DISCUSSION

Introduction

The Montauk 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 smaller portion of 

the land within the quadrangle is publicly owned by either Montauk State Park or the 

Ozark National Scenic Riverways (National Park Service). The map area has 

topographic relief of about 490 feet (ft), with elevations ranging from approximately 

830 ft where the Current River exits the middle-eastern edge of the quadrangle to 

about 1,320 ft in sec. 16, T. 31 N., R. 7 W., in the southwestern part of the 

quadrangle. The most prominent physiographic features within the quadrangle are the 

deeply incised valleys of the Current River and its major tributaries located in the 

center of the map area. The Montauk quadrangle is named for Montauk Springs, a 

cluster of several springs that resurge in sec. 22, T. 32 N., R. 7 W. These springs 

supply clean, cold water for the Montauk Fish Hatchery, and the addition of their flow 

to that of Pigeon Creek produces the headwaters of the Current River, the centerpiece 

of the Ozark National Scenic Riverways park.

STRATIGRAPHY

Mesoproterozoic Basement Rocks

Unexposed basement rocks underlying the quadrangle are part of a 

Mesoproterozoic granite or rhyolite terrane that is extensively distributed in the 

Midcontinent region. Outcrops of this terrane occur in the Alley Spring, Big Spring, 

Eminence, Powder Mill Ferry, Stegall Mountain, Winona, and Van Buren North 

quadrangles to the southeast and in the Saint Francois Mountains, 55 miles (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 the Neoproterozoic. Although the regional 

relief on this buried erosional surface has been reported to be as much as 2,000 ft 

(Kisvarsanyi, 1975), it is much less in the Montauk quadrangle.

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

southeast yielded radiometric ages of 1,461.8±5.5 Ma for granite exposed in the Big 

Spring quadrangle (Harrison and others, 2000), 1,480±42 Ma for granite obtained 

from a corehole in the Winona quadrangle (Van Schmus and others, 1993), and 

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

the southeast (Bickford and others, 1981). A U-Pb age of 1,470.4±2.7 Ma was also 

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

to the southeast (Harrison and others, 2000).

Paleozoic Stratigraphy

The Montauk quadrangle is underlain by about 2,000 ft of lower Paleozoic marine 

strata, chiefly comprising 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 three units are exposed 

in the map area. The idealized stratigraphic column (fig. 2) summarizes the general 

lithologies and thickness of the Paleozoic strata that are exposed in the Montauk 

quadrangle. No drill holes in the quadrangle penetrated below the Derby-Doerun 

Dolomite; therefore, the actual thicknesses of the lowest four Paleozoic formations 

listed here are unknown. Thicknesses listed here for units below the Potosi Dolomite, 

and as shown on the cross section (A–A'), are estimates based on the averages of 

thicknesses reported in two relatively nearby mineral exploration drill holes. The first 

drill hole, number 21956 (refer to the Missouri Department of Resources, Division of 

Geology and Land Survey, well database at http://www.dnr.mo.gov/env/wrc/ 

logmain/index.html), is located in the Round Spring quadrangle (Orndorff and Weary, 

2009) about 11.5 mi to the east-southeast of the southeastern corner of the Montauk 

quadrangle. The second drill hole, number 25824, is located in the Prescott 

quadrangle about 12.5 mi west of the Montauk quadrangle. 

During the Late Cambrian, 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 and probably overlaps them in the quadrangle. The 

thickness of the Lamotte is variable, but averages about 145 ft in the Montauk 

quadrangle. 

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). The Bonneterre is estimated to be about 315 

ft thick in the Montauk quadrangle.

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 Montauk 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- to medium-bedded dolomite 

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

be 125 ft thick in the Montauk quadrangle.

The Derby-Doerun is overlain by the Upper Cambrian Potosi Dolomite. 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), who 

reviewed descriptions of the Potosi, revised and redefined the formation into its 

current usage. The Potosi is a vuggy, fine- to medium-grained, light-brownish-gray to 

pale-yellowish-brown dolomite having siliceous druse as a replacement of the dolomite 

or as vug linings. A fetid odor emanating from freshly broken surfaces is a minor yet 

diagnostic characteristic indicative of trace organic material. With the exception of 

algal structures (stromatolites), the Potosi is characteristically unfossiliferous. A single 

drill hole (17427) penetrated the full thickness of the Potosi in the map area and 

documented a thickness of 325 ft (table 1). The lithologic difference between the 

Potosi and the overlying Eminence Dolomite 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. 

