Reinterpretation
0ftypesection
0f
JuanaLopez
Member
of Mancos
Shale
marks a distinct horizon and may be traced
over an immense area in New Mexico." However, Rankin (1944,p. 12, 19,20) was the first
to formally name the unit and identified it
(often incorrectly) in a number of measured
sectionsin northern New Mexico and southern
Colorado. Rankin (9a4, p. 12, 19, 20) applied the name "Juana Lopez sandstonemember of the Carlile shale" to l0 ft (3 m) of very
calcareous
sandstone containing Prionocyclus
byStephen
C.Hook,Paleontologist,
NewMexicoBureauof
Minesand
Mineral
Resources,
Socorro,
NM,and
wyomingensis near the top of the Carlile
William
A. Cobban,
Paleontologist,
U.S.Geological
Survey,
Denver,
C0
Shale. The type section for this new unit was
given as
sec.32, T. 15 N., R. 7 8.,
The Juana Lopez Member (Upper Creta- face (Dane, 1960;Molenaar,1973i Campbell, on the Mesita Juana Lopez Grant, six miles
ceous)of the MancosShale(SanJuan Basin), 1979). Microfossil studies by Lamb (1968) and northwest of Cerrillos, Santa Fe County, New
Carlile Shale(Denver and Raton Basins)or Thompson (1972), however, indicate that this Mexico" (Rankin, 1944,p. l9).
BentonShale(North Park Basin)is one of the surface may be younger toward the northern
Kauffman (in Dane and others, 1966, p.
most distinctiveand laterallypersistentlitho- edge of the San Juan Basin. Outcrop sections H6, H7, Hl3, H14) examinedthe type section
stratigraphicunits in New Mexico, Colorado, we visited indicate that the top of the Juana of the Juana Lopez Member in 1963and conUtah, and southwestern
Kansas.The member Lopez Member lies in a lower faunal zone cluded that l) the type section of the member
containsan abundantlate Turonian (middle along the south side of the San Juan Basin w a s a c t u a l l yi n s e c . 3 3 ,T . 1 5 N . , R . 7 E . ; 2 )
Cretaceous)invertebrate fauna whose ele- than it does along the east side. ln the the thicknessof the type sectionwas only 3 ft
mentshave restrictedvertical rangesand are southernSan Juan Basin near Hosta Butte, 32 l0 inches. the remainder of Rankin's estiwidely distributed in time-equivalentrocks mi (52 km) east of Gallup, the top of the mated l0 ft being composed of baked shale of
throughout the Western Interior of North Juana Lopez Member lies in the Prionocyclus Niobrara age; 3) the member at the type secAmerica(Daneand others,1966).
macombi Zone. whereas on the east side near tion was composed predominantly of calcareOn the east side of the San Juan Basin. Cuba, the top lies two zones higher in the nite and limestone(hence,the term sandstone
where Dane and others (1966)establisheda Pr ionocyc lus n ov i mexi canus Zone.
was dropped from the member name); 4) the
107-ft-thick(32.6 m) well-exposedreference
The distinctive lithologic and faunal nature
member at the type section was abnormally
section (fig. l), the Juana Lopez Member of the Juana Lopez was first noted by New- thin and containedan unusuallysmall percentrangesin thicknessfrom 90 to 135ft (27.4to berry (1861)near Las Vegas, New Mexico. In age of shale compared with sectionsmore than
4l.l m). Here, and at other localities,the a later publication, Newberry (1876, p. 33) 100 ft (30 m) thick in the San Juan Basin; 5)
memberconsistsof thin bedsof rusty-brown- presented a detailed measured section of
the member at the type section was succeeded
weathering,highly fossiliferouscalcarenire
in- Cretaceous rocks along the Canadian River disconformably by rocks equivalent to the
terbeddedwith dark-graynoncalcareous
clay south of Taylor Springs, New Mexico; it in- basal part of the Niobrara Formation; and 6)
shale. The calcarenitesoccur as both con- cluded the Juana Lopez as a distinct lithology
the Juana Lopez at the type section was the
tinuous bedsand discontinuouslenses,range characterized as ".
. ferruginous, lami- lithologic and time equivalentof only the upfrom a fractionof an inch to 2 ft (0.6 m) thick, nated, sandy limestone . . [that is] a great per part of the member at the reference secare fine to coarsegrained,and are composed store-houseof fossils." Newberry (1876, p. tion.
of bioclasticdebris.This debrisgenerallycon- 107) also noted that "this ferruginous stratum
In connection with work on the Juana Losistsof the fragmentedprismaticlayer of Inpez Member of the Carlile Shale in the Taylor
oceramusshells.Fragmentedremainsof fish
Springsarea of northeastNew Mexico, 109 mi
teeth,bones,and scalesare common and led
(175 km) northeast of the type section(Hook
Rro
early workersto describethesebedsas "fishand Cobban, 1980,fig. 6), we visited the type
tooth conglomerates"or "fish- bone beds."
