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Tertiary and later igneous rocks of the San Juan Basin
Eugene Callaghan, 1951, pp. 119-123
in:
San Juan Basin (New Mexico and Arizona), Smith, C. T.; Silver, C.; [eds.], New Mexico Geological Society 2nd
Annual Fall Field Conference Guidebook, 163 p.
This is one of many related papers that were included in the 1951 NMGS Fall Field Conference Guidebook.
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NEWMEXICOGEOLOGICAL
SOCIETY* SECONDFIELDCONFERENCE
*. SAN JUANBASIN
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TERTIARY
AND LATERIGNEOusROCKSOF THE
SAN JUANBASIN
As summarized
by Crossand Larsen..(1935
, pp.
50-54)the volcanic
sequence
of the San JuanMountains
..
beganwithLakeForkandesite
of probably
earlyMiocene
age.Anyearlier
volcanic
centerthatsupplied
debris
EugeneCallaghan
to theAnimasandMcDermott
formations
wasdestroyed
NewMexicoBureauof Minesand MineralResources
by erosion
during
earlyTertiary
time.Theandeslte
wasfollowed
by thelatitic
Sanjuantuffandby a
hostofrhyolitic
andandesitic
flows
andtuffPublished
workon theSanJuanBasinindicates
I latitic
breccias.
Theseeruptions
werefollowed
by an erosion
thatthecentral
partoftheBasin
isnearly
freeof
interval
whichproduced
theSanJuanpeneplain.
bodiesof igneous
rock.However,
theoutermargin
Volcanism
laterthanthepeneplain
supplied
thin
isringed
by scattered
igneous
masses.
Mostof these
accumulations
of rhyolite
andtuffandwasfollowed :
masses
areerosional
remnants
andmanyarelandmarks
thathavea prominence
outof proportion
totheirareal
intheRioGrande
drainage
areaby basaltic
eruptions
extent.
Theexactageof almostallof theigneous
whichyielded
rocksgrouped
as theHinsdale
formation
age.Someintrusive
rocksin theOuray
rockbodiesis unknown.
Theaccumulation
of greatpiles of Pliocene
of volcanlcs
in theSanjuanMountains
thatextend
region
areearlier
thantheerosion-surface
atthebase
of thevolcanic
succession.
Later,
granular
intrusive
beyond
themargin
of theBasinfromthevicinity
of
almost
wholly
restricted
tothe,
lower
part
rocks¯
are
Durango
Colorado
to
that
of
Chama,
New
Mexico,
I
is believed
to havebegunin Ollgocene
timeandto
of thevolcanic
sequence.
Recentworkin theSan
Juanregion
hasindicated
thatthebeginning
of :..’:.
haveextended
wellintoPliocene
time.Thecentral
volcanic
accumulation
maywellhavebeenin
partof MountTaylor
maybe as oldas lateMiocene
Oligocene
time.
butmostoftheassociated
volcanics
areprobably
much
younger.
Largepartsof theJemezvolcanic
sequer~ce
TheLaPlata
Mountains
northwest
of Durango
eastof theBasinareprobably
of Pleistocene
age.Many
Colorado,
contain
a variety
of porphyritic
rocksinterJof thevolcanic
necksandassociated
flowson thewest
mediatebetweendioriteandmonzonit%
and evensideof theBasinareprobably
of Pliocene
age.The
granular
syenite,
monzonite,
anddiorite
intruded
valleyflowsalongU. S. Highway
66 areprobably
of
intosedimentary
rocksas youngas theMancos
shale.
latePleistocene
ageandtheMcCartys
flowmaybe
No extrusive
rocksoccurin thisarea.Eckel,
(!9491
lessthan1,200yearsold.Evidence
of volcanic
p.32)i states
thatthese
intrusive
rocksmaybelate
activity
earlier
thanthemld-Tertlary,
hasbeenfound
Cretaceous
or earlyTertiary.
An earlymid-Tertlary
at thenorthsideoftheBasin.
Thosewhotookthe
fieldtripoftheSociety
in1950willremember
the
ageis suggested
by analogy
withothereruptive
areas
in New MexicoandColorado.
andesitic
volcanic
debris
in thesedimentary
Anlmas
andMcDermott
formations
of uppermost
Cretaceous
UteMounta!n
southwest
of Cortez,
Colorado,
is
andpossibly
Paleocene
age.
a steep-sided
masswhich¯rises
3,000feetabovethe:
surrounding
surface.
It isbriefly
described
by(Coffin.
AsidefromtheSonJuanandJemezvolcanics,
19211p.121)asa stockandtherockwasinterpreted
whichareconsidered
as being.outside
theBasin,
the.
as anandeslte
porphyry.
flowrocksandvolcanic
necksaredominantly
of basaltic forCoffin
or basaltic-andesite
composition
andappearance.