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- to coarse-grained, light-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 algal laminae. Other beds may be 

crossbedded. Exposures of 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. As 

much as 40 ft of the Eminence Dolomite is exposed along Big Creek in the 

southeastern part of the map area and along the Current River in the east-central part 

of the map area. These are the farthest upstream exposures of the Eminence 

Dolomite along the Current River. Drill holes record an average of about 250 ft of 

Eminence Dolomite in the subsurface of the Montauk quadrangle.

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

Caves and springs typically are concentrated just below the Gunter Sandstone 

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

Springs probably rises from conduits developed in the Eminence, the top of which is 

estimated to be only about 10 to 20 ft below the surface at that locality (Lowell and 

others, 2010).

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. 2). 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 in central Missouri. The 

Gasconade Dolomite is divided into one formal and two informal units. The lower, 

formal 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. 

The Gunter Sandstone Member was named by Ball and Smith (1903) for exposures 

along the Niangua River at Gunter (now Hahatonka Springs), Camden County, in 

central Missouri. In the map area, the member comprises the basal 10 to 20 ft of the 

Gasconade Dolomite. It consists of interbedded dolomite, sandstone, orthoquartzite, 

and sandy dolomite. The quartz sandstone content of the Gunter varies within the 

Montauk quadrangle, but on average, it is less than in many other areas to the 

southeast. Quartz sandstone and (or) orthoquartzite compose up to about 20 percent 

of the interval in some places and as little as zero percent in others. In areas, where 

quartz sandstone or orthoquartzite are absent from the stratigraphic interval, it is 

instead occupied by dolomite or sandy dolomite containing floating quartz grains. 

Where exposed in the Montauk quadrangle, the contact between the basal Gunter 

Sandstone Member and the underlying Eminence Dolomite commonly exhibits an 

irregular surface overlain with discontinuous lenses of white quartz sandstone and (or) 

orthoquartzite up to 2 ft thick (fig. 3). These lenses are rarely seen in-situ but 

commonly weather out of the surrounding dolomite and produce belts of float marking 

the approximate location of the Gasconade-Eminence contact (fig. 4).

Bridge (1930) named the Van Buren Formation for the medium-bedded, non-cherty 

and cherty dolomite overlying the Gunter Sandstone Member and extending upward 

to a thin oolite bed in the middle part of the middle unit of the Gasconade Dolomite 

(fig. 2). 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. 2). The Cryptozoon

chert typically comprises parallel columns, a few inches wide, of convex-upward algal 

laminae. The chert bed weathers into distinctive polygonal boulders (fig. 5). Triangular 

symbols on the map indicate locations where the Cryptozoon chert was observed in 

outcrop or as float.

The upper unit comprises thick-bedded dolomite that is conspicuously devoid of 

chert. Where not buried in residuum from the overlying Roubidoux Formation, the 

upper unit tends to form dolomite glades on south- and west-facing slopes.

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).

Similar to the Eminence Dolomite, the upper unit of the Gasconade Dolomite is 

one of the main cave-bearing intervals in the Montauk quadrangle. Caves and springs 

typically are concentrated just below the overlying sandstones of the Roubidoux 

Formation (Orndorff and others, 2006).

The total thickness of the Gasconade Dolomite in the Montauk quadrangle ranges 

from about 330 to 495 ft. The lesser thickness is based on outcrop and well data from 

areas in the eastern part of the quadrangle and northeast of the Big Creek fault. 

Thickness data from a single well (number 17427; table 1) combined with overlying 

local outcrop thickness indicates the greater thickness of 495 ft. In cross section A–A'

it was assumed that this greater thickness occurs consistently in areas to the southwest 

of the Big Creek fault, implying that vertical offset along this fault in the Early 

Ordovician accommodated deposition of a thicker Gasconade sequence on the 

southwest side.

