4

4

Qc

U D

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qac

Qac

Qac

Qac

Qc

Qc

Qc

Qc

Qc

Qc

Qls

Qls

Qls

Qls

Qls

Qls

Qp

Qp

Kb

Kb

Kd

Kd

Kd

Jm

Jm

Jm

Jm

Jm

Jm

Jm

Jm

Jm

Jm

Jsj

Jsj

Jsj

Jsj

Jsj

Jsj

Jsj

Prgu

Prgu

Prgu

Prgu

Prgu

Prgu

Prgu

Prgu

Pgf

Pgf

Pgf

Pgf

Pgf

Pgf

Pgf

Pgsu

Pgsu

Pgsu

Pgsu

Pgsu

Pgsl

Pgsl

Pgsl

Pgsl

Pgsl

Pc

Pc

Pc

Pc

Pc

Pc

f

f

f

f

YgSH

YgSH

YgSH

Yghp

Yghp

XgR

XgR

Xfl

YXp

YXp YXp

Xh

Xh

Xh

Yd

Yd

Yd

Yd

Sheep

Creek

fault

zone

U

D

Shell Creek fa

ult

U

D

U

D

U

D

U

U D D

Jimmy

East

Creek

fault

We

st Jim

my

Creek

fault

U

D

U

D

P

s

anhandle Creek hear zone

Cornelius Creek shear zone

BOULDER RIDGE FAULT ZONE

U

D

Shell Creek

a

nticline

East Bull Mountain syncline

3

4

4

3

5

3

25

33

3

3

4

4

50 15

15

20

3

3

12

5

7

25

4

10

15

29

35

20

32

40

40

18

22 37 15 25 28

5

35

25

33 27

20 80

30 40

20

50

15

15

12

13

10

10

24

5

15

15

4

40

22 25

25

33

30

30

30

30

26

4

75

60 70 45

80

80

50

60

60

40

70

28 15

55

80 60

INTRODUCTION

New geologic mapping within the Sand Creek Pass 7 1⁄2 ́ quadrangle defines geologic 

relationships within the northern Front Range of Colorado along the Wyoming border 

approximately 35 km south of Laramie, Wyo. Previous mapping within the quadrangle was 

limited to regional reconnaissance mapping (Tweto, 1979; Camp, 1979; Burch, 1983). 

Workman (2008) mapped the Eaton Reservoir 7 1⁄2 ́ quadrangle to the east, Burch (1983) 

mapped granite of the Rawah batholith to the south, Camp (1979) mapped the Laramie 

River valley to the west, and Ver Ploeg and Boyd (2007) mapped the Laramie 30 ́ × 60 ́ 

quadrangle to the north. Field work was completed during 1981 and 1982 by W.A. 

Braddock, during 2007 and 2008 by J.B. Workman, and the mapping was compiled at 

1:24,000-scale by J.B. Workman. Minimal petrographic work was done and no isotope 

work was done in the quadrangle area, but detailed petrographic and isotope studies were 

performed on correlative map units in surrounding areas as part of a related regional study 

of the northern Front Range. Stratigraphy of Proterozoic rocks is primarily based upon 

field observation of bulk mineral composition, macroscopic textural features, and field 

relationships that allow for correlation with rocks studied in greater detail outside of the map 

area. Stratigraphy of Phanerozoic rocks is primarily based upon correlation with similar 

rocks to the north in the Laramie Basin of Wyoming and to the east in the Front Range of 

Colorado.

DESCRIPTION OF MAP UNITS

[Thin, discontinuous surficial deposits were not mapped. Unit symbols for Proterozoic 

igneous rocks in some cases utilize nonstandard symbols (such as YgSH) which are intended 

to emphasize composition while differentiating broadly accepted lithodemic unit names. 

Lower case letters after the age designation indicate rock composition and textural features 

whereas upper case letters indicate major named intrusive body or rock suite.]

Alluvium (Holocene)—Poorly sorted coarse sand and pebbles to boulders in 

stream channels, flood plains, and low terraces. Flood-plain and terrace 

deposits are generally less than 5 m above present stream level. Unit may 

locally include colluvium not mapped separately. Composition reflects 

bedrock exposed upstream

Alluvium and colluvium, undivided (Holocene to middle? Pleistocene)— 

Mapped along and near Dry Creek, the northeastern foot of Bull Mountain, and the southeastern part of the map area where colluvium (Qc) 

higher on the slopes grades into alluvium (Qa) downslope and the contact 

between the two deposits has not been identified. Unit may include 

pediment deposits east of Dry Creek

Colluvium (Holocene to middle? Pleistocene)—Non-sorted clay, silt, sand, 

and angular to subrounded clasts ranging in size from granules to large 

boulders. Includes deposits transported downslope from source material 

chiefly by sheetwash, landslide, debris flow, hyperconcentrated flow, rock 

fall, and possibly periglacial processes

Landslide deposits (Holocene to middle? Pleistocene)—Non-sorted clay, silt, 

sand, and angular to subrounded debris on slopes and at base of slopes 

characterized by hummocky topography. Upslope regions characterized 

by headwall scarps and closed depressions and downslope areas by lobate 

toes. Formed along edges of Bull Mountain where unstable slopes are 

underlain largely by shales, siltstones, and claystones that are overlain by 

more resistant sandstones of the Dakota Group (Kd)

Pediment deposits (middle Pleistocene)—Poorly stratified and non-sorted to 

poorly sorted, clast- and locally matrix-supported gravelly and sandy alluvial 

sediments that overlie gently sloping surfaces cut on Red Peak and upper 

part of Goose Egg Formations (Prgu) at the northeast corner of 

quadrangle. Composition of clasts generally reflects upslope bedrock with 

sandstone-rich gravel and non-pebbly to pebbly silty sand derived from 

Mesozoic rocks to south on Bull Mountain. Unit locally includes minor 

unmapped sheetwash deposits, colluvium, and probable hyperconcentrated-flow deposits. Lower limit of pediment surface is about 18 m 

above East Shell Creek to the north of the quadrangle boundary. Pediment 

surface is about 60 m above Sand Creek in northeast corner of 

quadrangle. Estimated thickness is 1–5 m

Benton Group (Upper and Lower Cretaceous)—Black siliceous and 

non-siliceous marine shales interbedded with thin discontinuous bentonite 

and fine-grained sandstone beds. Includes all dark shales conformably 

deposited above upper sandstone of the Dakota Group (Kd) and below 

Niobrara Formation (latter unit exposed to west of the map area). Unit is 

poorly exposed and includes Frontier Formation and Mowry Shale (Shaw, 

1957). Upper part of unit not exposed in map area. Frontier Formation 

is equivalent, in descending order, to Carlile Shale, Greenhorn Limestone, 

and Graneros Shale of eastern Front Range (Berman and others, 1980; 

