Footwall drainage evolution and scarp retreat in response to increasing
fault displacement: Loreto fault, Baja California Sur, Mexico
Estelle Mortimer
Barbara Carrapa
Institut für Geowissenschaften, Universität Potsdam, Karl-Liebknecht-Strasse 25, Potsdam D-14476, Germany
ABSTRACT
The displacement rate history of the Loreto fault, Gulf of California, is well documented; it
is characterized by episodically accelerating displacement (10 k.y. frequency) superimposed
upon a period of 200 k.y. of increasing fault displacement rate. A detailed conglomerate provenance analysis in the Loreto basin records the footwall unroofing history, which has been
profoundly affected by increased fault displacement rate. Clast count data capture an immediate drainage response to a significant increase in slip rate, along with the occurrence and systematic increase in abundance of a new source rock type. Our data record an increase in the
rate of incision and catchment expansion associated with this increase in displacement rate. A
provenance signal from a shorter-term (100 k.y.) displacement rate increase has a 30–40 k.y.
lag time. No change in provenance is recorded for high-frequency (10 k.y.) variations, which
are filtered out by the system.
Keywords: drainage evolution, fault displacement rate, provenance, Baja California.
INTRODUCTION
Characterizations of surface process
responses to varying rates of surface deformation are a fundamental problem in landscape
evolution research. The spatial-temporal evolution of normal fault arrays is well understood, and the displacement rate on discrete
structures is thought to be episodic due to
fault growth and linkage (Dawers and Anders,
1995; Gupta et al., 1999; Mortimer et al.,
2005). Footwall uplift and associated hangingwall subsidence have a profound influence
on drainage catchment size, pattern, and
yield (Leeder and Jackson, 1993; Densmore
et al., 1998; Densmore et al., 2004; Cowie
et al., 2006). Theoretical models suggest that
the catchment response to varying displacement rate may be initially rapid but requires
~50 k.y. to reach 90% topographic equilibrium (Allen and Densmore, 2000). Despite
these insights, a lack of field investigations
leaves fundamental questions unanswered
regarding the effect of varying displacement
rate on surface processes.
This unique study provides a high-resolution
field investigation into the response of a catchment to varying displacement rate. The Loreto
fault has an exceptionally well-constrained
displacement rate history (Dorsey et al., 1995;
Dorsey and Umhoefer, 2000; Mortimer et al.,
2005). We use a detailed conglomerate provenance investigation in hanging-wall sediments to investigate: (1) catchment response
to 200, 100, and 10 k.y. increases in displacement rate; (2) variations in incision rate; and
(3) lag time between fault activity and catchment response.
GEOLOGICAL SETTING OF THE
LORETO BASIN
The Loreto basin (Fig. 1), Gulf of California,
is a Pliocene half-graben bounded to the west
by the NNW-SSE–striking, 35-km-long Loreto
fault (Dorsey et al., 1995). Deposition is recorded
by ca. 3.5 Ma (Umhoefer et al., 1994), although
it probably commenced earlier (Umhoefer,
2006, personal commun.). The sedimentary fill
consists of four sequences (Dorsey et al., 1995;
Dorsey and Umhoefer, 2000; Umhoefer et al.,
1994). Sequence 1 consists of nonmarine conglomerates and sandstones deposited during
slow subsidence (0.43 mm/yr; Dorsey et al.,
1995). Sequence 2 contains 16 footwall-derived
fan deltas deposited between 2.50 ± 0.04 and
2.21 ± 0.05 Ma (Mortimer, 2004), during more
rapid subsidence (~3 mm/yr; Mortimer et al.,
2005). Sequence 3, deposited in the latest stage
of rapid subsidence, consists of carbonate and
siliciclastic deltas (Dorsey and Umhoefer,
2000). Sequence 4 is made up of shallow marine
carbonates deposited during slow subsidence
(0.4 mm/yr; Dorsey et al., 1995).
LORETO FAULT DISPLACEMENT
HISTORY
The long-term displacement rate is represented by the subsidence history. A higherresolution characterization of the displacement rate history (Fig. 2) has been constrained
through detailed investigations of hanging-wall
sedimentary rocks (Dorsey et al., 1997; Dorsey
and Umhoefer, 2000; Mortimer et al., 2005).
Dorsey et al. (1997) invoked episodic fault slip
in the generation of high-frequency fan-delta
stacking during sequence 2. Mortimer et al.
