A FRESH LOOK AT CONTROLS ON SAND DEPOSITIONAL POROSITY

It has been over forty years since Beard and Weyl (1973) published their seminal work on the influence of mean grain size and sorting on sand depositional porosity. This work has been cited in over 400 scientific papers in the past ten years alone. Despite its significance for sedimentology, diagenesis, rock physics, and other fields, the extent to which these data are representative of the broad range in sand compositions and textures is unknown given that it is based on sand samples from one source terrain: medium-sized rivers from the Texas Gulf Coast.

We evaluated the sensitivity of depositional porosity to composition and texture using artificial sand packs made from three additional sources: (1) quartz sand from the St. Peter sandstone, (2) Animas River sand that contains abundant feldspar and rock fragments, and (3) sand from an arroyo in Rio Rancho, NM that also is rich in feldspars and rock fragments. We created grain packs for each of the three sources by sieving the sands into size fractions that we then recombined to achieve fine, medium, and coarse mean grain sizes for five sorting states ranging from very well to poorly sorted. Our results bracket those of Beard and Weyl’s and are comparable in that sorting is the primary control on depositional porosity variations for a given sand source with all three of the sand sources showing ~9 vol% lower porosities in the poorly sorted packs compared to very well sorted packs. Depositional porosities also may be affected by grain size but to a lesser degree (2-3 vol% higher in fine grained packs in two of the three sand sources but no difference between medium and coarse packs).

A key new finding is that the sand source influence on depositional porosity is of similar magnitude (up to 10 vol%) to the sorting effect for a given sand source. Packs from the St. Peter source have the lowest porosity values (~30-39 vol%) whereas Animas River packs have the highest porosities (~40-49 vol%) for very well to poor sorting (Rio Rancho: ~35-45 vol%; comparable Texas Gulf Coast: ~35-45%). The source dependent porosity differences likely reflect differences in grain shapes and compositions. Compared to the smooth, rounded quartz grains of St. Peter samples, the abundant feldspars and lithic fragments in Animas River sands are rougher and more angular.