Latest Miocene-earliest Pliocene evolution of the ancestral Rio Grande

26 Pages·2016·3.85 MB·English

by  Daniel J. Koning

Latest Miocene-earliest Pliocene evolution of the
ancestral Rio Grande at the Española-San Luis Basin
boundary, northern New Mexico
Daniel J. Koning1, Scott Aby2, V.J.S. Grauch3, Matthew J. Zimmerer1
1 New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology,
801 Leroy Place, Socorro, NM, 87801, USA; [email protected]
2 Muddy Spring Geology, P.O. Box 488, Dixon, NM 87527, USA
3 U.S. Geological Survey, MS 964, Federal Center, Denver, CO, 80225, USA
Abstract playa-lake systems (Mack et al., 1997 and 2006; Connell
et al., 2005). This southward expansion resulted in the
We use stratigraphic relations, paleoflow data,
fluvial integration of several previously closed basins
and 40Ar/39Ar dating to interpret net aggradation,
in south-central New Mexico. Previous studies in the
punctuated by at least two minor incisional events,
Socorro and Palomas Basins bracket the age of this
along part of the upper ancestral Rio Grande fluvial
system between 5.5 and 4.5 Ma (in northern New integration between ~6 and 4.6 Ma (Chamberlin, 1999;
Mexico). The studied fluvial deposits, which we Chamberlin et al., 2001; Mack et al., 1998 and 2006;
informally call the Sandlin unit of the Santa Fe Koning et al., 2015, 2016a). Workers have suggested this
Group, overlie a structural high between the San Luis downstream-directed integration occurred because of 1)
and Española Basins. The Sandlin unit was deposited orogenic-enhanced precipitation due to rift-flank uplift
by two merging, west- to southwest-flowing, of mountains in the late Miocene (Chapin and Cather,
ancestral Rio Grande tributaries respectively sourced
1994), 2) fluvial spillover because of reduced subsidence
in the central Taos Mountains and southern Taos
rates in rift basins (Cather et al., 1994; Connell et al.,
Mountains-northeastern Picuris Mountains. The
2005), 3) fluvial spillover due to paleoclimatic changes
river confluence progressively shifted southwestward
affecting water vs. sediment discharges (Kottlowski,
(downstream) with time, and the integrated river
(ancestral Rio Grande) flowed southwards into the 1953; Chapin, 2008; Connell et al., 2012), or 4) epei-
Española Basin to merge with the ancestral Rio rogenic doming and mantle-driven uplift (Kottlowski,
Chama. Just prior to the end of the Miocene, this 1953; Repasch, 2015a, b). Note that this southward
fluvial system was incised in the southern part of the expansion of the Rio Grande occurred prior to the 0.43
study area (resulting in an approximately 4–7 km Ma integration of Lake Alamosa in the northern San Luis
wide paleovalley), and had sufficient competency to Basin (Machette et al., 2013).
transport cobbles and boulders. Sometime between
As a step in understanding the downstream elon-
emplacement of two basalt flows dated at 5.54±
gation of the ancestral Rio Grande, this study explores
0.38 Ma and 4.82±0.20 Ma (groundmass 40Ar/39Ar
episodes of aggradation and incision that occurred in the
ages), this fluvial system deposited 10–12 m of
latest Miocene-earliest Pliocene at the boundary between
sandier sediment (lower Sandlin subunit) preserved
in the northern part of this paleovalley. The fluvial the Española and San Luis Basins (Fig. 1). There, we
system widened between 4.82±0.20 and 4.50±0.07 describe a gravelly sand deposited by two merging Rio
Ma, depositing coarse sand and fine gravel up to 14 Grande tributaries that drained the southern San Luis
km north of the present-day Rio Grande. This 10–25 Basin. The river below this merger, which we define as
m-thick sediment package (upper Sandlin unit) buried the ancestral Rio Grande due to its geographic similarity
earlier south- to southeast-trending paleovalleys to the modern Rio Grande and coarser texture of its sed-
(500–800 m wide) inferred from aeromagnetic data.
iment compared to underlying (late Miocene) deposits,
Two brief incisional events are recognized. The
flowed southwestward and joined with the ancestral Rio
first was caused by the 4.82±0.20 Ma basalt flow
Chama in the middle to southern Española Basin (Koning
impounding south-flowing paleodrainages, and the
and Aby, 2005; Koning et al., 2016b). This axial river
second occurred shortly after emplacement of a
4.69±0.09 Ma basalt flow in the northern study area. then flowed into the northern Santo Domingo Basin,
Drivers responsible for Sandlin unit aggradation may where a Rio Chama-dominated axial-fluvial system was
include climate-modulated hydrologic factors (i.e., established by 6.96 Ma (Smith et al., 2001; Smith, 2004).
variable sediment supply and water discharge) or a Whether the northern of these tributaries was fluvially
reduction of eastward tilt rates of the southern San connected with the northern San Luis Basin in the latest
Luis Basin half graben. If regional in extent, these Miocene-earliest Pliocene is not known. If the ancestral
phenomena could also have promoted fluvial spillover
Rio Grande only drained the southern San Luis Basin,
that occurred in the southern Albuquerque Basin at
then the Rio Chama (modern drainage area of 8,142 km2;
about 6–5 Ma, resulting in southward expansion of
U.S. Geological Survey, 2009) would have been the larger
the Rio Grande to southern New Mexico.
tributary at this time. Note that we use the term “river”
Introduction in a broad sense to convey a geographically large fluvial
system (e.g., having a drainage area >500 km2) that may
An intriguing event in the history of the Rio Grande rift,
be ephemeral, consistent with modern geographic usage
occurring in the latest Miocene or earliest Pliocene, was
in New Mexico of “river” and its Spanish variant, “rio.”
a remarkable downstream elongation of the Rio Grande
The studied fluvial sediment is locally underlain and
from the Albuquerque Basin southward to southern
overlain by four basalt flows whose 40Ar/39Ar ages (from
New Mexico, where it alternately avulsed into several
groundmass) bracket the unit between 5.5–4.5 Ma.
