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Surface offset (meters)

Scarp height (meters)

1 

2 

3 4 5 6 7 

8 

9 

10 11 

12 13 14 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

0 5 10 15 20 25 30 35 40 45 

SH = 3.3 m

SO = 1.8 m

max. angle = 21°

No vertical exageration

s

d

Om

Manitou Formation (Om)

Chaffee Group (MDc)

Red cast unit of

Manitou Formation (Om)

Dotsero Formation (d)

Figure 1. Close-up photograph of an outcrop in the upper reaches of Sacramento 

Creek showing the typical textural elements found in the Sacramento Porphyry 

(Tsp) of the Gray Porphyry group.

Figure 2. Photograph of vertical dikes in the Sacramento Porphyry (Tsp). 

These dikes are well exposed in the upper reaches of Little Sacramento Creek.

Figure 3. View to the east of large stock of the 

White porphyry group (TKwp) exposed on the 

steep west face of White Ridge. Several thick 

sills and laccoliths compose the stock in this 

area. Pinedale Till (Qtp) can be seen low in the 

photo in the upper reaches of Fourmile Canyon. 

Figure 12. Fault scarp profile CO-MS-1. SO, surface offset ; SH, scarp height (both in meters ); and max. 

angle is maximum slope angle of the fault scarp.

Figure 11. Exposures of gneiss (YXmr) in this part of the Mosquito Range are 

limited to a highly mixed complex of deformed biotite gneiss and medium-grained 

granitic intrusions and pegmatites. Commonly mapped as migmatite, the mixed 

rock is probably better considered as older gneiss intruded by younger plutonic rocks.

Figure 5. View to the northeast of typical large-scale bedding characteristics of the gray-brown facies of 

the Minturn Formation (mg). Subparallel bedding is the rule at this scale in the unit, but on a finer scale 

each larger bed is internally cross bedded.

Figure 4. Typical exposure of the sandstone of the white facies of 

the Minturn Formation (mw). Hammer in lower right part of 

photograph provides scale.

Figure 6. Photograph of the Leadville Limestone (Ml) taken just to the 

west of Empire Hill in the west half of the quadrangle. Note the 

irregular, wavy, and poorly defined bedding and uniform gray color of 

the rock. Fine-grained, brown sandy laminations can also be seen. 

Figure 7. Photograph of the upper part of the Cambrian through the Devonian section that is well exposed in the 

cirque cliffs east of Horseshoe Mountain.

Figure 8. Photograph of 

the Ordovician Manitou 

Dolomite (Om) taken in 

the cirque floor east of 

Horseshoe Mountain. 

Orange-brown sandy 

laminations help define 

the bedding in this area. 

10 in. × 12 in. notebook for 

scale.

Figure 9. The best exposures of the Cambrian part of the platform rock section occur in the southern 

cliff faces of Fourmile Canyon, just to the east of the quadrangle boundary. The Cambrian section, 

which is variable in thickness, is at its thickest here. s, Sawatch Quartzite; d, Dotsero Formation; 

and Om, Lower Ordovician Manitou Dolomite.

Figure 10. Photograph of typical coarse-grained, porphyritic Mesoproterozoic granitic 

rock (Yg) that is found in many parts of the quadrangle. Here it is a biotite monzogranite. 

Coarse potassium feldspar phenocrysts are approximately the size of a nickel.

CREST OF

MOSQUITO RANGE

WESTON FAULT

MOSQUITO-UNION FAULT

No vertical exaggeration

Thin surficial units not shown

Elevation above Sea Level Spring Creek

Union Creek

3250

2750

3750

2500

3000

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B

METERS

4250

8000

9000

10000

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mw mg

mw

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s

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Om

Yg

Yg

Yg

Yg

YXmr

MDc

Ml

Qac TKwp

b

b

d s mg

mg

s d

Om

Om

MDc

Ml

TKwp

BEND IN

SECTION

LONG AND DERRY HILL

WEST SHERIDAN

WHITE RIDGE

CREST OF

MOSQUITO RANGE

No vertical exaggeration

Thin surficial units not shown

Elevation above Sea Level

MOSQUITO FAULT

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MDc Ml

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TKwp

TKwp TKwp

TKwp

TKwp TKwp

Qrf

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Om MDc

mg

mg

mw

mg

mh

Qtp

Tgu

Tgu

TKwp

TKwp TKwp

TKwp

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TKwp

TKwp

TKwp

TKwp

TKwp

TKwp

TKwp

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TKwp

TKwp

TKwp

TKwp

TKwp

dsu

TKwp

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TKwp

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Qtp

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mg

mg

mg

mg

Qa

Qa

Qls

Qtp Qtp Qrf

Qrf

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MDc

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Qa Qa

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Qrf

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Qac

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Qtp

Qtp

Qtp

Qtp

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Qtp

Qtp

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Ql

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Qtp

Qls Qtp

Qrf

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Qrf Qrf

Qrf

Qrf

Qac

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s

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d

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d

d

d

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d

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Om

Om

Om

Qrf Ml

MDc

Qtp

Ql

Qa

Qac

Yg

Yg Yg

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Yg

Yg

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Yg

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Yg

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Yg

Yg

Yg

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Yg

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Yg

Yg

Yg

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Yg

Yg

Yg

MDc

MDc

Yg

Yg

Om

Om

Om

Om

Om

Qa

Om

Om

Om

Ml

Ml

MDc

MDc

MDc

MDc

MDu

MDu

Ml

Ml

Ml

Ml

Ml

Ml

Ml

Ml

Ml

Ml

Ml

Ml

Ml

Ml

Ml

Ml

Ml

Qrg

Qrg

Qac

Qac

Qrf

Qrf

Qrf

Qls

Qrf

Qrf

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Qls

Qrf

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Qls

Qrf

Qrf

Qrf

Qrf

Qrf

Qac

Qrf

Qtp

Qtp

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qa

Qtp

Qtb

Qtb

Ql Ql

Ql

Qtb

Qac

Qtp

Qtp

Qtp

Qac Qd

d

d

s

s

d

d

d

d

d

d

d

d

d

s

s

s

s

s

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s

Ql

Ql

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Qac

Qrf

Qrf

Qrf

Qrf

Qac

Qrf

Qrf

Qrf

Qrf

Qrf

Qrf

Qrf

Qrf

Qrg

Qac

Qtp

Qtp

Qtp

Qls

Qtp

Qtp

Ql

Qa

Ml

Qac

MDc

Om

Qac

Qtp

Qrf Qrf

Qrf

Qls

Qls

Qac

Qac

Qac

Qac

Qac

Qls

Qac

Qac

Qrf

Qrf

Qrf

Qac

Qac Qrg

Qrg

Qrg

Qrg

Qrg

Qrg

Qa

Qa

Qd

Qd

mg

mg

mg

mg

mh

mh

mh

mg

 b

b

b

b

b

b

b

mh

mw

mg

mg

mg

mh

Tsp

Ql

Ql Ql

Tsp

Tsp

Tsp

YXmr

YXmr

Tsp

MDc

MDc Ml

Om s

d

Ql Ql

MDc

MDu

Om Om

Om

Om

Om

Om

Om

Om

Om

Om

mw

mw

mg

mg

mg

mg

b

b

b

mw

mw

mg

Om

MDc

Om

MDc

MDc

MDc

Om

Om

Om

Om

Om

Om

Om

MDc

Om

Om

d

d

s

 s

MDc

d

d

d

dsu

dsu

d

s

d

dsu

dsu

dsu

dsu

s

s

s

s

s

YXmr

YXmr

YXmr

YXmr

YXmr

YXmr

YXmr

YXmr

DESCRIPTION OF MAP UNITS

[Due to the addition of a shaded relief base, colors on the List of Map Units and the Correlation of Map Units 

may not exactly match unit colors on the map]

