Geology and_ Geochronology of Central Interior Region of the

26 Pages·2012·5.81 MB·English

Geology and_G eochronology of
Precambrian Rocks in the
Central Interior Region of the
United States
GEOLOGICAL SURVEY PROFESSIONAL PAPER 1241-C
Geology and Geochronology of
Precambrian Rocks in the
Central Interior Region of the
United States
By RODGER E. DENISON, E. G. LIDIAK, M. E. BICKFORD, and
EVA B. KISV ARSANYI
CORRELATION OF PRECAMBRIAN ROCKS OF THE
UNITED STATES AND MEXICO
Edited by JACK E. HARRISON and ZELL E. PETERMAN
GEOLOGICAL SURVEY PROFESSIONAL PAPER 1241-C
Lithology, distribution, correlation, and
isotope ages of Precambrian terrane for five
subareas between the Appalachians and the
Rocky Mountains
UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1984
DEPARTMENT OF THE INTERIOR
WILLIAM P. CLARK, Secretary
U.S. GEOLOGICAL SURVEY
Dallas L. Peck, Director
Library of Congress Cataloging in Publication Data
Main entry under title:
Geology and geochronology of Precambrian rocks in the central interior region of the United States.
(Geological Survey Professional Paper; 1241-C)
Includes bibliographic references.
Supt. of Docs. No.: I 19.16:1241-C
1. Geology, Stratigraphic-Precambrian. 2. Stratigraphic correlation. 3. Geology-United States.
I. Denison, Rodger E. II. Series.
QE653.G476 1984 551.7'1'0973 8:wi00332
For sale by the Branch of Distribution
U.S. Geological Survey
604 South Pickett Street
Alexandria, VA 22304
CONTENTS
Page Page
Abstract ...... . C1 Area III-Southern Missouri, Southern Kansas, Oklahoma, and
Introduction . . . . . · 1 Northwestern Arkansas . . . . . . . . . . . . . . . . C8
Aclrnowledgrnents 2 Proterozoic time (interval occurring 900-1,600 m.y. ago) . 8
Area I-North and South Dakota 2 Formation of rhyolitic to dacitic volcanic rocks and as
Archean time . 2 sociated epizonal plutons 1,485 to 1,350 m.y. ago . 8
Gneiss .......... . 2 Mesozonal granite rocks along the Nemaha Ridge in
Greenstone . . . . . . . . 4 Kansas and Oklahoma, and in the eastern Arbuckle
Granite and granodiorite 4 Mountains, Oklahoma ............ . 10
Proterozoic time (interval occurring 1,600-2,500 m.y. ago) 4 Phanerozoic time (Paleozoic Era, Cambrian Period) . . . 11
Metamorphic rocks . . 4 Area IV-Texas and Eastern New Mexico . . . . . . . . . . 11
Granites ........................ . 4 Proterozoic time (interval occurring 1,600-2,500 m.y. ago) . 11
Silicic volcanic rocks . . . . . . . . . . . . . . . . . . 5 Torrance metamorphic terrane and "older granitic
Proterozoic time (interval occurring 900-1,600 m.y. ago) . 5 gneisses" ..................... . 11
Granite .......... . 5 Proterozoic time (interval occurring 900-1,600 m.y. ago) . 11
Mafic and ultramafic rocks . . . . . . . . . . . . . . 5 Rocks formed 1,200 to 1,400 m.y. ago . . . . . . . 11
Sioux Quartzite . . . . . . . . . . . . . . . . . . . . . 5 Rocks formed 1,000 to 1,200 m.y. ago . . . . . . . 13
Area 11-Nebraska, Iowa, Northern Missouri, Northern Kan Proterozoic time (interval occurring 600-900 m.y. ago) . 14
sas, and Eastern Colorado . . . . . . . . . . . . . . 5 Metarhyolite of Devils River uplift . . . . . . . 14
Archean time . . . . . . . . . . . . . . . . . . . . . . . . 5 The Van Horn Sandstone . . . . . . . . . . . . . 14
Proterozoic time (interval occurring 1,600-2,500 m.y. ago) 5 Phanerozoic time (Paleozoic Era, Cambrian Period) . 14
Metavolcanic, metasedimentary, and foliated granitic
Area V-Eastern Midcontinent . . . . . . . . . . . . . . 14
rocks formed 1,650-1,800 m.y. ago . . . . . . . 7
Proterozoic time (interval occurring 900-1,600 m.y. ago) . 14
Proterozoic time (interval occurring 900-1,600 m.y. ago) . 7
Granite-rhyolite terrane 14
Anorogenic plutonic rocks. formed about 1,450-1,480
