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Ministry of Energy Mines and Responsible for Core Review

BCAge 2004A-1:
A Database of Isotopic Age Determinations for Rock Units from British Columbia
Release 3.0, October 2004 (Open File 2004-3)

compiled by: K. Breitsprecher and J.K. Mortensen


Pacific Centre for Isotopic and Geochemical Research,
 Earth & Ocean Sciences, University of British Columbia
e-mail: kbreitsp@eos.ubc.ca, jmortensen@eos.ubc.ca

 

Recommended Citation:
Breitsprecher, K. and Mortensen, J.K., 2004. BCAge 2004A - a database of isotopic age determinations for rock units from British Columbia.  British Columbia Ministry of Energy and Mines, Geological Survey, Open File 2004-3 (Release 2.0), 7766 records, 9.3 Mb

 

The original source is to be cited when quoting dates. The database (this compilation) should be cited when referring to assemblages of data produced by querying it.

 

Overview

BCAge 2004 is an MS-Access based database containing all reported non-proprietary isotopic age determinations for bedrock units from British Columbia. It contains 7759 age determinations from 4828 rock samples, summarizing 622 published articles, theses or unpublished reports. The data is fully relational, and may be queried by the user on the basis absolute age (in Ma), location (NTS mapsheet, terrane polygon, or decimal lat-long, as specified by the user) or source (e.g. author, journal title, etc.). The product is not stand-alone; the end user must install Microsoft Access, version 2000 or XP (2002). XP users must up-convert the product – see information link on main menu for instructions.

 

What’s new for Release 3 (October 2004A-1):

2004A-1 (Release 3, October 2004) is a corrective-release, not an update; no new data has been added subsequent to the July 2004 release (see below). Corrections were made to some sample locations, and to a linkage issue between about 2 dozen rocks and their respective ages. Also, a small number of records were removed, where such were identified to be the result of redundant reporting. The above-noted revisions were identified during the course of integrating the BCAge dataset to the national (GSC Geochronology Knowledgebase) meta-data format. Approximately 100 records are affected by this update; the vast majority of those affected were corrected with respect to sample location.

 

New for Release 2 (July 2004A):

BCAge 2004A (July release) contains “new” data, as reported and/or GeoRef-indexed after the previous release, to May 2004; the previous release (February 2004) contained reports indexed to June 2003. Corrections to sample or location information have been made as required, based on reported or otherwise noted errors, and/or as additional information has been made available to the compilers. Ages and reliability ratings to pre-existing data may be modified and/or upgraded, as work on them progresses from “preliminary” to “final” findings, and or reaches higher levels of publication.

 

A new TERRANE field has been added. This field can be accessed under the “search by location” menu, or can be applied as a secondary filter (right-click drop down) on the “byAge” query screen. Please note that this is a first-pass assignment of samples to terranes; this information was generated by GIS overlay of data points onto BCGSB MapPlace terrane polygons. It is intended at this time as a means of extracting data by location within irregularly-shaped but geologically meaningful regional boundaries, whereas the previous version was restricted to searching by locations constrained to less-meaningful rectangular grids.

Because the terrane information was generated by a GIS system, it is prone to errors resulting from mismatches stemming from data coordinates, and in the level of detail resolved by the GIS terrane layer. Further, the stratigraphic aspects of stacked terranes is completely ignored by such a method. The field should not be taken to reflect any worker’s assessment of the correct terrane for the rock.

Data content

The data-base consists of 3 linked tables: 1) a rock table, which contains one record for each sample dated; 2) an age table, containing one record for each age determination reported; and 3) a sources table, containing one record for each report containing ages. The relational nature of the database ensures that multiple age determinations from a single sample (e.g. either as replicate analysis, as ages determined using different isotopic systems, or as multiple, multi-sourced grains in a clastic sedimentary rock), are linked to that single sample, and display as such to the user. Furthermore, this structure allows for ages from a single sample which appeared in different reports (e.g. Ar-Ar ages in one publication and U-Pb ages in another) to appear simultaneously to the user, because they are linked to a single record in the “rock” table.