SURFICIAL DEPOSITS

Mine waste

Mine dumps and waste (Holocene)—Unconsolidated material deposited by mining 

and milling processes. Prospect-pit tailings are also included in this unit. Deposit 

ranges from silt and sand to subangular to subrounded boulders. Unit locally 

contains clasts from units Qtb and Qtp. In the Breece Hill‒Ball Mountain‒Iowa 

Gulch region, unit is possibly more extensive than mapped. Only deposits greater 

than 20 m2 are shown. Thickness is 1 to 10 m

Alluvial deposits

Stream alluvium (Holocene and latest Pleistocene)—Pebble- to boulder-sized clasts 

in a silty sand or sandy silt matrix. Pebbles are generally rounded, where 

boulders tend to be subangular. Deposit commonly is overlain with organic-rich 

clayey silt and includes floodplain and overbank deposits. Locally includes 

sheetwash deposits. In several places peat deposits are included where perennial 

runoff has ponded in a closed depression. In Fourmile Creek, Twelvemile Creek, 

Big Union Creek, and Empire Gulch, locally contains Pinedale Till (Qtp). 

Thickness 2 to 5 m

Mass-movement deposits

Rockfall deposits (Holocene)—Coarse, angular clasts that are primarily clast 

supported with little to no matrix. Largest clasts are generally at the toes of 

deposits. In some places, the deposit mantles relict rock glaciers and appears to 

have flow features, being lobate in form. Chiefly forms at the base of cliffs 

deposited by gravitational collapse. Deposit occurs as various landforms such as 

talus cones, rock fans, and block fields. Deposits usually have little to no soil 

development. Thickness ranges from less than 1 to 20 m

Landslide deposits (Holocene and late Pleistocene)—Poorly sorted, fine- to 

coarse-grained deposit typically characterized by hummocky topography that 

forms lobes. Deposits mainly lack sorting or stratification. Processes of 

deposition involved downslope movement of unconsolidated material. Includes 

previously deposited till that has been remobilized by gravitational creep. 

Dormant landslides may become remobilized periodically. Most of the landslides 

post-date the last glaciation (Pinedale) (< 18‒20 ka). Thickness varies from 1 to 20 m

Alluvial and colluvial deposits

Alluvium and colluvium, undivided (Holocene and late Pleistocene)—Poorly to 

moderately sorted, pebble- to cobble-sized, subangular to subrounded clasts 

within a fine- to coarse-grained silt and sand matrix. Forms gently to moderately 

sloping surfaces. Some channelized pebble-sand alluvium is included in this unit. 

Locally includes sheetwash deposits. Outwash and till of Bull Lake (late middle 

Pleistocene) and Pinedale (late Pleistocene) glaciations are locally within unit. 

Contains fine-grained silty sand deposits proximal to springs. Thickness ranges 

from 1 to 5 m

Lacustrine deposits

Lake deposits (Holocene and latest Pleistocene)—Consists chiefly of silts and clays 

deposited by settling out of suspension. These fine-grained sediments most likely 

have an eolian origin. Contains organic-rich paludal deposits. Coarser clastic 

sands and gravels occur proximal to the inlets of these lacustrine environments. 

Deposits primarily occur in either glacially scoured bedrock depressions or 

undrained kettles in late Pleistocene till. All deposits post-date the last glacial 

period. Thickness 1 to 5? m 

Glacial deposits

Rock glacier deposits (Holocene and latest Pleistocene)—Clast-supported, angular 

to subangular boulders, cobbles, pebbles, sand, and silt. Unit has little to no 

sorting and is nonstratified, except at the toes of side slopes where slope 

processes have sorted finer-grained sediment. Deposits generally are lobate and 

hummocky in form due to downslope movement of remnant interstitial ice from 

Pinedale glaciation. The downslope toes of deposits are generally very steep 

(angle of repose) due to Holocene reactivated creep, showing relatively fresh 

displacement. Forms primarily in the cirque headwall of glaciated drainages. 

Commonly contains rockfall deposits. Thickness 5 to 20 m

Pinedale Till (late Pleistocene)—Unconsolidated subangular to rounded cobbles and 

boulders in a clay, silt, sand, and gravel matrix. Sediment was deposited directly 

by ice with little to no reworking by glacial meltwater. Forms terminal, lateral, 

recessional, and ground moraines in predominantly east-west drainages flowing 

off the crest of the Mosquito Range. Surface topography is characterized as 

hummocky with undrained closed depressions. Sorting or stratification is 

commonly absent except where lenses of silty sand were deposited by subglacial 

streams. Clasts are polymictic consisting of all lithologies within the map area 

and generally show very little weathering. Pedogenic soils in deposit are 

commonly weakly to moderately developed Bw horizons, but locally may also 

have weak Bt horizons. Most sediment was likely deposited within oxygen 

isotope stage 2 (~12–20 ka) (Briner, 2009; Young and others, 2011; Mason and 

Ruleman, 2011), but locally contains clasts from late middle Pleistocene unit 

Qtb. Thickness 5 to 60 m

Bull Lake Till (late middle Pleistocene)—Subangular to subrounded cobbles and 

boulders in a clay, sand, silt, and gravel matrix deposited directly by glacial ice. 

Identified and mapped only on the south side of Twelvemile Creek. Clasts consist 

predominantly of Proterozoic igneous and metamorphic lithologies derived from 

the crest of the Mosquito Range. Surface boulders are generally partly buried, 

and the surface morphology is smoothed from the original depositional form. 

Soils generally have moderately formed argillic horizons and slightly red 

chroma. Granitic clasts show moderate weathering, tend to be friable and easy 

crushed with a hammer. Thickness unknown

EARLY TERTIARY TO LATE CRETACEOUS

INTRUSIVE ROCKS

Gray porphyry group—Sills in the Mount Sherman quadrangle fall into one of two 

broad groupings of intrusive rock units. The younger group of sills is the Gray 

porphyry group, which includes the Lincoln, Sacramento, Evans, and Johnson 

Gulches, Mount Zion, and Iowa Gulch Porphyries according to Behre (1953) and 

Pearson and others (1962). Behre (1953) identified the Sacramento, Johnson, and 

Evans Gulches units in the quadrangle and the others are exposed to the north. 

