Lexicon of rock-stratigraphic units in Union, Colfax, Mora, and eastern Taos Counties, New Mexico
— Marjorie E. MacLachlan

This lexicon includes rock-stratigraphic (lithostratigraphic) units and geologic names that have been applied to exposed rocks in Union, Colfax and Mora Counties, and the eastern part of Taos County. Due to limited space many well-known terms and place names are abbreviated (see list below), and informal and economic terms have been omitted. Cambrian and Ordovician sedimentary rocks occur in the subsurface of Union County; none are known from the subsurface of Colfax County (Foster, 1966 New Mexico Geological Society Guide-book, 17th Field Conference). The names applied to these lower Paleozoic rocks are from the Midcontinent area of the U.S. (Baldwin and Muehlberger, 1959, New Mexico Bureau of Mines and Mineral Resources Bulletin 63). Rocks from many other parts of the geologic column are either missing or unrecognized in the four counties. For these reasons, no formal names of Silurian, Devonian, Early and Middle Triassic, Early and Middle Jurassic, Eocene, or Oligocene rocks appear in this lexicon. However, a section approximately 9,500 ft thick (Johnson, Dixon, and Wanek, 1966, New Mexico Geological Society 17th Field Conference) thought to repre-sent Eocene and Oligocene time is present in adjoining south-east Colorado.

 

A formal stratigraphic name is one that has been defined according to the Code of Stratigraphic Nomenclature (1970) and whose definition has appeared in a publication. Names in abstracts were not considered. Of course, many published names predate publication of the Code and do not have adequate definitions by modern standards. Ideally, each formal name should have as part of its definition the description of: 1) a type section, locality, or area, 2) lithology, 3) contacts with adjacent units, 4) areal distribution, and 5) age. 

 

In this lexicon, the name, its stratigraphic rank, time-stratigraphic unit assignment, and areal extent are given in that order in the heading for each unit. Citation of the naming paper, with a brief description of the stratigraphic unit from it, is followed by information from published references in which use of the unit was modified or clarified in some way in one or all four counties of the conference area. If the state name is omitted after the location of the type or derivation of the geographic name, the locality is within New Mexico. Information in most of the naming papers written before and shortly after 1900 was abstracted from the Wilmarth Lexicon (U.S. Geol. Survey Bulletin 896); the other information was abstracted from original sources. Some of the stratigraphic units described in this lexicon have not been adopted by the U.S. Geological Survey. The initial letters of all formal units are capitalized.