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Drift Composition and Surficial Geology of the Trutch Map Area (94G), Northeastern British Columbia

Geological Survey of Canada Open File D3815

Geochronology

Cosmogenic 36Cl and Radiocarbon dating


Contents

Part I: Cosomgenic 36Cl Ages Part II: Radiocarbon Dates
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Part I: Cosmogenic 36Cl Ages of Glacial Samples from Northeastern British Columbia: Report of Results Prepared for Dr. Jan Bednarski, Geological Survey of Canada, October 1999

Recommended Citation:

Phillips F.M.1
1999: Cosmogenic 36Cl Ages of Glacial Samples from Northeastern British Columbia, Report of Results, prepared for Dr. Jan Bednarski, Geological Survey of Canada, October 1999 in Drift composition and surficial geology of the Trutch map area (94G), northeastern British Columbia, J.M. Bednarski (comp.); Geological Survey of Canada Open File D3815 

1Fred M. Phillips, Department of Earth & Environmental Science, New Mexico Tech, Socorro NM 87801 (Email: phillips@nmt.edu; Http://www.ees.nmt.edu/Hydro/faculty)


Introduction

This report describes the results of 36Cl buildup dating of three samples of glacial origin collected by Jan Bednarski from the Rocky Mountains in northern British Columbia. The samples were expected to have exposure ages in the general late Quaternary range.

The "in situ" cosmogenic nuclide dating methods rely on the production of rare nuclides in rocks exposed to cosmic radiation at the surface of the earth. Several meters of rock are sufficient to block most of the cosmic rays that cause high rates of nuclear transmutations (Lal and Peters, 1967). Many geological agents, such as glaciers, frequently excavate many meters deep and bring to the surface rocks that have previously had little exposure to cosmic rays. This initiation of exposure to stronger radiation at the surface "sets the clock" for cosmogenic nuclide dating. By measuring the amount of cosmogenic nuclide that has accumulated, the time of exposure can be calculated. The principles of the cosmogenic nuclide dating methods have been described by (Cerling and Craig, 1994; Gosse and Phillips, 1999; Lal, 1991).

This dating study employed 36Cl. Chlorine-36 is the only unstable, long-lived, isotope of chlorine, with a half-life of 301,000 years. It is produced in rocks by high- energy neutron reactions on 40K and 40Ca, and by low energy neutron absorption by 35Cl. The 36Cl dating method was initially demonstrated by (Phillips et al., 1986) and is described io (Phillips, 1995; Zreda and Phillips, 1994; Zreda and Phillips, 1998).

Sample Processing and Analysis

Sample processing was performed by Mr. Terry Thomas. The processing was initiated by crushing the rock to centimeter-sized grains. The grains were ground to fine sand size in a TEMA mill and the individual mill batches carefully mixed to ensure a homogeneous sample. The ground sample was leached in dilute nitric acid to remove any meteoric 36Cl or 36Cl fallout from nuclear weapons testing. Aliquots of the sample were removed and sent to the New Mexico Bureau of Mines and Mineral Resources for X-ray fluorescence analysis for major elements and U and Th, and to XRAL Laboratory in Ontario, Canada, for prompt gamma emission spectrometry analysis of 8 and Gd, Masses of the remaining sample material, ranging from 50 to 200 g, were dissolved to extract the sample C1. The rock powder was placed in one-liter Teflon bottles and -100 g of deionized water, «100 g of nitric acid, and «500 g of hydrofluoric acid were added. Complete dissolution was accomplished in 2 to 4 days. The supernatant (over the reprecipitated fluorosilicate residue) was centrifuged and then -3 g of 0.998 mg "Cl (g H2O ’ isotopica11y-labeled carrier (99’lo "Cl) was added, along with excess AgNO, to precipitate both the natural and cam*er Cl. Exact sample and carrier masses are given on the appended worksheets. The resulting AgCl precipitate was purified of S (which is an interfering isobar in the "Cl analysis) using standard procedures (Zreda, 1994). The purified and dried AgC1 was then sent to PRIME Lab (Purdue University) for accelerator mass spectrometry analysis of t4e "C1/Cl ratio (Elmore et al., 1979) and the "Cl/Cl ratio. Based on the "Cl/Cl, "CliC1, sample mass, and mass of "Cl carrier added, the program CHLOE (Phillips and Plummer, 1996) was used to calculate the "Cl/Cl ratio and C1 concentration of the samples. A11 analytical results are given in Table 1.

