Mappability, stratigraphic variation, and diagenetic problems in sedimentary map unit definition and field mapping

摘要

Mapping, recognizing, and defining sedimentary formations in poorly exposed terrains, especially in units with significant stratigraphic variations and disparate definitive diagenetic characteristics, focuses attention on both the problems of field mapping and the contrast between classical field mapping practices and modern stratigraphic and sedimentological analyses. Field mappers must contend with issues of scale and mappability in defining formations, diagenetic versus primary depositional characteristics in delineating contacts, and facies changes, while making standard 1:24,000–1:25,000 scale maps. Formations were defined as mappable units, even before codification of stratigraphic practice, and pre–twentieth-century “legacy formations” remain as widely recognized units. Many “modern formations” recognized and defined in the late twentieth century also meet stratigraphic code definitions. However, postlegacy analyses of exceptionally well-exposed sections using new techniques and insights, while yielding knowledge of stratigraphic variability, may yield newly named, but not necessarily mappable “formations.” Resulting problems of formation recognition, formation definition, and field mapping of units and contacts on 1:24,000 or 1:25,000 scale maps are faced repeatedly by STATEMAP and other mappers. In some cases, mapping of “hybrid” formations, composed of two or more previously defined units is required. In other cases, stratigraphic position and general character must override details of type section stratigraphy in mapping formations. We recommend latitude in correlation with type sections and suggest that, while well-exposed sections provide important insights into paleoenvironments and stratigraphic variations, unless units are demonstrably mappable at the 1:24,000 or 1:25,000 scale, these variations should not be used to subdivide previously defined and mappable formations into new unmappable “formations.”