K

K K

K K

0 2 4 6 8 10 12 14 16 18

METERS

20 22 24 26 28

METERS

0

2

4

6

NORTHEAST SOUTHWEST

RITO SECO SOUTH (RSS) TRENCH

Unit 4: 

Fluvial channel

deposits

2

2

3

4-1

4-2

4-3

7

5-2

5-1

9-3

9-4

8

7

6-1

6-1

6-2

8

1

1

1

9-1

9-4

9-4

9-4

9-1

9-3

9-3

9-1 9-3

9-3

9-1

9-2

9-2

9-2

6-2 6-2

Location of soil

profile RSS

(table 3)

1

135°

145°

Canal berm

Spoil

135°

RSS-L2; NA

RSS-L1; 5.5±0.4 ka

RSS-C1; 5.5±0.2 ka

RSS-L4;

12.4±1.8 ka

RSS-C3; NA RSS-C2; NA RSS-C4; NA

RSS-C5;

1.5±0.1

RSS-L3

13.5±0.9 ka

Filled canal

EH1

Figure 4. Map of south wall of trench across Sangre de Cristo fault at the Rito Seco South (RSS) site. Full description of units is given in table 1. Data for luminescence and radiocarbon samples are given in table 5.

Scarp height: 2.7 m

Surface offset: 2.2 m

(from profile RSS-A, fig. 3)

EXPLANATION OF UNITS

Unit 1—Canal fill

Unit 2—Cumulic A horizon

Unit 3—Slope colluvium, hanging wall

Unit 4—Fluvial channel deposits

 B horizon (prismatic structure)

 Channel fill, sandy facies

 Channel fill, basal facies

Unit 5—Colluvium from PE1

 Colluvium on footwall

 Colluvium on hanging wall

Unit 6—Slope colluvium

 A horizon on debris flow

 Bwk horizon on debris flow

Unit 7—Debris flow, contains reworked

 charcoal

Unit 8—Fluvial gravel

Units 9-1–9-4—Tertiary Santa Fe Group; see table 1 for description

 of subunits; patterned areas show sand-rich beds 

1

2

3

4-1

4-2

4-3

5-2

5-1

6-1

6-2

7

8

9

0 2 4 6 8 10 12 14 16 18

METERS

20 22 24 26 28 30 32

METERS

0

2

4

6

8

10

EAST WEST

RITO SECO NORTH (RSN) TRENCH 

Unit 4:

Fluvial 

channel

deposits

? ?

EH3

EH2

EH4(?)

EH4(?)

EH3

EH2

EH1

EH1

K

K

K

K

K

tree throw K

K

K

K

K

K

? ?

RSN-L2;

9.2±0.6 ka

RSN-L3;

18.5±1.1 ka 

RSN-L1;

5.1±0.2 ka

RSN-L4;

23.5±2.7 ka

RSN-L5;

27.3±1.2 ka

Disturbed surface

7-1

2-1

8-1

8-2

8-2

8sz

8-2

7sz

2-2

3-1

5-1

6

7-1

7-2

7-2

7-1

3-3

5-2 5-2

5-3 5-3

3-2

2-2

1

4-1

4-2

?

RSN-L6;

27.7±2.8 ka

RSN-C1; None

RSN-L7;

51.2±3.8 ka

7sz

3-1

7-1

7-1

7-1

Scarp height: 6.6 m

Surface offset: 6.2 m

(from profile RSN-A, fig. 3)

Figure 5. Map of south wall of trench across Sangre de Cristo fault at the Rito Seco North (RSN) site. Full description of units is given in table 2. Data for luminescence and radiocarbon samples are given in table 5.

Disturbed surface

7-1

7-1

8-1

6

6

7-1 7-1 7-1

5-2 3-1

3-1

3-2

Filled canal

Unit 1—Canal fill

Unit 2—Colluvium from PE1 (most recent event)

 A horizon on slope colluvium

 Colluvium from event PE1

Unit 3—Colluvium from PE2 (penultimate event)

 Proximal colluvium 

 Distal colluvium, calcic (weak B horizon)

 Basal distal colluvium

Unit 4—Fluvial channel deposits

 Channel-fill, upper sand facies 

 Channel-fill, basal facies

Unit 5—Colluvium from PE3

 Proximal colluvium

 Upper distal colluvium, calcic

 Basal distal colluvium

Unit 6—Colluvium from PE4(?), calcic, clay-rich

 B horizon at top (shown by symbol)

Unit 7—Fluvial gravel

 Cobble-boulder gravel; pattern areas are sand rich

 Medium-grained sand

 Sheared gravel

Unit 8—Tertiary Santa Fe Group

 Cobble-pebble sandstone

 Interstratified sand and gravel

 Sheared gravel

1

2-1

2-2

3-1

3-2

3-3

4-1

4-2

5-1

5-2

5-3

6

7-1

7-2

7sz

8-1

8-2

8sz

EXPLANATION OF SYMBOLS FOR FIGURES 4 AND 5

Fault-related structures Contacts Symbols

Sharp—Well-defined

 contact between units

 and within units

Gradational or distinct

Inferred or indistinct

Preferred age from 

 luminescence (L) or

 radiocarbon (C) sample;

 NA, not analyzed 

Strike of fault, fracture,

 or contact

Fault—Arrows show

 slip direction; thin line weight

 indicates older, less active strands

Fault-related free face

Event horizon—Ground surface at

 time of prehistoric earthquake;

 numbers 1–4 correlate to

 PE1–PE4 events

135°

RSS-L4

12.4±1.8 ka

K

Cobbles and boulders

Krotovina—Burrows and

 root fillings; large disturbed

 area labeled as “tree throw”

 interpreted to be site of root

 mass from ancient tree EH

8

6

4

2

0

100 80 60 40 20 0

METERS

METERS

Profile RSN-A

(Located along south side of RSN trench)

EAST WEST

Scarp height: 6.55 m

Surface offset: 6.15 m

Maximum slope angle: 17.2°

VERTICAL EXAGGERATION 3X

80 60 40 20 0

METERS

6

4

2

0

METERS

NORTHEAST SOUTHWEST

Profile RSS-A

(Located along south side of RSS trench)

Scarp height: 2.67 m

Surface offset: 2.22 m

Maximum slope angle: 17.2°

VERTICAL EXAGGERATION 3X

METERS

-1

60 40 20 0

4

2

0

METERS

NORTHEAST SOUTHWEST

Profile RSS-B

(Located approximately 75 m southeast of RSS trench)

Canal

VERTICAL EXAGGERATION 3X

Scarp height: 1.17 m

Surface offset: 0.90 m

Max. slope angle: 10.3°

60 40 20 0

4

2

0

METERS

EAST WEST

Profile RSN-B

(Located approximately 75 m south of RSN trench)

6

8

80

Scarp height: 6.28 m

Surface offset: 6.15 m

Maximum slope angle: 21.0°

VERTICAL EXAGGERATION 3X

Figure 3. Topographic profiles across fault scarps 

adjacent to and near trenches at the Rito Seco site. See 

figure 2B for locations. Green diamonds are data points. 