24 New Mexico Geology May 2016, Volume 38, Number 2
We synthesize these ages with stratigraphic relations Geologic Setting
and aeromagnetic data to interpret a basic chronologic
The study area is located on the western margin of the
sequence of latest Miocene erosion, net aggradation
southernmost Taos Plateau, overlying a structural high
between 5.5 and 4.5 Ma punctuated by episodic inci-
between the San Luis and Española Basins of the northern
sion, followed by erosion. Complexities in this sequence
Rio Grande rift (Fig. 1). This structural high is manifested
of events are described below. Finally, we briefly discuss
by a northwest-trending Bouguer gravity high (Cordell,
the implications of this fluvial history to the concomitant
1979; Manley, 1979a; Ferguson et al., 1995; Koning et al.,
downstream elongation of the Rio Grande.
N 106°0'0"W 105°45'0"W 105°30'0"W
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deposits Ta Xpc
Qlc Landslides and colluvium Xg Xg Ta Latir XvTm
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Qp Piedmont alluvial deposits T Xvm Tbom s Pl Ta Tv io G fi eld XXgvmos Til
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TTTTblpoppmr PBLPoiaecssñu aarPil stiTin cafo onrsrokm cF Rkaoshtri moy(Oonal lti(itigOoeon l(i- gOM(oOlii-oglMigcoieo-oMn-cMeieo)inoceec)ennee)) 36°15'0"N QEp spao jCaleitenñQBQppMQlc CTAbpLQMlc ELFQTTlcsbfp PXXXpgipsXscsuXvrmisXTqs TlMp XtgQnaXTXvsXsfgfps Xg RIFTQp COMaLrOaepaR ADsO
VPol Yg DE
TTvill LrLrooaaccttkikirrs s vv (o(oOOllcclliaiaggnnooii-c-cMM ffiiieioeolcldcdee inennextert))urussivivee Rio ChamaTsBf MQTabQpRipo Granade BaTsfsinTsf Tsf QTsQa RIOGMRANEXICO MTNEEXEXIWCAOS
Ritito Conglomerate and El Rito
Trer 0 5 10 20 30 mi
Fm (Oligocene and Eocene)
Paleozoic 0 10 20 30 40 km
Permian and Pennsylvanian, Paleoproterozoic
undivided
m Madera Group Xg Granitic plutonic rocks Xvf Rhyolite and felsic volcanic schist
M Mississippian rocks, undivided Xps Pelitic schist, includes Pilar Phyllite Xpc Calc-alkaline plutonic rocks
Highway Taos-1 Ccolausntt site Xq Quartzite Xvm Msuabfoicr dmineatatev ofelclsainci cm reotcakvso lwcaitnhi c rocks
Fault Contact Xs Metasedimentary rocks
Figure 1. Geologic map of the southern San Luis Basin and northern Española Basin. Study area indicated by the black, rectangular box. The location of the inter-basin structural
high is shaded a light bluish-green color bounded by a dashed red line in the lower left of the figure; this area coincides with positive isostatic residual gravity values from Grauch
and Keller (2004). Site abbreviations: BM=Black Mesa; CA=Cerro Azul; MV=Mesa Vibora; ELF=Embudo local fauna site of Aby and Morgan (2011); LM=La Mesita; OC=Ojo
Caliente; SA=Sandlin Arroyo; VP=Vallito Peak. Usage of Servilleta Basalt follows terminology of Lipman and Mehnert (1979), Abiquiu Formation and Ritito Conglomerate is used
as defined by Maldonado and Kelley (2009), and usage of Picuris formation follows Rehder (1986) and Aby et al. (2004). Geologic map is modified from Koning and Mansell
(2011), who in turn updated the 1:500,000 geologic map of New Mexico (NMBGMR, 2003).
May 2016, Volume 38, Number 2 New Mexico Geology 25
2004a) and local outcrops of Proterozoic bedrock (Kelley, Our study describes scattered exposures along a
1978; Koning et al., 2007a). Bounding the southeast side 12-km long escarpment, south of U.S. Highway 285, that
of this structural high is the left-lateral Embudo fault, coincides with the west side of the southern Taos Plateau.
