LATE QUATERNARY SURFACE FAULTING ON THE CHERAW FAULT, SOUTHEASTERN COLORADO

By

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

1997

GEOLOGIC INVESTIGATIONS

MAP I–2591

Pamphlet accompanies map

Manuscript approved for publication October 8, 1996Any use of trade names in this publication is fordescriptive purposes only and does not implyendorsement by the U.S. Geological SurveyFor sale by U.S. Geological Survey Information Services,Box 25286, Federal Center, Denver, CO 80225

 



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MAPPED PART OF NORTHEAST WALL OF TRENCH ACROSS CHERAW FAULT

(mirror image)

S 34° E N 34° W

12 10

16 14

18

METERS METERS

213

METERS

213

METERS

Zone of jumbled blocks

and colluvium

Land surface at time of oldest faulting

event is below bottom of trench

Land surface at time of

most-recent faulting event

Main fault zone

C14 - CF-1

9.9 ka 

C14 - CF-2

12.1 ka 

Land surface at

time of penultimate

faulting event

?

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?

C14 - CF-3

9.8 ka 

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|{ TL-CF-1 13.2 

± 1.2 ka

TL-CF-6

16.8 

± 1.8 ka

TL-CF-9 not reliable

TL-CF-218.3 

± 2.0 ka

TL-CF-3

21.3 

± 2.5 ka

TL-CF-5not reliable

TL-CF-418.4 

± 3.9 ka

TL-CF-713.5 

± 1.2 ka

TL-CF-8

13.6 

± 1.1 ka

?

?

?

?

?

?

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N 34° W

14

12 10

16

24 22 20 18

METERS

METERS

213

METERS

Projected top of Upper Cretaceous Smoky Hill Shale Member (bedrock)

MAPPED PART OF SOUTHWEST WALL OF TRENCH ACROSS CHERAW FAULT

Zone of jumbled blocks

and colluvium

Main fault zone

Secondary

 fault zone

Land surface at time of

most-recent faulting event Land surface at time of penultimate faulting event 

Land surface at time of oldest faulting

event is below bottom of trench

C14 - CF-4

9.4 ka 

C14 - CF-5

8.4 ka 

0

-224-4

00 50

0

TANCE, IN METERS

H-1

G-1

F-1

NORTHWEST

Scarp height = 3.6 m

Unit 6csa

Units

1, 4-6

Top of unit 7

Top of unit 7

?

Top of unit 9 (bedrock)

Top of unit 9 (bedrock)

f trench. See description and explanation of trench units for patterns in auger holes. 









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z||

Throw

>3.0 m

Throw

3.2 m

Units

1, 4-6

Units

1, 5

Cherawfault

99

00

 20 S., 

n of 

5esi

5esi

5esi

4asa

4csi

5esi

1r

1r

7fsa

7fsa

7fsa

7fsa

9

9

9

7fsa 7fsa

7fsa

7fsa

7fsa

7fsa

7fsa

7fsa

7fsa

5psi

5psi

5psi

5psi

5psi 5psi

5psi

6psi

6csa

6csa

6csa

6csa 6csa 6csa

6csa

6psi

6psi

6psi

6psi

6psi

6psi

5psi

4pB

4pB

4pB

4pB

4pB

4pA 4pA

4pA

4pA

4pA

4pA

4pA 4pA

3csa

3csa

3csa

2c

3asi

3asi

3asi

3asi

3asi

3asi

5pA

5pA

5pA

5pA

5pA 5pA

5pA

5pA

3cu

7fsa

8fg

7fsa

7fsa

7fsa

7fsa

7fg

7fg 7fg

7fg

7fg

7fg

5fg

2c-3c

4csi

5esi

5psi

5psi

5psi

5psi

5psi

6psi

6psi

6psi

6psi

6psi

6csa

6csa

5psi

5psi

3asa

3asa

3asi

3asi

3asa

7fsa

4pA

4pB

4pB

4pB

5pA

5pA 5pA

4pA

4pA

4pA

7fsa

7fsa

7fsa

7fsa

8fg

7fsa

1r 

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zy

COLLUVIAL

DEPOSITS

RESIDUAL

DEPOSITS

ALLUVIAL

DEPOSITS

PALUDAL

DEPOSITS

FLUVIAL

DEPOSITS

EOLIAN

DEPOSITS

2c

3cuzy|{3csa

3asizy|{3asa

2c-3c

HoloceneUpper Pleistocene

QUATERNARY

UPPER

CRETACEOUS

CORRELATION OF TRENCH UNITS

SURFICIAL DEPOSITS

1r

yz{|

4pB

4asa

4csi

4pA

5pA



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9

7fsa

5esi 5psi

6psi

|{6csa

2c

3cu 3csa 3asi 3asa

2c-3c

4pB

4asa

4csi

4pA5pA

5fg7fg8fg

9

7fsa

5esi 5psi

6csa 6psi

most-recent faulting event

Oldest faulting event

Erosional unconformity

BEDROCK

Penultimate faulting event



Crossbedded



EXPLANATION OF TRENCH SYMBOLS

Faults—Arrows indicate relative sense of motion. Dashed where inferred, queried where uncertainStratigraphic contacts—Dashed where subtle or gradational. Queried where inferred.

Note: contacts between units 2 and 3 and older units result from deposition of younger

colluvium and alluvium against exposed fault planes

Thermoluminescence sample and estimated age—See table 2Area sampled for radiocarbon (C14) analysis and estimated age of faulting event—See table 1

?