W.A. Cobban, oral commun., 1998). Lower 30 m of unit consists of rusty 

dark-brown to black, wavy bedded, siliceous, fissile shale containing fish 

scales intercalated with thin beds of bentonite, equivalent to Lower 

Cretaceous Mowry Shale of eastern Front Range (Cole and Braddock, 

2009). Estimated total thickness is 168 m from well logs in the Laramie 

River valley (Camp, 1979)

Dakota Group (Lower Cretaceous)—Light-gray to brown, fine-grained, 

ripple-marked, cross-stratified quartz sandstone deposited along alluvial 

plain interbedded with black to dark-gray, laminated, carbonaceous, 

shallow marine shale and siltstone and gray to brown, planar to trough 

crossbedded chert-pebble conglomerates at base. Unit unconformably 

overlies less resistant clays of the Morrison Formation (Jm). Includes, in 

descending order, Muddy Sandstone, Thermopolis Shale, and Cloverly 

Formation as mapped by Ver Ploeg and Boyd (2007). Muddy Sandstone 

consists of approximately 20 m of white to brown-weathering, fine- to very 

fine grained quartz sandstone with minor interbedded sandy, carbonaceous 

shales. Thermopolis Shale consists of approximately 20 m of dark-gray 

shale which grades upward to siltstone and very fine grained quartz 

sandstone of overlying Muddy Sandstone. Cloverly Formation consists of 

12 m of light-gray, fine- to medium-grained quartz sandstone at top which 

grades into 10-m-thick section of dark-gray laminated shale overlying 

approximately 20 m of gray to brown, trough crossbedded quartz- and 

chert-pebble conglomerates at the base, filling channels in underlying 

Upper Jurassic rocks. Total thickness in map area estimated to be 83 m, 

but slumping and colluvial cover make thickness difficult to measure. 

Camp (1979) estimated total thickness of 91 m based on well data from 

Laramie River valley to west of map area

Morrison Formation (Upper Jurassic)—Greenish-gray, gray, and maroon 

siliceous claystone interbedded with gray thin-bedded, fine-grained, 

crossbedded sandstone and discontinuous beds of gray, micritic limestone 

deposited in fresh-water rivers, lakes and swamps. Sandstone and 

limestone beds increase up section. Upper contact with overlying Dakota 

Group (Kd) marked by channel-cutting erosional unconformity. Lower 

contact is marked by one or more cherty shale beds which may represent a 

minor erosional unconformity or may be gradational with the upper 

sandstones of the Sundance Formation (Curtis, 1963). Forms recessive 

slope below more resistant sandstones and conglomerates of Dakota 

Group. Total thickness estimated to be 78 m on eastern slope of Bull 

Mountain

Sundance Formation (Upper and Middle Jurassic) and Jelm Formation of 

Chugwater Group (Upper Triassic), undivided—Sundance Formation 

consists of calcareous nearshore marine sandstone and laminated siltstone. 

Pipiringos (1957, 1968) and Pipiringos and O’Sullivan (1976) define three 

members on west side of Bull Mountain (just west of map area). These are, 

in descending order, Windy Hill Sandstone Member, Pine Butte Member, 

and Canyon Springs Sandstone Member. Windy Hill Sandstone Member 

consists of gray-white, buff-weathering, thin-bedded to crossbedded 

calcareous sandstone. Unconformity separates the Windy Hill from 

underlying Pine Butte Member. Pine Butte Member consists of white to 

greenish-white and yellow, crossbedded sandstone overlain by blue-green to 

gray calcareous shale interbedded with lenses of calcareous sandstone and 

sandy oolitic limestone. Canyon Springs Sandstone Member is white, 

yellow-weathering, fine- to very fine grained, thin-bedded to crossbedded, 

well-sorted calcareous and glauconitic sandstone and laminated siltstone 

equivalent to Entrada Sandstone of eastern Front Range (Pipiringos and 

others, 1969). Canyon Springs Sandstone Member regionally becomes 

younger to south and overlies a regional unconformity marked by a thin 

horizon of chert pebbles within map area (Pipiringos, 1968). 

The Chugwater Formation of Darton (1908) has been modified 

significantly by many authors. High and Picard (1967) proposed raising the 

Chugwater to group status and its members to formational status as used 

here. Pipiringos (1968) described Jelm Formation as grading laterally into 

upper part of Crow Mountain Member of the Chugwater of Love (1939) in 

south-central Wyoming and therefore includes Jelm Formation as part of 

the Chugwater Group as used here. Jelm Formation of Knight (1917) and 

Red Peak Formation of Love (1939) are recognized in map area. Jelm 

Formation is mapped here with the Sundance Formation and the Red Peak 

Formation is mapped here with the upper part of the Goose Egg 

Formation. 

Upper part of Chugwater Group (Gypsum Spring and Popo Agie 

Members of Love (1939)) was removed by regional unconformity at base of 

Sundance Formation within map area. The regionally extensive Alcova 

Limestone of Lee (1927), which sits at or near base of Jelm Formation 

equivalent in central Wyoming, pinches out in Laramie Basin and is not 

present in map area

Jelm Formation of the Chugwater Group consists of non-marine 

sandstone with minor shale, silt, and limestone-pebble conglomerate divided 

into two members, as defined by Pipiringos (1968), within map area. 

These are, in descending order, Sips Creek and Red Draw Members. Sips 

Creek Member consists of greenish-white, orange-weathering, fine- to 

medium-grained, trough crossbedded fluvial sandstone overlying a basal 

greenish-gray shale and limestone-pebble conglomerate. Red Draw 

Member consists of reddish-brown to reddish-pink, orange-weathering, very 

fine to fine-grained, crossbedded calcareous sandstone (possibly eolian) 

interbedded near top with dark reddish-brown, laminated siltstones and 

shales. Siltstone and shale beds thicken to north into Wyoming (Pipiringos, 

1957). Base of unit contains rounded siltstone pebbles and boulders. 

