ISOTOPIC AGE DETERMINATIONS

 

RETURN TO MAIN MENU

 

       22 Ma
MJA-LM1 East summit of Lawson Mountain dacite dome.
NW1/4 of NW1/4 of Sec. 34, T. 11 S., R. 20 E. (Lawson NW).
44o 34'36"N, 120o19'26"W
UTM coordinates = 4939187N, 712502E
Elev. (ft) = 4400
Ar-Ar, U.S.G.S. lab at Denver, 2000, whole rock.
21.86 + 0.07, plateau age, Appel, 2001.

 

       29 Ma 
Picture Gorge Ignimbrite of the John Day Fm., Carroll Ridge, Painted Hills.
NE1/4 of SE1/4 of Sec. 36, T. 10 S., R. 20 E. (Painted Hills SE)
44o39'18"N, 120o15'58"W
UTM coordinates = 4948052N, 716770E
Elev. (ft) = 2400
Ar-Ar, Swisher at Berkeley, 1993, feldspar crystal.
28.7 + 0.07, 28.7 + 0.05, Bestland et al., 1994.
Previous age determinations of this unit:
26.94 + 0.23, Fiebelkorn et al., 1983.
25.3, Hay, 1963. 

       29 Ma 
EMT-11 John Day basaltic dike in road cut, W. of Bridge Cr.
SE1/4 of SE1/2 of Sec. 1, T. 12 S., R. 21 E. (Mitchell SE)
44o33'10"N, 120o08'38"W
UTM coordinates = 4937021N, 726853E
Elev. (ft) = 3190
K-Ar, Armstrong lab at Yale, 1971, whole rock.
29.33 + 0.59, 29.45 + 0.59, Enlows and Parker, 1972.
29.3 + 0.4 average recalculated by Fiebelkorn et al., 1983.

 

       32 Ma 
KA-845 John Day basaltic lava SE of Painted Hills.
Center of NW1/4 of Sec. 9, T. 11 S., R. 21 E. (Sutton SW)
44o38'01"N, 120o14'38"W
UTM coordinates = 4945804N, 720575E
Elev. (ft) = 2120
K-Ar, U. of Cal. Berkeley lab, whole rock.
31.5, Evernden et al., 1964.

 

       33 Ma 
M-859 John Day basaltic dike N. of Mitchell Rock.
NE1/4 of SW1/2 of Sec. 26, T. 11 S., R. 21 E. (Mitchell NE)
44o35'06"N120o10'23"W 
UTM coordinates = 4940504N, 724421E 
Elev. (ft) = 2860
K-Ar, Armstrong lab at Yale, 1971, whole rock.
33.48 + 1.27, 33.30 + 1.23. Enlows and Parker, 1972.
34.3 + 0.9, average recalculated by Fiebelkorn et al., 1983.

 

       34 Ma 
MJA-BD1 Fresh margin of John Day mafic dike, N. of U.S. 26.
SW1/4 of SW1/4 of Sec. 32, T. 11 S., R. 22 E. (Keyes NW)
44o33'57"N, 120o07'16"W
UTM coordinates = 4938534N, 728604E
Elev. (ft) = 3120
Ar-Ar, U.S.G.S. lab at Denver, 2000, whole rock.
30.42 + 0.18, total gas age; 34.1 + 0.6, regression age, Appel, 2001.

 

       34 Ma 
KFO-901 Andesite lava from summit of hill near center of Keyes Quad.
S1/2 of SW1/4 of Sec. 35, T. 11 S., R. 22 E. (Keyes NE)
44o33'59"N, 120o03'21"W
UTM coordinates = 4938788N, 733787E
Elev. (ft) = 4690
K-Ar, Armstrong lab at Yale, 1971, whole rock.
35.59 + 3.41(feldspar), 30.09 + 4.71(whole rock), Enlows and Parker, 1972.
33.7 + 3.1, recalculated by Fiebelkorn et al., 1983 (Probably too young; lies stratigraphically between MJA-HEC (49 Ma) and MJA-LEC (49 Ma); near E-16-77 (Ma 49).

 

       38 Ma 
KA-818 Andesite lava 1 mi. NE of Sand Mountain.
SW1/4 of Sec. 8, T. 11 S., R. 21 E. (Sutton SW)
44o37'42"N, 120o14'24"W
UTM coordinates = 4945151N, 718938E
Elev. (ft) = 2800
K-Ar, U. of Cal. Berkeley lab, whole rock.
37.5, Evernden et al., 1964. Probably too young because this flow is part of the sequence of Clarno lavas which includes MJA-CBU2 (43 Ma).