(2005) documented clinoform nucleation,
from shoal-water to Gilbert-type deltas, and
systematically increasing foreset height, which
record 5–15 k.y. repeated episodes of accelerating (from <2 to 8 mm/yr) and later decelerating displacement rate. An overall increase
in foreset height up-section possibly reflects a
second-order displacement rate increase at progradational unit 6 (Mortimer et al., 2005).
FOOTWALL SOURCE AREA GEOLOGY
Paleocurrent analyses (Mortimer et al.,
2005) indicate NNE transport and a SSW
footwall source. Source rocks, stratigraphic
ages, and geologic relations have been documented by McLean (1989) and Umhoefer
et al. (2001). The principal source rock unit
(after Umhoefer et al., 2001) is the Miocene Comundú Group, which overlies preCretaceous metamorphic rocks, Cretaceous
granites, and Eocene-Miocene sandstones.
The clastic Comundú Group lower unit
(MCL) is composed of 300 m of conglomerates and sandstones, deposited unconformably
on metamorphic rocks and granites (Umhoefer
et al., 2001). The Comundú Group lower unit
is conformably overlain by 400–700 m of the
Comundú Group middle unit (MCM) within
the Sierras Gigantas, but cross sections and map
relations indicate ≤500 m in the investigated
area. Comundú Group middle unit rocks consist of andesitic breccias and lavas (Umhoefer
et al., 2001; McLean, 1989). Lower unit or
metamorphic rocks crop out along the southern
margin of the granite. A possible onlap of the
Comundú Group middle unit onto metamorphic
rocks has been suggested 10 km north of Loreto
© 2007 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
GEOLOGY,
2007
Geology,
JulyJuly
2007;
v. 35; no. 7; p. 651–654; doi: 10.1130/G23690A.1; 4 figures; Data Repository item 2007161.
651
tern
Ea s
Loreto Fault
EL AM
io
nA
nt
on
Mex
1
N=24
Ar
ro
yo
igh
Sa
2
5 km
al H
ctur
Stru
3
A
Gulf of California
300
SA
Displacement rate
111°30W
N=153
C′
400
40
0
600
A′
Loreto Fault
70
0
1
LORETO
Nopolo Fault (NF)
2.5 x vertical exag.
Height (m)
B B′
Height (m)
700
A′
CC′
Pliocene
?
0
B
26°00 N
Alluvium
Fluvial deposits
Lava Flows
Pliocene
5 km
Wind gap
AA′
0
B′
LF
AA′ C′
C
600
Height (m)
Key to mapped units
Quaternary
Calcarenite
Carbonate delta
Marine mudstone
Clastic deltas
Alluvial Fan
Miocene Comundú Gp
Upper (MCU) flows
Upper (MCU) lavas
Middle (MCM)
Lower (MCL)
Eocene (?)
sandstone
?
0
NF
Cretaceous
granite
LFPre-Cretaceous
metamorphic
Figure 1. Loreto basin geological map and cross sections (after Dorsey et al., 1995; McLean,
1989; this study). Location of conglomerate clast counts is indicated by white star in
sequence 1 (S1). Quaternary deposits: Arroyo El Leon (EL), Arroyo San Antonio, (SA), Arroyo
Amarillo (AM), Arroyo Las Flores (AF). Three study areas in the Loreto basin are (1) PU1–10;
(2) PU11–13; (3) PU14. Rivers (blue) and drainage divides (dashed blue) are also shown.
(Umhoefer et al., 2001). Instead, McLean
(1989) and this study identify a faulted contact.
The Comundú Group middle unit is overlain by
upper Comundú Group (MCU) lava flows and
breccias, which may be deposited directly onto
metamorphic rocks. All of the footwall rocks are
intruded by andesitic dikes.
METHODS
Provenance is a powerful tool for quantifying sediment source(s), drainage evolution, and lag time (e.g., Graham et al., 1988).
Conglomerate clast counts were undertaken
in sequence 1 (Fig. 1), sequence 2 delta topsets (progradational units [PUs] 1, 4, 6, 7–9,
11–14), Quaternary deposits, and PU6 shoalwater and Gilbert-type delta topsets. Quaternary sediments (Fig. 1) drain Pliocene deltas
652
Time
Figure 2. Displacement rate characterization
of Loreto fault. Rapid displacement commenced at start of sequence 2 (Dorsey et al.,
1995). Superimposed are high-frequency displacement rate acceleration and deceleration
(Mortimer et al., 2005) recorded by clinoform
nucleation. Possible second-order increase
in displacement rate occurred at PU6,
100 k.y. into sequence 2 (Mortimer et al.,
2005; see text for discussion).
