This document presents a detailed study on the distribution and thickness of Chilcotin group basalts and Quaternary drift across the interior plateau of British Columbia, revealing that basalt cover is thinner than previously believed and predominantly thick in paleovalleys. Significant geological findings include the identification of a volcanic dam formed 1.06 million years ago and the reversal of the Fraser River's flow, impacting mineral transport and drainage patterns. The research combines various geological data to enhance subsurface understanding, providing valuable insights for natural resource exploration.

1. Distribution and Thickness of basalts and drift on the Interior Plateau
Distribution et épaisseur des basaltes et des sédiments glaciaires de la région du plateau intérieur
1 2 2 3 1
Graham D.M. Andrews , Jackie Dohaney , Kelly Russell , Alain Plouffe , Robert G. Anderson
Geological Survey of Canada 1. Geological Survey of Canada, 625 Robson Street, Vancouver, British Columbia. V6B 5J3 Canada; email: Graham.Andrews@NRCan.gc.ca
2. EOS-UBC, 6339 Stores Road, Vancouver, British Columbia. V6T 1Z4 Canada
Commission géologique du Canada 3. Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario. K1A O8E Canada
Prospectivity on the Interior Plateau has hitherto been hampered A B
by a lack of data to determine the areal distribution and vertical
WATER WELL MODELING 093A/05
093J/03 093J/02
Y
093J/01
A - intraglacial c. 1.06 Ma B - deglaciation post-1.06 Ma C - intraglacial >30 ka
RO
buried
thickness of Chilcotin Group basalt lavas and Quaternary drift. McLeese Lake paleochannel ?
CK
Water Well Records (n = 10,486) Bedrock Intersection Points (n = 3,189) Y
Peace Drainage
M
TN
y
S
e
Vall
CA
RI
Research at UBC supported by the GSC TGI-3 and MPB programs
BO
O
buried (Arctic)
ser
M
0 km 10 TN
S
paleochannel
Skeena Skeena
Fra
093K 093J 093I 093K 093J 093I INTE
Fraser-Peace
RIOR
1:500,000
and Geoscience BC reveals that basalt cover is thinner and less V V C
O
A
ST
PLAT
EAU
S
093G/15
Ferndale 093G/16 Drainage Drainage Basin Drainage
extensive than previously thought (<40%) across the Interior PG PG Miocene M
TN
S
Basin (Arctic) Basin
093F 093H 093F 093H 093B/08 alley
093G 093G ako V
Plateau. It is only thick (>20 m) in paleovalleys that can be readily RO
CK
Y
M
093B/01 X’ 093A/04 N ech
Prince George
Q TN
identified in geological and geophysical surveys. Similarly,
S
Q
CA
RI
BO
O
4 - Prince X’
M
TN
George area
Quaternary drift, although areally extensive, is only thick in
S
093C 093B 093A 093C 093B 093A INTE
RIOR
WL
WL Williams Lake X
paleovalleys. We have mapped cover thickness variations in
C
O
A
ST
PLAT
EAU
S
Beaverley
bedrock intersection point
Fraser Valley
M
TN 093G/14
S
‘C ‘C
several areas on the Interior Plateau (below) using a combination 100 100 150 Mile House
093G/09
“push down” point
oa oa
ey
092N 092O 092N 092O
ll
sta sta
Va
buried drift thickness (m) n = 1576
ko
of field observations, water well records, and archived 0 50 km
092P
0 50 km
092P
Ba l’ Dr Ba l’ Dr
ila
paleochannel ? < 10 10 - 20 20 - 50
C Y’
Ch
geological, geochronological, geophysical, and MINFILE data “COVER” 50 - 100 > 100
sin ain sin ain
Fras
0 km 10
Water well logs recorded by the Province of British a a
e
e
092P/13 bedrock (m asl)
ge ge
er V
g
g
1:500,000
X
(A) Typical till veneer (<1 m) covering granite
sin ina
sin ina
sets; the results of which are presented here and will be Columbia provide valuable subsurface data, in particular
a lley
bedrock intersection point
Springhouse 093G/11 093G/10 400 850 1300
bedrock (092P). (B) Forested till blanket (>1 m) 2 - Williams Lake
Ba Dra
Ba Dra
“push down” point
for identifying the cover - bedrock surface. Wells are
published soon. covering “thin” (<20 m) Chilcotin Group basalt unevenly distributed across the Interior, concentrated
area 092O/16 drift thickness (m) n = 1954
X X’
bia
bia
1200
X X’ <5 5 - 10 10 - 20
(092P). ( C ) Gravel-rich esker (~10 m)
lum
Williams Lake
lum
around cities and along major highways (above).
