Hydrodynamic Fractionation of Grains on the Basis of Density and Shape in Distributive Deepwater Settings: An Experimental Study

Recent discoveries in distributive submarine fans reveal spatially varying mineralogical compositions that affect primary and secondary porosity and permeability.

Turbidity currents longitudinally fractionate, or sort, grains on the basis of size. How do they fractionate grains on the basis of mineralogy? Each of the common sandstone-forming minerals such as quartz, feldspar, and mica has distinctive densities and shapes, which affect settling velocity of grains in a suspension. This study uses measurements from two experiments conducted in the Tulane Deepwater basin to document how turbidity currents spatially fractionate grains on the basis of density and shape. The basin is 6 m long, 4 m wide, and 2.2 m deep, and is outfitted with an XYZ data collection carriage which houses an acoustic Doppler profiler and a laser topography scanning system. Both experiments used engineered sediment with similar grain-size distributions (D50~100 μ). Experimental turbidity currents had 2% excess density from suspended sediment and 2% excess density from dissolved salt which resulted in currents with Reynold’s numbers of 18,000. Deposits from both experiments were lobate in form. Samples of the deposits were collected using a common grid and analyzed with QEMSCAN, an automated and quantitative scanning electron microscope.

The first experiment examined how grains fractionate on the basis of density. The experimental currents had equal proportions by volume of spherical ballotini (ρ=2.50 g/cm3) and spherical zirconia silicate (ρ=3.85 g/cm3). Analysis of the deposit documents that the proportion of high-density particles to low-density particles decreases toward the lateral and distal margins of the deposit. At all locations, high-density particles are smaller than adjacent low-density particles. The second experiment examined how grains fractionate on the basis of shape. The experimental currents had equal proportions by volume of spherical ballotini (ρ=2.50 g/cm3) and angular crushed glass (ρ=2.50 g/cm3). Analysis of the deposit documents that the proportion of angular particles (crushed glass) to spherical particles (ballotini) increases toward the lateral and distal margins of the deposit.

Results indicate turbidity currents can spatially fractionate minerals on the basis of density and shape. This process explains spatial variations in mineralogical composition described in some submarine fans, and provides empirically derived patterns useful for prediction.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California