 

Each entry includes the following information:
    1.  Internal database reference number(s);
    2.  Age and error (in Ma); interpretation (meaning) of age;
    3.  Material dated, isotopic system used, and laboratory in which the sample was dated;
    5.  The rock type and geological unit (where this is known);
    6.  Reliability rating and comments;
    7.  Reference(s); and
    8.  Sample location (NTS map area, latitude/longitude, UTM zone/easting/northing).

 

Data compiled in BCAge remains the ownership of the authors who produced or reported that information. All ages returned by BCAge queries are linked to a complete listing of the original source of the data and should be cited to that original report. Users who wish to cite the results of a query, however, such as “Previously reported ages for Unit X range from 85 to 115 Ma”, may cite this compilation as the source of that information.

 

Accuracy of the data returned by BCAge should be verified by the user from the original source(s). BCAge is intended as a search engine to locate reports containing original isotopic age data; the level of funding for the project did not allow for double-checking of data after input. Further, in many cases, accuracy is limited by the quality of the original reports. Of particular note is significant errors in sample locations in some original reports, which have been noted during the process of data entry. Problems encountered during compilation include locations drawn on sketch maps with erroneous coordinates, UTM coordinates reported one digit short of a proper coordinate, and loosely constrained sample locations (the latter is particularly true for older reports, which commonly constrain locations only to the nearest 5, or even 15, minutes). Where noticed and/or possible, such errors in original reporting have been corrected during compilation. As GIS-based mapping is becoming the new norm, even a small amount of rounding in the reported coordinates can place a sample in the wrong polygon for its lithologic type. Sample locations in BCAge have NOT been rigorously checked for consistency with GIS or other map product, except for those samples on mapsheets 82L & 82K.  Also, note that many reports do not provide a detailed description of the sample nor its geologic association. In such cases these fields are blank or contain only the cursory information which was available in the report. The user should not rely on all fields to contain information for all records. Finally, the ‘geologic unit’ is limited also by both the level of original reporting and by the knowledge and nomenclature of the day at the time the report was issued. At this time, the ‘geologic unit’ field has not been reassessed or assigned to modern criteria or mapping.

 

Quality of age determination

A number of issues with reliability and consistency of reporting for K-Ar age determinations have been addressed from the earliest BCAge release. Older K-Ar age determinations have been recalibrated as necessary to current IUGS decay constants (Steiger and Jaeger, 1977).  Such records contain a comment in the "Age Note" field to this effect.  All error levels have been adjusted from the original report to reflect the 2 sigma level (95% confidence interval).  All K/Ar ages have been assigned a cooling age interpretation, although many in older reports have been originally interpreted as igneous crystallization ages. Many of the K/Ar determinations may well reflect igneous cooling, yet we now know that Ar-reset by thermal events is a large-scale phenomenon in the Cordillera, and thus for consistency, BCAge simply flags all such older (K/Ar) ages as “cooling age”. Assessment of whether that cooling is due to thermal reset (eg. metamorphism) or igneous cooling must be made by the individual worker using the ages.

 

Each age has been assigned a "reliability rating" so that users can assess the degree to which they wish to rely on the age for their study. There are 3 levels of rating:

-        "A" rating – use with confidence, the age is apparently analytically sound.

-        "B" rating – use with caution, some aspect of the reporting (interpretation, analytical data, methodology) suggests the age should be regarded as an estimate (see chart below for specific examples). Author cites age determination as ‘estimate’, ‘problematic’ or otherwise questions quality of the data/material analyzed.

-        "C" rating – do not use, the age is unreliable.  This is applied if the author cites the age as meaningless (geologically or analytically), if the workers have used methods now known to be wholly unreliable, or if  the age determination has been superceded by redating of the same rock by more modern methods.

Excluded data:  “Model” ages are not included in this compilation due to their derivation from assumed values (model-dependence) rather than from analytical results alone. Rb-Sr ages based on a single, relatively non-radiogenic analysis and an assumed initial Sr isotopic ratio are herein considered to be model ages.