Those
Themanysmallbodies
of igneous
rockthatoccur.
atthewestsideof the.Basin
areabnormal
inhaving
the
along
the
western
side
of
the.
Basin
between
theFour
lowsilica
content
of basaltic
rocksbuta highcontent
of
-~
Corners
and Galluphavebeenstudied
by Howel
potash
whichis suggested
mineralogically
in theircon
Williams
(19361.
pp~111-172).
Aside
fromthediorite
...
tentofbiotite
and,insomelocalities,
ofleucite.
porphyry
laccollth
ofCarrizo
Mountain;.
therocksare:
These
rocksaretherefore
classified
as trachybasolt
alkaline
andarecharacterized
by a hlghcontent
of
andminette~.
Rhyolite,
trachyte;
latite,
andandeslte
potasheventhoughthecontent
of silica
is low.ThoUgh
occurin thecoreof MountTaylorandmakeup the
identical
mineraiogically
greater
proportion
of therocksof theJemezandSan
I theflawrocksarecalled
trachybasalt,
andthedikerocksarecalled
minette,
JuanMountains.
Thelarger
intrusive
bodies
areof
whichis a basaltic-appearing
rackwithabundant
biotite
dioritic
ormonZonltic
composition,
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NEW MEXICO GEOLOGICAL SOCIETY * SECOND FIELD CONFERENCE * SAN JUAN BASIN
rockfragments
fromthepre-Cambrlan
basement.
This
andotherigneous
masses
arethesource
of thepyrope
garnets
andtheolivine
orperldot
crystals
thatare
foundin thisregion.
Zilditloi
Mountain
hasa lava
cap250feetthickcomposed
of trachybasalt.
Fluted
Rockis a smalllaccolith
ofminette.
flakes.
Mostof thevolcanic
neckscontain
large
amounts
of fragments
of pre-Cambrian
basement
rocks
andsomeconsist
largely
of sedimentary
rockdebris,
Fromdatain theHoplButtes
areainArizona,
itis
judged
thattheageof mostifnotallof these
igneous
bodiesis Pliocene.
Noteson thevarious
bodlesltaken
:.
fromWilliams’
account
follow.
Thevolcanic
necksin theChuskaValleyalong
thehighway
between
GallupandShlprock
areperhaps
themoststriking
of all.Shiprock,
whichrisesover
1400feetaboveitsbaseis composed
of tuff-breccla
traversed
by a fewdikesof minette.
Thecrudebedding
of thetuff-breccia
anditsdiptoward
thecenter
suggest
thattherewereprobably
no lavaflowsfromthisneck.
Theradiating
en echelon
dikesofminette
arealsoa
source
of greatinterest.
Bennett
PeakandFordButtes
aremadeup of fragments
of sedimentary
rocksrarely
morethanan inchin diameter
andcutby a fewsmall
dikesofmlnette.
TheCarrizo
laccolith
whichcove~san areaof
100square
milesistheonlylargeintrusive
bodyin
thegroup
on thewestsideOftheBasin.
Itconsists
entirely
of diorite
porphyry
in whichtherearelarge
phen0crysts
of hornblende
andandeslne
andsmaller
intergrowth
of
ina microgranular
quartz
and biotite
quartz
andorthoclase.
Thedikesandnecksin theRedrock
Volleyarea,
including
MittenRockandThumbRock,arecomposed
mainlyofminette
lavaratherthanbreccia,
though
somesuchas ThumbRockcontain
manyforeign
inclusions.
Igneous
bodiesin theChuskaMountains
areboth
necksandremnants
of flo~s.BlackPinnacle
is a
neckof minette.
Sandstone
in contact
withit was
changed
to quartzite
orto buchite,
a silica
glass
formed
by melting
of thesandstone.
TubbyButteis
a neckcomposed
of tuff-breccia
containing
large
blocks
ofsandstone
and.shale.
Itiscutbya dikeof
platyminette~
Lavacapsoccuron Palisades
and
Sonsela
Buttes.
In Washingto
Pass
flows
are
either
n
coarse-gralned
withlargecrystals
ofolivine,
auglte,
biotite,
andsanidlne
or fine-gralned
withsporadically
distributed
phenocrysts.
FIowsof
trachybasalt
alsooccur
in theLukachukal
Mountains.
TheTwinConesnearGallup
consist
of 90 percent
of co¯minuted
Cretaceous
sandstone
andonlya small
fraction
of admixed
minette.
A fewlargeblocksof
alaskite
andJurassic
sandstone
aswellas other
sedimentary
rocksoccurinthesenecks.