A man-made canal along the base of the scarp has locally 

modified the slope profiles, but has not substantially 

affected measurements of scarp heights or surface offsets. 

Scarp heights and surface offsets are based on definitions 

of Bucknam and Anderson (1979).

Trend of profile: 83°

Trend of profile: 80°

Trend of profile: 35°

Trend of profile: 40°

METERS

Canal

Canal

Canal

Tsf

Tsf

Tsf

Qac

Qac

Qac

Qai

Qx

Qa

Qay1

Qay1

Qay1

Qay2

Canal

R i t o

S e c o

Rito Seco Road

EXPLANATION

Qa Quaternary alluvium, modern

Quaternary alluvium, young,

 late phase

Qay2

Qai Quaternary alluvium, intermediate

Quaternary fan alluvium and colluvial

 deposits, undifferentiated

Qac

Quaternary deposit,

 modified by human activity

Qx

Earth-filled dam

Fault—Bar and ball on downthrown side;

 dashed where inferred

Quaternary alluvium, young,

 early phase

Qay1

Tsf Tertiary Santa Fe Group

Figure 2A. Geologic map of Quaternary and Tertiary deposits in the vicinity of the Rico Seco study site overlain on aerial 

photograph. Red lines show generalized traces of the Sangre de Cristo fault. Outline shows area of detailed site map shown 

in figure 2B. 

0 100 200 300 METERS

Qai

Qa

Qx

Qay1

Qay1

Qay1

Qac

UTM COORDINATE, EAST (METERS)

UTM COORDINATE, NORTH (METERS)

465100 465200 465300 465400 465500

4121600

4121500

4121400

4121300

4121200

RSS-B

RSS-A

RSN-B

RSN-A

Rito Seco Road

WP 05A

WP 05

WP 01A

WP 01B

WP 04

WP 02

WP 03A

WP 03B

WP 06

WP 01

WP 03

Rito Seco

South (RSS)

trench

Rito Seco

North (RSN)

trench Soil pit

Earth-filled

dam

R i t o S e c o

EXPLANATION

Qa Quaternary alluvium, modern

Qai Quaternary alluvium, intermediate

Quaternary fan alluvium and colluvial

 deposits, undifferentiated

Qac

Qx Quaternary deposit, modified by human activity Earth-filled dam

RSS-B

Trench

Scarp profile

Fault—Bar and ball on downthrown

 side; dashed where inferred

WP 02 GPS waypoint

Qay1 Quaternary alluvium, young, early phase

Figure 2B. Geologic map of the Rito Seco site showing locations of trenches, fault scarps, scarp profiles, and GPS 

waypoints. Scarp profiles are shown in figure 3. Maps of trench walls are shown in figures 4 and 5. GPS data are listed in 

table 4. 

Town of

San Luis, Colo. 

Cristo 

fault 

de 

Sangre

San Luis

Valley

Study

area

Great

Sand Dunes

National Park

Sangre de Cristo

M

o

u

n

ta

ni s

PP

Denver

Sangre de

Cristo fault

Figure 1A. Map showing Cenozoic (yellow) and Quaternary (red) faults and historical earthquakes (orange dots) in Colorado. Image is from the Colorado Geological Survey. 

Figure 1B. Map showing the Sangre de Cristo Range and San Luis Valley in south-central 

Colorado. White arrows show location of the Sangre de Cristo fault, which bounds the western 

side of the range. PP, Poncha Pass. Outline shows area of figure 1C. Image obtained from NASA 

Applied Sciences Directorate Earth Science Enterprise Scientific Data Purchase Program map 

server at https://zulu.ssc.nasa.gov/mrsid.

Figure 1C. Map showing the San Luis section of the Sangre de Cristo fault; dashed red line 

shows trace of the fault. Outline is location of the Rito Seco study area shown in figure 1D. 

Image obtained from NASA Applied Sciences Directorate Earth Science Enterprise Scientific 

Data Purchase Program map server at https://zulu.ssc.nasa.gov/mrsid.

Figure 1D. Aerial photograph of the Rito Seco study site. Solid red lines show 

generalized traces of the Sangre de Cristo fault; bar and ball on downthrown side. 

Dashed fine red line shows probable fault along eastern side of mesa. Outline shows 

approximate area of most of the geologic site map shown in figure 2A. The green 

triangle labeled WP indicates location of waypoint Ritoseco-Rd 19.5 in table 4. 

Range crest

Crestone Pk.

4,358 m

San Luis Valley

Rito Seco

Range crest

Study area

Culebra Pk.

4,281 m

Town of

San Luis

Approximate

location of fault

Figure 1E. Oblique view to the east of the Crestone section of the Sangre de Cristo range 

showing the steep range-front morphology and proximity of the range crest. Image available 

at http://earth.google.com (last accessed December 2006).

Figure 1F. Oblique view to the east of the San Luis section of the Sangre de Cristo range 

showing the relatively subdued range-front morphology and distance to the range crest. 

Image available at http://earth.google.com (last accessed December 2006).

WP 

0 1 2 KILOMETERS

Blanca Peak

105°25’

37°12’30”

102°03’

37°00’

41°00’

109°03’

Culebra reentrant

0 100 KILOMETERS

0 50 KILOMETERS

105°30’

0 10 KILOMETERS

37°30’

105°30’

37°30’

Approximate

location of fault

 

Table 5. Luminescence and radiocarbon sample data for the Rito Seco North (RSN) and Rito Seco South (RSS) trenches.