section of the Juana Lopez. After remeasuring
-l
Gray septarianlimestoneconcretionsarecom..-/-\
the type section of the member in a 1965 railt L o sl mon in the lower and middle parts of the
Alomos
road cut of the Atchison, Topeka, and Santa
member.Although shaleis the dominant lis a N r aF E
Fe Railway Company in NW%SE% sec. 33,
f
thology, the hard, platy-weatheringcalcareT. l5 N., R. 7 E., we suggestmodification of
S o n r oF e
s ) o / s A N o o v At\o ;
nites are more conspicuousin outcrop, often
z
all but the first of Kauffman's conclusions.
l . - - .t y p e
oo,/
littering the slopeswith debris,and generally
; ;x i s e c t i o n
This paper presentsour measured type section
d
stand in topographicrelief above the softer
.O/
l ; : o c e r r ,t l o s
of the Juana Lopez Member (table l) and
+/
shales(Daneand others,1966).
compares this newly rerheasuredsection with
(
M e s r t od e J l o n o
t
L o p e zG t o n l
/
|
In the subsurfacethe calcarenites
producea
_ _
l_
_ _
the reference section establishedby Dane and
| ^
:8ER
ALtL Lo
,^
r^h,
\
distinctive "kick" (resistivedeflection) on
| "--1,:____:others (1966) on the east side of the San Juan
NC|A\., \
electriclogs(Lilly, 1952,fig. 2; Bozanic,1955,
tQt"
Basin.
to6"
p l s . l - 3 ; L a m b ,1 9 6 8 f, i g . 5 ; M o l e n a a r , 1 9 1 3 ,
p. 94). The top of this resistivezone(referred FIGURE l-LocnrroNs oF rHE TypEANDREFERENcE
Lithology and thickness
sECTtoNSop tHE Junne Lopez MeN4genop rHr
to by oil-companygeologistsas Sanosteeor
A 1965 railroad cut of the Atchison,
MnNcos Ssele, Snrrl Fe eNo SnNoovnLCoun.
SanasteeMember of the Mancos Shale)has
Topeka, and Santa Fe Railway Company exrrEs, New Mexrco (modified from Dane and
long beenconsidereda time-synchronous
suro t h e r s , 1 9 6 6 ,f i g . l ) .
poses not only the entire type section of the
Juana Lopez Member of the Mancos Shale,
but also severalhundred feet of Mancos Shale
both above and below this member (fig. 2).
Two points are readily apparent from a cursory examination of this cut. First, the ledgeforming calcarenites in the upper part of the
member (fig. 3) are approximately l0 ft (3 m)
thick and therefore closely match Rankin's
Q9aa, p.20) original estimate.In fact, these
upper beds are probably all Rankin intended
to include in his definition, although he shows
the Juana Lopez to be 50 ft thick on his
graphic section no. 6 (Rankin, 1944, fig. 5).
Second, the basal calcarenites, although thin
and rather inconspicuous on the south side of
the cut (fig 2), are well exposed on the north
side (fig. 4) and can be traced laterally (to the
south) in outcrop. These basal calcarenites
also carry the Lopha lugubris-Prionocyclus
macombi fauna characteristic of the lower
Juana Lopez in the San Juan Basin and are
separatedfrom the upper, ledge-forming beds
by approximately 90 ft (27 m) of shale. Hook
measured the Juana Lopez in this railroad cut
on July 16-18, 1979 (table l) and obtained a
thickness of 106 ft (32.3 m). This measured
thickness is almost identical to that obtained
by Dane and others (1966, p. HlO-Hl2) for
the referencesection (fig. 5). The Juana Lopez
was measured at the type locality by the
Jacob's staff method (Kottlowski, 1965, p.
6l-75). The thickness obtained is thought to
be accurate to within l0 percent of true
thickness. Several factors other than those
listed by Kottlowski (1965, p. 122) may contribute some error. First, a reliable dip and
strike measurementcould be obtained only on
the uppermost calcarenites (fig. 3); this measurement was then used to measure the thickness of the entire member. Second, several
minor faults with displacementsof less than 5
ft (1.5 m) cut the member; in unit 19 (table 1)
it was impossible to offset on a key bed to
avoid one of these faults. Third. at least two
igneous dikes, which weather to small chips
and are covered by shale talus, cut unit 3l
(table l).
uNtr oF JueNe Loprz
CALCARENITE
FIGURE 3-Clost-up vtEw oF Flc 2 sHowtNc upprR LEDGE-FoRMTNG
MrvsEn Upper contact of Juana Lopez Member shown as dashed line; 5-ft-long (l.5 m) Jacob's staff
for scale.
The Juana Lopez Member at its type section
can be subdivided into three informal units
similar in lithology and thickness to those at
the reference section: a lower calcarenite unit
with interbedded shales, approximately l0 ft
(3 m) thick; a middle shale unit with scattered
lenses of calcarenite that hosts septarian
limestone concretions, approximately 8l ft
(24.7 m) thick; and an upper calcarenite unit
with interbedded shales, approximately l5 ft
(4.5 m) thick. These same informal subdivisions at the reference section are approximately ll ft (3.3 m), 83 ft (25.3 m), and 12 ft
(3.7 m) thick (fie. 5).