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 Roubidoux is divided into a lower, middle, and upper part. The lower part, 20 to 

30 ft thick, is sandstone rich and commonly produces a topographic bench that sheds 

sandstone float onto the slopes below. The middle part is mainly 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 chert and sandstone. 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 an early Early Ordovician age (early to 

middle Ibexian). 

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

removed by erosion. A thickness up to about 220 ft of the Roubidoux is exposed in 

the southwestern part of the quadrangle. The Roubidoux is reported to be as thick as 

250 ft in other parts of Missouri (Thompson, 1995).

On the basis of new information, mapping of the Roubidoux Formation and the 

Gasconade Dolomite along the eastern edge of the Montauk quadrangle does not 

necessarily match that along the western edge of the previously published Cedargrove 

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) derived 

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

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

were mapped as geomorphic features. They occur along the sides of the stream valley 

bottoms, standing at higher elevations than the present-day flood plain. Along the 

Current River and its major tributaries, surficial terrace deposits occur at several 

discrete levels. Detailed differentiation and mapping of these terraces across the map 

area was beyond the scope of the present project. Therefore, all terrace deposits were 

mapped as a single unit (Qt).

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

(Ebens and Connor, 1980). Nevertheless, the Montauk region includes areas of soils 

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

mappable geologic unit because (1) they are thin and (2) they are intermingled with 

underlying clayey residuum due to bioturbation. In the Montauk quadrangle small areas 

of loess soils were observed to occur in saddles between hills and on flat uplands. 

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

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

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

stony residual soil 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, thus producing a thin mantle of 

colluvium along most valley walls. Because this veneer typically is nearly ubiquitous but 

thin, typically less than 5 ft, it is not mapped as a separate unit.

Karst

Karst features in the Montauk quadrangle include caves, springs, sinkholes, 

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

carbonate bedrock surfaces. Although caves can be found anywhere in the 

stratigraphic section they tend to be concentrated at particular stratigraphic horizons. 

Two stratigraphic horizons where caves tend to occur include (1) the upper part of the 

Eminence Dolomite, just beneath the Gunter Sandstone Member of the overlying 

Gasconade Dolomite, and (2) the upper part of the Gasconade Dolomite, near the 

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

others, 2010). Few cave entrances are exposed in the upper part of the Eminence 

Dolomite in the Montauk quadrangle, since only a few tens of feet of that formation 

crop out above the levels of the Current River and Big Creek. As previously 

mentioned, Montauk Springs probably rises from caves or karst conduits in the upper 

part of the Eminence Dolomite. Sinkholes are relatively sparse in the Montauk 

quadrangle. 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. Sinkholes observed during the course of field studies for 

this area, as well as those 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.

STRUCTURAL GEOLOGY

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

vertical jointing and strike-slip faulting having predominant northeast and northwest 

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

Montauk 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 measurement in the quadrangle, 

14° to the southwest, occurs in rocks exposed along the southeast bank of the Current 

River in sec. 6, T. 31 N., R. 6 W. This relatively high dip value is probably associated 

with deformation of the rocks located near the terminus of the Bell Hollow fault. 

Several broad open folds and most of the higher dip values measured in the 

quadrangle also occur in that vicinity. Small folds were observed only along the valleys 

of the Current River and Pidgeon Creek, where there is sufficient bedrock exposure to 

see the fold limbs. The locations of these folds are marked with minor-fold symbols on 

the geologic map.

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 joint planes in a master joint set. Joints that 

are confined to individual beds are classified as non-throughgoing. Joints extending 

across bedding planes into underlying and (or) overlying beds are classified as 

throughgoing. Most joints in the carbonate rocks are open (containing no mineralized 

infilling) and have narrow apertures. Some joints, however, have been solutionally 

widened by as much as 10 inches (in.). Cumulative orientations of all the joint 

measurements observed in outcrop are shown in figure 6. The two primary joint sets, 

one with a north-northwest trend and the other with an east-northeast trend, 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. 

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. Most of these 

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

fault movement. Shears were observed in-place in a massive sandstone outcrop in the 

Roubidoux Formation near the intersection of State roads VV and V in sec. 27, T. 32 

N., R. 7 W., in the central part of the quadrangle. One set of these shears had a 

measurable trend of 300° and dip of 77° to the northeast. This trend is approximately 

parallel to the enclosing traces of the Gladden Creek fault to the north and the Big 

Creek fault to the south.