 

       40 Ma 
Basal Member "A" ignimbrite of the John Day Fm., Painted Hills.
NW1/4 of NE1/4 of Sec. 11, T. 11 S., R. 20 E. (Painted Hills SE)
44o38'07"N, 120o17'22"W
UTM coordinates = 4945776N, 714990E
Elev. (ft) = 2270
Ar-Ar, Swisher at Berkeley, 1993, feldspar crystal.
39.7 + 0.03, Bestland et al., 1994.
Previous age determinations of this unit:
36.4 + 1.1, 36.1 + 1.0, Swanson and Robinson, 1968.
37.4 + 1.1, 37.1 + 1.0, recalculated by Fiebelkorn, et al., 1983.

       42 Ma 
MJA-CM2 Summit of large dacite intrusion, N. of Cougar Gulch.
Center NE1/4 of Sec. 34, T. 11 S., R. 20 E. (Lawson NE).
44o34'34"N, 120o18'39"W
UTM coordinates = 4939153N, 713517E
Elev. (ft) = 4075
Ar-Ar, U.S.G.S. lab at Denver, 2000, whole rock.
41.57 + 0.11, total gas age, Appel, 2001.

 

       42 Ma 
WDW-3-58 Dacite intrusive, summit of White Butte.
NW1/4 of NE1/4 of Sec. 20, T. 12 S., R. 21 E. (Mitchell SW)
44o31'14"N, 120o13'33"W
UTM coordinates = 4933190N, 720452E
Elev. (ft) = 5640
K-Ar, Armstrong lab at Yale, 1971.
46.11 + 3.96 (hornblende), 40.55 + 0.90 (whole rock), Enlows and Parker, 1972; 41.6 + 0.9 whole rock recalculated by Fiebelkorn et al., 1983.

 

       42 Ma 
BEH-GRD1 NE end, garnet-bearing rhyolite dike, S. slope of Scott Butte.
SE1/4 of NE1/4 of Sec. 33, T. 10 S., R. 22 E. (Toney SW)
44o39'41"N, 120o05'01"W
UTM coordinates = 4949227N, 731204E
Elev. (ft) = 3780
K-Ar, O.S.U. Duncan lab, 1990, biotite separate.
42.0 + 0.5, not published, (probably too young, see MJA-SB1 and SB2)

 

       42 Ma 
MJA-CAU1 Late Clarno andesite lava, ridge crest W. of West Branch Cr.
N. central Sec. 26, T. 11 S., R. 20 E. (Lawson NE).
44o35'26"N, 120o17'46"W
UTM coordinates = 4940828N, 714613E
Elev. (ft) = 3980
Ar-Ar, U.S.G.S. lab at Denver, 2000, whole rock.
42.4, total gas age; 42.04 + 0.92, regression age, Appel, 2001.

 

       43 Ma 
MJA-CAU2 Late Clarno andesite lava, ridge crest W. of West Branch Cr.
N. central Sec. 26, T. 11 S., R. 20 E. (Lawson NE).
44o35'26"N, 120o17'46"W
UTM coordinates = 4940793N, 714653E
Elev. (ft) = 4000
Ar-Ar, U.S.G.S. lab at Denver, 2000, whole rock.
42.2 + 0.10, total gas age; 42.95 + 0.8, regression age, Appel, 2001.

 

       43 Ma 
MJA-CBU2 Late Clarno andesite lava, N. of Meyers Cr. mouth.
NE1/4 of NE1/4 of Sec 16, T.11 S., R. 21 E. (Mitchell NW).
44o37'08"N, 120o12'20"W
UTM coordinates = 4944160N, 721689E
Elev. (ft) = 2250
Ar-Ar, U.S.G.S. lab at Denver, 2000, whole rock.
42.88 + 0.20, plateau age, Appel, 2001.

 

       44Ma 
MJA-CB1 Mafic intrusive, summit of butte 1 mi. N. of Mitchell.
SW1/4 of SW1/4 of Sec. 25, T. 11 S., R. 21 E. (Mitchell NE).
44o34'48"N, 120o09'04"W
UTM coordinates = 4940029N, 726166E
Elev. (ft) = 3850
Ar-Ar, U.S.G.S. lab at Denver, 2000, whole rock.
43.80 + 0.10, (total gas age), 52 + 3.9 (regression age), Appel, 2001.

