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Enigmatic columnar structures of sandstone, cherty sandstone, and chert in the Phosphoria Formation were studied at 18 localities in southwest Montana. The structures have circular to elliptical cross sections, diameters of 0.5 to 8 in. and are up to 13 ft long. Most have irregular external annulations, perhaps due to compaction; others are smooth sided.
The structures are in and can be traced through a variety of host rocks. They are most common and best developed in the nonglauconitic littoral to sublittoral facies of the Shedhorn Sandstone. In the sandstone they are almost always oriented perpendicular to bedding. In the intercalated shale or chert host rock, approximately 50% are inclined at very low angles to the bedding. Up to 95% of a host bed may consist of the columns.
The high-density packing of the columns, their morphology, highly variable composition, and association with several host-rock types indicate the structures are organic, probably burrows, rather than inorganic in origin. Their great length, sparse bulbous bases, and the presence of other poorly preserved patterns suggest that the organisms that formed the structures mainly were escaping sediment influx by moving upward rather than burrowing downward. If so, the structures may have important implications about rates of sedimentation on the Permian platform of western United States, if the life span of the organism that produced them can be determined. However, no organism capable of producing the burrows has been found preserved within one.
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