These U-shaped Arenicolites occurrences are found in fine-grained sandstone within an ancient storm dominated shoreface (Wall Creek Member, Frontier Formation, Powder River Basin, Wyoming). Note the diagnostic U-shape of the sand-infilled burrows. Arenicolites can be easily mistaken with Skolithos when only the vertical tube is visible.
The vertically oriented burrows in this image are not Skolithos occurrences. The burrows are identified as Arenicolites. In fact, both images are from the same outcrop location. The relative orientation of the viewer with the trace fossil can make proper identification difficult. Note the stratification change of the host sediments occurring mid-way up. A lat stratified sandstone bed rests directly upon completely bioturbated sandstones. The underlying sandstones have been reworked by the activities of organisms moving through the ancient substrate ultimately obliterating primary sedimentary structures. The vertical portions of the u-shaped Arenicolites occurrences seen in the flat stratified sandstone bed are part of a particular trace fossil suite - Glossifungites demarcated surface - found 75 cm higher in section and occurring across the top surface of a sandstone tempestite.
These Arenicolites examples are found in fine- to medium-grained sandstone sediment-gravity deposits. A Monocraterion occurrence is also visible and is labeled (Mo) - Panther Tongue, Utah.
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