Faults in the Montauk quadrangle are difficult to observe directly, in part due to 

vegetation cover and in part due to residuum cover, and have to be inferred on the 

basis of indirect evidence. This evidence includes (1) vertical offset of strata in which 

the observed dip angle is insufficient to explain the offset for a given distance (vertical 

stratigraphic offset along faults may reverse as horizontal slip offsets gentle folds along 

opposing sides of the fault; this is referred to as scissor faulting), and (2) cataclastic 

deformation bands observed in float blocks of sandstone. 

Although only inferred 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 amount of horizontal slip on faults within the Montauk quadrangle is 

unknown.

The southeast- to northwest-trending Big Creek fault traverses the southern part of 

the quadrangle. This fault is named for Big Creek in the southeastern corner of the 

quadrangle, where strata have noticeable vertical offset. Cataclastic deformation bands 

were observed in float blocks along the trend of this fault in secs. 29 and 33, T. 32 N., 

R. 7 W. This fault probably continues to the northwest into the Licking quadrangle. 

Likewise, the Big Creek fault continues to the southeast across the southwestern 

corner of the adjoining Cedargrove quadrangle, though it was not recognized during 

earlier mapping there (Weary, 2008). Vertical offset along the length of the Big Creek 

fault scissors. To the southeast of the intersection with the Bell Hollow fault the 

northeastern block is up in relation to the southwestern block by as much as 60 ft. 

Between the intersection with the Bell Hollow fault and an area about 1.75 mi to the 

northwest of the intersection with the South Ashley Creek fault, vertical offset on the 

Big Creek fault reverses, with the northeastern block down by as much as 50 ft. 

Farther to the northwest along the Big Creek fault, the sense of movement reverses 

once again, and for the remainder of the trace of the Big Creek fault the northeastern 

block is again up in relation to the southwestern block by as much as 70 ft.

The Gladden Creek fault is located in the northeastern part of the map area. Like 

the Big Creek fault, it has a subparallel trend to the northwest. This fault was originally 

mapped in the Cedargrove quadrangle, which adjoins the eastern edge of the Montauk 

quadrangle (Weary, 2008). Offset along the fault diminishes to the northwest, where 

its location becomes unmappable. Vertical offset across the fault ranges from about 90 

ft in the southeastern part to 0 ft in the northwestern part.

Two faults were mapped in the southwestern part of the quadrangle, the Long 

Hollow fault and the South Ashley Creek fault. The Long Hollow fault trends to the 

north-northwest. This fault was mapped based on slight vertical offset of strata across 

it, and alignment of areas with sandstone float containing cataclastic deformation 

bands. Along the fault the northeastern block is downthrown by as much as 60 ft.

The South Ashley Creek fault is located just to the east of the Long Hollow fault. 

This fault trends to the northeast, where it terminates against the Big Creek fault. 

Along the fault the northwestern block is downthrown by about 50 ft. 

The trace of the Bell Hollow fault originates at an orthogonal junction with the Big 

Creek fault in the southeastern part of the quadrangle. The fault trends from there to 

the northeast for about 1.9 mi, where it becomes unidentifiable. Vertical offset along the 

fault ranges from almost nothing to as much as 60 ft down on the northwestern side.

HYDROLOGY

With the exception of the Current River and its major tributaries, most water flow in 

the Montauk quadrangle is subterranean, with surface flow occurring only at times of 

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

from nearby springs or seeps but then is lost to subterranean drainage downstream. 

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 preceding sections, Montauk Springs is a cluster of large perennial 

springs and, with the exception of the Current River, the most significant hydrologic 

feature in the quadrangle. Montauk Springs is estimated to produce an aggregate flow 

of about 53 million gallons of water per day (82 ft3/sec; 2.3 m3/sec) (Vineyard and 

Feder, 1982). The influx of groundwater at Montauk Springs marks the official 

headwaters of the Current River. The channel of the surface stream continues 

upstream to the northwest for another 9 mi as Pigeon Creek. Downstream from 

Montauk Springs, the Current River flows southeastward through the Ozark Plateaus 

physiographic province for about 90 mi before entering the Mississippi embayment 

near the Arkansas State line. Except for a few areas of Potosi Dolomite outcrop in 

the Eminence-Stegall Mountain area, the Current River flows almost exclusively on the 

Eminence Dolomite from about 2 mi downstream of its headwaters at Montauk 

Springs to a point about 5.5 mi southeast of the town of Van Buren. This is a linear 

distance of about 53 miles.

