ABSTRACT

Twelve pillow-shaped structures with long diameters ranging from 20 feet (6 m) to 73 feet (22 m) are present in a well exposed outcrop of the Upper Cretaceous Panther Sandstone Member of the Star Point Formation in east-central Utah. These post-depositional, non-tectonic structures bear resemblance to similar features previously reported in the literature, but differ strikingly because of their large size.

Six pillows which were examined in detail show primary lamination that can be traced along the bottoms and sides. Primary physical and biogenic sedimentary structures, represented by groove casts, micro-crosslamination, and trace fossils, are preserved in the distorted primary lamination and on the bedding planes. Smaller ball-shaped structures, 1 foot (0.3 m) to 7 feet (2 m) in diameter, comprise the matrix between and above the large pillows. These were formed from a sandy-silt which was compressed by the subsidence of the pillow structures and flowed as a viscous fluid. Both the balls and pillows are considered to have formed in place due to a fortuitous set of depositional conditions, which brought together beds of sand and sandy-silt of the proper thicknesses.