which transfers extensional strain between the San Luis A prominent landmark in the study area is Cerro Azul,
and Española Basins (Muehlberger, 1978, 1979; Kelson a steep-sided, 170-m tall hill that abuts the southwestern
et al., 2004; Koning et al., 2004a). A component of end of the Taos Plateau. The top of the Taos Plateau rises
northwest-down throw along the Embudo fault permitted 30–60 m above gentle, rolling topography in the northern
minor sediment accumulation on the structural high adja- Española Basin to the west. Immediately south of the
cent to the fault. study area, the Embudo local fauna (ELF) site lies at the
The Taos Plateau occupies the southern San Luis Basin south end of the Taos Plateau (Fig. 1). There, a Blancan
(Fig. 1) and is underlain by a succession of more than 210 (4.9–1.6 Ma) fossil assemblage was found at the top of a
m of Pliocene-age fluvial sediment and interlayered basalt 12–15 m thick, fining-upward interval of sand and gravel
flows. The southern San Luis Basin coincides with an that unconformably overlies the Ojo Caliente Sandstone
east-tilted graben, whose master fault system (the Sangre (Aby and Morgan, 2011).
de Cristo fault) has tilted Pliocene strata gently to the
east and south (Bauer and Kelson, 2004). The package of Methods
Pliocene basalt and sediment has commonly been referred
to as the Servilleta Formation (Butler, 1946, 1971; Descriptions and differentiation of distinctive Santa Fe
Montgomery, 1953; Lambert, 1966; Galusha and Blick, units were conducted as part of geologic mapping (Koning
1971; Drakos et al., 2004), but some workers prefer to use and Aby, 2003; Koning et al., 2007a). In addition, three
separate names for the basalts vs. sediment (e.g., Lipman stratigraphic sections were measured. Gravel counts and
and Mehnert, 1979; Dungan et al., 1984). In this paper, paleoflow measurements were made at the stratigraphic
we follow the latter convention and use Servilleta Basalt sections and elsewhere in the mapped area. Paleoflow
(Lipman and Mehnert, 1979). The Servilleta Basalt is part directions were determined by measuring clast imbrica-
of the Taos Plateau volcanic field, which also includes tion (n=129) and channel-margin trends (n=3).
minor andesite, basaltic andesite, and dacite flows (Ozima High-resolution aeromagnetic data were interpreted
et al., 1967; Lipman and Mehnert, 1979; Thompson and after application of a reduction-to-pole transformation
McMillan, 1992). (Bankey et al., 2007) that corrects for shifts of anomalies
Numerous studies have investigated the geochronol- away from the centers of their magnetic sources. The
ogy of the basaltic rocks of the Taos Plateau volcanic approach to interpreting remanent magnetization of
field. Reported ages of the basalts range from 4.8 to 2.5 volcanic rocks from aeromagnetic data follows that of
Ma (Ozima et al., 1967; Lipman and Mehnert, 1979; Grauch et al. (2004, 2006) and Grauch and Keller (2004).
Baldridge et al., 1980; Appelt, 1998). Within this time Basaltic groundmass from three lava flows and one
span, Dungan et al. (1984) interpret three short-lived crystallized lava lake were dated using the 40Ar/39Ar
episodes of volcanism (compared to inter-eruptive time technique. Sample preparation included crushing, sieving,
intervals) based on three major geochemical varieties washing away clay-sized material in deionized water, dis-
of basalts separated by gravelly sediment. But 40Ar/39Ar solving carbonate with hydrochloric acid, and removing
dating of 86 lava samples (using primarily groundmass) phenocrysts using a Frantz magnetic separator and optical
indicates a relatively regular temporal spacing of basaltic picking under a binocular microscope. The groundmass
volcanism (Appelt et al., 1998). Repasch et al. (2015a, b) separates and the 28.201 Ma Fish Canyon interlaboratory
present a 4.5 Ma basalt age from Black Mesa, which they standard (Kuiper et al., 2008) were loaded into aluminum
use to interpret the presence of the ancestral Rio Grande discs and irradiated at the US Geological Survey TRIGA
by that time. reactor in Denver. Analyses were completed at the New
In the southern San Luis Basin, pre-Servilleta Basalt Mexico Geochronology Research Laboratory. Samples
basin-fill stratigraphy has been studied using data from were incrementally heated and released gases were
sparse, deep water-supply wells (e.g., Drakos et al., 2004). cleaned by a getter pump in an all-metal, fully automated
Correlative basin-fill strata to the south have received extraction line. Isotopic ratios were measured using a MAP
much study in the well-exposed Española Basin, which 215-50 mass spectrometer. More information regarding
serves as the type area for the Santa Fe Group (Spiegel and the 40Ar/39Ar technique can be found in McIntosh et al.
Baldwin, 1963; Galusha and Blick, 1971). In the northern (2003). Data tables and additional analytical parameters
Española Basin, the upper Santa Fe Group consists of a are in Appendix 1 (Data Repository 20160002).
