MISSOURI

Geology mapped in 2003 and 2004

Printed on recycled paper

GEOLOGIC MAP OF THE ALLEY SPRING QUADRANGLE, SHANNON COUNTY, MISSOURI

By

David J. Weary and Randall C. Orndorff

2012

Any use of trade, product, or firm names in this publication is for descriptive

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

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

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

Base from U.S. Geological Survey, 1965;

photorevision as of 1985

Polyconic projection. 1927 North American datum.

10,000-foot grid ticks based on Missouri coordinate system,

east zone

1,000-meter Universal Transverse Mercator grid ticks, zone 15,

shown in blue

0°

APPROXIMATE MEAN

DECLINATION, 2012 TRUE NORTH MAGNETIC NORTH

MAP LOCATION

U.S. DEPARTMENT OF THE INTERIOR SCIENTIFIC INVESTIGATIONS MAP 3161

U.S. GEOLOGICAL SURVEY

.5 0 1 KILOMETER1

SCALE 1:24 000

1 1/2 0 1 MILE

CONTOUR INTERVAL 20 FEET

DISCUSSION

INTRODUCTION

The Alley Spring 7.5-minute quadrangle is located in south-central Missouri within

the Salem Plateau region of the Ozark Plateaus physiographic province (Fenneman,

1938; Bretz, 1965) (fig. 1). Most of the land in the quadrangle is privately owned

and used primarily for grazing cattle and horses and growing timber. A large minority

of the land within the quadrangle is publicly owned, either by the Missouri State

Forests or by the Ozark National Scenic Riverways, administered by the National Park

Service. The map area has topographic relief of about 535 feet (ft), with elevations

ranging from about 630 ft where the Jacks Fork River exits the lower east edge of the

quadrangle, to about 1,165 ft approximately 0.25 mile (mi) southeast of the

intersection of State Highway 19 and State Road D in the northeastern part of the

map area. The most prominent physiographic feature of the quadrangle is the valley

of the Jacks Fork River in the southern portion of the map area. The namesake for

the quadrangle, Alley Spring, is a first-magnitude spring flowing at an average rate of

about 81 million gallons per day (125 cubic feet per second or 3.5 cubic meters per

second) (Vineyard and Feder, 1982). Alley Spring is located in the south-central part of

the quadrangle (fig. 2).

STRATIGRAPHY

Mesoproterozoic Basement Rocks

The basement rocks underlying the quadrangle are part of a Mesoproterozoic

granite/rhyolite terrane, which is extensive in the Midcontinent region. Outcrops of

this terrane also occur in the Big Spring, Eminence, Powder Mill Ferry, Stegall

Mountain, and Van Buren North quadrangles to the east and southeast and in the St.

Francois Mountains 50 mi to the northeast (fig. 1). These granites and their

surrounding volcanic deposits were deformed, faulted, tilted, and subjected to uplift

and erosion beginning in the Mesoproterozoic and continuing through Neoproterozoic

time. The regional relief on this buried erosional surface has been reported to be as

much as 2,000 ft (Kisvarsanyi, 1975).

The rhyolite of Storys Creek (map unit Yscr) is exposed in one small area of the

Alley Spring quadrangle in sec. 15, T. 29 N., R. 4 W., about 0.5 mi south of State

Highway 19 near the central east edge of the map area. The rock is aphanophyric,

having about 3 to 5 percent phenocrysts of alkali feldspar. The matrix is colored

medium gray to medium dark gray or brownish gray and the phenocrysts are pale pink

to pale reddish purple. Pale-pink flow bands in the matrix are also present locally.

Some of the fractures in the rock contain thin vein quartz. Table 1 presents a single

sample analysis of the chemistry of the rhyolite of Storys Creek. This is the

westernmost outcrop of Mesoproterozoic rocks in the Eminence-Van Buren volcanic

field (Lowell and others, 2005; Lowell and others, 2010).

Uranium-lead (U-Pb) isotopic analyses on these rocks in nearby quadrangles yielded

radiometric ages of (1) 1,461.8±5.5 Ma for granite exposed in the Big Spring

quadrangle (Harrison and others, 2000), (2) 1,480±42 Ma for granite from a corehole

in the Winona quadrangle to the southeast (Van Schmus and others, 1993) and (3)

1,473±15 Ma for granite from a corehole in the Round Spring quadrangle to the

north (Bickford and others, 1981). A U-Pb age of 1,470.4±2.7 Ma was obtained from

the “Rhyolite of Shut-In Mountain” in the Stegall Mountain quadrangle to the

southeast (Harrison and others, 2000).

Paleozoic Stratigraphy

The Alley Spring quadrangle is underlain by about 1,990 ft of lower Paleozoic

marine strata, chiefly dolomite, chert, sandstone, and orthoquartzite. From oldest to

youngest, these units are the Upper Cambrian Lamotte Sandstone, Bonneterre

Formation, Davis Formation, Derby-Doerun Dolomite (usage of Missouri Geological

Survey, 1979), Potosi Dolomite, and Eminence Dolomite; and the Lower Ordovician

Gasconade Dolomite and Roubidoux Formation. Only the last four units crop out in

the quadrangle. The columnar section, figure 3, summarizes the general lithologies

and thickness of the Paleozoic strata that crop out in the Alley Spring quadrangle. No

drill holes in the quadrangle penetrated below the Derby-Doerun Dolomite; therefore

thicknesses of the lowest three Paleozoic formations listed here, and portrayed in the

cross section, are estimates based on reported thicknesses from neighboring

quadrangles.

During Late Cambrian time, the Ozark region was flooded by an extensive

epicontinental sea that covered most of the midwestern part of the United States.

Because of topographic relief on the Mesoproterozoic paleosurface, initially only low

areas were covered by the sea, and high areas remained above water as islands. In the

map area the basal Cambrian unit, the Lamotte Sandstone, onlaps the flanks of these

Mesoproterozoic bedrock hills. The Lamotte is estimated to reach a thickness of about

275 ft in the map area.

During deposition of the Bonneterre Formation, algal reefs developed along the

flanks of the remnant islands of Mesoproterozoic rocks in the Late Cambrian sea

(Lyle, 1977). These reefs, especially to the northeast along the Viburnum Trend (fig.

1), later became the focus of precipitation of Mississippi Valley-type base-metal

deposits (Thacker and Anderson, 1977). Most of the islands were entirely buried near

the end of deposition of the Bonneterre. However, they affected depositional patterns

even after they were completely buried, owing to differential compaction of sediments

deposited on and around their flanks. The Bonneterre is estimated to be about 390 ft

thick in the map area.

The Davis Formation probably was named for exposures on Davis Creek, St.

Francois County, Mo. (Buehler, 1907). The Davis Formation conformably overlies the

Bonneterre Formation and comprises shale, siltstone, sandstone, dolomite, and

limestone conglomerate. Shales within the Davis have low primary permeability and

act as aquitards (part of the St. Francois confining unit) between the underlying St.

Francois aquifer and the overlying Ozark aquifer (Kleeschulte and Seeger, 2000). The

Davis is estimated to be about 175 ft thick in the Alley Spring quadrangle.

The Derby-Doerun Dolomite (usage of Missouri Geological Survey, 1979) was

originally two separate formations described from outcrops near mines operated by

the Derby Lead Co. and the Doe Run Lead Co. in St. Francois County, Mo.

(Thompson, 1995). The formation comprises thin-bedded to medium-bedded dolomite

intercalated with thin-bedded siltstone and shale. The Derby-Doerun is estimated to be

about 200 ft thick in the study area.

