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TIERRASBLANCAS

LAGUNITA

SAN BARTOLO

LAS ARENAS

SAN PEDRO DE LOS METATES

Pastores faultPastores fault

CerroSantaLucía 6

6015

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A'

A

99°51'W99°52'W99°53'W99°54'W

19°53'N

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99°51'W99°52'W99°53'W99°54'W

19°53'N

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SCALE 1:10,000

1 1 2 3 KILOMETERS0.5

CONTOUR INTERVAL 10 METERS

GEOLOGIC MAP OF THE TIERRAS BLANCAS AREA, ESTADO DE MEXICOLonnie T. Mercer, Bart J. Kowallis, Eric H Christiansen, and Wade E. Miller, Brigham Young University

Oscar Carranza-Castañeda, Universidad Nacional Autónoma de MéxicoIsabel Israde-Alcantara, Universidad Michoacana de San Nicolás de Hidalgo

Description of Map Units

Map Symbols

MEASURED SECTION LOCALITY

CONTACT -- Dashed where inferred or approximately located.

inclined horizontal

VOLCANIC VENT

STRIKE AND DIP OF BEDS20

FAULT -- Dashed where inferred or approxi-

mately located, dotted where covered; bar and ball on downthrown side.

SAMPLE LOCALITY – See map text for description of samples

22

LA

N

DIGITAL MAP AND CHART SERIES DMCH018DOI: 10.1130/2014.DMCH018

Published byThe Geological Society of America, Inc.3300 Penrose Place • P.O. Box 9140Boulder, Colorado 80301-9140

© 2014 The Geological Society of America. All rights reserved. For permission to copy, contact editing@geosociety.org.

50

0

100

150

200 m

5x vertical exaggeration

Tv

Tv

Tv

TlTl

Tl

Ttb

Ttb

TtbQva

Qva

QaoQa

Qa Alluvium, unconsolidated sand and gravel in drainages and main graben valley.

Colluvium, unconsolidated sand and gravel on slopes.

Andesite lava �ows, high-K2O; 0.4 ± 0.1 Ma; dark gray to black groundmasses with 5-10% plagioclase phenocrysts and sparse pyroxene; abundant xenocrysts of quartz and partially disaggregated clots of glassy feldspar, quartz, and biotite; felsic inclusions are medium grained and up to 1 cm across.

Basaltic andesite lava �ows and cinder cones, high-TiO2; approx. 0.8 to 1.1 Ma; commonly have gray to dark gray groundmass with <5% plagioclase and pyroxene phenocrysts.

Tierras Blancas beds, mammal fossils indicate an age of Late Pliocene to Early Pleistocene; divided into four lithofacies: 1) sandy mudstone, 2) diatomaceous mudstone, 3) massive mudstone, and 4) volcaniclastic sandstone.

Lagunita beds, sparse mammal fossils indicate an Early Pliocene age; gray to brown volcanic breccias, very light brown tu�aceous mudstones, two thin distal pyroclastic fall layers, and a 3 m thick ash �ow tu�; has �ve lithofacies: 1) tu�aceous mudstone, 2) volcaniclastic sandstone, 3) lapilli ash-�ow tu�, 4) fallout tu�, and 5) volcanic breccia (debris �ow/lahar).

TrRhyolite and dacite lava �ows of Cerro Santa Lucia, medium-K2O, probably Late Pliocene to Early Pleistocene, light gray groundmass with phenocrysts of plagioclase, quartz, biotite, and amphibole.

Intermediate to silicic volcanic and volcaniclastic rocks, Miocene to Pliocene; one low-silica dacite lava �ow with light tan to light gray groundmass and no prominent phenocrysts was analyzed from this group.

TaAndesite lava �ow, geochemically distinct, medium-K2O magnesian, undated but probably Late Pliocene to Early Pleistocene, no clear age relationship with Tierras Blancas and Lagunita beds, so it may be older or younger than they are.

QaoOlder alluvium, at higher elevations than the current drainage; may include a package of semi-consolidated sand and tu� beds (1.20 ± 0.13 Ma) at the base of the unit (Cemeterio Beds). The age of a fallout ash in this unit is older than the Qva and Qvb ages, but some of the alluvium in this unit is likely also younger than those �ows.

QaQc

Qc

Qc

QcQc

Qvb

Qvb

Qvb

Tv

Quaternary

Tertiary

A’ ASouth North

Quaternary alluvium and colluvium

Andesite lava flows associated with scoria cones, volcanic rock samples: Arenas, Cerro, Chosto, Ejido, Garabato, Lanzados, San Bartolo, Sur, Tejocote.

Basaltic andesite lava flows associated with scoria cones. Reverse magnetic polarity correlated to the Matuyama chron (1.1-0.78 Ma), samples Cascada, Cerrito, Metates, Mondragon, Rio, TB

Silicic/intermediate lava flows from a parasitic lava dome (Cerro Santa Lucía) on the eastern flank of the early Pliocene San Pedro stratovolcano (Norato-Cortez, 1998), volcanic rock samples D1, D2, DSB, Loma

Cementerio beds and older alluvium: Interbedded, tuffaceous, fluvial/alluvial sediments and a tuff unit, volcanic ash samples MX-2000-06, AG-2002-04, AG-2002-08; sandstone samples AG-01-01, AG-03-01, AG-04-03, AG-04-04, AG-06-01, AG-08-01, AG-08-02, AG-08-03, AG-10-01

Tuff - white, distal pyroclastic fall deposit, volcanic ash bed sample AG-2002-14

Intermediate to silicic volcanic rocks exposed in the footwall of the Pastores fault (Aguirre-Díaz et al., 2000), volcanic rock sample Curva

Metasedimentary rocks, mafic and felsic dikes. Exposed in footwall of the south-central Acambay graben (Aguirre-Díaz et al., 2000).

Angular unconformity

10 m

unconformity

Sandy mudstone - tan, massive sandy mudstone. Samples AG-06-03 and AG-10-03

Lag

un

ita

bed

s (T

l)u

nn

amed

vo

lcan

ics

(Tv)

Volcanic breccias - gray to brown, lahar/debris flow breccias

Tuff - brown, distal pyroclastic fall deposit. Volcanic ash bed sample AG-2002-12

Lapilli tuff - light gray to white, thin (3 m) ash flow tuff

Tuffaceous mudstones and volcaniclastic sandstones,white to light gray, interbedded fluvial and alluvial sediments

Late

Plio

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ater

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QaQc

Qao

Qvb

Qva

Diatomaceous mudstone - white, laminated lacustrine sediments; all diatom samples

Diatomaceous mudstone - white, laminated lacustrine sediments

Mudstone - brown, massive marginal lacustrine deposits

Volcaniclastic sandstone - thin (5-30 cm) dark gray, fluvial/sheet sandstone, samples AG-02-02, AG-02-03, AG-03-02, AG-05-02.

0.4 ± 0.1 Ma

1.20 ± 0.13 Ma