Download petrology and Structure of the Southern Wet Mountains, Colorado

\U WATERSHEDS
Trans. 13th Ann. Mtg
n '$
P'.gi
ROBERT E. BOYER
Deft. Geology, University of Texas, Austin, Texas
:iysical approach to
y Bull. 23, 4th ser., 190 p.lf
Pa. Geol. Survey 4th ser.'4?
petrology and Structure of the Southern
Wet Mountains, Colorado
juency: U. S. Geol. Survey f
Folio 33
;|
•-
i
10
. Div. Geol. Sci. Bull. 5jM
Set. Bull. 55, 269 p.
•$.
asins: U. S. Geol. Survey >}
;
tream channels and some «
and their relation to the -;:
?y: New York, McGraw-
;
!. Survey 4th ser., Bull.
Geol. Survey Folio 160
abstract): Am. Geophys.
topographic maps: Am.
587-591
'eophys. Union Trans.,
Abstract: The Precambrian core of the southern
\\'et Mountains consists in part of metasedimentary
•niciss and schist concordantly foliated with granite
'neiss and composite rocks—migmatite and lit"nar-lit gneiss. These rocks developed by alteration
during intrusion of the San Isabel Granite batholith
and associated smaller plutons. Migmatite borders'
the batholith and grades outward into lit-par-lit
i-neiss as a result of more intensive reconstitution
and metasomatism nearer the igneous mass. Foliation in the metamorphic and composite units tends
to "wrap around" the plutons and is discordant to
the foliation (planar flow structure) of these bodies.
Lineation, which developed in the metamorphic
and composite rocks during folding and neocrystallization of the sequence, plunges northward. Lineation (linear flow structure) of the granite plutons
jlso generally plunges northward and supports the
belief that the plutons are of common origin.
Field relations and petrographic investigations,
including heavy-mineral studies, indicate a genetic
relation among the granite bodies; the smaller
plutons are probably contaminated differentiates
of the magma which formed the San Isabel batho-
CONTENTS
Perth Amboy, N. J.:
sigle, No. 55: Pa. Geol. I
>f drainage basins: Am. |
b Orchard Mountains
y: Geol. Soc. America
f
lion Trans., v. 38, p. !
f
America Bull., v. 69, :
1240 p.
i, Ohio: Hydrologic
-i basin, U. S. Geol.
River basins: U. S.
Univ. Press, 403 p.
lith. Lead-alpha age dating of zircon from the San
Isabel Granite indicates that this batholith represents
part of the orogenic activity which occurred about
1500 m. y. ago.
The southern Wet Mountains form a southeastplunging anticline with the dominant fracture pattern striking N. 60° W., parallel to the range axis,
and a minor trend N. 40° E. Complex structural
relations reflect both Precambrian deformation and
later superimposed fracturing. Late Paleozoic uplift (Ancestral Rocky Mountains) occurred along
pre-existing lines of weakness; Laramide deformation and Tertiary block faulting, although influenced by .these trends, formed additional fracture
patterns. The Wet Mountains were uplifted to
present heights from mid-Tertiary to Pleistocene.
Probably the Wet Mountain massif acted as a
resistant block to regional compression and yielded
by upward movement, although uplift may be the
result of solely vertical stresses. Movement occurred in stages along prominent high-angle boundary faults and by slight displacements along innumerable joints.
I
I
;
i
:
Introduction
Acknowledgments
General geology
Precambrian geology
Metamorphic rocks
General features
Biotite schist and biotite-hornblende schist
Hornblende gneiss and amphibolite . . .
Lime-silicate gneisses and related rocks . .
Granite gneiss
Composite rocks
General features
Lit-par-lit gneiss
Migmatite
Igneous rocks
General features
Gneissic granite
Granite of Cliff Creek
Granite of Bear Creek
Granite of Williams Creek
San Isabel Granite
Quartz diorite
Pegmatites
Heavy-mineral studies
Structure
1048
1048
1050
1053
1053
1053
1053
1053
1054
1054
1055
1055
1055
1055
1056
1056
1056
1056
1056
1056
1056
1057
1057
1058
1061
Regional structure
Local structural features
Foliation and lineation
Faults
Joints
Geologic history
Precambrian history
Paleozoic-Mesozoic history
Laramide history . . . "
Post-Laramide history
References cited. . . '.
Figure
1. Index map of south-central Colorado . . . .
2. Stratigraphic section of the Wet MountamsHuerfano Park area
3. Sketches of apatite grains from Precambrian
granites and associated rocks
4. Sketches of zircon grains from Precambrian
granites and associated rocks
5. Generalized tectonic map of south-central
Colorado
6. Lineation in the Precambrian rocks ot the
southern Wet Mountains, Colorado . . .
Geological Society of America Bulletin, v. 73, p. 1047-1070, 6 figs., 4 pis., September 1962
1047
1061
1061
1061
1062
1063
1066
1066
1066
1067
1067
1068
1049
1051
1057
1057
1060
1064
fl
C;Y AND STKUCTUKK, ;•
Plalc
1
Facing
C*C
•"I" . . . KH7
<• I'oliatum in the Precambri.n rocks i
southern W,t Mmmuins, C,,Ua,|,,
Table
1062
and
1063
INTRODUCTION
The Wet Mountains
aciatcil rocks .
,( ,1 "/I- -7) rcc"8'»zed the mu,,
of the fohation in the Greenl,,,ri, \
reg.on. Hdls (1900, p. 1) su,d!»l ,TC
and summarized the history of tl
aiiis Recent reconnaissance i,,UM
(Hurbauk and Goddard 1937- (
1°^, p. 182) describe the structure''"
logic lustory. U. S. Geological Si
studies (Christman and others,
have been focused about 20 mi
ot tile map region. These reiio,
S >
Utt
a Kl
older metasedimentary"un i ts "Th^r | '
ajford excellent opporUn.i for tvalu±Tof
the respective roles of imLinn
«m. and neocrystalliJdon
°"' mctas°lnatrohation :m,l i;,, :.._
, .
scale of 600 leet^o Ihe'i'ncb b^Sn,'!!
others 1955) and an expk,na,
,
cwala and Brock (1956)
-Singewald
""'S
and Hrock (1956)
F eMmapping,
g on U.
S F
1'ield
Sc
1lotoeranhs
'
TP(\
/"-?n
' nnm
°" U" S.
' Forest
°™ *»'
Photographs
1:20,000),
was -.ccom,, '
'"«
ln "'«= "<-,s of 1955 thSffi;
P
K was transfcrr,.,!
i^ an
„
,large , .
Ping
transferred to
?crv,ce drainage
M
,
-a
(M.A. theses) by University of Muh,
dents (Boydsum and others, 1- '
•
J">. l(->58; Read, 1956). The
part 01 a project of th e Departnu
<> ogy University of Michigan, ,„
I luerfano Park quadrangle, Co1(;u,I,,
ACK.NOWI.KDGMHNTS
The writer is indebted to E. N. (
. and
During early reconnaissance, Kndlich
orn I'.k "r T ""i1™1' fonncd lhe
Um ant ranilcs and
and .scattered
i« i ' 1Coccurrences
' « of fdsic and
S»«*«.
ami
mafic
d'kcs were mcmioued by Cms., (18%, , 78(,)
suggestions and constructive
invaluable. Appreciation is expre.x
Kriggs, F. Win. Ueinrich, W. R
t;. S. lurneaure, and J. H. Zlmlh,,t[
aui and advice, both in the field a,,!
office, have been of great assista,.,
Compton and James Gilluly rcai| ,
version of the manuscript and (
suggestions for its improvement.
A National Science Foundation
for the year 1956-1957 enabl,
30 MILES
Figure 1. Index map of south-central Colorado
ham School of Grad
1050
K. li. UOYKR-WiTKOIXX;Y AND STKUCTUKK, SOUTHERN WKT MTNS., Col,
1051
CiKNKRAI. CiUULOCY
versity of Michigan, partially defrayed field
expenses for the summers of 1955 and 1957.
A grant from the Pcnrose Hcqucst of the
Geological Society of America covered expenses during the summer of 1958. Grants
from the Geology Foundation and the University Research I n s t i t u t e of The University
<>1 Texas paid for field expenses during the
summer of 1959. The Geology Foundation
paid the costs of drafting the illustrations.
unconformahly upon the Sangre di (
Formation. The Entrada flanks (lie »,,.
end of the Wet Mountains as a massiu, >.
ous, cross-bedded unit. The Morrison 1
lion, varicolored clays, shales, ami uM!:,.«
l:
with thin beds of sandstone ami In I K . *U
crops out along most of the sedimcm.u\\ t'l^
ary, commonly in contact with Piu a..-. ^
rock (PI. 1).
__ The basal Cretaceous unit, the |' Uii..
Formation, is mapped with tinGENERAL GEOLOGY
Dakota Sandstone (PI. 1). The IXiU,J w_l
A nearly flat surface, evidence of probably
stone forms prominent hogbacks thai, II'. _
extensive erosion, forms the crest of the southaround the flanks of the range. At siu,.j!
ern Wet Mountains. This surface ranges from
the Dakota is in high-angle iiorin.il L..
half a mile to 4 miles in width and extends for
tact with the Precambrian. Along il,,.» -j
8 miles at an elevation of about 11,400 feet.
zones the Dakota commonly display I,,
It correlates best with the Flattop peneplain
veins with opal cement in the fraci'uu.
ot the Southern Rocky Mountains (Lee 19??
