David R. Pyles1, Jane G. Stammer1, Kyle M. Straub2 1. Chevron Center of Research Excellence, Department of Geology and Geological Engineering, Colorado School of Mines
2. Department of Earth and Environmental Sciences, Tulane University
Submarine fans contain compensationally stacked channels and lobes that form a distributive, radially dispersive map pattern. Sediment in submarine fans is predominantly delivered by turbidity currents. Grains in the turbidites are longitudinally sorted by size, which is the primary control on settling velocity (ws). However, grain density and shape also dictate ws. Specifically, angular particles have higher drag force (Fg) and therefore lower ws than spheres of equivalent volume; just as spherical, relatively high-density particles have a higher ws than spherical lower-density particles of equivalent volume. Each of the common sandstone forming minerals such as quartz, feldspar, and mica has a distinctive density due to chemical composition and a distinctive shape partly due to mineral cleavage. Here we use measurements from physical experiments and the stratigraphic record to document the efficacy of turbidity currents to spatially fractionate mineral grains on the basis of grain density and grain shape and we discuss how this process can be applied to address practical geoscience problems.
1) Two physical experiments were conducted in the Tulane University’s Deep-Water basin. Three grain types with similar grain-size distributions were used. The first experiment used equal proportions of spherical high- and low-density (control) grains. The proportion of high- to low-density grains decreased by ~ 50% toward the margins of the deposit. At all locations, high-density grains were smaller than low-density grains; however, when normalized by calculated ws, the populations approximately align, indicating that the different grain types were hydrodynamically equivalent. The second experiment used equal proportions of spherical (control) and angular grains of the same density. The proportion of angular to spherical grains increased by ~ 500% toward the margins of the deposit.
2) The Upper Cretaceous Point Loma Formation, San Diego, California contains turbidite lobes that compensationally stack to build a submarine fan. Spatial variations in the mineralogy of one turbidite bed exposed over a lateral distance of ~ 2.5 km are evident. The amount of feldspar, plagioclase, biotite, and terrestrially derived organic material increases relative to quartz toward the lateral margin of the deposit. These minerals are more angular and/or less dense than quartz. In contrast, the amount of hornblende and ilmenite decreases relative to quartz toward the lateral margin of the bed. These minerals are denser than quartz
This process is important because large-scale spatial changes in mineralogy, such as those documented herein, can result in spatial changes in primary and secondary porosity, permeability, Young’s modulus and Poisson’s Ratio, which strongly influence the storage capacity and recoverability of fluids in subsurface reservoirs.