m.y. ago ..................... . 7 Sedimentary rocks . . . . . . . 16
Anorthosites in southwestern Nebraska . . . . . . . 7 Basaltic rift zones . . . . . . . 16
Rift-related sedimentary rocks 16
Mafic volcanic and hypabyssal rocks and related arkosic
Subsurface Grenville province 16
sedimentary rocks associated with the Central North
American rift system 8 Metallogenic significance of the Precambrian basement 17
Metamorphism . . . . . . . . . . . . . . . . . . . . . 8 References cited . . . . . . . . . . . . . . . . . . . . . . 18
ILLUSTRATIONS
Page
PLATE 1. Correlation chart for Precambrian rocks of the Central Interior region, United States. In pocket
FIGURES 1-5. Sketch maps showing geology of basementrocks in:
1. North and South Dakota . . . . . . . . . . . . . . . . . . . . . . C3
2. Nebraska, Iowa, and parts of Missouri, Kansas, and Colorado. 6
3. Oklahoma and parts of Kansas, Missouri, and Arkansas . 9
4. Texas and eastern New Mexico . . . . . . . . . . . 12
5. Central Interior region east of the Mississippi River . . 15
III
CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO
·GEOLOGY AND GEOCHRONOLOGY OF PRECAMBRIAN ROCKS IN THE
CENTRAL INTERIOR REGION OF THE UNITED STATES
By RODGER E. DENISON! E. G. LIDIAK,2 M. E. BICKFORD,3 and EVA B. KISVARSANYI4
ABSTRACT INTRODUCTION
Rocks of the buried Precambrian crust in the Central Interior re
gion range from more than 2,700 to less than 1,000 million years Our understanding of the Precambrian in the Central
in age and from granite and granulitic gneiss to gabbro and basalt Interior region is based upon widely separated outcrop
in rock type. The oldest rocks occur in the Dakotas and clearly are areas and samples from irregularly distributed but
buried portions of the Canadian Shield; they are mostly older than
numerous wells drilled largely in search of oil and gas.
2,700 million years, and some may be as old as 3,600 million years.
Flawn (1956) showed that it was possible .t o make a
The central part of this region, including Nebraska, northern Mis
souri, and northern Kansas, is underlain by diverse igneous and map of the buried Precambrian terrane based on drill
metamorphic rocks whose ages are mostly 1,600 to 1,800 million hole samples, and the larger scale study of Muehlberger
years; scattered anorogenic granitic plutons whose ages are about and others (1967) led to publication of the basement
1,~1,500 million years are also known in this terrane. rock map of the United States (Bayley and
The most distinctive feature of the Continental Interior is the great
Muehl berger, 1968). These works remain the founda
terrane of felsic igneous rocks that makes up the basement from Ohio
and Wisconsin across southern Missouri and Kansas and into the tion of our present understanding. The geology and
Texas Panhandle and far western Texas. These rocks, which include geochronology of the scattered surface exposures are
abundant rhyolite and mesozonal and epizonal granitic bodies, range now better known, but the geochronology of the sub
in age from 1,500 to 1,200 million years, and the general tendency
surface has received little attention, and the basic re
~ for ages to decrease from northeast to southwest; older rocks are
not known anywhere within this terrane. Toward the east in Ohio, ference work is the series of reports by Goldich and
eastern· Kentucky, and eastern Tennessee, and toward the south in his coworkers (1966).
central Texas, the basement terrane consists of medium-grade The rocks in the Central Interior region are here di
metamorphic rocks and associated granitic plutons that formed vided into four general types: (1) deep-seated granitic
mainly 1.000-1.100 million years ago.