The Kentnn Group, predominantly :-..
p. 17; Little, 1925, p. 498), dated as middle
composed of the Graneros, (Jretnli,,;.
lertiary, an age compatible with Miocene
Carlile formations. Overlying it art ij,V" f V
volcanic rocks (Greenhorn Peaks) resting on
Hays (Timpas) Limestone and thc.Si,,,,!
this surface. The Flattop peneplain is assigned (Apishapa) Marl which make tip tin- \.
an_elevation of 11,000-12,000 feet.
Group. The Pierre Shale at the i,,,, ,
The gentle southwest surface dips result Cretaceous System in Iluerfano 1'jil.
.W
from tilting which accompanied uplift. This present.
explains the steep northeast flank of the range,
Resting unconformahly upon iL
in contrast to a more gentle slope on the south- Shale in Iluerfano Park is the Palcouu i
west side. Several terrace levels along the Canyon Formation, in turn ovcrl.m, :
southwest face extend for more than 8 miles;
Eocene Cuchara, Iluerfano, and K,,,,.
break-in-slope faults indicate that these ter- mations. Of the Paleocene and Eom, ;
races are structural. The terraces were once a
tions, only the Farisita is present HI ::.
continuation of the erosion surface hut have
area. The lower Earisita sands wcu .1...
been displaced along the faults by the upwardat the same time as the upper HuuL . t
moving range core and subsequently modified
mation, as the two interfinger alonu i.1.
by erosion. A late Tertiary age is suggested for side of Iluerfano Park.
the faulting, thus supporting a middle Tertiary
A polymictic conglomerate foriih .,
age ol the erosion surface.
tered patches in T. 24 S., R. d9 \\
The Precambrian complex of the southern
top of the WetMountains (i'l. I, i"
Wet Mountains is bounded by upper Paleozoic parently flat-lying patches reslinj; „ , t
M
or younger sedimentary rocks which wrap
volcanic rocks and granite do nm'n,:
around its southern end (PI. 1). (Figure 2 is a
feet in thickness. The fragments u,^. < ;
stratigraphic section of the southern Wet
granite, gneiss, and schist of the I'u,»=.>•-*
Mountains—Iluerfano Park area.)
complex. This conglomerate is thou,!.; „
The Sangre de Cristo Formation, the oldest
Pliocene, although it may be PleistiM.y
sedimentary rock exposed, is dated by Brill
Much float and a few outcrops o| ;,|» ,
(1952, p. 822) as late Pennsylvania!! or early
mafic dikes occur throughout tin j::. 'T»
Permian (Wolfcampian). It rests unconforma- dikes show two distinct trends (I'l i bly on the Precambrian complex at the south- dominant strike is N. 50°-60° W., |i.;A
»
ern end of the Wet Mountains. Boydston and
most of the faults; a subordinate u,..: »
others (1954, M.A. thesis, Univ. Mich., p.
seen in tbe extreme western pan <il •:,- *•t
28) noted that this formation is 445 feet thick
strikes N. 10°-.50° E. Joint sets run:,.*
in Red Canyon (see. 18, T. 26 S., R. 68 W.), trends. The dikes generally din Ti.h , an
bU
whence it thins markedly northward and east- ing 70°-80°.
ward and thickens westward.
Mafic dikes, which range from ji.Ur »
The upper Jurassic Eutrada Sandstone rests basalt and diabase, are common in i!,
c:
ALLUVIUM
tec-***
CONGLOMERATE
VOLCANIC
ROCKS
MIOCENE
5
|
FARISITA
FORMATION
PALEOCENE
POISON CANYON
FORMATION
PIERRE SHALE
'::
.:
UTPTO
\
6
t
*
§
SM5KY HILL
MARL
§
FORT HAYS
LIMESTONE
u
CARLILI
SHALE
*"
g
GREENHORH
LIMESTONE
3
GRANEROS
SHALE
r
:'-V::*"*
88
''?'.'».°'»
,
PUHCATOIM
FORMATION
win
FORMATION
ENTRADA
SANDSTONE
nuity
:
SANGRE DE CRISTO
rally
1 000 T
°<-.-j-;fi
« • . . '- •
— -1rl~-
ss
^^^2
:
^K
t--:::
400
1 300
1 800
700
50
200
225*
250
.; .•:•;!:;,':'
50 ^
DESCRIPTION
Unoonaolldated gravel, sandj
ailt and olay along streams
Poorly sorted gravel along
foothills
Silica cemented conglomerate
Kith Precavbrlan fragments
Porphyritic vesicular (In part)
andeslte lava underlain by
lacustrine rhyolite tuff
Poorly consolidated heterogeneous
yellon-brown conglomerate
Unite to buff arkoalc sandstone
intcrbtdded Kith reddish
broin to green nudstone
Conglomeratic arkose with
partings of nudstone
Brown conglomeratic arkose
and midstcne
Calcareous oonoretlonary gray
shale
White to yellow calcareous
foramlniferal shale
Unite lithographic llnestcne
with shale partings
Dark calcareous shale and alltstone with arkDsie sandstone
at top (Codell mercbor)
Blulah-gray Iith.ogri4>hlo liasstone and ealcareeua shala
Dark carbonaceous shale with
bentonlte partings
Vhlte-to-yellow quarts lancVstcne
Grcy-rhlte conglomeratic sandstone
_ • _--_. - T_
s^f^i
:
? '"5f--'
0 ' •
. '
325 «
35 *
'",'
' ' a •' • .
1 OOOf
llthlc sandstone
Massive oaloaroous quartx
sandstone
Reddlsh-brewn nudstone above
gray and brown conglomeratic
arkosic sandstone, ailtstono,
and shale .
• '. • • ' ' •
' _ 'Jj,'
Unknown Gneisses and schists intruded
by granites
i
Muiigrapliic section of the Wet Mountuins-Huerfano Park area. Asterisks indicate ap(>ro\i-
CRYSTALLINE
ROCK
;.
300 *
2-800
i—v
•*Wa»
vmo.nf
25 •
a •••.
.*" '^~ r~- .'\
DAKOTA
SAtlDSTONE
Lorn
Varies
25*
HUERFANO
FORMATION
EOCENE
CUC KARA
FORMATION
•
« . o ; -' • ^-"p"
- .
TERRACE GRAVEL
PLIOCENE
1
LITHOLOGY TWCKNES
IN FEET
FOR11ATION
AGE
^tLx
1053
1052
R. li. I10YKK —I'l.TROLOGY A N D STRUCTURE, SOUTIIKRN \VKT MTNS., t.nl.
half of the area. Most are siliceous and include
rhyolitc, rhyolite porphyry, trachyte, trachyte
porphyry, hostouite, ami quart/, latile porPty/yThe age of the dikes is uncertain, as no dikes
extend into the sedimentary rocks Iroin the
Precambrian complex. The dikes are not metamorphosed, and generally their strikes are
straight and do not follow gneissoid or schistose
structures, which suggests that they lormed
late in the informational history, probably in
the Tertiary. The presence of a diabase sill,
which cuts the Greenhorn Limestone, and a
mafic dike, which follows a fault in the Smoky
Hill Marl (I'l. 1), support a Tertiary age lor
the dikes in the I'rccunibriun complex. Lovering and Tweto (1953, p. 17), Phair (1952), and
other workers in the neighboring Front Range
report a probable Tertiary age for similar
dikes. An age of approximately 60 in. y. was
established on pitchblende ores associated with
bostonite and quartz monzonite porphyry
dikes in the Central City district (Lovering
and Goddard, 1950, p. 47). However, the work
of Singewald and Brock (1956, p. 583) farther
north in the Wet Mountains supports a Precambrian age. These authors dated an albitic
stock essentially contemporaneous with syenite
dikes at 600 m. y. L. I. Briggs (Written communication, 1958) noted syenite (bostonite)
fragments in the Farisila Formation in Huerfano Park; the conglomeratic Farisita presumably predates Laramide intrusive igneous activity in the area.
A quartz-poor rhyolite slock is exposed in
Maes Canyon over more than halt ol sec. 18,
T. 25 S., K. 68 W. (PI. 1). Foliation in the surrounding Precambrian rocks has been deflected
around the stock and dips steeply away from it
(PI. 4); this indicates a widening of the stock
with depth. Contacts of the stock with adjacent
rocks are sharp, and the bordering rocks arelittle metamorphosed.
A second hypabyssal rhyolite stock, centered
at Badito cone, cuts the Graneros Shale.
Rhyolite porphyry extends south from the
stock; linear flow structures (PI. 1) are subparallel to the erosion surface and indicate
movement of this rock either as an extrusion
or as a laccolith. Santana Butte, partly included on the map in sec. 25, T. 25 S., R. 69
W., is a similar shallow intrusive rock which
cuts the Niobrara Formation. Several bodies
of similar rock in adjacent Huerfano Park are
dated as probably Kocene (Briggs and God-
quartz inclusions. The preponderance ol biouu-s. Minor local brecciation of the lile over chlorite anel the scattered garnet
dard, 1956, p. 43); this is compatible v
o
jtlnhuted
to
later
faulting.
Probably
limited findings in the map area.
porphyroblasts indicate an iiuermedialc grade
»M> originated, at least in part, outside of regional inetamorphism. Transition to higher
Crystal vilric rhyolile Uilf was .le|->.