fluviatile package called the Chamita Formation overlying http://geoinfo.nmt.edu/repository/index.cfml?rid=20160002
an eolian unit called the Ojo Caliente Sandstone Member
of the Tesuque Formation. The Vallito Member of the Lithologic units
Chamita Formation (defined by Koning and Aby, 2005),
is particularly relevant for our study and interpreted to
Stratigraphic nomenclature
represent deposition from a sandy, bedload-dominated
fluvial system draining the southern San Luis Basin during At least three informal names have been applied to
late Miocene time (Koning and Aby, 2005). Previous Pliocene-Pleistocene clastic deposits elsewhere in the
mapping by several workers illustrate the stratigraphy and San Luis Basin: the Lama formation (Pazzaglia and
structure of the Santa Fe Group within 20 km south and Wells, 1990), Blueberry Hill deposit (Bauer and Kelson,
southwest of the study area (Manley, 1976, 1979b; May, 1997; Kelson et al., 2001; Bauer et al., 2004), and the
1980; Steinpress, 1981; Koning and Aby, 2003; Koning Servilleta Formation (Butler, 1946, 1971; Montgomery,
and Manley, 2003; Koning, 2004; Koning et al., 2004b). 1953; Lambert, 1966). Until the Pliocene sediment in the
26 New Mexico Geology May 2016, Volume 38, Number 2
418500 419000 419500 420000
Xoq Qe Qe\QTae-Ts
Qa Valley-floor sandy alluvium (Holocene) Qe/Ts
Tsu
Qao Older sand and gravel alluvium (Pleistocene) Cerro Azul
38 Qe
29
Qe Eolian sand (Holocene - upper Pleistocene) 30 Tsui GTM-756
Eolian sand overlyingolderunits: 32Tsl A-68NCSS 00
EolianQ sea/nQdT aaen-dT sslopewashQ oev/Tesrlyingolder Qunei/tTss:bcau Qct GTM-752a GMTMC-7S5S2bTsbcau 40152
Qes/Qao Qes/QTae Qes/Tto Qa Qe
Qes/QTae-Ts Qes/Tcv Qes/Tsbcau
Qct/Tsl
Qct Colluvium & talus (Holocene - upper Pleistocene)
Colluvium and basaltic talus overlying units: Qes/Qao QTae
Qct/Tsl Qct/Tto Qes/Tto
0
0
# # # Qls Landslide deposit (Pleistocene) Qa Qa 44
# # # 1
0
# # # 4
QTae Post-Sandlin alluvium and eolian sediment Qa
Qe/Tsbcau
Ts Sandlin sedimentary unit (Pliocene)
Tsu Upper unit, undivided
Tsun Northern petrofacies J54B Qa
A-58
Qes/Tto
Tsus Southern petrofacies SCSS 0
0
6
Tsui Basal inset sandy gravel 418000 Qa 13
J46f 0
Tsl Lower unit J54C QTae Qa Qe/Ts 4
Tcv (Cuhpapmeri t-am Fidmdl,e V Maliloitoce Mnee)mber Qa A-52A-54
417500 Tsbcal
Tesuque Fm, Ojo Caliente Tsl A’
Tto TJ-7
Sandstone Member
(upper -m iddle Miocene) Qe
Qa
0
0
417000 8
Tsui 2
1
0
4
Qe
Qa
e
n
416500 n li Qa
Qe/Tsctio Qe 1:24,000
e
Qa s s
s Tsbelf
Cro Qct/Tto P00aleoflow direction from:
0
Qes/Tto 2
01 channel trend
4
imbrication
Embudo local
fauna site Servilleta Basalt (Pliocene)
Tsbelf Embudo local fauna site basalt flow
A Tsbcal Tsbelf TsbscrSouth Comanche Rim basalt flow
0 Tsbcau Upper Cerro Azul basalt flow
Qls 20
1 TsbcalLower Cerro Azul basalt flow
1
0
4Proterozoic basement Basalt geochronologic sample
QTae? Qls Xoq Ortega J54C Clast count site
Quartzite J46f
0 1,000 2,000 4,000 6,000 8,000Feet UTM coord, NAD 83, zone 13
0 250 500 1,000 1,500 2,000Meters
Figure 2. Geologic map of the southern half of the study area. Contour interval is 20 feet. Paleoflow data are from Appendix 6 and Koning et al. (2007a).
The thick black line depicts the location of the vertically exaggerated cross section of A-A’’’ (Figure 4). SCSS, MCSS, and NCSS = south, middle and
north Cerro Azul stratigraphic sections, respectively. Usage of Vallito Member (Chamita Formation) follows definition of Koning and Aby (2005); usage of
Servilleta Basalt follows terminology of Lipman and Mehnert (1979).
May 2016, Volume 38, Number 2 New Mexico Geology 27
419000 419500 420000 420500 419000 419500 420000 420500
Qa Qes/Tcv
Tsun
Qes/QTae
Tsus U
Qes/Tto
.S QTae
Tsus . H
0
0 w 0
Tsus 00 y A’’’ 35
Qa Qe/QTae-Ts 4019 28 402
QTae 5
Qa
Qes/Tcv
0
Qes/Tcv 00 00
Tsus 5 3
8 2
1 0
0 Tsbscr 4
4
J82 Tsun
Qa Qct
Qes/Tto Qes/Tto J54E
Qa 0
Tsus 0 0
0 5
0 2
18 C 02
Qes/Tto 40 Tsun J1 ro 4
Qes/Tcv s
s
Qa Qe/QTae-Ts J78B s
Qes/QTae e 0
0 c 0
Qa Qes/Tcv 1750 Qa tio 0220
0 n 4
4

Qa Tsun Qes/QTae-Ts lin
Qes/Tto Qa e
J84
0
J44D 0 0
0 5
Tsus Tsun 70 Qes/Tcv 21
Qa Qe 01 40
4
Tto
Tcv Qes/QTae
01
Qa Tto Tcv A’’ Tcv J19 GTM-5 0
Tcv 500 Qes/ 100
Tto Tsus 016 402
4 Tsun
Xoq
Tsu
Qe/QTae-Ts
0
00 Qa Qes/Tcv 50
0 0
6 2
1 0
0 4
Qe 4
Qes/QTae
Cerro AzulTsu Qe Qe/Ts 00 J72 Tsus 000
30 TsuiGTM-756 0155 J16J 4020
Xoq 32 ne 4 Qes/QTae-Ts
Tsl A-68NCSS n li
MCSS o 02
GTM-752aGQTeM-752b ecti 000 Qa 9500
s 5 1
Qa s 01 40
Qct/Tsl Tsbcau os 4 Tsus
r
C Tsus
Qes/Tto QTae Qa Qes/QTae
0
0 Tsun 0
Qa Qe/Ts 50 90
14 Qa Qa Qes/Tcv 01
0 4
4
UTM coord, NAD 83, zone 13
0Figure 31. G,0eo0lo0gic ma2p, 0of0 th0e middle and north4er,n0 p0a0rts of the study area6 (,l0ef0t a0nd right panels, res8pe,0ct0iv0elyF).e Seete Figure 2 for explanation of 1ma:p2 u4nit,s0. C0on0tour
interval is 20 feet. The thick black line depicts the location of the vertically exaggerated cross section of A-A’’’ (Figure 4).