and metamorphic rocks similar to those exposed in the
A belt of basalt, interflow arkosic sandstone and siltstone, and re
lated mafic intrusive rocks can be traced with the aid of geophysical shield areas; (2) anorogenic mesozonal and epizonal
data from the Lake Superior region southw~ into central Kansas. granite; (3) rhyolite and epizonal granite; and (4 ) basalt
This feature, the Central North American rift system, is widely be and gabbro of "rift" type.
lieved to be an abortive continental rift that formed about 1,100 mil The first type is typical of rocks exposed in the Pre
lion years ago. Geophysical data suggest that other areas in the east
cambrian shields. These are diverse and strongly de
em part of the interior are also underlain by rift basalts and related
formed rocks together with undeformed massive plu
rocks.
The Central Interior region was dominated by eugeosynclinal tons that are characteristically older than about 1,600
sedimentation and orogenic tectonics prior to about 1,600 million m.y. (million years). The marked density and magnetic
years ago. After that time the region apparently stabilized, and the contrasts of these rocks allow extrapolation by
sedimentation was characterized by the deposition of sheets of
geophysical methods in areas where drill control is lack
quartzose sandstone about 1,600 million years ago. Subsequent igne
ing. About 18 percent of the Central Interior region
ous activity, sedimentation, and tectonics have been dominantly
anorogenic except along the margins of the stable interior. is underlain by rocks of this type.
The second type is characterized by anorogenic meso
10De EDeJV Square, Dallu, TX 76008.
IJ>epertment of GeolORY aDd P1aDetary Seleneee, Univendty or PlttabUJ'Ih, Pittsburgh, zonal to epizonal granitic plutons, formed 1,300 to 1,500
PA Ilia). m.y. ago, and associated with relatively minor
'Department of GeolCJRY, Univendty or KaMu, Lawnmee, KS 66CM6.
4Miuouri Department or Natural Reeoun!ee, Rolla, MO 66401. metasedimentary and metaigneous rocks. Silver and
Cl
C2 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO
others (1977) and Emslie (1978) have discussed the im Precambrian rocks of the Central Interior region is the
portance of these rocks, which form a discontinuous great preponderance of granite and related volcanic
band from northeastern New Mexico to central Mis rocks. These rocks, which generally have petrographic
souri and probably eastward to the Grenville Front in features indicating that they were emplaced at shallow
the Central Interior region. We estimate that 13 per to intermediate crustal levels, make up about two
cent of the Continental Interior is underlain by these thirds of the Continental Interior. Mafic rocks occwr
rocks. mostly along the Central North American rift system.
The third type is characterized by large tracts of Igneous rocks of intermediate composition are excep
rhyolite and associated epizonal granite. After these tionally rare. The greenstone belts that so characterize
tracts were recognized in the subsurface (Muehlberger the older shield areas are confined to the buried exten
and others, 1966; Muehlberger and others, 1967), addi sions of the shield in Area I, and thus make up only
tional drilling showed that much of the area east of about 1 percent of the Central Interior region.
the Mississippi River and west of the southward exten Sedimentary rocks are also notably rare. Shelf-type
sion of the Grenville province is underlain by similar sedimentation evidently began about 1, 700 m.y. ago,
rocks. Although the origin of the epizonal granite-rhyo but the major deposits were of clean sandstone. Car
lite terrane remains unclear, certain conclusions may bonate rocks are virtually unknown, and all the sedi
be drawn from our present understanding: mentary rocks make up only an estimated 7 percent
(1) The rocks are preserved in structural depressions; of this great region.
surrounding rocks represent deeper crustal levels of
ACKNOWLEDGMENTS
emplacement.
(2) The rhyolites are invariably associated with The cooperation of the various State geological sur·
coeval hypersolvus granites that commonly display veys, mining companies, and oil companies in providing
micrographic quartz-perthite intergrowths. access to samples has been essential to this report.