Pipiringos (1957) described an erosional disconformity between Jelm and 

underlying Red Peak Formations within Chugwater Group which correlates 

with a regional unconformity above Lykins Formation on east side of Front 

Range (Pipiringos and O’Sullivan, 1976), but Camp (1979) described this 

contact in Laramie River valley as conformable and irregular due to 

slumping under differential compaction. Jelm Formation thins significantly 

to south and may pinch out in vicinity of Cameron Pass (Kellogg and 

others, 2008). 

Pipiringos (1957) measured total thickness between base of Morrison 

Formation (Jm) and top of Red Peak Formation on west side of Bull 

Mountain at 74 m. Camp (1979) estimated 79 m of Jelm Formation and 

9 m of Sundance Formation in Laramie River valley. On eastern slope of 

Bull Mountain in map area, exposure is extremely poor due to pervasive, 

unmapped thin colluvial cover derived from upslope slumping of Dakota 

Group (Kd) sandstones and conglomerates. Unconformity separating units 

is not exposed and therefore is not mappable. Total thickness of undivided 

map unit estimated to be 100 m in map area

Red Peak Formation of Chugwater Group (Lower Triassic) and upper part 

of Goose Egg Formation (Lower Triassic and Upper Permian), 

undivided—Combined unit characterized by red to red-brown, planarlaminated to ripple crossbedded siltstone, planar-bedded, fine-grained 

sandstone, and micaceous shale interbedded with gypsum beds near base. 

Red Peak Formation of Chugwater Group consists of four gradational 

facies subdivisions, as defined by Picard (1967). All facies are recognized in 

map area and described in descending order. Upper platy facies is characterized by slope-forming horizontally bedded siltstone interbedded with 

lesser silty claystone and discontinuous lenses of mud-pebble conglomerate 

deposited in a tidal-flat environment. Alternating facies is characterized by 

resistant massive beds of moderate- to well-sorted nearshore marine 

siltstone alternating with thin, fissile beds of poorly sorted siltstone, silty 

claystone, and very fine grained sandstone deposited in tidal-flat environments. Lower platy facies is similar to upper platy facies and is characterized by slope-forming, convolute-bedded siltstone interbedded with lesser 

mud-pebble conglomerate and silty claystone with minor, very fine grained 

sandstone. Basal silty claystone facies is characterized by poorly exposed 

valley-forming silty claystone interbedded with minor ledges of thin-bedded 

siltstone and sandstone with abundant ripple cross-bedding deposited in a 

transitional environment. Red Peak Formation conformably overlies upper 

part of the Goose Egg Formation.

Underlying upper part of the Goose Egg Formation is here informally 

defined as all rocks above the Forelle Limestone Member of Goose Egg 

Formation (Pgf) and below Red Peak Formation. Burk and Thomas (1956) 

originally reassigned this interval to upper part of Goose Egg Formation and 

Maughan (1964) defined four members at Red Mountain (northwest of map 

area) within upper part of the formation deposited in tidal-flat or lagoonal 

environments adjacent to the Phosphoria sea. They are, in descending 

order, Lower Triassic Little Medicine and Freezeout Shale Members, and 

Upper Permian Ervay and Difficulty Shale Members. Maughan (1964) 

describes a local unconformity above the Ervay Member related to late 

movements of the ancestral Front Range highlands in the southern Laramie 

Basin. This valley-forming sequence is very poorly exposed within map 

area, and therefore contacts between individual members and contact with 

overlying Red Peak Formation are not mapped. The top of the Little 

Medicine Member is recognized, where exposed, as the top of thin carbonate bed approximately 22 m above the top of the Forelle Limestone 

Member (Pgf) or as the top of the highest gypsum bed in section.

Total thickness of combined unit estimated to be 209 m on eastern 

slope of Bull Mountain within map area. Camp (1979) estimated total 

thickness to be 177 m in Laramie River valley 

Lower part of Goose Egg Formation (Permian)—The Goose Egg Formation 

was defined by Burk and Thomas (1956) for exposures in central Wyoming. 

Maughan (1964) describes eight members, reducing some units from 

formational rank and reassigning some units previously included in overlying 

Chugwater Formation of Darton (1908). Upper part of formation (Little 

Medicine, Freezeout Shale, Ervay, and Difficulty Shale Members), described 

above as lower part of unit Prgu, cannot be differentiated from overlying 

rocks of Chugwater Group in map area. Members of lower part of the 

Goose Egg Formation, described in detail below, include the Forelle 

Limestone and Satanka Shale Members

Forelle Limestone Member (Upper Permian)—Gray, microcrystalline, algal, 

wavy-laminated dolomite interbedded with purple and tan, laminated shaly 

dolomite deposited in hypersaline nearshore environment. Camp (1979) 

described three subunits in the Laramie River valley which are also present 

in map area. These divisions are, in descending order, gray dolomite 

interbedded with purple shaly dolomite, purple and tan shaly dolomite, and 

gray dolomite at base. Estimated thickness is 8 m

Upper part of Satanka Shale Member (Upper and Lower Permian)— 

Reddish-brown to reddish-orange, massive, horizontally bedded sandy shale 

and siltstone interbedded with lesser amounts of reddish-brown to 

pale-orange, fine-grained, laminated sandstone and thin blocky gypsum and 

thin, purplish-gray, wavy-laminated dolomitic limestone near base of unit. 

Deposited in tidal flat or lagoonal environment. Unit is equivalent, in 

descending order, to Glendo Shale and Minnekahta Limestone Members of 

the Goose Egg Formation, as defined by Burk and Thomas (1956). 

Pearson (1972) described a thin gypsum bed within his Satanka Shale in 

the southern Laramie basin which he correlated to Minnekahta Limestone. 

We loosely correlate the dolomite and gypsum beds at base of map unit to 

Minnekahta Limestone Member, but Satanka terminology, as defined by 

Pearson (1972), is preferred due to uncertainty of correlation. Campbell 

(1963) correlated Minnekahta Limestone Member to Falcon Limestone 

Member of Lykins Formation on east side of Front Range. Base of unit is 

defined by a thin gray dolomitic limestone with white to gray chert nodules. 