The age of the Gray porphyry group is only generally known. No ages have been 

determined from rocks within the quadrangle, but a nearby fission-track age 

suggests that the Sacramento Porphyry is 43.9 Ma (Thompson, 1990). Other 

units in the Gray porphyry group are undated locally. Similar megacrystic 

porphyritic sills to the north and east of the Leadville mining district have been 

dated, and all ages are within the Eocene. Mach (1992) dated a porphyritic 

monzogranite sill from the Gray porphyry group near Copper Mountain, 19 km 

to the north, at 36.7 Ma (fission track on apatite) to 48.6 Ma (fission-track on 

zircon). Marvin and others (1989) determined that another megacrystic porphyry 

near the town of Frisco is 44.1 Ma, based on K/Ar. Bryant and others (1981) 

dated several related sills near Keystone and Boreas Pass at 43.1 to 50.6 Ma 

Sacramento Porphyry (Eocene)—The Sacramento Porphyry (Emmons, 1886; 

Behre, 1953) is porphyritic monzogranite; gray green when fresh; weathers to 

gray brown or yellow gray. Subhedral-to-euhedral phenocrysts are composed of 

plagioclase (as large as 1.5 cm), biotite (as large as 0.75 cm), and hornblende 

(0.75 cm). Subhedral to anhedral quartz is also present (as large as 2.5 cm). 

Phenocrysts are supported in a fine-grained, quartz-rich matrix that is pale green 

in color (fig. 1). Primarily exposed on the east flank of the Mosquito Range in 

the northeast corner of the quadrangle, where it forms a large stock-like body 

with a roof that is subparallel to bedding in the overlying Pennsylvanian 

sedimentary rocks. Interior of stock, which is well exposed in the high reaches of 

Little Sacramento Gulch, is composed of near-vertical dikes, each 0.5 meter to 

several meters in width (fig. 2). Considered by Thompson (1990) to be the same 

age as the Evans Gulch Porphyry and older than the Johnson Gulch Porphyry 

Gray porphyry group, undifferentiated (Eocene)—Includes the Evans Gulch and 

Johnson Gulch Porphyries, both of which are exposed in the northwest corner of 

the quadrangle around Breece Hill and Ball Mountain (Behre, 1953). These two 

porphyries are similar in composition, but represent different plutons. Each of 

these porphyries can be separately identified, but poor exposures and widespread 

cover by mine waste inhibited detailed mapping in this area; so they are 

undifferentiated in this report. Evans Gulch Porphyry is light gray-green to light 

green and is fine grained with crystals smaller than 2 mm. Larger crystals are 

oligoclase and andesine (25 percent), quartz (5 percent), and biotite and 

hornblende are also common. Less common are apatite, rutile, and zircon, and all 

are set in a very fine-grained groundmass. Johnson Gulch Porphyry is chiefly 

gray and is characterized by quartz crystals as large as 5 mm and smaller, but 

more common plagioclase crystals; rare orthoclase occurs as do biotite and 

hornblende

White porphyry group, undifferentiated (early Paleocene and Late Cretaceous)— 

Probably includes rocks equivalent to the Pando Porphyry of Tweto (1951). 

Mostly occurs as light-white or light-yellowish-brown sills that are present 

throughout the quadrangle and as a thick stock or laccolith in the Mount Sherman 

and upper Fourmile Creek drainage areas. Forms numerous sills that intrude 

subparallel to bedding in all of the units of the Phanerozoic sedimentary section 

and at many locations between the Upper Cambrian Sawatch Quartzite (s) and 

the Proterozoic crystalline basement. Several thick sills, distinguished by slight 

color variations, grain size and weathering/erosional character, comprise the 

stock or laccolith in upper Fourmile Creek (fig. 3). Fine grained to 

cryptocrystalline with a sugar texture in hand specimens. Composed of quartz, 

plagioclase, and biotite, with quartz being the largest grains at 2 mm or smaller. 

Liesegang bands common. Considered to be earliest Paleocene to Late 

Cretaceous in age because northwest of the quadrangle, the Pando Porphyry has 

been dated to 70 Ma by K/Ar (Pearson and others, 1962) and at 64.1 ± 0.9 Ma 

by a 238U/206Pb SHRIMP age on zircon (Wayne Premo, USGS, written 

commun., 2012) 

PALEOZOIC SEDIMENTARY ROCKS

Paleozoic sedimentary rocks ranging in age from Pennsylvanian to Cambrian are exposed 

throughout the Mount Sherman quadrangle. Marine and nonmarine Pennsylvanian units include the 

Minturn and Belden Formations. We subdivided the Minturn into three parts, in descending order: 

(1) a white and light brown primarily nonmarine facies (mw), (2) a brown- and gray-colored 

chiefly transitional marine facies (mg), and (3)a dark-colored and dense hornfels facies (mh). 

The Belden Formation (b) is a carbonate-rich black shale that is predominately marine. Other 

Pennsylvanian rock units have been identified in the nearby region such as the Coffman 

Formation, a black shale that interfingers with the Minturn.Variously named marine limestone 

members within the Minturn were not differentiated in the quadrangle, although those lithologies 

are present in places. Locally, the dark-gray marine Leadville Limestone (Ml) makes up most of the 

Mississippian System. The local units that compose the Mississippian and Devonian are the Dyer 

Dolomite and Parting Formation of the Chaffee Group (MDc). These formations are not 

differentiated in this report. The Manitou Dolomite (Om) makes up the Lower Ordovician. 

According to Myrow and others (1995, 1999, 2003) a thin unit of red-colored beds at the base of 

the Manitou they called the Taylor Pass Member is Cambrian in age. The red-colored beds are 

present in most places, but are not differentiated on our map. Two Cambrian quartzite units, the 

Dotsero Formation (d) and Sawatch Quartzite (s), are differentiated on the map, except in places 

where exposures are poor, or in regions of steep terrain that rendered it hard to display both units 

Minturn Formation (Desmoinesian and Atokan)