The Upper Cambrian Potosi Dolomite, the oldest Paleozoic unit exposed in the

Alley Spring quadrangle, crops out locally in the valleys of the Jacks Fork River, Storys

Creek, and Mahans Creek in the southeastern part of the quadrangle. As much as 65

ft of Potosi is exposed. The name Potosi was originally used by Winslow (1894) in a

much broader sense for rocks exposed near the town of Potosi in Washington County,

Mo. Dake (1930) reviewed and revised descriptions of the Potosi and redefined the

formation into its current usage. The Potosi is a vuggy, fine-grained to

medium-grained, light-brownish-gray to pale-yellowish-brown dolomite having siliceous

druses as a replacement of the dolomite or as vug linings. A fetid odor on freshly

broken surfaces is a minor yet diagnostic characteristic indicative of trace organic

material. In outcrop, the Potosi characteristically is more intensely jointed than the

overlying formations. Pronounced cavern development, comprising minor caves and

other solution features, exists at or near the contact between the Potosi and the

overlying Eminence Dolomite; this suggests a permeability contrast across that

boundary. With the exception of algal structures, the Potosi is characteristically

unfossiliferous. A single drill hole (21242) penetrated the full thickness of Potosi in the

quadrangle and documented a thickness of 260 ft (table 2). The lithologic difference

between the Potosi and Eminence is thought to be the result of a secondary alteration

of the Potosi that includes dissolution of carbonate, precipitation of silica, and

introduction of hydrocarbons. This alteration front causes the contact between these

two units to vary up and down stratigraphically.

The Eminence Dolomite was first described by Ulrich (1911) from exposures in the

vicinity of the town of Eminence, Mo. The Eminence comprises mostly massive to

thick-bedded, medium-grained to coarse-grained, light-gray to medium-gray dolomite

and cherty dolomite; chert occurs as nodules and angular fragments. Many of the beds

are stromatolitic, with pits and vugs developed between the laminae, and others are

crossbedded. At outcrops the thick, non-cherty dolomite beds commonly weather into

irregular pinnacles, especially beds lying subjacent to sandstone of the overlying

Gunter Sandstone Member of the Gasconade Dolomite. Bridge (1930) reported the

Eminence to be abundantly fossiliferous, especially with trilobites and gastropods in

residual cherts. No such fossils were observed during the course of this study. About

200 ft of Eminence Dolomite is exposed in the southeastern part of the map area

along the Jacks Fork River, Mahans Creek, and Storys Creek. Lesser exposures of the

Eminence also occur locally along the northern edge of the quadrangle in Spring

Valley, Jacks Fork Hollow, and the valley of Grassy Creek. Drill holes record a

subsurface thickness range of 225 to 380 ft in the quadrangle.

The Eminence Dolomite is one of the main cave-bearing formations in the map

area. Caves and springs typically are concentrated just below the Gunter Sandstone

Member of the overlying Gasconade Dolomite (Orndorff and others, 2006). The

conduit of Alley Spring is a water-filled cave in the upper part of the Eminence

Dolomite (Ozark Cave Diving Alliance, unpub. data, 2005).

Previous maps produced by this U.S. Geological Survey (USGS) project placed the

Cambrian-Ordovician boundary in the upper part of the Eminence Dolomite

(McDowell, 1998; Orndorff and others, 1999; McDowell and Harrison, 2000;

Orndorff and Harrison, 2001; Harrison and others, 2002; Weems, 2002; Harrison

and McDowell, 2003). However, recent international agreement on redefinition of this

boundary requires that it be placed at the contact between the Eminence and

Gasconade Dolomites (Cooper and others, 2001). Bridge (1930) speculated that an

important unconformity exists at the base of the Gasconade (fig. 3). Conodont

biostratigraphy suggests that there may be multiple unconformities within the basal

Gasconade interval (Repetski and others, 2000).

The Gasconade Dolomite was named by Nason (1892) for exposures along the

Gasconade River in Laclede, Pulaski, and Phelps Counties, central Missouri. In the

map area the basal 10 to 20 ft consists of interbedded sandstone, orthoquartzite, and

thin-bedded dolomite and sandy dolomite named the Gunter Sandstone Member by

Ball and Smith (1903) for exposures along the Niangua River at Gunter (now

Hahatonka Springs), Camden County, in central Missouri. The quartz sandstone

content of the Gunter varies within the Alley Spring quadrangle. Quartz sandstone and

(or) orthoquartzite represent up to about 50 percent of the interval in some places and

as little as zero percent in others. In the areas having no quartz sandstone or

orthoquartzite, those stratigraphic intervals are occupied by dolomite or sandy

dolomite containing floating quartz grains.

The Gasconade Dolomite is divided into three informal units. The lower unit is the

Gunter Sandstone Member. The middle unit includes the former Van Buren Formation

of Bridge (1930) (which occupies the lower part of the middle unit), overlain by

dolomite up to and including a persistent Cryptozoon chert bed. The upper unit is

thick-bedded dolomite. Where not buried in residuum from the overlying Roubidoux

Formation, the upper unit tends to form dolomite glades on south-facing and

west-facing slopes. The total thickness of the Gasconade Formation in the Alley

Spring quadrangle is about 300 ft.

Bridge (1930) used the name Van Buren Formation for the medium-bedded

non-cherty and cherty dolomite above the Gunter Sandstone Member and below a thin

oolite bed in the middle part of the middle unit of the Gasconade Dolomite (fig. 3).

Bridge’s Van Buren Formation was defined by its faunal content, but the name is no

longer used (Thompson, 1995). Pratt and others (1992) described a lower part and

an upper part of the Gasconade Dolomite that are divided by a persistent Cryptozoon

chert, which is located about 80 to 100 ft below the top of the Gasconade, at the top

of the upper part of the middle unit (fig. 3). Outcrops of the chert bed are rare in the

Alley Spring quadrangle; however, float of Cryptozoon chert boulders, indicating this

horizon, was observed in many locations. The Cryptozoon chert typically comprises

parallel columns, a few inches wide, of convex-upward algal laminae. The chert bed

breaks on weathering into distinctive polygonal boulders. Triangular symbols on the

map indicate locations where the Cryptozoon chert was observed in outcrop or as

float.

Fossils are rare in the Gasconade within the map area, though the presence of

planispiral gastropods and cephalopods from cherts within this unit have been

reported at Fort Leonard Wood Military Reservation, about 55 mi to the northwest,

and at a locality near the town of Van Buren, about 25 mi to the southeast (Harrison

and others, 1996; J.E. Repetski, USGS, oral commun., 2000).

The upper unit of the Gasconade Dolomite is one of the main cave-bearing intervals

in the Alley Spring quadrangle. Caves and springs typically are concentrated just below

the overlying sandstones of the Roubidoux Formation (Orndorff and others, 2006).

The Roubidoux Formation was named for exposures in the area of Roubidoux

Creek about 50 mi to the northwest in Pulaski County, Mo. (Nason, 1892). The

Roubidoux consists of interbedded dolomite, cherty dolomite, sandy dolomite,

dolomitic sandstone, quartz sandstone, orthoquartzite, and bedded chert. The contact

between the Roubidoux and the underlying Gasconade is probably conformable and is

placed at the base of the first significant orthoquartzite or sandstone in the Roubidoux.

The lower 20 to 30 ft of the Roubidoux is sandstone-rich and commonly produces a

topographic bench that sheds sandstone float onto the slopes below. The middle part

of the Roubidoux is chiefly thick-bedded gray dolomite, which resembles the dolomite

of the upper part of the Gasconade. The upper part of the Roubidoux is

sandstone-rich and forms another topographic bench. Upon weathering the

Roubidoux produces a sandy, orange-colored soil that in places is littered with tabular

sandstone and orthoquartzite float blocks as well as white chert cobbles and boulders.

Fossils generally are rare in the Roubidoux Formation, though impressions of snails

(mostly Lecanospira) locally are common in cherts and sandstones. Heller (1954)

reported the occurrence of brachiopods, cephalopods, and trilobites from this

formation elsewhere within the Salem Plateau. Harrison and others (1996) reported

the occurrence of conodonts indicative of early Early Ordovician (early to middle

Ibexian) age.