[—, not available]

 aPosition on south wall of trench in horizontal (X) and vertical (Y) directions where described sample was collected.

 bSamples analyzed at the Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, Calif.

 cRadiocarbon samples RSS-C2, RSS-C3, and RSS-C4 were not analyzed because they are from a stratigraphically younger unit and determining 

their age would not improve the time constraint on the most recent surface rupture (PE1) on the fault.

 dSample analyzed at National Ocean Sciences Accelerator Mass Spectrometry Facility, Woods Hole Oceanographic Institution, Woods Hole, Mass.

 eSamples calibrated using Radiocarbon Calibration Program CALIB, v. 4.3, Quaternary Isotope Laboratory, University of Washington, Seattle, 

Wash. Original version of program is described by Stuiver and Reimer (1993). Ages are B.P. (before 1950) with two-sigma error limits: Listed as the 

maximum calibrated age, calibrated age (in parentheses), minimum calibrated age.

 fRadiocarbon age for sample RSN-C1 is anomalously young for its stratigraphic position and is likely young organic material filling a burrow 

excavated into unit 3-1. Because the sample does not date the actual deposit in which it is located (unit 3-1), it has no value in interpreting the 

faulting history and therefore, the calibrated age is not applicable (NA), and we do not provide a preferred age.

 gThis age is the mode of the calibrated age, shown in parentheses in the Calibrated age column; this age is rounded to the nearest 100 years. The 

plus-or-minus uncertainty is a range that broadly captures the maximum and minimum calibrated age ranges. This preferred age is shown on the 

trench maps (figs. 4 and 5).

 a Position on south wall of trench in horizontal (X) and vertical (Y) directions where described sample was collected.

 b Infrared-stimulated luminescense (IRSL) age determination; age given in calendar years (cal. yr) referenced to the present.

 c Blue-light optically stimulated luminescense (OSL); age given in calendar years (cal. yr) referenced to the present.

 d Preferred age is usually the average of the IRSL and OSL determinations rounded to the nearest 100 years. The plus-or-minus values are similar 

averages. The preferred age is shown on the trench maps (figs. 4 and 5).

 e The age is queried because the sample yielded erratic results during analysis. Preferred age for this sample is from the OSL measurement.

 f This IRSL age is the average of two measurements: 55,530±5,250 and 46,940±2,310.

 g Luminescense analysis of this sample suggests that it could be approaching saturation and thus, this number could be a minimum estimate of its 

actual age. Preferred age for this sample is from the IRSL measurements.

 h Preferred age for RSS-L3 and RSN-L1 is based only on results from the blue-light OSL analysis. IRSL ages for these samples were not 

determined. 

Radiocarbon sample data Luminescence sample data

Sample ID

RSS-C1

RSS-C2

RSS-C3

RSS-C4

RSS-C5

RSN-C1

Locationa

X/Y (m) 

23.6/0.9

22.55/1.35

23.1/1.32

20.32/1.42

17.15/2.3

15.75/3.65

Unit

number 

4-2

4-1

4-1

4-1

3

5-1

Radiocarbon age 

(

14C yr B.P.)

4,850±70b

not analyzedc

not analyzedc

not analyzedc

1,610±35d

210±35b

Calibrated agee

(cal. yr B.P.)

5,723 (5,529) 5,334 

—

—

—

1,585 (1,522) 1,411

308 (NA) 2f

Sample ID

RSS-L1

RSS-L2

RSS-L3

RSS-L4

RSN-L1

RSN-L2

RSN-L3

RSN-L4

RSN-L5

RSN-L6

RSN-L7

Locationa

X/Y (m)

23.6/0.9

23.7/1.2

7.6/3.8

17.5/0.65

23.3/3.6

23.1/3.3

27.4/0.7

21.6/3.1

21.55/2.85

18.3/2.25

18.1/0.95

Preferred ageg

(ka, kiloannum)

5.5±0.2 

—

—

—

1.5±0.1

Nonef

Unit

number

4-2

4-2

7

8

2-2

3-2

4-2

3-2

5-2

5-3

7-2

IRSL ageb

(cal. yr) 

5,456±314

not analyzed

not analyzed

10,380±940

not analyzed

8,750±540

17,020±1,000

21,270±1,290

25,780±1,650

19,070±2,420(?)e

51,235±3,780f

Blue-light OSL agec

(cal. yr) 

5,312±429

not analyzed

13,521±903

14,500±2,680

5,130±247

9,790±660

20,030±1,270 

25,650±4,010

28,880±770

27,700±2,750

42,230±4,830g

Preferred aged

(ka, kiloannum)

5.5±0.4 

—

13.5±0.9h

12.4±1.8

5.1±0.2h

9.2±0.6

18.5±1.1

23.5±2.7

27.3±1.2

27.7±2.8e

51.2±3.8

Waypoint name

(labeled WP in

figs. 1D and 2B)

Ritoseco 01

Ritoseco 01A

Ritoseco 01B

Ritoseco 02

Ritoseco 03

Ritoseco 03A

Ritoseco 03B

Ritoseco 04

Ritoseco 05 

Ritoseco 05A

Ritoseco 06

Ritoseco-Rd 19.5

37.24090

37.24082

37.24083

37.24033

37.23998

37.23994

37.23011

37.24097

37.24108

37.24228

37.23912

37.24246

Comments

Survey station on bench on north side of RSN trench.

West corner of south wall of RSN trench.

East corner of south wall of RSN trench.

Base station used as control point to locate RSN and RSS trenches.

Survey station on north side of RSS trench.

West corner of south wall of RSS trench.

East corner of south wall of RSS trench.

Center of east edge of soil pit.

Control point on crest of earth-filled dam, west of RSN trench.

At center of Rito Seco road where dam crest projects northward to center of road.

Control point at southwest corner of reservoir, at base of dam. 

At intersection between north-south County Road 19.5 and east-west Rito Seco Road.

Table 4. GPS data and surveying control for the Rito Seco trench sites.

GPS bearing notes: Ritoseco 02 to 01=325°±1°, distance 74 m.

Ritoseco 03B to 03A=21°±2°, Brunton compass bearing=215°

 (south wall of RSS trench). 

