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The Jackpile sandstone of local usage--exposed near Laguna, New Mexico--is in the uppermost unit in the Morrison formation of Jurassic 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 probably 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. Terrestrial plant remains are locally abundant.
The so-called Jackpile sandstone is a northeast-trending tabular body as much as 12 miles wide, at least 30 miles long, and up to 200 feet thick. It splits into distributary-like bar fingers to the northeast. Cross-beds in the Jackpile sandstone dip mostly northeast, suggesting that the sediments were transported northeastward. 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 stratigraphic 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 Jackpile deposition.
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