The top of the Roubidoux Formation is not present in the map area, having been

removed by erosion. The Roubidoux is reported to be as much as 250 ft thick in other

parts of Missouri (Thompson, 1995). A thickness of about 140 ft of the Roubidoux is

exposed in the northwestern part of the quadrangle.

Surficial Geology

Alluvial deposits (Qa) and terrace deposits (Qt) were mapped within the stream

valleys and are similar in composition. The terraces comprise Holocene to possibly

late Pleistocene alluvial deposits (Haynes, 1985; Albertson and others, 1995) that

were derived mostly by washing out and reworking of the clay, silt, and chert fractions

from the adjacent upland residual soils and to a lesser extent by reworking of loess.

Terraces were mapped as geomorphic features along the sides of the stream valleys at

higher elevations than the present-day flood plain. Along the Jacks Fork River and its

major tributaries, several levels of surficial terrace deposits occur. Detailed

differentiation and mapping of these terraces across the map area were beyond the

scope of the present project. Therefore, all terrace deposits were mapped as a single

unit (Qt).

Pleistocene loess is reported as thin or absent over most of the Salem Plateau

(Ebens and Connor, 1980). Nevertheless, the Alley Spring region includes areas of

soils having a loess component (Gott, 1975). In most areas, these soils do not

constitute a mappable geologic unit, because they are thin and intermingled with

underlying clayey residuum due to bioturbation. Small areas having these soils were

observed in the Alley Spring quadrangle in saddles between hills and on flat uplands.

These areas are not extensive enough to be mapped as a separate unit. Some of the

larger sinkholes are filled with deposits of Holocene and Pleistocene(?) muck (Qm) that

are large enough to be mapped as a separate unit. Only one area, however,

surrounding Indian Pond (sec. 14, T. 29 N., R. 5 W.), was delineated on this map.

In upland areas, a pervasive mantle of residuum represents the weathering-in-place

of fresh bedrock into soil. Generally, dolomites have been leached away, leaving a

residue of angular pebbles, cobbles, and boulders composed of chert, quartzite, and

sandstone floating in a matrix of clayey silt to silty clay. On most slopes, the residual

soils have undergone downhill creep, producing a thin mantle of colluvium along most

valley walls. Because this veneer typically is nearly ubiquitous but thin, it is not mapped

as a separate unit.

Many of the high ridges and hills in the quadrangle underlain by Gasconade

Dolomite bedrock are capped with residual deposits consisting of quartz sandstone

slabs and boulders, white chert boulders and cobbles, and sandy soils clearly derived

from the in-place weathering of the overlying Roubidoux Formation and ranging from

0 to about 40 ft thick. These deposits also occur as colluvium on steep hillsides. They

have been mapped as a separate unit designated QTrr (Quaternary and Tertiary

Roubidoux residuum).

KARST

Karst features in the Alley Spring quadrangle include caves, springs, sinkholes,

disappearing and losing streams, and small-scale solutional features on exposed

carbonate bedrock surfaces. Caves can be found anywhere in the stratigraphic section

but are concentrated at particular stratigraphic horizons. Many caves are in the upper

part of the Eminence Dolomite (just beneath the Gunter Sandstone Member of the

overlying Gasconade Dolomite), and in the upper part of the Gasconade Dolomite,

near the contact with the Roubidoux Formation (Orndorff and others, 2006; Lowell

and others, 2010). Most sinkholes are filled with sediment and residuum, and many

were formed by subsidence of the land surface into caves below. Sinkholes can be

found in all of the formational units, but they are larger and more numerous in the

lower part of the Roubidoux Formation where it collapses into caves in the upper part

of the underlying Gasconade Dolomite. Sinkholes observed during the course of field

studies in the quadrangle as well as sinkholes implied by closed contours on the

topographic map are delineated on the geologic map. Most locations were checked

using a global positioning system (GPS) to provide precise coordinates; sinkhole

shapes and sizes are approximate and based on visual estimates.

Alley Spring, the largest karst spring in the quadrangle, has an average discharge of

81 million gallons per day (Vineyard and Feder, 1982). There is at least a 25-year

history of scuba diving in the spring and in the large water-filled cave that is a conduit

supplying groundwater to the spring. Recent (2000 to present) explorations by the

Ozark Cave Diving Alliance (OCDA) have extended the mapped portion of the cave.

The point of farthest exploration to date is about 2,000 ft west-southwest of the

spring, at an elevation approximately 100 ft below the surface of the rise pool. Dye

traces indicate that some of the water feeding Alley Spring flows from as far away as

37 mi to the west-northwest (Aley and Aley, 1987).

Preliminary analysis of audio-magnetotelluric data collected near Alley Spring has

yielded inconclusive results as to the exact location of the conduit(s) to the west and

northwest of the known cave (Pierce and Weary, 2009; Lowell and others, 2010).

Recent cave diving by the OCDA produced video and photos and supplied rock

samples from various parts of the system to allow the USGS to relate the conduit

system to the local geology. Cave diving in various Ozark springs has shown in many

instances that the rise pool is supplied by a conduit that descends obliquely to depths

as great as 300 ft before leveling off and continuing subhorizontally. The angle of

descent typically is 25° to 35°. The cave diving survey of Alley Spring shows that the

conduit becomes horizontal beneath the Gunter Sandstone Member of the Gasconade

Dolomite (about 130 ft below the surface of the rise pool). This demonstrates a

stratigraphic control on cave level similar to that observed in dry caves studied

throughout the region (Orndorff and others, 2006).

STRUCTURAL GEOLOGY

The regional style of deformation in southeastern Missouri is brittle, with vertical

jointing and strike-slip faulting predominant along northeast and northwest trends

(Clendenin and others, 1989; Harrison and Schultz, 2002). Strata within the Alley

Spring quadrangle are generally subhorizontal to gently dipping. Gentle folding occurs

as compressional or transpressional strain near faults and possibly as subsidence into

large karstic voids. The steepest dip measured in the quadrangle was 17° to the

northwest in rocks exposed along State Highway 106 in sec. 33, T. 29 N., R. 4 W.

This high dip is probably associated with deformation of the rocks between the

Hartshorn, Mahans Creek, and Possum Trot Hollow faults. It also is possible that there

is a buried knob of igneous Mesoproterozoic basement rocks in the southeast corner

of sec. 28, T. 29 N., R. 4 W., and the overlying Paleozoic sedimentary rocks are

dipping off of it as described in earlier studies in the area (Bridge, 1930; Harrison and

others, 2002). Most of the steeper dips measured in the quadrangle are found in this

general area.

The rocks of this quadrangle are pervasively jointed. Joints were characterized by

their orientation, spacing, persistence, and aperture. Spacing refers to the mean

perpendicular distance between parallel joints in a joint set. Joints confined to

individual beds are classified as non-throughgoing, and joints extending across bedding

planes into overlying and (or) underlying beds are classified as throughgoing. Most

joints in the carbonate rocks are open (containing no mineralized infilling) and have

narrow apertures. However, some joints have been widened by solution and are as

much as 10 inches (in.) wide. Cumulative orientations of all the joint measurements

observed in outcrop are shown in figure 4. The two primary joint sets trend

north-northwest and east-northeast and are indicative of development under a regional

stress field. Cox (1995) interpreted similarly oriented joints throughout the Ozarks as

far-field deformation related to the Ouachita and Appalachian orogenies.

Cataclastic deformation bands occur locally in sandstones within the quadrangle. All

were found in float blocks, diminishing their usefulness for indicating direction of fault

movement. The deformation bands are of tectonic origin and are created by

millimeter-scale displacements through shearing that involves a combination of

pore-space collapse and fracturing of grains (Davis, 1999; Harrison and Schultz,

2002). Because development of deformation bands is a strain-hardening mechanism

(Davis, 1999), the bands are usually more resistant to weathering than the surrounding

rock and are commonly found in raised relief on the surfaces of sandstone boulders.