The calcarenites are hard, brittle, thinbedded, platy-weathering rocks that range
from a fraction of an inch to 9 inches (0.2 m)
thick. Bedding surfaces are generally irregular
and highly burrowed and bioturbated. The
calcarenites are generally fine grained, with
most grains being bioclastic debris derived
from Inoceramus and Lopha shells; comminuted fish remains are common. These calcarenitesrange in color from medium to dark
to brownish gray on fresh surface and weather
from slightly darker gray to dark yellowish
A L S OI N T H I SI S S U E :
p.22
p 27
p.30
C h i n l eF o r m a t i o n
P r o t e r o z o i ct e c t o n i cs e t t i n g
Service/News
SOON:
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K i r t l a n dc r o c o d i l e
D a k o t a - M a n c o sZ,u n i B a s i n
New AAexfic@
GEOLOGY
.
Science
andSeruice
Volume 2, Number 2, MaY 198O
Acting Editor:
JoanC
Pendleton
publishcd quarterly bY
Ncw Mexico Bureau of Mines & Mineral Resources
a division of New M€xico Institute ofMining & Technology
BOARD OF REGENTS
Ex Officio
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of Publ ic l6truclion
Leonud De|-ayo, Srperintendent
Appointed
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Judy Floyd, Pres, 1917-1981,Las Cruces
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Steve Torres, 1961-1985, Socorto
New Mexico Institute of Mining & Technology
Kenneth w. Ford
Prcsident
New M€xico Bureau of Mines & Mineral Resources
FrankE.Kottlowski
Direclor...,,.
.Ceorges.Austin
DepulyDiector...
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BureouEditoFGeologist.....,
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New Mexico Bureau of Mines & Mineral Resources,
FICURE 2-Rarlnono cur op Arcsrsou. Toprxn. nNoSaNrnFp Rerluv Coupnrv nr rvpEsEcrtoNoF
View is southeast
toward southsideof cut; upper
rHt Juela LopEzMEvgEnoF rnp MlNcos SHer-E.
and lowercontactsof JuanaLopezMembershownasdashedlines.
May 1980
New Mexico Geology
Socorro, NM 87801
Ciculalion: l,2N
of New Mexico Printitrg Plant
Prirler.'University
orange where iron content is higher. Most
have a petroliferous odor, especiallyin the upper calcareniteunit.
Dark-gray fissile noncalcareous shale is the
dominant lithology at the type section. These
shales are conspicuously darker than those of
the overlying calcareous shale member of the
Mancos (fig. 3). Limestone concretions, common throughout the shales, occur both as
isolated bodies and in zones a few feet thick
(fig. 6) and are usually septarian. Scattered
thin lenses of more resistant calcarenite are
also common. Bentonite beds occur throughout the upper two informal divisions of the
type section of the Juana Lopez and range
from a fraction of an inch to 6 inches thick
(0.15 m). The bentonites are white (fig. 6),
weather dark yellowish orange, and are often
secondarily gypsiferous. The thickest bentonites occur in the upper part of the Juana
Lopez,
The contact between the Juana Lopez
Member and the underlying shale member of
the Mancos is conformable at the type section;
Dane and others (1966, p. Hl0) established
that there was a lateral gradation between the
basal Juana Lopez and the underlying shale in
the San Juan Basin. The upper contact of the
Juana Lopez at its type section is conformable
and gradational. Not only are the shales interbedded in the upper calcarenite unit calcareous(table I, units 43, 45,47,50), but also
the base of the overlying shale unit is silty,
with the silt content decreasing upward. No
physical evidence of an unconformity was
observed at either the lower or upper contacts
of the Juana Lopez at its type section. However, we have recorded an unconformity at the
base of the Juana Lopez Member in the Taylor
Springs area of northeast New Mexico (Hook
and Cobban, 1980,fig. 4).
Fauna and age
Fossils were collected throughout the Juana
Lopez Member at the type section (fig. 5).
With the exception of the Dl08l4 collection
and one concretion from the Dl08l I level. all
fossils shown on fig. 5 were collected in place
from the railroad cut (fig. 2). Although all
lithologies in the Juana Lopez are fossiliferous, fossils are generally more abundant and
better preservedin the calcarenites.
