16.10 MB Geologic Map of the Sierra Bacha, Coastal Sonora, Mexico

GEOLOGIC MAP OF THE SIERRA BACHA, COASTAL SONORA, MEXICO
112°30'0"W
0.5
4
1
5 KILOMETERS
3 MILES
2
CONTOUR INTERVAL: 20 METERS
Nor th
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M
30°N
1
20°N
e
x
ic
o
360000
358000
364000
19
200
374000
376000
112°15'0"W
378000
3284000
29°0'0"N
3282000 29°40'0"N
¹
¹
D
D
D
D
D
¢
3274000
0
29°35'0"N
D
D
¤
D
D
D
¢
¢
3270000
D
D
Ta1
@
30
i
400
0
PO
i
ZO
@
@
0
73
CO
YO
TE
FA
@
UL
QTg
Kt
T
Â
Mzc
approximate strike and
dip direction
Tba
Tc2
horizontal bedding
eutaxitic foliation
vertical eutaxitic foliation
metamorphic foliation
Ttd
vertical metamorphic
foliation
strike and dip of fault
vertical fault
strike and dip of minor fault; arrow shows
trend of kinematic indicator and direction
of hanging wall transport
Tb3
Tr2
joint
@
¢S
Tc2
Tc2
Kgd
358000
362000
360000
364000
112°25'0"W
366000
368000
370000
372000
112°20'0"W
na
Tc3
374000
TUFF OF DESEMBOQUE - Pink to white, welded rhyolite ash-flow tuff. The base is characterized by a 0.5- to 1-mthick, discontinuous, porphyritic black vitrophyre overlain by a 5-m-thick pink, ashy crystal-rich zone; both of these
ones contain abundant euhedral phenocrysts (plagioclase > quartz > biotite > alkali feldspar). The base grades
upward into a 10- to 15-m-thick, moderately to densely welded zone with abundant 4- to 12-cm-long yellow pumice
fiamé and 10-15% phenocrysts (feldspar > biotite ≈ quartz). The upper 30 m show extensive vapor phase alteration
and abundant quartz-filled spherules in a pink groundmass that decrease in abundance upsection. The uppermost
part of the unit is gray to white and not welded. Gastil and Krummenacher (1977) determined an age of 10.4 ± 0.2 Ma
[K-Ar] for this unit, which they report as an unnamed rhyolite tuff (their sample S2G-114A). Total unit thickness is
40 to 50 m.
Thick sequence of dark purple to black, aphanitic vesicular basalt flows with only up to 1-2% plagioclase phenocrysts
in a glassy groundmass. Total exposed thickness up to 1100 m.
Gray to light purple, aphanitic rhyolite lava flows. Gray lenses with glassy or frothy textures create a locally welldefined flow foliation. Secondary quartz precipitation along foliation is pervasive in some locations. This resistant unit
consists of one or two 60- to 90-m-thick flows. The base of each flow contains a unique pink to red to white ash-fall
tuff below a discontinuous 1- to 3-m-thick, greenish-black vitrophyre. An unpublished age of 11.7 ± 0.2 Ma [Ar/Ar] has
been determined for this unit (A. Iriondo, personal communication, 2011). Total unit thickness is up to 240 m.
paved highway
Black, glassy, plagioclase-phyric basalt flows with up to 10% phenocrysts (plagioclase > orthopyroxene >> amphibole) and cindery pyroclastic breccias up to 5 m thick. Individual flows are typically 10 to 20 m thick. Total unit
thickness is up to 210 m.
Tst
Well-laminated lithic tuff and tuffaceous sandstone with yellow to white to light green pumice and dark volcanic lithics
in a yellow ashy matrix. Individual beds are 30 to 100 cm thick and show normal grading of pumice and lithics. Total
unit thickness is up to 25 m.
10.4 ± 0.2
[K-Ar]
radiometric age (± 2σ error) in Ma;
isotopic system in brackets
-J
Td3
Kt
Ta3
ec
o
Qal
Td2
Kt
@
@
356000
Stratified to massive, pebbly sandstone to sandy pebble conglomerate and interbedded pebble-cobble conglomerate.
Heterolithic clast assemblage includes andesite, dacite, basalt, rhyolite, and granitic and metamorphic basement
lithologies. In the western Cerro Colorado, this unit shows locally well-developed planar stratification and mixed
matrix- and clast-support in 5- to 15-cm-thick, and mostly subangular to subrounded intermediate volcanic clasts with
less common basement clasts and outsized boulders up to 35 cm in diameter. In the Lomas Ona-Jeco (southeastern
map area), this unit is mostly massive with local planar stratification of 20- to 50-cm-thick bedsets defined by
variations in grain size. This unit is interpreted to be pre-kinematic based on its consistent dip with underlying units
throughout the study area. The Tuff of Desemboque (Ttd, 10.4 Ma) is interbedded in the upper part of this unit. Total
unit thickness is up to 350 m.
Tb2
400
Ttd
354000 112°30'0"W
BASALT OF ARIVAIPA - Horizontal to sub-horizontal, 10- to 15-m-thick basalt flows containing plagioclase microphenocrysts in a fine-grained aphanitic groundmass consisting of plagioclase >> olivine ≈ pyroxene ≈ magnetite.
Individual flows contain a 1- to 3-m-thick red to black basal flow breccia, vesicular upper and lower contacts, and
well-defined vertical joints with 0.5 m-spacing. Gastil and Krummenacher (1977) obtained a radiometric age of
6.4 ± 1.9 Ma [K-Ar] for this unit where it dips shallowly to the northeast in the Cerro Prieta above an angular
unconformity with older late Miocene volcanic strata. Maximum exposed thickness is 90 m.