Table 1. Analytical results from northern British Columbia samples

Sample
Latitude (°N)
Elevation (m)
PRIME Lab No.
36Cl/(1015CI)
±
36Cl
Conc.(104 atoms/g)
±
CI (ppm)
98BJB0031
57.39
1859
R99-0814
268
14
100.7
5.3
22.1
98BJB0032
57.39
1859
R99-0815
143
30
58.5 
12.3
24.1
98BJB0049
57.48
1722
R99-0816
227
26
114.0
13.0
29.6

Sample
SiO2 (wt.%)
TiO2 (wt. %)
AI2O3 (wt. %)
MnO (wt.%)
MgO (wt. %)
CaO (wt. %)
K2O (wt. %)
Na2O (wt. %)
98BJB0031
99.31
0.03
0.00
0.00
0.02
0.01
0.01
0.02
98BJB0032
99.73
0.02
0.00
0.00
0.02
0.01
0.02
0.03
98BJB0049
99.64
0.03
0.10
0.00
0.02
0.01
0.03
0.02

Sample
Fe2O3 (wt. %)
P2O5 (wt. %)
LOI (wt. %)
Th (ppm)
U (ppm)
B (ppm)
Gd (ppm)
98BJB0031
0.04
0.01
0.16
<1
1
<5
<1
98BJB0032
0.06
0.05
0.16
<1
1
<5
<2
98BJB0049
0.14
0.02
0.15
1
1
<5
<1

As can be seen from the composition, the samples all appear to be a very pure quartzite. The low Ca and K concentrations mean that nearly all production will be from neutron absorption by Cl. (The actual proportion of production by Cl is about 98% for these rocks.)

Calculated Exposure Ages

CHLOE was used to calculate the exposure ages of the samples. CHLOE uses the elevation/latitude scaling factors of (Lal, 1991), but scaling for secular variations in production rates was not attempted. The newly revised thermal and epithermal neutron distribution equations of (Phillips et al., 1999) are now used in CHLOE. New production rates from (Phillips et al., 1999) have been revised to be consistent with the improved neutron formulation, and are based on the data of (Phillips et al., 1996).

Cosmogenic surface exposure ages vary depending on whether the surface is stable or not. However, independent data on rates of surface erosion for a particular rock surface are seldom available, and thus CHLOE calculates exposure ages as a function of a range of assumed rates of surface mass loss. In Table 2, I present calculated ages assuming erosion rates of 0, 1, and 5 mm ka-1. The ages decrease as the assumed erosion rates increase because low-energy neutron production increases with depth (down to ~25 cm) and erosion causes the rock surface to move down into regions of higher production, and hence lower calculated ages. Given the extremely resistant nature of pure quartzite, the assumed erosion rate of 5 mm ka-1 is probably much greater than the actual erosion rate, and the actual value probably lies between 0 and 1 mm ka-1. Cosmogenic surface exposure ages also depend on any surface coverage of the sample, which will tend to block cosmic radiation. Snow shielding may be a factor for these samples, but since I have no information on snow coverage, it was not included as a correction in the age calculations.

Table 2. Chlorine-36 exposure ages for northern Alberta samples.

Age (yr BP), given assumed erosion rate
Sample Zero Erosion 1 mm/ka 5 mm/ka
98BJB0031 14020±760 12880±650 10750±480
98BJB0032 7300±1600 7000±1500 6200±1200
98BJB0049 13100±1560 12100±1340 10200±1000

As is illustrated in Figure 1, two of the samples are in good agreement. Samples 98BJB0031 and 98BJB0049 both have ages that are concordant, at the 1 ? level, between 13260 and 14780 years(for the zero erosion ages). Sample 98BJB0032, however, is much younger than the other two and, even given analytical uncertainties, is several thousand years out of agreement. There is no apparent reason for the different result on sample 98BJB0032 in the analytical data. The field notes indicate that 31 and 32 were from the same locality, while 49 was from a different locality. The most likely explanation is that 32 was either covered (e.g., by till or ice) or was derived from the splitting of an originally larger boulder.