Faults in this area are difficult to observe directly, due to vegetation and residuum

cover, and have to be inferred on the basis of indirect evidence. This evidence includes

(1) vertical offset of strata having insufficient observed dip to explain that offset

(vertical stratigraphic offset along faults may reverse in sense as the result of horizontal

offset of gentle folds along both sides of the fault, sometimes referred to as scissor

faults) and (2) cataclastic deformation bands observed in float blocks of sandstone.

Although only indications of vertical offset were observed on faults in the

quadrangle, the faults are interpreted to be predominantly strike-slip on the basis of

observations of similar faults exposed in subsurface base-metal mines in nearby

quadrangles.

The Hartshorn fault traverses the southern part of the quadrangle and trends

northwest. This fault has been mapped by the USGS in reconnaissance for tens of

miles northwestward from Alley Spring. The mapping of this fault was originally based

on small vertical stratigraphic offsets observed in sparse outcrops and well logs and

also on interpretation of regional aeromagnetic data (R.W. Harrison, USGS, unpub.

data, 2010). Recently acquired electrical resistivity profiles confirm the location of the

fault in the vicinity of Alley Spring (fig. 5, cross section B–B') (Pierce and Weary,

2009; Lowell and others, 2010). Total offset on this fault is difficult to determine; the

horizontal slip is undetermined and the vertical offset varies locally, ranging from as

much as 100 ft to as little as 20 ft, with the southwestern block up in relation to the

northeastern block in the Alley Spring quadrangle. The Hartshorn fault continuing to

the northwest may be the same structure as the Arthur Creek fault, mapped by

Satterfield (1976) near the town of Raymondville, about 20 mi northwest of the Alley

Spring quadrangle. The Hartshorn fault trace also approximates the trace of a

topographic lineament named the Alley Hollow lineament by Aley and Aley (1987).

Another regional strike-slip fault that traverses the Alley Spring quadrangle is the

Sutton Creek fault, which crosses the northeastern part of the map area along a

northwesterly trend. This fault was mapped in the Eminence quadrangle adjoining to

the east (Orndorff and others, 1999) and is mentioned in a report on the copper

deposits of Shannon County (Bowles and Davidson, 1921). It extends southeastward

into the central part of the Eminence quadrangle and, like the Hartshorn fault, extends

for tens of miles to the northwest of the Alley Spring quadrangle. The Sutton Creek

fault is probably the same structure as the Oscars fault, mapped in the Raymondville

area by Satterfield (1976). In the Alley Spring quadrangle, the block on the southwest

side of the fault is down relative to the northeastern block, with vertical offsets ranging

from 20 to 40 ft. Geometric relations to adjoining faults indicate left-lateral movement

on the Sutton Creek fault.

Two smaller faults in the Alley Spring quadrangle, the Venice and Possum Trot

Hollow faults, are interpreted to be splays off the Sutton Creek fault. The Venice fault

diverges from the Sutton Creek fault in the northeastern part of the Alley Spring

quadrangle and terminates against the McCabe Hollow fault in the western part of the

Eminence quadrangle (Orndorff and others, 1999). The Venice fault has an apparent

vertical offset ranging from approximately 20 ft to about 140 ft, with the southwestern

block down relative to the northeastern block. Vertical offset of the Mesoproterozoic

basement rocks may be greater, implying pre-Gasconade movement along this fault.

This fault bounds the southwestern margin of the small outcrop area of the

Mesoproterozic rhyolite of Storys Creek (map unit Yscr). The Possum Trot Hollow

fault diverges from the Sutton Creek fault near the northern edge of the Alley Spring

quadrangle and trends toward the southeast, where it continues into the south-central

part of the Eminence quadrangle and terminates against the Shawnee Creek fault

(Orndorff and others, 1999). Vertical offsets on the Possum Trot Hollow fault range

from 20 to 90 ft, down on the southwestern side, along most of its trace.

A splay off the Possum Trot Hollow fault extends east-southeastward into the

Eminence quadrangle, where we interpret it to join the Venice fault. This fault, herein

named the Storys Creek fault, has a vertical offset of as much as 60 ft. The southern

block is down relative to the northern block east of Storys Creek, while the reverse is

true west of Storys Creek. The Venice, Possum Trot Hollow, and Storys Creek faults

were unrecognized at the time of mapping of the Eminence quadrangle (Orndorff and

others, 1999).

A relatively short, northeast-trending fault is mapped in the southeastern part of the

Alley Spring quadrangle and is herein named the Mahans Creek fault. This fault trace

runs between the Possum Trot Hollow fault to the north and the Hartshorn fault to the

south. The Mahans Creek fault separates blocks that are up to the northwest and

down to the southeast, with a relative vertical displacement across the fault ranging

from about 60 ft to 140 ft. The northeastern part of the northwestern block adjacent

to the Mahans Creek fault is warped into an anticline that exposes the Potosi Dolomite

in bluffs along the Jacks Fork River.

The Pyrtle Hollow fault crosses the northwest corner of the Alley Spring quadrangle

and offsets the northwestern block down as much as 150 ft relative to the

southeastern block. This fault extends to the northeast into the Round Spring

quadrangle, where it terminates against the southeast-trending Sutton Creek fault

(Orndorff and Weary, 2009). The Pyrtle Hollow fault probably extends to the

southwest into the Summersville NE 7.5-minute quadrangle, where stratigraphic offset

was recognized in earlier mapping and interpreted to be caused by a solutionally

let-down synform (Anderson and Wharton, 1975).

At the north-central edge of the map area the Jacks Fork Hollow fault, mapped in

the Round Spring quadrangle (Orndorff and Weary, 2009), is inferred to extend to the

southwest where it terminates against the Sutton Creek fault. Vertical offset is believed

to be as much as 120 ft, with the northwestern block up in relation to the

southeastern block.

ECONOMIC GEOLOGY

Generally low-quality and low-volume deposits of iron (Crane, 1910) and

manganese (Grawe, 1943) have been reported in the Alley Spring region. Gravel,

derived from alluvial stream bottoms, and fill earth, derived from residuum, are widely

available and are used locally for road-base and dam construction. Dolomite bedrock in

the quadrangle, especially the Gasconade Dolomite, could be quarried for road metal

or dimension stone.

HYDROLOGY

With the exception of the Jacks Fork River and its major tributaries, most water

flow in the Alley Spring quadrangle is subterranean, with surface flow occurring only at

times of abundant rainfall. Many small streams have short reaches where water flows

from springs or seeps and then is lost to subterranean drainage downslope. Locations

of springs that were encountered during fieldwork for this map are indicated by spring

symbols. No formal estimate of spring flow was made. As noted in the preceding karst

section, Alley Spring is a very large perennial spring and, with the exception of the

Jacks Fork River, is the most significant hydrologic feature in the quadrangle. Within

the map area, the Gasconade, Eminence, and Potosi Dolomites are the major

stratigraphic units that produce groundwater (Dreiss, 1983). Collectively, these units

are part of the regional Ozark aquifer that underlies the Salem Plateau (Imes and

Emmett, 1994).

REFERENCES CITED

Albertson, P.E., Mienert, Dennis, and Butler, Grant, 1995, Geomorphic evaluation of Fort

Leonard Wood: U.S. Army Corps of Engineers, Waterways Experiment Station, Technical

Report GL–95–19, 241 p.

Aley, T.J., and Aley, C., 1987, Groundwater study, Ozark National Scenic Riverways:

Protem, Mo., Ozark Underground Laboratory, National Park Service contract CX

6000-4-0083, 222 p.