Crudely bedded|{

Planar beddedKrotovina (fossil animal burrow)Location of horizontal and vertical control points (1 meter grid)

yz

DESCRIPTION OF TRENCH UNITS

yz

|{

1r

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2c

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5psi

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9

4asa

Unit 1—Modern surficial deposits

Residuum (Holocene)—Mixture of loose, light-brown, poorly sorted sand and silt containing 

weak A horizon. Soil probably formed on underlying units

Units 2 and 3—Deposits that resulted from the most-recent faulting event

Silty colluvium (Holocene)—Poorly sorted, loose, light-brown clayey silt; forms thick mantle at 

base of scarp. Has weak blocky structure where thin on upthrown fault block. Unstratified, nonorganic

Undifferentiated colluvium (Holocene)—Poorly sorted, loose mixture of light-brown clayey 

silt and intact blocks of sediment derived from upthrown fault block. Unit 2 and all subunits of unit 3 mapped on southwest trench wall. Combination of units 2 and 3 mapped on northeast 

wall of trench

Sandy colluvium (Holocene)—Moderately well sorted, loose, very light brown silty sand; 

probably reworked eolian material. Fills fissures and irregular topography formed by ground rupturing during most-recent faulting event

Undifferentiated colluvium (Holocene)—Heterogeneous mixture of poorly sorted clayey silt 

(reworked units 5 and 6) and colluvial collapse blocks too small to map separately. Only mapped along main fault on southwest wall of trench

Silty alluvium (Holocene)—Poorly sorted, loose, brown sandy silt that fills elongate fissures. 

Upper part of unit typically has thin layers of light-brown sand that give the unit a bedded appearance (see symbols) 

Sandy alluvium (Holocene)—Poorly sorted, loose, very light brown sandy alluvium that fills 

openings between disturbed blocks of silt (units 5 and 6). Typically unbedded, but one area on 

the northeast trench wall has crossbedding that is characteristic of deposition by flowing water (see symbols)

Unit 4—Deposits that resulted from the penultimate faulting event

Soil A horizon (Holocene and upper Pleistocene)—Organic-rich A horizon developed on 

paludal silts deposited at base of fault scarp. Bulk organic carbon and charcoal extracted from this horizon yielded radiocarbon dates as reported in table 1

Soil B horizon (upper Pleistocene)—Slightly oxidized layer (cambic B horizon) developed in 

paludal silts deposited at base of fault scarp

Silty colluvium (upper Pleistocene)—Homogeneous light- to medium-brown clayey silt; 

unbedded, friable to hard (where dry). Interpreted as colluvium reworked from unit 5 on upthrown fault block. As such, it is related to the most-recent faulting event on the southern (secondary) fault zone

Sandy alluvium (upper Pleistocene)—Poorly sorted, loose, very light brown sandy alluvium 

that fills a small graben at the southern (secondary) fault zone. Has crossbedding that is characteristic of deposition by flowing water (see symbols)

Unit 5—Deposits that resulted from the oldest faulting event in trench

Soil A horizon (upper Pleistocene)—Organic-rich, brown A horizon developed on paludal silts 

deposited at base of fault scarp. Bulk organic carbon extracted from this horizon yielded a radiocarbon date as reported in table 1. Locally, weak cambic B horizon is present below this unit but it was not mapped because of its subtle character

Eolian silt (upper Pleistocene)—Brown sandy silt (loess) and fine- to medium-grained sand; 

massive. Mainly eolian deposit but may contain some colluvium; transitional and probably coeval with unit 5psi

Paludal silt (upper Pleistocene)—Brown silt; massive when moist, blocky when dry. Upper of 

two thick (0.4–1.1 m) silt layers (eolian?) deposited in marshes or ephemeral lakes that formed at the base of the fault scarp, probably as a result of backtilting. Later sedimentation overlapped the fault zone. Basal contact of unit was defined on the basis of blocky structure and may not be stratigraphically significant

Fluvial gravel (upper Pleistocene)—Thin lens of sandy pebble gravel that is interbedded with 

paludal silt (unit 5psi). Unit represents occasional fluvial deposition at base of scarp

Unit 6—Deposits that resulted from the oldest faulting event in trench 

Paludal silt (upper Pleistocene)—Brown to dark-brown, massive silt. Lower of two thick 

(0.6–1.3 m) silt layers (eolian?) deposited in marshes or ephemeral lakes that formed at the base of the fault scarp, probably as a result of backtilting. Unit is restricted to downthrown fault block. Base is interbedded with fault-scarp colluvium (unit 6csa)

Sandy colluvium (upper Pleistocene)—Unbedded to poorly bedded, light-brown, fine- to 

medium-grained sand interbedded with the basal part of unit 6psi. Lack of bedding and presence of gravel clasts suggest that the unit is derived from the erosion of unit 7 on the upthrown fault block. Grades downward into unit 7 in auger holes and where exposed in trench during excavation

Units 7 and 8—Deposits that predate the oldest faulting event in trench

Fluvial sand (upper Pleistocene)—Interbedded light-brown, fine- to medium-grained sand and 

minor beds of sandy pebble gravel. Sandy gravels are moderately to well sorted and indicate stream deposition. Bedding forms in fluvial sand are stratified, planar, and crossbedded (see symbols) although some thick sandy parts of unit are massive

Fluvial gravel (upper Pleistocene)—Light-brown, fine- to medium-grained sandy gravel. 

Moderately well sorted. Contains 5- to 10-cm-diameter clasts reworked from older Quaternary and Pliocene alluvium (Rocky Flats and Nussbaum Alluviums; Sharps, 1976) in upper part of drainage area. Pebble imbrication and bedforms indicate a southerly transport direction

Fluvial gravel (upper Pleistocene)—Poorly sorted sandy alluvium containing angular to 

subrounded, pebble- to cobble-size clasts of shale (bedrock). Clast shape and monolithologic composition indicate short transport distances by a stream. Unit predates the three events recorded in trench 

Unit 9—Bedrock

Smoky Hill Shale Member of the Niobrara Formation (Upper Cretaceous)—Orangishbrown to olive-green shale; gypsiferous, thinly bedded. Exposed in lower 0.5 m of uplifted 

(southern) block, adjacent to main and secondary fault zones in trench

5esi

?