Figure 1. Comparison of 36Cl exposure ages from northern British Columbia glacial samples.

Conclusions

Three samples of glacial origin from the Rocky Mountains in northern Alberta were analyzed for 36Cl surface exposure dating. Nearly all of the 36Cl production in all three samples was from thermal and epithermal neutron activation of 35Cl. Ages were calculated for assumed erosion rates varying from zero to 5 mm ka-1, but due to the normally extremely resistant nature of pure quartzites, the actual erosion rate is probably less than 1 mm ka-1. Two of the samples gave results in good agreement, with the third significantly younger. Based on the two older samples, the age of exposure of the samples probably lies in the range 13260 to14780 years.

References

Cerling T. E. and Craig H. (1994) Geomorphology and in-situ cosmogenic isotopes: Ann. Rev. Earth Planet. Sci. 22 273-317.

Gosse J. and Phillips F. M. (1999) Terrestrial cosmogenic nuclides: Theory and application: Quat. Sci. Rev. submitted.

Lal D. (1991) Cosmic-ray labeling of erosion surfaces: in situ production rates and erosion models: Earth Planet. Sci. Lett. 104 424-439.

Lal D. and Peters B. (1967) Cosmic ray produced radioactivity on the earth: In Handbuch der Physik, Vol. 46/2 (ed. K. Sitte) Springer-Verlag, Berlin, pp. 551-612.

Phillips F. M. (1995) Cosmogenic chlorine-36 accumulation: A method for dating Quaternary landforms: In Dating Methods for Quaternary Deposits (ed. N. W. Rutter and N. R. Catto) Geological Association of Canada, GEOTEXT2, p. 61-66.

Phillips F. M., Leavy B. D., Jannik N. O., Elmore D., and Kubik P. W. (1986) The accumulation of cosmogenic chlorine-36 in rocks: A method for surface exposure dating: Science 231 41-43.

Phillips F. M. and Plummer M. A. (1996) CHLOE: A program for interpreting in-situ cosmogenic nuclide data for surface exposure dating and erosion studies: Radiocarbon (Abstr. 7th Int. Conf. Accel. Mass Spectrom.) 38 98-99.

Phillips F. M"Stone W, D., and Fabryka-Martin J. T. (1999) An improved approach to calculating low-energy cosmic-ray neutron fluxes near the land/atomsphere interface: Chem. Geol. (Isot. Geosci. Sect.) submitted.

Phillips F. M., Zreda M. G., Flinsch M. R., Elmore D., and Sharma P. (1996) A reevaluation of cosmogenic 36Cl production rates in terrestrial rocks: Geophys. Res. Lett. 23 949-952.

Zreda M. G. and Phillips F. M. (1994) Surface exposure dating by cosmogenic chlorine- 36 accumulation: In Dating in Surface Contexts (ed. C. Beck) University of New Mexico Press, Albuquerque, pp. 161-184.

Zreda M. G. and Phillips F. M. (1998) Quaternary dating by cosmogenic nuclide buildup in surficial materials: In Dating and Earthquakes: Review of Quaternary Geochronology and Its Applications to Paleoseismology, Vol. NUREG/CR-5562 (ed. J. M. Sowers, J. S. Noller, and W. R. Lettis) Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission, Washington DC, pp. 2-101 – 2-128.



Part II: Radiocarbon Dates

Sample GSC-6432

FIELD No. : 99-BJB-0049 LABORATORY No. : GSC-6432
SITE No. :
SUBMITTERS: J. Bednarski
COLLECTORS: J. Bednarski
WEIGHT USED : g (wet )
11.9 g (dry)
COLL. DATE: 99.07.02 (YY.MM.DD)
SAMPLE Wt : 350 g
TREATMENT : hot base, hot acid, and distilled water rinses; noncalcareous
Processed 7.9 g which yielded 7.0L C02
MATERIAL: wood
TAXA: Picea; identified by J. Bednarski
DATED
COUNT DATA: one count (s) for 3800 minutes in the 2-L counter with a mixing ratio of 1.00

Sample count of 12.757 ± 0.065, cpm,
Background of 1.171 ± 0.025, cpm, and a
Standard of 17.959 ± 0.103 cpm.