Within the map area, the Roubidoux Formation and 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).

ECONOMIC GEOLOGY

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

manganese (Grawe, 1943) have been reported in the region of the Montauk 

quadrangle.

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.

The large karst springs in the quadrangle also have an economic value. Montauk 

Springs supplies large quantities of clean, cold water that support a trout hatchery, 

owned by the State of Missouri since 1926. The Montauk State Park waters and the 

nearby downstream reaches of the Current River support a popular and nationally 

famous trout fishery. The Current River is renowned as a canoeing, fishing, hunting, 

and camping destination as it flows for many miles through mostly public lands, 

primarily the Ozark National Scenic Riverways park (National Park Service). There is 

direct economic benefit derived from millions of annual tourist dollars as well as indirect 

benefit from ecological services provided by Montauk Springs and the Current River.

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

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Missouri Department of Natural Resources, Division of Geology and Land Survey, 2d 

Series, v. 40, 190 p.

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

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i2719.]

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[Report], v. 6, 387 p.

Qa

Qt

Or

Og

e

p

dd

d

b

l

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 clasts of chert, sandstone, and quartzite in a 

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

much as 20 ft

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

sand-size, subrounded to subangular clasts of chert, sandstone, and 

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

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

occur in 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 is at least 10 ft and possibly more along 

larger stream courses

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 to medium grained, thin bedded to 

laminated, evenly to irregularly bedded; interbedded with quartz 

sandstone that is white to pale orange (where fresh), light brown or 

reddish brown (where weathered), very fine to coarse grained, poorly 

sorted, thin 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 is 

commonly replaced by white, gray, or brown chert, frequently 

stromatolitic and occasionally oolitic or fossiliferous. The lower and 

upper parts of the formation are sandstone rich; the middle part is 

predominantly dolomite. Fossils are rare except for occasional 

impressions of the snail Lecanospira. Bedding surfaces of sandstone 

beds commonly contain ripple marks formed by currents. The lower 

contact of the formation was mapped at the base of the lowest 

orthoquartzite or sandstone bed encountered above the Gasconade 

Dolomite. The uppermost part of the formation is missing and truncated 

throughout the quadrangle by erosion. Up to about 220 ft of the 

Roubidoux Formation is exposed in the Montauk quadrangle

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

and orthoquartzite. The formation is divided into an upper unit, a middle 

unit, and a lower unit (fig. 2). The lower unit is the formal Gunter 

Sandstone Member; the middle and upper units are informal. None of 

these units are shown separately on this map. Dolomite of the upper unit 

is light gray, medium to coarse grained, thick bedded, vuggy; weathers to 

a pitted surface. Dolomite of the middle unit is light gray, fine to coarse 

grained, medium to thick bedded, with white to light-gray chert nodules 

and lenses. A locally persistent oolite bed occurrs in the middle part of 

the middle unit. The uppermost bed of the middle unit (top of the upper 

part) is a persistent white Cryptozoon chert, 2 to 10 ft thick. The basal 

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

the lower, formal unit of the formation. The Gunter typically consists of 

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

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

The contact with the underlying Eminence Dolomite is placed at the base 

of lowest sandstone or sandy dolomite of the Gunter Sandstone Member. 

Thickness of the Gasconade Dolomite ranges from 330 to 495 ft in the 

Montauk quadrangle

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

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

massive- to thick-bedded, commonly massive; minor amounts of chert, 

locally oolitic or drusy, in nodules and angular fragments. Many of the 

beds are stromatolitic, with pits and vugs developed between the algal 

laminae. Other beds may be crossbedded. Thickness is estimated to be 

about 250 ft in the Montauk quadrangle

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

Dolomite, light-brownish-gray to pale-yellowish-brown, fine- to mediumgrained, thick-bedded, vuggy and porous, with irregular blebs of silica and 

quartz druse; locally stromatolitic; has a bituminous odor typical of freshly 

broken rock. Thickness in the Montauk quadrangle is about 325 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- to 

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

amounts of chert and sparse sulfide minerals; vuggy and porous. 

