CAN COMPACTION ACCOUNT FOR LOWER-THAN-EXPECTED CROSS-BED DIPS IN THE COCONINO SANDSTONE (PERMIAN), ARIZONA?

Large-scale cross-bedding in sandstones is often interpreted as a series of eolian dune foresets. However, many Permian sandstones, including the Coconino, have cross-bed dips that are much lower than the angle of repose in modern dunes (32-34°). Many authors have suggested that compaction is the primary cause of the low dip angles. For this study, 214 cross-bed dips were measured in the Coconino throughout northern and central Arizona; the mean, range and standard deviation of the dips were 20.2, 3.0-32.0 and 5.7°, respectively. These numbers are consistent with other published data. Approximately 400 thin sections that covered the entire outcrop range and thickness of the Coconino were also available for study. Percent porosity was calculated by point counting on 14 representative thin sections; the mean, range and standard deviation of the porosity were 24.2, 12.0-32.4 and 6.3%, respectively. Cement was counted as pore space.

If a compacted sandstone with a dip of 25° had an initial dip of 33° and a porosity of 40%, the final porosity would have to be 12% using the formula tan(ϴ₁)/tan(ϴ₀) = (v_s + v_v1)/(v_s + v_v0). Using this same formula and assuming initial porosities of 20-40% and initial dip angles of 30-34°, the Coconino has dip angles which are too shallow to be explained solely by compaction. Because the Coconino remains so porous after diagenesis, either depositional porosity was unrealistically high, initial cross-bed dips were uncharacteristically low, or unknown factors have acted to reduce dips while preserving or creating void space. Our theoretical results show that compaction probably only accounts for a few degrees of dip angle reduction.

Thin sections were examined for any of the common effects of severe compaction. Fractured or shattered grains were relatively rare, and mica grains generally showed little or no deformation. Undulose extinction in quartz grains, often the product of strong differential stresses during diagenesis, was not usually abundant. These observations, along with the mathematical results, suggest that compaction may not be sufficient to explain the low cross-bed angles in the Coconino Sandstone. The lack of evidence for compaction in the thin sections and the theoretical calculations indicate that the cross-bed dips in the Coconino may be close to their original angles.