Camp (1979) estimated thickness to be 43 m in Laramie River valley. 

Estimated thickness on eastern side of Bull Mountain is 52 m

Lower part of Satanka Shale Member (Lower Permian)—Reddish-orange 

to dark maroon, thin laminated, sandstone and siltstone with increasing 

interbedded dolomitic sandstone and gypsum near top of section. Percentage of sandstone in lower part of unit increases to south across map area. 

Deposited in tidal flat or lagoonal environment. Unit is loosely correlated 

with Opeche Shale Member, as defined by Burk and Thomas (1956). 

Maughan and Wilson (1963) correlate the lower part of the map unit at 

Chimney Rock with the Owl Canyon Formation of Condra and others 

(1940). The regional unconformity above the Owl Canyon Formation on 

the eastern side of the Front Range (Cole and Braddock, 2009) is inferred 

to exist in the lower part of the Satanka Shale Member in map area, but 

has not been identified due to poor exposure. Satanka terminology, as 

defined by Pearson (1972), is preferred due to uncertainty of correlations. 

Estimated thickness at Sand Creek Pass is 35 m

Casper Formation (Lower Permian to Middle Pennsylvanian)—White to 

buff, yellow-weathering, fine- to medium-grained, well-sorted quartz-rich, 

carbonate-cemented eolian sandstone interbedded with discontinuous, thin 

lenses of freshwater limestone. Unit displays prominent ripple and 

large-scale trough (festoon) cross-laminations with significant soft-sediment 

deformation at several levels. Darton (1908) originally defined Casper 

Formation to include all rocks below Satanka Shale and above Precambrian 

rocks, but Knight (1929) restricted name to exclude arkosic rocks assigned 

to Fountain Formation (f), as used here. Equivalent to Ingleside Formation of Butters (1913) on east side of Front Range. Contact with overlying 

Satanka Shale Member is sharp and conformable. Contact with underlying 

Fountain Formation (f) is generally sharp, although gradational in some 

exposures. Maughan and Wilson (1963) described a local unconformity at 

base of Casper Formation in area related to onlapping and beveling of 

basin fill along flanks of Ancestral Front Range uplift. In northern parts of 

map area near Chimney Rock, Casper Formation appears to be in 

conformable contact with underlying Fountain Formation (f), but to south, 

where exposures are poor, upper unit of Fountain Formation thins, which 

could be a result of non-deposition or of erosion, but no angular discordance is noted across this contact. Within map area, unit thins significantly 

to south and pinches out north of Sand Creek Pass. At Chimney Rock 

location, thickness estimated to be 29 m 

Fountain Formation (Middle to Lower Pennsylvanian)—Maroon to pink and 

light-greenish-gray, coarse-grained, trough crossbedded, arkosic, fluvial 

sandstone and conglomerate interbedded with minor amounts of pink and 

light-green micaceous siltstone and mudstone. Unit is informally divided 

into upper and lower members. Sandstones within upper member are finer 

grained and contain well-rounded clasts of quartz and feldspar, conglomerate beds are thinner and less common, and siltstone is only locally present. 

Contact between upper and lower members is conformable in vicinity of 

Chimney Rock and placed below lowest occurrence of sandstone with 

abundant, well-rounded clasts. Lower member is characterized by more 

abundant coarse-grained arkosic sandstone and conglomerate interbedded 

with siltstone and mudstone and minor fine-grained quartzose sandstone. 

Lower member has very few resistant beds and forms low rolling hills to 

west of unconformable contact with underlying Proterozoic Sherman 

Granite (YgSH). Thin, discontinuous limestone bed containing chert 

nodules is exposed below base of unit just east of map area (Workman, 

2008). Ichthyolith fossils indicate a maximum age of Mississippian for 

limestone but have a long range and could just as well be Pennsylvanian (A. 

Harris, written commun., 2005). Sando and Sandberg (1987) assigned 

similar limestones exposed at northern end of Laramie Range in Wyoming 

to Big Goose Member of Mississippian Madison Limestone of Peale 

(1893). Very coarse grained arkose conglomerates occur at base of unit. 

Maughan and Wilson (1963) indicated episodic uplift of Ancestral Front 

Range during the Permian and Pennsylvanian caused beveling and onlapping relationships towards flanks of uplift (southwest in this area) which 

caused local erosion both between upper and lower members and at 

contact with Casper Formation (Pc). Upper member thins to south 

across map area, but poor exposure prevents confirmation of contact 

relationships. Estimated thickness of unit is 73 m near Sand Creek Pass. 

Knight (1929) reported a thickness of 134 m in vicinity of Chimney Rock. 

On west slope of Bull Mountain, where formation is thickest within 

Laramie River valley, Camp (1979) reported a thickness of 169 m for 

lower unit of formation and a total thickness of 212 m

Mafic dikes (Mesoproterozoic)—Gray to black, fine- to medium-grained, 

sparsely porphyritic, ophytic to subophytic hornblende-biotite diorite and 

diabase dikes that form a northwest-trending dike swarm across much of 

northern Front Range. Composition varies locally and regionally, but 

generally consists of zoned and altered plagioclase phenocrysts in a matrix 

of pyroxene, hornblende, biotite, iron oxides, and alteration products. 

Dikes mostly exposed cutting Sherman Granite (1,412 ± 13 Ma; 

Alienikoff, 1983), but cut Paleoproterozoic metamorphic rocks in some 

locations. Southeast of map area in Livermore Mountain quadrangle, 

correlative dikes are cut by main phase of granite of Log Cabin batholith 

(1,390 ± 30 Ma; Braddock and others, 1989) and by pegmatites of similar 

age (Braddock and Connor, 1988). Extent and orientation of dikes is 

largely mapped from aerial photographs in conjunction with field 

observations

Pegmatite (Mesoproterozoic? and Paleoproterozoic)—Coarse to very coarse 

grained, white to pink, inequigranular leucogranitic intrusions forming dikes 

and small bodies. Contains quartz, plagioclase, microcline, and variable 

amounts of biotite and (or) muscovite. Graphic granite texture is common, 

and brecciation is extensive. Unit is poorly exposed and generally mapped 

based upon abundance in float material. Locally may include aplitic dikes. 