The invertebrate fauna at the type section,
like that at the reference section (fig. 5; Dane
and others, 1966, p. H7-Hl0), is dominated
by a few speciesof mollusks. The lower calcarenite unit is dominated by the oysterLopha
lugubris (Conrad), the bivalve Inoceramus
dimidius White, and the ammonite. prionocyclus macombi Meek; the middle shale by
Lopha lugubrl's; and the upper calcarenite by
the bivalves Inoceramus dimidius and L
perplexus Whitfield, and the ammonites Scaphites warrenl Meek and Hayden, S. whitfieldi
Cobban, Prionocyclus wyomingensis Meek,
and P. novimexicanus (Marcou). A concretion
in unit l0 (Dl08l I level) also containeda new
species of the ammonite Coilopoceras and a
rare occurrence of the ammonite Hourcquia,
previously reported only from Madagascar,
Japan, Russia, and possibly France. The
ranges and occurrencesof these speciescom90). The Mesita de Juana Lopez has simply
pare very favorably with those in the reference beena placenamesinceat least1746,but there
section (fig. 5).
is no documentaryevidenceconcerningJuana
The fauna collected at the railroad cut inLopezor why the areabearsher name(Myra
dicates that the type section of the Juana Ellen Jenkins, oral communication, 1979).
Lopez Member lies entirely within the upper The Mesitade JuanaLopezGrant wasgranted
Turonian (middle Cretaceous) zones of Prioto Domingo Romero, Miguel Ortiz, and
nocyclus macombi, Prionocyclus wyomingenManuel Ortiz in January 1782(Pearce,1965,
sis, and Prionocyclus novimexicanus. These p. 100).
three prionocyclid species serve as name
Conclusions
bearers for the basal three faunal zones of the
upper Turonian in the Western Interior (Obral) The JuanaLopezMemberof the Mancos
dovich and Cobban, 1975; Hook and Cobban,
Shaleat its type sectionin a 1965railroad cut
1979; and,Cobban and Hook, 1979).The zone of the Atchison, Topeka, and SantaFe Railof Prionocyclus macombi probably extends at
wayCompanyin theNW%SEVIsec.33,T. l5
least l0 ft (3 m) below the base of the Juana N., R. 7 E., SantaFe County,New Mexicois
Lopez into the underlying shale (USGS Meso106ft (32.3m) thick and composedprimarily
zoic locafity D10810), and the zone of Prionoof dark-graynoncalcareous
fissileshalewith
cyclus novimexicanus extends a few feet into
interbeddedcalcarenitebeds and lenses;the
the overlying shale (USGS Mesozoic locality
thickestcalcarenitesoccur at the top of the
D10820; fig. 5; and Kauffman ln Dane and member; thin beds of laterally persistent
others, 1966, p. Hl3, as Prionocyclus wyomcalcarenitemark the baseof the member.2)
ingensis elegans Haas). Thus, faunally, it is The memberis succeeded
conformablyby calnot possible to recognize an unconformity at
careousshalesof late Turonian(middleCretaeither the top or the base of the Juana Lopez ceous)ageand conformablysucceeds
noncalMember at its type locality.
careousshalesof early late Turonian age. 3)
Descriptions and illustrations of Lopha
The Juana Lopez is the lithologic and time
Iugubris, Inoceramus dimidius, Prionocyclus
equivalent of the entire member at the
macombi, Prionocyclus novimexicanus, and
referencesection establishedby Dane and
Scaphites whitfieldi can be found in Hook and
others(1966)on the eastsideof the San Juan
Cobban (1979, 1980).Illustrations of most of
Basin. 4) The Juana Lopez Member lies enthe fauna appear in Kauffman (1977).
tirely within the late Turonian (middleCretaHistorical note
Although the name Juana Lopez is geologically well known, little is known about the
historical person. She was apparently Juana
Lopez del Castillo (Myra Ellen Jenkins, oral
communication, 1979), whose family is
thought to have come from Bernalillo in 1706
to help found Albuquerque (Pearce, 1965, p.
ceous) ammonite zones of Prionocyclus
macombi, Prionocyclus wyomingensis, and
Prionocyclus novi mexicanus.
In addition, it appearsthat Rankin's(1944)
original definition of the Juana Lopez was
restrictedto the upper calcareniteunit (approximately the upper 10 ft-3m-of
the
JuanaLopez Memberas presentlyrecognized
and mapped). The expandedscope of the
Juana Lopez apparently originated with
FIGURE 4-Bnsnl cALCARENTTE
uNrr oF JuaNa Lopez MeMaen ExposEDoN NoRTHsrDEoF RAILRoAD
cur Contact of Juana Lopez Member with underlying lower shale member of Mancos Shale shown as
dashedline; 5-ftJong (1.5 m) Jacob's staff for scale.
New Mexico Geology
May 1980
Division,State
Bozanic(1955)and was carried to its logical kins, Chief, HistoricalServices
conclusionin surfacesectionsby Dane and RecordsCenterand Archives,and StateHistorian, SantaFe, New Mexico, for historical
others(1966).
AcrNowr-socMENTS-We thank the At- informationon JuanaLopez.C. M. Molenaar
chison,Topeka,and SantaFe RailwayCom- and E. R. Landis,both of the U.S. Geological
pany for permissionto measurethe type sec- Survey,Denver,Colorado,criticallyreviewed
tion of the JuanaLopez and Myra Ellen Jen- this manuscript.