Ritoseco 02 to 03=178°±2°, distance 45 m.

Ritoseco 04 to 01=255°.

Ritoseco 04 to 02=184°.

Ritoseco 04 to 03=181°.

105.38976

105.38987

105.38955

105.38929

105.38924

105.38924

105.38908

105.38918

105.39261

105.39317

105.39114

105.42561

0465421

0465410

0465439

0465461

0465465

0465466

0465480

0465471

0465168

0465118

0465296

0462240

4121482

4121472

4121474

4121418

4121380

4121375

4121394

4121489

4121503

4121636

4121284

4121668

Ritoseco 01B to 01A=258°±2°, Brunton compass bearing=262° (south wall of RSN trench).

Ritoseco 03 to 01=334°, distance 109 m.

Ritoseco 05 to 02=106°, distance approximately 300 m.

Ritoseco 06 to 02=50°, distance 219 m.

Datum: 1927 North American Datum (NAD27).

Latitude (N.) and longitude (W.)

(decimal degrees)

UTM coordinates (meters)

Zone 13

Easting Northing Horizona

Profile RSN–Soil pit, hanging wall, 20 m east of fault

(Amalia soil series, Ustic Calciargid)

A

Bt

Bk1

Bk2

C

Profile RSS–Trench footwall, south side, near 5.5 m

(Kibin soil series, Pachic Arginstoll)

A

Bkt

2Bkt2

2Btk3 

3C1

4C2

Unit

 number

NA

NA

NA

NA

NA

6-1

6-1

7

7

8

9

Depth top

(cm)

 0

 15

 38

 79

 165

 0

 23

 46

 74

 114

 185

Depth base

(cm)

 15

 38

 79

 165

 NE

 23

 46

 74

 114

 185

 NE

Lowerb

boundary

C, S

C, W

C, W

C, W

NE

C, S

C, W

C, W

C, W

C, W

NE

Colorc

moist

7.5 YR 8/2

7.5 YR 3/3

7.5 YR 3/4

7.5 YR 7/3

7.5 YR 4/4

10 YR 3/1 

10 YR 3/3

7.5 YR 5/4

7.5 YR 4/4

7.5 YR 7/6

7.5 YR 5/4

Textured

L

L, vgr

LS, xgr

LCOS, xgr

COS, xgr

L

L, gr

L, gr

L, gr

COS, xgr

SL, vgr

Gravele

(percent)

10

45

75

75

70

10

25

17

17

80

45

Structuref

1–2, f–m, gr

2, f–m, sbk

sg

sg

sg

1, f, gr,

2, f–m, sbk

2, f–m, pr–sbk

2, m, sbk

sg

ND

Dryg

consistence

ND

sh

ND

ND

ND

ND

ND

ND

ND

ND

ND

Wetg

consistence

ss, ps

ms, mp

ND

ND

ND

ND

ND

ND

ND

ND

ND

Claye

(percent)

14

26

<5

<5

<5

13

24

26

26

<2

10

Comments

Subrounded clasts, non-calcareous.

Subrounded clasts, non-calcareous, thin clay films on ped faces.

Subrounded clasts, slightly calcareous, CaCO3

 disseminated in matrix and forms coats on bottom of

 clasts.

Subrounded clasts, strongly calcareous, CaCO3

 disseminated in matrix and forms coats on bottom of

 clasts.

Subrounded clasts, non-calcareous, few thin CaCO3 coats under clasts.

Non-calcareous. 

Few thin CaCO3 coats on base of clasts.

Strongly calcareous, CaCO3

 coats on clasts and masses in matrix.

Strongly calcareous, few thin CaCO3

 coats under clasts.

Few thin CaCO3

 coats under clasts.

Santa Fe Group; seams and coats of CaCO3

.

Table 3. Soils data from the Rito Seco trench sites.

[Described by Alan Stubbe, National Resource Conservation Service, San Luis, Colo. office, Sept. 16, 2005]

a

Horizon nomenclature from Soil Survey Staff (1975) and Birkeland (1999).

b

Boundary: Distinctness: C, clear; Topography: S, smooth; W, wavy; NE, not exposed.

c

Colors: From Munsell Soil Color Chart (Kollmorgen Instruments, Corp., 1994). 

d

Texture classes: L, loam; LS, loamy sand; SL, sandy loam; COS, coarse sand; LCOS, loamy coarse sand; gr, gravelly (15–35 percent); vgr, very gravelly (36–60 percent); xgr, extremely gravelly (61–90 percent). e

Percentage of gravel and clay estimated in the field.

f

Structure: Grade: 1, weak; 2, moderate; Size class: f, fine; m, medium; Type: sbk, subangular blocky; pr, prismatic; sg, single grain; gr, granular; ND, no data.

g

Consistence: Dry: sh, slightly hard; Wet: ss, slightly sticky; ms, moderately sticky; ps, slightly plastic; mp, moderately plastic; ND, no data.

Table 2. Unit descriptions for the Rito Seco North (RSN) trench.

Unit

number

1

2-2

3-1

3-2

3-3

4-1 

4-2

5-1

5-2

5-3

6

7-1

7-2

7sz

8sz

 

 

2-1

7-1

8-1

8-2

Locationa

X/Y (m)

23.5/4.0

23.5/3.6

15.5/4.5

23.4/3.3

20.5/3.1

27.0/1.9 

27.2/0.7

15.5/3.4

19.3/2.75

19.0/2.45

21.65/2.3

22.0/1.0

18.1/0.9

13.5/3.0

11.5/5.0

3.0/9.1

5.0/7.5

9.1/5.4

10.35/4.25

Matrixb

 texture

SL

S–SL

SL(?)

LS 

S

S

L–SL

S

S

SL

LS–L

S

S

S–LS

Var.

SL

S

S

S

Gravel

(percent)

Var.

10–20

35–50

<5

20–30

<<5

<2

5–10

15–25

5–20

NA

50–90

0

ca. 50

0–>50

5–20

Var.

10–90

<50

Var.

Clast size (mm)

(Max. avg.)

ND

4 (0.5–1)

15 (3–5)

0.6 (0.6)

<1 (ND)

3 (0.5–1)

 16 (0.5–1)

ND (<5)

10 (1–2)

8 (1–5)

NA

30 (1–5)

NA

>10 (1–5)

Var.

