PHYSICAL COMPACTION AS A CAUSE OF REDUCED CROSS-BED DIP ANGLE IN THE NAVAJO SANDSTONE

The Jurassic Navajo Sandstone is world-famous for its stunning large-scale cross-beds. Hunter (1977) and others delineated cross-beds formed by avalanching down dune slipfaces from those produced by related eolian processes. The current dip of the avalanche or sandflow cross-strata is about 24°, significantly lower than the angle of repose on comparable modern dunes. This discrepancy has been attributed to post-depositional modification by compaction, but few attempts have been made to test this interpretation quantitatively. As a pilot study, we made thin sections in 3 mutually perpendicular planes (horizontal, vertical parallel to strike, and vertical perpendicular to strike) from an oriented sample collected from sandflow layers in cross-bedded sandstone just outside Zion National Park (Utah). We then quantified the minus-cement porosity in each direction via point-counting and determined it was 28.5% in the horizontal direction vs. 31.3% in the vertical direction. Assuming a depositional porosity of 43%, this discrepancy corresponds to a differential shortening of about 29% in the vertical direction. With this amount of compaction, the current angle of 24° would correspond to a pre-compaction dip of about 32°, in reasonable agreement with measurements from modern dunes. Dissolution is evident on the edges of some detrital quartz grains, but this could not have been significant in lowering the dip angle because our data indicate well under 1% of grain volume was lost to chemical compaction. In summary, our data indicate the low dip angle of the Navajo cross-beds at our study site can be confidently attributed to physical compaction. They also suggest significant compaction (and therefore burial) took place prior to cementation (largely by sparry carbonate). A better understanding of porosity occlusion in eolian sandstones has a number of potential uses, e.g., interpreting the migration of petroleum and other fluids through the Navajo (Chan et al. 2000) and reflectors observed in Navajo sandstone with GPR (Jol et al. 2003).

REFERENCES CITED: Chan M.A. et al. (2000) AAPG Bulletin, 84(9): 1281-1309. Hunter R.E. (1977) Sedimentology, 24: 361-387. Jol H.M. et al. (2003) The Leading Edge, 22(9): 882-887.