Figure 5. Computer-scanned composite of two aerial photographs showing the Cheraw fault scarp near the trench site. Photographs are CNW-SHH-118 (left) and CNW-SHH-112 (right) taken on August 14, 1967 (U.S. Department of Agriculture.Agricultural Stabilization and Conservation Service). Colorado State Highway 96 is present in the northwest corner of the image.

96

Area offigure 3

0 0.5 1.0 Miles0 0.5 1.0 Kilometers

N

Cheraw

fault

scarp

Cheraw

fault

scarp

Field

sample

No.

C14-CF-1

C14-CF-2C14-CF-3C14-CF-4C14-CF-5

Laboratory

sample

No.

Beta-93290Beta-93291Beta-93552Beta-93553Beta-83716

Sampledmaterial,

unit

Soil organics,

4pA

Soil organics,

5pA

Soil organics,

4pA

Soil organics,

4pA

Charcoal,

4pA

Measured age,

yr B.P. 

±1σ

 9,260

±90

10,550

±100

9,160

±90

8,650

±90

7,730

±50

C13/C12

ratio

-15.4-15.7-15.4-15.7-27.1

Conventional

age, C14 yr

B.P. 

±1σ

9,420

±90

10,700

±100

9,320

±90

8,880

±90

7,690

±50

Calibrated

age and 

±2σ

range, yr B.P.

10,390

10,180-10,63510,730-10,865

ca. 12,600

10,320

10,165-10,330

9,865

9,525-9,970

8,415

8,390-8,445

Apparent

mean

residence

time, in yr

500500500500

NA

<9,890 (<9.9 ka)

 9,680-10,13510,230-10,365<9,820 (<9.8 ka)

 9,665-9,830<9,365 (<9.4 ka)

 9,025-9,470<8,415 (<8.4 ka)

 8,390-8,445 

Table 1. Data for radiocarbon dating of samples from trench across the Cheraw fault[Measured, conventional, and calibrated ages are reported as yr B.P., with reference to 1950. The estimated time of faulting events is calculated by subtracting the apparent mean residence time of sampled material from dendrochronologically calibrated age, and reported as yr B.P. Estimated times shown in bold are those used in discussion and mapped parts of trench. See text for discussion of apparent mean residence time. Dendrochronological calibration is not available for samples that have conventional radiocarbon ages greater than about 10,000 years, but radiocarbon ages in the range of 10,700 yr (C14-CF-2) 

typically have a calendric age of about 12,600 yr B.P. (Stuiver and Reimer, 1993; Lowell and Teller, 1994). NA, not applicable]

Field

sample

No.

TL-CF-1TL-CF-2TL-CF-3TL-CF-3TL-CF-4TL-CF-5TL-CF-6TL-CF-7TL-CF-8TL-CF-9

Laboratory

sample

No.

TL-CF-1TL-CF-2

TL-CF-3a

TL-CF-3b

TL-CF-4a

TL-CF-5TL-CF-6TL-CF-7TL-CF-8TL-CF-9

Sampledmaterial,

unit

Silt, 5psi

(upper)

Silt, 5psi

(lower)

Silt, 6psi

(upper)

Silt, 6psi

(upper)

Silt, 6psi

(lower)

Silt, 6psi

(middle)

Silt, 5psi

(middle)

Silt, 5esi

(middle)

Silt, 5esi

(middle)

Silt, 4pB

Environmental

dose, Greys

74.93

±1.74%

104.56

±3.31%

115.36

±5.18%

116.53

±2.68%

103.69

±9.47%

51.01

±1.85%

94.05

±3.14%

74.47

±1.68%

77.74

±3.8%

95.60

±2.7%

Moisture

content:

field,

saturation

15.5%24.3%14.5%17.4%16.5%21.5%16.5%21.5%18.9%19.5%20.5%23.1%15.8%24.2%13.7%14.3%14.5%14.6%12.4%23.7%

Dose rate:

Greys 

± 1

σ

at moisture

content

5.66

±4.1%

12.0%

5.72

±4.2%

12.0%

5.45

±4.3%

15.0%

5.45

±4.3%

15.0%

5.62

±4.4%

15.0%

5.56

±4.6%

15.0%

5.60

±4.2%

12.0%

5.53

±4.0%

10.0%

5.72

±3.8%

8.0%

5.63

±3.7%

8.0%

Total bleach

TL age,

ka 

±2σ

(A)

13.2

±1.2

18.3

±2.0

21.2

±2.9

21.4

±2.2

18.4

±3.9

9.2

±0.9

16.8

±1.8

13.5

±1.2

13.6

±1.1

17.0

±1.6

Partial bleach

TL age,

ka 

±2σ

(B)

12.0

±3.6

18.0

±3.1

15.5

±13.4

21.3

±16.7

16.2

±10.5

5.1

±1.5

20.4

±3.1

12.8

±1.3

11.5

±1.1

16.4

±1.5

Favored

TL age,

ka 

±2σ

(A)

13.2

±1.2

(A)

18.3

±2.0

(A)

21.3

±2.5

(A, average

of -3a,- 3b)

21.3 

±2.5

(A, average

of -3a,- 3b)

18.4

±3.9

(A)

Not reliable

(A)

16.8

±1.8

(A)

13.5

±1.2

(A)

13.6

±1.1

(A)

Not reliable

 (A)

Table 2. Data for thermoluminescence age estimates of samples from trench across the Cheraw fault

Estimated time

of faulting

event and 

±2σ

range (yr B.P.)