Ta4
29°30'0"N
S
¢
@
Mostly massive, moderately sorted, volcaniclastic, pebble-cobble conglomerate and interbedded pebbly sandstone.
This unit has mixed matrix- and clast-support and rare boulders up to 50 cm in diameter. The clast assemblage is
heterolithic with angular to subrounded volcanic and basement clasts in a grussy, red sand matrix. This unit is
characterized by bedding dips that decrease systematically up-section from about 39° to 0°, which is best exposed in
the hanging wall of the concealed Noriega fault just east of the Cerro Pelón. In addition to the fanning dips, this unit
has a significantly higher percentage of tonalite, metasedimentary, and metavolcanic clasts which further distinguish it
from Tc2.
dike
LT
O
Qal
Tcf
Kgd
as
Dissected, subhorizontally stratified pebbly sandstone and clast-supported sandy pebble conglomerate with local
primary dip of up to 8° in the eastern Sierra Tordilla. Gravel fraction consists of mostly subangular to rounded
pebbles, sparse cobbles, and rare boulders up to 90 cm in diameter. Bedsets are 7 to 35 cm thick and defined by
mostly massive, clast-supported, pebble-rich beds lacking sedimentary structures except rare channel scour and
upper plane-bed stratification. The clast assemblage is polymict consisting of volcanic (56%) and granitic and
metamorphic clasts (44%). This unit is interpreted as widespread, locally derived debris flows and alluvium that cover
and obscure older pre- and syn-extensional strata. Maximum exposed thickness is approximately 160 m.
Light gray-purple, rubbly, vesicular basaltic-andesite lava flow with up to 10-20% altered red pyroxene phenocrysts.
This unit only occurs in the Cerro Prieta. Total unit thickness is 45 m.
U
FA @
ÂÂ
Tc3
QTg
A
56
200
Ttd
@
m
0
@
Kgd
Qal
QTg
Trp
G
Tc2
o
58
50
approximate strike and
dip of bedding
0
IE
@
Ttd
30
R
58
Tc2
41
Lo
O
Tb2
30
N
Ttd
Tc2
Kgd
@
60
@
Kt
o
o15
¹
Mzs
9
3266000
Tc3
52
54
Kt
Qc
Unconsolidated gravel colluvium consisting of pebbles to boulders of variegated volcanic clasts, except in the Cerro
Las Burras and Cerro Prieta where clasts are only basalt. Typically confined to slopes near steep topography.
strike and dip of inclined bedding
200
Kt
59
Qal
Unconsolidated sand and gravel alluvium
settlement
¹oo
Kt
intermediate dike (dashed where
inferred)
unimproved dirt road
Qal
376000
378000
Gray trachydacite lava flows containing 10-15% altered phenocrysts (plagioclase > alkali feldspar ≈ amphibole >>
apatite). Exposures of this unit are rare and discontinuous in the central and eastern study area. Total unit thickness
is up to 430 m.
Black, aphanitic basaltic-andesite lava flows containing rare lath-shaped plagioclase phenocrysts up to 2 mm long
and vesicles commonly filled with white or yellow zeolite. Microlitic groundmass consists of plagioclase and altered
pyroxene; no olivine is observed. This unit is well exposed ~3 km south of Pozo Coyote where it is intercalated with
2- to 3-m-thick red tuffaceous sandstones and pyroclastic breccias. An isochron age of 11.76 ± 0.08 Ma [Ar/Ar] was
determined for this unit (Darin, 2011). Total unit thickness is up to 240 m.
Cerro Colorado
Rio San
Ignacio
SW
1000
?
500
Kt
Ttc
Kt
Z
PO
O
Ta2
CO
T
YO
E
Qal Ttsi
Ttsi
QTg
Qal
Td2
Td1
Ta2
Ta2
Ta1
Ta1
FA
Tst
Td1
Tba
QTg
Qal
Qal
Td2
Tc3?
Ttsi
Ta1
T
UL
Kt
?
Tb3
Tc3?
Tba?
QTg
Qc
Qsi
Ta1
Td1
Kt
?
Kt?
Tdt?
Tr?
LI
BE
AD
T
R
LT
U
FA
Qal
Ttsi
Tdt
Tr?
Tb3
Tb3
Tta
Qal
Tta?
?
Ttsi?
Tdt?
Kt?
Qal
0
SEA LEVEL
Tb3
Lewis, C.J., 1994, Constraints on extension in the Gulf extensional province from the Sierra San Fermín, northeastern Baja California, Mexico [Ph.D. thesis]: Cambridge, Massachusetts, Harvard University, 361 p.
?
-500
-1000
?
?
Oskin, M., and Stock, J., 2003a, Pacific-North America plate motion and opening of the Upper Delfin Basin, northern Gulf of California, Mexico: Geological Society of America Bulletin, v. 115, p. 1173-1190.
Oskin, M., and Stock, J., 2003b, Cenozoic volcanism and tectonics of the continental margins of the Upper Delfín basin, northeastern Baja California and western Sonora, in Johnson, S.E., Paterson, S.R., Fletcher, J.M., Girty, G.H., Kimbrough, D.L.,
Tr?
Kt?
Gastil, R.G., Krummenacher, D., Doupont, J., and Bushee, J., 1974, The batholith belt of southern California and western Mexico: Pacific Geology, v. 8, p. 73-78.
?
-1500
and Martín-Barajas, A., eds., Tectonic evolution of northwestern Mexico and the southwestern USA: Geological Society of America Special Paper 374, p. 421-428.
Oskin, M., Stock, J., and Martín-Barajas, A., 2001, Rapid localization of Pacific-North America plate motion in the Gulf of California: Geology, v. 29, no. 5, p. 459-462.
