ABSTRACT

Distinctive patterned surfaces, each characterized by a branching system of ridges, are common in certain massive siltstone layers in the Permo-Triassic redbeds of Wyoming and northern Colorado. Most of the patterns are on the surfaces of asymmetric conical bodies of siltstone, about a decimeter in both height and maximum diameter. The cones are always oriented with apices down, and with bases parallel to stratification.

Lack of associated fossils and organic matter, degree of pattern variability, and the unvarying orientation of the specimens in the strata argue against organic origin of the patterns. Drainage systems are more irregular and dendritic than the redbed patterns. Furthermore, experiments in which drainage patterns were developed in conical depressions demonstrated that the pattern cannot be confined to the depression as is the case with the Permo-triassic specimens.

A stream of water welling upward through uncompacted silt erodes a conical depression at the surface, and down-slope movement of silt grains settling out of suspension erodes a system of branching grooves on the surface of the depression. Plaster casts of these patterned depressions have many of the distinctive characteristics of the Permo-triassic patterned cones. The experiments are successful in all details only when the silt is uncompacted and water saturated, but not submerged. The same conditions are thus implied for the cone-bearing layers in the redbeds at the times of origin of the enclosed cones. Origin of the upwelling water in the Permo-triassic strata is uncertain.