0 250 500 1,000 1,500 2,000Meters
28 New Mexico Geology May 2016, Volume 38, Number 2
A A' Blue lines = base of Tsus A''
immediately NE of Cerro
Elev (m) Azul projected onto the line Elev (ft)
South of section. Lower line uses North 7,200
a 1° E dip in the projection. QTae Tsun
Inferred elevation range of maximum J54B: 4.82±0.20 Ma Qe 7,100
aggradation of Sandlin unit Tsu
2,150 Qe Tsbcau Tsui Tcv Tsus
Tsbcal Tsbcal Tsui Tsl Note similar elevation 7,000
Tslbg of upper Sandlin unit
2,100 J54C: 5.54±0.38 Ma abnadse S (aTnsdulsin o unn riitg thotp) 6,900
Tto lower Sandlin paleovalley (Tssidl oen o lfe Cfte) rorno Aeizthuel r
Xoq (Xoq) 6,800
Cerro Azul
(projected)
2,050
Tto 6,700
A'' A'''
Undifferentiated
Elev (m) Servilleta Basalt Elev (ft)
South projected in from 770 m North 7,300
Inferred elevation of maximum to the east (see Koning U.S. Highway 285
2,200 aggradation of Sandlin unit et al., 2007a) paleovalley
7,200
Basalt boulder QTae
QTae
Tsun Tsbscr
Tsu 7,100
Tsus
2,150 QTae Site GTM-5 Tcv
Tsun 4.69±0.09 Ma
Cobbly bed with Basalt cobbles 7,000
1–40% Paleozoic and boulders
Tcv sedimentary clasts become more undivided
2,100 c(mororesltalyt e1s– t5o% T)s —us cnoomrthmon to Miocene strata 6,900
Tto 6,800
Tto
2,050
6,700
10x vertically exaggerated 0 2,000 5,000Feet
0 500 1,000 1,500 2,000Meters
Figure 4. Vertically exaggerated, north-south cross-section showing stratigraphic relations in study area. Cross-section line depicted as A-A’’’ in
Figures 2–3. In A’’-A’’’, all lithologic contacts are projected onto the line of section using a 1°E apparent dip. See Figure 2 for explanation of unit labels.
southern San Luis Basin is formally assigned a name with Member along the escarpment has sparse very fine to
an appropriate type section, we informally and provi- medium pebbles composed of rhyolitic lavas and tuffs,
sionally refer to the predominantly Pliocene deposits in greenish Paleozoic sedimentary clasts, dacite, quartzite,
the study area as the Sandlin unit of the Santa Fe Group, and granite (most to least abundant). The tan, cross-strat-
named after a nearby drainage to the west (Fig. 1). ified sandstone of the Tesuque Formation corresponds to
The Sandlin unit can be divided into upper and the Ojo Caliente Sandstone Member (Galusha and Blick,
lower subunits based on the elevation of the deposit and 1971; May, 1980). Detailed descriptions of these strata
stratigraphic position relative to the upper basalt flow are given in Appendix 2.
near Cerro Azul (Figs. 2–4), which we informally call the We correlate three stratigraphically distinctive
upper Cerro Azul flow and describe below. The elevation mafic flows in the study area to the Servilleta Basalt
of the lower Sandlin subunit progressively decreases from (Figs. 2–4). All three flows contain olivine phenocrysts,
7,030–7,080 ft (2,143–2,158 m), at the southeast corner of supporting a field-based designation of “basalt.” The
Cerro Azul, southwards to 6,960–7,020 ft (2,121–2,140 northern basalt underlies the northernmost Sandlin unit
m) at the southern end of Taos Plateau; it is capped by near U.S. Highway 285 and continues northward along
the upper Cerro Azul flow. The elevation range of the the Comanche Rim. For the purposes of this study, we
exposed upper Sandlin subunit increases in an irregular informally call it the south Comanche Rim flow (Tsbscr).
fashion from 7,080–7,180 ft (2,158–2,188 m), at Cerro The other two basalts are found south of Cerro Azul.