Oskin, M.E., 2002, Tectonic evolution of the northern Gulf of California deduced from conjugate rifted margins of the Upper Delfín Basin [Ph.D. thesis]: Pasedena, California, California Institute of Technology, 481 p.
Ramos-Velázquez, E., Calmus, T., Valencia, V., Iriondo, A., Valencia-Moreno, M., and Bellon, H., 2008, U-Pb and (40)Ar/(39)Ar geochronology of the coastal Sonora batholith: New insights on Laramide continental arc magmatism: Revista
Mexicana De Ciencias Geológicas, v. 25, no. 2, p. 314-333.
-2000
Ta1
Ttc
Tr
Trb
Tbu
Tss
Tcu
Tru
Ttu
Tcf
Kg
Kgd
Kt
Kd
Mzs
Mzv
Mzc
Purple, aphyric peraluminous trachyandesite flows containing 1 to 2% microlitic plagioclase and probable pyroxene in
an aphanitic groundmass. Some flows contain a 1- to 2-m-thick red basal flow breccia. Dikes of similar composition
and agglomeritic breccia zones are common in the southwesternmost Cerro Colorado. A similar aphyric andesite flow
is also observed just west of the Cerro Pelón in the hanging wall strata of the Bacha fault. Total unit thickness is up to
330 m.
Gray to purple, plagioclase-phyric, peraluminous vesicular basaltic-trachyandesite lava flows. Locally abundant
vesicles are commonly filled with white to yellow zeolite. Exposures are typically weathered and rubbly. The age of
this unit is partly constrained by the interbedded Tuff of Cerro Colorado (Ttc, 14.5 Ma). Total unit thickness is up to
290 m.
TUFF OF CERRO COLORADO - Yellow to red, crystal-lithic rhyolite tuff. This unit either directly overlies the
basement nonconformity or is interbedded within Ta1 throughout most of the Cerro Colorado. The base contains a 2to 8-m-thick, yellow to orange, nonwelded member with yellow, red, and purple tephra and subangular volcanic lithics,
grading upward into a brick-red, partially welded crystal-lithic tuff with up to 5% yellow and gray pumice and up to
10% phenocrysts (quartz > feldspar). Locally this unit contains multiple cooling units several meters thick and/or a
purple densely welded upper member with significantly less pumice and smaller lithic fragments. The uppermost
welded members form resistant ridges flanked by colorful talus slopes on both sides. A maximum eruption age of
14.5 ± 0.3 Ma [U-Pb] has been determined for this unit (Darin, 2011). Total unit thickness is 10 to 40 m.
Gray to light purple rhyodacite flows (Tr) with 5 to 15% phenocrysts (quartz > plagioclase >> biotite) and
compositionally similar rhyolitic breccias (Trb) found only in the Cerro Las Burras. Total unit thickness is up to 1100 m.
Thin, discontinuous exposures of indurated, glassy basalt flows. Some flows have an aphanitic texture while others
contain up to 10% phenocrysts, including altered olivine, pyroxene, and less common plagioclase. Total unit thickness
is up to 60 m.
Red to orange, massive to laminated, fine- to coarse-grained sandstone consisting of moderately sorted, subangular
to subrounded grains of quartz >> alkali feldspar ≈ biotite, and up to 10% red lapilli locally. Overall grussy composition
resembling the underlying granitic basement units. Typically found in depositional contact with Mesozoic basement
units or intercalated with Ta1. Typical unit thickness is 1 to 3 m, with a maximum of 15 m.
Undifferentiated conglomerate (Tcu), rhyolite lava- and ash-flows (Tru), and rhyolite ash-fall tuffs (Ttu). Ttu is yellow to
white to pink, nonwelded to welded, thinly laminated tuffs with an ashy, quartz-rich matrix, volcanic lithics, and up to
10% euhedral phenocrysts of plagioclase, biotite, quartz, and sanidine. Individual tuffs are up to 20 m thick.
Well-stratified, clast-supported pebble-cobble conglomerate and interbedded pebbly sandstone. Conglomerate beds
contain a white to red granular, grussy matrix and sparse boulders up to 60 cm in diameter. Well to very well rounded
clasts include quartzite, tonalite, chert, limestone, and various metamorphic lithologies along with subordinate
volcanic clasts. This unit is correlated with the distinctive conglomerate unit of Gastil et al. (1973) and Bryant (1986)
based on the exotic clast assemblage, the presence of similarly unique Permian limestone clasts containing fusulinid
and gastropod fossils, and proximity to previously mapped outcrops along strike to the southeast in the Sierra Seri
(Gastil & Krummenacher, 1976). Correlatable outcrops in Baja California are overlain by and intercalated with
volcanic flows as old as ca. 20-21 Ma (Lewis, 1994; Stock, 1989); additional stratigraphic constraints on Baja (e.g.,
Dorsey & Burns, 1994; Oskin & Stock, 2003b) suggest an Oligocene to middle Miocene age for this unit.
Medium- to coarse-grained tonalite (Kt), granodiorite (Kgd), granite (Kg), and quartz diorite (Kd). Kt is composed of
plagioclase > quartz > biotite >> alkali feldspar, and contains locally abundant metasedimentary enclaves typical of
S-type granites formed from sedimentary protoliths. Kgd is composed of plagioclase > quartz > alkali feldspar ≈
biotite. Kg contains microcline megacrysts up to 3 cm in length and quartz > alkali feldspar >> biotite ≈ plagioclase;
it is only found in the central study area as 10- to 30-m-thick dikes and a single concentric pluton that forms a
prominent peak in the Cerro Pelón, where cross-cutting relationships and the presence of tonalite xenoliths within Kg
indicate that it postdates emplacement of Kt. Kd consists of fine- to medium-grained quartz diorite containing
plagioclase > amphibole ≈ biotite > quartz ± orthopyroxene; it is only present as a single intrusion in the Sierra
Tordilla. A Late Cretaceous age has been assigned to these units based on radiomentric ages from proximal areas
with similar lithologies representing the coastal Sonora batholith (Gastil et al., 1974; Gastil and Krummenacher, 1977;
Ramos-Velázquez et al., 2008).