White facies—White to light gray coarse-grained arkosic sandstone and 

pebble-to-cobble conglomerate (fig. 4). Exposed near the central and eastern map 

border at highest stratigraphic levels of Minturn in the quadrangle. Resistant and 

well indurated in most places. Quartz, feldspar, quartzite, and granite clasts are 

scattered throughout and make up about 15 percent of the generally sandier rock 

body. Low-angle cross stratification that is defined by clast alignments and 

parting along bedding surfaces is ubiquitous at scales of centimeters to tens of 

centimeters. On a larger scale of several meters, bedding is subparallel and more 

massive. Nonmarine; probably deposited on an alluvial fan of low relief by 

braided streams. Total thickness is unknown. About 800 m is exposed in 

quadrangle

Gray-brown facies—Mostly coarse-grained arkose that includes minor amounts of 

micaceous pebble and cobble conglomerate in lenses; found throughout eastern 

and northwestern parts of the quadrangle; gray to gray red-brown in color in 

most of quadrangle, red brown in northeast corner of quadrangle. Clasts are 

chiefly quartz and chert. Sandstone is moderately resistant in most areas. Layers 

of dark-gray calcareous shale are common, especially lower in the section. Shale 

layers are fissile and form recesses and swales commonly covered by vegetation 

(fig. 5). Gray limestone beds are described as commonly occurring in the 

Minturn Formation in nearby areas (for example, Widmann and others, 2004 and 

2006; McCalpin and others, 2012), but are uncommon in Mount Sherman 

quadrangle. Bedding includes low-angle cross-stratification upon close 

inspection. Fine-grained sand beds are mostly parallel bedded and coarser 

grained beds are mostly cross stratified. Beds vary between several cm and 1 m 

in thickness. Calcareous shale and limestone beds are marine; gray arkose is 

likely deltaic marginal marine. About 1,000–1,100 m thick

Hornfels facies—Dense dark-gray to black, thermal contact metamorphosed 

sandstone or shale, the protolith of which is the gray arkose facies of the Minturn 

Formation. Occurs as irregular blobs, stringers, and lenses preserved between sills 

of the upper parts of the White porphyry group around Mount Sherman, White 

Ridge, and Dyer Mountain. Most bodies are only a few meters in thickness

Belden Formation (Atokan and Morrowan)—Interbedded black shale, very 

fine-grained sandstone, and dark-gray limestone at the base of the Mississippian 

section in the southern half of the quadrangle. Recessive units covered by thin 

surficial deposits, soils, and vegetation in most areas. Shale outcrops form 

saddles and topographic lows between the more resistant sandstone and 

limestone beds, and are locally calcareous and (or) carbonaceous, and may 

contain plant and marine fossils and silicified wood. Interbeds of fine-grained, 

micaceous, and arkosic sandstone that are generally olive drab to light gray in 

color. Limestone beds are dark gray to black, locally contain marine fossils and 

microlaminations, and are about 2 m thick. At least 500–600 m thick in southern 

part of quadrangle where unit comprises the footwall of the Weston fault

Leadville Limestone (Mississippian)—Bluish-gray to black, thick-bedded, 

fine-grained dolomite characterized by localized irregular, alternating 1- to 5-mm 

bands of dark-gray to black dolomite and white, coarse-grained, vuggy dolomite, 

known as “zebra rock,” and well-rounded to fragmented black-chert pebbles and 

cobbles (fig. 6). Bedding is irregular and wavy. The rock is mottled or brecciated, 

with the breccia typically occurring within, and not across, beds. Abundant 

dissolution features characterize the unit prompting interpretations that the unit 

represents paleokarst (Maslyn, 1977; Beaty and others, 1990). Thin, 

discontinuous lenticular beds of tan-weathering, medium-grained, white quartz 

sandstone and orthoquartzite, and localized beds containing abundant subangular 

rip-up clasts of limestone are found near the base of the formation. Heavily 

intruded by sills of the White porphyry group (TKwp), especially at the top of 

unit. Thickness is highly variable due to incomplete preservation except on north 

flank of Mount Sheridan where a complete section, about 100 m thick, is exposed

Chaffee Group (Lower Mississippian and Upper Devonian)—Includes Dyer 

Dolomite and Parting Formation, but the two units are undifferentiated on this 

map. Dyer Dolomite is thinly bedded to unbedded, finely crystalline and 

microcrystalline dolomite. The lower third of the unit is medium gray, whereas the 

upper part is slightly darker. Weathering and alteration produces an irregular 

pattern of yellowish to brownish gray outcrop in many places (fig. 7). Thin (<5 cm) 

lenses of shale and (or) chert occur locally. Parting Formation is predominantly 

bright-white to subdued-gray, fine- to medium-grained, well-sorted orthoquartzite 

and moderately well-sorted, quartz-rich sandstone with a few interbeds of light 

yellowish-brown dolomitic sandstone. Locally, this unit is coarse grained and 

contains quartz pebbles, particularly near its base. Orthoquartzite beds appear 

unstratified on a fresh surface, but on weathered surfaces thin lenses of less 

well-cemented sandstone are revealed. Although similar in character, the Parting 

Formation generally is not as pure a quartzite or as white as the Sawatch Quartzite 

(s). Finer grained facies weather orange brown, whereas coarse-grained facies 

tend to be light gray or purple. Irregular thickness, 0–40 m

Leadville Limestone and Chaffe Group, undivided (Mississippian and Upper 

Devonian—Mapped together where exposures are poor

Manitou Dolomite (Lower Ordovician)—Cliff-forming unit composed of light- to 

dark-gray, thin- to thick-bedded dolomite with rare interbeds of dark gray 

limestone (fig. 7). Generally weathers to light yellow-brown color. Characterized 

in many places by orange-brown laminations that occur as indistinct layers (2 to 

30 cm thick) of sandy dolomite and by whitish-gray to black laminated chert 

nodules, blebs, and stringers (fig. 8). The lowermost 1–3 m of the Manitou 

Dolomite contains thick beds of whitish dolomite with abundant brick-red ovoid 

spots. These beds have a red hue from a distance (fig. 7) and have been called the 

“red-cast beds” (Emmons, 1886). The red-cast beds were formerly considered to 

be the upper part of the Peerless Formation (Behre, 1953). Myrow and others 

(2003) placed these beds within the Taylor Pass Member of the Manitou 

Dolomite. The Manitou is 60 to 70 m thick at Horseshoe Mountain

Dotsero Formation (Upper Cambrian)—Fine- to medium-grained quartz sandstone 

overlain by dolomite-cemented sandstone and thin, sandy to silty dolomite beds. 

Red-brown to purple-brown color in most exposures. The Dotsero is well 

exposed in cliffs and on the steepest slopes, but is poorly exposed elsewhere. 

Bedding in the lower and more quartzitic part of the unit is parallel and uniform, 

about 0.5 m in thickness. The lower part of the unit typically forms a small cliff 

that looks nearly unbedded from a distance. Beds in the upper calcareous part of 

the unit are subparallel and wavy, ranging from a few cm to 25 cm in thickness 

(fig. 9). Thickness is 25–30 m

Sawatch Quartzite (Upper Cambrian)—White, thick-bedded, medium-grained, 

well-cemented orthoquartzite and quartzitic sandstone with moderately well 

sorted and subrounded grains. Well exposed in cliffs and on steep slopes. In 

gentle terrain, exposures are limited to piles of white, loosely aggregated 

boulders. A thin (10–20 cm) bed of quartz-pebble conglomerate with 

well-rounded clasts, commonly occurs at the base of the unit. Parallel, uniform, 

and even beds range between a few cm and 1 m in thickness and are defined by 

parting, slight differences in resistance, and by variations in color from white to 

light brown (fig. 9). Thickness is 35–45 m

Sawatch Quartzite and Dotsero Formation, undivided (Upper Cambrian)— 

Mapped together where exposures are poor

PROTEROZOIC IGNEOUS AND METAMORPHIC ROCKS

Intrusive granitic rocks and high-grade gneiss compose the Proterozoic rocks in the quadrangle. 