Azul, northwards to 7,270–7,310 ft (2,216–2,228 m) These are informally referred to as the lower and upper
near U.S. Highway 285; it is not overlain by the upper Cerro Azul basalt flows (Tsbcal and Tsbcau) based on
Cerro Azul flow. Within the upper Sandlin subunit, two their proximity to Cerro Azul.
gravel-based petrofacies are differentiated (Figs. 2–4) and
described below. Key gravel types
The Chamita and Tesuque Formations (Santa Fe
We found gravel composition to be very useful in interpret-
Group) underlie the Sandlin unit (Figs. 2–4). The sandy
ing provenance. To match clast types to potential source
sediment of the Chamita Formation is correlated to the
areas, we studied previous geologic mapping of highlands
Vallito Member based on lithologic similarity (Koning
surrounding the southern San Luis Basin and conducted
and Aby, 2005)—particularly its predominant fine- to
gravel counts of near-source Pliocene deposits deemed
medium-grained sand texture, common slightly orangish
representative of three general source areas (Table 1; Fig.
color, and clast composition—but exposure does not
5): Arroyo Hondo (representing the north-central Picuris
permit direct physical correlation. Locally, the Vallito
May 2016, Volume 38, Number 2 New Mexico Geology 29
SOURCES
Central Taos Mountains Eastern Picuris Mountains & Northern Picuris Mountains
Southern Taos Mountains
Northerly sources Southerly sources
T-32
Taos-3 1 T-31 T30b
Taos-2 1 Taos-1 1 T30a
Dunn Bridge Rio Pueblo de Taos Mouth of Arroyo Hondo
STUDY AREA
J19 Increasing influence of southerly sources
J1
Northern petrofacies, upper Sandlin 0 50 100%
subunit (Tsun)
TJ-7 1
GTM-752b
GTM-756 1
Inset gravel at base of upper Sandlin
subunit (Tsui) J44D
1 J84
Site Names
J#, A-#, T-#, Taos-#, GTM-# J82
J16J
Quartzite
Tertiary intermediate volcanic J72
Tertiary felsic volcanic J78B
Undivided volcanic Southern petrofacies, upper Sandlin subunit (Tsus)
Paleozoic sedimentary
A-54
Pilar Phyllite
4 A-58
Granitic
tr GTM-752a
Servilleta Basalt
Proterozoic (does not include Phyllite) 5 A-68
Lower Sandlin subunit (Tsl)
Other
A-52
Quartz-porphyry marker clast
not differentiated but seen in nearby outcrops J46f
Lower Sandlin basal gravel (Tslbg)
1 % gabbro-intermediate intrusives in Proterozoic
Figure 5. Bar graphs illustrating the results of clast counts near potential source areas (top) and in the study area (bottom). Graphs
are grouped according to subunit (following Table 2). Horizontal placement of the subunits reflects inferred relative contributions
from the source areas listed above. Note that northerly source areas in the study area also include gravel recycling from the Tusas
Mountains–Taos Plateau border region (not reflected in the near-source clast counts).
Mountains; site T30), Rio Pueblo de Taos (representing volcaniclastic sandstone or white-tan, subrounded
the northeastern Picuris Mountains and southern Taos sandstone of the Ojo Caliente Sandstone Member of
Mountains; sites Taos-1, T31, and T32), and near Dunn the Tesuque Formation.
Bridge in the Rio Grande Gorge (representing the central Two other useful clast types for determining
Taos Mountains; sites Taos-2 and Taos-3). provenance include the Pilar Phyllite and a quartz-
Four gravel types are particularly important for bearing porphyry that we informally call the “quartz-
identifying basin-fill provenance (Figs. 5–6). The first porphyry marker clast.” The Pilar Phyllite (Fig. 6B) is
gravel type consists of greenish-brownish sedimentary a gray-black, carbonaceous, quartz-muscovite phyllite
rocks composed of sandstone, siltstone, and limestone, with slaty cleavage. It is restricted to the Picuris
the latter being much less abundant than the other Mountains (unit Xps of Fig. 1; Montgomery, 1953;
types (Fig. 6A) Bauer, 1993; Bauer and Kelson, 1997; Kelson and
These lithologies match quartzose to feldspathic Bauer, 1998), consistent with being observed in the
Paleozoic sedimentary rocks restricted to the southern Arroyo Hondo near-source clast count sites but not
Taos Mountains, east and southeast of Taos (unit those farther to the north (Table 1).
PP of Fig. 1; Montgomery, 1953; Bauer, 1993; Bauer The quartz-porphyry marker clast is a durable rhy-
and Kelson, 1997; Kelson and Bauer, 1998; Bauer et olite that commonly becomes slightly polished during
al., 2000; Kelson et al., 2001); hereafter we refer to transport (Fig. 6C–E). Its color ranges from light
this gravel suite as “Paleozoic sedimentary clasts.” gray, light yellow, or light greenish yellow. The clast’s
These clasts differ from reworked Santa Fe Group phenocryst assemblage is dominated by quartz and
sandstone clasts, which typically are composed of either contains ≤1% sanidine and ≤1% mafic minerals. The
30 New Mexico Geology May 2016, Volume 38, Number 2
A B
C D
Figure 6. Photographs illustrating three useful clasts for determining prove-
nance. A) Paleozoic sedimentary clasts from the southern Taos Mountains,
which are all sandstone in this photograph. B) Pilar Phyllite from the Picuris
Mountains. C-E) The quartz-porphyry marker clast from: C) the lower basal
gravel at site A-52 (Table 2), D) site Taos-1 (Table 1), E) site Taos-2 (Table 1).