(3) The rhyolites are not known to be associated with James D. Hansink of Rocky Mountain Energy provided
any significant volume of other volcanic or sedimentary an opportunity for Denison and Lidiak to review the
rocks. basement of the Continental Interior in 1976, and this
(4 ) The rhyolites are essentially undeformed and only work has been an important base for the results pre
locally recrystallized. sented here. W. R. Muehlberger and W. R. Van
(5) The several tracts of rhyolite-granite are similar Schmus reviewed the manuscript and gave many help
but not the same in age; the ages show no simple pat ful comments. Financial support was provided by the
tern of variation. U.S. Geological Survey and the U.S. Department of
The rhyolite-epizonal granite association underlies an Energy. Lidiak is pleased to acknowledge support from
estimated 52 percent of the Continental Interior, al NASA grant NSG--5270.
though much of this area is east of the Mississippi
River where drill-hole control is poor. The abundance
AREA I-NORTH AND SOUTH DAKOTA
of these rocks is the major difference between the
buried Precambrian of the Continental Interior and the Characterization of the buried basement complex in.
exposed shield areas. Gravity and magnetic observa North and South Dakota is based on gravity and mag-·
tions are of limited value in extrapolating these rocks netic anomalies and on lithologic study of several
into areas where drill control is poor. hundred basement well samples (Muehlberger and.
The extension of Keweenawan basaltic and gabbroic others, 1967). Lithology and ages of rock units are
rocks from the Lake Superior region into the Central shown on plate 1. These data indicate that the eastern
Interior region along the Central North American rift Dakotas are mainly part of the subsurface extension
system has yielded the fourth major rock association. of Archean rocks of the Canadian Shield. The western
Basaltic rocks and related arkose can be traced as far Dakotas are a continuation of mainly Proterozoic rocks
as east-central Kansas on the basis of scattered well of the Canadian Shield.
samples and gravity and magnetic data. That smaller
ARCHEAN TIME
areas east of the Mississippi River are also underlain
by similar mafic igneous rocks can be inferred from
GNEISS
geophysical measurements. These rocks are possibly
time correlative with the Keweenawan associations. We Gneiss of Archean age is apparently widespread in
estimate that about 10 percent of the interior is under eastern North and South Dakota. The oldest rocks (fig.
lain by rocks of this type. 1) are granitic and granulitic gneisses that crop out in
Perhaps the single most significant feature of the the Minnesota River valley (Goldich and others, 1961;
GEOLOGY AND GEOCHRONOLOGY OF PRECAMBRIAN ROCKS C3
EXPLANATION
PROTEROZOIC
e.~.:;_~·~:t<~~~
SIOUX QUARTZITE
!'<-;':_.
GRANITE
'..-'):''
:).:~-~~:{ SILICIC VOLCANIC ROCKS
}~{:}~{:} METAMORPHIC BELTS
ARCHEAN
GRANITES AND GRANODIORITIES
GREENSTONE BELTS
GRANITIC AND GRANULITIC
GNEISSES .
---- CONTACT OR INFERRED BASEMENT
CONTACT
----FAULT OR INFERRED FAULT
AREA OF EXPOSED ROCKS
100 200 KILOMETERS
FIGURE 1.-Sketch geologic map of basement rocks in North and South Dakota. Unpatterned areas have no control.
Goldich and others, 1970) and extend along a series of highs; five wells to basement encountered granitic and
prominent gravity and magnetic anomalies from near gneissic rocks. In North Dakota the gravity anomalies
Duluth, Minn., west-southwestward to east-central are less distinct, but 22 wells to basement demonstrate
South Dakota (Lidiak, 1971; Morey and Sims, 1976). the granitic and gneissic character of the terrane. Min
Detailed radiometric studies of the Minnesota River eral assemblages in the gneisses indicate widespread
valley rocks (Goldich and others, 1970; Goldich and metamorphism to the amphibolite facies.
Hedge, 1974) have shown that they are at least 3,500 Sparse radiometric data indicate that the gneisses
m.y. old and that one phase appears to be 3,700 m.y. are of probable Archean age (Burwash and others,
old. Metamorphism and granite emplacement about 1962). A further indication of age is the continuation
2,700 m.y. ago and a thennal event about 1,800 m.y. of geophysical anomalies associated with Archean
ago have partly obliterated the earlier geologic history gneisses of the Canadian Shield into the eastern
of the gneisses. Dakotas. The higher grade of metamorphism in the
Granitic gneiss is shown on figure 1 as the predomi gneisses compared to Archean greenstones suggests
nant rock type in five other areas of northeastern South that at least some of the gneisses predate the 2, 700-
Dakota and in a large area of northeastern North m.y.-old Algoman orogeny.