Mostly black to gray, low-grade hornfels facies metasedimentary rocks (Mzs) with a coarse-grained to granular
texture and abundant primary quartz and secondary muscovite. Metavolcanics (Mzv) include dense, glassy black
basalt and andesite(?) flows commonly containing plagioclase and amphibole phenocrysts and showing a streaky
black and white schistosity locally. Metacarbonates (Mzc) consist primarily of 5- to 25-cm-thick beds of low-grade
meta-limestone and intercalated calcite-cemented sandstone; this unit contains 1- to 3-cm-long unidentified,
fragmentary, silicified fossils(?) with both cylindrical and 4- to 6-sided prismatic/angular forms that are very rare, but
locally abundant in densely packed layers up to 10 cm thick (29.52353°N, 112.31983°W). Low-grade metamorphism
is likely the result of contact metamorphism during emplacement of the Late Cretaceous coastal Sonora batholith
(Ramos-Velázquez et al., 2008); protolith ages are inferred to be Mesozoic, although older ages are possible.
Very fine-grained quartzite. Outcrops are usually weathered and rubbly. Only exposed in one locality, just southwest
of Pozo Coyote. This lithology is not observed elsewhere in Mesozoic strata of Sonora, leading some to interpret it as
an unusual facies of Paleozoic strata west of the miogeoclinal margin (Gastil and Krummenacher, 1977; Stewart and
Poole, 2002).
*Petrologic descriptions of volcanic units are based on a combination of whole-rock geochemical analysis, petrography, and
texture. Geochemical data are presented in Darin (2011).
Dorsey, R.J., and Burns, B., 1994, Regional stratigraphy, sedimentology, and tectonic significance of Oligocene-Miocene sedimentary and volcanic rocks, northern Baja California, Mexico: Sedimentary Geology, v. 88, no. 3-4, p. 231-251.
Gastil, R.G., Lemone, D.V., and Stewart, W.J., 1973, Permian fusulinids from near San Felipe, Baja California: American Association of Petroleum Geologists Bulletin, v. 57, no. 4, p. 746-747.
?
?
Kt
Ta2
Porphyritic dacite lava flows with abundant diagnostic euhedral plagioclase phenocrysts (10 to 20%) up to 6 mm long,
sub- to euhedral biotite (2 to 5%) and quartz (2 to 5%) phenocrysts in a gray to red groundmass. Locally exposed
basal flow breccias are up to 3 m thick. Intercalated with Tdt in the Sierra Bacha and Sierra Tordilla. Total unit thickness is up to 270 m.
Gastil, R.G., and Krummenacher, D., 1977, Reconnaissance geology of coastal Sonora between Puerto Lobos and Bahía Kino: Geological Society of America Bulletin, v. 88, p. 189-198.
500
?
C
BA
HA
FA
Tdf
Monolithologic, lithic-rich dacite tuffs with subordinate interbedded tuff breccias and dacite lava flows (Tdf). Tuffs and
breccias have a nearly homogeneous composition of porphyritic dacite (Tdf) clasts. The matrix is ashy and varies
from yellow, orange, and white to pink. Outcrops are typically hard and non-friable. This unit is interpreted as a
proximal to medial dacite stratovolcano facies. Typical unit thickness is 100 to 200 m, with a maximum of 660 m.
Darin, M.H., 2011, Late Miocene extensional deformation in the Sierra Bacha, coastal Sonora, Mexico: Implications for the kinematic evolution of the proto-Gulf of California [M.S. Thesis]: Eugene, Oregon, University of Oregon, 95 p.
1000
Tr2
i
Kt
T
UL
Td2
Td1
Tst
s
Tt
ELEVATION
(meters above sea level)
T
UL
Tdt
Bryant, B.A., 1986, Geology of the Sierra Santa Rosa Basin, Baja California, Mexico [M.S. thesis]: San Diego, California, San Diego State University, 75 p.
NE
?
Kt
Ttsi
Ta2
Ta3
Tr2
?
-1500
Tss
Qal
Mzs
Tss
Tb3
Tb1
2
Ta
-1000
FA
I
R
SE
Ttc
Qal
A′
Rio San
Ignacio
2
Td
-500
Ta2
1
Ta
?
Qsi
Ta
1
1
Ta
0
GROUND SURFACE
Qal
Qal
Qal
Tc3
Tc1
Massive, poorly sorted, volcaniclastic pebble-cobble conglomerate and breccia with a gray, ashy matrix. This unit has
a monolithologic assemblage of clasts from underlying dacite flows and tuffs (Tdf, Tdt) in the northwestern study area.
Virtually all clasts are porphyritic dacite, which is diagnostic of this unit. Total unit thickness is up to 750 m.
Gastil, R.G., and Krummenacher, D., 1976, Reconnaissance geologic map of coastal Sonora between Puerto Lobos and Bahía Kino: Geological Society of America Map and Chart Series MC-16, scale 1:150,000, 1 sheet.
3
Ta
Tbu
sf
Tt
Tdt
Tss
Tb2
Cerro Las Burras
Bennett, S.E.K., Oskin, M.E., and Iriondo, A., 2013, Transtensional rifting in the proto-Gulf of California near Bahía Kino, Sonora, Mexico: Geological Society of America Bulletin, v. 125, p. 1752-1782.