*i' aica; plugs in the Silver Clifl district grades such as hornblende gneiss and amphibothe eroded Precambrian surface ol tin .._«
Wet Mountains. The once cxicmiu 'J
* ici'iix-nt the volcanic center.
lite rich in plagie>clase and hornblenele is ce>mmains only where protected by Miou.,: u
mon. Despite the lack of relict struciures, ihe
ite lava, which is probably related. I vis)
.\MHR1AN GEOLOGY
schisls probably had a sedimentary origin.
cirque in the south face of South (.:.•:,
Mineral assemblages such as calcile, scapolite,
Peak exposes a section of lull alum: . • r
and eliopsiele; almandite, biotiie, and nuiscovjlje.itiircs. A thick series of melamor- vite; or epielole, manganian zensite, anel tremothick. The lulls apparently thicken v c . «
as several hundred feet are cxpo-ml »i,. ;
[•••"- xilnncntary rocks intimately associated lile-aclinolile all support a metasedimentary
CCC road in sec. 21, T. 24 S., K. 7tl U ;•
|«S |:iiiiiic material constitutes much of the history, as do quartz-rich schisls anel those w i t h
These tuffs are in part lacustrine; | i...j «s5ciu Uel Mountains. These rocks repre- prominent microcline anel biotiie. The occurss A Kitimcntary seemence prol)ably mler- rence of heavy minerals as stubby, wellcross-bedding, coarse-grained zones, i... _•
*^rJ u n l i volcanic rocks and perhaps con- rounded, altered grains suggests transporlalion
nehng melicate that they are in pan ::. ^
•4c: minisivc re>cks. They have undergone prior to inetamorphism. Me>st likely the origiBurbank and Goddarel (1937, p. ')¥> • .:*»ft'*!r to high-graele regional metanior- nal seeliments were primarily pelilcs wilh some
Miocene age for lulls and associate.I 1.1 <
*o iS.^i prexluced amphibolite, hornblende
in northern Iluerfano Park. Tlie\ • .
carbonate rocks anel perhaps lulls.
vt ••>:> liiotue gneiss anel schist, and granite
underlying arkosic anel congloimi., , i
Hornblende gneiss and amphibolite. 1 lornwhich may be as old as Oligocene, aie <,.. «* jn» Hit |'.u.igiieisses and paraschisls are the blenele gneiss anel ampliibolile e)ccur as distinct
*••«
ll.ey
aie
similar
and
possibly
related
te>
free from ve>Icamc debris. The lull *.. &
types, although gradations between the two are
* BJ>A Idaho Springs Formation in the Front common. They show considerable textural
beds are overlain by lava flows, U i i l W
l*f: ilcHiibeel by Lovering anel (Joelelarel variation, with fine-, medium-, and coarseby Pliocene and Pleistocene grav.elScattered erosional remnants o! ai.a,. m y*. c W), Lovering and Tweto (1953, p. grained phases. The degree of foliation is also
• U V u . c l y (1948, Ph.D. thesis, Univ. Mich., highly variable, and compositional banding is
occur over the crest of die \\Vt M.»..:..j
•»», t.l Suik and others (1949, p. 17).
The andesite is underlain by rluoli:.. J
generally found with layers up to 1 inch thick
T\« UKLiniorphic re>cks crop out promi- of hornblende-rich anel plagie>clase-rich segregaplaces; elsewhere il lies directly on i!,. :-.
Precambrian surface. The l.it};<.| :--:-iae- •ttN e»n die southwest flank of the range. tions. Microscopic study revealeel layering
covers approximately 2 square mile- •. t* *»t »!«^iiiljiii aie biotite anil (or) hornblende- within ferromagnesian bands. One specimen of
than 100 feet thick, and fonm (•::-:•» *A »Jj\t anel gneiss anel granite gneiss, wilh hornblende gneiss has bands of hornblenele,
«r«Ttii Jin|i!nli(ilii(.- and lime-silicate gneiss. biotiie, and hornblende-biotite inlerlayered
Peaks.
%r:«l !««hes of granite gneiss, differentiated
Variations in composition, t c M n i e .»•«
with ejuartz-ohgoclase.
pcarance reveal that the lavas repuv.. ..* * fli tuy> ol megascopic compositional variaHornblende, 45-60 per cent of the rock, is
••». «t;e uupptel as separale unils. Foliation characteristically porphyre>blaslic, locally poiflows. All are anelesites anel \\eie i». jAa^r-ul-»
eil
most
ot
the
gneiss
anel
schist
entiated, except for two Hows a! the d..••.»»
kiloblasuc, w i t h included apaiite, oligoclase,
. i :!«r< ol the associated composite rocks. magnetite, anel sphene. Biotite laths anel shreels
Peaks, where the younger andesiu- !u «.»..»
1
•.-..-.I ol lohaiiem of the porphyroblaslic replace die hornblende. Where the bieilite exmore mafic plagioclase, is relalinh o.- ••<•,
":r t l *' N < » ihllers. paralleling the lohalion ceeds 10 per cent the rock is more properly
anel is elark gray rather than pale- K.: u. &
untlerlying hcmatitizcd andesite. 'IIn .«*
r {^..uc |KK!U-S (PI. 4).
termed a hornblenele-biotite schist. Calcic e>ligojinl biotite-hornblende schist. clase (Abyj) generally forms anhedral to subbetween these flows is sharp w i t h i,<>f««
feimt kl.m anel bioiilc-liornblende schist, heelral matrix crystals but may le»rm porphyrosurface. Harley and Salolli (1955. M \ v»,
A»^ (t^iics line) hornblenele gneiss, are blasts. The me>re feilialeel hornblende gneisses
Univ. Mich., p. 40) suggested llut tin •»
% medium -grained to medium hne- contain up to 10 per cent biotiie anel some
anelesile may have been n\iili/ul i)_:rj e
: «ai) itincly inlerfingereel wilh lit-far-lit quartz, whereas the amphibolites have neither.
trusion by auto-oxidation. The ip a \'
™ »aj (x-gnulilic granilic material. Bio- Magnetite anel sphene are much more abundant
andesite is uncertain, although the *.>.IF>«
4*. Ac rrient abuiulanl terromagnesian mmclosely associated with the lull'. Tl< L"»«
ihe amphibolites.
iwft «e:un is light- to dark-brown flakes in The
probably Miocene, but perhaps Pluurr Tf
hornblende gneisses and some amphiboMunii! «a luiuls anel slringers; in some schists lites arc metasedimentary; original sedimentary
tuff and lava may be related, w i t h L:-.ra»
*»oi>rnjcly shreeldeel, stretched, and bent features are obscureel by metamorphic textures.
for erosion between eruptions.
H tduiiun planes. 1 lornblcnele averages A sedimentary parentage is indicated by acSeveral agglomeratic volcanic hirwa*
<b» 10 per cent in ihe biolile-hornblenele cesseiry epiele>le anel lemrmalme; mineraleigical
associated witJi the flows. A pi|>cliUtat»
IKS it not present in the biotiie schist. banding ot bie)lile, hetrnblende, e>lige>clase, anel
sec. 27, T. 24 S., R. 69 W., suj^eMn «t»i
vent. Prominent breccia Hows w h u h H-slni:
W«i i »sj cjifjoclasc (Ah?)) occur as anhedral quartz; rounded grains e>f accessory zircon;
<Mfe nnrT^tiul grams. Local ceMicentraiions of abundance of quartz; and concordant field re(sec. 16, T. 24 S., R. 70 W.) proluU. h»
jhnanehte cetntain abundant
by auto-brecciation during flow a i d « r « t .
1054
R. E. HOYER-1'ETKOI.OGY AND STRUCTURE, SOUTHERN WET MTNS.,
lationships. Most likely the parent rock was
mixed calcareous and argillaceous sediments.
Other amphibolites hud a mafic igneous parentage; they show almost equal amounts of hornblende and oligoclasc with no other essential
minerals or any segregation or banding. Probably these represent mafic volcanic rocks or
possibly sills interlayerecl with the sedimentary
rocks.
Lime-silicate gneisses and related rocl(s. Calcium-rich metumnrpliic rocks include various
lime-silicate rocks, of which most are gneisses.
These rocks occur as small conformable lenses
and thin beds within the metasedimentary and
associated composite rocks and consequently
were not mapped separately. The lime-silicate
gneisses are typically coarse-grained; their
banding ranges from well defined gneissic to
poorly foliate. Calcite, epidote, and quartz occur in all; tremolite-actinolite, diopside, green
hornblende, and andraditc were noted at .several outcrops.
The minerals of the lime-silicate rocks furnish
strong evidence that the parent rocks were
arenaceous and argillaceous limestone and dolomite. For example, one typical assemblage contains andradite, calcite, diopside, epidote,
quartz, and scapohte. The scattered lime-silicate rocks are further proof that most mctamorphic rocks were developed from intercalated shale, limestone, sandstone, and related
sedimentary rocks. Prccambrian regional metamorphism which accompanied folding along
with minor metasomatism caused by the
igneous intrusions formed these rocks.
Granite gneiss. Biotile granite gneiss occurs
associated with bands anil lenses of biotitehornblende schist and gneiss, and in places cut
by aplitic and pegmatitic stringers. The gneiss
is biotite-rich in some places, hornblende-rich
in others. Quartz, microcline, and oligoclase
(Ab 75 ) are essential; in places plagioclase exceeds potassium feldspar, and the rock approaches being a diorite gneiss. A small area of
hornblende granite gneiss is present; this rock
is probably derived from metamorphism of a
sill-like pluton in the sedimentary rock.
Porphyroblastic granite gneiss occurs with a
pronounced foliation produced by abundant
biotitc and stretched quartz grains and feldspar
porphyrohlasts. Prominent pink microcline
porphyroblasts up to 1 cm across make the rock
porphyroblastic. Microcline constitutes as
much as 40 per cent; quartz is slightly less
abundant. Oligoclase (AI)76) and biotite arcpresent in amounts up to 20 per cent.
I'RECAMBRIAN GEOLOGY
1055
by injection. Deep-canyon exposures reveal the
pert »cre favorable to granitization. Con- layering to be consistent in dip as well as strike.
Several features point to an ijjiiun..