Age is poorly constrained within map area where unit only intrudes 

Paleoproterozoic rocks. In surrounding areas, Paleoproterozoic pegmatites 

related to Rawah batholith, as well as Mesoproterozoic pegmatites related 

to Log Cabin batholith, have been documented (Peterman and others, 

1968; Cole and Braddock, 2009)

Sherman Granite (Mesoproterozoic)—Red to pink, coarse-grained, equigranular to mildly porphyritic hornblende-biotite syenogranite. Composed of 

quartz, microcline, oligoclase, 5–10 percent sodium-bearing hornblende 

and biotite, and accessory allanite, zircon, sphene, muscovite, and rutile. 

Quartz, microcline, and plagioclase form an interlocking matrix of anhedral 

grains with sparse phenocrysts of subhedral to euhedral microcline and 

smaller euhedral hornblende and biotite. Microcline contains inclusions of 

hornblende, rutile, biotite, quartz, and plagioclase. Hornblende contains 

abundant inclusions of sphene. Subtle alignment of tabular microcline 

phenocrysts is locally present, but poorly developed and not visible in 

thin-section (Burch, 1983). Rock is commonly found as weathered to 

rounded exposures and forms a pink grus of variable thickness composed 

of angular quartz and feldspar grains. Alteration is common with biotite 

replaced by magnetite, magnetite altered to hematite, and plagioclase 

altered to epidote and sericite. Extensive epidote alteration is present in 

Cow Creek area (southeast corner of quadrangle) extending from contact 

with older metamorphic rocks north for approximately 2 km. Altered rocks 

form a prominent topographic high with much less grus development. 

Contacts with older intrusive and metamorphic rocks are generally 

sharp and characterized by a breccia zone up to 200-m-wide cut by quartz 

and epidote veins. Southern boundary of Sherman Granite is defined by 

Cornelius Creek shear zone that is marked by zones as wide as 1,500 m of 

cataclastic deformation accompanied by extensive quartz veins and 

alteration (pattern on map). No mylonite fabric was observed within field 

area along structure. Brittle nature of this shear zone implies a postProterozoic age (Abbott, 1972). No Sherman Granite is exposed to south 

of shear zone, so main intrusive contact between Sherman Granite and 

Rawah batholith has been removed by faulting. Small inclusions (less than 

1 m) of older rocks are generally not found more than several meters from 

main intrusive contact. Due to greater erosional resistance of older granitic 

and gneissic rocks (Yghp and Xfl) intruded by Sherman Granite, contact 

commonly lies at base of a steep ridge and is consequently covered by an 

apron of colluvium derived from older rocks on ridgeline. 

Apparent offsets west of map area along Sheep Creek fault zone imply 

that contact between Sherman Granite and older rocks to south dips to 

south-southeast below these older rocks and sub-parallel to structural grain 

in older rocks. Sherman Granite does not show evidence of solid-state 

foliation, nor does it appear to deform foliation in older metamorphic 

rocks; it truncates foliation and structural grain at most places along 

contact. Unit is cut by mafic dikes (Yd) within map area and by granite of 

Log Cabin batholith in southern part of Cherokee Park quadrangle (Eggler 

and Braddock, 1988) east of map area. U/Pb zircon age is 1,412 ± 13 

Ma (Alienikoff, 1983) and Rb-Sr isochron age is 1,430 ± 20 Ma (Zielinski 

and others, 1981)

Porphyritic hornblende monzogranite (Mesoproterozoic?)—Tan to reddishtan, fine-grained, xenomorphic, porphyritic hornblende monzogranite. 

Composed of fine-grained matrix of plagioclase, microcline, quartz, and 

minor amounts of hornblende and biotite with medium- to coarse-grained 

phenocrysts of predominantly quartz and hornblende and lesser amounts of 

microcline and plagioclase. Reddish color in some exposures comes from 

hematite alteration of hornblende and biotite. Unit is cut by Sherman 

Granite (YgSH) and cuts older metamorphic rocks (Xfl and Xh). Maximum 

relative age is uncertain, but unit lacks foliation and cuts across structural 

grain of Paleoproterozoic rocks to east in Diamond Peak quadrangle 

(Braddock and others, 1989), therefore a Mesoproterozoic age is assumed

Granite of Rawah batholith (Paleoproterozoic)—Pink to gray, fine- to 

medium-grained, xenomorphic, inequigranular to equigranular, biotite 

granite. Composed of quartz, microcline (locally orthoclase present), 

plagioclase, biotite, magnetite, illmenite, and, locally, hornblende. Accessory minerals are sphene, apatite, zircon, allanite, and, less commonly, 

sillimanite, rutile, and garnet. Secondary epidote or clinozoisite and 

muscovite are present along with local clay alteration. Biotite percentage 

varies widely from absent to 7.5 percent and biotite-poor phases are 

generally not foliated. Unit contains abundant inclusions of older metamorphic rocks. Hornblende gneiss (Xh) inclusions are commonly rotated with 

foliations not parallel to regional wall-rock foliation and structural trends. 

Included bodies range from centimeter- to kilometer-scale in main body of 

Rawah batholith south of map area (Burch, 1983). Biotite gneiss 

inclusions are rarer than hornblende gneiss inclusions and generally smaller. 

Inclusions of felsic gneiss may be common, but are difficult to recognize in 

the field due to similar composition and texture to granite of Rawah 

batholith. More geochemical work would be needed to clarify these 

relationships. Burch (1983) described biotite-rich foliated phases of Rawah 

batholith throughout its exposure in Front Range which have been reinterpreted as older felsic gneisses (Xfl) by Workman (2008) within Eaton 

Reservoir quadrangle to the east because their foliation parallels structure in 

older metamorphic rocks, and because these rocks are quite similar to 

biotite granite gneisses that are interlayered with hornblende gneiss (Xh). 