The granitic rocks, which are mostly coarse-grained and only slightly deformed, might correlate with 

a number of different plutons regionally. Most of the local coarse-grained and slightly deformed 

Proterozoic intrusive rocks, like those in the Climax quadrangle to the north, belong to the Berthoud 

Plutonic Suite (1,400 Ma) (McCalpin and others, 2012). Similar rocks occur in the Sawatch Range 

where they are called the St. Kevin Granite (Tweto, 1974) that has been dated at 1,390 Ma (Pearson 

and others, 1962) and at 1,444 ± 15 Ma by a concordia 238U/206Pb SHRIMP age on zircon (Wayne 

Premo, USGS, written commun., 2012). Granitic rocks are widespread to the south around the town 

of Granite, but remain unnamed and undated there. No attempt was made to correlate the granitic 

rocks in the quadrangle to any specific rock bodies that are outside the quadrangle.

For the most part, the metamorphic rocks in the quadrangle are biotite gneiss and schist. They 

are metasedimentary and metavolcanic rocks of the Proterozoic gneiss complex of Tweto (1987). 

These rocks are poorly dated locally, but are thought to have been deposited between 1,770 and 1,700 

Ma and metamorphosed between about 1,750 and 1,670 Ma (Wallace, 1995) based upon regional 

studies

Granitic rocks (Mesoproterozoic)—Pink to pinkish-gray, undeformed to moderately 

foliated, medium- to coarse-grained granitic rock; mostly monzogranite, but 

ranges from granodiorite to granite in composition; consisting of roughly equal 

proportions of microcline, quartz, and plagioclase with lesser amounts of biotite 

and muscovite (fig. 10). The rock has a seriate-to-porphyritic texture defined by 

alignment of tabular microcline phenocrysts, many of which exhibit Carlsbad 

twinning. Some euhedral laths of microcline exceed 3 cm in length. Quartz is 

present as anhedral grains and as aggregates of small, sutured grains. Weathered 

surfaces have a somewhat rusty coloration 

Monzogranite, gneiss, and dikes, undivided (Paleoproterozoic)—Medium- to 

dark-gray, medium-grained gneiss intruded at all scales by monzogranite and 

pegmatite dikes (fig. 11). Monzogranite is similar to unit Yg. Gneiss is 

characterized locally by the layering of schistose, dark-colored laminae 

containing biotite, hornblende, plagioclase, quartz, and light-gray to 

pinkish-gray, medium-grained to pegmatitic material composed primarily of 

quartz, plagioclase, and microcline. Gneiss is well foliated and exhibits 

numerous ptygmatic folds, boudinage, and sigmoidal structures 

Qd

Qa

Qrf

Qls

Qac

Ql

Qrg

Qtp

Qtb

Tsp

Tgu

TKwp

mw

mg

mh

b

MI

MDc

MDu

Om

d

s

dsu

Yg

YXmr

CORRELATION OF MAP UNITS

unconformity

unconformity

unconformity

unconformity

TERTIARY

MISSISSIPPIAN

QUATERNARY

Holocene

Eocene

MESOPROTEROZOIC

PALEOPROTEROZOIC

Pleistocene

Paleocene

Late Cretaceous

DEVONIAN

ORDOVICIAN

PENNSYLVANIAN

CAMBRIAN

CRETACEOUS

Mass-movement

deposits

Glacial

deposits

Mine

dumps

Alluvial

deposits

Lacustrine

deposits

Alluvial and

colluvial

deposits

EARLY TERTIARY TO

LATE CRETACEOUS

INTRUSIVE ROCKS

PALEOZOIC

SEDIMENTARY ROCKS

PROTEROZOIC IGNEOUS AND

METAMORPHIC ROCKS

SURFICIAL DEPOSITS

Qtp

Qtb

Qd

Qa

Qls Qac Ql Qrg

Tsp Tgu

TKwp

Ml

MDc

Om

MDu

b

mh

mg

mw

s

Yg

YXmr

d

dsu

Qrf

SCALE 1: 24 000

1 1/ 2 0 1 MILE

1000 0 1000 2000 3000 4000 5000 6000 7000 FEET

1 .5 0 1 KILOMETER

0 500 1000 1500 METERS

CONVERSION FACTORS

Multiply By To obtain

inches (in.)

inches (in.)

0.3937

0.03937

3.281 feet (ft)

0.6214 mile (mi)

0.3861

millimeters

centimeters (cm)

meters (m)

square meters (m2

)

kilometers (km)

square kilometer (km2

)

10.76 square foot (ft2

)

square mile (mi2

)

CO-MS-1

Surface offset (1.8 m)

85

20

?

Contact—Dashed where approximately located

Fault—Dashed where approximately located; dotted where concealed

Probable Quaternary fault—Fault showing displacement during Quaternary time 

(undifferentiated). Queried where identity or existance is questionable

High-angle reverse fault—Dashed where approximately located; dotted where 

concealed. Sawteeth on upper (tectonically higher) plate

Anticline

Syncline

Bedding

Vertical bedding—Showing strike

Inclined bedding—Showing strike and dip

Overturned bedding—Showing strike and dip

Location of fault scarp profile—Surface offset in meters (m) in parenthesis

Water body 

EXPLANATION

REFERENCES CITED

Beaty, D.W., Landis, G.P., and Thompson,T.B., 1990, Carbonate-hosted sulfide deposits of the 

central Colorado Mineral Belt—Introduction, general discussion, and summary, in Beaty, 

D.W., Landis, G.P., and Thompson, T.B., eds., Carbonate-hosted sulfide deposits of the central 

Colorado mineral belt: Economic Geology Monograph v. 7, p. 1‒18.

Behre, C.H., Jr., 1953, Geology and ore deposits of the west slope of the Mosquito Range, 

Colorado: U.S. Geological Survey Professional Paper 235, 176 p.

Bookstrom, A.A., 1989, The Climax-Alma granite batholith of Oligocene age and the porphyry 

molybdenum deposits of Climax, Colorado, U.S.A.: Engineering Geology, v. 27, p. 543‒568.

Briner, J.P., 2009, Moraine pebbles and boulders yield indistinguishable 10Be ages—A case study 

from Colorado, USA: Quaternary Geochronology, v. 4, p. 299‒305.