)
more
material
permeated
these
zones,
age. Foliations that trend nearly ai n.:
It is difficult to visualize any but very mobile
in place was more complete.
to those of the other metainorphk im.,
material moving within confined boundaries
cate crosscutting relations. Ouiiiop< . •• i
for the distances required to develop the masjr»u.v Rocly
sive, in places blulllike, much like il,.r» j
sive unit, and no evidence was found to support
ietrjlfeatiires.
Bands,
stringers,
and
lenses
granites. Also, although the rink i> »:.
sloping or forced intrusion. Emanations associ4j«fjnu)r|)liic
rocks
intimately
layered
with
ated, neither segregation bandim; 1,1. •
ated with the granites permeated much of the
*. iiulcrial compose a large part of the country rock along fractures and bedding
bcilding was seen. The accessory ini:r.-.i i
tni Wet Mountains. These rocks are reis unlike that of the metasedinicni.m M i
planes of the melasedimentary units. Simuli t» as composite and include varieties
resembles that of the neighlioiiu. ',.ua(
taneously, and probably more important, the
mifjc
from
regularly
banded
lit-par-lit
Zircons and apatites tend to lie nn.jc ;
heated metamorphic rocks were reconstituted
in
highly
contorted
migmatiles.
Tranand subhedral, with less nielainui » j
in place. These combined effects developed the
between these varieties are characterthan those of the metascdimeni.ui ., i
lit-par-lit
gneiss.
«v I.*! all their contacts are gradational.
posite rocks. Finally, normal /.uiin.. .* <t
Migmatite. With more complete "in situ
L-jmul metamorphism accompanied foldclase was noted, a feature coiniiiiu, ;. ustewing" and increase in granitic material the
H d i!^ Prccambrian sedimentary sequence,
gneisses. Quite probably, the |«MJ I,.:.-»,
lit-par-lit gneiss grades into migmatite that conat *.>ntiMicnl granitic intrusions yielded
texture is due to potassium mci.iMUi. ,j.
tains crinkled, wavy, and contorted streaks,
emanations which permeated the
sociated with the later granite LX«!H>
schlieren, and irregular lenses of metasedimenllni added material along with reconOther granite gneiss bodies wliul. •.-.-•m
tary rock (PI. 3, fig. 1). Locally the rock rei nf metamorphic rock in place dea group of rocks with minor lcxiur.il *.*: aw
sembles granite, but relict ferromagnesian
ilicse
"mixed
rocks."
The
composite
alogical differences were mapped *• . nf
bands and gradation into lit-par-lit gneiss testify
;c ihus younger than the paragneisses
unit. These bodies are typiulK :_
to its mixed origin. Textures of the migmatite
medium-grained, 'rarely coarse, an,i ;.•»« mt ;«jjsiliisls but older than the major
range from medium-grained to coarse-grained,
porphyritic. Anhedral and gcnculK i... •at yst^ iniriisions.
and slightly porphyritic varieties are common.
Tit
c.iic
uniformly
banded
lit-par-lit
gneiss
quartz constitutes 30-35 per cent ol \ii K
Quartz and microcline are the most abundant
•
•nil.
c\|K>sed
on
the
dissected
southwest
microcline is the predominant |xiij ...-. »
minerals; oligoclase (Abvt) also occurs. Biotite
spar, although orthoclase al-.ii n,.-, "j &>* i< ilic range. Northward the contortion and lesser hornblende are concentrated in
potassium feldspar makes up 25 ii .. 3 • »**i. inclamorphic bands decrease in con- the incompletely assimilated metasedimentary
Ofcn. jml the rock grades into migmatite. bands. The hornblende developed at the exThe plagioclase is oligoclase, gener^L. ..<r
from Ab?2 to Al);e, but some nmie »«i. «• T«» i--.»nuiiie contains fine stringers and
pense of biotite, which indicates an increase in
clase, Abn4> "'so occurs. Biotitc, w h u l . ; *.j «8&irru •>! inclamorphic rocks incorporated
and pressure.
• ft* Ktumtitulcd material and commonly temperature
averages 10 per cent, occurs as Ihkn .1.---5
The complete gradation between migmatite
in streaks and lenses and yicKK i!.c ,-jr»
wrr^iict granite.
and lit-par-lit gneiss supports their common
It f-*> In gneiss. The lit-par-lit gneiss con- origin. The migmatite was closer to the source
structure.
»ti
iui«l«
nl
lineto
coarse-grained
granitic
The granite gneiss generally luni.s. ;-,•«
of the invading material; therefore the rocks
«»tit.'J iiitciljyered with biotite and biotite- were in a higher-temperature environment with
masses, bands, and tongues, \\lmh i..^-^
with the other metamorphic aa.l i,ca» tiKifti.»k s^lust and gneiss (PI. 2, fig. 1). The more thorough soaking by granitic emanations.
»rt»>i^l U\trs, less than 1 inch to several As a result the over-all viscosity of the rock
rocks so complexly that comnionlv Ki^l to*r 'luiL, (.onimonly pinch and swell along was lower, enabling it to shear and flow fold
ol one are incorporated in anotlia U oir
fcw «:i-.Lc. The granitic bands are composed more readily. As the rock was closer to the upplaces the granite gneiss represent cj^»
t.Mucly ol quartz and microcline with
outcrops of the granitic C(ini|xinci.! a «
ward moving magma, the velocity gradient
* «/.f.KUs<: (Ab 74 ). Samples of different
lit1-far-lit gneiss. The conformily ol lii.»*
of flow was also greater, thus causing the sedi»«jiil f>iaiutic layers from all parts of the mentary bands to be distorted and strung out.
the granite gneiss and associated rukj f.t;
luutiin pUgioclase of similar composition. Because a higher proportion of invading maindicates that the granite formeil ciilr: mS
»i tJicly hornblende are minor conor syntectonically wijth regional nicii!^i-«»terial occurs in the migmatite, the melasedi»ar«i <4 the granitic bands.
that developed the gneissic texliim. Ii ht-w;
mentary rocks of many outcrops were largely
TV It fjr lit gneiss developed from a thick digested and assimilated. There was extensive
suggests a genetic relationship ki»m s»
«< iciinisuuiiccl and metasomatized sedigneisses and the composite rocky '11* i«i«
reconstitulion in place, probably by differential
rwL.1.. The general east strike and
sharp contacts, intimate inleiliup;^ «
uOWMifet Deep northerly dip of the foliation melting.
similar plagioclase composition* j:«l kr»
The writer visualizes the San Isabel Granite
8". « »!CJf»l uniform, perhaps isoclinal, fold- batholith as the source of the magmatic emanamineral suites all support their pencil, as*
•§
*rS<\|x>scd
outcrops
show
graded
bedRegional metamorphism, rcconstiium »J
tions which permeated the composite rocks; a
ta| mni fucllciu cross-bedding of paraschist
migmatizalion associated with ilic S»i S*t
zone of migmatite bounds the south side of the
Mi pKj^anu 7«ncs (PI. 2, fig. 2) that indicate batholith and grades outward into lit-par-lit
batholith intrusion gave rise lo il« p«*
Wrr»rf tie tcijucncc have been overturned.
gneiss, which probably repiexriili rjf
gneiss (PI. 1). Reconnaissance revealed comtwtoraity of the banding prohibits an origin
sedimentary facies whose coni|«r»:« t.
1056
K. E. BOYER_I>UTROUX;Y AND STRUCTURE. SOUTHERN WET MTNS., U)l,
PRECAMHRIAN GEOLOGY
1057
posite rock north of the batholitli, suggesting
lhat zonation persists around the pluton.
ally the granite of Bear Creek is ass,*,.
the composite or metamorphic iui.,
Igneous
where transecting them. The simiLnu
granites of Bear Creek and William, I .
Generalfeattires. The youngest Prccambrian
their gradational contacts and IMIK;,
rocks are igneous bodies, mainly granitic
port their close relation.
which cover approximately half the map area'
Granite oj Williams Cm{. |M,K .;.
I he most extensive of these is here named the
San Isabel Granite batholith. Several smaller coarse porphyritic granite of \V|||U:;.
plutons, namely the granites of Bear Creek, crops out over much of the mmlmc.(--lift Creek, and Williams Creek, and the of the area. It is pink red and clox-h ,,
gneissic granite were also mapped. The granites the granite of Bear Creek. An .
display similar plagioclase composition but medium-grained phase is most u u j .
though several hy-ge outcrops (,i .
ditter in appearance and in heavy-mineral conpegmatitic
facies were mapped (PI. i ,
tent, so that different units were mapped
separately, although they may have a common and preponderant microcline forniL;...'
crysts in the pegmatitic pltisc „[ ,).:
origin.
Gneissic granite. Gneissic granite is exposed Extensively kaolinized and seiun,...'
over several square miles of the southeast end clase, Ab75, does not exceed 10 I K , „
rock; biotite is 5 per cent or less.
ot the southern Wet Mountains. It is generally
San Isabel Granite. The San IsaU; i _,
medium-grained with abundant quartz and
microchne, each exceeding 30 per cent of the which covers 20 square miles of il,, ...