Northern boundary of Rawah batholith is defined by Cornelius Creek 

shear zone, as described above. No granite of Rawah batholith is exposed 

to north of shear zone; therefore main intrusive contact between Rawah 

batholith and older metamorphic complex to north has been removed by 

faulting. McCallum and Hedge (1976) reported a whole-rock Rb/Sr age of 

1,710 Ma. Correlative rocks south of map area along Cache la Poudre 

Canyon have produced a zircon age of 1,717 ± 8 Ma (W. Premo, written 

commun., 2005)

Leucocratic felsic gneiss (Paleoproterozoic)—Pink to red, fine-grained, 

leucocratic felsic gneiss. Composed of quartz, plagioclase, and microcline 

with less than 5 percent biotite and (or) hornblende and magnetite. 

Composition ranges significantly from aplitic rocks lacking any visible dark 

minerals to granitic gneiss containing significant hornblende or biotite. 

Map unit defined by greater than 50 percent leucocratic felsic gneiss where 

interlayered at centimeter- to meter-scale with unmapped hornblende 

gneiss and amphibolite (Xh), biotite felsic gneiss, and quartzofeldspathic 

biotite schist. Foliation defined by fine-scale compositional layering, by 

alignment of fine-grained biotite or hornblende crystals, and by elongated 

medium-grained porphyroblastic augen aggregates composed of intergrown 

fine-grained quartz and feldspar. Foliation is generally poorly defined and 

absent in more felsic, massive exposures. Strongly layered exposures are 

interpreted as felsic portion of Paleoproterozoic volcanic sequence that was 

strongly metamorphosed and foliated prior to intrusion of Rawah batholith 

(XgR). Zircon ages from SHRIMP analysis of sample collected to east of 

map area along Cherokee Park road indicate an age of primary volcanism 

at 1,766.6 ± 8.3 Ma (W. Premo, written commun., 2006). Current usage 

represents a field term for rocks of similar composition and texture

Hornblende gneiss and amphibolite (Paleoproterozoic)—Dark-gray to black, 

fine- to coarse-grained, interlayered hornblende gneiss, amphibolite, and 

biotite-hornblende gneiss. Composed predominantly of hornblende with 

varying quantities of plagioclase and minor quartz and biotite and (or) 

magnetite, and rarer orthoclase; interpreted as mafic-composition metavolcanic rock. Map unit defined by greater than 50 percent hornblende 

gneiss or amphibolite where interlayered at centimeter- to meter-scale with 

unmapped leucocratic felsic gneiss (Xfl), biotite felsic gneiss, and quartzofeldspathic biotite schist. Lenses of calc-silicate gneiss and localized lightto dark-green alteration bands of epidote- or chlorite-bearing rocks are 

common. 

Foliation in unit defined both by compositional layering, and alignment 

of elongate, prismatic hornblende crystals within a matrix of subhedral to 

euhedral plagioclase. Poikioblasts of hornblende contain small inclusions of 

opaque minerals, sphene, biotite, and epidote that parallel foliation. Less 

commonly, poikioblasts of plagioclase that contain inclusions of zircon, 

sphene, and hornblende form a spotted texture. Quartz and rare orthoclase occur as minor phases within matrix of rock between larger 

hornblende and plagioclase crystals. Plagioclase-poor and plagioclase-rich 

compositional layers 1–10 cm thick are common and define bands that 

outline tight recumbent folds with centimeter- to meter-scale amplitudes. 

Rocks of this unit are common as inclusions throughout the younger 

granite of Rawah batholith (XgR) and as small inclusions along contact 

zone with younger Sherman Granite (YgSH). Foliation and compositional 

layering are truncated by younger granite of Rawah batholith. Workman 

(2008) reported a U/Pb zircon age from SHRIMP analysis of exposures to 

east of map area along Beaver Creek that indicate an age of primary 

volcanism at 1,779 ± 5 Ma

REFERENCES

Abbott, J.T., 1972, Rb-Sr study of isotopic redistribution in a Precambrian mylonite-bearing 

shear zone, northern Front Range, Colorado: Geological Society of America Bulletin, v. 

63, p. 487–494.

Aleinikoff, J.N., 1983, U-Th-Pb systematics of zircon inclusions in rock-forming minerals: A 

study of armoring against isotopic loss using the Sherman Granite of ColoradoWyoming, USA: Contributions to Mineralogy and Petrology, v. 83, p. 259–269.

Berman, A.E., Pooleschook, D., Jr., and Dimelow, T.E., 1980, Jurassic and Cretaceous 

systems of Colorado, in Kent, H.C., and Porter, K.W., eds., Colorado geology: Denver, 

Colorado, Rocky Mountain Association of Geologists 1980 Guidebook, p. 111–128.

Braddock, W.A., Cole, J.C., and Eggler, D.H., 1989, Geologic map of the Diamond Peak 

quadrangle, Larimer County, Colorado, and Albany County, Wyoming: U.S. Geological 

Survey Geologic Quadrangle Map GQ-1614, scale 1:24,000.

Braddock, W.A., and Connor, J.J., 1988, Geologic map of the Livermore Mountain 

quadrangle, Larimer County, Colorado: U.S. Geological Survey Geologic Quadrangle 

Map GQ-1617, scale 1:24,000.

Burch, A.L., 1983, Petrology and U-Th potential of the eastern portion of the Precambrian 

Rawah batholith, Larimer County, Colorado: Fort Collins, Colorado State University, 

unpublished M.S. thesis, 378 p.

Burk, C.A., and Thomas, H.D., 1956, The Goose Egg Formation (Permo-Triassic) of 

eastern Wyoming: Geological Survey of Wyoming Report of Investigations no. 6, 11 p.

Butters, R.M., 1913, Permian or “Permo-Carboniferous” of the eastern foothills of the 

Rocky Mountains in Colorado: Colorado Geological Survey Bulletin 5, p. 65–94.

Camp, W.K., 1979, Structure and stratigraphy of the Laramie River Valley, Larimer 

County, Colorado: Fort Collins, Colorado State University, unpublished M.S. thesis, 

225 p.

Campbell, J.A., 1963, Permo-Triassic red beds, northern Denver Basin, in Bolyard, D.W., 

and Katich, P.J., eds., Guidebook to the geology of the northern Denver Basin and 

adjacent uplifts: Denver, Colorado, Rocky Mountain Association of Geologists 1963 

Guidebook, p. 105–110.

Cole, J.C., and Braddock, W.A., 2009, Geologic Map of the Estes Park 30 ́ × 60 ́ 

quadrangle, north-central Colorado: U.S. Geological Survey Scientific Investigations 

Map 3039, scale 1:100,000.