Ta2
Tss
Qal
Sonora
Highway
003
Densely foliated rhyolite flow containing up to 5% phenocrysts (quartz ≈ sanidine > feldspar) in a pink to purple
groundmass. Found only in the Cerro Colorado. Maximum unit thickness is 50 m.
Pzq
Bennett, S.E.K., 2013, The role of rift obliquity in formation of the Gulf of California [Ph.D. thesis]: University of California - Davis, 205 p.
A
Tr1
Purple to gray, aphanitic trachydacite lava flows containing up to 5% blocky sanidine and plagioclase phenocrysts
and accessory pyroxene in a microlitic plagioclase groundmass. Like Td1, this unit consists of multiple 20- to 60-mthick flows with 1- to 4-m-thick red basal flow breccias in the Cerro Colorado. This unit commonly displays a welldefined 1- to 4-cm-spaced flow foliation, which helps to distinguish it from Td1. Total unit thickness is up to 330 m.
REFERENCES CITED
TUFF OF ARIVAIPA - Deep maroon to red, partially welded, crystal- and lithic-rich rhyolite tuff only exposed in the
Cerro Las Burras. Up to 5% quartz and plagioclase phenocrysts and undeformed yellow, ashy pumice are diagnostic
of this tuff. This unit contains a 1- to 4-m-thick, white to red breccia at its base, which grades upward into a partially
welded lithic-rich zone containing 15% brown, red, purple, and black, subangular volcanic lithics and rare granitic
xenoliths. The upper 2 to 3 m of the unit are nonwelded and contain abundant yellow and white pumice clasts with no
phenocrysts. A white, discontinuous, ashy, quartz-rich layer up to 10 m thick is observed approximately 30 m below
the top of the unit. Total unit thickness is 50 to 80 m.
Tb1
Pink, porphyritic trachydacite lava flows containing up to 15% phenocrysts of feldspar (up to 5 mm long) and
accessory quartz and biotite. Flows commonly display a non-planar, distorted flow foliation. The base of each lava
flow consists of a 1- to 10-m-thick yellow ash-fall tuff, which is usually overlain by a discontinuous 1- to 3-m-thick
vitrophyre. Individual flows are up to 130 m thick. Total unit thickness is up to 650 m.
380000
112°15'0"W
TUFF OF SAN IGNACIO - Bright pink to white or orange, ashy, crystal-poor, spherulitic rhyolite ash-flow tuff. This unit
contains uncommon cm-scale, white to pink pumice fiamé and unique subrounded to angular, vesicular, plagioclasephyric andesite lithics with an average diameter of 1 cm (max. 5 cm). Abundant 0.5- to 3-cm-diameter quartz-filled
spherules and lithophysae in a pink to white, ashy groundmass are especially diagnostic of this tuff and pervasive
throughout the vapor-phase alteration zone. Very rare small pumice fragments and partially dissolved potassium
feldspar, quartz, and biotite phenocrysts are also observed. The base is commonly a 1- to 5-m-thick, dense black to
brown vitrophyre with rare feldspar micro-phenocrysts. A 0.5- to 1.5-m-thick, orange to brown, laminated basal surge
deposit in the central and southern Cerro Colorado locally contains abundant subrounded tonalite (Kt) pebbles and
cobbles. The basal unit is overlain by a 5- to 8-m-thick, pink to orange, crystal- and lithic-rich welded zone with
uncommon phenocrysts (quartz >> feldspar > biotite). The welded zone grades upward into a spherulitic, partially
welded zone of vapor-phase alteration 10 to 30 m thick in most exposures. Internal rheomorphic flow deformation in
the form of disharmonic folds and recrystallized pumice fiamé is characteristic of this unit in the Cerro Las Burras.
Flow-banding is densely-spaced (<1 cm) and rheomorphism is local and irregularly distributed within the unit; pumice
lineations show various degrees of deformation, with some stretched up to 30 cm long (1:50 aspect ratio). A high
precision age of 12.56 ± 0.09 Ma [U-Pb] has been determined for this unit (Darin, 2011). Typical unit thickness is 20 to
40 m; maximum thickness is 350 m in the Cerro Las Burras.
Indurated, glassy basalt flows containing up to 10% altered olivine and subordinate pyroxene phenocrysts. Total unit
thickness is up to 40 m.