Event 1:Event 2:

ca. <12,100

(<12.1 ka)

Event 1:Event 1:Event 1: 
U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

LATE QUATERNARY SURFACE FAULTING ON THE

CHERAW FAULT, SOUTHEASTERN COLORADO

By

Anthony J. Crone, Michael N. Machette, Lee-Ann Bradley,

and Shannon A. Mahan

Pamphlet to accompany

GEOLOGIC INVESTIGATIONS

MAP I–2591
CONTENTS

Introduction 1

Geologic and Seismologic Framework 1

Bedrock Geology 1

Surface Expression of the Fault 2

Description of the Trench Site 2

Trench Stratigraphy 3

Interpretation of Faulting History 3

Cumulative Throw at the Trench Site 4

Oldest Faulting Event Recorded in the Trench 4

Penultimate Faulting Event Recorded in the Trench 5

Most-Recent Faulting Event Recorded in the Trench 5

Conclusions 6

Acknowledgments 6

References Cited 6
1

INTRODUCTION

Although relatively infrequent, earthquakes can

unexpectedly occur in the stable interior of continents

such as the midwestern part of the United States and can

cause damage, injuries, and fatalities in populated

regions (Johnston and Kanter, 1990). This study of the

Cheraw fault in southeastern Colorado is part of an ongoing effort to better understand the long-term behavior of

potentially seismogenic faults in the interior of continental plates where the historical rates of seismicity are low

compared to plate-margin settings (Johnston, 1994).

For decades, earthquake-hazard assessments in stable continental regions such as the central and eastern

United States have been based largely on the assumption

that the spatial distribution of historical seismicity is an

accurate guide to the location of future large, potentially

devastating earthquakes. However, data from recent historical earthquakes coupled with paleoseismologic studies of faults in the stable interior of North America

(Crone and Luza, 1990) and Australia (Crone and others,

1992; Machette and others, 1993) indicate that modern

seismicity is not a reliable indicator of all potentially

hazardous faults that might rupture in the future. These

studies show that some potentially active faults are currently aseismic and may have remained so for many tens

of thousands of years. After a lengthy period of quiescence and with little or no precursory activity, these

faults may generate moderate- to large-magnitude earthquakes that are associated with as much as 35 km of surface rupture. Thus, studies that document the history of

movement on Quaternary faults in stable continental

regions are needed to characterize both their short-term

and long-term behavior and to improve hazard assessments in these largely aseismic areas.

Earthquake-hazard assessments in stable continental

regions are hindered by an incomplete inventory of prehistoric surface ruptures. For many years, little consideration was given to the possibility that prehistoric Quaternary surface-rupturing earthquakes had occurred in the

stable interior of the United States. However, in the

early 1980’s, a Quaternary scarp was recognized on the

Meers fault in southwestern Oklahoma, and subsequent

studies documented evidence of two Holocene surfacefaulting events (Crone and Luza, 1990; Kelson and

Swan, 1990). The study described herein establishes the

Cheraw fault as the second aseismic fault in the stable

interior of the west-central United States to have demonstrable late Quaternary offset. We suspect that additional

studies in the continental interior will identify other faults

that have moved in the recent geologic past and that

could generate damaging earthquakes in the future.

The main purpose of this report is to present the

basic stratigraphic evidence and numerical ages that

document Holocene and latest Pleistocene surface-faulting events on the Cheraw fault. The results reported here

expand upon and supersede the findings reported by

Crone and Machette (1995). The evidence for the number and timing of faulting events is derived from several

sources of information including detailed topographic

mapping of our study site, subsurface stratigraphy determined from a series of 1.5- to 7.6-m-deep auger holes, a

110-m-long exploratory trench, and analysis of samples

for radiocarbon and thermoluminescence dating.

GEOLOGIC AND SEISMOLOGIC

FRAMEWORK

The Cheraw fault was recognized during regional

geologic mapping by the U.S. Geological Survey more

than 25 years ago (Scott, 1970; Sharps, 1976; Kirkham

and Rogers, 1981), but no detailed paleoseismological

studies were conducted perhaps because of the fault’s

relatively remote location. The fault is about 100 km

east of Pueblo, Colo. (the closest major urban area), and

about 140 km east of the range front of the Rocky Mountains. The fault is within a sparsely populated portion of

the Colorado Piedmont (fig. 1). It trends N. 45° E., has a

down-to-the-northwest sense of throw, and has a mapped

length of about 44 km. In many places, the fault’s trace

is mapped as approximately located, inferred, or concealed (Sharps, 1976). Along its central part, the fault

displaces early Quaternary piedmont surfaces (mainly

formed by early Pleistocene Rocky Flats Alluvium;

Scott, 1982) that are widespread in southeastern Colorado, and in a few locations including where we excavated our exploratory trench, movement of the fault

formed a scarp on late Pleistocene deposits.

Historical seismicity of southeastern Colorado is

sparse, and no historical events can be directly associated with the Cheraw fault. The most notable earthquake

in the region was an intensity IV event in November

1955, located 25–30 km northwest of La Junta, Colo., and

about 25 km west-southwest of the trench site (Kirkham

and Rogers, 1981). Thus, the record of historical seismicity offers no insight into the seismogenic potential of

the fault, as is the case with many intraplate Quaternary

faults.

BEDROCK GEOLOGY

Sharps (1976) mapped the Cheraw fault as primarily

exposed in the Upper Cretaceous Smoky Hill Shale

Member of the Niobrara Formation and showed it as

inferred beneath Quaternary alluvium in several localities. However, Scott (1970) and Kirkham and Rogers

(1981) interpreted discordant elevations of the Rocky

Flats Alluvium across the fault as evidence of Quaternary

movement. There is little evidence that throw on the

fault has substantially offset the bedrock (tens of meters),

thus the fault does not appear to have a long history of

recurrent movement. For example, the cumulative throw

on the fault is probably less than the thickness of the

Smoky Hill Shale Member (150–215 m thick in the area)

because neither the Fort Hays Limestone Member nor the

Pierre Shale (all Upper Cretaceous), which are strati-
2

graphically below and above the Smoky Hill Shale

Member, respectively, are exposed along the fault. Similarly, Sharps’ (1976) structure-contour map shows only

about 20–25 ft (6–8 m) of normal, down-to-the-northwest

displacement on the fault, even though drill-hole control

for the structure-contour map is relatively sparse in this

area. Therefore, stratigraphic relations of bedrock units

suggest that the Cheraw fault does not have a long (that

is, Neogene) history of movement. Furthermore, stratigraphic data from shallow auger holes indicate that only

about 7–8 m of cumulative Quaternary offset has occurred since deposition of the early Pleistocene Rocky Flats

Alluvium (Crone and Machette, 1995).