Anderson, K.H., and Wharton, H.M., 1975, Reconnaissance geologic map of the

Summersville NE 7.5' quadrangle, Missouri: Missouri Department of Natural Resources,

Division of Geology and Land Survey, scale 1:24,000.

Ball, S.H., and Smith, A.F., 1903, The geology of Miller County: Missouri Geological Survey

[Reports], 2d Series, v. 1, 207 p.

Bickford, M.E., Harrower, K.L., Hoppe, W.J., Nelson, B.K., Nusbaum, R.L., and Thomas,

J.J., 1981, Rb-Sr and U-Pb geochronology and distribution of rock types in the

Precambrian basement of Missouri and Kansas: Geological Society of America Bulletin, v.

92. p. 323–341.

Bowles, J.H., and Davidson, L.E., 1921, The copper deposits of Shannon County, Missouri:

Rolla, Mo., Missouri School of Mines and Metallurgy, unpublished M.S. thesis.

Bretz, J.H., 1965, Geomorphic history of the Ozarks of Missouri: Missouri Geological Survey

and Water Resources [Reports], 2d Series, v. 41, 147 p.

Bridge, Josiah, 1930, Geology of the Eminence and Cardareva quadrangles: Missouri

Geological Survey [Reports], 2d Series, v. 24, 228 p., 3 pls. in pocket, scale 1:62,500.

Buehler, H.A., 1907, The lime and cement resources of Missouri: Missouri Geological Survey

[Reports], 2d Series, v. 6, 255 p., plate in pocket.

Clendenin, C.W., Niewendorp, C.A., and Lowell, G.R., 1989, Reinterpretation of faulting in

southeast Missouri: Geology, v. 17, p. 217–220.

Cooper, R.A., Nowlan, G.S., and Williams, S.H., 2001, Global stratotype section and point

for base of the Ordovician System: Episodes, v. 24, p. 19–31.

Cox, R.T., 1995, Intraplate deformation during the Appalachian-Ouachita orogeny as

recorded by mesoscale structures on the Ozark Plateau of North America: Columbia, Mo.,

University of Missouri-Columbia, unpublished Ph.D. dissertation, 229 p.

Crane, G.W., 1910, The iron ores of Missouri: Missouri Geological Survey [Reports], 2d

Series, v. 10, 434 p.

Dake, C.L., 1930, The geology of the Potosi and Edgehill quadrangles: Missouri Geological

Survey [Reports], 2d Series, v. 23, 233 p., 2 pls. in pocket, scale 1:62,500.

Davis, G.H., 1999, Structural geology of the Colorado Plateau region of southern Utah, with

special emphasis on deformation bands: Geological Society of America Special Paper 342,

157 p.

Dreiss, S.J., 1983, Linear unit-response functions as indicators of recharge areas for large

karst springs, in Back, William, and LaMoreaux, P.E., eds., V.T. Stringfield Symposium;

Processes in Karst Hydrology: Journal of Hydrology, v. 61, no. 1/3, p. 31–44.

Ebens, R.J., and Connor, J.J., 1980, Geochemistry of loess and carbonate residuum: U.S.

Geological Survey Professional Paper 954–G, 32 p.

Fenneman, N.M., 1938, Physiography of the eastern United States: New York, McGraw-Hill,

714 p.

Gott, J.D., 1975, Soil survey of Mark Twain National Forest area, Missouri (parts of Carter,

Oregon, Ripley, and Shannon Counties): U.S. Department of Agriculture, Forest Service

and Soil Conservation Service, in cooperation with Missouri Agricultural Experiment

Station, 56 p., 40 map sheets, scale 1:24,000.

Grawe, O.R., 1943, Manganese deposits of Missouri: Missouri Geological Survey and Water

Resources, 62d Biennial Report, Appendix 6, 77 p.

Harrison, R.W., and McDowell, R.C., 2003, Geologic map of the Wilderness and Handy

quadrangles, Oregon, Carter, and Ripley Counties, Missouri: U.S. Geological Survey

Geologic Investigations Series Map I–2801, scale 1:24,000.

Harrison, R.W., and Schultz, A.P., 2002, Tectonic framework of the southwestern margin of

the Illinois basin and its influence on neotectonism and seismicity: Seismological Research

Letters, v. 73, issue 75, p. 685–718.

Harrison, R.W., Lowell, G.R., and Unruh, D.M., 2000, Geology, geochemistry, and age of

Mesoproterozoic igneous rocks in the Eminence-Van Buren area; a major structural outlier

of St. Francois terrane, south-central Missouri [abs.]: Geological Society of America

Abstracts with Programs, v. 32, no. 3, p A–14.

Harrison, R.W., Orndorff, R.C., and Weary, D.J., 2002, Geology of the Stegall Mountain

7.5-minute quadrangle, Shannon and Carter Counties, south-central Missouri: U.S.

Geological Survey Geologic Investigations Series Map I–2767, scale 1:24,000.

Harrison, R.W., Orndorff, R.C., and Weems, R.E., 1996, Geology of the Fort Leonard Wood

Military Reservation and adjacent areas, south-central Missouri, with contributions on

Quaternary terraces by Paul E. Albertson, Dennis Mienert, and Grant Butler: U.S.

Geological Survey Open-File Report 96–60, 255 p., 10 pls. in pocket, scales vary.

Haynes, C.V., Jr., 1985, Mastodon-bearing springs and late Quaternary geochronology of the

lower Pomme de Terre Valley, Missouri: Geological Society of America Special Paper 204,

35 p.

Heller, R.L., 1954, Stratigraphy and paleontology of the Roubidoux Formation of Missouri:

Missouri Geological Survey and Water Resources [Reports], 2d Series, v. 35, 118 p., 2 pls.

in pocket.

Imes, J.L., and Emmett, L.F., 1994, Geohydrology of the Ozark Plateaus aquifer system in

parts of Missouri, Arkansas, Oklahoma, and Kansas: U.S. Geological Survey Professional

Paper 1414–D, 127 p.

Kisvarsanyi, E.B., 1975, Data on Precambrian in drill holes of Missouri including rock type

and surface configuration: Missouri Geological Survey Report of Investigations 56, 20 p.

Kleeschulte, M.J., and Seeger, C.M., 2000, Depositional environment, stratigraphy, and

vertical hydraulic conductivity of the St. Francois confining unit in the Fristoe Unit of the

Mark Twain National Forest, Missouri: U.S. Geological Survey Water-Resources

Investigations Report 00–4037, 65 p.

Lowell, G.R., Harrison, R.W., and Unruh, D.M., 2005, Contrasting rift-margin volcanism in

the St. Francois terrane of Missouri at 1.47 Ga [abs.], in Abstracts of the 15th Annual V.M.

Goldschmidt Conference: Geochimica et Cosmochimica Acta, v. 69, no. 10S, p. A-81.

Lowell, G.R., Harrison, R.W., Weary, D.J., Orndorff, R.C., Repetski, J.E., and Pierce, H.A.,

2010, Rift-related volcanism and karst geohydrology of the southern Ozark dome, in Evans,

K.R., and Aber, J.S., eds., From Precambrian rift volcanoes to the Mississippian shelf

margin; geological field excursions in the Ozark Mountains: Geological Society of America

Field Guide 17, p. 99–158. Digital Object Identifier: 10.1130/2010.0017(06)

Lyle, J.R., 1977, Petrography and carbonate diagenesis of the Bonneterre Formation in the

Viburnum Trend area, southeast Missouri: Economic Geology, v. 72, no. 3, p. 420–434.

McDowell, R.C., 1998, Geologic map of the Greer quadrangle, Oregon County, Missouri:

U.S. Geological Survey Geologic Investigations Series Map I–2618, scale 1;24,000.

McDowell, R.C., and Harrison, R.W., 2000, Geologic map of the Powder Mill Ferry

quadrangle, Shannon and Reynolds Counties, Missouri: U.S. Geological Survey Geologic

Investigations Series Map I–2722, scale 1:24,000.