Siltstone and shale, thin-bedded, interbedded with the dolomite. The 

Derby-Doerun is estimated to be about 125 ft thick in the Montauk 

quadrangle

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

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

Shale, dark-green, fissile, thin- to thick-bedded; composes as much as 50 

percent of the formation. Limestone, light-gray, fine-grained to 

cryptograined, dense, locally glauconitic. The Davis contains 

intraformational sedimentary breccias and grainstones. The unit is 

estimated to be about 175 ft thick in the Montauk quadrangle

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

Dolomite, limestone, siltstone, and shale. Dolomite, light-gray, fine- 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 the 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 (lead, zinc) deposits in the Ozark 

region. The unit is estimated to be about 315 ft thick in the Montauk 

quadrangle

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

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

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

contains interbeds of red to purple silty shale and (in the upper part) 

scattered lenses of arenaceous dolomite. Felsite pebble or boulder 

conglomerate is commonly present at the base. Thickness is variable, but 

estimated to average about 145 ft in the Montauk quadrangle

Igneous rocks undifferentiated (Mesoproterozoic)—Shown in cross 

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

basement rocks in the Montauk quadrangle

Figure 6.—Compass-rose diagram showing the dominant joint trends in the lower 

Paleozoic rocks of the Montauk quadrangle. Joint sets were statistically weighted by 

counting moderately spaced joint sets twice, and widely spaced sets once. This system 

approximates the importance of each set relative to the volume of the rocks. Interval 

is 10 degrees. Numbers on the diagram are percent of total. n, number of joints.

S

N

W E

n=212

8

4

12

16

Figure 5.—Typical exposure of the Gasconade Dolomite Cryptozoon chert horizon 

as large rubble blocks. Note the convex-upward algal laminae and polygonal fracture 

surfaces on the boulder in the foreground. This photograph was taken at a locality 

in the Alley Spring quadrangle about 24 mi to the southeast of Montauk.

1 foot

Figure 4.—Orthoquartzite lenses at the base of the Gunter Sandstone Member of the 

Gasconade Dolomite. The hammer lying on the outcrop is approximately 19 inches 

long. Note the float boulder of orthoquartzite in the near background.

Figure 3.—Basal quartz sandstone bed of the Gunter Sandstone Member of the 

Gasconade Dolomite exposed just above the Current River in the northern part of sec. 

36, T. 32 N., R. 7 W. The heavy white dashed line marks the unconformable contact 

with the underlying Eminence Dolomite and the Cambrian–Ordovician boundary.

Gunter Sandstone Member 

of the Gasconade Dolomite

basal quartz sandstone bed

Eminence Dolomite

1 foot

Figure 2.—Idealized stratigraphic column of outcropping lower Paleozoic formations 

in the Montauk quadrangle showing formal and informal units of the Gasconade 

Dolomite. Ss., sandstone.

100 FEET

Eminence

Dolomite

(e)

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

Figure 1.—Map showing location of the Montauk 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 by Ordovician to Pennsylvanian rocks and Quaternary periglacial 

sediments, and the Mississippi embayment by Cretaceous to Quaternary sediments 

(adapted from Imes and Emmett, 1994). Approximate location of the Viburnum 

Trend mining district is also shown.

River

Missouri

Central Lowland

Montauk

quadrangle

Salem Plateau

OZARK

PLATEAUS

0 50 Miles

St. Francois

Mountains

Mississippi

embayment

Springfield

Plateau

Viburnum Trend

95° 90°

40°

36°

Table 1.—Summary of drill-hole data in the Montauk quadrangle, Missouri.

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

Potosi Dolomite; |dd, Derby-Doerun Dolomite. Datum is mean sea level. ND, no data. 

SL, sea level]

1Top formation is the highest bedrock unit identified in the well; ground surface may be mapped as 

the overlying formation.