<10 (ND)

30 (Var.)

<5 (±1)

<5 (±1)

Sorting

ND

Mod.

Mod.

Mod.–well

Well

Well

Well

Mod.–well

Mod.

Mod.

Mod.–well

Mod.–well

Well

Poor

Var.

Mod.

Mod.–well

Mod.–well

Mod.

Colors: dry (d), 

 moist (m)

10YR 6/2 to 6/3 (d)

7.5YR 5/3.5 (d)

ND

7.5YR 6.5/3 (d)

7.5YR 6.5/3.5 (d)

10YR 6/3.5 (d)

7.5YR 6/4 (d)

7.5YR 6.5/3 (d)

7.5YR 6.5/3 (d)

7.5YR 6/4 (d)

5YR 5/5 (d)

Var.

5YR 7/5 (d)

5YR 5/7 (d)

Var.

7.5YR 3/4 (d) to

7.5YR 4/4 (m)

Var. to 7.5YR 6/4 (d) 

7.5YR 6/3 (d)

Var.

Dryd

consistence

ND

SO

SH

SH–H

LO

LO

LO

LO

SO

SO

SH

SO

SO

LO

H–VH

LO

LO

H–VH

H–VH

CaCO3

g

content

ND

Non-calc.

Stage II

Stage I+

Minor

Non-calc.

Stage I

Minor 

Stage I+ to IIStage I

Stage I+ to II

Non-calc.

Non-calc.

Non-calc.

ND

ND

Var.

ND

ND

Lowerf

boundary

A, W

C, W

A, S

A, W

A, W

A, S

NE

A, S

A, S

A, W

A, W

NE

A, I

NA

NA

A, S

A, S

NE

NE

Genesis

Canal fill

Colluvium from PE1

Proximal colluvium from PE2

Distal colluvium from PE2

Basal distal colluvium from 

PE2

Channel fill, upper sand facies

Channel fill, basal facies

Proximal colluvium from PE3

Upper distal colluvium from 

PE3

Basal distal colluvium from 

PE3

Colluvium from PE4(?), 

Bkt horizon 

Fluvial gravel

Fluvial gravel, sandy facies

Fluvial gravel, sheared

Santa Fe Gp., sheared

A horizon on slope colluvium

Fluvial gravel

Santa Fe Gp., bedded 

sandstone

Santa Fe Gp., interstratified 

sand and gravel facies

Comments

Filled, man-made canal channels. Mainly pebbly, medium- to fine-grained sand.

Contains 40–60 percent clasts between 12 and 18 m (3–6 cm common diameter). Grades upslope to finer grained slope-wash 

facies. 

Contains blocks of footwall units. Fining upward sequence, lower part is slightly redder than top. Contains estimated 3–5 percent 

CaCO3

.

Probably contains considerable eolian sand and silt reworked into the colluvium. Contains 5–10 burrows. Blocky soil structure. 

Thins and is finer grained to west; slightly coarser to the east. Pinches out at 22.2 m.

Increasing amount of pebbles downward. Channel cut and filled after deposition of colluvium from PE3 and before deposition of 

colluvium from PE2.

 Initial infill in paleochannel. Probably contains substantial eolian sand and silt.

Unit fines upward and westward; gravelly component ends near 16 m.

CaCO3

 variably distributed in unit and decreases with depth.

Consists of coarse-to-fine-to-coarse sequence from bottom to top; middle fine interval has <5 percent clasts. Probably has 

significant amount of eolian sand and silt.

Buries minor relief on underlying unit 7. Has relatively strong color, structure, and carbonate content.

Clasts are fresh Precambrian rocks; clast imbrication indicates westward-flow direction. Clast-supported deposit. 

Deposit is mottled with 5YR 6/8 (d) orange color.

Matrix is highly oxidized and clay enriched; clasts have clay skins. Unit has strong fabric parallel to faults.

Highly oxidized pebble-cobble conglomerate, sand, and silt beds that are bounded by faults. Clasts are Precambrian rocks.

Grades downslope into unit 2-2.

Unit varies from cobble gravel to fine sand. CaCO3

 is probably deposited by ground water (nonpedogenic). Unit is present on both 

sides of fault.

Sandstone beds are 10-cm thick, strike N. 5° E., and dip 43° W.

Less oxidized than other units in Santa Fe Group. Bedding is well defined.

FOOTWALL BLOCK HANGING WALL BLOCK

SHEAR

ZONE a

Position on south wall of trench in horizontal (X) and vertical (Y) directions where described sample was collected.

b

Texture classes (<2mm): L, loam; LS, loamy sand; SL, sandy loam; S, sand.

c

Clast shape: SBA, subangular; SBR, subrounded; RD, rounded.

d

Dry consistence: LO, loose; SO, soft; SH, slightly hard; H, hard and weakly cemented; VH, very hard and moderately cemented.

e

Stratification: M, massive and non-stratified; W, weak and discontinuous; MO, moderately distinct and mostly continuous; S, strongly stratified, continuous bedding.

f

Lower boundary: Distinctness: A, abrupt (<1 cm); C, clear (1–5 cm); Shape: S, smooth; W, wavy (pockets wider than deep); I, irregular (pockets deeper than wide); NE, not exposed.

g

Calcium carbonate: Nomenclature in stages I–II from Birkeland (1999), + and - indicate strong and weak development of carbonate stage, respectively; Non-calc., non-calcareous.

Abbreviations: Var., variable; Mod., moderate; NA, not applicable; ND, not determined; (?), value uncertain; >, greater than; <, less than; <<, much less than; ±, plus or minus; ca., approximately. 

Clastc

shape

ND

SBR

SBA–SBR

SBA

ND

ND

SBA–SBR

ND

SBR–RD

SBA–SBR 

NA

SBR–RD

SBR

SBA–SBR

SBA–SBR

SBA–SBR

SBA–SBR

SBA–SBR

SBR–SBA

Stratificatione

W

M

W

M–W

M

W

M

W

M–W

M–W

M

W–MO

MO

NA

NA

M

M–MO

MO–S

MO–S

Table 1. Unit descriptions for the Rito Seco South (RSS) trench.