,
rock. Ohgoclase (Ab8s), considerably more part of the mapped area, has two d
sodium-rich than in the other granites, con- (PI- 1). The more common t\|« . ,
stitutes 10-15 per cent. Mafic-mineral content porphyrilic granite which typicJIh .^
is low, and biotite is the main constituent Un- m large bluffs and ridges; less cdiim,,.. ,
like the other granites, the gneissic granite formly medium-grained phase oui.:- to
generally is prominently foliated. In some places mineral contents, microtexlures. j;..: ta
it contains bands of pegmatitic and aplitic mineral suites indicate that the m,, ,rf
material, elsewhere bands of gneiss and schist. are facies of the same granite, as do i|.r. sua
contacts which are gradational out '-,** <m
1 he gneissic granite weathers into sharp
Anhedral and interstitial quari/ ...x.-*
angular-jointed blocks, not into rounded blocks
or smooth-faced boulders as do the other fegran- approximately 25 per cent of ilk- s_: ,„
Granite. Microcline, 25-30 p tl l( .. M
ites.
large pink phenocrysts in the poi,,!,,.-,. M
Granite of CliffCreel^. Two distinct textural
lac.es of the granite of Cliff Creek have been (PI. 3, fig. 2). Approximately 21) pu ..-. „,
mapped, an aplitic variety and a medium- rock is subhedral to dihedral olij>... ^
grained phase. Scanty patches of biotite impart Biotile, averaging 15 per cent, is'i^; ,-„
a foliation, although the rock is markedly low brown to medium brown but nui ,L ,' v»
in manc-mmeral content. Anhedral and inter- as in the other granites and the K , 4 ...
I he high content of accessory nuu
stitial quartz and microcline each constituteticularly apatite and sphnic, i, :
about 30 per cent of the rock; oligoclase
(Ab74) is more abundant than in the other diagnostic feature of San Isabel (,1.,.
Contacts of the San Isabel Claim,
granites. At many places the granite of Cliff
migmatite and tit-par-lit gneiss .:,
Creek is associated with the San Isabel Granite
Contacts are not sharp, and local transitions Regional foliation of fche composiu i:,
suggest that they may be slightly different morphic rocks clearly wraps arimml i!,
pluton (PI. 4) and attests to forcible e
phases irom the same magma.
Granite of Bear Cree^. The granite of Bear Reconnaissance showed that the t
l-reek is a consistently fine-grained rock diac- San Isabel batholith lies to the noi
nostically pinkish-red on fresh break. In places the part mapped is merely a pcrij-u^p.
" is very friable- and generally displays a faintly The smaller granite bodies therefoic cm nr
gne.ssic texture. Quartz and microcline arc the sent border facies or minor dill'crcniu!.***
most abundant constituents. Oligoclase (Ab!6) major intrusive rock. These pluiomutaB
docs not exceed 20 per cent of ,|lc rock, much tudly contemporaneous with die *w ka>
less than in ihe granite of Cliff Creek Gener- Ciranite, although their better I'ulu-a«
gests a slightly greater age. Also, ihcx «*
VERY
RARE
i.-"-j '--"1 (*\ <"> C..'
:"-' O ' ^> 'O <
^ .^iirc i. Sketches of apatite grains from Precaivtbrian granites and associated rocks (X 2-4)
T-- .:. pan represent moblized metamorphic
i«l Sideling the batholith as indicated by
Im t-iucnt heavy-mineral characteristics.
•J««^: Ji irite. A few small bodies of quartz
ttm ii.,p out in the area; the largest are sur-
AZUf>.
.Mlllictous
peglllULllcs
101111
4itcicil outcrops, although commonly
the exposures are confined to float. The pegmatites are controlled by joints, faults, and especially foliation in the composite and metamorphic rocks. All appear vertical or steeply
dipping. Few of the pegmatites are zoned;
l i i a g l i c i i i c a i K i i i c i u a i i L C . viia^iaiu^iiai i i j i i i a i i ^
suggest that these pegmatites represent coarse
> CD
rrp> ^
^2 o w O O
a ^ «» O «Z>
O O •• 9 O
O
G7 CZ? C9
OQ O * O
O CO
«=> O
CO C3
O
^O
Sketches of zircon grains from Precambrian granites ami associated rocks (X
R. K. UOYER-I-ETROLOC3Y AND STRUCTURE. SOUTIIKRN WET MTNS.. CC
I'KUCAMUKIAN GKOl.OtiY
crystalline zones of the migmatite. The pegmatites are probably related to the granitic intrusions, thus are the youngest Precambrian
rocks in the southern Wet Mountains.
bordering the batholith. The total he.,,
nonmagnetic heavy-mineral fractiun, ..•
only slightly more than 2 per cent ,.,:
cent of the total sample respective!; 11 .:e .a
Heavy-Mineral Studies
Both apatite and zircon are cliara'iit:
Nine samples of the granites and one each altered; metamict alteration is iu:
prominent on the zircon and was j|.
ot the migmatite, porphyroblastic granite
gneiss and granite gneiss were examined for on the apatite (Figs. 3, 4). Both ..„•,
their heavy-mineral content. About 60 g of noticeably rounded, and subhedrjl ,;•.
each sample was crushed to pass through a 60 rare. The surface of the apatite is „„.
mesh (0.250 mm) sieve. Each sample was then pitted. Sphene is considerably lc« „;..
divided and a fraction weighed to 50 g Fach than apatite and zircon. Some s|»ln.^
fraction was then washed, decanted, dried and are rounded and extensively alien,'
the magnetic fraction separated by a bar maE- coxene. The biotite, 0 index apr,,.u_
net and weighed. Heavy medium separations 1.635, is medium w dark brown ,,i :.,
were then run by centrifuge on the nonmag- brown and generally does not C U u j . ,
netic fraction in tetrabromoethane, specific cent of the nonmagnetic heau ,.„. „
gravity 2.92 ± 0.01. The heavy and light Chlorite after biotite is common; him. .
portions obtained were dried, weighed and leucoxene are prominent alicratmi.. ., t
magnetic heavy minerals.
mounted on glass slides.
The heavy minerals of the j, rjl .. „
As only a few samples were used, conclusions
somewhat
resemble those of iht r ' ,
must be cautious, although most features support field interpretations. Table 1 summarizes (Figs. 3, 4; Table 1) except ilu, '.'~',
absent. The migmatite contains al)ui..L M
the heavy-mineral data. Apatite, biotite
sphene, and zircon were particularly useful- biotite and about equal amonim „• .,
figures 3 and 4 illustrate the findings on apatite sphene, and zircon (Table 1). In du•,.. ^
both apatite and zircon are roundul J.
and zircon respectively.
The heavy minerals of the San Isabel Granite monly have metamict alteration (!:., ,
emphasized several features noted in thin sec- Although not abundant, growth |,v..j>
occur on the zircons in the form o! „_•. ,
tions. The heavy-mineral content exceeds 12
per cent of the total sample, and the nonmag- and aggregate crystals which lUKi.l
Eckelmann (1955, p. 947) cj|C. .„ n i l
netic heavy minerals average about 8 per cent
autochthonous granite.
Apatite generally forms fresh, unaltered grains
Both the total and nonnujjnu.. >,,•
ot which approximately 20 per cent are elongate and subhedral (Fig. 3). The zircon is clear mineral content are exceptional!) }..,: ,
and unaltered and mainly in slender subhedral porphyroblastic granite gneiss (Tj!-i.
sharp contrast is the very low u^
grains (Fig. 4), commonly with tiny needlelike
inclusions. Sphene, which either exceeds or is magnetic heavy minerals. Zircon k.,7,,
as abundant as both apatite and zircon is subhedral with less alteration (ft, . <,
yellow to amber or smoky brown; the grains tinctly subhedral needlelike grain? ^,
are irregular and only slightly altered to about 30 per cent of the apatin- wK- ,.
little altered (Fig. 3). The signilkm, >_:*,*
leucoxene. Biotitc, ft index approximately 1 62
is commonly pale brown or green; it is the ism of some apatite and zircon iii^: .^ t
most abundant accessory and exceeds 50 per porphyroblastic granite gneiss ij'(..-;,u«»
mcta-igneous rock. The biotite is jut u *
"i",1 ."' / he nonmagnetic heavy minerals
brown. Fluorile, suggestive of i s , K ,o r,
Chlorite (after biotite) is minor. The magnetic and
epidote are minor.
heavy fraction shows very little leucoxene or
Radioactive age dating indicate, -.u »
hematite alteration.
San Isabel Granite was emplaccd kt* i
f rTrt-^T"?!""31 features of 'he granites
of Chff Creek, Bear Creek, and William Creek neighboring Pikes Peak Granite. /j;.« M
the San Isabel Granite yielded A kj,-«
differ from those of the San Isabel Granite
probably because of contamination of the age of 1430 + 200 m. y (T \V Sicrt !•»
smaller pluions by assimilated metasedimentarv No. IWA-70, Geochemistry an,l (-•«
material; this further supports the idea that Branch, U. S. Geological Suru-vj Tn *
they are in part mobilized metamorphic rock agrees with a period of activity in the U*
City-Gunnison area suggested In (,A
1059
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GRANITE GNEISS
MIGMATITE
PORFHYROBLAST1C
GRANITE GNEISS
GRANITE OF
WILLIAMS CREEK
GRANITE OF
B E A R CREEK
GRANITE OF
GRANITE
£
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CLIFF CREEK
1058
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1060
I'RECAMHRIAN GEOLOGY
K. K. liOYKK—1'KTROLOGY AND STRUCTURE, SOUTHERN WET MTNS., COI.O.
Kulp (1960, p. 220), based on A40/K40 age
.iicJMiremcnts. They conclude that (p. 222)
. . . a major orogenic event may have occurred
iUmt 1500 m. y. ago in the area of Colorado.
." The San Isabel Granite batholith probably
.i|>rcsents part of ibis activity.
>TRUCTURF,
(v'.-j/om// Structure
CANON
• CANON
CITY
cirr
\ EMBAYMENT
IO Miles
Figure 5. Generalized tectonic map ot south central Colorado
UTE»»TU«!
1061
The Wet Mountains structural elements
trend dominantly northwest, essentially parallel
to the elongation of the range. In the map area,
fault, dike, and joint sets strike generally N.