1 cm = 200 m
1 cm = 200 m
1000 20 - 50 > 50 1000
Creek Fraser Valley
Chilako Valley
deposited on an eroded bedrock surface. (D)
m asl
m asl
The base of the drift and the base of the Chilcotin
Co
800
Co
800 bedrock (m asl) km 100
0 km 100 0 km 100 0
Chilcotin Group veneer
“Thick” (>20 m) basalt lava pile filling a basalts have been identified in over 7,000 and 3,000 600
400 850 1300
600
paleovalley at Chasm Provincial Park (092P). wells respectively. These points can be interpolated to
x10 vertical exaggeration 28889 m
1 mm = 200 m
400
sea decaying ice mass ice thickness (m) new river course
(E) Ancestral Chilcotin River filled by “thick” form bedrock elevation maps (below and right) 092P/14
092P/15
Y
x10 vertical exaggeration 48131 m
1 mm = 200 m
Y’
D - present-day impounded lake catchment boundary new canyon abandoned channel
basalt and pillow-breccia succession near Fraser Valley buried paleochannel present course
1 cm = 200 m
depicting the drift thickness (points). These maps are 800 alpine ice-cap continental divide river course Dog Creek dam
Fraser Valley
m asl
Alexis Creek (093B). improved by integration of outcrop and surficial geology X’ 600
parallel ancient drainage
map data-sets. Cross-sections can be drawn to depict
the present-day and buried bedrock surfaces. La Canim L
ak e
x10 vertical exaggeration 49167 m
1 mm = 200 m anti-parallel ancient drainage REVERSAL OF THE FRASER RIVER
D The take-home messages are: c
la
Ha Lac la Hache
5 - Vanderhoof area A newly identified volcanic dam, formed at 1.06 Ma, indicates northward
ch
1. - the use of this data-set to constrain the depth to e 093K/01
0 km 10 X’ 093J/04 flow of the Fraser River and the absence of the Fraser Canyon at that time.
bedrock (data and maps available soon!) Forest Grove
092P/10
1:500,000 This requires reversal of the Fraser River’s flow and carving of the canyon
2. - the discontinuous and thin nature of the Chilcotin Ste
war
t Va
RO
CK
Y
M
TN
in the last 1 million years. Erosion of the Fraser Canyon caused the base
lley
level of the drainage basin to drop, carving deep canyons in some
S
5 basalts, and thick drift only in major valleys. CA
4
RI
BO
O
tributaries, and abandoning others. We attribute the reversal to ice-
M
TN
3. - the presence of abandoned valleys and major INTE
RIOR
S
100 Mile House
erosion along the Fraser River after eruption of C
O
A PLAT
EAU
damming during the end of a c. 1 Ma glaciation.
ST
The implications of this for exploration are as follows:
S
E basalts and before the last glaciation (see next Vanderhoof M
TN
S
panel - Reversal of the Fraser River). Nechak
o Valley
1. - Reversal means that the Fraser River and some tributaries (e.g., the
1 Lone Butte Thompson River) the have changed their course in the last 1 Ma. Fluvial
093G/02 093G/01 092P/11
Tachic
k Lake
sediments and detrital minerals deposited before 1 Ma were transported
2 bedrock intersection point
Fra
bedrock intersection point RO
CK RO
Bridge Lake away from source (e.g., mineral deposit) apparently upstream! Tributaries
ser
Y CK
“push down” point M
TN
S
Y
M
TN
092P/07 “push down” point
Va
S
CA
ke in the west appear unchanged, but those in the east may be completely
lle
CA
RI
drift thickness (m) n = 750 Cluculz
X’ drift thickness (m) n = 2150 BO RI
La drift thickness (m) n = 777 Lake
y
O BO
M
TN O
n
ee
M
< 10 10 - 20 20 - 50 S TN <5 5 - 10 10 - 20
50 - 100 > 100
INTE
RIOR INTE
RIOR
S
G
r <5
20 - 50
5 - 10 10 - 20
> 50 20 - 50 > 50 reversed.
3 bedrock (m asl)
C
O
A
ST
PLAT
EAU
S
C
O
A
ST
PLAT
EAU
S
X bedrock (m asl) bedrock (m asl) 2. - Reversal probably occurred multiple times after initial reversal at c. 1
Quesnel X
450 m M
TN
S
M
TN Ma, therefore, detrital minerals may have been carried upstream and
Ra
W subaerial lavas E X 400 850 1300
S
400 850 1300 093F/16 093G/13
yfie
subaerial lavas 400 850 1300
downstream.
ld
70 Mile House 0 10 X’
Va
km X
1 - Quesnel area 3. - Contemporary and later glacial deposits recycled this alluvial
lle
bedded hyaloclastite 092P/06 1300
y
basement - 1:500,000
pillow breccia Stewart Valley
Mesozoic X X’
sediment and redistributed it - therefore tills in the reversed valleys and
1 cm = 200 m
70 m 1100
Pliocene Chilcotin River valley floor
sedimentary rocks 900 X 3 - 100 Mile House area X’
Nechako Valley
m asl
basement -
1 cm = 200 m
adjacent areas, especially older tills, may have very complex detrital
1 cm = 300 m
Fraser Valley 1500 900
Mesozoic 700 Green Lake
m asl
perched ?
m asl
sedimentary rocks 1300
700
source histories.
alley
0 km 10 remnants
Quaternary deposits 500 1100
of Chilcotin Group
Chilcotin Group veneer extensive Chilcotin Group
er V
Quaternary deposits
1:500,000
300 900 500
4. - This event abandoned channels of the old drainage on the Interior
Fras
x10 vertical exaggeration 24341 m x10 vertical exaggeration x10 vertical exaggeration 46721 m
093B/15 093B/16
Present Chilcotin River valley floor Quaternary deposits 1 mm = 200 m 61360 m
1 mm = 300 m
1 mm = 200 m 0 km 100 Plateau, many subsequently buried. These are potential placer deposits.
Natural Resources Ressources naturelles
Canada Canada Please see accompanying student posters by Jackie Dohaney (new map of Chilcotin basalt distribution) and Rebecca-Ellen Farrell (new map of Chasm Provincial Park)