Arroyo de Gua
Mex
A
2.21 Ma
Highway 1
B
600
repeated clinoform
nucleation
2.5 Ma
3.5 Ma
~2.4 Ma
Sequence 2
Sequence 1
S1
C
Pu2
1
B′
LF
Pu6
(Arroyo Amarillo) or footwall and Pliocene
rocks (Arroyo San Antonio; Arroyo El Leon).
An additional count was undertaken in footwall
Quaternary deposits that source only MCL
rocks (Arroyo Las Flores). The aim was to: (1)
investigate sequences 1 and 2 and Quaternary
provenance to constrain catchment evolution;
(2) evaluate Pliocene-Quaternary clast reworking; and (3) assess clasts reworked into MCL
footwall rocks. Topsets were sampled on a
vertical section from Mortimer et al. (2005) in
three areas: (1) PU1–9; (2) PU11–13; and (3)
PU14 (Fig. 1) parallel to paleotransport direction, thus removing along-strike variability.
At each location, ~100 clasts were counted
within a regular grid (10 cm grid; 1 m × 1 m),
moved along strike. Only topsets were counted
to compare like depositional environments.
Thirteen clast types (see the GSA Data Repository1) were identified and assigned to possible
footwall source(s). Of these, five types were
unique to specific source units and were used to
infer specific source areas: metamorphic, granitic, Eocene sandstone, MCL, and MCM.
PROVENANCE DATA
Clast count data (Fig. 3) show that metamorphic clasts comprise 15%–25% of clast populations throughout the section. In Arroyo Las
Flores, which sources only MCL, they comprise 40%. Granitic clasts first appear in
PU13 (1%) and are present in PU14 (1%) and
Quaternary deposits (5%). Eocene sandstone
clasts occur at very low abundance (1%–2%)
in PU7, 13, and 14.
MCL-derived clasts first occur in PU1 (3%),
and their abundance notably increases after
PU9. In footwall-derived Quaternary deposits,
they comprise 45%–50%, but only 25% in those
reworking Pliocene deltas. An absence of MCM
clasts in Arroyo Las Flores agrees with stratigraphic relations. MCM clasts are present from
sequence 1 to Quaternary deposits, with abundance between 40%–50% in the Pliocene, and
20%–25% in Quaternary deposits.
Non–source-specific clasts comprise the
remaining populations. Those derived from
dikes are present (<25%) in all counts. Those
from Miocene volcanic flows are absent in
Arroyo Las Flores, suggesting derivation from
upper MCM and MCU rocks. Reworked shells
1
GSA Data Repository item 2007161, clast types
and inferred source lithology recorded in the Loreto
basin, is available online at www.geosociety.org/
pubs/ft2007.htm, or on request from editing@
geosociety.org or Documents Secretary, GSA, P.O.
Box 9140, Boulder, CO 80301, USA.
GEOLOGY, July 2007
and unknown clasts occur at very low abundance (1%). There is little variation between the
shoal-water and Gilbert-type delta of PU6.
IMPLICATIONS FOR DRAINAGE
EVOLUTION
The NNE sediment transport indicates
a footwall source to the SSW; however, no
present-day drainage exists across the footwall.
Instead, Pliocene boulder debris-fan and Quaternary cobble alluvial fan deposits (Dorsey and
Umhoefer, 2000; this study) onlap MCL rocks
(Fig. 1), suggesting that a drainage did exist during the Pliocene.
Metamorphic rocks were exposed during
deposition of MCL, and their occurrence without MCL clasts in sequence 1 (Fig. 3) suggests
that they continued to form topographic highs.
Reworking and recycling of metamorphic clasts
are indicated by their high relative abundance in
Pliocene deltas and Arroyo San Antonio. Their
notable occurrence in Arroyo Amarillo probably
reflects reworking of Pliocene deposits.
The dominant trend in the data is the upsection increase in MCL clasts, which records
an increase in exposure of MCL rocks. The
likely mechanism for this is unroofing, both
through vertical incision and lateral catchment
expansion while overlying MCM rocks were
removed. In this unroofing scenario, MCM clast
abundance should decrease as MCL increases;
this is only observed in the Quaternary deposits,
while MCM abundance in the Pliocene (including reworked Arroyo Amarillo) counts is relatively constant. This continued MCM abundance
probably reflects lateral catchment expansion in
the Pliocene, while the MCL increase was generated by incision. The decrease in MCM abundance in the Quaternary deposits suggests that
unroofing continued until this time.