NW -48
Unit
m i l e s ( 7 7 . 2 k m) -
SE
T y p es e c t i o n
N w r / 4 S E 7 o s e 3c 3 , T .1 5 N , R 7 E
R e f e r e n c es e c l i o n
r/.
SEr/4sw sec l4,T lgN,R lw
54
)J
!I
!I
!-I-
!I
dE
sEcrtoN oF ruB JunNe LopEz
TABLE l-Tvpp
MEunrn op tne Malcos Snale NW%SE% sec.
33. T. 15 N., R. 7 8., SantaFe County, New Mexico, exposed in railroad cut of Atchison, Topeka,
and Santa Fe Railway Company. Measured by S. C'
Hook on July 16-18, 1979, using a Jacob's staff,
Abney hand level, and tape. Fossil collections made
on June 28, 1979, by W. A. Cobban and S. C.
Hook, and by Hook on July l6-18, 1979.
! I
- l
.'......._Dl0834
lnoceromus
vallarsdorfensis
n occ I o mu 5 ta, P I e x u st
/Dlo92o
t o nocvc lu s nov i h€ x ic o nu t
,/-P/
II
52
51
--,>\-
-,4
_v_
_\<
-4-s- -<- t
L---'1
e<
50
colcoreous 3hole
L:J
o
E
o
-D44|
Lucino
sp
@-<E
r _919615
---
Lopho
rtil
tugubrts
=
ffi
o
lJ
b€nlonile
........._D44lO Lopho
J
=
-o--u-
0to8ll
U S G SM e s o z o i c
f o s s r ll o c o l i l y
nuhberdtlool)
/ugublts
49
48
47
-044O9
Lqhd tugubris
..._(DlO8l4)
Prionocyc
lus mocombi
OO
46
45
O--
oO367t Lopho lugubrisl
I noceramus d imidius ,
Prionoctclus mocombi
9@e
Description
Mancos Shale
Upper shale member (Part):
Top of measured section
Bentonite, white
Shale, calcareous, olive-gray;
weathersslightly lighter
USGS Mesozoic locality D10834:
waltersdorJensis
Inoceramus
Andert
Bentonite, white; weathers dark
yellowish orange
Shale, olive-gray, calcareous,
silty; silt content decreases uPward; weathersslightly lighter
USGS Mesozoic locality D10820:
I nocera m us perplexrs Whitfield
Baculi tes y o k oya ma i Tokunaga
and Shimizu
novimexicanus
Prionocyclus
(Marcou)
Total measured thickness, uqqer
shale member
Top of Juana Lopez Member:
Calcarenite, olive-gray; weathers
light olive brown and dark Yellowish orange; fine-grained, wavY
bedded, in beds % to 2 inches
thick (1.3 to 5 cm); Petroliferous
odor: hard resistant unit, basal 6
inches (15 cm) forms ledge; contains abundant fragmented fossils
U S G S M e s o z o i cl o c a l i t y D l 0 8 l 9 :
I noceramus pe rplexus Whitfield
novimexicanus
Prionocyclus
(Marcou)
Shale, dark-gray, calcareous
Bentonite, white; weathers dark
yellowish orange
Calcarenite, olive-gray; weathers
light olive brown and dark yellowish orange; fine-grained, in beds
as much as 3 inches thick (3.7 cm);
petroliferous odor; hard resistant
unit; contains abundant fragmented fossils
U S G S M e s o z o i cl o c a l i t y D l 0 8 l 8 :
I noceramus dimidlus White
Prionocyclus sp.
Shale, dark-gray, calcareous
Calcarenite, olive-gray; weathers
light olive brown to dark yellowish orange; fine-grained, thinbedded; petroliferous odor; hard,
resistant unit; contains abundant
fragmented Inoceramus shells and
comminuted fish remains
Inoceramus sp.
Prionocyclus sp.
calcareous,
Shale, dark-gray,
hard, platy-weathering
1J
FICURE S-Ourcnop sEcrroNs or tHr JunNe Loprz Mrvspn AT rrs REFERENcE
AND TypE sEcrtoNs
oF FossrLSReference section from Dane and others (1966, p. Hl0-H12) with
sHowrNc occuRRENCES
faunal names undated; upper member of Mancos Shale not measuredor described.
20
May 1980
New Mexico Geolog!
Thickness
in.
ft
Calcarenite, olive-gray; weathers
light olive brown to dark yellowish orange; fine-grained, thinbedded. hard resistantunit
l0
42
4l
40
39
38
37
36
35
34
JJ
32
3l
30
29
USGS Mesozoic locality Dl08l7:
Inoceramus dimidizs White
Scaphites warreni Meek and
Hayden
Prionocyclus sp.
Shale, dark-gray calcareous,silty,
hard, platy-weathering
Bentonite;
weathers
dark
yellowish orange; gypsiferous
Calcarenite, olive-gray; weathers
to light olive brown to dark yellowish orange; fine-grained, thinbedded; petroliferous odor; hard
resistantunit
USGS Mesozoic locality Dl08l6:
I n oceramus dimidltts White
'
Scaphites sp.