improved dirt road
46
39
200
@
Unconsolidated fine- to very coarse-grained sand and minor gravel alluvium of the Rio San Ignacio
@
Kg
10.4 ± 0.2
[K-Ar] Tc2
Kgd
Qsi
300
Tc3
24
25
Kt
Kt
Tts
Tdt
Â
Tts
Tdt
42
3268000
Ta2
@
S
Kgd
Kt
30
73
Mzs
19
Qal
3272000
D
D
D
D
D
D
D
D
S
¢
¢
D
20
D
@
D
D
D
D
D
D
D
D
D
S
D
D
D
D
D
¢¢
D
DD
D
D
D
¢
¤ ¢
¢
D
D
¢
¤
D
D D
D
D
D
D
DD
D
D
¢
D
@
@
D
¢
D
D
@
D
D
@
@
@
3276000
0
D
@
A
N
IG
N
A
S
20
@
LT
U
FA
IO
C
200
@
S¢
¢¢
¢
D
D
D
0
3276000
29°35'0"N3274000
¢S
S
0
20
60
200
@
SS
D
D
@
Mzs
Kt
Tta
felsic dike (dashed where inferred)
0
Kt
p
Â
D
Â
D
S
o
D
D
ci
¢
D
na
D
3282000
D
Ig
Qal
j
n
20
Ta1 14.5 ± 0.3
[U-Pb]
Tss
S
¹
41
42
p
Sa
S
38
l
o
¢
Ta1
Qal
Qa
Ri
¢S
S
D
¹
3270000
Ttc
¢
Ttsi
¹
3268000
¢60
Mzv
¢
50
77
¨
66
¢71
2
Ta
0
34
o¹
o
3266000
¦ ¨
38
40
33
Kt
Qal
Mzs
112°0'0"W
@
Kt
e
Ta1
S
38
p
Qal
Kt
Mzs
28
29°30'0"N
p
2
Td1
@
51
TUFF OF SAN FELIPE - Maroon to brown to orange, densely welded tuff containing abundant yellow to white
pumice, 10-15% anorthoclase phenocrysts, rare zoned pyroxene, and absolutely no phenocrystic quartz. Abundant
flattened pumice fiamé reach lengths of up to 25 cm and form a well-defined eutaxitic foliation. Trachyte-rhyolite
inclusions containing abundant alkali feldspar in a dark glassy groundmass are a common diagnostic feature of this
unit. This unit is a well-documented and regionally extensive rhyolite ignimbrite exposed over an area of greater than
4,000 km² in northern Baja California and Sonora, Mexico that serves as a key stratigraphic marker and geologic
tie-point across the Gulf of California (e.g., Stock et al., 1999; Oskin et al., 2001; Oskin and Stock, 2003a). Bennett et
al. (2013) report a high-precision age of 12.50 ± 0.08 Ma [Ar/Ar] for this unit. It is only exposed in two localities within
the study area, representing the northernmost identified outcrops of the Tuff of San Felipe on the eastern rifted
margin of the Gulf of California. Northeast of El Desemboque, the unit is ~25 to 70 m thick with a thin (<1 m)
discontinuous black vitrophyre. The lower densely welded, fiamé-rich member (20 to 40 m thick) grades upward into
a non-welded zone with intact, undeformed pumice (5 to 30 m thick).
Source: i-cubed 15m eSAT imagery, courtesy of Esri
@
¹
Td
3
Tr
2
p
oS
So
o
¢¹ ¢
D
30
Ta1
Td
Mzv
Ttc
5636
38 54
¹40
42
Ta1
Ttc
Tr1
54
@
Td1
Qal
Ttsi
D
¢S
70
Tb3
40
@
Ta1
Ttc
D
Ta2
300
@
Mzv
D
31
Ta2
Kt
¹
@
75 65
55
200
¹
o
j
¹ ¹ oD jD
D
79
@
Td1
@
74
o
¹ ¹ ¹o
D
3272000
@
@
0
Ta1
75
D
@
D
26
Ta1
19
78
40
km
@
20
¹
Td1
Mzs
47
40
km
¹
e
0
o
0
D
0
o
Tc2
Mzs
Kt
@
27
Tc2
Ttd
Mzs
Td2
Tc3
Kt
Kt
@
j
D
n
30
o40 Qc
43
Kt
38
@
0
Kt
D
0
D
D
ló
Tss
QTg
Td2
o
10
20
D
Pe
Mzs
Kt
Tdt
Qal
26
85
@
Qal
j37
QTg
57
Mzs
D
30
o21
55 81
Kt
32
¹51¹45
Qal
85
35
34
30
44
D
j
71
Tdt
Tcu
El Desemboque
¹
0
Kg
Kt
Kt
o
¹
¹D
¹
o
o
¹
pp o
66
Tc3
Qc
D
oo
ro
0
35
43 37
o
75
¹
30
66
D
50
o32
¹¹o32
Td1
9
Tr2
Qal
o
Kt
Ttd
36
Kt
26 28
S
Kt
23
Tss
86
30
50
72
D
Tb3
45
¹
4341
60 o
53 Ta1
40o
o
50 ¹ Ttc
Ttu
o34 Ttc
Tss D
60
Ta2
83
T
S
s
Tr1
¹ s 58
77
44
Ttsi
@
er
Ta1
o
¹
Kt
¹
j
D
0
Tbu
¢41
50
u
39 80
Kg
Kg
Kt
Ta2
Tb
00
Ta2
¹
Ta2
p28
Ta1
o
40
78
o
o
Kg
o
Qal
Qal
@ Qal
@
Qal
60
76
j
D
t
Tss
D
Td
40
Qal
Ta2
Ttsi
54
Ta
2
D
C
Ta
1
Ttsi
32
Qal
Tr2
Td Ta1
2