60 W. (PI. 1) as they do farther north (Singewald ami Brock, 1956, p. 584). The northweststriking fault trend of the Front Range parallels
that of the Wet Mountains; Lovering and Goddard (1938, p. 46) a t t r i b u t e these faults to
strong lateral compression in an east or castnortheast direction. Briggs and Goddard (1956,
PI. 1) also found northwest-striking t h r u s t
faults dipping southwest in Huerfano Park
which have moved eastward. In the northern
Wet Mountains, Christman and others (1959,
PI. 15) found a dominant northeast strike and
sleep north dip of the foliation in the metamorphic rocks. The San Isabel Granite batholith
lies southeast of this area, so the trend of foliation is like that around the batholith in the
southern Wet Mountains.
I'lie Wet Mountains form tbe central u n i t
-I jn en echelon pattern with the Sangre de
i iMo Range to the west and the Front Range
:o [lit north (Fig. 5). A close genetic relation of
lie Wet Mountains and the Front Range has
• nig been recognized (Burbank and Lovering,
r<U, p. 28-1; Lovering and Goddard, 1950, p.
;
"). Burbank and Levering (1933, p. 28-1-285)
.uu-d t h a t the Wet Mountains are part of the
Neozoic Colorado Front Range. They believe
:lui they ". . . may be considered as a strip of
.k uplifted foreland of tbe Sangre de Cristo
Kjnge which failed under compression not Local Structural Features
-Hinplelely relieved by the thrusting of the
Foliation and lineation. The Precambrian
\iiij;re de Cristo belt."
metamorphic and composite rocks have promiThe southern Wet Mountains form a south- nent foliation and lineation. Tbe foliation is
.-j>l plunging anticline outlined by the up- primarily formed by alignment of biotite, but
turned sedimentary beds that bound the range. also by quartz and feldspar in the igneous bands
Vliing its southwestern flank the range is of the lit-par-lit gneiss. Foliation represents the
UHindcd by high-angle normal and reverse modified original stratification, subsequently
Liulis which separate it from the Wet Mountain loldeil and steeply tilled to the north. RcconVjlley and Huerfano Park (Fig. 5). Most cx- slilution and metasomatism which formed the
•cnsivc of these is the Wet Mountains fault composite rocks and the granite gneiss bodies
• huh Johnson (1959, p. I l l ) maps as nearly concordant with the bedding occurred in the
.rtucal along the entire southwestern flank late stages or just after this folding.
..I lilt range. Tbe northeastern border is characFoliation in the older Precambrian units
•cri/ed by thrusting which Burbank and God- forms a distinct arc around the San Isabel
tinl (1937, p. 937) have described as "moder- balholilh to the north (PI. 4). Deflection
.:c overturning or overthrnsting . . . toward caused by granite intrusion is particularly
.he cast" but which Gabelman (1953, p. 18-4) noticeable near sec. 19, T. 24 S., R. 69 W.,
.mrprcts as a west-dipping underthrust.
where composite rocks contact the granite of
\i its southeast end the Wet Mountain
Bear Creek, and in sec. 10, T. 25 S., R. 68 W.,
>:iucuiral unit bifurcates as it plunges steeply where they diverge around the San Isabel
iciicjtli the Great Plains (Fig. 5). The southern Granite. A contour diagram of the foliation
yur of the split, the Greenhorn anticline, plots of the older units illustrates the concen;Junges southward into Raton basin, whereas tration ol readings (PI. 4). Two maxima were
:hc main branch plunges southeastward and obtained with most readings in an arc. The
lutim the Apishapa uplift (McGinnis, 1956, lo!iation strikes easterly and dips uniformly
northward; dips range between 45° and vertical
f 12). Osborne and others (1938, p. 88) couriered this southeast-plunging arch to suggest and average approximately 7H°. The foliation
i ujincwhat elevated basement connection be- altitudes are lairly constant with depth as seen
:*ccn the ancestral Wet Mountains and the in deep canyons.
•njthcastern Colorado late Paleozoic highlands
Lineation, on the loliate planes of the older
*hi<;li may have acted as a buttress to resist
Precambrian units, is commonly shown by
i*c thrusts of the Sangre de Cristo and Wet stretching of biotite, quartz, and microcline.
Mountain ranges.
Slight crcmilalion of minerals, particularly
1062
K. K. HOYliK —I'K'I'KOLOCJY AND STKUCTUKK, SUU'I'llKltN WKT MTNS., COLO.
biotite, accentuates the lincatiuu. Aligned
elongate minerals, especially microcline and
ampliibole, also contribute. The lineations of
the older units and the granites trend ilomi*
nately northward (Fig. 6). All were plotted on
a single contour diagram; the maximum concentration has a N. 30° E. bearing and plunges
60° NE.
Lineation within the older Precambrian units
developed essentially contemporaneously with
the foliation of these units. The lineation, in
the foliation and oriented essentially down-dip,
apparently developed during the folding. Slippage during folding and neocrystallization
along bedding planes probably developed the
lineation. Minor movements and slight readjustments after the main intrusions, but while
the rock was still somewhat plastic, produced
some crinkles and crenulations and local deflections of the lineation.
All the Precambrian granite intrusive rocks
display foliation and lineation. The foliation
is a planar flow structure developed mainly by
parallelism of biotite, but also by alignment of
elongate microcline crystals. Plate 4 illustrates
the trend of the primary foliation within the
granite plutons and the pronounced discordance
to that of the older Precambrian units. The
contour diagram of the foliations (planar flow
structure) of the granites on Plate 4 exemplifies
this discordance. The concentration of these
foliations differs markedly from that of the
older units. The average foliation for the granites strikes N. 40° E. and dips 45° NW. In the
western part of the area the strike is north to
northeast but swings more to the east in the
San Isabel Granite. That the smaller plutons
may be satellites of the San Isabel Granite is
supported by this foliation trend. The predominantly north dip indicates that the major
plulon is centered north of the area as supported by reconnaissance studies.
Lineation in the granites is commonly
obscure. It is a primary linear flow structure
caused by orientation of prismatic minerals.
Elongate feldspar, mainly microcline, is the
chief factor, although hornblende, where pres-
ent, is well oriented. The lineation diagram i<
Figure 6 includes readings from both tlic iM;
units and the granites. This diagram indiuu
a generally uniform north plunge of the jjiam;;
lineations, with local deflections, parlicuLi]
near the faults that bound the batholith on iLeast side. The nor/h plunge of lineations wiiL..
the smaller plutons supports a common nup:..
chamber for all the granites.
Vaults. The fault zones, some confined u
the Precambrian core and others extending n>;.
the sedimentary rocks (PI. 1), are clurjiui
ized by breccia, abundant fracturing, htiiuii;:
mineralization, and slickensides. Several pern.:
of faulting have been recognized, bul nur:
faults have had Laramide or younger muu
merit. Most fault zones are impregnated will
finely disseminated hematite and thus M
similar to the "breccia reefs" of the FIUI.I
Range (Lovering and Goddard, 1938, p. 3v..
Epidote is locally abundant in the fault /uno
The epidote may be of Precambrian age, ai«:
shear zones containing it are probably Prccaii,
brian like the epidote-bearing faults in iL
Front Range, whereas the Laramide faults u>..
tain hematite (E. N. Goddard, personal u.r..
munication, 1957). Occurrences of both epklu::
and hematite along some faults probably IIUIL
Precambrian shear zones reactivated in ui,
inide or later time.
The younger faults are commonly marked U
breaks in topographic slopes, particularly An.'r
the southwest flank of the range where sexcu!
parallel faults form structural terraces wilt
distinct scarps. Most faults are on the ranjr
flanks, but a few occur along the crest nhcic
they commonly form topographic depression
The faults have two distinct trends (PI. I t
(I) that of the dominant set averages N. Wr
\V. and ranges to E.-W.; anil (2) the subordi
nate set which ranges in strike from N. IU°\V
lo N. 30° E. Set 1 parallels the elongation c.;
the range and includes those f a u l t s along iln
Precambrian-sedimentary rock contact on butt,
the northeast and southwest Hanks. Mjm
dikes, particularly on the northeast lljnl,
parallel this fault trend. A second grou)
J'LATK 2. /,/'/'I'AR /./'/' GNEISS, T U R K E Y CREEK CANYON,
SOUTHERN WET MOUNTAINS, COLORADO
Eigure 1. Parallel layering of lit par lit gneiss which crops out in Turkey Creek canyon, see. 15, E JS
S., R. 69 W. Variations in widths of granite layers are pronounced, with some pinching and sutllu^
along strike.
Eigurc 2. Relict cross-bedding in metasedimentary zones within the lit-far-hl gneiss at above Iwjliu
Eoliations dip steeply toward bottom of picture, indicating that rock bns been overturned.
Figure 2
COLORADO SCHOOL OF MINES
BOYER, PLATE 2
Geological Society of America Bulletin, volume 73
GOLDEN COLORADO
faULUHH,
^
STRUCTURE
lly branch faults of the main set, strike
iximately N. 30° W. Foliation is a promicontrol for the northwest trend of faultFault-strike changes closely correlate with
ition trends as seen by comparison of the
It patterns on Plate 1 with the foliation patis on Plate 4. Furthermore, changes in
lion control the branch faults to this
inant trend. Set 2 is confined to the east
of the range, where faults that parallel the
icnhorn anticline abruptly cut off the Prejnbrian complex on the east. Here again the
.lucnce of the foliation can be detected in the
of faulting. Sets 1 and 2 intersect beof the split in the range core at its soulhI end. Some faults of Set 1 continue easturd into the sedimentary rocks, but many end
{.iinst Set 2.