Unit

number

1

2

3

4-1

4-2

4-3 

5-1

5-2

7

8

6-1

6-2

7

8

 

9-1

9-2

9-3

9-4

Locationa

X/Y (m)

24.75/2.0

20.5/2.45

21.0/2.0

21.1/1.6

23.0/0.95

23.1/0.55 

14.75/2.75

17.0/2.0

17.0/1.4

18.5/0.7

3.8/4.9

3.8/4.7

4.0/4.4

8.5/3.4

14.3/1.3

15.5/2.25

15.5/1.5

East of 12 m

Matrixb

 texture

S

L to SL

SL, pebbly

CL, sandy 

S

S, gravelly

S, gravelly

S, gravelly

S, silty, gravelly

S, gravelly

LS

SL

LS

S

S, silty

LS

S

S(?)

Gravel

(percent)

Var.

5–25

30–40

<5

<5

>10(?)

>50(?)

>50(?)

25–40

>50

5–20

5–20(?)

20–40

>50

0

2

Var.

>>50

Clast size (mm)

(Max. avg.)

8 (2)

ND (0.5)

3–5 (0.5–1)

5–8 (0.5–1)

1 (<1)

 10 (1–2)

20 (>2?)

15 (3–5)

10 (1–2)

35 (10–15)

3–5 (1)

3–5 (1)

10–15 (1)

30–35 (10–15)

NA

1 (<1)

10 (3–5)

35 (10)

Sorting

Well

Mod.–well

Poor

Mod.–well

Mod.–well

Well

Mod.–well(?)

Mod.(?)

Poor

Poor–mod.

Mod.

Poor

Mod.

Mod.(?)

Well

Well

Mod.–well

Poor–mod.

Dry color 

ND, lt. gray

10YR 3/4

7.5YR 5/3.5

7.5YR 4/4

7.5YR 5/3.5

7.5YR 5/4

7.5YR 4/4.5

10YR 6/3.5, var.

7.5YR 3/3.5

10YR 6/6 to

5YR 5/8

7.5YR 4/4

7.5YR 5/4

7.5YR 6/3.5

5YR 5/8 to

7.5YR 6/6

7.5YR 7/3.5

2.5YR 7/2

7.5YR 7/3 to

7.5YR 6/8

5YR 5/8 to

7.5YR 7/4

Dryd

consistence

LO

SO–SH

SO–SH

SH–H

SO

SH

LO–SO

SH–H

SO–SH

LO–SO

SO

SH

SO–SH

LO

H–VH

H

H

H

CaCO3

g

content

ND

ND

ND

ND

ND

ND

ND

ND 

Stage I

Non-calc.

Non-calc.

Stage I+ to IIStage I+ to IIND

ND

ND

ND

ND

Lowerf

boundary

A, S

C, S

C, S

A, S

C, W

NE

A, S

A, S

C, W

NE

A, S

C, S

C, W

A, S–W

A, S

A, S

NE

NE

Genesis

Canal fill

Cumulic A horizon

Slope colluvium

“Prismatic” B horizon

Channel fill, sandy 

facies

Channel fill, basal 

facies

Colluvium on footwall

Colluvium on hanging 

wall

Debris flow

Fluvial gravel

A horizon on debris 

flow and colluvium

B horizon on debris 

flow

Debris flow

Fluvial gravel

Santa Fe Gp., sandy 

facies

Santa Fe Gp., 

lacustrine facies(?)

Santa Fe Gp., gravel 

facies or bed

Santa Fe Gp., 

conglomerate facies

Comments

Filled, man-made canal channels. Mainly pebbly, medium- to fine-grained sand.

Organic-rich slope deposit, large clasts most abundant in lower part. Contains abundant rootlets and charcoal fragments.

Abundant root tubes, krotovina (filled burrows), and charcoal fragments. Coarser-grained upslope (to east); contains 40–50 percent 

gravel at 16–17 m interval. 

Distinct prismatic structure. Peds are 3–6 cm in diameter, 10 cm tall, and have weak clay films. Contains abundant charcoal 

fragments and significant amount of loess. 

Locally contains ripple marks and laminated fine sand that is capped by organic-rich silt (mud that collected in bottom of 

paleochannel).

Scattered gravel clasts in sand matrix.

Matrix-supported cobble gravel; clasts derived from unit 8. Unit has maximum thickness of 35 cm.

Weakly cemented. Unit dips 15–30° to west. Contains 20–25 cm diameter blocks of Santa Fe Group. Maximum thickness of 70 cm.

Disseminated CaCO3

 gives unit whitish color. Predates faulting. Eroded by younger fluvial channel (unit 4). 

Heavily Fe stained, locally Mn stained. Clast imbrication indicates westward-flow direction. Predates faulting.

Weak, platy, moderate to massive soil structure; abundant root hairs and organic matter.

Weak B horizon. Medium, moderate, subangular-blocky soil structure; CaCO3

 veinlets in matrix and weak stage II on clasts; no clay 

films on peds.

Ranges from granular, silty sand to cobbly silty sand; CaCO3 in upper 40 cm. Abundant charcoal in krotovinas.

Clast-supported gravel, heavily Fe stained, no Mn staining. Clasts are fresh, mainly Precambrian rocks. Imbrication shows 

westward-flow direction.

Highly fractured. Possibly eolian facies of formation. Locally Mn stained.

Locally Fe stained; burrowed.

Metamorphic rock clasts are weathered; igneous clasts are intact. Colors vary. Subtle imbrication indicates westward-flow direction.

Clasts are 5–10 percent volcanic rocks, 25–30 percent metamorphic rocks; volcanic and metamorphic rocks are generally grussified. 

FOOTWALL BLOCK HANGING WALL BLOCK a

Position on south wall of trench in horizontal (X) and vertical (Y) directions where described sample was collected.

b

Texture classes (<2 mm): CL, clay loam; L, loam; LS, loamy sand; SL, sandy loam; S, sand.

c

Clast shape: A, angular; SBA, subangular; SBR, subrounded; RD, rounded.

d

Dry consistence: LO, loose; SO, soft; SH, slightly hard; H, hard and weakly cemented; VH, very hard and moderately cemented.

e

Stratification: M, massive and non-stratified; W, weak and discontinuous; MO, moderately distinct and mostly continuous; L, laminated.

f

Lower boundary: Distinctness: A, abrupt (<1 cm); C, clear (1–5 cm); Shape: S, smooth; W, wavy (pockets wider than deep); NE, not exposed.

g

Calcium carbonate: Nomenclature in stages I–II from Birkeland (1999), + and - indicate strong and weak development of carbonate stage, respectively; Non-calc., non-calcareous.