Prionocyclus
wyomingensis
Meek
Shale, dark-gray, calcareous,silty
Calcarenite, olive-gray; weathers
to light olive brown and dark yellowish orange; fine-grained, wavy
b e d d e d ,i n b e d s % t o 2 i n c h e s( 1 . 3
cm to 5 cm) thick; petroliferous
odor; hard resistant unit; only
scraps of /noceramzs observed
0
6
Shale,dark-gray,noncalcareous
0
ll
Siltstone, olive-gray; weathers
dark yellowish brown; slightly calcareous,micaceous,very thin bedded, resistantunit
0
lv,
Shale, dark-gray, noncalcareous
ll0
Bentonite. white
06
Shale, dark-gray, noncalcareous
26
Calcarenite, olive-gray; weathers
slightly darker; fine-grained, burrowed, in wavy, irregular beds as
much as I inch (2.5 cm) rhick;
hard, resistant unit; contains
abundant fragmented 1xocerd mus
Shale, dark-gray, noncalcareous;
contains zone of flat limestone
concretions I ft (0.3 m) in diameter and 2 inches (5 cm) thick,
that are I ft (0.3 m) above base;
also has two basaltic dikes I ft
thick
13
0
Bentonite, white
0l
Shale, dark-gray, noncalcareous;
contains a zone of gray septarian
limestone concretions as much as
3 ft (1 m) in diameter and I fr (0.3
m) in thickness, 2 fr (0.6 m) above
28
21
26
25
Lq
23
22
2l
20
l9
l8
t7
t6
15
14
l3
l2
base. and also several calcarenite
lensesas much as I inch (2.5 cm)
thick in middle of unit
l0
Bentonite. white
0
Shale, dark-gray, noncalcareous
8
Bentonite, white
0
Shale, dark-gray, noncalcareous;
calcarenite lens I ft (0.3 m) from
top
U S G S M e s o z o i cl o c a l i t y D l 0 8 l 5 :
Lopha lugubris (Conrad)
Bentonite, orange-weathering
Shale, dark-gray, noncalcareous,
with zone of poorly fossiliferous
concretions as much as 2 ft (0.6 m)
long and 6 inches (15 cm) high developed at base, and scattered
concretionsin upper half
36
Bentonite, orange-weathering
02
Shale, dark-gray, noncalcareous
20
Bentonite, orange-weathering
0l
Shale, olive-black, noncalcareous,
with minor thin calcarenitesin upper half; one limestone concretion
I ft (0.3 m) long, 2 inches (5 cm)
thick observed 20 ft 6.1 m) above
base: crossed minor fault of unknown displacement (probably
f ess than 5 ft (1.5 m\) at 22 ft (6.7
1t
m)
U S G S M e s o z o i cl o c a l i t y D l 0 8 l 4
(float):
Prionocyclus macombi Meek
Bentonite?, orange-weathering;
gypsiferous zone
Shale, olive-black, noncalcareous,
with zone of gray-weathering septarian limestone concretions as
much as I ft (0.3 m) in diameter
and 6 inches (15 cm) thick that lie
2-3 ft (0.6- I m) above baseof unit l 5
Bentonite. white: weathers vivid
dark yellowish orange
0
Shale dark-gray, noncalcareous
Bentonite, white; weathers vivid
dark yellowish orange
Shale, dark-gray, noncalcareous,
with minor calcarenite lenses as
much as 2 inches(5 cm) thick
U S G S M e s o z o i cl o c a l i t y D l 0 8 l 3 :
Lopha lugubris (Conrad)
Inoceramus dimidius White
Prionocyclus sp.
Calcarenite, light-brownish-gray;
FIGURE 6-Mtoole sHALEuNIT on Junue Lopnz MBnasenoN sourH srDEoF RATLRoAD
cur showing interbedded lensesof calcarenite,zones of limestone concretions, and two prominent bentonite beds that
are approximately l0 ft (3 m) apart. Note small fault offset of lower bentonite.
0
lV2
0
Y2
ll
l0
0
I
9
8
weathers brownish gray; finegrained, in irregular thin burrowed beds; hard resistantunit
USGS Mesozoic locality Dl08l2:
Lopha lugubris (Conrad)
Shale, dark-gray, noncalcareous,
with scattered lensesof calcarenite
Calcarenite, light-brownish-gray;
weathers brownish gray; finegrained, with minor interbedded
noncalcareous shale, in irregular
beds as much as 2 inches (5 cm)
thick; gray limestone concretions
as much as l0 inches (25 cm) long
and 4 inches (10 cm) thick occasionally developed at top of unit;
resistantunit
USGS Mesozoic locality Dl08l l:
Lop ha lugubris (Conrad)
Coilopoceras n. sp.
P r io noc y clus mo co m b i Meek
Hourcquia sp.
Shale, dark-gray, noncalcareous
medium-gray;
Calcarenite,
weathers grayish red to dark yellowish brown; very fine grained,
burrowed, irregular top and bottom: hard resistantunit.