SURFACE EXPRESSION OF THE FAULT

Where expressed in the field, the Cheraw fault typically forms a subtle, northwest-facing escarpment that is

most recognizable where it opposes the regional topographic gradient (fig. 2), which slopes to the southeast

toward the Arkansas River, some 30 km to the south

(fig. 1). Aerial photographs clearly show areas where

alluvium is ponded against the scarp because the scarp

has dammed local drainages. Eolian processes were

active in eastern Colorado, particularly during the Holocene (Madole, 1994) and latest Pleistocene, and in many

places the scarp has been mantled by windblown sand

and loess. Thus, in some low-lying areas along the fault’s

trace, material has preferentially accumulated on the

downthrown (northwest) side of the scarp, so the scarp

height is only a fraction of the actual throw due to a

combination of erosion and deposition. The highest and

oldest scarps, which record the longest history of faulting,

are about 7–8 m high and are formed on the Rocky Flats

Alluvium (mapped by Sharps, 1976).

The trenching study results indicate that eolian deposition has subdued the scarp’s morphology. At the trench

site where we document Holocene surface rupture, the

scarp has a maximum slope angle of about 11° and a

height of about 3.6 m. Based on fault-scarp morphology

studies (Bucknam and Anderson, 1979), this scarp’s morphology is much more subdued than comparable Holocene fault scarps in climatically similar parts of the

Basin and Range Province in Utah. However, this comparison is complicated by the fact that the Cheraw scarp

is the product of multiple surface-faulting events, whereas the Utah scarps result from single events. Nevertheless, partial burial of the Cheraw scarp by eolian deposits

and by alluvium ponded on the downthrown side has

resulted in a subdued morphology that makes it appear to

be an older feature than was documented by the

trenching study.

DESCRIPTION OF THE TRENCH SITE

In order to assess the location of the fault relative to

the local topography, we made a detailed topographic

map of the Cheraw trench site (fig. 3) by measuring 915

topographic data points in an area of approximately 0.4

km2 using a Sokkia Set 5A Total Station. We contoured

these data using commercial software (MacGridzo) to

generate a detailed topographic map (fig. 3) with 1-foot

(30-cm) contours; this map documents the location of the

trench, scarp, and soil pits relative to cultural features

(mainly roads and fences), which are the only notable

landmarks in the area.

The scarp at the trench site is 3.6 m high and about

120 m wide (figs. 3 and 4). In late 1994, we excavated a

110-m-long, northwest-southeast-oriented trench across

the scarp, and in 1995, we completed mapping of critical

portions of the trench (fig. 4). The trench is located in

the SE1/4 sec. 34, T. 20 S., R. 54 W. (USGS Houston

Lakes 7.5-minute quadrangle), Kiowa County, Colo., on

the property of the Timberlake Grazing Association. We

selected this trench site because it is along part of the

fault where a prominent scarp is present on Quaternary

deposits that are inset into, and therefore substantially

younger than, the Rocky Flats Alluvium. Aerial photographs (fig. 5) of the trench site show the presence of distinct fluvial channels (relatively dark areas) on the upthrown side of the scarp that define a through-flowing

drainage system, which was disrupted by movement on

the fault. The sharpness of these abandoned channels in

the photographs suggest that they are likely late Pleistocene in age rather than middle Pleistocene or older.

Thus, on the basis of our aerial photographic interpretations, we suspected that a detailed trenching study at this

site would provide evidence for late Pleistocene or

younger movement on the fault.

We started to excavate the trench near the crest of

the broad scarp and anticipated intercepting one or more

faults near the mid-portion of the scarp (fig. 4). However,

the first fault (most headward in the trench) was not

encountered until about 85 m from the beginning of the

excavation (secondary fault zone at 22-m mark on

mapped part of southwest wall of trench), and the main

fault zone was intercepted 5–6 m farther downslope. The

location of the faults relatively close to the base of the

scarp is unusual, although in retrospect their location

might have been expected owing to pervasive burial of

the downthrown fault block (northwest side of fault).

Nevertheless, all of the pertinent stratigraphic and structural details of the faulting were exposed in the northwesternmost 25 m of the trench, which is the portion that

we mapped in detail and most of which is illustrated on

the accompanying plate.

We systematically examined the entire trench several times for evidence of deformation, and ultimately

confirmed that faulting was present only in the northwestern part shown on the plate. We carefully cleaned

both walls of this part of the trench with mason’s trowels

to clarify and highlight the stratigraphic relations. Next

we established a grid of string lines on both walls to use

as a frame of reference for mapping. We identified key

stratigraphic units and structural features on the walls and

marked those features using multicolored tape. We then
3

mapped the features at a scale of 2 inches to 1 meter

(approximate scale of 1:19.7). Finally, we described and

sampled the stratigraphic units and collected samples for

radiocarbon and thermoluminescence analyses.

TRENCH STRATIGRAPHY

The trench exposed distinctly different stratigraphy

on the southeastern (upthrown) and the northwestern

(downthrown) sides of the fault zones. Southeast of the

main fault zone, the trench exposed a sequence comprised of bedrock (unit 9), overlain by fluvial sand and

gravel (units 7 and 8), and capped by massive, sandy silt

and fine- to medium-grained sand (units 5esi and 4csi).

The bedrock is gypsiferous shale of the Smoky Hill Shale

Member of the Niobrara Formation, which was exposed

at the bottom of the trench on the southeastern side of the

two fault zones. A poorly sorted fluvial gravel (unit 8fg)

that contains abundant rip-up clasts of the shale is directly above the bedrock at the main fault zone. Upward,

medium- to coarse-grained fluvial sand (unit 7sa) and

gravel (unit 7fg) dominate; stratification in these units

varies locally from well stratified to crossbedded to cutand-fill structures. This entire sequence is capped by

eolian silt (loess; unit 5esi) and, between the main and

secondary fault zones, a wedge of colluvial silt (unit

4csi).