Most faults dip 75° or steeper. On the north,t flank the N. 60° W. faults dip northeast,
a die southwest slope to the southwest. The
Drill-striking faults dip eastward. Movement
generally dip-slip normal. As the dips are
;ccp, slight variations in dip may change a
»rmal fault to a reverse fault in a few miles
uong strike. At depth these faults are thought
a icinam high-angle faults, dipping essentially
itrtical (PI. 1) as would be consistent with
«p\vard movement of the crystalline core by
itrtical uplift.
The sedimentary rocks, especially the return Dakota Sandstone, are characteristically
dragged along the fault contacts into nearly
icrtical altitudes. Faults bounding the Preuraluian complex have displaced the sedimenury rock of Huerfano Park at least 1500 feel
'Biiggs and Goddard, 1956, p. 44); those on
ihc range cresl, although difficult to evaluate,
(rncrally have a throw of a few hundred feet
w less. The faults forming the structural terCiccs on the southwest flank of the range have
^placements of as much as 1000 feet.
Faulting occurred in the Precambrian, late
Pilco/oic, Laramide through Tertiary, and
fovsibly Quaternary. Most Precambrian faults
ire of the N. 60° W.-trending set, developed
1063
late in the emplacement of the San Isabel
batholith. Disturbance began with folding of
the metasedimentary sequence, followed by
the granite intrusion. During intrusion and
migmatization the plasticity permitted deformation without rupture; as the complex cooled,
fracturing and jointing occurred. None of these
faults are followed by dikes, veins, or pegmatites, so the faulting postdated the main intrusion. Many Precambrian faults were rejuvenated during later disturbances; other
faults also developed from the new stress orientations. The role of late Paleozoic deformation
(Ancestral Rockies) is particularly difficult to
evaluate and is believed to have been confined
to regional upwarps and movements along
older faults.
Laramide and later disturbances resulted in
a series of movements, the youngest in Pliocene
or Pleistocene time. Although much of this
movement was along pre-existing fault zones
and their extensions, new fractures also formed
under regional compression. Movement of the
Wet Mountain massif, however, was primarily
vertical. The steep dip of the faults and the
high-angle drag of sedimentary rocks along the
faults attest lo the near-vertical boundaries of
the Precambrian block. The Wet Mountain
massif was resistant to regional compression
and moved upward en bloc between the northwest-trending faults. Regional compression is
indicated by the intense folding and thrust
faulting prominently displayed in the Tertiary
units of 1 luerfano Park. 1 lowever, uplift of the
Wet Mountain Block may have been the result
of solely vertical stresses as has been suggested
for the Front Range by ). C. Harms (1961,
paper presented at Am. Assoc. Petroleum
Geologists Annual Meeting, Denver, Colo.).
Joints. Joints abound in the crystalline
rocks. Four dominant and two lesser joint sets
are recognized. Set 1 is nearly horizontal; the
rest dip 65° to vertical and have strikes as
follows: (2) N. 45°-60° W.; (3) N. 45°-50° E.;
(4) nearly E.-W.; (5) N. 10° E.; (6) N. 10°20° W.
1'1-ATK 3. MIGMAT1TK AND SAN 1SAHK1. C.RAN1TK
FROM SOUTUKKN Wl-.T MOUNTAINS, COLORADO
MIC3MATITE
BOYER, PLATE 3
Geological Society of America Hullelin, volume 73
furc I. Migmaiitc showing waviness and contortion ol mafic stringers. Common schlicren ol iucumptctoly segregated mafic-rich and malic-deficient zones indicate partial digestion and assimilation of
mcusedimemary rock by intruding material. Outcrop in sec. 22, T. 25 S., R. 69 \V.
t ijuie 2. Microcline phenocrysts in a matrix rich in quartz and bioiite give San Isabel Granite a porisounced porphyritic texture. Indistinct foliation present at ibis outcrop on north bank of Lake Isabel
,Ui. sec. 1,T. 2-1 S., R. 6'J W.
1064
R. K. HOYIiR-l'iiTROUKIY AND STRUCTURE, SOUTIIK KN WET MTNS., com
'''''•'•''•'•'•'•.vX
•'•'•'•'•'?••'.•,•,•.*/
"
SI'KUCTLIKl.
'\'.'v
/""'•'•'•
t;:y:':V^:^r, . / , , t.
'. V • • : • • •••;.'-'-<••''• . . ' . V - ' . - l
<>y ••••'•<'. ^ T
/
^ *'
s
63
EXPLANATION
^
'- .V
x»f •'
19 70/70 i°°
'tftl^>t /
IV. .
'iffgfcf-f
Tertiary rhyolite slock
' ' X ' -XN
,\ '
\
'
IJ.,',
rf\
Post-Precambrian
sedimenlary rocks
<
Younger granites and ;3
associated rocks
1 5
<
:.-!>
Metamorphic and
composite rocks
Contact
ri'i ft^i i
1-1
[.liiU I ___
Bearing and plunge
of lineation
CONTOUR D I A G R A M Of [ INE4I
IN P R E C A M B R I A N ROCKS
l?17 UNiAllONS PLOTTED C.
IOWER HfMISPhlffiE)
Figure 6.
I. intuition in the I'rccambrij.: :
;u
southern Wet Mounlains, Colorado
H)o5
1066
GEOLOGIC HISTORY
R. li. UOYKK—PETROLOGY AND STKUCTUKK, SOUTHERN WET MTNS., COM).
The San Isabel Granite consistently displays
joint sets 1, 2, and 3. In and near sec. 4, T. 24
S., R. 69 W., the granite has weathered into
prominent ridges which trend N. 45° W. and
are cut by joints striking N. 45° E.; both sets
dip nearly vertically. The metamorplric and
composite rocks are prominently jointed parallel to the foliation. The trend of this set, westnorthwest in the west, changes gradually to
southwest near the east end of the range (PI.
4). Abundant joints trend northwest, nearly
parallel to the dominant fault and dike trend.
Joint sets 3 and 5 are also well represented in
these rocks. The composite rocks also prominently display the nearly horizontal set (Set 1),
dipping up to 15° W. and striking northeast.
A few exceptionally good outcrops of lit-par-lit
gneiss show an older joint set filled by granitic
material like that making the igneous bands of
the gneiss. This jointing probably developed in
the original sedimentary rock.
The general lack of joint mineralization suggests that most joints formed alter the granitic
intrusions. Jointing within the San Isabel and
associated granites lormed during their cooling.
The N. 45°-60° W. set, which parallels the
platy flow structure ol the granite, is most
prominent. These appear to be primary (the
longitudinal joints of Balk, 1937, p. 34) and
developed during cooling and contraction of
the granite. The joints striking N. 45°-5()° E.,
called cross joints (Balk, 1937, p. 27), are also
primary, formed essentially by lengthening of
the rock in the direction of flow-age during its
final consolidation.
The Hat-lying fractures, also primary, do not
appear to embrace any How lines and apparently
developed by differential expansion during
erosion of overburden. Unloading released
vertical stress, but not horizontal stress, thereby
tunning the joints. Exfoliation has probably
not been important. Except for the joint set
(illed with igneous material, all jointing oi the
composite and metamorphic rocks postdates
intrusion. The set paralleling the loliation
represents pre-existing bedding along which
fracturing took place during either the orogeny
of the Ancestral Rockies, the Laramide, or
both. The nearly horizontal set developed, like
the Hat-lying joints within the granites, by
differential stress developed during unloading.
The northeast- and northwest-striking joints,
which intersect at 30°-80° in the northern
quadrant, may he complementary shear sets
whose angles of intersection change in different
rocks. These joints probably developed at the-
time of uplift, perhaps during formation <>l il*
Ancestral Rockies, but more likely during iL
Laramide and younger uplifts of the ranjjr. \:
the time the massif was uplifted, much num..
displacement occurred along the abumLii.:
joints, enabling considerable aggregate \\\\.\\
in areas with no prominent faults.
GEOLOGIC HISTORY
Precambrian History
The oldest Precambrian units, the incuscJi
mentary rocks, are similar to the wiilcspu.,,:
Idaho Springs Formation of neighboring aiuand were probably deposited in early 1'rtcji:,
brian time. They represent many ililli-ru.:
sedimentary sequences, very likely intcrcalju,:
with flows and possibly concordant intrunu
masses, whose present mineralogical siniilaritio
reflect a common chemistry and metamorplu.
history.
Folding of the sedimentary rock was ilu
first step in a complex Precambrian liisiun
The regional structure is obscure, but iL
general east-west strike and north dip of il.i
loliation indicates that the map area is on il.c
flank ot a large fold, perhaps an anticline
Probably during or just after the folding, il.t
San Isabel Granite batholith and the assoiiaiul
smaller plutons were intruded. Magmatit c-nij
nations permeated the metasedimentary niin>
along fractures and bedding planes, and lU
heated rock was reconstituted in place to luiiu
the composite rocks; migmatite devdo|<,l
closest to the intrusion, lit-par-lit gneiss lanlicr
away. As the complex cooled, plastic lluuj^c
stopped, and the rock was jointed and broken
by faults; faulting was accompanied by cpiduu
zation along the fracture planes.
Following the igneous activity the \Vn
Mountains, as a highland, were eroded during
a long period. Osborne (1956, p. 62) bclii-us
the erosion was to base level or near ii. Tin
Precambrian rock exposed in the Wet Moun
tains characterizes the transitional mcso/onc
catazone environment of Buddington (l°5'>,
p. 708) and therefore testifies to exltnsiu
erosion.