Base of member
Total thickness, Juana Lopez Mernber
7
'/4
6
1V,
6
J
l-6
8
l0
106
Lower shale member (part):
Shale, medium-dark-gray;
weathers olive gray, noncalcareous, slightly silty; contains scattered septarian concretions
throughout as much as I ft (0.3 m)
in diameter and zone of concretions as much as lVz ft (46 cm) in
diameter in basal 5 ft (1.5 m)
60
USGS Mesozoic localitv Dl08l0
l0 ft (3 m) below top:
Prionocyclus sp.
USGS Mesozoic locality D10809
20 ft (6 m) below top:
Prionocy clus hydlti (Stanton)?
Shale. with thin interbeds of
sandstone(not described)
70
Total measuredthickness,
l o w e r s h a l em e m b e r
130
%
0
2V,
2V,
References
Bozanic, Dan, 1955, A brief discussion on the subsurface Cretaceous rocks of the San Juan Basin:
Four Corners Geological Society, Guidebook lst
field conference,p. 89-107, I fig., 4 pls., 2 charts,
4 maps
C a m p b e l l , C . V . , 1 9 7 9 ,M o d e l f o r b e a c hs h o r e l i n ei n
Callup Sandstone (Upper Cretaceous) of northwestern New Mexico: New Mexico Bureau of
M i n e s a n d M i n e r a l R e s o u r c e sC
, i r c . 1 6 4 ,3 2 p . , 3 2
figs., I table
Cobban, W. A., and Hook, S. C., 1979, Collignon iceras wool lgori wo o I Igori (Mantell) ammonite
fauna from Upper Cretaceous of Western Interior, United States: New Mexico Bureau of
M i n e s a n d M i n e r a l R e s o u r c e sM
; em. 37, 52p.,l2
figs., l2 plates
Dane, C. H., 1960, The boundary between rocks of
Carlile and Niobrara age in San Juan Basin, New
Mexico and Colorado: American Journal of Science(Bradley volume), v. 258-4, p. 46-56, 3 figs.
Dane, C. H., Cobban, W. A., and Kauffman, E.
C., 1966, Stratigraphy and regional relationships
of a reference section for the Juana Lopez
Member, Mancos Shale, in the San Juan Basin,
New Mexico: U.S. Ceological Survey, Bull.
1224-H, 15p., 3 figs.
rtF
New Mexico Ceolog!
May 1980
2l
Hook, S. C., and Cobban, W. A., 1979,Prionocyclusnovimexicanas(Marcou)-Common Upper
Cretaceousguide fossil in New Mexico: New Mexico Bureauof Mines and Mineral Resources,Annual Repon 1977-78,p. 3,f42, 5 figs.
1980, Some guide fossils to the Upper
CretaceousJuana Lopez Member of the Mancos
and Carlile Shales, New Mexico: New Mexico
Bureauof Mines and Mineral Resources,Annual
Report1978-79,p. 38-49,7 figs.
Kauffman, E. c., 1977, Illustrated guide to biostratigraphically important Cretaceous macrofossils, Western Interior Basin, U.S.A.: The
Mountain Geologist,v. 3 and 4, p. 225-274' 32
pls.
Kottlowski, F. E., 1965, Measuring stratigraphic
sections: New York, Holt, Rinehart, and
p.
Winston,253
Lamb, G. M., 1968,Stratigraphyof the lower Mancos Shalein the San Juan Basin: GeologicalSocietyof America,Bull., v. 79,p. 827-854,5 figs.
Lilly, O. J., 1952,The Doswelloil field, Rio Arriba
County, New Mexico, rz GeologicalSymposium
of the Four Corners Region: Four CornersGeologicalSociety,p. 99-103,4 figs.
Molenaar, C. M., 1973,Sedimentaryfaciesand correlation of the Gallup Sandstoneand associated
formations, northwestern New Mexico, in
Cretaceousrocks of the southern Colorado Pla-
teau, J. E. Fassett,ed.: Four CornersGeological
Society,Mem., p. 85-110,l9 figs., I table
Newberry, J. S., lE6l, Geotogicalreport' in lves,
J. C., Repon upon the Colorado River of the
west: U.S. 36th Congress,lst session,SenateExecutive Document and House Executive Document90,pt.3,154p.
1876,Geologicalreport, in Macomb' J. N.'
Reportof the exploringexpeditionfrom SantaFe'
New Mexico, to the junction of the Grand and
GreenRiversof the Great Coloradoof the West in
1859:U.S. Army EngineerDepartment,p. 9-l18,
I I pls.
obradovich, J. D., and Cobban, w. A., lts, A
time-scalefor the Late Cretaceousof the Western
Interior of North America: Ceological Association ofCanada,Spec.Paper13,p. 3l-54, 3 figs.
Pebrce,T. M., ed., 1965,New Mexicoplacenames:
Albuquerque, University of New Mexico Press,
1 8 7p .