GEOLOGY, July 2007
6GD
6S W
A
S1-
S1B
Cumulative clast count data
MCL only
Figure 3. Results of clast
Upper Deltas only
count data for each loca100%
tion. Clast types are: metaEocene Sandstone
morphic (M/m); granite
(Kg); andesitic breccia
MCL
80%
(Ab); dikes (Ad) with hornclast type
blende (Adh); volcaniM/m
Kg
clastic (Vcc); altered
Ab
volcanic (Alt); Miocene vol60%
Ad
canic (Mv); Eocene sandAdh
Vcc
stone (Es); intrusive
MCM
Alt
Mv
unknown (Ti); shells (Sh);
40%
Es
Quaternary lava (Lv);
Sh
MCM
Lv
unknown (Ukn). Clasts
In
and footwall sources are
Ukn
20%
detailed in GSA Data
Repository (see text footMetamorphic
Granite
note 1). Five distinctive
0%
footwall sources—metaEl Leon
Amarillo
7
9 11 12 13 14
1
4
Las Flores
morphic, granite, MCL
San Antonio
(Comundú Group lower
Delta Progradational Units (PUs)
Quaternary (Qt)
Seq. 1
unit), MCM (Comundú
Up Sequence
Group middle unit), and
Eocene sandstone—are highlighted to show trends.
The catchment delimited by modern drainage
divides is too small to provide the sediment volume preserved in the basin, particularly given
the amount of footwall rocks remaining. The
Pliocene catchment must, therefore, have been
larger. The outcrop of metamorphic rocks in the
Pliocene catchment may be traced to a low point
on the modern divide (C-C′; Fig. 1), which may
have contained a Pliocene drainage later captured by Arroyo San Antonio.
The occurrence of Eocene sandstone in PU7
indicates the level of vertical incision by this time.
Its absence in PU8–12 and reoccurrence in PU13–
14 probably reflect limited exposure and low
preservation potential. Granitic rocks first occur
in PU13, indicating late-stage sourcing. Map relations suggest that granite underlies MCL rocks in
the footwall (Fig. 1). This is further supported by
the occurrence of granite clasts in Arroyo El Leon
(Quaternary), which record a granite source in this
region despite a lack of modern outcrop.
INCREASED UNROOFING IN RESPONSE
TO INCREASED DISPLACEMENT
The provenance data record an unroofing
trend in response to footwall uplift. Here, calculated incision rates are based on the first appearance of clast types, and depositional unit (time)
is based on the age and duration of PUs of Mortimer et al. (2005). MCM and MCL unit thickness has been assumed to be constant. Incision
rates provide an estimate of vertical incision that
most likely reflects higher-order streams.
Fault activity initiated by ca. 3.5 Ma
(Umhoefer et al., 1994). Between this initiation and the first appearance of MCL rocks
(ca. 2.5 Ma), ~450–500 m of overlying MCM
rocks had to be removed. This yields an incision
rate of ~0.45–0.5 mm/yr (slower if deposition
commenced earlier).
During sequence 2, 300 m of MCL rocks
overlying the granite were removed in the north
of the catchment by the first granitic clast appearance in PU13 (190 k.y. in sequence 2); this yields
an incision rate of 1.6 mm/yr. The appearance
of Eocene sandstone in PU7 requires 300 m
of MCL removal in ~100 k.y., which equates to
an incision rate of 3 mm/yr. This greater incision rate in the center of the catchment possibly
resulted from greater stream power. However,
these estimates do not account for lateral catchment expansion into tributaries where MCM
rocks remained overlying MCL rocks.
A fourfold increase in incision rate occurred
between sequence 1 and PU1 (20 k.y. into
sequence 2), which we interpret to be related to
increased displacement rate. A possible increase
in displacement rate is inferred to have occurred
during PU6 (~100 k.y. into sequence 2). While no
change in provenance is apparent, a pronounced
increase in MCL clast abundance occurs between
PU9 and PU11 (30–40 k.y. later).
DRAINAGE EVOLUTION IN RESPONSE
TO INCREASING FOOTWALL UPLIFT
We propose a model of footwall drainage evolution in response to increasing uplift (Fig. 4).
At the start of fault activity (ca. 3.5 Ma), the
displacement rate was low. Incision into MCM
rocks (~0.45 mm/yr) was focused along drainages perpendicular to the fault, while metamorphic rocks formed topographic highs. During
sequence 1, the catchment began to expand laterally, establishing tributaries within the MCM.