together with the quartz-porphyry marker clast; and 2) the
marker clasts generally lack chatoyant sanidine whereas the
white quartz-porphyry in the Peña Tank rhyolite generally
contains 0.5–8% conspicuous chatoyant sanidine 0.5–2.0
mm long (Koning et al., 2007b; Aby et al., 2010; McIntosh
et al., 2011). Although present in the Rio Pueblo de Taos
clast count data, the quartz-porphyry marker clast is more
abundant near Dunn Bridge (Table 1; Fig. 5)—consistent
with a source in the central or northern Taos Mountains
and difficult to explain, based on geographic relations, if
the source were in the southeastern Tusas Mountains. The
E quartz-porphyry marker clast was not observed in gravel
sourced from the Picuris Mountains (sites T30a and T30b,
quartz-porphyry marker clast is likely derived from a Table 1; Fig. 5).
silicic unit in the Latir volcanic field in the central Taos Gravel composed of gabbroic and intermediate
Mountains (Lipman et al., 1986), but the specific intru- intrusives (inferred to be granodiorite, quartz diorite,
sion, ignimbrite, or flow has not been identified. Quartz or diorite based on hand sample identification) are best
porphyries have not been observed in the Picuris Mountains correlated with the central Taos Mountains. Gabbroic
or southern Taos Mountains. The quartz-porphyry marker and intermediate intrusives in the central Taos Mountains
clast cannot be derived from the white quartz-porphyry include: granodiorite of Jaracito Canyon, tonalite of Red
observed in the Peña Tank Rhyolite (unit Tpr, Fig. 1) in River, and undivided mafic-ultramafic rocks (Lipman
the southeastern Tusas Mountains because: 1) the red-gray and Reed, 1989); note that these are included in unit Xpc
lithologic types of the Peña Tank Rhyolite are not observed on Fig. 1. In near-source clast count data, slightly more
May 2016, Volume 38, Number 2 New Mexico Geology 31
TABLE 1. Clast count data from Pliocene sedimentary deposits east-northeast of study area
Site: T30a T30b Taos-1 T-31 T-32 Taos-2 Taos-3
Mouth of Mouth of Rio Pueblo de Rio Pueblo de Rio Pueblo de Rio Grande at Rio Grande at
Location: Arroyo Hondo Arroyo Hondo Taos Taos Taos Dunn Bridge Dunn Bridge
North-central North-central S Taos Mtns, S Taos Mtns, S Taos Mtns, Central Taos Central Taos
Provenance: Picuris Mtns Picuris Mtns NE Picuris NE Picuris NE Picuris Mtns Mtns
Mtns Mtns Mtns
Quartzite 59% 57% 14% 25% 17% 3% 1%
Tertiary
intermediate
volcanic* 8% 13% 9%
Tertiary felsic
volcanic* 16% 16% 21%
Volcanic
(undivided)] 27% 11% 24% 58% 51% 29% 4%
Paleozic
sedimentary 0% 0% 19% 16% 17% 0% 0%
Vein quartz 0% 0% 3% 0% 0% 1% 2%
Proterozoic
amphibolite 0% 0% 0% 0% 0% 1% 1%
Pilar Phyllite 11% 31% 0% 0% 0% 0% 0%
Non-foliated
granitic** 0% 0% 16% 40% 39%
Foliated
granitic** 0% 0% 19% 16% 8%
Granite
(undivided) 0% 0% 35% 0% 15% 67% 47%
Servilleta
Basalt 0% 0% 1% 0% 0% 0% 0%
Proterozoic
schist 3% 1% 0% 1% 0% 0% 0%
Gabbro-
intermediate
intrusives 0% 0% 1%*** 0% 0% 1% 1%
Mylonite 0% 0% 1% 4% 2%
Other 0% 0% 2% 0% 0% 0% 3%
Quartz-
porphyry* 0% 0% 1% 4% 7%
Comments: Other = Other =
chlorite-rich 1 epidote-rich
intrusive, intrusive;
epidote-rich 2 Fe-rich rocks
intrusive
Easting^ 432949 432969 435670 435291 435779 436773 436316
Northing^ 4019449 4019454 4022633 4022399 4022643 4043635 4043401
Note: * This clast type was subsumed into "Tertiary volcanics (undivided)" where shaded.
** This clast type was subsumed into "Granite (undivided)" where shaded.
*** The gabbro may possibly be amphibolite.
^ UTM coordinates in meters; datum is NAD83 (zone 13).
gabbro-diorite is seen near Dunn Bridge than the Rio likely the Latir volcanic field, but erosion and fluvial
Pueblo de Taos, and none are observed at Arroyo Hondo activity in the late Oligocene through Miocene has
(Table 1; Fig. 5). transported the volcanic gravel to the west and south.
Other gravel types cannot be attributed to a single- Granitoids, quartzites, vein quartz, and amphibolites are
source locality. For example, volcanic gravel is found in found in both the Picuris Mountains and central Taos
the central Taos Mountains, Picuris Mountains, and the Mountains (Fig. 1; Montgomery, 1953; Lipman and Reed,
Tusas Mountains–Taos Plateau border region (i.e., the 1989; Bauer, 1993). Granitic rocks are also found in the
Cordito Member of the Los Pinos Formation; Manley, Tusas Mountains–Taos Plateau border region (e.g., Tres
1981). The original eruptive center for this gravel was Piedras granite of Just (1937) and Barker (1958)).
