--> Characterizing the Roles of Sediment Concentration and Substrate Composition on the Properties of Basal Sandy Layers in Transitional Flow Deposits

2019 AAPG Annual Convention and Exhibition:

Datapages, Inc.Print this page

Characterizing the Roles of Sediment Concentration and Substrate Composition on the Properties of Basal Sandy Layers in Transitional Flow Deposits

Abstract

We summarize the results from recent laboratory experiments that highlight predictable variations in the attributes for basal sandy layers of transitional flow deposits. The aspect ratios for basal sand layers deposited on a flat, inclined, non-erodible surface were found to vary systematically as a function of initial sediment concentration (sand + mud). At a volumetric sediment concentration of 5 percent, the downslope length and half-width of the deposit were sub-equal, similar to the expected ratio for traditional turbidites. A departure from turbidite shape occurred as sediment concentrations produced effective viscosities for mixtures that were 15, 30, and 55 times greater than water. As volumetric sediment concentrations increased to 15, 20, and 25 percent, the fraction of basal sand deposited in the downslope direction was 1.5, 4, and 5 times greater than that deposited in the strike direction. This relative lengthening of the deposits was primarily the consequence of higher effective viscosities that substantially reduced the rates of lateral spreading, which in turn limited deposit half-widths. Variogram analyses of geostatistical models (Vmud) built using acoustic volumes and core from the deposits of flows with sediment concentrations of 15, 20, and 25 percent revealed a similar anisotropy in correlation lengths. The ratio of Vmud correlation lengths for the basal sand layers in the major (parallel to flow) to minor (perpendicular to flow) directions were 1.5, 2.7, and 5.2, respectively. These predictable trends for basal sand layers were substantially altered when transitional flows ran across erodible substrates. In cases where this substrate was mud, its incorporation into the depositing flow had two important effects: (1) mixing of mud up into the settling sand acted to reduce the volume of the high permeability basal layer as some of its sand was transferred into a sandy mud layer; and (2) spatially variable entrainment of the substrate increased the complexity of the basal sand-layer shape, producing multiple fingers extending in the downslope direction. These experiments clearly demonstrated how substrate properties can wield a first order control on the form of transitional flow deposits and particularly their basal sand layers. Implications for natural systems and deposits will be discussed.