 

       44 Ma 
KFO-1112 Basal Clarno lava flow, U.S. 26 at the Narrows.
NE1/4 of SW1/4 of Sec. 21, T. 11 S., R. 21 E. (Mitchell NW)
44o35'20"N, 120o12'48"W
UTM coordinates = 4941777N, 721159E
Elev. (ft) = 2400
K-Ar, Armstrong lab at Yale, 1971, whole rock.
42.31 + 0.84, 43.35 + 0.87, Enlows and Parker, 1972.
44.0 + 0.6, recalculated by Fiebelkorn et al., 1983. Probably too young; lies stratigraphically below MJA-LWC (52 Ma).

       45Ma 
MJA-HM2 Alkali-basalt intrusion, 1200 ft. S.E. of Old Hudspeth Mill.
Center, Sec. 20, T. 11 S., R. 21 E. (Mitchell NW).
44o36'01"N, 120o14'09"W
UTM coordinates = 4942031N, 719375E
Elev. (ft) = 2640
Ar-Ar, U.S.G.S. lab at Denver, 2000, whole rock.
44.40 + 0.08, total gas age; 45.26 + 0.31, regression age, Appel, 2001.

 

       46 Ma 
KFO-1702B Mafic intrusive, summit of Marshall Butte. 
Center of Sec. 29, T. 11 S., R. 22 E. (Keyes NW)
44o35'14"N, 120o06'49"W
UTM coordinates = 4940891N, 729165E
Elev. (ft) = 4538
K-Ar, Armstrong lab at Yale, 1971, whole rock.
45.03 + 0.90, 44.8 + 0.90, Enlows and Parker, 1972.
46.1 + 0.07, average recalculated by Fiebelkorn et al., 1983. Probably too young; see MJA-MB1 (47 Ma).

 

       47 Ma 
MJA-MB1 Mafic intrusive, summit of Marshall Butte.
Center of Sec. 29, T. 11 S., R. 22 E. (Keyes NW)
44o35'14"N, 120o06'49"W
UTM coordinates = 4940891N, 729165E
Elev. (ft) = 4538
Ar-Ar, U.S.G.S. lab at Denver, 2000, whole rock.
47.21 + 0.14 (total gas age), 46 + 1.2 (regression age), Appel, 2001.

       48 Ma 
MJA-MC2 Phlogopite-bearing kersantite dike of Mud Creek.
NE1/4 of NE1/4 of Sec. 4, T. 12 S., R. 21 E. (Mitchell NW)
44o33'45"N, 120o12'19"W
UTM coordinates = 4937907N, 721958E
Elev. (ft) = 3000
Ar-Ar, U.S.G.S. lab at Denver, 2000, mica separate.
47.79 + 0.24, total gas age, Appel, 2001.

 

       49 Ma 
MJA-MB3 Mafic intrusive on S. E. slope of Marshall Butte.
Center of Sec. 29, T. 11 S., R. 22 E. (Keyes NW)
44o35'05"N, 120o06'44"W
UTM coordinates = 4940671N, 729234E
Elev. (ft) = 4200
Ar-Ar, U.S.G.S. lab at Denver, whole rock.
48.69 + 0.17, total gas age; 49.21 + 0.84, (regression age), Appel, 2001. Same intrusive unit as MJA-MB1 (47 Ma).

 

       49 Ma 
MJA-LEC Clarno lava, N. side of U.S. 26 at Mitchell.
SE1/4 of SE1/4 of Sec. 36, T. 11 S., R. 21 E. (Mitchell NE)
44o33'00"N, 120o08'38"W
UTM coordinates = 4938587N, 726813E
Elev. (ft) = 3000
Ar-Ar, U.S.G.S. lab at Denver, 2000, whole rock.
48.84 + 0.14, 49.62 + 0.12, total gas ages, Appel, 2001.

 

       49 Ma 
MJA-HEC Mitchell Mtn. Pass, E. of Mitchell on U.S. 26.
SE1/4 of SE1/4 of Sec. 2, T. 12 S., R. 22 E. (Keyes SE)
44o33'12"N, 120o02'49"W
UTM coordinates = 4937384N, 734909E
Elev. (ft) = 3840
Ar-Ar, U.S.G.S. lab at Denver, 2000, whole rock.
49.42 + 0.13, 48.98 + 0.09, total gas ages, Appel, 2001.