Reliability Rating Criteria

 

RELIABILITY CODE

DESCRIPTION

 A/B>

apparently analytically reliable

 B

Analytical reliability questionable, as noted by authors in original report, or by compilers following guidelines below:

K-Ar

Biotite ages with low measured K2O contents (<5%), reflecting either impure concentrates and/or alteration of the biotite. In such cases the age should be considered a minimum age at best.

No correction was made for the presence of atmospheric Argon. This includes many of the analyses carried out at the GSC laboratory prior to about 1965. Although less important for mica and relatively high-K whole-rock samples, this introduces considerable error into ages for low-K samples such as amphiboles.

Age was determined from whole-rock sample, a method now known to yield inaccurate results.

Rb-Sr

ages calculated using 2-point Rb/Sr isochrons, for which possible geological error  cannot be evaluated (this does not apply to Rb-Sr ages for whole rock-mica pairs).

Rb/Sr "errorchron" ages, for which the calculated MSWD > 2.5, indicating scatter in the data considerably outside of expected analytical error.  The MSWD is given for such ages (in some cases recalculated from the original analytical data, where available).  "Errorchrons" usually indicate that the samples analysed are either not closely related, or at least some of the samples have experienced post-crystallization isotopic disturbance(s).  These ages should be used with considerable caution.

Ar-Ar

Age is not a plateau age. Note that definitions of “plateau” vary between labs; this compilation is limited to the definition as used by the lab/author producing the report.  No attempt has been made to validate ages reported as “plateaus” to analytical data, except where spectra are visibly disturbed (significant Ar-loss, excess Ar, or recoil is suspected beyond that reported).  If such disturbed ages are called “plateau” but are based on <50% Ar-release and/or non-contiguous steps, a B rating is assigned.

U-Pb

Ages are inferred from scattered discordant U-Pb zircon analyses which do not define a discordia line.

Igneous age based on single fraction

Detrital grains with 207Pb/206Pb age in excess of 5%, but not more than 15%, discordant. For Mesozoic and younger grains, the 207Pb/235U age is used to measure discordance rather than 207Pb/206Pb age.

 C

Geological error, scientific error, or methodological (details given in the comments section)

Methodology now known to be wholly inaccurate. Examples:

bullet

some very early U/Pb dates are upper or lower intercepts of discordia which includes fractions from widely (100’s km) separated samples

bullet

Rb-Sr ages which are derived from suites now known to be wholly unrelated by other dating methods.

bullet

Rb-Sr ages from suites of rocks with 100’s km distribution

bullet

Rb-Sr ages derived from materials, such as feldspars, in which Rb or Sr mobility is now well documented

bullet

K-Ar ages for which the material dated is reportedly “heavily chloritized biotite”

Detrital grains with Pb/Pb age in excess of 5%, but not more than 15%, discordant. For Mesozoic and younger grains, the 207Pb/235U age is used to measure discordance rather than 207Pb/206Pb age.

 

Usage Notes

 

System  The database was compiled on, and is known to be compatible with, the following configuration:

Software:  Microsoft Access, version 2000 or Office XP (=Access 2002). XP users must perform a one-time data-base conversion. See convert.htm file on the CD for instructions.
Op System:  Windows 2000 or XP

Querying the database  The database menu includes some search options which accommodate the most common types of geological data querying: select records by age, by location, or by source (reference).  A working knowledge of MS-Access will be helpful to the user who wishes to query at a higher level, such as combining age and location.  Some tips are provided under “selecting records for export” below.

 

Two data retrieval options are presented on the main menu.