On the northwestern (downthrown) side of the main

fault zone, the trench exposed a sequence of deposits

that we interpret as a mixture of scarp-derived colluvium

and fine-grained alluvium that accumulated in relatively

moist, occasionally marshy (paludal) conditions at the

base of the scarp. The oldest deposit exposed on the

downthrown side was a massive, fine- to medium-grained

sand (unit 6csa) that we interpret as scarp colluvium.

This colluvium is overlain by and interfingers with massive silt (unit 6psi) and massive to blocky silt (unit 5psi)

that we interpret as paludal deposits, which were ponded

against the scarp. Much of the silt in these paludal

deposits is probably reworked loess that originally mantled the landscape during the late Pleistocene. Unit 5psi

is capped by a moderate to weakly developed A horizon

(unit 5pA), which formed during a time when the sedimentation rate at the base of the scarp was relatively low

and the surrounding landscape was relatively stable. On

the northwestern side of the main fault zone, the A horizon on unit 5 is buried by paludal silt that contains a

lower B horizon (unit 4pB) and an upper A horizon (unit

4pA). Between the main fault zone and the secondary

fault zone, unit 5 is buried by sandy alluvium (unit 4asa)

and silty colluvium (unit 4csi) that we interpret as debris,

which accumulated along the scarp at the same time as

the paludal silt was collecting in the adjacent marsh

lower on the slope. Units 3 and 2 are sandy alluvium and

silty colluvium that locally fill fissures and form wedgeshaped deposits of scarp colluvium derived from erosion

of the fault scarp and adjacent scarp slope. In places

within the main fault zone, deposits of unit 3 appear to

be stratigraphically below units 5 and 6 because unit 3

sediment deposited beneath and around blocks of the

older units that were suspended in open fissures, which

formed during the most-recent faulting event. The

youngest deposit mapped in the trench is a layer of

residuum (unit 1) that mantles the modern land surface.

INTERPRETATION OF FAULTING

HISTORY

We interpret stratigraphic relations in the trench as

recording evidence for three surface-faulting events on

the Cheraw fault in Holocene and latest Pleistocene [≤30

k.y. (thousand years)] time. The timing of these events is

constrained by four radiocarbon dates on soil organic

concentrates and one date on charcoal (table 1 on plate),

and nine thermoluminescence (TL) age estimates (table

2 on plate) on samples of silt and fine-grained sand.

Reconstruction of paleolandscapes and the thicknesses

of pre- and post-faulting deposits help establish general

limits on amounts of throw associated with each event

(table 3). Finally, we summarize our preferred estimates

for the times of these events in table 4.

The four samples of bulk organic material from the

buried A horizons and the sample of charcoal from the

upper buried A horizon (unit 4pA) yielded stratigraphically consistent radiocarbon age determinations (table 1).

The soil organic matter in the buried A horizons is within

about 0.5 m of the modern surface and has not been oxidized, which we interpret as direct evidence that the

most-recent faulting event is geologically young.

The radiocarbon dates from organic matter in the buried

soils must be adjusted for the apparent mean residence

time (AMRT) of organic carbon in the A horizons at the

time of burial. The accumulation and decomposition of

organic matter in A horizons is a dynamic process that

ideally reaches a steady state when organic-matter

accumulation and decomposition rates are equal

(Matthews, 1980). In this steady-state condition, the

organic matter remains in the soil for some length of

time, which is commonly described as the AMRT

(Scharpenseel, 1971). Many factors affect the AMRT of

carbon in a soil, including vegetation type, climate, and

depth within the soil, and the AMRT can range from

hundreds to thousands of years (Paul and others, 1964).

In semi-arid regions of the western United States that are

climatically similar to southeastern Colorado, the AMRT

for a soil is commonly on the order of several hundred

years (Machette and others, 1992); that is, a radiocarbon

analysis of organics in a modern A horizon would yield

an age of several hundred years. For this study, we use

500 years for the AMRT of the soils. For the samples of

buried soils listed in table 1, the AMRT is subtracted

from the apparent radiocarbon age to better estimate the

time when the soil was buried.
4

Table 3. Interpretation of amounts of vertical offset on the Cheraw fault, Colorado

[CW, colluvial wedge. The minimum offset values on the main fault zone for the most-recent and penultimate faulting events are

based on the maximum thickness of the colluvium that accumulated at the base of the scarp; these values do not compensate for the

effects of any backrotation immediately adjacent to the scarp. The possible offset values on the main fault zone for the most-recent

and penultimate events are derived by multiplying the maximum thickness of the colluvial wedge from these events by a factor of

two, which is a common way to estimate the height of paleoscarps. The total offset on the secondary fault zone is about 0.8 m, most

or all of which occurred during the penultimate faulting event]

Faulting event Minimum offset on

main fault zone

Possible offset on main

fault zone

Offset on secondary fault

zone

Range of total

offset

Most-recent ~0.5 m (maximum thick- ..........

ness of CW).

~1.0 m (twice thickness

of CW).

0.0–0.1 m (permissible). ~0.5–1.1 m.......

Penultimate.......... ~0.4 m (maximum thickness of CW.~0.8 m (twice thickness

of CW).

0.7–0.8 m............................. ~1.1–1.6 m.......

Oldest...................... ~1.5 m (thickness of

distal fill).

~1.5 m (thickness of

distal fill).

None...................................... ~1.5 m................

Total of 3 events. ~2.4 m..................................... ~3.3 m ~0.8 m .................................. ~3.2–4.1 m .................................. .......

The TL age estimates are from samples of finegrained deposits that we believe are either primarily insitu loess or loess that was locally reworked and redeposited on the downthrown side of the scarp. Except for

two samples, the TL age estimates are stratigraphically

consistent within the error limits of the determinations.