Abbreviations: Var., variable; Mod., moderate; NA, not applicable; ND, not determined; (?), value uncertain; >, greater than; >>, much greater than; <, less than. 

Clastc

shape

ND

SBA–SBR(?)

SBA–SBR(?)

ND

SBA–SBR(?)

A–SBR

RD

SBA–SBR(?)

SBR–RD

RD 

ND

ND

SBR–SBA

SBR–RD(?)

ND

NA

SBR–SBA(?)

SBR–SBA(?)

Stratificatione

M–L

M

M

M

M–W

M

M

W

M

MO

W

M

M–W

W–MO

M–W

ND

MO

M–W

INTRODUCTION

 

 The Sangre de Cristo fault is a major, north-northwest-striking, west-dipping normal fault located about 250 km south of Denver, Colo., (fig.1A) that separates the Sangre de Cristo Mountains to the east from the 

San Luis Valley to the west. It extends from Poncha Pass (PP, fig. 1B) southward along the range front, through Great Sand Dunes National Park, and into northern New Mexico as far south as Taos. It is the longest 

fault in Colorado that has clear evidence of latest Quaternary faulting (Colman, 1985). Slip on the fault has uplifted the rugged peaks of the Sangre de Cristo Mountains to elevations in excess of 4,300 m from the San 

Luis Valley to the west, where the elevation ranges between about 2,134 and 2,438 m. Historically, the fault has been seismically quiescent even though large fault scarps are present on Holocene and latest Pleistocene 

deposits along much of its length (Kirkham and Rogers, 1981; McCalpin, 1982, 1986; Colman, 1985). 

 Based on the presence of Holocene fault scarps, its 205-km-long length (Widmann and others, 2002), and 5–7 km of net Cenozoic throw (Tweto, 1979; Colman, 1985; Wallace, 2004), the Sangre de Cristo fault 

could be the most hazardous fault in Colorado, but the history of Quaternary movement on the fault is poorly documented. Recent compilations of Quaternary faults have simply divided the Sangre de Cristo fault into a 

northern part, which is located in Colorado, and a southern part, which is located in New Mexico (U.S. Geological Survey, 2005). Both parts are further subdivided into sections on the basis of geomorphic and 

geometric evidence. In Colorado, the fault has been divided into four sections; from north to south, the Crestone, Zapata, Blanca, and San Luis sections (Widmann and others, 2002; U.S. Geological Survey, 2005). The 

geomorphology of the fault and the adjacent range front suggest that at least two parts of the fault in Colorado have markedly different long-term histories of faulting. The northern part, which includes the Crestone, 

Zapata, and Blanca sections, is marked by a steep range front, deeply incised valleys, and abundant fault scarps on latest Pleistocene and Holocene deposits (McCalpin, 1982, 1986; Colman and others, 1985); the 

range crest is typically 3–8 km east of the fault trace (figs. 1B and 1E). At the southern end of Blanca Peak (fig. 1C), the range front steps abruptly eastward and forms the Culebra reentrant (Wallace, 2004; Kirkham 

and others, 2005). The San Luis section extends from this step southward to the New Mexico border. Along the San Luis section, the range front is more subdued (figs. 1C and 1F), and the fault trace is complex 

(Kirkham and Rogers, 1981). In contrast to the range front to the north, a set of foothills separate the range front from the range crest, and the range crest is typically 13 km to more than 20 km east of the fault trace 

(fig. 1F). Fault scarps are discontinuous and are best expressed on middle and early Pleistocene and older deposits (Colman and others, 1985). Thus, the geomorphology suggests significant differences in Cenozoic fault 

behavior between the San Luis section and sections to the north and south. There is abundant evidence of latest Pleistocene and Holocene movement along almost all of the Sangre de Cristo fault (McCalpin, 1982, 

1986; Kirkham and Rogers, 1981; Menges, 1990; Bauer and Kelson, 2004), but few studies have been able to determine the times of individual prehistoric earthquakes. In particular, the San Luis section of the fault 

has been largely unstudied in comparison to the sections to the north in Colorado (McCalpin, 1982, 1986; Kirkham and Rogers, 1981) and to the south in New Mexico (Menges, 1990; Bauer and Kelson, 2004).

 

 To better understand the fault’s late Quaternary history and seismic hazard, we conducted a paleoseismic study at the Rito Seco site in September 2003 (Crone and others, 2004; Crone and Machette, 2005). The 

site is located about 5.4 km northeast of the town of San Luis in Costilla County, Colorado (fig. 1D). Rito Seco is a large, ephemeral drainage that has deposited Quaternary alluvium of various ages across the trace of 

the fault (figs. 2A and B) and thus provides a suitable setting for paleoseismic studies. We excavated a 27-m-long, 3-m-deep trench (RSS) across a 2.7-m-high scarp formed on young alluvium that is inset into older Rito 

Seco alluvium (figs. 2B and 4). About 100 m to the north, we excavated a 32-m-long, 5- to 6-m-deep trench (RSN) across a 6.6-m-high scarp formed on older alluvium (figs. 2B and 5). We also excavated a 2.5-m-deep 

soil pit in the older alluvium on the upthrown side for the fault, about 20 m east of the northern trench (fig. 2B). In both trenches, we exposed Tertiary sediment of the Santa Fe Group in the footwall of the fault and 

Quaternary alluvial gravels in both the hanging wall and footwall. Because we exposed correlative Quaternary gravels on both sides of the fault, we can make accurate measurements of the total vertical displacement of 

these deposits (figs. 4 and 5).

 In this report, we present detailed maps of the trenches and a compilation of field and laboratory data used to support our interpretation of the history of four (PE1–PE4) prehistoric surface-faulting earthquakes at this 

site (figs. 4 and 5). We will present a detailed discussion of our interpretations in a subsequent report.