Remnants
of basaltic
flowsoccurat Powell
MounrainandLookout
Mountain
on eithersideof U.S.
Highway66 betweenGallupand Grants.
Onthesouthsideof theBasinthemostprominent
feature
is theMountTaylor
volcanic
field,
ably
described
by C. B. Hunt(1938,pp.51-80).Mount
Tayloris a lowconeseton a platform,
theMesa
Chivato,
whichis surrounded
by steepslopes
leading
downto thevalley
of theRioPuerco
andto thatof
OftheSeveral
igneous
structures
nearFt.
is followed
by Uo S.
I Defiance, theRioSanJosewhosecourse
Highway
66.Theplatform
is an olderosion
surface
themostinteresting
isBuellPark.Thiscircular
decuton Cretaceous
rocks
which
reach
upward
to a
presslon
in sandstone
is regarded
byWilliams
as having
r
pointabout2,000feetbelowthecrestof themountain.
beenformedby cauldron
subsidence.
A cylindrical
Volcanic
necksoccuron all.sides
of theMountTaylor
blockabout2½ milesin diameter
dropped
atleast
1000feet,probably
aftertheeruptlons
of lavaand
areabutareespecially
numerous
~nthevalley
ofthe
RioPuerco.
Huntshowsthatthecentral
coreof Mount
greenlapilll
tufts.
Outlet
neckand:TheBeastare
Taylor
consists
of rhyolite
tuffrrhyolite
trachyte,
and
necksdiffering
somewhat
in internat
structure.
Outlet
latlte.
Therhyolitic
vltric
tuffistheearliest
andmost
neckhasa coreof blocky
minette
surrounded
by a
widespread
of theeruptive
materials.
Huntregards
sheath
ofbreccia
oftheenclosing
sedimentary
rocks,
thismaterial
as of probable
lateMiocene
age.The
Someof thesedimentary
material
is vitrified.
The
sillclc
groupofrockswasalmost
covered
by porphyritic
Beastis mainly
tuff-breccla
withdikesthatshowtransitlonfromminette
to monchiquite.
TheGreenKnobs
andesite
aspossibly
thelatest
phaseoftheMountTaylor
of a neckwhichis madeup of a minette
eruptive
sequence,
in thesoutheastern
margin
of the
areremnants
mountain,
pumiceous
tuffcontaining
fragments
of sandthatweathers
readily
andcontai:ns
anabundance
of
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NEW MEXICO GEOLOGICAL SOCIETY * SECOND FIELD CONFERENCE * SAN JUAN BASIN
stone
rests
on thinflows
of basaltic
rockandisoverlainby water-laid
material
whichis succeeded
by the
porphyritic
andeslte.
Theremainder
of theplatform
is covered
by basalt
flowsthathadmanylocalsources.
Whereerosion
hasdestroyed
theplatform
andremoved
theflowsthenumerous
ventsor volcanic
necksstandout
in relief
abovethedissected
sedimentary
rocks.
Erosion
hascutas muchas 2000feetbelowthelevelof the
basalt
flowswhichHuntregards
as of probably
Pliocene
age.
SinceHunt’sworkwaspublished
themineof the
PumiceCorporation
of America
wasopenedin the
pumiceous
tuffneartheeastendofGrants
Ridge.
A geologic
section
of oneof thebasaltic
volcanoes
is wellexposed
at themine.Justeastof themine
thepumiceous
tuffgiveswayto rhyolite
andto
masses
ofperlite
orvolcanic
glass
thatareintimately
associated
withtherhyolite
onthenorth
sideofthe
ridge:.
Cursory
examination
suggests
thattherhyolite
andperllte
represent
oneor moredomeeruptions
at
thissiteandarenotconnec
tedwiththeMountTaylor
center.
Exploratory
workhasbeendoneon theperllte
witha viewto commercial
utilization.
andscoria
overlying
boththeSantaFeandlater
terrace
sandsandgravels
as in theAlbuquerque
area.
Of particular
interest
tothetraveler
alongU. S.
Highway
66 between
MesitaandBluewater
arethe
basaltic
valley
flows.Theflowat Bluewater
extends
froma localsource
northofthehighway
to thecarrot
fields
northwest
ofGrants.
Another
flowfloors
the
valley
southoftherailroad
at Grants
fromtheoverpass
westof Grantseastward
towardMcCartys.
Thisflow
haditssource
manymilestothesouthin whatis almost
a shield
areaof basaltic
flows.
A morerecent
basaltic
flowcalledtheMcCartys
by Nichols
(1946,pp.1049-1086)
is superimposed
overtheolderflow.TheMcCartys
flow
covers
a largeareato thesouthbutapproaches
theSan
JoseValley
ina verythinstream
andreaches
asfaras
McCartys.