Condra, G.E., Reed, E.C., and Scherer, O.J., 1940, Correlation of the formations of the 

Laramie Range, Hartville uplift, Black Hills, and western Nebraska: Nebraska Geological Survey Bulletin 13, 52 p.

Curtis, B.F., 1963, Jurassic stratigraphic relationships in the northern Denver Basin, in

Bolyard, D.W., and Katich, P.J., eds., Guidebook to the geology of the northern Denver 

Basin and adjacent uplifts: Denver, Colorado, Rocky Mountain Association of Geologists 1963 Guidebook, p. 111–118.

Darton, N.H., 1908, Paleozoic and Mesozoic of central Wyoming: Geological Society of 

America Bulletin, v. 19, p. 403–470.

Eggler, D.H., and Braddock, W.A., 1988, Geologic map of the Cherokee Park quadrangle, 

Larimer County, Colorado, and Albany County, Wyoming: U.S. Geological Survey 

Geologic Quadrangle Map GQ-1615, scale 1:24,000.

High, L.R., and Picard, M.D., 1967, Rock units and revised nomenclature, Chugwater 

Group (Triassic), western Wyoming: Mountain Geologist, v. 4, no. 2, p. 73–81.

Kellogg, K.S., Ruleman, C.A., and Shroba, R.R., 2008, Geologic map of the Clark Peak 

quadrangle, Jackson and Larimer Counties, Colorado: U.S. Geological Survey Scientific 

Investigations Map 3010, scale 1:24,000.

Knight, S.H., 1917, Age and origin of the red beds of southeastern Wyoming: Geological 

Society of America Bulletin, v. 28, p. 168–169.

Knight, S.H., 1929, The Fountain and the Casper Formations of the Laramie Basin—A 

study on genesis of sediments: University of Wyoming Publications in Geology, v. 1, 

no. 1, 82 p.

Lee, W.T., 1927, Correlation of geologic formations between east-central Colorado, central 

Wyoming, and southern Montana: U.S. Geological Survey Professional Paper 149, 

80 p.

Love, J.D., 1939, Geology along the southern margin of the Absaroka Range, Wyoming: 

Geological Society of America Special Paper 20, 134 p.

Maughan, E.K., 1964, The Goose Egg Formation in the Laramie Range and adjacent parts 

of southeastern Wyoming: U.S. Geological Survey Professional Paper 501-B, p. 

B53–B60.

Maughan, E.K., and Wilson, R.F., 1963, Permian and Pennsylvanian strata in southern 

Wyoming and northern Colorado, in Bolyard, D.W., and Katich, P.J., eds., Guidebook 

to the geology of the northern Denver Basin and adjacent uplifts: Denver, Colorado, 

Rocky Mountain Association of Geologists 1963 Guidebook, p. 95–104.

McCallum, M.E., and Hedge, C.E., 1976, Rb-Sr ages of granitic rocks in the Rawah 

batholith, Medicine Bow Mountains, northern Colorado: Isochron/West, v. 17, no. 12, 

p. 33–37.

Peale, A.C., 1893, The Paleozoic section in the vicinity of Three Forks, Montana: U.S. 

Geological Survey Bulletin 110, 56 p.

Pearson, E.F., 1972, Origin and diagenesis of the Owl Canyon and Lower Goose Egg 

Formation (Permian) of southeastern Wyoming and adjacent Colorado: Laramie, 

University of Wyoming, unpublished Ph.D. thesis, 170 p.

Peterman, Z.E., Hedge, C.E., and Braddock, W.A., 1968, Age of Precambrian events in the 

northeastern Front Range, Colorado: Journal of Geophysical Research, v. 73, no. 6, p. 

2277–2296.

Qa

Qac

Qc

Qls

Qp

Kb

Kd

Jm

Prgu

Pgf

Pgsu

Pgsl

f

Yd

YXp

YgSH

Yghp

XgR

Xh

Jsj

Pc

Xfl

Picard, M.D., 1967, Stratigraphy and depositional environments of the Red Peak Member 

of the Chugwater Formation (Triassic), west-central Wyoming: University of Wyoming 

Contributions to Geology, v. 6, p. 39–67.

Pipiringos, G.N., 1957, Stratigraphy of the Sundance, Nugget, and Jelm Formations in the 

Laramie Basin, Wyoming: Geological Survey of Wyoming Bulletin 47, 63 p.

Pipiringos, G.N., 1968, Correlation and nomenclature of some Triassic and Jurassic rocks 

in south-central Wyoming: U.S. Geological Survey Professional Paper 594-D, 26 p.

Pipiringos, G.N., Hail, W.J., and Izett, G.A., 1969, The Chinle (Upper Triassic) and 

Sundance (Upper Jurassic) Formations in north-central Colorado: U.S. Geological 

Survey Bulletin 1274-N, 35 p.

Pipiringos, G.N., and O’Sullivan, R.B., 1976, Stratigraphic sections of some Triassic and 

Jurassic rocks from Douglas, Wyoming, to Boulder, Colorado: U.S. Geological Survey 

Oil and Gas Investigations Chart OC-69.

Sando, W.J. and Sandberg, C.A., 1987, New interpretations of Paleozoic stratigraphy 

and history in the northern Laramie Range and vicinity, southeast Wyoming: U.S. 

Geological Survey Professional Paper 1450, 39 p.

Shaw, A.B., 1957, Correlation and nomenclature of the pre-Pierre Cretaceous of North 

Park, Colorado, in Guidebook to the geology of North and Middle Parks Basin, 

Colorado: Denver, Colorado, Rocky Mountain Association of Geologists 1957 Guidebook, p. 48–50.

Tweto, O., 1979, Geologic map of Colorado: U.S. Geological Survey, scale 1:500,000.

Ver Ploeg, A.J., and Boyd, C.S., 2007, Geologic map of the Laramie 30 ́ × 60 ́ 

quadrangle, Albany and Laramie Counties, southeastern Wyoming: Wyoming State 

Geological Survey, Map Series 77, scale 1:100,000.

Workman, J.B., 2008, Geologic map of the Eaton Reservoir quadrangle, Larimer County, 

Colorado and Albany County, Wyoming: U.S. Geological Survey Scientific Investigations Map 3029, scale 1:24,000.

