Product details

Abstract
11-1 Reynolds, S. J., Florence, F. P., Welty, J. W., Roddy, M. S., Currier, D. A., Anderson, A. V., and Keith, S. B., 1986, Compilation of radiometric age determinations in Arizona: Arizona Bureau of Geology and Mineral Technology, Bulletin 197, 258 p.

ABS
This compilation includes information for 1,688 K-Ar, Ar-Ar, Rb-Sr, fission track, isotopic-lead, and lead-alpha age determinations in Arizona. The data were compiled from original references and entered into a microcomputer using database-management software.

‘lhe compilation consists of five main lists and two maps. Part I is a master list of all information compiled for each date, including dating method, material dated, rock type, name of rock unit, location information, and comments about the geologic setting and significance of the date. The dates are listed in order of increasing age and are denoted with a record number (computer identification number) that is referenced in the cross indices in parts II, III, and IV. The master list contains information that may be imporrtant for interpreting the significance of each date and should be consulted after locating a date of interest in the cross indices. A double-asterisk “flag” following the reported age indicates that interpretation of the date is required because the rock has undergone a complex thermal history or there are questions concerning the analysis (e.g., too high a K content for the material reportedly dated). We cannot overstate the need to refer to the comments in the master list and, in many cases, to consult the original reference to appreciate the validity and significance of reported ages. Certain dates known to be inaccurate are included in the listings, but are flagged with the warning asterisks and discussed in the comment section. These dates are included for completeness and to alleviate confusion stemming from past discussion of such dates. Comments may also contain reference to age determinations made from different samples or by using different methods on the same sample or rock unit.

Part II is a cross index that arranges dates by geographic area, such as mountain range, and lists the record number, method, age, material dated, rock type, name of rock unit, and sample number for each age determination. This list simplifies the task of identifying the dates that exist for a given area. Most geograthic-area names were compiled from existing maps and are self-explanatory (e.g., Superstition Mtns. or Defiance Plateau). We have assigned names to some areas for which no suitable names existed. These newly assigned names include terms such as the “Carrizo Embayment” for the area between Fort Apache and the Canyon Creek fault, and the “Santa Maria Mtns.” for the mountain range south of the Santa Maria River and east of the Date Creek Basin (referred to on some maps as the Black Mtns.). In addition, we have used a variety of informal area names such as Lake Mead area, San Carlos area, Blue Range area, Cameron area, etc. All geographic names are listed by county in Table A, along with the two-letter code assigned to each county in the database.

Part III is a cross index that lists all dates on a particular formation or rock unit, such as the Galiuro Volcanics. This index lists the record number, method, age, material dated, rock type, sample number, and quadrangle for each age determination. This index is useful in determining the age range of a particular rock unit and in evaluating agreement or discrepancy among different dates for the same rock unit or sample. Constructing this list required us to assign informal rock-unit names, derived from geographic features, to previously unnamed rock units. For a date obtained from a rock from a drill hole, the informal rock-unit name generally incorporates the name of the well (e.g., Exxon State 32-1 basalt). In all cases, our informal names are listed in lowercase letters, except for the proper-name portion of the unit name (e.g., Springerville basalt). In contrast, all formal rock-unit names are listed with uppercase letters (e.g., Canelo Hills Volcanics). Many names listed with lowercase letters are widely used, but not formally named (e.g., Wilderness granite). It should be emphasized that our newly assigned rock-unit names are informal and, if used in the future, should retain their informal character until they are formally designated. In general, the accuracy of rock-type names, such as basalt, assigned in original sources was not evaluated. In some cases, rock-type names have been modified to reflect presently accepted igneous-rock classifications (Streckeisen, 1976).

Part IV is a list arranged by original sample number and is primarily useful in comparing multiple dates on a single sample or in locating a date for which only the Ssample number is krown. For references that report dates in tabular form with consecutive numbers starting at one or “A,” we have prefixed the sample numbers with a code denoting the original reference (e.g., M73:10 is sample number 10 in Marvin and others, 1973). This permits the user (and computer) to distinguish between what would otherwise be identical sample numbers for two entirely different samples. A list of these sample-number prefix codes is given in Table B. Part V is a complete list of references cited.

The Appendix lists age determinations that are of uncertain geochronologic significance. These include all lead-alpha age determinations, several invalid Rb-Sr isochron ages and single point Rb-Sr model ages based on questionable assumptions, and two K-Ar age determinations whose purpose was to identify excess argon problems. These dates are not included in any of the indices in parts II through IV.

The compilation is accompanied by two l:1,000,0000-scale maps upon which all dates listed in part I have been plotted. A photographically reduced version of the 1969 State geologic map serves as a base and aid to the reader. Plate 1 is a plot of all K-Ar and Ar-Ar age determinations, and plate 2 is a plot of all Rb-Sr, fission-track, and isotopic-lead ages. Dates listed in the Appendix are not plotted on either map.
We have tried to include in the compilation every published radiometric age determination for which there is analytical data. We have also included many unpublished dates that are cited as personal or written communications in published references. Some age determinations reported in theses, especially those from out-of-State universities, may have been overlooked. This bulletin is partly based on and supersedes Arizona Bureau of Geology and Mineral Technology Open-File Reports 77-1 (Keith, 1977), 85-8 (Reynolds and others, 1985), and 85-15 (Florence and Reynolds, 1985).

All ages for which analytical data are available have been recalculated using presently accepted decay constants (Roberts and others, 1968; Steiger and Jager, 19771. Ages published without analytical data have been recalculated using decay-constant conversion tables (Dalrymple, 1979), unless the original constant is unknown, in which case the date has not be recalculated. Many pre-1980 isotopic-lead age determinations were recalculated using the approximations listed in Table C. This table also lists all laboratories and decay constants used in this compilation.

Key words
absolute age. Ar/Ar. Arizona. catalogs. dates. fission-track dating. geochronology. inventory. K/Ar. maps. Rb/Sr. United States.