Rankin, C. H., lg4/, Stratigraphyof the Colorado
Group, Upper Cretaceous,in northern New Mexico: New Mexico Bureau of Mines and Mineral
Resources,Bull. 20, 27 p., 6 frgs.
Thompson, G. G., 1972, Palynologic correlation
and environmental analysis within the marine
MancosShaleof southwesternColorado: Journal
tr
of SedimentaryPetrology,v. 42,p.281-3N
Depositional
and
environments
paleocurrents
0l ChinleFormation
(Triassic),
SanJuanBasin,
eastern
NewMexico
TX
Houston,
Departmsnt
RiceUnivorsity,
byDennis
D.KurtzandJohnB.Andorson,
otGeology,
Strata of the Chinle Formation (Upper Triassic)crop out in the Nacimiento Mountains
of Rio Arriba and SandovalCounties on the
easternside of the San Juan Basin. The strata
are exposed in the San Pedro-Nacimiento
uplift, on the French Mesa-Archuleta arch,
and in the ChamaBasin(fig. l). As elsewhere
in the Southwest,the Chinle in north-central
New Mexico was depositedunder terrestrial
conditionsconsistingof two cyclesof fluvialto-marshlandor lacustrinesedimentation(Stewart and others,1972).Thesecyclesreflectthe
uplift and erosionof nearbyhighlandsand can
be usedto decipherthe effectsof theseuplifts
on sedimentation.
Although the Chinle Formation has been
mapped and studied principally to locate and
assesssedimentary copper deposits near its
base,relativelylittle work hasbeendoneconcerning the details of its depositionalhistory.
In describing the lithostratigraphic sequence
and the possibledepositionalhistory of the
Chinle in north-central New Mexico, this
report analyzes changesin sediment paleotransport directionsbetweenfluvial cyclesand
pebble lithologies to document changes in
sedimentsourceareasand dispersalpatterns.
These changesand concomitant changesin
May 1980
NewMexico Geology
fectssamplesfrom the lowestunit, whereonly
fine-grained beds have a calcareouscement,
and from the uppermostmernber,wheresandstone beds contain varying amounts of siltsizedmaterial. Interpretationsconcerningthe
remainder of the formation, however, and
thoseregardingoverall textural trendsare not
significantlyaf fected.
Unit descriPtlons
Wood and Northrop (1946)correlatedTriassicstrata in the Nacimiento Mountains with
the Chinle Formation (Upper Triassic)in Arizona,namedby Gregory(1917).The Chinleis
the only representativeof the Triassic in
nortl-central New Mexico. Vertebrate remains (Reesideand others, 1957) and plant
fossils (Daugherty, l94l) found in Chinle
$trata of the Nacimiento Mountains provide
paleontologicevidencefor Upper Triassicage.
The Chinle Formation in the eastern San
Juan Basin has been divided into four lithostratigraphic units (Wood and Northrop,
1946),listed in ascendingorder: Agua Zarca
Sandstone Member, Salitral Shale Tongue,
Poleo SandstoneLentil, and tlle upper shale
member.In the presentstudy the Poleo Sandstone Lentil is further divided into upper and
lower portions.
The Agua Zarca Sandstone Member, ftrst
describedby Wood and Northrop (1946)'
crops out in a northeast-southwest-trending
belt, thickest (approximately30 m) near San
Miguel Canyon(fi5,2, sectionl). Wherepresent,it is a prominentridgeformer. The Agua
Z,arcais thin or absent on the French MesaArchuleta arch and in the southeasternChama
Basin(fig. 3, sections6, 7, 8). Erosionalremnants of thick channels are present in the
ChamaBasin.
south-central
The Salitrol Shole Tongue, also first
describedby Wood and Northrop (1946),oc-
depositionalenergy form the basis for a discussionof the tectonicsignificanceof the unit.
Methods
Paleocurrent measurements were taken
from small fless than 4 cm), medium (4-15
cm), and large (more than 15 cm) crossbeds.
Data weregroupedaccordingto the sizeof the
crossbeddingand were statistically analyzed
using the von Misesdistribution(Till, 1974).
Though relatively few measurementswere
analyzed, each of these directions represents
an average for a set of several tens of individual crossbeds.The mean vector directions are thus reasonableestimatesof paleotransportdirection and paleoslope.
Sediment texture was analyzed by settling
tube. Thirty rock sampleswere disaggregated
by dissolution of carbonatecementin hydrochloric acid or by pounding. This techniqueis
suitable only for an approximation of mean
grain size and particle-sizedistribution but is
adequate for examining general textural
trendswithin a given unit or grosschangesbetween units. A sampling bias is present
becauseonly rocks with a calcareouscement
wereanalyzedand becauseonly the sand-sized
fraction (0.0d to 4.0d) was used.This bias af-
voLcANtcs
UPLIFT
Y#r
oF EAsTERN Sen JunN
FIGURE l-LocertoN
BastN, sgowttrlc MAJoR TEcroNIc
Bnsln-Cgeva
FEATURES,outcrops of Chinle Formation are
delineated and stippled.