Porosity and Packing of Holocene River, Dune, and Beach Sands

ATKINS, JOHN E., Conoco, Inc., Oklahoma City, OK, and EARLE F. McBRIDE,* University of Texas at Austin, Austin, TX

The porosity and packing of 174 samples of well-sorted surficial and shallowly buried (to 17 m), unconsolidated Holocene sands were determined by point counting the upper surface of thin sections of epoxy-impregnated samples in reflected light. Average depositional porosity for 124 surficial beach sands, river point-bar and braid-bar sands, and eolian dune sands is between 47% and 49% and the range is between 40% and 58%. Beach sands exhibit an average packing value (contact index = CI) of 0.79, river sands an average CI of 0.91, and eolian dune sands an average CI of 1.02. Packing gets tighter with depth, but we found no decrease in porosity with depth for river and beach sands buried to 17 m. Thus, packing is more sensitive to small changes in fabric than is porosity.

Beach sands typically contain 5.5% oversized pores (OSP), river sands 3.8% OSP, and eolian dune sands 4.0% OSP. Most OSP are packing defects rather than dissolution pores, although trapped air bubbles are common in some beach sands. OSP decrease linearly with depth to 17 m, our deepest sample. Extrapolation of our data indicates that they will be destroyed at a depth less than 100 m.

Significant differences in porosity, oversized-pore, and packing values exist between most point-bar and braid-bar deposits and between two heavily sampled point bars, but no significant differences in these values exist when braid-bar sands are compared to other braid-bar sands. Sands from different beaches have significant differences in porosity, oversized pores, and packing.

The average porosity is 46% for eolian ripple strata, 50% for grain-fall strata, and 51% for grain-flow strata. Ripple strata are tighter packed than grain-fall and grain-flow strata.

Work was supported by NSF Grant 8720040.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)