Abstract: Effect of Shape Sorting on Porosity of Carbonate Sands and Gravels

R. J. Vinopal, A. H. Coogan

An overall average porosity value for carbonate sands composed of varied-shaped particles has remained undetermined because of the unpredictable effect of different fossil particle shapes on porosity. In this study porosity was measured for 250 single, dual, and multicomponent packs of varied shaped, particulate sands and gravels of natural and artificial origin. Single-shape component packs, where the shapes approximately correspond to Zingg shapes as well as less regular shapes, have repeatedly determinable average porosities for moderately well-sized sediments as follows: (1) discs = 35 percent, (2) spheres = 43 percent, and (3) rods = 46 percent. Packs of irregularly contorted rods and coral sticks (Porites) average 59 percent porosity and algal-stick sands (Goniolith n) average 62 percent porosity. Porosities of single-shape packs of various pelecypod shells range from 65 (Transanella) to 85 percent (Anomia).

Dual-shape, two component packs (e.g., discs + rods) have porosities proportional to the percent of the dominant component of the mix, making the porosity of two component mixtures essentially predictable.

Packing of three or more varied-shaped particles (e.g., spheres + blades + rods) tends to block and cancel the interference effects of radical shape which in single-shape component packs increases porosity. Consequently, carbonate sands and gravels of grains with high shape variability tend to cluster about the center of the shape-influenced porosity range of 35 to 85 percent, or at about an estimated value of 60 percent porosity. The measured value for an artificially prepared multicomponent mixture is 67 percent porosity. The value for a carbonate beach gravel is 65 percent porosity.

Thus, lacking any other information on grain type or shape, a value of 65 percent may be presumed for the average interparticulate porosity of a carbonate sand or gravel of moderate size sorting before it is cemented, compacted, or leached.

AAPG Search and Discovery Article #90978©1975 GCAGS-GC Section SEPM Annual Meeting, Jackson, Mississippi