n Alas - Alaska Division of Geological & Geophysical Surveys - State ...
n Alas - Alaska Division of Geological & Geophysical Surveys - State ... n Alas - Alaska Division of Geological & Geophysical Surveys - State ...
Olipocene (?) Paleocene EXPLANATION 1 CRETACEOUS KILOMETERS as Surficbl depaslts (Ouatemary) Ts Sandstone, siltstone. conqlomerate, claystone. and coal (Miocene and Oligocene?) Tv Andesitic and basaltic volcanic mcks IMiotene lo Paleocene) Tt Falstc and intermediate luff, lava flows. and Intrusive rocks (Miocene lo Paleocene) rjmi Felsic, and in~ermediate, and mallc intrusive rocks (Tertiary) Tar Arkose Ridge Formation (Emene or Palearena) Tc Chlckaloon Formatlon (Eocene or Paleocene) Wg Granitic rocks (early Tertiary and Cretaceous) Km Matanuska Formation ICretaceous) Kv Vafdez Group (Cre1acmus)-ArgillHe and graywacke Kmc McHugh Complex (Cretaceous) JS Marine sedimentary rocks (Jurassic) JQ Granitic and rnetamwhic rocks {Jurassic) pJm Meravolcanic, metasedimentarj, and granitic mcks (pre.Jurassic) pls Mlca sch~st 1 PA€-JURASSIC C ~ f l t ~ t located - A Fault A Thrusl fault-Sawteeth m upper plate - + Fault-Showing ~elativa mg K.Ar sample horizonlal movsrnen~ Figure 53.--Geologic sketch mag of the Matanuska Valley and part of surrounding region, southern Alaska (madlfied from Ucikman, 1974), showlng 16 sample sites where volcanic rocks were collected for age determinations and other analyses (see tables 16, 17).
Upper Cretaceous shale in contact with a large felsic stwk yielded potassium-argon ages of 37.5fl.2 to 45.52 2.3 m.y., which correspond to the late Eocene. 'Ihese ages are oompatlble with the late Paleocene age of the fossil plants in the Chickaloon Formation (J. A. Wolfe, written commun., 1981), which these volcanic rocks intrude. Table 16. Porastlm-r on r r al volcunlc rocks frm the Munvsks - - - - - & % & u ~ ~ e ~ ~ ~ - [Potassim md rr on analyses by (SOLO 6llutIonlma~~-r~cmtrlc techniqurr: rratysrr, U.S. &blosccaf?urvey. Rnlo Pwt. Call,., md Krbqer Enterpriser. Cambrlage. Mrsr .I - Map ~b. FIeld bck Mrlcrt~l Kfi kqe lftq. 53) No. Y D ~ rnslyzed lut pet) (my.1 --6----.-----------d&------ 1.95 45.511.8 Rhyolltc ash- AIXaI t 3.31 61.dt1.5 flow tuff. feldspar. Basalt nor------Whole rcKP--- -28 66.2fI.7 8 ~~ASI-W~U My01 f te rtocl------do--------- 7 Slldt b l ~ k Of do--------hornttl red ShblC Of the h~anurk~ Fornation In corntact ulth rhyol l re stock at loca)lty 8. Volcrnlc &kZ - >--. ot Lkd YIuNIMIU~ hl 11 19~61-112 ~ ~ rnor-----!hat* r ~ t 12 ?9LB-I07 An6C~ite flw-----46--------- 13 ?PAW-IOL awit ~OU-------d+------- 10 19~&-11)2 PI UP-- .-.--.--- ---&--------- IS 19~62-116 Pyrnclrst tn - - - - -- - - ruff breccla. Potmiurn-argon ages of volcanic rocks in the southeastern Talkeetna Mountains-43.612.2 to 60.1i4.6 m.y. (late Paleocene to Late Eocene)-essentially over- Lap the potassium-argon ages of samples from the Arkose Ridge Formation (table 18). Stratigraphic rela- tions in the southeastern Talkeetna Mountains, how- ever, contradict the age equivalency of these volcanic rocks and the Arkuse Ridge Formation. The Talkeetna Mountains volcanic rocks overlie generally gently d e formed fluviatile gravel that rests unconformably on the strongly folded and faulted rocks of the Chickaloon Formation. Because plant fossils suggest that the Chickaloan and the Arkose Ridge are coeval, we had expected to obtain ages from the southeastern Tal- keetna Mountains volcanic rocks that were somewhat younger than those in the Arkose Ridge. Possibly the contradiction is due to a large uncertainty in the 60.1- m.y. age for the southeastern Talkeetna Mountains volcanic rocks, caused by its large content of potas- sium and radiogenic argon. Excluding this determina- tion, the age for the volcanic rocks would range from 43.6S!,2 lo 55.5B.5 rn.y., or from latest Paleocene to late Eocene. This age range would permit the volcanic rocks of the southeastern Talkeetna Mountains to be younger than the Arkose Ridge Formation, but it would certainly not prove that they are, In fact, younger. The age determinations do, however, indicate that the interval between deposition of the Arkose Ridge FOP mation and the volcanic rocks was short. All the dated southeastern Talkeetna Mountains samples are from bedded volcanic rocks, and e plug intruding them, from the north side of the Castle Mountain fault system. These bedded volcanic rocks are absent south of the meln northern (Caribou) strand of this fault system. h the southeastern Talkeetna Mountains, however, just east of the area shown in fig- ure 