Zielinski, R.A., Peterman, Z.E., Stuckless, J.S., Rosholt, J.N., and Nkomo, I.T., 1981, The 

chemical and isotopic record of rock-water interaction in the Sherman Granite, 

Wyoming and Colorado: Contributions to Mineralogy and Petrology, v. 78, p. 209-

219.

50

MAP EXPLANATION

Contact—Dashed where approximately located

Fault—Dashed where approximately located; dotted where concealed;

 U on upthrown block; D on downthrown block; tick shows dip of fault;

 arrow shows lineation on fault surface 

Fault zone—Characterized by brittle deformation in most locations with a

 variety of lithologies present as millimeter- to meter-scale clasts within

 fault breccia and cataclastic material; alteration is common; quartz veins

 are common; foliation and lineation measurements within fault zones

 taken on larger coherent blocks; faults shown within fault zones are

 younger and cut breccia along discrete planes

Anticline

Syncline

Strike and dip of bedding

Inclined

Horizontal

Strike and dip of foliation in metamorphic rocks

Inclined

Vertical

Lineation

Intraformational unconformity—Shown on correlation of map units

60

U

D

80 60

80

U

Jh

K

K K

K

Xg

Xg

XgR

XgR

YgS

Xm

Xm

Xm

Xm

Tn

Tn

Tn

Tn?

YgLC

Tv

QTg

Q

Q

Q

b

pYgS

pYgS

Xgb

Xm

Xm

Xm

Q

SHEEP CREEK FZ

BOULDER RIDGE FZ

BOULDER RIDGE FZ

CORNELIUS CREEK SZ

BULL MTN FT

GREEN RIDGE FT

MIDDLE MTN FT

LARAMIE RIVER FT

Map area

MCINTYRE CREEK FT

STINK CREEK FT

EAST JIMMY CK FT

PANHANDLE CREEK SZ

RED FEATHER LAKES SZ

RED FEATHER LAKES SZ?

ELKHORN CREEK SZ

JELM MTN FT

Jh

Jh

WYOMING

COLORADO

Ygs

105°37'30"

105°37'30"

105°45'

105°45'

105°52'30"

105°52'30"

106°00'

106°00'

41°07'30" 41°07'30"

41°00' 41°00'

40°52'30" 40°52'30"

40°45' 40°45'

SCALE 1: 250 000

0 5 10 15 20 25 KILOMETERS

INDEX MAP SHOWING SELECT GEOLOGIC FEATURES

(Geology modified from Camp (1979), Burch (1983), VerPloeg and

Boyd (2007), and J.B.Workman and others (unpublished data, 2008))

EXPLANATION

Q 

Fault zone

Shear zone

QTG

Tn

Tv

K

Jh

YgLC

Ygs

pYgs

Xgb

XgR

Xm

Surficial deposits (Quaternary)

Thrust fault—Teeth on upper plate

High-level gravels (Quaternary to Tertiary)

North Park Formation (Tertiary)

Volcanic rocks (Tertiary)

Cretaceous sedimentary rocks

Jurassic to Pennsylvanian sedimentary rocks

Log Cabin batholith (Mesoproterozoic)

Sherman Granite (Mesoproterozoic)

Pre-Sherman igneous rocks (Mesoproterozoic?)

Gabbro (Paleoproterozoic)

Rawah batholith (Paleoproterozoic)

Metamorphic rocks (Paleoproterozoic)

Breccia zone

Contact

Fault—FT

FZ

SZ

b

Qa

Qp

Qac

?

Qc

?

Qls

?

Chugwater Group

Goose Egg Formation

Unconformity

Unconformity

Kd

Kb

Unconformity(?)

Unconformity

f

Xfl Xh

YgSH

XgR

Yd

Yghp

?

YXp

Unconformity(?)

Jm

Jsj

U

Prgu

U

Pgf

Pc

Pgsl

U

Pgsu

CORRELATION OF MAP UNITS

QUATERNARY

CRETACEOUS

JURASSIC

TRIASSIC

PERMIAN

PENNSYLVANIAN

MESOPROTEROZOIC

PALEOPROTEROZOIC

Holocene

Late Pleistocene

Middle Pleistocene

Upper Cretaceous

Lower Cretaceous

Upper Jurassic

Middle Jurassic

Upper Triassic

Lower Triassic

Upper Permian

Lower Permian

Middle Pennsylvanian

Lower Pennsylvanian

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

SCIENTIFIC INVESTIGATIONS MAP 3133

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

ArcInfo coverages and a PDF for this map are available at 

http://pubs.usgs.gov/sim/3133/

Suggested citation:

Workman, J.B., and Braddock, W.A., 2010, Geologic map of

the Sand Creek Pass quadrangle, Larimer County, Colorado,

and Albany County, Wyoming: U.S. Geological Survey

Scientific Investigations Map 3133, scale 1:24,000.

GEOLOGIC MAP OF THE SAND CREEK PASS QUADRANGLE, LARIMER COUNTY, COLORADO, AND ALBANY COUNTY, WYOMING

By

Jeremiah B. Workman and William A. Braddock

2010

CONVERSION FACTORS

Multiply By To obtain

inches (in.)

feet (ft)

miles (mi)

2.54

0.3048

1.609

centimeters (cm)

meters (m)

kilometers (km)

1 .5 0 1 KILOMETER

SCALE 1:24 000

CONTOUR INTERVAL 40 FEET

NATIONAL GEODETIC VERTICAL DATUM OF 1929

1 1/ 2 0 1 MILE

Geology mapped by W.A. Braddock, 

1981–1982, and J.B. Workman,

2007–2009 

Digital map database prepared by

Jeremiah B. Workman

Editing and digital cartography by

Gayle M. Dumonceaux

Denver Publishing Service Center

Manuscript approved for publication

August 26, 2010

Base from U.S. Geological Survey, 1967

Polyconic projection and 10,000-foot grid ticks:

Colorado coordinate system, north zone and

Wyoming coordinate system, east zone

1000-meter Universal Transverse Mercator grid

ticks, zone 13, 1927 North American Datum

14°

APPROXIMATE MEAN

DECLINATION, 1967 TRUE NORTH

MAGNETIC NORTH

COLORADO

WYOMING

QUADRANGLE LOCATION