The displacement rate increased at the start
of sequence 2, possibly due to segment linkage,
whereby the displacement rate in this region of
overlap would increase most (Dawers and Anders,
1995; Gupta et al., 1999). The increased displacement rate led to increased footwall uplift, which
enhanced incision (1.6–3 mm/yr) and produced
a near-synchronous erosional response. Between
PU1 and PU6, the vertical incision rate did not
increase within higher-order streams, but the
catchment expanded laterally, removing MCM
rocks and exposing MCL rocks in tributaries.
The possible displacement rate increase in
PU6 may be reflected by increased MCL abundance between PU9 and PU11 (30–40 k.y. lag
time). During deposition of PU13, granite was
exposed in the north of the catchment, while
the headwaters must have extended west and
north of the modern-day catchment. Large
boulders in alluvial facies in the footwall and
immediate hanging wall indicate that a proximal source remained.
After sequence 2, we propose a drainage
reorganization with capture into the Arroyo San
Antonio and diversion or capture of the main
stream into Arroyo de Gua. Unroofing continued into the Quaternary, recorded by significant reduction of MCM and increase of MCL
653
Sequence 2 - PU1-6
Sequence 1
N
N
NF
NF
LF
study area
LF
a
Gulf of Californi
Sequence 2 - PU9-11
N
Sequence 2 - PU6-9
N
NF
NF
LF
LF
a
Gulf of Californi
a
Gulf of Californi
Post Sequence 2
Sequence 2 - PU11-14
N
N
NF
NF
LF
Gulf of Californi
a
LF
Uplifting Eastern
Structural High
Figure 4. Proposed drainage reorganization and evolution in response to footwall uplift
during an increase in displacement rate on the Loreto fault (see text for discussion).
clasts (Fig. 3). Fault scarp retreat had begun
during deposition of sequence 2, as suggested
by footwall alluvial deposits. The presence of a
pediment surface across the basin and within the
footwall, however, suggests that rapid escarpment retreat occurred during the Quaternary.
LAG TIME AT DIFFERENT FORCING
FREQUENCIES
Provenance data record a near instantaneous
(<15 k.y.) response to a longer-term (200 k.y.)
displacement rate increase. The proposed
second-order (100 k.y.) increase (PU6) resulted
in an increase in MCL clasts 30–40 k.y. later
(PU9−PU11). The lag time could have been
due to sediment storage in the fluvial system, or
the erosion response time for lesser-magnitude
events. It appears that greater-magnitude displacement rate variations generate a more rapid
catchment response. Recurring high-frequency
(10 k.y.) fluctuations are not recorded.
We conclude that for the Loreto fault, longterm, high-magnitude (fourfold) variations in fault
displacement rate, and therefore footwall uplift
rate, led to an increased incision rate and unroofing recorded almost instantaneously (<20 k.y.)
by the sedimentary system. In contrast, lowermagnitude variations generated a response with
a greater lag time (30–40 k.y.). High-frequency,
high-magnitude variations in fault displacement
rate are not recorded but are filtered out.
654
The unique Loreto basin example documents catchment evolution to varying displacement rate at typically irresolvable time scales.
Our new quantitative data provide insight
into catchment response previously exclusive
to landscape models. The rapid catchment
response to long-term, high-magnitude variations, while filtering out high-frequency fluctuations, is likely to be a characteristic of normal
fault footwall catchments in general.
ACKNOWLEDGMENTS
The authors thank B. Dorsey, A. Densmore,
P. Umhoefer, and N. Dawers for constructive
reviews; and P. Cowie, D. Commins and S. Gupta
for discussion. Field work was funded by Deutsche
Forschungsgemeinschaft (DFG) proposal MO1-1664.
REFERENCES CITED
Allen, P.A., and Densmore, A.L., 2000, Sediment
flux from an uplifting fault block: Basin
Research, v. 12, p. 367–380, doi: 10.1046/
j.1365-2117.2000.00135.x.
Cowie, P.A., Attal, M., Tucker, G.E., Whittaker, A.C.,
Naylor, M., Ganas, A., and Roberts, G.P., 2006,
Investigating the surface process response to
fault interaction and linkage using a numerical
modeling approach: Basin Research, v. 18, p. 1–
12, doi: 10.1111/j.1365-2117.2006.00283.x.