32 New Mexico Geology May 2016, Volume 38, Number 2
Stratigraphic relations Three stratigraphic relations in the 10–25 m thick upper
Sandlin subunit are noteworthy. One, in cross-section A-A’’’
Figures 2 and 3 present an updated version of previous
east of Cerro Azul, the basal contact of the upper Sandlin
geologic mapping (i.e., Koning and Aby, 2003; Koning
subunit projects to a similar elevation as the upper contact of
et al., 2007a). A north-south, vertically exaggerated
the lower Sandlin subunit (Fig. 4). Two, the northern petrofacies
cross-section illustrates stratigraphic relations found in
of the Sandlin subunit progrades southward over the southern
the study area (Fig. 4), as do three stratigraphic sections in
petrofacies north of Cerro Azul. Three, in the north part
the southern study area (Fig. 7, Appendix 3) whose loca-
of our study area, in the vicinity of U.S. Highway 285, the
tions are depicted in Figure 2. Sandlin strata dip gently
upper Sandlin subunit thickens in a 700-m wide paleovalley
east-southeast (≤1.0°) north of Cerro Azul and appear to
we informally name the U.S. Highway 285 paleovalley (Fig.
be subhorizontal to the south, based on kilometer-scale
4). Locally, the basal 1–2 m of the upper Sandlin subunit
cross-section relations and attitudes measured primarily by
in this paleovalley is cobbly and contains 1–40% Paleozoic
three-point procedures from the geologic map (Figs. 2–4).
sedimentary clasts as well as minor basalt boulders (Appendix
Bedding attitudes of the underlying Chamita Formation
4). The basal gravel of the U.S. Highway 285 paleovalley is
are subhorizontal or dip up to 2° E. The basal contact
inset ~8 m beneath the base of the south Comanche Rim flow
of the Sandlin unit is scoured, where exposed, and map
(Tsbscr). On the north side of the paleovalley, stratigraphically
and cross-section relations suggest irregular relief over
higher strata of the upper Sandlin subunit onlaps the south
102-103 m horizontal distances, consistent with erosional
Comanche Rim basalt flow (Fig. 4).
topography (Figs. 2–4).
south middle north
clay- sand peb- cob- clay- sand peb- cob- clay- sand peb- cob-
silt vffmcvcbles bles silt vffmcvc bles bles silt vffmcvcbles bles
meters uSATpoer-prpAve arirl tl ae C1gte8aer mrBoofa aAs4na.z8dlut2 ln ( ±Tfol so0tb w.s2ch 0aoo ufMw )t ahn–.e. 1meters5 uSTopepprv eairltl eC1te8ar. 1rBo am As aazlunt ld( Tf lnsoobwtc saohuf o)thwen meters Bsmsatairmdsadateillg etl ro maCcpeeaharliristcoyu sAraeeszdcu t tlahi ote n
13.1 m
thickness not constrained
Basal part of upper Sandlin
subunit, inset into upper
Cerro Azul basalt flow
5
10
Poorly exposed
MCSS-6
10
SCSS-5
Lower Sandlin subunit (Tsl)
MCSS-5
NCSS-4
MCSS-4
NCSS-3
MCSS-3 NCSS-2
SCSS-4 Cemented pebbly sandstone 0
5 SCSS-3: Burrowed sandstone NCSS-1
SCSS-2: Cemented sandstone
MCSS-2
5 SCSS-5 = strat section unit Servilleta Basalt
upper Cerro Azul flow
SCSS-1
MCSS-1 Gravel lower Cerro Azul flow
lower Cerro Azul flow of the Pebbly sand Inset basal gravel,
Servilleta Basalt (Tsbcal) –
upper Sandlin subunit
Ar-Ar age of 5.54 ± 0.38 Ma. Vallito Member of Chamita
Formation (Tcv) Cross-stratified
Lower Sandlin
pebbly sand
0 subunit
Ojo Caliente Member of
Tesuque Formation (Tto) Sand Vallito Member of
0 Chamita Fm
Figure 7. Cerro Azul stratigraphic sections. Locations depicted in Figures 2 and 3. Thicknesses of the south Cerro Azul section and pre-basaltic strata of the
north Cerro Azul section were measured using eye height and a Brunton compass, but subunit thicknesses of the exposed lower Sandlin unit in the north
Cerro Azul section were double-checked with a ruler. The middle Cerro Azul section was measured using an Abney level and Jacob’s staff.
Immediately east of Cerro Azul, a volcanic-rich gravel
Stratigraphic relations between the lower Sandlin subunit
appears to conformably underlie the bulk of the upper Sandlin
and Servilleta Basalt flows are important to our interpretations
subunit (Tsui, Figs. 3–4). Southeast of Cerro Azul, this gravel
(Figs 2, 4, 6). Two exposures reveal 1.0–1.5 m of cobble- and
is inset ~5 m below the base of the upper Cerro Azul basalt
boulder-bearing gravel beneath the lower Cerro Azul flow,
flow. This inset relation is demonstrated by meter-scale
which we call the basal gravel of the lower Sandlin subunit
boulders of basalt associated with this gravel, one of which
(Fig. 8). Located between the lower and upper Cerro Azul
is clearly part of a cemented fluvial deposit (Fig. 9). These
flows is a 10–12 m-thick interval of sand with minor gravel
boulders match the olivine-phyric basalt of the upper Cerro
that we assign to the lower Sandlin subunit (Fig. 7).
May 2016, Volume 38, Number 2 New Mexico Geology 33