Dakota. The belts in South Dakota are associated with Rocks of Archean age are also present in the Black
west-southwest-trending gravity lows and magnetic Hills (Zartman and Stern. 1967; Ratte and Zartman,
C4 COKRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO
1970; Kleinkopf and Redden, 1975). Two granulites IP'ffi.(Q>'lrJE.ffi.(Q>fl(Q>II<C 'lriiMIE. (IIN'lriE.ffi.V AIL OC<ClUUIIN<G
from neaJr the centell" of the Williston Basin may also JlSl®@®=~SlB)@@ MoJY'o A~)
be Archean in age because they occwr along a pll"omi
nent lineaJr gravity hlgh, which can be tJraeed north
MIE'II' AM<O>UIHIIIC JR?.OCIU)
waJrd to the Nelson Rivell" high of Innes (1SOO). The
Nelson Rivell" hlgh has been conelated with Alrchean The east-northeast-tll"ending anomalies of the
wanulitic gneisses that well"e involved in both the Keno Superioll" pll"ovince aJre tenninated in the centll"al
Tan and Hudsonian oll"ogenies (PatteJrson, 1S83; Bell, Dakotas by northwest-tll"ending anomalies of the Chm
1SOO; Gibb, 1S88; Kornik, 1S89). clhill pll"ovince (Muehlbell"gell" and othern, 1S87; lLidiak,
1971). The alinement of gJravity and magnetic anomalies
implies a northwest structwral tll"end of the basement
<GIRUEIEN§'II'<O>NIE Jrocks. Thll"ee metamonohlc belts aJre infened. The belt
Belts of gJroonstone and ll"elated ll"ocks aJre extensive in the c~ntrnl Dakotas is maJrked by magnetic highs
and both hlghs and lows in grnvity. The few wells to
in the basement of the eastern Dakotas. Seven belts
basement suggest that the aJrea is undell"lain by mafic
aJre shown on figwre 1; they aJre chaJracterized by wav
and silicic schist and gneiss. The belt in south-centn\1
ity ~ghs and less pll"onounced but lineaJr magnetic
South Dakota continues into N ebrnska and coincides
highs. Study of 18 samrnples indicates that amphibole
schists and gneisses are dominant; seJr]pentinite is pll"es with magnetic and grnvity lows. The rocks aJre. domi
nantly silicic schists (Lidiak, 1972). The thllrd metamoll"
ent in one basement well. Modal compositions suggest
phlc belt trends through the Black Hills and continues
mafic and vJ.trnmafic igneous antecedents. A stawrolite
into Montmla. Tlhls belt also coincides with a m2gnetic
schist in northeastern South Dakota indicates that
metasedlimentaJry Jrocks aJre associated with the low. The ll"'Cks aJre mainly medium grnde me~en­
greenstones. The ll"ocks aJre chaJracterized by taJry ll"odks, l~y intruded by gJranite. Gough and
metamo1rphlsm to the gJreenschlst Oll" lowell" amphibolite Camfield (1972) sllllggested that grnphltic schist may be
abwndlmt.
facies.
No age detell"llllinations have been published foll" any The presence of Alrchean granitoid ll"'Cks in the ·Black
of the supll"acrustal Jrocks in the Dakotas. They aJre Jre Hills suggssts tl'lwt these metamoll"]phlc belts probably
garoed as being of Alrchean age because the associated develo~dl on a siallic crust within a cnton rnthell" than
gravity and magnetic anomalies continue into northern along a continenW maJrgin. The time of deposition is
Minnesota, where they coincide with gJreenstone and infell"redl to have been about 1,S00-2,100 m.y. ago.
ill"on-fonnation of the Keewatin Gll"oup, dated at about Goldlich and othell"s (1SS6) concluded that the Jrocks
2, 700 m.y. (Hall"t and Davis, 1S89). in the western Dakotas were involved in oll'ogeny
1,700-1,SW m.y. ago. Most of the ages aJre of minell"als
from metamorphic ll"ocks. The basement may include
<GmANII'IT'IE ANI!J) <Grn.AN<O>I!J)II<O>lRUI'II'IE oldell" ll"ocks whose ages well"e Jreset by youngell"
metamoll"phlsm as well as Jrocks fonned at that· time.