Missouri Geological Survey, 1979, Geologic map of Missouri: Rolla, Mo., Missouri Division of

Geology and Land Survey, scale 1:500,000.

Nason, F.L., 1892, A report on the iron ores of Missouri: Missouri Geological Survey

[Report], v. 2, 366 p.

Orndorff, R.C., and Harrison, R.W., 2001, Geologic map of the Winona quadrangle,

Shannon County, Missouri: U.S. Geological Survey Geologic Investigations Series Map

I–2749, scale 1;24,000.

Orndorff, R.C., and Weary, D.J., 2009, Geologic map of the Round Spring quadrangle,

Shannon County, Missouri: U.S. Geological Survey Scientific Investigations Map 3073,

scale 1;24,000.

Orndorff, R.C., Harrison, R.W., and Weary, D.J., 1999, Geologic map of the Eminence

quadrangle, Shannon County, Missouri: U.S. Geological Survey Geologic Investigations

Series Map I–2653, scale 1;24,000.

Orndorff, R.C., Weary, D.J., and Harrison, R.W., 2006, The role of sandstone in the

development of an Ozark karst system, south-central Missouri, in Harmon, R.S., and Wicks,

C.M., eds., Perspectives on karst geomorphology, hydrology, and geochemistry; a tribute

volume to Derek C. Ford and William B. White: Geological Society of America Special

Paper 404, p. 31–38.

Pierce, H.A., and Weary, D.J., 2009, Interpretation of the hydrogeology of a large karst

spring in the Ozark National Scenic Riverways, Missouri, from audio-magnetotelluric (AMT)

soundings and geologic mapping [abs.]: Geological Society of America Abstracts with

Programs, v. 41, no. 7, p. 246.

Pratt, W.P., Middendorf, M.A., Satterfield, I.R., and Gerdemann, P.E., 1992, Geologic map

of the Rolla 1° x 2° quadrangle, Missouri: U.S. Geological Survey Miscellaneous

Investigations Series Map I–1998, scale 1:250,000.

Repetski, J.E., Orndorff, R.C., Weary, D.J., and Ethington, R.L., 2000, Conodont

biostratigraphy of the Eminence Dolomite–Gasconade Dolomite contact interval in the

Missouri Ozarks [abs.]: Geological Society of America Abstracts with Programs, v. 32, no. 3,

p. A-39–A-40.

Satterfield, I.R., 1976, Reconnaissance geologic map of the Raymondville 15' quadrangle,

Missouri: Missouri Department of Natural Resources, Division of Geology and Land Survey,

scale 1:62,500.

Thacker, J.L., and Anderson, K.H., 1977, The geologic setting of the southeast Missouri lead

district; regional geologic history, structure and stratigraphy: Economic Geology and the

Bulletin of the Society of Economic Geologists, v. 72, no. 3, p. 339–348.

Thompson, T.L., 1995, The stratigraphic succession in Missouri: Missouri Department of

Natural Resources, Division of Geology and Land Survey, v. 40, 2d Series, revised, 190 p.

Ulrich, E.O., 1911, Revision of the Paleozoic systems: Geological Society of America

Bulletin, v. 22, no. 3, p. 281–680 (especially see p. 630–631).

Van Schmus, W.R., Bickford, M.E., Anderson, J.L., Bender, E.E., Anderson, R.R., Bauer,

P.W., Robertson, J.M., Bowring, S.A., Condie, K.C., Denison, R.E., Gilbert, M.C.,

Grambling, J.A., Mawer, C.K., Shearer, C.K., Hinze, W.J., Karlsstrom, K.E., Kisvarsanyi,

E.B., Lidiak, E.G., Reed, J.C., Jr., Sims, P.K., Tweto, Ogden, Silver, L.T., Treves, S.B.,

Williams, M.L., and Wooden, J.L., 1993, Transcontinental Proterozoic provinces, in Reed,

J.C., Jr., Bickford, M.E., Houston, R.S., Link, P.K., Rankin, D.W., Sims, P.K., and Van

Schmus, W.R., eds., Precambrian; Conterminous U.S., v. C-2 of The geology of North

America: Boulder, Colo., Geological Society of America, p. 171–334.

Vineyard, J.D., and Feder, G.L., 1982, Springs of Missouri (rev. ed.): Missouri Department of

Natural Resources, Division of Geology and Land Survey, Water Resources Report 29,

212 p.

Weary, D.J., and Schindler, J.S., 2004, Geologic map of the Van Buren South quadrangle,

Carter County, Missouri: U.S. Geological Survey Geologic Investigations Series Map

I–2803, scale 1:24,000.

Weems, R.E., 2002, Geologic map of the Low Wassie quadrangle, Oregon and Shannon

Counties, Missouri: U.S. Geological Survey Geologic Investigations Series Map I–2719,

scale 1:24,000.

Winslow, Arthur, 1894, Lead and zinc deposits, part 1: Missouri Geological Survey [Report],

v. 6, 387 p.

Qa

Qm

QTrr

Qt

Or

Og

|e

|p

|dd

|d

|b

|l

Yscr

Yu

DESCRIPTION OF MAP UNITS

Alluvium (Holocene)—Gravel, sand, and clay along the bed and active

flood plains of modern stream valleys. Material consists mostly of

subrounded to subangular chert, sandstone, and quartzite clasts in a

matrix of sand containing silt and clay. Thickness ranges from 0 to as

much as 20 ft

Muck and clayey silt (Holocene and Pleistocene?)—Silt, reworked

loess, clay, and organic-rich muck filling sinkholes that intermittently

retain water. Thickness unknown

Terrace deposits (Holocene and Pleistocene?)—Large cobble-size to

sand-size, subrounded to subangular chert, sandstone, and quartzite

clasts within a matrix of sand, silt, and clay. In some locations a

significant fraction of the matrix is silt-size reworked loess. Deposited

on relatively flat areas along floors and walls of modern stream valleys,

but above normal seasonal floods of present streams. Terraces along

the Jacks Fork River and major streams usually are at least 6 to 10 ft

above stream level. Thickness at least 10 ft and possibly more along

larger stream courses

Residuum derived from the Roubidoux Formation (Quaternary and

Tertiary)—Sandstone and cherty sandstone slabs, and sandy chert

boulders and cobbles with sandy soils. Commonly occurs as boulder or

slab piles on hilltops, minor knobs, and ridgecrests where carbonates

from the Roubidoux Formation and some carbonates from the upper

part of the underlying Gasconade Dolomite have been removed by

nearly in-place weathering. These deposits also occur as colluvium on

steep hillsides. Unit mapped only where it overlies the Gasconade

Dolomite, because this residual material is ubiquitous in areas underlain

by the Roubidoux Formation. Contact with Gasconade Dolomite is

placed where identifiable clasts derived from the Gasconade become

part of the residuum, or where residuum thickness approaches zero.

Unit thickness may exceed 40 ft on some knobs and tapers down to

zero on flanking hillsides

Roubidoux Formation (Lower Ordovician)—Dolomite, cherty dolomite,

sandy dolomite, dolomitic sandstone, quartz sandstone, orthoquartzite,

and bedded chert. Dolomite is very light gray to medium-gray,

pale-orange, or pinkish-gray, very fine grained to medium-grained,

thin-bedded to laminated, evenly to irregularly bedded; interbedded

with quartz sandstone, white to pale-orange where fresh, weathering

light-brown or reddish-brown, very fine grained to coarse-grained,

poorly sorted, thin-bedded to thick-bedded, commonly crossbedded or

ripple-marked; well-rounded quartz grains commonly have angular

overgrowths; most beds are dolomite-cemented, some silica-cemented.