Bryant, Bruce, Marvin, R.F., Naeser, C.W., and Mehnert, H.H., 1981, Ages of igneous rocks in the 

South Park–Breckenridge region, Colorado, and their relation to the tectonic history of the 

Front Range uplift: U.S. Geological Survey Professional Paper 1199-C, p C15‒ C36.

Cappa, J.A., and Scott, L.M. 2007, Geologic Map of Lake County, Colorado: Colorado Geological 

Survey, Resource Series 42, scale 1:50,000.

Emmons, S.F., 1882, Geology and mining industry of Leadville, Lake County, Colorado: U.S. 

Geological Survey Annual Report, 2, p. 203‒292. 

Emmons, S.F., 1886, Geology and mining industry of Leadville: U.S. Geological Survey 

Monograph no. 12, 753 p.

Emmons, S.F., Irving, J.D., and Loughlin, G.F., 1927, Geology and ore deposits of the Leadville 

mining district, Colorado: U.S. Geological Survey Professional Paper 148, 368 p.

Johansing, R.J., and Thompson T.B., 1990, Geology and origin of Sherman-type deposits, central 

Colorado, in Beaty, D.W., Landis, G.P., Thompson, T.B., eds., Carbonate-hosted sulfide 

deposits of the central Colorado mineral belt: Economic Geology Monograph v. 7, p. 367‒394.

Landis, G.P., and Tschauder, R.J., 1990, Late Mississippian karst caves and Ba-Ag-Pb-Zn 

mineralization in central Colorado—Part II, Fluid inclusion, stable isotope, and rock 

geochemistry data and a model of ore deposition, in Beaty, D.W., Landis, G.P., Thompson, 

T.B., eds., Carbonate-hosted sulfide deposits of the central Colorado mineral belt: Economic 

Geology Monograph v. 7, p. 339‒366.

Mach, C.J., 1992, Geology and mineral deposits of the Kokomo District, Colorado: Fort Collins, 

Colo., Colorado State University, M.S. thesis, 154 p.

Marvin, R.F., Mehnert, H.H., Naeser, C.W., and Zartman, R.E., 1989, U.S. Geological Survey 

radiometric ages, compilation “C”— Part Five—Colorado, Montana, Utah, and Wyoming: 

Isochron/West, no. 53, p. 14‒19

Maslyn, R. M., 1977, Fossil lower karst near Molas Lake, Colorado: Rocky Mountain Association 

of Geologists, The Mountain Geologist, v. 14, no. 1, p. 17‒25

Mason, Cody, and Ruleman, C.A., 2011, Rate and timing of deglaciation using 10Be cosmogenic 

nuclide surface exposure dating, Mt. Massive Wilderness, Colorado, USA [abs.]: Geological 

Society of America Abstracts with Programs v. 43, no. 5, p. 65

McCalpin, J.P., Funk, Jonathan, and Mendel, David 2012, Leadville South quadrangle geologic 

map, Lake County, Colorado: Colorado Geological Survey, scale 1:24,000.

McCalpin, J.P., Temple, Jay, Sicard, Karri, Mendel, David, and Ahmad, Bashir, 2012, Climax 

quadrangle geologic map, Lake and Park Counties, Colorado: Colorado Geological Survey, 

Scale 1:24,000.

Myrow, P.M., Ethington, R.L., and Miller, J.F., 1995, Cambro-Ordovician proximal shelf deposits 

of Colorado, in Cooper, J.K., Finney, S.C., and Droser, M.L., eds., Ordovician odyssey—Short 

papers for the Seventh International Symposium on the Ordovician System: Society for 

Sedimentary Geology (SEPM), Pacific Section, , book 77, p. 375‒379.

Myrow, P.M., Taylor, J.F., Miller, J.F., Ethington, R.L., Ripperdan, R.L., and Allen, J., 2003, Fallen 

arches—Dispelling myths concerning Cambrian and Ordovician paleogeography of the Rocky 

Mountain Region: Geological Society of America Bulletin, v. 115, no. 6, p. 695‒713.

Myrow, P.M., Taylor, J.F., Miller, J.F., Ethington, R.L., Ripperdan, R.L., and Brachle, C.M., 1999, 

Stratigraphy, sedimentology, and paleontology of the Cambrian-Ordovician of Colorado, in

Lageson, D.R., Lester, A., and Trudgill, B., eds., Colorado and adjacent areas: Boulder, Colo., 

Geological Society of America Field Guide 1, p. 157‒176. 

Pearson, R.C., Tweto, Ogden, Stern, T.W., and Thomas, H.H., 1962, Age of Laramide porphyries 

near Leadville, Colorado, in Geological Survey Research 1962, short papers in geology, 

hydrology, and topography, Articles 60‒119: U.S. Geological Survey Professional Paper, 

450-C, p. C78‒C80.

Pierce, K.L., 2004, Pleistocene glaciation of the Rocky Mountains, in Gillespie, A.R., Porter, S.C., 

and Atwater, B.F., eds., The Quaternary Period in the United States: Amsterdam, Elsevier, p. 

63‒76. 

Ruleman, C.A., Bohannon, R.G., Bryant, B., Shroba, R.R., and Premo, W.R., 2011, Geologic map 

of the Bailey 30' × 60' quadrangle, north-central Colorado: U.S. Geological Survey Scientific 

Investigations Map 3156, scale 1:100,000.

Schildgen, Taylor, Dethier, D.P., Bierman, Paul, and Caffee, Marc, 2002, 26Al and 10Be dating of 

late Pleistocene and Holocene fill terraces—A record of fluvial deposition and incision, 

Colorado Front Range: Earth Surface Process and Landforms, v. 27, p. 773‒787.

Thompson, T.B., 1990, Precious metals in the Leadville mining district, Colorado, in Morris, H.T., 

Fisher, F.S., Shawe, D.R., and Thompson, T.B., eds., Gold-bearing polymetallic veins and 

replacement deposits—Part II: U.S. Geological Survey Bulletin, 1857-F, p. F32‒F49.

Tschauder, R.J., Landis, G.P., and Noyes, R.R., 1990, Late Mississippian karst caves and 

Ba-Ag-Pb-Zn mineralization in central Colorado—Part I, Geologic framework, mineralogy, 

and cave morphology, in Beaty, D.W., Landis, G.P., Thompson, T.B., eds., Carbonate-hosted 

sulfide deposits of the central Colorado mineral belt: Economic Geology Monograph v. 7, p. 

308–338.

Tweto, Ogden, 1951, Form and structure of sills near Pando, Colorado: Geological Society of 

America Bulletin, v. 62, no. 5, p. 507‒532.

Tweto, Ogden, 1974, Geologic map and sections of the Holy Cross quadrangle, Eagle, Lake, Pitkin, 

and Summit Counties, Colorado: U.S. Geological Survey Miscellaneous Investigations Series 

Map, I-830, 2 sheets, scale 1:24,000.

Tweto, Ogden, 1987, Rock units of the Precambrian basement in Colorado: U.S. Geological Survey 

Professional Paper 1321–A, 54 p.