ENCL MAT : clay, silt and sand overlain by colluvium, underlain by coarse gravel 
LOCALITY : along Trutch Creek, at the junction with a tributary stream from the north, 21.75 km east of Trutch (abandoned) townsite, northeast British Columbia 
COORDINATES 57.7212° N 122.5804° W
NTS SHEET No. 94G/10 elev. 640 m
AGE (uncorr.): 2750 ± 60 (70.991pMC)
?13C = -24.73%.
AGE (norm.) : 2750 ± 60
AGE (corr.) :
SIGNIFICANCE : deglaciation; geomorphic processes, landslide
ASSOC. DATES :
ADDITIONAL INFORMATION : 
MISCELLANEOUS INFORMATION : (GSC-6432; 99-BJB-0049)
Estimated Age : 5 ka BP
Project No. : 980004
COLLECTION SITE :
River cut bank on the north side of Trutch Creek. Currently this is a well-drained site. The trunk was protruding from the face, upright with horizontal root limbs, and apparently in growth position. The trunk diameter is about 30 cm with growth rings 0.5-3 mm.
STATE of PRESERVATI0N :
The wood was extracted from the exposure and stored in a fabric plastic bag. The sample was further air dried when returned from the field. Parts of the wood that were rotted and brittle with a dark brown colour were cut away. The submitted parts were firmer and dull grey to tan coloured. Some better-preserved parts of the trunk had a white fungus growth on the surface which were discarded. Only the inner parts of the trunk were submitted.
STRATIGRAPHIC RELATIONSHIPS :
The base of the section is a silty unit, but it is mostly covered. This is overlain by very coarse gravel containing boulders up to 1 m in diameter. Granites are present in the gravels suggesting a glaciofluvial origin. The coarse gravel unit is abruptly overlain by a unit of stratified silt and clay, with pebbly gravel stringers with some evidence of channel fills. This 3-4 m thick silt and clay unit contains isolated tree trunks and the apparently rooted trunk. These sediments are in turn overlain by the uppermost unit in the section, about 3 m of diamicton, probably colluvium.
SIGNIFICANCE : 
The wood date may either predate the last Laurentide ice advance into the area or , more likely, date a large landslide in the Trutch Creek valley. Several other dates in the area may show if there was a time during the postglacial when slumping was especially active.
OTHER INFORMATION : 99BJB0047

SampleGSC-6440

FIELD No. : 99-BJB-0047 LABORATORY No. : GSC-6440
SITE No. :
SUBMITTERS: J. Bednarski
COLLECTORS: J. Bednarski
WEIGHT USED : g (wet )
10.5 g (dry)
COLL. DATE: 99.07.02 (YY.MM.DD)
SAMPLE Wt : 168 g 
TREATMENT : hot base, hot acid, and distilled water rinses; noncalcareous
Processed 7.9 g which yielded 7.0L C02
MATERIAL: wood
TAXA: Picea; identified by J. Bednarski
DATED
COUNT DATA: one count (s) for 3970 minutes in the 5-L counter with a mixing ratio of 1.00

Sample count of 15.823 ± 0.075, cpm,
Background of 2.080 ± 0.033, cpm, and a
Standard of 28.482 ± 0.131 cpm.

ENCL MAT : stratified sand abruptly overlying fluvial gravels
LOCALITY : along Trutch Creek, 10 km east of Trutch (abandoned) townsite, northeast British Columbia 
COORDINATES 57.7175° N 122.7764° W
NTS SHEET No. 94G/10 elev. 724 m
AGE (uncorr.): 4720 ± 60 (70.991pMC)
?13C = -23.14%.
AGE (norm.) : 4750 ± 60
AGE (corr.) :
SIGNIFICANCE : deglaciation; geomorphic processes, landslide
ASSOC. DATES :
ADDITIONAL INFORMATION : 
MISCELLANEOUS INFORMATION : (GSC-6440; 99-BJB-0047)
Estimated Age : 5 ka BP
Project No. : 980004
COLLECTION SITE :
River cut bank on the north side of Trutch Creek. Tree trunk (15cm diameter) exposed in the face of a section enclosed by stratified sands. Some of the wood surface had clay-skin sheen.
STATE of PRESERVATI0N :
The wood was extracted from the exposure and stored in an open plastic bag. The sample was further air dried when returned from the field. The dry wood is very brittle with a dark reddish brown in colour. It is composed of extremely fine growth rings (0.025 mm width). Some parts of the wood sample had been penetrated by modern rootlets. None of these pieces of wood were submitted with the sample.