The exceptions are samples TL-CF-5 (9.2±0.9 ka) and

TL-CF-9 (17.0±1.6 ka). The estimated age of sample TLCF-5 is anomalously young compared to all of the other

TL estimates and radiocarbon ages, most of which are

stratigraphically younger. In contrast, sample TL-CF-9 is

anomalously old with respect to its stratigraphic position.

Based on the radiocarbon ages that stratigraphically

bracket sample TL-CF-9, it should have an age between

9.4 ka and 11.2 ka, which is much younger than the TL

estimate of 17 ka. Because of their inconsistent ages

with respect to stratigraphic and radiometric constraints,

we consider the TL age estimate of samples TL-CF-5 and

TL-CF-9 to be unreliable, and, therefore, we dismiss

them and do not discuss them further in this report.

CUMULATIVE THROW AT THE TRENCH

SITE

Two conspicuous zones of normal-slip faults offset

stratigraphic units in the trench and juxtapose relatively

light-colored fluvial sands (units 7 and 8) against relatively dark-colored silt and clayey silt (units 5 and 6).

The faults dip about 70° NW. The main fault zone,

which is near the base of the scarp, has a minimum estimated cumulative throw of about 2.4 m (table 3),

whereas the secondary, southeastern fault zone has a

cumulative throw of only about 0.8 m (table 3). A minor

amount of stratigraphic offset is accommodated by gentle

tilting between the two faults. Thus, stratigraphic relations in the trench suggest a minimum net throw of about

3.2 m across the entire fault zone.

This minimum net-throw value of about 3.2 m, which

is based on the thickness of faulting-related Quaternary

deposits adjacent to the fault zones, is comparable to the

net throw measured on the top of the Cretaceous bedrock.

On the upthrown side of the fault zone, we exposed

Smoky Hill Shale Member near the 22-m mark in the

trench, and on the downthrown side, we penetrated the

shale in auger hole G-1 at a depth of about 4.9 m (fig. 4).

Projecting the top of bedrock in the auger hole back to

the fault zone and correlating it with the shale exposed in

the trench indicates a throw of about 3.2 m (fig. 4).

OLDEST FAULTING EVENT RECORDED IN

THE TRENCH

The evidence of the oldest event in the trench is the

colluvium and paludal silt (units 5 and 6) that are present

only on the downthrown side of the scarp. Prior to this

event, a southerly flowing channel (fig. 5), which was

part of the area’s drainage network, had incised through

older Pleistocene alluvium and into the underlying Cretaceous bedrock; the channel was partially filled with

fluvial sand and gravel (units 7 and 8). The fault scarp

that formed during the oldest event dammed the channel

and created a relatively marshy depocenter on the northwest side of the fault. Colluvium from the scarp and

paludal silt accumulated in this depocenter. We estimate that about 1.5 m of throw occurred during this faulting event. This estimate is based on a reconstruction of

the thickness of the colluvium and alluvium in the distal

part of the depocenter, away from the localized backrotation and deformation adjacent to the fault that can exaggerate the thickness of post-faulting deposits.
5

Table 4. Interpretation of timing of movements on the Cheraw fault, Colorado

[14C, radiocarbon; TL, thermoluminescence. Evidence for events that predate the oldest event recorded in the trench is based on the

inference that tens of thousands of years would be required for a scarp from older events to be eroded and for a through-flowing

drainage channel to be established across any scarp that existed prior to the oldest event recorded in the trench]

Faulting event Main fault zone Secondary fault zone Best estimate of

event’s age

Type of evidence

Most-recent.... <8.4 ka.............. Probably not active 8 ka.................... ..................................... 14C, soil, and stratigraphy.

Penultimate... <12.1 ka <13.0 ........... ±1.1 ka, <13.5±1.2 ka 12 ka................. ..................... 14C, TL, soil, and

stratigraphy.

Oldest >18.4 ............... ±3.9 ka,

>21.3±2.5 ka.

Not active (top of bedrock offset same

as net offset from events 1 and 2). No

stratigraphic evidence of offset.

20–25 ka.......... Stratigraphy and TL.

Older event(s). _ _ _ _ ≥100 ka............ _ _

All of this throw occurred on the main fault zone; we did

not find stratigraphic evidence of vertical movement on

the secondary fault zone during this event.

The TL age estimates provide the best constraints on

the minimum age of the oldest event. These estimates

suggest that unit 6psi has an age of about 18.4–21.3 ka

(samples TL-CF-4 and TL-CF-3, respectively, table 2).

If we assume that the deposition of unit 6csa and much of

6psi required 2–4 k.y., then the oldest faulting event documented in the trench occurred at about 20–25 ka (table

4). The buried A horizon (unit 5pA) on the paludal silt

indicates that a period of relatively slow sedimentation

and landscape stability occurred well after the oldest

event (recorded in the trench). Organic matter from this

soil yielded a conventional radiocarbon age of

10,700±100 yr B.P., which corresponds to a calendric age

of about 12,600 yr B.P. (sample C14-CF-2, table 1). We

subtract 500 years from this number as the estimated

AMRT for soil organic matter, which yields a minimum

constraining age of 12.1 ka for the oldest event and a

maximum constraining age for the penultimate event.

PENULTIMATE FAULTING EVENT

RECORDED IN THE TRENCH

The interval of relatively slow sedimentation on the

downthrown side of the fault following the oldest event

ended abruptly when the penultimate faulting event rejuvenated the scarp. This rejuvenation accelerated sedimentation on the downthrown side and resulted in deposition of a northwestward-thinning wedge of paludal silt.

Following degradation of the scarp from the penultimate

event, an A horizon (unit 4pA) and a cambic B horizon

(unit 4 pB) formed in this wedge of reworked silt.

Stratigraphic evidence indicates that the total throw

from the penultimate event was 1.1–1.6 m (table 3). At

the main fault zone, units 4pA and 4pB have a combined

thickness of about 0.4 m, which must have been the minimum height of the fault scarp. If we apply a commonly

used guideline that the height of a fault scarp is roughly

twice the thickness of deposits that accumulated at the

base of scarp, then we estimate that the scarp on the

main fault zone could have been 0.8 m high. Thus, we

use a value of 0.4 m as the minimum vertical offset from

this event and 0.8 m as a possible maximum value.

