New Mexico Geological Society Annual Spring Meeting — Abstracts

In 1985, a decision to revise and publish an updated version of the 1965 New Mexico State Geologic Map was made. The project is being carried out jointly with the U.S.G.S. Justification for the revision derives from (1) much detailed geologic mapping during the last 22 years, (2) new radiometric dating methods (Ar40/Ar39) that have permitted a better understanding of the sequence of mid-Tertiary volcanics, (3) a keener understanding of Precambrian events, and (4) refinements to our biostratigraphic knowledge.

The recompilation is currently underway at the NMBMMR using a computerized digitizing system. The computer assisted approach to the project was found to be superior to a more conventional approach such as graphical data transfer instrument, e.g. a Bausch and Lomb zoom transfer scope.

Stylistic changes will include (1) color selections and contrasts, (2) more attention to portrayal and definition of structure, and (3) legend format.

Nomenclature changes have been most apparent in, but not limited to, the Precambrian, Jurassic, Cretaceous, and the mid-Tertiary. The Precambrian rocks will be assigned to one of five age or genetic groups: (a) sedimentary, (b) plutonic, or (c) one of three suites of Early Proterozoic metamorphic rocks. Changes in the Jurassic include proposals by the U.S.G.S. to replace the middle part of the San Rafael Group with the name Wanakah Fm. The change reduces the rank of the Todilto and Summerville Fms. to Members of the Wanakah. The Zuni Sandstone will be redefined and restricted geographically. Changes to the cretaceous will include restricting the stratigraphic range of the Mesaverde Group in the west-central part of the state, and elevation of the Dakota Sandstone to Group status with both upper and lower cretaceous components in the east-central part of the state.

Extensive revisions of the Tertiary volcanics are largely the result of accurate dating of ignimbrites which permit correlations across the Datil-Mogollon volcanic field.