Paleozoic- A lesozuic History
During the earlier part of the Paleozoic lln
Wet Mountains were a positive clement on tinColorado sag of the Transcontinental anh
Lovering and Goddard (1950, p. 57) stau.l
that during Paleozoic and Cretaceous time
"the Front Range highland and Wet Mnun
iius highland may be regarded as a compound
witive unit trending north-northwest." Boriring the Wet Mountain highland to the
•ot was the Central Colorado basin which
.lit in its southeastern part, bounding the
.jliland.
In late Mississippian or early Pennsylvania!!
jie the Wet Mountains were uplifted as a part
.1 the Ancestral Rockies, Holmes (1952)
^ualizcd several thousand feet of vertical
movement. Major uplift is dated as near the
::,il of Morrow time by Maher (1953, p. 2489).
.bwndrop of the adjacent basin area accom;aiiied the uplift and rapid deposition that
jok place. Accumulation of sediments was
;;tatest in Pennsylvanian and Permian time,
Jihough erosion of the range continued during
;iost of the Mesozoic. The coarse conglomerate
j the Pennsylvanian-Permian Sangre de
i'risto Formation testifies to rapid deposition.
In Red Canyon, the Sangre de Cristo Forma:ion rests unconformably on the basement
inks; it pinches out on the mountain Hank,
:iuckcns rapidly to the west, and apparently
.narks the edge of the zeugeosyiicline (Brill,
l')52, p. 831) bordering the range.
The neighboring lowlands have no Triassic
jiul only relatively thin Jurassic rocks. 1 lowtier, moderate erosion of the Wet Mountains
i> visualized until just before the late Cretaicous. The seas then advanced slowly, and
marine deposition continued until near the
end of the Cretaceous period and the beginning
»f the Laramide orogeny.
j
1067
faults bordering the Wet Mountains. Overlap
of Farisita on upturned Huerfano and upper
Cretaceous beds near the southeast flank of the
range is further evidence of the late age of the
uplift.
Detailed heavy-mineral studies indicate that
the Eocene formations of Huerfano Park are
apparently not primarily derived from the
Wet Mountains (L. I. Briggs, personal communication, 1957). In recent investigations of
the Farisita, Briggs found rounded pebbles and
cobbles of Precambrian rocks correlative with
types in the southern Wet Mountains. This
indicates that the Precambrian core was exposed, at least in part, during deposition of
these Farisita beds. Therefore, a pre-Farisila
(late early Eocene) uplifting of the Wet Mountain block and stripping of its sedimentary
cover is indicated.
History
In the late Cretaceous, probably in Pierre
nine, regional unrest began as the first stage
ul the Laramide orogeny. Burbank and Godilard (1937, p. 972) believe that uplift of the
southern \Vel Mountain massif occurred at the
same time as that of the Sierra Blanca and the
('ulebra massifs. An erosional unconformity
Ixriwcen the upper Cretaceous Pierre Shale
anil the Paleoccne Poison Canyon Formation
in Hueifano Park gives evidence ol the unrest
that began as thrusting in nearby basins and
culminated in uplift of the range. The Laramide
orogeny probably attained its peak in late or
post-Bridger time, as formations as young as
the Eocene Farisita have been displaced by
Post-Laramide History
The extensive erosion surface, probably developed in mid-Tertiary time, is tentatively
correlated with the Flattop peneplain ol the
From Range. Assuming the Wet Mountains
were essentially at the same level as the top ol
the Iluerlano Formation at the end ol the
Eocene epoch, erosion preceded extrusion ol
the Miocene volcanic rocks, patches of which
occur on the range crest. Development of this
erosion surface could have started in Eocene
time, the Huerfano beds acting as a protective
covering. Post-Huerfano erosion occurred, and
the Miocene tuffs were deposited on the erosion
surface. Overlying the lulls are the volcanic
rocks of Miocene or possibly Pliocene age.
Most probably the major uplift of the Wet
Mountains took place after the volcanism, thus
accounting for the volcanic rocks on the lofty
Greenhorn Peaks. A major northwest-1 rending
fault, apparently an extension of a break-in
slope fault along the southwest Hank, cuts the
volcanic rocks in sec. 16, T. 24 S., R. 70 W.
This late Tertiary faulting is like that in neighboring areas; Burford (1960, Ph.D. thesis,
Univ. Michigan, p. 182) noted Pliocene or
younger faults in the Sangre de Cristo Range.
Following the uplift came renewed erosion accented by Pleistocene mountain glaciation and
continuing to the present.
1068
K. E. UOYER-l'ETROLOGY AND STRUCTURE, SOUTHERN WET MTNS., COLO.
REFERENCES CITED
Balk, Robert, 1937, Structural behavior of igneous rocks: Geol. Soc. America Mem. 5, 177 p.
Briggs, L. I., and Goddard, E. N., 1956, Geology of Huerfano Park, Colorado, p. -10-45 in Guide l».,i
the geology of the Raton Basin, Colorado: Denver, Rocky Mm. Assoc. Geologists, 148 p.
Brill, K. G., Jr., 1952, Stratigraphy in the Permo-Pennsylvanian zeugogeosyncline of Colorado ami 1,1,.::
ern New Mexico: Geol. Soc. America Bull., v. 63, p. 809-880
Buddington, A. F., 1959, Granite emplacement with special reference to North America: Cu-ul v.
America Bull., v. 70, p. 671-748
Burbank, W. S., and Goddard, E. N., 1937, Thrusting in Huerfano I'ark, Colorado anil related \iiuV.-...,
of orogeny in the Sangre de Cristo Range: Geol. Soc. America Bull., v. 48, p. 931-976
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ore deposition of Colorado and southern Wyoming, p. 272-316 /// Ore deposits of the Western .Vy
(Undgren volume): Am. Inst. Mining Metall. F.ng., 797 p.
Christman, R. A., Brock, M. R., Pearson, R. C., and Singewald, Q. D., 1959, Geology and dim.
deposits
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Christman, R. A., Heyman, A. M., Dellwig, L. F., and Gott, G. B.,J953, Thorium investigations!'"
52, Wet Mountains, Colorado: U. S. Geol. Survey Circ. 290, 40 p.
Cross, Whitman, 1896, Geology of Silver Cliff and the Rosita Hills, Colorado: U. S. Geol. Sunn I " . :
Ann. Kept., pt. 2, p. 263-403
Endlich, F. M., 1874, Report upon the geology of the San I.uis district, p. 305-361 in Haydcn, I V
U. S. Geol. and Gcog. Survey Terr. 7th Ann. Kept, for the year 1873, 718 p.
Gabelman,
J. W., 1953, Definition of ;i mineral belt in south-central Colorado: Kcon. Geology, \. <•
p. 177-210
Giffin, C. E., and Kulp, J. L., 1960, Potassium-argon ages in the Precambrian basement ol Colouii
Geol. Soc. America Bull., v. 71, p. 219-222
Gilbert, G. K., 1897, Description of the Pueblo quadrangle: U. S. Geol. Survey Geol. Atlas, Folio 3(j,' j
Hills, R. C., 1900, Description of the Walsenburg quadrangle: U. S. Geol. Survey Geol. Atlas, Folio Ml,:,
Holmes, C. N., 1952, Tectonic and stratigraphic history of the Ancestral Rockies of Colorado (Atntuii
Geol. Soc. America Bull., v. 63, p. 1263
Johnson, R. B., 1959, Geology of the Huerfano Park area, Huerfano and Custcr counties, Colorado: I >
Geol. Survey Bull. 107! D, p. 87-119
Lee, W. T., 1922, Peneplains of the From Range and Rocky Mountain National Park, Colorado: I'. >
Geol. Survey Bull. 730-A, p. 1-17
Little, H. P., 1925, Erosional cycles in the Front Range ol Colorado and their correlation: Gecil. .V
America Bull., v. 36, p. 495-512
Lovering, T. S., and Goddard, E. N., 1938, Laramide igneous sequence and differentiation in the I'rui,:
Range, Colorado: Geol. Soc. America Bull., v. 49, p. 35-68
1950,
319
p. Geology and ore deposits of the Front Range, Colorado: U. S. Geol. Survey Prol. Paper _'-'•
Lovering, T. S., and Tweto, Ogden, 1953, Geology and ore deposits ot t h e Boulder County (im^ii;.
district, Colorado: U. S. Geol. Survey Prof. Paper 245, 199 p.
Maher,
J. C.,
1953, Paleozoic history of southeastern Colorado: Am. Assoc. Petroleum Geologists Bull
v. 37,
p. 2475-2489
McGinnis, C. J., 1956, A brief description of the physiography of the Raton Basin, Colorado, p. 10 -13;-,
Guide
148 p. book to the geology of the Raton Basin, Colorado: Denver, Rocky Mtn. Assoc. GeoIogiMOsbornc.H. W., 1956, Wet Mountains and Apishapa uplift, p. 58-64 in Guide book to the geology ol ilu
Raton Basin, Colorado: Denver, Rocky Mtn. Assoc. Geologists, 148 p.
Osborne, H. W., and others, 1938, Along the Front Range of the Rocky Mountains, Colorado: Kim*
Geol. Soc. Guidebook I 2 t h Ann. Field Conf., 110 p.
Phair, George, 1952, Radioactive Tertiary porphyries in the Central City district, Colorado, .nul i l u u
bearing upon pitchblende deposition: U. S. Geol. Survey T. K. 1. 247, U. S. Atomic Energy Coiim,
Tech. Inf. Service, Oak Ridge, Tenn., 53 p.
Poldervaart, A., and Eckelmann, F. D., 1955, Growth phenomena in zircon ol autochthonous giamto
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1069
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RECEIVED BY THE SECRETARY ol' THE SOCIETY, MARCH 1, 190!