38
Tr1
Ta1
@
Ta1
65
Tb2
Qal
@
39
o ¹
¹¹
Mzs
@
A
Kt
Ttsf
Kt
Ttsf
Tbu
Qsi
Gulf of California
Mzc
o
52
82
Tss
84
Kt
0
0
o
Kg
Tbu Tdt 45
Mzs
Ta2
@
D
o45
Tba
Tba
Td2
¹35
76
¹ TstTd2 Ttu 53
65
o
41
52
¢S 43
Qal
Td1
Td1
D
20
70
39
Tr2
Ta1
¨
0
LT
@
77
Kt
3
¹
¹¹¹ ¹
U
Mzs
59
Kt
60
58
34
Ttsi
38
D
Kt
44
Qa
30
Kt
43
Mzc
Tb2
@
@
FA
400
D
¨
Mzc
Kt
Tr1
Qal
Ta1
Ta1
S
Td2
34
Ta1
66
53
Kt
I
Ta2
71
@
Kt
¹40
49
Td1
54
38
D D
D
0
Tb3
300
Tb3
Td2
Ta
2
l
Tc3
Tdt
R
Ttsi
0
20
Tba
p8
Td1
Td1
12.56 ± 0.09
[U-Pb]
Tst
49
Qal
200
15
Ttsf
SE
1
Tr2
Qal
D
Kt
Kt
Td
Ta2
j
¨
Kt
10
@
Qal
76
Ttsi
47
Mzc
T
Qal
Kt
@
35
30
D
Kt
100
o
45
¹j
D
Tss
Ta1
DD
Kt
Td2
45
Ta2
Td2
D
D
D
D
Ttc
Pzq
UL
@
0
@
Tst
D
D
@
Kt
FA
20
Ttsi
p39
Ta2
D
300
100
o
53
Tss
A
Qal
1
Ttsi
Tb1
Ta1
Ta
1
Ta1
CH
Tc1
45
Td
D
Kt
0
Tb2
Ta3
Tst Ta3
Td2
300Td2
52
46
18
Td1
@
Pzq
¹ 57¹4546
¹4748
54
2
l
Ta Qa
Ttc
100
Ta2 4549
Tss
47
Ttsi
Â15
51
Td2
j
@ Tdt
Td2
¹
@
Kg
49
60
o
44
Ttsi
Ta1
BA
@
Ta1
Ta2
Qal
Kt
QTg
Tc1
Ta3
Ttsi Tb1
D
51
Kt
Td2
20
oD
p
Kt
Tdt
Tb2
Td1
49
¹50
¹42 ¹42Tst
@
Ta3
QTg
Tr2
36
Tr2
o p
@
Kg
44
¹43
¹
¹¹
Td2
p35
QTg
¹¹ ¹
54
Tr2
D
QTg
20
Td3
Qal
40
9
Tdt
la
33
Qc
Qal
p
Tc3
Tc2
Tb1
o
o11
Tc1
Cerro Colorado
Td2
T
20
fault, dashed where approximate,
dotted where concealed, ‘?’ where queried;
ball and bar on downthrown side; arrows
indicate relative lateral motion
Qal
Tba
Tba
53
Qsi
@
Kt
p
Qal
o
0
Qal
UL
50
Qc
Td2
0
15
11.7 ± 0.2
[Ar/Ar]
Ttsi
QTg
Tc1
0
@
FA
20
11.76 ± 0.08
[Ar/Ar]
Tdf
Qal
il
Td3
@
Ta3
QTg
Tdt
Tdf
40
30
rd
@
Tc1
40
o
TE
Tc2 Tc3
Kt
10
QTg
p
T
Kg
YO
Qsi
QTg
@
a
O
Pzq
lithologic contact, dashed where
approximate, dotted where concealed,
‘?’ where queried
QTg
Td3
o
50
C
200
Qal
100
o
Tdt
0
o
0
40
¹
20
rr
Qal
p
Kt
p
@
ie
QTg
60
o
f
Td
QTg
@
Tr2
¹
QTg
Qal
20
Qsi
100
f
0
S
Qal
Kt
¹
QTg
Tdt
62
Qal
39
Tbu
400
0
Tdt
Tbu
Kg
Tb3
Qal
QTg
QTg
ZO
Td
10
@
@
Tc2
O
o
45
10
o
50
Mzs
QTg
7
oo
8
Tc1
Kd
@
@
Qal
¹66
Td3
P
p
45
Qal
@
0
¹¹
0
Tb2
Tdt
Kg
20
LT
20
63
20
Pozo Coyote
Tr2
Tbu
60
¹¹
U
¹
0
34
Mzs
Td1
Tba
QTg
Td3
Tru
100
QTg
@
54
PALEOZOIC
EXPLANATION OF MAP SYMBOLS
Qal
55
Td3
27
Qc
200
Tb3
Tb2
Kg Kgd
Kt Kd
0
FA
Tr2
66.0
Mzs Mzc
Mzv
Tb3
Tdt
8
Qsi
@
33.9
20
Td3
¹55
63
Qc
Td1
30
Mzs
53
QTg
500
Qal
@
f
Tc1
Kd
Tcu
S e
¢
TE
Td
0
Tba?
5
¹49¹57
?
DESCRIPTION OF MAP UNITS*
Qal
24
@
YO
o
O
a
20
Tr
¹
Tdt
o
o
10
Trb
Qsi
Tc2
Td1
Cretaceous
Tb3
p
Tc2
47
0
0
Ttsi
Qal
Mzs
57
Tta
C
h
QTg
Qal
53
@
c
48
¹
Td1
Ttsi
Late
Cretaceous
Tb3
56
26
Ttp
ZO
a
QTg
Tc1
Kt
400
100
23.0
Tcf
?
TUFF OF CERRO PRIETA - Pink, densely welded trachydacite tuff with a 1- to 2-m-thick basal vitrophyre, eutaxitic
foliation defined by flat recrystallized pumice fiamé, and 10 to 15% phenocrysts (sanidine > biotite > quartz). This unit
bears a slight resemblance to the Tuff of San Ignacio (Ttsi) in both mineralogy and the abundance of spherules in the
vapor phase alteration zones. However, this unit overlies both Td1 and Ttsi, has a higher abundance of phenocrystic
quartz and biotite, and has a distinctly different bulk geochemical signature (Darin, 2011).
Tbu
44
Cerro Las Burras
?
14.5
Ttc*
¹Approximate ages from timescale of Walker et al. (2012). Radiometric
ages of dated units from within field area (indicated by asterisks) are
italicized; see Darin (2011) for summary.