Dolomite locally replaced by white, gray, or brown chert, frequently

stromatolitic and occasionally oolitic or fossiliferous. Lower and upper

parts of unit are sandstone-rich; middle part chiefly dolomite. Fossils

rare except for occasional impressions of the snail Lecanospira.

Bedding surfaces of sandstone beds commonly contain ripple marks

formed by currents. Lower contact of unit mapped at lowest

orthoquartzite or sandstone bed encountered above the Gasconade

Dolomite. Up to about 140 ft of the Roubidoux Formation is exposed

in the Alley Spring quadrangle. The uppermost part of the formation is

missing and truncated throughout the quadrangle by erosion

Gasconade Dolomite (Lower Ordovician)—Dolomite, chert, sandstone,

and orthoquartzite. Formation is divided into three informal units (fig.

3), not shown separately on this map. Dolomite of upper unit,

light-gray, medium-grained to coarse-grained, thick-bedded, vuggy;

weathers to a pitted surface. Dolomite of middle unit, light-gray,

fine-grained to coarse-grained, medium-bedded to thick-bedded, with

white to light-gray chert nodules and lenses. Uppermost bed of the

middle unit is a persistent white Cryptozoon chert, 2 to 10 ft thick.

Basal 10 to 20 ft of the Gasconade is the Gunter Sandstone Member,

the lower unit of the formation. The Gunter typically comprises

light-gray to white sandstone, sandy dolomite, or orthoquartzite

interbedded with light-gray to tan, fine-grained, thin-bedded dolomite.

The Gasconade Dolomite is about 300 ft thick in the Alley Spring

quadrangle. Contact with the underlying Eminence Dolomite is placed

at base of lowest sandstone or sandy dolomite of the Gunter Sandstone

Member

Eminence Dolomite (Upper Cambrian)—Dolomite, light-gray

occasionally mottled red, pink, or green, medium-grained to

coarse-grained, medium-bedded to thick-bedded, commonly massive;

minor amounts of chert, locally oolitic or drusy, in nodules and angular

fragments, mostly in upper half of formation. The Eminence is about

200 ft thick in outcrops exposed in the quadrangle, but reaches a total

thickness of as much as 380 ft in drill holes (table 2)

Potosi Dolomite (Upper Cambrian)—Dolomite, light-brownish-gray to

pale-yellowish-brown, fine-grained to medium-grained, thick-bedded,

vuggy and porous, with irregular blebs of silica and quartz druse; locally

stromatolitic; bituminous odor typical of freshly broken rock. About 65

ft of Potosi is exposed in the quadrangle. Total thickness in the

quadrangle is probably variable, ranging between about 220 and 500 ft

Derby-Doerun Dolomite (usage of Missouri Geological Survey,

1979) (Upper Cambrian)—Shown in cross section only. Dolomite,

siltstone, and shale. Dolomite, buff to brown or light-gray, fine-grained

to medium-grained, thin-bedded to medium-bedded, argillaceous, silty,

with minor amounts of chert and sparse sulfide minerals; vuggy and

porous. Siltstone and shale, thin-bedded, interbedded with dolomite.

The Derby-Doerun is estimated to be about 200 ft thick in the Alley

Spring quadrangle

Davis Formation (Upper Cambrian)—Shown in cross section only.

Siltstone, sandstone, dolomite, shale, and limestone conglomerate.

Shale, dark-green, fissile, thin-bedded to thick-bedded; composes as

much as 50 percent of formation. Limestone, light-gray, fine-grained

to cryptograined, dense, locally glauconitic. The Davis contains

intraformational sedimentary breccias and grainstones. Unit is probably

about 175 ft thick in the Alley Spring quadrangle

Bonneterre Formation (Upper Cambrian)—Shown in cross section

only. Dolomite, limestone, siltstone, and shale. Dolomite, light-gray,

fine-grained to medium-grained, medium-bedded; commonly contains

algal structures; locally glauconitic and shaly. Limestone, brownish-gray

to pink, fine-grained, thin-bedded, fossiliferous, locally oolitic; more

common in lower part of formation. Siltstone, quartzose, light-gray to

dark-gray, laminated. Shale, dark-green, thin-bedded; occurs as sparse

thin interbeds and partings. The Bonneterre is a major host rock of

Mississippi Valley-type base-metal deposits (lead, zinc) in the Ozark

region. Unit is probably about 390 ft thick in the Alley Spring

quadrangle

Lamotte Sandstone (Upper Cambrian)—Shown in cross section only.

Sandstone, quartzose, light-gray, yellow, brown, or red, dominantly

medium-grained, moderately sorted to well sorted, well indurated;

locally contains interbeds of red to purple silty shale and, in upper part,

scattered lenses of arenaceous dolomite. Felsite pebble or boulder

conglomerate is commonly present at base. Thickness is variable,

estimated to range from 0 to 275 ft based on drill-hole data from

nearby quadrangles

Rhyolite of Storys Creek (Mesoproterozoic)—Rhyolite with

aphanophyric texture; contains about 3 to 5 percent phenocrysts of

alkali feldspar. Matrix is medium-gray to medium-dark-gray or

brownish-gray; phenocrysts are pale-pink to pale-reddish-purple.

Pale-pink flow bands in the matrix are also present locally

Igneous rocks, undifferentiated (Mesoproterozoic)—Shown in cross

section only. Probably rhyolite or granite. No drill holes reached

basement rocks in the Alley Spring quadrangle

Qa

Qt Qm

|e

Og

Or

|p

|dd

|d

|b

|l

Yscr Yu

QTrr

ORDOVICIAN

TERTIARY

CAMBRIAN

MESOPROTEROZOIC

CORRELATION OF MAP UNITS

These units except Yscr

are in cross section only

Lower

Ordovician

Holocene and

Pleistocene

Upper

Cambrian

QUATERNARY

Unconformity

Drill hole—Number refers to well records from Missouri Department of Natural

 Resources; see table 2 for summary of well data

Cataclastic deformation bands in sandstone float

Spring

Widely spaced (>6 ft)

Closely spaced (<2 ft)

Moderately spaced (2–6 ft)

Moderately spaced (2–6 ft)

Horizontal

Inclined 5

80

80

Strike and dip of inclined flow-layered foliation in Mesoproterozoic rock 48

77

x

27566

Sinkhole

Structure contour—Drawn on base of Roubidoux Formation; projected where above

 land surface. Contour interval 20 ft

1240

Contact—Approximately located; dotted where concealed

Fault—Long-dashed where approximately located; short-dashed where inferred;

dotted where concealed. Bar and ball on downthrown side; paired arrows indicate

strike-slip displacement; in cross section, paired arrows indicate relative direction of

vertical movement. Faults exposed in mines in the nearby quadrangles have strikeslip displacement; however, except for the Sutton Creek fault, strike-slip movements

 are not shown on this map because they cannot be documented in surface exposures

Strike and dip of beds

Strike and dip of joints—Apertures are narrow (<0.5 in.), except where noted by

 w (solutionally widened)

Non-throughgoing, inclined

Widely spaced (>6 ft)

Closely spaced (<2 ft)

Moderately spaced (2–6 ft)

Non-throughgoing, vertical

Widely spaced (>6 ft)

Throughgoing, vertical

PLANAR FEATURES

OTHER FEATURES

EXPLANATION OF MAP SYMBOLS

Cryptozoon-bearing chert bed of Gasconade Dolomite—Locality where chert

bed was observed in outcrop or in highest float

Sample locality for geochemistry of Mesoproterzoic rocks—See table 1 for analysis Yst-58

[Point of observation is at intersection of multiple symbols]

Figure 5. —Audio-magnetotelluric resistivity section B–B' across the Hartshorn fault. From Lowell

and others (2010). Fault location indicated by dashed vertical line. Red triangles indicate location

of sounding stations. Warmer colors (red) indicate higher apparent resistivity. Cooler colors (blue)

 indicate lower apparent resistivity and may indicate saturation by ionic water along the fault.

