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Abstract

The “Jackpile” sandstone, a term of local usage, is exposed near Laguna, New Mexico. It is the uppermost unit in the Morrison formation, of Jurrassic age. The petrography, sedimentary structures, and shape of the unit, its relation to tectonic structures, and analogies to similar ancient and modern sandstones suggest that it was deposited by a northeast-flowing stream system that was largely confined by contemporaneous structural depression. Continued downwarping after deposition, followed by erosional truncation, emphasized the structural localization of the unit.

The sandstone is fine to medium grained, friable, and moderately well sorted; coarser grained beds are more abundant near the base of the unit. The composition ranges from a calcite-cemented subarkose near the base to kaolinite-indurated quartz sandstone near the top. This compositional variation probably is a result of weathering prior to deposition of the Dakota sandstone. Terrestrial plant remains are locally abundant.

The “Jackpile” sandstone is a northeast-trending tabular body as much as 13 miles wide, at least 33 miles long, and locally more than 200 feet thick. It splits into distributary-like fingers to the northeast, and cross-beds in the sandstone dip mostly northeast. The unit wedges to the northwest and southeast along an angular unconformity bounded by the overlying Dakota sandstone, and broad folds in the strata below this unconformity parallel the southeastern limit of the “Jackpile” sandstone.

Other stratigraphie units in the Morrison formation tend to thicken in the area of the “Jackpile” sandstone. This suggests that structural downwarping was active in the area before, as well as during and after, deposition of the unit.

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