53, mdic dikes and silk in the underlying country rock have a contiguous distribution both north and south of the Caribou fault (Grante, 1985). Contiguity, lithology, and the observation (Grantz, 1960) that the mafic dikes on opposite sides of the fault have the same strong west-northwestward preferred orientatjon suggest that these dikes form e related volcanic field. None of these eestern dikes have yet been dated, and so we cannot compare their ages to those of the volcanic rocks discussed here, although Grantz (1 960) assigned them to the Kocene. The three suites of volcanic rocks in the Matanuska Valley erea have an essentially bimodal dis- tribution of silica, with peaks between 45 to 59 and 69 to 78 percent SiOZ, although some intermediate com- positions also occur. Strontium-isotopic ratios of six samples of volcanic rock from the Matanuska Valley and from the Arkose Ridge Formation range from 0.7037 to 0.7042; one sample from the southwestern 'l#ke@a Mountains is slightfy more radiogenic ( Sr/ Sr=0.7047). ?his rock, wNch may be part of the Arkose Ridge Formation, rests on granitic rvlcks through which it was probably extruded; it may be slightly contaminated by the probably more radiogenic strontium of the granite. The strontium-isotoplc ratios are fairly primitive and narly as low as those measured in igneous rocks from ensimatic island arcs or along continental margins, where the crust Is young (Peterman and Hedge, 1974). We believe thnt these low isotopic ratios are consistent with derivation of the magmas from the upper mantle, with minimal strontium-isotope exchange w lth older crust01 rocks. Preliminary rare-earth+lernent data suggest little fractionation for the more mdic rocks at shallow crustal levels but are inconsistent for the more felsic varieties (M. L. Silberman and K. J. Wenrich, unpub. data, 1981). The strontium-isotopic ratios for the volcanlr: rocks of the Matanuska Valley area contrast sharply
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- Page 57 and 58: 1 slgnlficarrtly more umnlum (73&1,
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- Page 65 and 66: & Fclsic in~rutirt rucks 0 Eio~ite
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- Page 69 and 70: (Mg3.09 pe2+ 0.69 pe 0.~1~0.01~~0.9
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- Page 73 and 74: EXF'lANATIOW 66600' Contan-Apprnimn
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- Page 79 and 80: (Alnus ap.), heaths (Ericaceae, + E
- Page 81 and 82: terrane extends at least 300 krn to
- Page 83 and 84: Table 19.--6tneral petrography of M
- Page 85 and 86: were measured on 8 12-in. mass spec
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- Page 103 and 104: Mineral qItWation end r ~ k t k W e
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- Page 133 and 134: 40 40 Forbes, R. B., and Engels, J.
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- Page 139 and 140: Quartz Alkali feldspar Plagioclase
- Page 141 and 142: F'@m 85.--Sketch map of Juneau area
Olipocene (?)<br />
Paleocene<br />
EXPLANATION<br />
1<br />
CRETACEOUS<br />
KILOMETERS<br />
as Surficbl depaslts (Ouatemary)<br />
Ts Sandstone, siltstone. conqlomerate, claystone.<br />
and coal (Miocene and Oligocene?)<br />
Tv Andesitic and basaltic volcanic mcks<br />
IMiotene lo Paleocene)<br />
Tt Falstc and intermediate luff, lava flows. and<br />
Intrusive rocks (Miocene lo Paleocene)<br />
rjmi Felsic, and in~ermediate, and mallc intrusive<br />
rocks (Tertiary)<br />
Tar Arkose Ridge Formation (Emene or Palearena)<br />
Tc Chlckaloon Formatlon (Eocene or Paleocene)<br />
Wg Granitic rocks (early Tertiary and Cretaceous)<br />
Km Matanuska Formation ICretaceous)<br />
Kv Vafdez Group (Cre1acmus)-ArgillHe and graywacke<br />
Kmc McHugh Complex (Cretaceous)<br />
JS Marine sedimentary rocks (Jurassic)<br />
JQ Granitic and rnetamwhic rocks {Jurassic)<br />
pJm Meravolcanic, metasedimentarj, and granitic<br />
mcks (pre.Jurassic)<br />
pls Mlca sch~st<br />
1 PA€-JURASSIC C ~ f l t ~ t located - A<br />
Fault<br />
A Thrusl fault-Sawteeth m upper plate - + Fault-Showing ~elativa<br />
mg K.Ar sample<br />
horizonlal movsrnen~<br />
Figure 53.--Geologic sketch mag <strong>of</strong> the Matanuska Valley and part <strong>of</strong> surrounding region, southern <strong>Alas</strong>ka<br />
(madlfied from Ucikman, 1974), showlng 16 sample sites where volcanic rocks were collected for age<br />
determinations and other analyses (see tables 16, 17).
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