Movement on the secondary fault zone also occurred

during this event. The total cumulative throw on the secondary fault is about 0.8 m. The stratigraphic evidence

suggests that most, if not all, of this offset occurred during the penultimate event; however, we cannot preclude

the possibility that a small amount of throw (<10 cm)

occurred during the younger, most-recent event.

The time of the penultimate event is bracketed by

radiocarbon ages on organic matter from the A horizon of

the two buried soils (units 4pA and 5pA). When this

faulting event occurred, deposition of the wedge of paludal silt buried unit 5pA, which has an estimated age of

12.1 ka. Thus the event occurred shortly after 12.1 ka.

The age of the A horizon that caps unit 4 establishes a

minimum time for the event. This A horizon has an

estimated age of 9.4–9.9 ka (samples C14-CF-1, C14-CF-3,

and C14-CF-4, table 1). The radiocarbon ages also indicate that the colluvial wedge of paludal silt (unit 4csi)

from the penultimate event was deposited over a time

span of about 2–3 k.y.

MOST-RECENT FAULTING EVENT

RECORDED IN THE TRENCH

The most obvious evidence of the most-recent faulting event on the Cheraw fault is offset of the youngest

buried soil (units 4pA and 4pB). This event rejuvenated
6

the scarp again and resulted in deposition of colluvium

and alluvium, which we mapped as units 2 and 3. We

estimate that about 0.5–1.1 m of offset occurred during

this event (table 3). Most of the offset occurred on the

main fault zone, although a small amount of offset could

have occurred on the secondary fault zone. If movement

did occur on the secondary fault zone, then we suspect

that the vertical offset must have been 10 cm or less

because a greater offset would have likely left a stratigraphic record of the offset. The maximum thickness of

colluvium from this event is slightly less than 0.5 m,

which corresponds to the minimum size of the resultant

scarp. Doubling this value suggests that the scarp may

have been 1.0 m high. If we assume that as much as 0.1

m of offset also occurred on the secondary fault zone,

then the cumulative throw during this event could have

been 1.1 m.

The radiocarbon dates and TL age estimates only

establish a maximum limit on the time of this event

because they provide age control only for units that were

offset by this event. Three radiocarbon analyses on concentrated organic matter from the buried soil (unit 4pA)

that was faulted by this event yielded dates that indicate

the most-recent event occurred after 9.4–9.9 ka (table 1).

Furthermore, accelerator mass spectrometry (AMS)

radiocarbon analysis of charcoal fragments in this A horizon yielded a calibrated age of 8,415 yr B.P. (C14-CF-5,

table 1). These charcoal fragments could have been

deposited within the A horizon or could have been

younger charcoal incorporated into the soil (unit 4pA)

prior to its burial by fault-scarp colluvium (unit 2c).

Because the age calibration of the charcoal fragments

does not require the subjective estimate of AMRT as do

soil organics, we have more confidence in the charcoal

age compared to the soil ages. Thus, we favor using the

charcoal age of 8.4 ka as an estimate of the maximum

time for the most-recent faulting event.

We have no absolute ages for deposits (units 1, 2,

and 3) that postdate this event, so we cannot define a

minimum limit for the time of the event. However, the

most-recent event probably occurred within a few hundred years after deposition of the charcoal because we

would expect small charcoal fragments located less than

10 cm below the ground surface to be either oxidized or

dispersed by bioturbation within a few centuries. Thus,

our best estimate for the time of the most-recent event is

about 8 ka.

CONCLUSIONS

The Cheraw fault is located on the Piedmont in

southeastern Colorado, a region that has been seismically

quiescent region during historic time, yet our paleoseismic investigation of the fault indicates that three earthquakes have produced surface rupture during the past 20–

25 k.y. and that the most-recent faulting event occurred in

the early Holocene. Our study also suggests that the

three faulting events on the Cheraw fault may be part of

a pattern of temporal clustering of earthquakes in which

one or more earthquakes occur in a relatively short period

of time (10–15 k.y.), and that this interval of high activity

is bounded by long intervals (>100 k.y.) of relative low

activity.

Clearly, if significant historical seismicity had been

associated with the Cheraw fault, it would have been

recognized as a potential source of future large earthquakes. However, recent paleoseismic studies (Crone

and Luza, 1990; Kelson and Swan, 1990; Crone and others, 1992; Machette and others, 1993) and historical

surface-faulting earthquakes in stable continental regions

(Johnston, 1994; Crone and others, in press) show that

faults similar to the Cheraw fault can remain aseismic for

many thousands of years and rupture unexpectedly with

little or no precursory warning. This type of fault behavior indicates that the record of historical seismicity is

imperfect in identifying all potentially seismogenic faults

in the stable interior of continents.

ACKNOWLEDGMENTS

This study was supported, in part, by a grant from the

U.S. Nuclear Regulatory Commission and by the National

Earthquake Hazards Reduction Program of the U.S. Geological Survey. We thank Mr. Larry Hansen, President of

the Timberlake Grazing Association, for access and permission to excavate the trench on the association’s property. Paula B. Maat (formerly of the U.S. Geological

Survey) collected the thermoluminescence samples in

the field and made the in-situ measurements of radiation

for dose rates. We also thank Juan Carlos Moya (University of Colorado, Boulder) and Lin Wei-Hsiung (Central

Geological Survey of Taiwan, Taipei) for their assistance

in the field during initial studies of the Cheraw fault, and

Theodore Barnhard (USGS) and Thomas Machette for

assistance in making the detailed topographic survey. In

addition, many colleagues and interested geologists gave

helpful suggestions during two field trips that we conducted to the site in 1995. However, we accept responsibility for any errors or misinterpretations that exist in this

report.

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