Dawers, N.H., and Anders, M.H., 1995, Displacement-length scaling and fault linkage: Journal
of Structural Geology, v. 17, p. 607–614, doi:
10.1016/0191-8141(94)00091-D.
Densmore, A.L., Ellis, M., and Anderson, R.S.,
1998, Landsliding and the evolution of normalfault-bounded mountains: Journal of Geophys-
ical Research, v. 103, p. 15,203–15,219, doi:
10.1029/98JB00510.
Densmore, A.L., Dawers, N.H., Gupta, S., Guidon,
R., and Goldin, T., 2004, Footwall topographic
development during continental extension: Journal of Geophysical Research–Earth Surface,
v. 109, FO3001, doi: 10.1029/2003JF000115.
Dorsey, R.J., and Umhoefer, P.J., 2000, Tectonic and
eustatic controls on sequence stratigraphy of
the Pliocene Loreto basin, Baja California Sur,
Mexico: Geological Society of America Bulletin, v. 112, p. 177–199, doi: 10.1130/00167606(2000)112<0177:TAECOS>2.3.CO;2.
Dorsey, R.J., Umhoefer, P.J., and Renne, P.R., 1995,
Rapid subsidence and stacked Gilbert-type fan
deltas, Pliocene Loreto basin, Baja California Sur,
Mexico: Sedimentary Geology, v. 98, p. 181–204,
doi: 10.1016/0037-0738(95)00032-4.
Dorsey, R.J., Umhoefer, P.J., and Falk, P.D., 1997,
Earthquake clustering inferred from Pliocene
Gilbert-type fan deltas in the Loreto basin, Baja
California Sur, Mexico: Geology, v. 25, p. 679–
682, doi: 10.1130/0091-7613(1997)025<0679:
ECIFPG>2.3.CO;2.
Graham, S.A., Tolson, R.B., DeCelles, P.G., Ingersoll, R.V., Bargar, E., Caldwell, M., Cavazza,
W., Edwards, D.P., Follo, M.F., Handschy, J.F.,
Lemke, L., Moxon, I., Rice, R., Smith, G.A.,
and White, J., 1988, Provenance modeling as a
technique for analyzing source terrane evolution
and controls on foreland sedimentation: Spec.
Publs. Int. Ass. Sediment, v. 8, p. 425–436.
Gupta, S., Underhill, J.R., Sharp, I., and Gawthorpe,
R., 1999, Role of fault interactions in controlling syn-rift sediment dispersal patterns:
Miocene Abu Alaqa Group, Suez Rift, Sanai,
Egypt: Basin Research, v. 11, p. 167–190, doi:
10.1046/j.1365-2117.1999.00300.x.
Leeder, M., and Jackson, J.A., 1993, The interaction
between normal faulting and drainage in active
extensional basins, with examples from the
western United States and central Greece:
Basin Research, v. 5, p. 79–102.
McLean, 1989, Reconnaissance Geological Map of
the Loreto and part of the San Javier Quadrangles, Baja California Sur, Mexico: U.S. Geological Survey Map MF-2000, scale 1:50000.
Mortimer, E., 2004, Tectonic controls on the growth
of coarse-grained delta clinoforms in the
Pliocene Loreto Basin, Baja California Sur,
Mexico (Ph.D. Thesis): Edinburgh, Scotland,
University of Edinburgh.
Mortimer, E., Gupta, S., and Cowie, P.A., 2005,
Clinoform nucleation and growth in coarsegrained deltas, Loreto basin, Baja California
Sur, Mexico: A response to episodic accelerations in fault displacement: Basin Research,
v. 17, p. 337–359, doi: 10.1111/j.1365-2117.
2005.00273.x.
Umhoefer, P.J., Dorsey, R.J., and Renne, P.R., 1994,
Tectonics of the Pliocene Loreto basin, Baja
California Sur, Mexico, and evolution of the
Gulf of California: Geology, v. 22, p. 649–
652, doi: 10.1130/0091-7613(1994)022<0649:
TOTPLB>2.3.CO;2.
Umhoefer, P.J., Dorsey, R.J., Willsey, S., Mayer,
L., and Renne, P.R., 2001, Stratigraphy and
geochronology of the Comundú Group near
Loreto, Baja California Sur, Mexico: Sedimentary Geology, v. 144, p. 125–147, doi: 10.1016/
S0037-0738(01)00138-5.
Manuscript received 25 January 2007
Revised manuscript received 28 February 2007
Manuscript accepted 4 March 2007
Printed in USA
GEOLOGY, July 2007