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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 |
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)
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).
Table 1. Analytical results from northern British Columbia samples
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36Cl/(1015CI) |
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Conc.(104 atoms/g) |
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98BJB0031 |
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14 |
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5.3 |
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98BJB0049 |
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26 |
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13.0 |
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98BJB0032 |
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98BJB0049 |
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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.)
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.
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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.
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.
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,
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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 |
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,
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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 |
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,
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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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