FORMATIONS — IN CONCEPT AND IN THE FIELD

“A formation is body of rock identified by lithic characteristics and stratigraphic position... and is mappable at the Earth’s surface” (North American Stratigraphic Code, revised 2005). Many mappers apply the name “formation” to map units in ways that do not strictly conform to the Code because of practical difficulties in applying the rubric to the rocks being mapped. This is true for "formations" that are defined on the basis of detailed descriptions of well-exposed roadcut or cliff exposures. Tracing such units across a terrain where outcrops are sparse is commonly not feasible. Thus, units so defined may be unmappable; yet mappability is tacitly expected by many non-mappers. Mappers also may encounter situations in which formations named nearly a century ago, are present, but are not mappable locally as separate units. The practical solution to such problems is to combine two or more defined formations into a map unit assigned a designation such as "Ersatz Formation and False Formation, undivided". Such a solution is useful in mapping rocks, but is one not strictly in agreement with the Code. Additional difficulties arise where rocks of two formations interfinger or are gradational within a significant stratigraphic thickness, so that placement of the contact between them is subjective. This is especially true in instances where formation names are extended for long distances from their described type sections and lateral facies changes exist. Contact placement is further complicated where diagenetic changes at the boundary impose different textures or compositions on depositionally and lithologically similar rocks. In such cases, we may chose secondary criteria for defining contacts. Thus, although defining map units that exhibit one or more of the complications noted here, as “formations,” is not always in strict accord with the Stratigraphic Code. The solutions to the mappability problems are practical for those of us who map rocks. New terminology is unnecessary. We point out these examples to remind non-mappers that while historical precedence, lithologic detail, and stratigraphic units defined in road cuts and other well exposed sections provide guides for mappers, mappers must define units and apply the formation designation in ways that yield code required mappability in such terrains.