 

Quick-hit opens a form to view either the rock or reference data table; ages associated with each age or report appear in a drop-down at the bottom of the ‘rock’ form, or can be seen by clicking “view associated ages” on the ‘reference’ form. This method permits the user to browse the entire dataset without querying a subset. The user can either

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browse the data one record at a time using the CD-buttons at the bottom of the form

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find a specific rock or reference, using the binoculars “find” button on the MS-Access menu bar. For example, if you wish to look up ages from a specific report on the ‘reference’ form, click once in the “authors” field, click the binoculars icon, enter an author’s name, under “match” option, select “any part of field”. The form will drop to the NEXT record in the list meeting this criteria. To find another record, click “find next”.

bullet

select a subset of the data, by using the right-click drop-down filter menu from any field. For example, from the ‘rock’ form, click on ‘sample description’ field, right-click, enter *basalt* to select all rocks which have the word “basalt” in their description. Note again, not all rocks are fully described; thus such searches are limited by the level of detail provided in the original report.

 

PowerSearch  returns subsets of data, based on age, location, or reference constraints input by the user, which can be filtered further by the right-click method outlined above.  Note: Never leave a prompt on the power-search options blank; MS-Access will assume this means you want records where that field is “blank”. If you wish to ignore one of the prompted fields, enter a wildcard (asterisk *), or numeric min/max as prompted.

 

By age  applies a temporal filter, based on user-entry of minimum and maximum absolute-ages in Ma (millions of years). This query searches the ‘age’ field, and does not factor in analytical error; the user should thus “pad” the min-max ages appropriately. From the ‘age determination’ form, you may use the right-click filtering options, as outlined in “quick-hit” section above, to further refine the results of your search. For example, you might start by requesting all ages between 65 and 110 Ma in age. Then, you might use the “mapsheet” field to filter this subset for ages on the mapsheet in your target area (eg.) 105G. Then, you might narrow your results to exclude all ‘C’ rated ages from the “ratings” field. Each filter acts on the previous subset; with each filter applied, you should see a decreased number of records returned on the CD-button bar at the base of the form. To return to the entire data-set and begin a new filtering process, right-click “remove filter/sort”. Hyperlinked buttons on this form provide pop-up information on reference or rock which is not included in the summary fields on the age-determination form. 

 

The PowerSearch ByAge function is the recommended approach for anyone wishing to export their results to Excel.  Note that searching “by rock” and “by reference” does not drag age-determinations associated with rocks/references into the exported data. ByAge has been designed for exporting purposes to include all summary fields (those visible on the form) in the Excel sheet.  To export, apply all desired filters, then (from MS-Access menu), click “File”, “Export”, choose “Excel” as a file time, and click “save-all”. Save-all means that all records selected – including those ‘behind’ the one currently up on the view-form - will be exported.

 

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by location returns all rock samples within a user-specific area.  You may constrain the area by mapsheet (eg. 105G), by terrane (eg. Cassiar) or by a decimal-lat/long rectangular box (eg. 61.364). The mapsheet filter has prompts for both NTS reference (105G) or name (Glenlyon). You need only enter one or the other, but do be sure to * the prompt you are ignoring.  A handy aspect of this search function is the user can immediately see which sample has multiple age determinations, by one or more methods, regardless of whether those ages were reported in separate sources. As with Quick-hit “rock” form, these results can be further filtered on any field in the upper part of the form. Note that exporting this query will not attach ages in the drop-down subform (bottom of the form) in your Excel file. See ‘byAge’ function above for exporting.

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by source  applying a source filter returns all references matching your criteria, with hyperlinks to ages reported in that reference. Note that searching by reference is not Boolean; if you wish to enter a keyword to be found anywhere in the field, enter it with *asterisk*  wildcards both sides. For example, Armstrong, R.L. returns records where R.L. Armstrong is the sole author, armstrong* returns records where Armstrong is the first author, *armstrong* returns records where Armstrong is anywhere in the authorship string.

 

Errors and omissions

The compilers have made every effort at comprehensive inclusion and correctness of data. We welcome input from users who note errors, or omissions, or who have newer and better information regarding sample locations and lithologies. Please send recommendations for changes to breitspr@telus.net or jmortensen@eos.ubc.ca, providing therewith the sample-ID as it is given in the database.