CoaJrSe-grained, two-feldsp~ gJranite (21 wells) is in
The metamoll"]phlc belts possibly date from eaJrliell"
tell"pll"eted as the pll"incipall ll"ock typ8 in lalrge aJreas of
Protell"ozoic time, but thls dating can be demonstrnted
the eastern Dakotas (fig. 1). GJranodiorite and tll"ondhje
only in the Black Hills, whell"e All"chean grnnite gneiss
mite (seven welllls) and mnphlbolite-glmdle gneiss (seven
wellls) are allso present but are apjpaJrently suboll"dlinate (Zartman and Stem, 1S87) is unconfoJrmably o~ell"lain
by a thick ·metasedimentaey succession that was folded,
to gR-anite. These ll"ocllm lie in the subsurlace extension
metamoll"]phosed, and· intruded by gJranite 1,700-1,SOO.
of the Superioll" pll"ovince of Canada and aJre charnc
m.y. ago dwing the Black Hills oll"ogeny (Goldich and
terized by gravity and magnetic lows. The wanite and
othell"s, 1SOO). ·
Jrelated ll"'Cks in the eastern pall"t of the aJrea of figwre
1 have the samrne genem tll"end as the greenstones and
aJre intell"]pll"eted as being pall"t of an All"Chean gJreenstone
<Gm.ANII1I'IW
granite tenane that was involved in the .Algoman
oll"ogeny. K-Alr (potassiu.m-aJrgon) and Rb-Sll" (rubidiu.m GJranite (11 wells) occwrs at scattell"ed localities in the
strontiu.m) ages on minell"alls from both granite and western Dakotas. AppaJrent ll"adliometric ages on minell"
gneiss Jrefiect mrminlly the widespread igneous activity als and whole Jrock samrnpl8s aJre in the rnnge 1,~1,810
and metmnoll"]phlsm that occWl"ll"ed during thls Oll"ogeny m.y. (Goldi~h and othem, 1SSS). The gJranites pll"obably
(Bmwash and othem, 19$2; Petell."'man and Hedge, 1SM; foll"med during the ~oll" ~rioo of oll"ogeny in _the :west
Goldich and othem, 1SOO). · em Dalkot2a~
GEOLOGY AND GEOCHRONOLOGY OF PRECAMBRIAN ROCKS C5
SILICIC VOLCANIC ROCKS layers of rhyolite and quartz sandstone (Beyer, 1893).
Similar silicic volcanic\,rocks in South Dakota yield ages
Silicic volcanic rocks are present in eastern South
of 1,680-1,700 m.y.; the Baraboo Quartzite of Wiscon
Dakota. These rocks are essentially unmetamorphosed
sin, often considered to be correlative with the Sioux
and apparently overlie the older plutonic complex.
Quartzite, rests upon rhyolite whose U-Pb zircon age
Three determinations yield ages of 1,680-1,700 m.y.
. is 1,760± 10 m.y. (Van Schmus, 1978). ·
(Goldich and others, 1966). Petrographically similar
rhyolitic volcanic rocks occur in adjacent northwestern
Iowa (included in undifferentiated felsic rocks on fig.
AREA II-NEBRASKA, IOWA,
2).