 Vertical exaggeration x5. See geologic map for location of section.

Hartshorn fault

B Ohm meters

Distance (meters)

Horizontal scale 1:24,000

Elevation

(meters above sea level)

400

100,000

59,948

35,938

21,544

12,916

7743

4642

2783

1668

1000

599

359

215

129

77

46

28

17

10

300

ASP012

ASP011

ASP025

ASP024

ASP027

ASP028

ASP026

ASP021

200

100

0

400

300

200

100

10000 2000 3000

0

46

B'

Figure 4.—Compass-rose diagram showing the dominant joint trends in the Paleozoic

rocks of the Alley Spring quadrangle. Joint sets were statistically weighted by counting

closely spaced joint sets three times, moderately spaced sets two times, and widely

spaced sets one time. This system approximates the importance of each set relative to

the volume of the rocks. Interval is 15 degrees. Numbers on diagram are percent of

total. n, number of joints.

n = 679

W

N

E

S

10

12

8

6

4

Figure 3.—Columnar section of outcropping Paleozoic formations

in the Alley Spring quadrangle showing informal units of the

 Gasconade Dolomite. Abbreviation used: Ss., sandstone.

100 FEET

Eminence

Dolomite

(|e)

(|p)

Sandstone and orthoquartzite;

 locally dolomitic

Dolomite

Bedded chert

Thin oolite bed (<1 ft)

Cherty dolomite

Sandy dolomite

EXPLANATION

Gasconade Dolomite (Og)

LOWER ORDOVICIAN

upper unit lower unit middle unit

Gunter Ss.

Member

lower part

middle part

upper part

Roubidoux

Formation

(Or)

UPPER CAMBRIAN

Potosi Dolomite

Figure 2.—The rise pool of Alley Spring. The spring rises from thick beds in the

lower part of the Gasconade Dolomite, visible in the background. Alley Spring has

an average discharge of 81 million gallons per day (125 cubic feet per second or

 3.5 cubic meters per second) (Vineyard and Feder, 1982). Most of the outflow is to

 the right of this view. The spring is in E 1/2 sec. 25, T. 29 N., R. 5 W.

Figure 1.—Map showing location of the Alley Spring 7.5-minute quadrangle and the

geomorphic subdivisions of Missouri. The St. Francois Mountains region is underlain mostly by

Cambrian and Mesoproterozoic rocks; the Salem Plateau by Cambrian and Ordovician rocks;

the Springfield Plateau by Mississippian rocks; the Central Lowland province by Ordovician to

Pennsylvanian rocks and Quaternary periglacial sediments; and the Mississippi embayment by

Cretaceous to Quaternary sediments (adapted from Imes and Emmett, 1994).

River Missouri

95° 90°

40°

36°

Central Lowland

Salem Plateau

Mississippi

embayment

Springfield

Plateau

St. Francois

Mountains

Alley Spring

Quadrangle

0 50 Miles

OZARK

PLATEAUS

Viburnum Trend

Table 2.—Summary of drill-hole data in the Alley Spring quadrangle, Missouri.

[Or, Roubidoux Formation; Og, Gasconade Dolomite; |e, Eminence Dolomite; |p, Potosi

Dolomite; |dd, Derby-Doerun Dolomite. Datum is mean sea level]

Drill-hole Collar Contact Bottom

number Location elevation, elevation, elevation,

in feet in feet in feet

Drill-hole data on file with Missouri Department of Natural Resources,

Geological Survey and Resource Assessment Division

18092 SE1/4 NE1/4 sec. 31 1038 898 Or–Og 635

T. 29 N., R. 4 W.

21242 SE1/4 NE1/4 sec. 20 770 595 Og–|e

T. 29 N., R. 4 W. 305 |e–|p

 45 |p–|dd -5

21662 NE1/4 NE1/4 sec. 21 760 675 Og–|e1

T. 29 N., R. 4 W. 295 |e–|p 155

26662 SW1/4 NE1/4 sec. 36 750 605 Or–Og2 100

T. 29 N., R. 5 W.

28085 SE1/4 NW1/4 sec. 4 720 645 Og–|e

T. 28 N., R. 4 W. 420 |e–|p 120

28153 SE1/4 NE1/4 sec. 5 820 520 Og–|e

T. 28 N., R. 4 W. 270 |e–|p 60

28154 SE1/4 NE1/4 sec. 5 700 550 Og–|e

T. 28 N., R. 4 W. 270 |e–|p 190

1 Field mapping indicates this well starts below the Og–|e contact. The contact as shown

in this table may actually represent the base of the sandstone-rich interval in the Eminence

Dolomite reported in areas to the east of the Alley Spring quadrangle (Bridge, 1930; Weary

and Schindler, 2004).

2

Field mapping indicates this well starts in the lower part of the Gasconade Dolomite.

This contact may actually be the Og–|e contact.

Sample Location UTM North UTM East Unit SiO2 Al2O3 Fe2O3 MgO MnO CaO TiO2 Na2O K2O P2O5 LOI Total

 number

Yst-58 sec. 15, 643985 4117056 Yscr 76.38 12.32 2.71 0.10 0.05 0.48 0.23 6.71 2.17 0.03 0.35 101.53

T. 29 N., R. 4 W.

Table 1.—Major-oxide analysis of a representative Mesoproterozoic volcanic rock (map unit Yscr) in the Alley Spring quadrangle, Missouri.

[Sample locality shown on map. Data in weight percent. Whole-rock geochemistry using fusion-inductively coupled plasma emission spectrometry with 0.01

percent detection limits provided by Actlabs, Inc., Wheat Ridge, Colo. LOI, Loss on ignition] A A'

Surficial deposits not shown NO VERTICAL EXAGGERATION

400

400

800

800

1200

1200

SEA LEVEL

400

400

800

800

1200

1200

SEA LEVEL

JACKS FORK RIVER

POSSUM TROT HOLLOW FAULTSTORYS CREEK FAULTStorys Creek

HARTSHORN FAULT

Possum Trot Hollow

Drewell HollowMissouri Route 106McCormack Hollow

VENICE FAULT

Graveyard Hollow

DRILL HOLE 21242 (projected from 1,575 ft to NW)

DRILL HOLE 21662 (projected from 2,550 ft to SE)

FEET FEET

Or Og Or Or

Og

|p

|e

|dd

|d

|b

Yu

|l

Og

|p

|e

|dd

|d

|b

Yu

|l

Og Og

|p

|e

|dd

|d

|b

Yu

|l

|p

|e |p

|e

|dd

|d

|b

Yu

Yscr

Yu? Yscr?

|l

980

960

1100

1040

1020

1000

960

980

920

860

980

960

960

960

940

920

1000

1020

900

920

940

960

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1040

1160 1140

1120

880

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1220

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1160

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1120

1080

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1160

1000

960

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1020

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1000

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#

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Yst-58

x

18092

26662

21242

21662

28085 28154

28153

JACKS

FORK HOLLOW FAULT

SUTTON CREEK

FAULT

VENICE

 FAULT

POSSUM

TROT

HOLLOW

FAULT

MAHANS

CREEK

FAULT

HARTSHORN

 FAULT

STORYS

POSSUM

TROT

HOLLOW

CREEK

FAULT

FAULT

PYRTLE HOLLOW FAULT

4

9

3

15

4

4

3

3 9

6

3

3

10

3

4

8 13

9

80

80

3

3 3 3

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w

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4 3

6

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4

w

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5

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48

57

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4 9 7

5 3 3

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4 4

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10 6

9

3

7

15

10

w

3

4

3

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w

3

3

3

4 3 3

4

3

3 4

4

3

3

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15

17

5

4

3

3

3

3

6

3

77

A

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Or

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Og

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|p

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Yscr

QTrr

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|e

|p |p

|e |e

QTrr

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