ACKNOWLEDGMENTS

 We thank Mr. Sandy Cooper, former owner of the Shalom Ranch, San Luis, Colo., for his cooperation and permission to work on ranch property at the Rito Seco site. We thank Robert M. Kirkham and Alan 

Wallace for sharing their knowledge of the area’s geology, and Robert M. Kirkham and Neil Lindsay for their field assistance. We also thank Margo Johnson for preparing the initial drafts of figures 4 and 5, and Alan R. 

Nelson and Stephen F. Personius for technical reviews that improved the quality of this map.

REFERENCES CITED

Bauer, P.W., and Kelson, K.I., 2004, Cenozoic structural development of the Taos area, New Mexico, in Brister, Brian, Bauer, P.W., Read, A.S., and Lueth, V.W., eds., Geology of the Taos Region: New Mexico 

Geological Society Guidebook, 55th Annual Field Conference, p. 129–146.

Birkeland, P.W., 1999, Soils and Geomorphology (3d ed): New York, Oxford University Press, 430 p.

Bucknam, R.C., and Anderson, R.E., 1979, Estimation of fault-slope ages from a scarp-height-slope-angle relationship: Geology, v. 7, p. 11–14. 

Colman, S.M., 1985, Map showing tectonic features of late Cenozoic origin in Colorado: U.S. Geological Survey Miscellaneous Investigations Map I–1566, scale 1:1,000,000.

Colman, S.M., McCalpin, J.P., Ostenaa, D.A., and Kirkham, R.M., 1985, Map showing upper Cenozoic rocks and deposits and Quaternary faults, Rio Grande rift, south-central Colorado: U.S. Geological Survey 

Miscellaneous Investigations Map I–1594, scale 1;125,000.

Crone, A.J., and Machette, M.N., 2005, Paleoseismic history of the Sangre de Cristo fault near San Luis, Colorado: Geological Society of America Abstracts with Programs, v. 37, no. 7, p. 558.

Crone, A.J., Machette, M.N., Bradley, L.-A., and Mahan, S.A., 2004, Late Quaternary earthquake history of the Sangre de Cristo fault near San Luis, Colorado: Geological Society of America Abstracts with Programs, 

v. 35, no. 5, p. 138.

Kirkham, R.M., Brister, B.S., Grauch, V.J.S., and Budhan, J.S., 2005, Culebra graben—A major intrarift structure in the San Luis basin, Rio Grande rift: Geological Society of America Abstracts with Programs, v. 36, 

no. 6, p. 14.

Kirkham, R.M., and Rogers, W.P., 1981, Earthquake potential in Colorado: Colorado Geological Survey Bulletin 43, 171 p.

Kollmorgen Instruments Corp. (Macbeth Division), 1994, Munsell Soil Color Charts (revised ed.): New Windsor, N.Y., unpaginated, [405 Little Britain Rd., New Windsor, N.Y. 12553]. 

McCalpin, J.P., 1982, Quaternary geology and neotectonics of the west flank of the northern Sangre de Cristo Mountains, south-central Colorado: Colorado School of Mines Quarterly, v. 77, 97 p.

McCalpin, J.P., 1986, Quaternary tectonics of the Sangre de Cristo and Villa Grove fault zones, in Rogers, W.P., and Kirkham, R.M., eds., Contributions to Colorado seismicity and tectonics—A 1986 update: Colorado 

Geological Survey Special Publication 28, p. 59–64.

Menges, C.M., 1990, Late Quaternary fault scarps, mountain-front landforms, and Pliocene–Quaternary segmentation on the range-bounding fault zone, Sangre de Cristo Mountains, New Mexico: Geological Society of 

America Reviews in Engineering Geology, v. VIII, p. 131–156.

Soil Survey Staff, 1975, Soil Taxonomy: U.S. Department of Agriculture Handbook no. 436, 754 p.

Stuiver, Minze, and Reimer, P.J., 1993, Extended 14C database and revised CALIB radiocarbon calibration program: Radiocarbon, v. 35, p. 215–230 (program available at http://radiocarbon.pa.qub.ac.uk/calib/) (last 

accessed 11/09/2006).

Tweto, Ogden, 1979, The Rio Grande rift system in Colorado, in Reicker, R.E., ed., Rio Grande rift—Tectonics and magmatism: Washington, D.C., American Geophysical Union Monograph, p. 33–56.

U.S. Geological Survey, 2005, Quaternary fault and fold database of the United States: Available at http://earthquake.usgs.gov/regional/qfaults/ (data accessed 11/09/2006).

Wallace, A.R., 2004, Evolution of the southeastern San Luis basin margin and the Culebra embayment, Rio Grande rift, southern Colorado, in Brister, Brian, Bauer, P.W., Read, A.S., and Lueth, V.W., eds., Geology of 

the Taos Region: New Mexico Geological Society Guidebook, 55th Annual Field Conference, p. 121–132.

Widmann, B.L., Kirkham, R.M., Morgan, M.L., and Rogers, W.P., 2002, Colorado late Cenozoic fault and fold database and internet map server: Colorado Geological Survey Information Series 60a (available at 

http://geosurvey.state.co.us/pubs/ceno/) (last accessed 11/09/2006).

DATA RELATED TO LATE QUATERNARY SURFACE FAULTING ON THE SANGRE DE CRISTO FAULT, RITO SECO SITE, COSTILLA COUNTY, COLORADO

By

Anthony J. Crone, Michael N. Machette, Lee-Ann Bradley, and Shannon A. Mahan

2006

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

SCIENTIFIC INVESTIGATIONS MAP 2955

 Version 1.0

Any use of trade names is for descriptive purposes only and does not imply

endorsement by the U.S. Government

This map was produced on request, directly from digital files, on an electronic plotter

For sale by U.S. Geological Survey Information Services, Box 25286, Federal Center

Denver, CO 80225; 1-888-ASK-USGS

A PDF file is available online at: http://pubs.usgs.gov/sim/2006/2955

Suggested citation:

Crone, A.J., Machette, M.N., Bradley, L.-A., and Mahan, S.A., 2006,

Data related to late Quaternary surface faulting on the Sangre de Cristo fault,

Rito Seco site, Costilla County, Colorado: U.S. Geological Survey Scientific 

Investigations Map 2955, 1 sheet.

EXPLANATION OF UNITS