Nichols
regards
thisflowas probably
less
than1,200
yearsold.Allthefeatures
of flowsOfthis
type,suchasspatter,
cones,
ropysurface,
pressure
ridgesandtreetrunkmolds,areverywellshown.An
olderflowis exposed
almost
continuously
fromthe
vicinity
of Laguna
tothatof Mesita.
TheJemezMountains
in Sandoval
andRioArrlba
counties
eastandsoutheast
of Cubaareeastof the
¯
Naclmiento
uplift
andarethusnotin theBasin.
The
Manyof thebasaltic
necksin theMountTaylor
"entire
Jemezmassis beingCarefully
mapped
byC. S.
region
arecomposed
of columnar
masses
of basalt
but
¯ arecomposed
RossandRobertSmith,
of theU. S. Geological
Survey.
others
ofbasaltic
breccia
whosebedding
Suffice
to saythattheJemezareahasbeena volcanic
tendsto dipinward
toward
thecenter
in a manner
center
fora considerable
period.
Mostof thefeatures
similar
tothatofthetuffbreccias
ofthenecks
inthe
of
the
mountains
are
of
post-Pliocene
age.Themost
western
partof theBasin.An example
is theneck
prominent
feature
is thegreatcaldera
of theValle
nearCubero.
Themostimpressive
of thevolcanic
Grande
whichhasbeenin partfilled
byrhyolitic
¯ necksis Cabezon
Peakwhichrises2000feetabove
domes.Visitors
to LosAlamosandFrljoles
Canyon
theRioPuerco.
Theexposed
basaltic
coreis 800feet
in Bandelier
National
Monument
arefamiliar
with
high
and
1500
feet
in
diameter,
¯
thegreatcliffs
oftancolored
welded
tuffwhichissued
as a kindofflowthrough
breaks
in thewallof the
Basaltic
rocksoccur
atseveral
erosional
levels
in
TheJemezregion
is doubtless
thebest
theareato thesouthof theMountTaylor
region.
Basalts caldera.
example
ofthevarious
facies
oftherhyolitic
eruptive
atthreelevels
aredistinguished
intheLucero
uplift
process
intheUnited
States
andthereport
of Ross
(Kelley
andWood,1946).Onecapsthehighest
part
andSmithis eagerly
awaited.
of theMesaLucero.
Another
occursat a lowerlevel
as theprominent
capon theMesaRedonda
nearHighway6, fourmilessouthwest
of Correo
andthethird
theCerroColorado
flowwhichcamefromthesouth
intothevalley
of theRioSanJoseat Correo
and
followed
thevalley
for12 milessoutheastward.
This
latest
flowis considerably
eroded
andis wellabove
thepresent
stream|evelbelowCorreo.
In theRio
Grandetrough
region
outside
theSanJuanBasin
the¯Santa
Feformation
contains
basaltic
flows
(Wr:ight,
1946,pp.412-413)
andtherearebasalt
flows
122
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NEW MEXICO GEOLOGICAL SOCIETY * SECOND FIELD CONFERENCE * SAN JUAN B~SIN -
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Selected
References
Coffin,
R- C. ~ (1921)Radium,
uranium,
andvanadium
deposits
of southwestern
Colorado:
Colo.Geol.Survey
Bull.16
Cross,
Charles
W.¯ andLarsen
A briefreview
of thegeology
oftheSanJuanregion
I EsperS.,(1935)
southwestern
Colorado:
U. S. Geol.SurveyBull.843
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EckellE. B. andothers(1949).
Geology
andoredeposits
of theLaPlato
district,
Colorado:
U.S.Geol.Survey
Prof.Paper219,p.179.
HuntC. B. (1938)Igneous
geology
andstructure
of theMountTaylorvolcanic
field:U. S. Geol.Survey
Prof.Paper189-B,
pp.51-80.
Kelley,
V. C.,andWood,G. H.,(1946)Lucerouplift~
Valencia,
Socorro,
andBernalillo
Counties,
New
Mexico:U. S. Geol.SurveyOiland GasInvs.Prelim.Map no.47.
Nichols~
R. L. (1946)McCartys
basaltflow~Valencia
County~NewMexico:Geol.Soc.Am.,Bull.,vol.
pp.1049-1086.
Williams~
Howel(1936)Pliocene
volcanoes
af theNavajo-Hopi
country:
Geol.Soc.Am. Bull.,vol~47,pp.
111-172.
Wright,
H. E. Jr.,(1946)Tertiary
andQuaternary
geology
of theLowerRioPuertoarea,NewMexico:
Geol.
Soc.Am.,Bull.,vol.57, pp.383-456.
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