NORTHERN MISSOURI, NORTHERN
PROTEROZOIC TIME (INTERVAL OCCURRING KANSAS, AND EASTERN COLORADO
900-1,600 M.Y. AGO)
Basement rocks in Area II include a variety of igne-
ous, metamorphic, and sedimentary rocks whose ages
GRANITE range from at least 1,800 m.y. to about 1,000 m.y. The
distribution and petrography of these rocks have been
Four Rb-Sr whole rock or feldspar ages on granite
determined primarily from study of cuttings and cores
from southern South Dakota and adjacent Nebraska are
from deep drilling, but geophysical data have also been
in the range 1,480-1,510 m.y. (Goldich and others,
used to extend terranes mapped on the basis of well
1966). These rocks are probably related to the
samples. Large numbers of well samples are available
anorogenic granites ·discussed in the following sectio11
in Nebraska and Kansas because ofo il and gas explora
on Nebraska, Iowa, and surrounding area.
tion; these have been studied by Lidiak (1972) in N e
braska and by Scott (1966) and Bickford and others
MAFIC AND ULTRAMAFIC ROCKS .
(1979) in Kansas. The Missouri basement is reasonably
Diabase, diorite, gabbro, and pyroxenite occur in well known beeause of drilling for minerals and has
scattered wells in North and South Dakota. Except for been studied by Kisvarsanyi (1974, 1975). Relatively lit
deuteric alteration in the diabases, the rocks are unal tle is known about the Precambrian rocks of Iowa and
tered and thus intrusive into the plutonic complex. eastern Colorado, because a smaller number of wells
Their age is unknown, but they probably reflect several have penetrated the basement there.
intrusive episodes. They are tentatively regarded as
postdating regional metamorphism and thus being less
ARCHEAN TIME
than 1, 750 m.y. old.
No rocks of Archean age are known in Area II, al
though we infer such rocks to underlie parts of north
SIOUX QUARTZITE em Iowa because of the proximity of. the ancient rocks
that are exposed in the Minnesota River valley (Goldich
The Sioux Quartzite is a uniform, mildly folded, sub
and others, 1970; Goldich and Hedge, 1974).
horizontal formation that is nonconformable on the un
derlying plutonic complex. It is extensively developed
in the surface and subsurface of southeastern South
PROTEROZOIC TIME (INTERVAL OCCURRING
Dakota and extends into adjacent Minnesota, Iowa, and
1,600-2,500 M. Y. AGO)
Nebraska. The formation is composed mainly of
silicified quartz sandstone that is conglomeratic near Only one radiometric age greater than 1,800 m.y. has
the base, and minor thin beds of red shale and argillite. been reported for any rocks from Area II, and most
The presence in the essentially l.mdeformed quartzite are 1,700 m.y. or less (Goldich and others, 1966). We
of diaspore plus quartz and pyrophyllite plus quartz have, however, indicated on the chronometric chart (pl.
(Berg, 1938) suggests hydrothermal or burial meta 1) that both sedimentary and volcanic rocks may have
morphism under static conditions. formed as early as 2,000 m.y. ago. This speculation is
Pebbles of iron-formatio~ in the Sioux Quartzite indi based upon the presence of silicic metavolcanic rocks
cate that the unit may be no older than about 1~900 and various metasedimentary rocks (schists, quartzites)
m.y. (Goldich, 1973), and· a Rb-Sr age determination in the basement of both Kansas and Nebraska. These
on a rhyolite from Sioux County, ·Iowa, suggests that are associated spatially with gneissoid granitic rocks
it may be at least 1,520 m.y. old (Lidiak, 1971). A that have yielded ages of about 1,700 m.y. If
nearby well is reported to have penetrated alternating metamorphism occurred later than 1,700 m.y. ago, it
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r. ..... ;.... . EXPLANATION z
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FELSIC ROCKS UNDIFFERENTIATED =·:·=·==·:·:·==···:·=:·:~· J MEMToAsMtlOy RmPeHtaICse RdiOmCenKtaSr y; ---- COCNOTNATCATC TO R INFERRED BASEMENT -at>z=<j
GRANITE grade variable, mostly --?--FAULT OR INFERRED FAULT 0
low to medium Queried where extension uncertain 0
RHYOLITE
SIOUX QUARTZITE } Sedimentary
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GABBRO KEWEENAWAN rocks
>v· 7.7. , " (" : .,. : ._J ~' I BASALT • AGE ROCKS
FIGURE 2.--Sketch geologic map of basement rocks in Nebraska, Iowa, and parts of Missouri, Kansas, and Colorado. Unpatterned areas
have no well control. CNARS, Central North American rift system.