The wood was saturated by groundwater in the past. Small (0.25 mm) epigenetic salt (gypsum?) crystals formed in pores, displacing cellular material with growth. There are also yellow deposits and staining along some pores. Small dark nodules appear along the longitudinal grain of the wood that may be part of the wood structure. None of the deposits on the wood react to 10% HCl.

The wood sample was cut up so that the least stained pieces were submitted, but all pieces have some precipitate crystals remaining.

STRATIGRAPHIC RELATIONSHIPS :
The wood trunk comes from stratified sands, containing organic detritus that abruptly overlie fluvial gravels. The sands appear as lenses in a silty diamicton. Sandy shear planes appear to cut through the diamicton. Although separated by a covered interval, these brownish units appear to be overlain an indurated grey diamicton. The overlying diamicton may be till or a massive slump block.
SIGNIFICANCE : 
The wood date may either predate the last Laurentide ice advance into the area or date a large landslide in the Trutch Creek valley. Several other dates in the area may show if there was a time during the postglacial when slumping was especially active. 
OTHER INFORMATION : 99BJB0049

Sample GSC-6450

FIELD No. : 99-BJB-0100 LABORATORY No. : GSC-6450
SITE No. :
SUBMITTERS: J. Bednarski
COLLECTORS: J. Bednarski
WEIGHT USED : g (wet )
10.0 g (dry)
COLL. DATE: 99.07.26 (YY.MM.DD)
SAMPLE Wt : 290 g 
TREATMENT : hot base, hot acid, and distilled water rinses; noncalcareous
Processed 8.0 g which yielded 7.2L C02
MATERIAL: wood
TAXA: Picea; identified by J. Bednarski
DATED
COUNT DATA: one count (s) for 5200 minutes in the 5-L counter with a mixing ratio of 1.00

Sample count of 8.929 ± 0.052, cpm,
Background of 2.103 ± 0.024, cpm, and a
Standard of 28.472 ± 0.132 cpm.

ENCL MAT : silty diamicton
LOCALITY : beneath mountain slope on the south side of Crehan Creek, 12.5 km due west of Sleeping Chief Mountain summit northeast British Columbia
COORDINATES 57.8078° N 123.7981° W
NTS SHEET No. 94G/13 elev. 1006 m
AGE (uncorr.): 9320 ± 80 (31.278pMC)
?13C = -23.69%.
AGE (norm.) : 9340 ± 80
AGE (corr.) :
SIGNIFICANCE : geomorphic processes, landslide
ASSOC. DATES :
ADDITIONAL INFORMATION : 
MISCELLANEOUS INFORMATION : (GSC-6450; 99-BJB-0100)
Estimated Age : 5 ka BP
Project No. : 980004
COLLECTION SITE :
The collection site is at the foot of a large landslide beneath the steep northwest face of a mountain (summit ~1650 m). The tree trunk protruding beneath an indurated diamicton block. The surface of the wood had an orange-brown coating and brown staining. The inside of the wood is dark reddish brown. Growth rings are about 2 mm wide with well-developed reaction wood indicating disturbance during life.
STATE of PRESERVATI0N :
The wood was extracted from the exposure and stored in an open plastic bag. The sample was further air dried when returned from the field. The dry wood is a dark reddish brown in colour and the most brittle and stained parts were cut away. Some of the pores contain 1 mm long lenticular nodules of a powdery white precipitate (no reaction to 10% HCl). The nodules usually lie en echelon along the grain of the wood.
STRATIGRAPHIC RELATIONSHIPS :
The landslide covers a large area and, although initially it may have been a single catastrophic event, it probably has a complex history of activity spanning a long interval of time. The toe of the landslide where the wood was removed is composed of diamicton blocks 10’s of metres thick. The wood was removed from beneath one of the blocks. The diamicton forms vertical faces, is till-like in appearance.
SIGNIFICANCE : 
The sample may date the initial landslide or a subsequent movement.
OTHER INFORMATION :


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