 

       49 Ma 
E-16-77 Clarno andesite plug on summit of Peggy Butte.
SE1/4 of SW1/4 of Sec. 22, T. 11 S., R. 22 E. (Keyes NW)
44o35'39"N, 120o34'26"W
UTM coordinates = 4941862N, 732236E
Elev. (ft) = 4845
K-Ar, U.S.G.S. lab at Menlo, 1978, Hornblende separate.
48.9 + 5.2, Fiebelkorn et al., 1983.

 

       49 Ma 
EMT-285 Minette lamprophyre on Spetch Rim.
NE1/4 of NE1/4 of Sec. 12, T. 12 S., R. 20 E. (Lawson SE)
44o32'45"N, 120o15'54"W
UTM coordinates = 4935942N, 717239E
Elev. (ft) = 3940
Ar-Ar, O.S.U. Duncan lab, 1992, phlogopite separate.
49.0 + 0.5, not published.

 

       49 Ma 
MJA-SR1 Minette lamprophyre on Spetch Rim.
NE1/4 of NE1/4 of Sec. 12, T. 12 S., R. 20 E. (Lawson SE)
44o32'45"N, 120o15'54"W
UTM coordinates = 4935942N, 717239E
Elev. (ft) = 3940
Ar-Ar, U.S.G.S. lab at Denver, 2000, phlogopite separate.
48.65 + 0.17, plateau age, Appel, 2001.

 

       49 Ma 
MJA-BB1 Minette lamprophyre on W. slope of Black Butte.
NW1/4 of NW1/4 of Sec. 1, T.12 S., R. 20 E. (Lawson SE)
44o33'41"N, 120o16'52"W
UTM coordinates = 4937610N, 715941E
Elev. (ft) = 3820
Ar-Ar, U.S.G.S. lab at Denver, 2000, phlogopite separate.
48.97 + 0.14, plateau age, Appel, 2001.

 

       49 Ma 
JD-15-88 Upper crystal-rich unit of Clarno ignimbrite, 0.5 mi. SE of Old Hudspeth mill.
S1/2 of Sec 20, T. 11 S., R. 21 E. (Mitchell NW)
44o35'50"N, 120o13'51"W
UTM coordinates = 4941716N, 719818E
Elev. (ft) = 2680
Ar-Ar, O.S.U. Duncan lab, 1992, feldspar separate.
49.4 + 0.9, not published. 

 

       51 Ma 
MJA-SB1 NE end, garnet-bearing rhyolite dike, S. slope of Scott Butte.
SE1/4 of NE1/4 of Sec. 33, T. 10 S., R. 22 E. (Toney SW)
44o39'41"N, 120o05'01"W
UTM coordinates = 4949227N, 731204E
Elev. (ft) = 3780
Ar-Ar, U.S.G.S. lab at Denver, 2000, biotite separate.
50.93 + 0.14, plateau age, Appel, 2001.

 

       51 Ma 
MJA-SB2 SW end, garnet-bearing rhyolite dike, S. slope of Scott Butte.
Center of Sec. 33, T. 10 S., R. 22 E. (Toney SW)
44o39'34"N, 120o05'14"W
UTM coordinates = 4949029N, 730956E
Elev. (ft) = 3700
Ar-Ar, U.S.G.S. lab at Denver, 2000, biotite separate.
50.97 + 0.11, plateau age, Appel, 2001.

 

       52 Ma 
MJA-LWC Third lava from Clarno Fm. base, U.S. 26 at the Narrows.
NE1/4 of SW1/4 of Sec. 21, T. 11 S., R. 21 E. (Mitchell NW)
44o35'54"N, 120o12'55"W
UTM coordinates = 4941900N, 721025E
Elev. (ft) = 2350
Ar-Ar, U.S.G.S. lab at Denver, 2000, whole rock.
52.32 + 0.18, total gas age, Appel, 2001.

 

       223 Ma 
Quartz, crossite, lawsonite blueschist of basement rock in Meyers Canyon.
SE1/4 of SW1/4 of Sec. 13, T. 11 S., R. 21 E. (Mitchell NE)
44o36'38"N, 120o09'20"W
UTM coordinates = 4943411N, 725696E
Elev. (ft) = 2810
K-Ar, U.S.G.S. lab, 1976, white mica and anthophyllite separate.
223 + 3.2, Hotz et al., 1977.

 

 

RETURN TO MAIN MENU