axisymmetric, producing elongated lobes. This eccentricity is controlled by the viscosity and strength of flows, with increases in either one acting to produce elongated deposits. Complexity in discrete inter lobe divisions is produced by two stages of deposit remobilization. Early remobilization occurs along decollements at the top of basal sand. These decollements are lubricated by pore fluid escaping from the basal sands and collecting beneath the capping muddy intervals. This remobilization produces an underlying relatively clean sand overlain by argillaceous sand and mud, similar to linked debrites of literatures. The later-stage remobilization is produced by drag from the fluidal muddy tails of flows. This remobilization is focused along lobe axes, etching channel-like grooves into the uppermost deposits. The remobilized material ends up being worked into sets of compressional ridges mantling the surfaces of lobes. Analysis of these ridges reveals a range of wavelengths and curvatures, connected to flow strength. These architectures are only reflected in the muddy capping intervals and not translated to the top of basal sand. These stratigraphic discordances between sub-layers within the lobes were developed by vertical and lateral stratification within the transitional flows. Quantification of the simplicity and complexity tied to lobes can be used to build realistic geologic models characterizing heterogeneous reservoirs from the bed to fan scales.