Wallace, A.R., 1995, Isotopic geochronology of the Leadville 1° × 2° quadrangle, west-central 

Colorado—Summary and discussion: U.S. Geological Survey Bulletin 2104, 51 p.

Widmann, B.L., Bartos, P.J., Madole, R.F., Barba, K.E., and Moll, M.E., 2004, Geologic map of the 

Alma quadrangle, Park and Summit Counties, Colorado: Colorado Geological Survey 

Open-File Report 04‒03, scale 1:24,000.

Widmann, B.L., Kirkham, R.M., Houck, K.J., and Lindsay, N.R., 2006, Geologic map of the 

Fairplay West quadrangle, Park County, Colorado: Colorado Geological Survey Open-File 

Report 06‒7, scale 1:24,000.

Young, N.E., Briner, J.P., Loenard, E.M., Liccardi, J.M., and Lee, Keenan, 2011, Assessing climatic 

and non-climatic forcing of Pinedale glaciation and deglaciation in the western United States: 

Geology, v. 39, p. 171‒174.

REGIONAL GEOLOGY AND STRUCTURE

The Mount Sherman 7.5' quadrangle is in the central Mosquito Range, a high north-south 

trending range that separates the relative lowlands of South Park (about 2,800 m elevation) in the 

east and the Upper Arkansas River valley (about 3,090 m elevation) to the west. Mount Sherman, 

the highest point in the quadrangle, is 4,278 m in elevation and other peaks along the crest of the 

range typically exceed 4,100 m in elevation. The crest of the range has been deeply glaciated. 

Stratiform rocks in the Mosquito Range, which are Cambrian to Pennsylvanian in age, provide 

a platform for Late Cretaceous sedimentary rocks and middle Tertiary sills. These Paleozoic and 

Mesozoic sedimentary rocks dip moderately (20°–30°) to the east. Proterozoic basement rocks are 

exposed beneath the sedimentary rocks throughout the more highly elevated western parts of the 

quadrangle. To the east, in central South Park, the Proterozoic basement is buried to depths of 

3,000+ m by deformed Phanerozoic sedimentary rocks (Ruleman and others, 2011). To the west, 

the Upper Arkansas River Basin south of Leadville, is a deep graben in which Upper Tertiary fill of 

the Dry Union Formation and Quaternary basin fill of indeterminate thickness rests on Proterozoic 

crystalline rocks (Tweto, 1974; McCalpin and Funk, 2009). 

Structural tilting in the Mosquito Range resulted in the complete removal of the platform of 

stratiform and sedimentary rocks in the Arkansas River Valley (Tweto, 1974; Cappa and Scott, 

2007; McCalpin and Funk, 2009), an area that is now a large graben. This Cambrian to 

Pennsylvanian age platform of stratiform rocks is also absent in the Sawatch Range to the west of 

the Arkansas River valley (Cappa and Scott, 2007). Tertiary sills in the Mosquito Range are tilted 

as well, and one of the youngest documented sills, the Chalk Mountain Rhyolite of the Climax area, 

approximately 18 km north of Mount Sherman, has been dated at 29 Ma (McCalpin and others, 

2012) and related dikes in that area are as young as 25.5 Ma (Bookstrom, 1989). 

The subparallel dip of sills and the Cambrian to Pennsylvanian age platform of stratiform 

rocks suggests that the tilting deformation likely postdates intrusion, so regional east dips 

developed sometime after the late Oligocene. Since the tilting and the erosion of the Paleozoic and 

Mesozoic rocks in the Arkansas River valley were probably coeval, most of the tilting probably 

occurred prior to, or in the early stages of graben formation. Regional deformation in this part of 

Colorado has long been considered to be part of the Laramide orogeny, but this study has revealed 

that much of the deformation is considerably younger than that. We consider the young regional 

stratal tilts to be a product of rift-flank uplift occurring in the Oligocene after the youngest igneous 

rocks (for example, Gray porphyry group ~ 36.7 Ma) were emplaced in the region and unrelated to 

Laramide compression. This implies that most of the topographic relief in the region is a product of 

the Rio Grande rift. 

Laramide faults are important in the quadrangle, and include the high-angle reverse faults 

known as the Weston and London faults, which are present in the south-central half and northeastern corner of the map, respectively. The Weston fault dips about 40°–50° to the east, is 

up-to-the-east, and has a vertical separation, measured on the Sawatch Quartzite (s), of about 

1,250 m. The fault displaces the sills of the White porphyry group so deformation postdates the 

early Paleocene, but beyond that, the age of the fault is poorly constrained. Little evidence for this 

fault can be found in the northern half of the quadrangle where all the faults are normal and cut the 

Weston fault. 

The Mosquito fault, the most prominent normal fault in the quadrangle, and the corresponding 

Mosquito-Union fault, are west-dipping, high-angle faults in the west-central part of the quadrangle 

that formed during the same extensional tectonism that created the graben of the Arkansas River 

Valley. The Mosquito fault is up-to-the-east and has an approximate vertical component of offset of 

about 250 m on the basal Sawatch Quartzite surface in the West Sheridan/Long-and-Derry Hill region. 

As offsets on related branches of the fault are considered, the basal Cambrian platform strata are about 

600 m higher on Mount Sheridan at the crest of the range than they are at the west border of the 

quadrangle. The difference is accommodated by down-to-the-west displacement of east-tilted blocks in 

a domino-style of tectonism in the Mount Sherman quadrangle. The total of 600 m of vertical 

separation is typical of most of the fault along its length, as it has that same approximate displacement 

in the Climax 7.5' quadrangle to the north, where the offset is focused on a single fault trace 

(McCalpin and others, 2012). The age of displacement is not known, but the fault cuts all of the sills 

and its trace is characterized by west-facing Quaternary scarps in the southern part of the quadrangle. 

These fault scarps primarily displace colluvium of unknown age and can be mapped as linear features 

by using the alignment of springs and seeps. One location towards the western boundary of the map 

yielded a credible fault scarp profile for a single-event fault (location CO–MS–1 on map; fig. 12).

Evidence of Pleistocene glaciations is recorded in the abundant large drainage basins and 

cirques within the quadrangle. Although most glacial deposits of Pinedale and Bull Lake ages were 

deposited outside of the quadrangle, multiple sets of inset glacial trimlines along the walls of the 

valleys indicate successive glaciations. Absolute ages of glaciations were not acquired within the 

quadrangle, but studies proximal to the mapped area and within the Rocky Mountains indicate that 

the penultimate glaciation (Bull Lake) culminated around 122 ka (Schildgen and others, 2002; 

Pierce, 2004). Deposits mapped as Bull Lake Till occur only within the Twelvemile Creek drainage 

and are perched above younger glacial deposits and scoured areas of Pinedale age.

The most recent glaciation (Pinedale) has been dated between 20.9 ± 1.0 ka and 14.4 ± 0.8 ka. 