112°30'0"W
¹
Tc2
12.5
Tb3
Tdt
@
Ttu
Tb3
Trb
Tr
Qal
@
0
49
LT
@
¹57
Ta4 @
PO
30
Qal
Tdf
100
Tru
Trp
LT
B
Tdf
U
@
Tc
2
Tc
2
¯
¯
5
76
¹
Kt
FA
¹39
48
U
a
47
D
oD
o
45
Tc1
Ttp
FA
D
Qal
A
rr
Tdf
Tru
D
0
H
Tru
¹50
Trp
20
C
Qal
43
Tb3
TA
@
A
ie
64
1
Td1
Tc
2
ER
¹
B
Tdt
¹
S
Tdf
Td
B
76
@
o53
42 34
¹62 Tru
Trp
Tb3
Tru
¹
Tc1
Tdf
50
Tc2
¹
¹
D
o
40 39
o
500
300
46
Tru
50
56
58
Tta
LI
A′
Tb3
S
¢
400
46 40
Tba
@
45
@
40
Tc2
6.4 ± 1.9
[K-Ar]
¹
¹
¹
55
¹ ¹
¹
¹ ¹¹
Kt
o
jo
o
Kt
Tc1
0
@
Tdt
30
@
¢
72
Trp
Tc1
0
¹40 ¹
20 9
¹
50 68
45¹
Tc2
56 ¹
45
40 42 p43
Qc
¹
p
Qal
0
p9
Tb3
Ta4
Tru
Ta1
11.8
MESOZOIC
Tba
0
Tdf
Ttsf
UNCONFORMITY
3280000
0
3
Oligocene
3
4
3278000
@
20
p
Kt
30
Qal
20
@
Qc
00
¹
0
o
40
Tb3
Tc2
ay
5
Paleogene
@
30
300
Tc1
Tc2
hw
S
D
o
Qc
Hig
200
Qal
Tc1
70
Qc
ra
S
D
D
t
Tc1
no
Tss
@
Td
Cerro
Prieta
46
D
61
60
oo
D
¹
D
D
Tc2
@
S
56 46
D
D
So
D
3284000
59
t
Tdf
Tdt
D
47
29°40'0"N
Td1
D
D
SS
33
Tr
Trb
3
10.4
Ttd*
Tb3
Tr2 Ta4
Tb2
Td3 Trp Ta3*
Td2
Tru
Td1 Ttp
Ttu
Tcu Ttsf* Ttsi*
Tta Tr1 Tb1
Tdt
Ta2
Miocene
@
Tc1
Tdt
Td
62 69 45
Tc2
Stony, purple-gray, aphanitic trachydacite lava flows with <2% altered sanidine phenocrysts and accessory pyroxene
in a microlitic plagioclase groundmass. In the Cerro Colorado, this unit consists of multiple ~20- to 60-m-thick flows
each containing a 1- to 4-m-thick reddish (oxidized) basal flow breccia. Individual flows are separated by 2- to 3-mthick beds of yellow to green tephra in the northern Cerro Colorado. Total unit thickness is 80 to 300 m.
UNCONFORMITY
Tst
0
¹
S
D
Tc1
20
Qal
Tba?
Qal
0
0
S
40
¹¹
15
20
Ttp
6.4
Tba*
380000
@
300
Kt
D
3280000
372000
Tc2
Neogene
29°0'0"N
Tc1
Tdt
65
Qal
3278000
112°20'0"W
370000
368000
Tc2
Mzs
200
366000
5.3
Tc1
70
Tc1
112°25'0"W
362000
2.6
?
(AT MAP CENTER, 2011)
o
Kt
356000
Qc
UNCONFORMITY
CENOZOIC
PLUTONIC & METAMORPHIC
ROCKS
Age¹ (Ma)
VOLCANIC ROCKS
Tc3
29°30'0"N
112°30'0"W
354000
Qal
Td1
QTg
Pliocene
29°30'0"N
0
2
U.S.A.
Other Geologic Maps
2 - Gastil & Krummenacher (1976)
3 - Oskin (2002)
4 - Bennett et al. (2013)
5 - Bennett (2013)
Magne
tic
1
True North
Base map (contours & water drainage) constructed from
1:50,000 scale digital topographic maps from the National
Institute of Statistics and Geography of Mexico (INEGI)
0.5
2
Qsi
Quaternary
40°N
120°W
10.4º
SCALE 1:30,000
0
Map Area (This Study)
1 - cf. Darin (2011)
SEDIMENTARY ROCKS
112°0'0"W
110°W
Geology by Michael H. Darin & Rebecca J. Dorsey
Datum: WGS 84
Projection: UTM Zone 12N
CORRELATION OF MAP UNITS
LOCATION MAP
-2000
Stewart, J.H., and Poole, F.G., 2002, Inventory of Neoproterozoic and Paleozoic strata in Sonora, Mexico: U.S. Geological Survey Open-File Report 02-97, 50 p.
Stock, J.M., 1989, Sequence and geochronology of Miocene rocks adjacent to the Main Gulf Escarpment: Southern Valle Chico, Baja California Norte, Mexico: Geofísica Internacional, v. 28, p. 851-896.
Stock, J.M., Lewis, C.J., and Nagy, E.A., 1999, The Tuff of San Felipe: an extensive middle Miocene pyroclastic flow deposit in Baja California, Mexico: Journal of Volcanology and Geothermal Research, v. 93, no. 1-2, p. 53-74.
Walker, J.D., Geissman, J.W., Bowring, S.A., and Babcock, L.E., compilers, 2012, Geologic Time Scale v. 4.0: Geological Society of America, doi: 10.1130/2012.CTS004R3C.
DIGITAL MAP AND CHART SERIES DMCH021
DOI:10.1130/2014.DMCH021
Published by
The Geological Society of America, Inc.
3300 Penrose Place • P.O. Box 9140
Boulder, Colorado 80301-9140
Darin, M.H., and Dorsey, R.J., compilers, 2014, Geologic map of the Sierra Bacha, coastal Sonora, Mexico:
Geological Society of America Digital Map and Chart Series 21, doi:10.1130/2014.DMCH021.
For permission to copy, contact [email protected].
© 2014 The Geological Society of America. All rights reserved.