These deposits are hummocky ground and lateral moraines seen in all of the main drainages within 

the quadrangle. Mason and Ruleman (2011) studied the recessional moraine complex on the 

western flank of Mount Massive, right across the Arkansas River valley, and determined that the 

Pinedale deglaciation occurred between 19.5 ± 1.8 ka and 12.6 ± 1.2 ka to the west. These ages 

indicate that deglaciation occurred over an approximate 6.5 ky period at a rate of about 1 m/yr.

Within the quadrangle, the post-glacial deposition has been limited to minimal Holocene 

deposition confined to small floodplains and sheetwash on hill slopes. Within most cirque basins 

rock glaciers are presently active as evidenced by their fresh hummocky appearance. Rock falls and 

landslides have occurred, likely as a function of post-glacial gravitational collapse of valley walls.

ECONOMIC GEOLOGY

There are two large carbonate-hosted, sulfide-ore deposits located in the Mount Sherman 

quadrangle (Emmons, 1882, 1886; Emmons and others, 1927; Behre; 1953). The first, the Leadville 

deposit, is in the Breece Hill, Ball Mountain, and California Gulch region (northwest part of 

quadrangle), and is commonly referred to as a Leadville-type deposit (Beaty and others, 1990). The 

second is the Sherman deposit (called a Sherman-type deposit) and is primarily on the eastern flank 

of Mount Sherman in the upper reaches of Fourmile Canyon (Beaty and others, 1990). Both 

deposits occur where sills of either or both of the Gray or White porphyry groups intrude the 

Mississippian Leadville Limestone. The Leadville-type deposit is primarily associated with the 

Gray porphyry group. Carbonate stratiform rocks from Devonian to Pennsylvanian age are 

mineralized. In the Sherman-type deposit, only the Leadville Limestone is mineralized as a result of 

intrusion by the White porphyry group (Beaty and others, 1990). The Leadville-type deposits 

formed at moderately high temperatures (300–500°C) (Behre, 1953) in response to migrating 

mid-Tertiary hydrothermal fluids which derived sulfur and lead from igneous sources (Beaty and 

others, 1990). Tschauder and others (1990) and Landis and Tschauder (1990) argued that the 

Sherman-type mineralization formed during the Late Mississippian as a Mississippi Valley-type 

deposit in a post-depositional karst system. However, Johansing and Thompson (1990) concluded 

that those same deposits formed during the Tertiary from basinal brines that moved along thermal 

gradients controlled by igneous intrusions. 

Quartz-gold-pyrite and silver-metal ores emplaced in carbonate rocks in the mid-Tertiary 

characterize the Leadville ore body (Tschauder and others, 1990). The Sherman ores are concentrated 

in white barite and dolostone, and they have a high silver and low gold, pyrite, and chalcopyrite 

content (Tschauder and others, 1990). 

Our mapping does not resolve the disagreement over Sherman-type genesis, but our mapping 

clearly shows that both major deposit types are associated with large intrusions (80–100 km2). We 

made an observation that might be of interest: We noted that the preserved volume of Devonian and 

Mississippian carbonate rock in intruded areas is highly variable, and sills seemed to favor this 

stratigraphic horizon. We think there has been a process of dissolution of carbonate rock in 

association with the sill injection. We interpret this dissolution to have facilitated the intrusion of 

sills as it would have provided space for magma as solid carbonate was dissolved and carried away 

by hot fluids. This might lend support to the Johansing and Thompson (1990) interpretation, 

although the sills that intrude Mount Sherman are mostly of the White porphyry group, which the 

latter authors think are older than the mineralization. 

CO-MS-1

Surface offset (1.8 m)

WESTON

FAULT

MOSQUITO-UNION

FAULT

MOSQUITO FAULT

LONDON

FAULT

WESTON

FAULT

CONTOUR INTERVAL 10 METERS

NATIONAL GEODETIC VERTICAL DATUM OF 1929

To convert meters to feet multiply by 3.2808

To convert feet to meters multiply by .3048

Geology mapped by R.G. Bohannon and C.A. Ruleman 

during 2010–2012.

D. Paco Van Sistine prepared the digital compilation of 

the geologic map and the GIS database

Edit and cartographic layout by L.J. Binder

Denver Publishing Service Center QUADRANGLE LOCATION

COLORADO

Base created by R.G. Bohannon from U.S. Geological Survey 

National Elevation Dataset, 2012

(NED, http://nationalmap.gov/elevation.html) 

Projection: Universal Transverse Mercator, zone 15

North American Datum of 1927 (NAD 27)

APPROXIMATE MEAN

DECLINATION, 2012 TRUE NORTH

MAGNETIC NORTH

9°

Geologic Map of the Mount Sherman 7.5’ Quadrangle, Lake and Park Counties, Colorado

By

Robert G. Bohannon and Chester A. Ruleman

2013

For more information concerning this publication, contact:

Center Director, USGS Geology and Environmental Change Science Center

Box 25046, Mail Stop 980

(303)236-5344

Or visit the Geology and Environmental Change Science Center Web site at:

http://esp.cr.usgs.gov/

Any use of trade, product, or firm names is for descriptive purposes only

and does not imply endorsement by the U.S. Government.

Although this information product, for the most part, is in the public 

domain, it also contains copyrighted materials as noted in the text. 

Permission to reproduce copyrighted items for other than personal use

must be secured from the copyright owner. 

Publishing support provided by: 

Denver Publishing Service Center

Manuscript approved for publication August 29, 2013

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This report is available at:

http://pubs.usgs.gov/sim/3271

Index map showing map area in yellow, and adjacent 

7.5-minute quadrangles. For U.S. Geological Survey 

publications the publication number, author(s), and year 

of publication are shown. For non-USGS publications 

the State agency, author(s), and publication year are 

shown. 

39°22’30”

106°22’30” 106°15’00” 106°07’30” 106°00’00”

39°15’00”

39°07’30”

36°30’00”

LEADVILLE

NORTH

CLIMAX ALMA

LEADVILLE

SOUTH

MOUNT

SHERMAN

FAIRPLAY WEST

GRANITE SOUTH PEAK JONES HILL

Colorado Geological

Survey

(McCalpin, Funk,

and Mendel, 2012)

SIM 3271

(Bohannon and

Ruleman, 2013)

Colorado Geological

Survey

(McCalpin and

others, 2012)

Colorado Geological

Survey, OFR 04–03

(Widmann and

others, 2004)

Suggested citation:

Bohannon, R.G., and Ruleman, C.A., 2013, Geologic map of the Mount Sherman 7.5’ 

quadrangle, Lake and Park Counties, Colorado: U.S. Geological Survey Scientific 

Investigations Map 3271, scale 1:24:000, http://pubs.usgs.gov/sim/3271/.

A

A'

B

B'

Colorado Geological

Survey, OFR 06–07

(Widmann and

others, 2006)

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Map 3271