Drill-hole

number

17427

19085

22975

23537

24772

26850

2768

Location

NE1/4SE1/4 sec. 8,

T. 31 N., R. 7 W.

SE1/4SW1/4 sec. 23,

T. 32 N., R. 7 W.

NE1/4NE1/4 sec. 27,

T. 32 N., R. 7 W.

NW1/4SW1/4 sec. 10,

T. 32 N., R. 7 W.

SE1/4SW1/4 sec. 27,

T. 32 N., R. 7 W.

SE1/4SE1/4 sec. 22,

T. 32 N., R. 7 W.

NE1/4SE1/4 sec. 22,

T. 32 N., R. 7 W.

Collar

elevation,

in feet

1035

981

926

1208

1178

1016

1034

Contact

elevation,

in feet

620 Og–e

325 e–p

SL p–dd

821 Og–e

866 Og–e

621 e–p

ND Or–Og

858 Og–e 

1123 Or–Og

831 Og–e

571 e–p

849 Og–e

Depth to

bedrock,

in feet

115

20

5

90

50

25

15

Bottom

elevation,

in feet

-30

581

426

708

993

416

612

Top

formation1

Og

Og

Og

Or

Or

Og

Og

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

Geological Survey and Resource Assessment Division

Long Hollow

South Ashley Creek

South Ashley Creek

South Ashley Creek

North Ashley Creek

North Ashley Creek

North Ashley Creek

Lick Hollow

CURRENT RIVER

Horse Hollow

Inman Hollow

A

NO VERTICAL EXAGGERATION

400

400

800

800

1200

1600

SEA LEVEL

LONG HOLLOW FAULT

BIG CREEK FAULT

GLADDEN CREEK FAULT

FEET

Og Og Og Og Og Og Og Og

Og Og Og Og Or Or Or Or Or

p

e

dd

d b

l Yu

400

400

800

800

1200

1600

SEA LEVEL

FEET

p

e

dd

d b

l Yu

p

e

dd

d b

Yu l

Or Or Or Or

p

e

b

Yu

Or Or

DRILL HOLE 24772

(projected from 520 ft to NW)

DRILL HOLE 22975

DRILL HOLE 17427

400 feet = 121.9 meters

Surficial deposits are not shown

A'

dd

d

l

1140

1200

1220

1240

1120

1160

1160

1180

1200

1200

1220

1240

1260

1180

1200

1140

1140

1160

1160

1140

1120

1100

1100

1080

1120

1180

1200

1220

1240

1260

1120

1140

1160

1180

1200

1280

1220

1200

1180

1160

1260

1240

1220

1160

1180

1160

1140

1120

1100

1080

1120

1140

1100

77 

23537

2768

26850

22975

19085

24772

17427

SOUTH ASHLEY CREEK FAULT

LONG HOLLOW FAULT

BIG CREEK FAULT

BIG CREEK FAULT

GLADDEN CREEK FAULT

BELL

HOLLOW

FAULT

4

2

6 5

14

2

2

2

3

8

5

2

3

3

2

2

3

4

2

3

3 4

3

2

2 10

2

2

2

2

2

3

2

4

10

10

7

9

7

13

4 4

3

w

5

3

4

2

2

5

2

4

2

3

5

3

2

2

2

A 

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CONTOUR INTERVAL 20 FEET

NATIONAL GEODETIC VERTICAL DATUM OF 1929

1 .5 0 1 KILOMETER

SCALE 1:24 000

1 1/2 0 1 MILE

ORDOVICIAN

CAMBRIAN

MESOPROTEROZOIC

In cross

section only

Unconformity

Lower Ordovician

Upper Cambrian

Holocene and

Pleistocene QUATERNARY

CORRELATION OF MAP UNITS

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5

77 

24772

1240 Structure contour—Drawn on base of Roubidoux Formation; projected 

where above land surface. Contour interval 20 ft 

PLANAR FEATURES

[Point of observation is at intersection of multiple symbols]

Strike and dip of beds

Inclined

Horizontal

Strike and dip of cataclastic deformation bands

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

noted by w (solutionally widened)

Throughgoing, vertical

Widely spaced (>6 ft)

Non-throughgoing, vertical

Widely spaced (>6 ft)

Moderately spaced (2–6 ft)

OTHER FEATURES

Drill hole—Number refers to well records from Missouri Department 

of Natural Resources; see table 1 for summary of well data

Cryptozoon-bearing chert bed of Gasconade Dolomite—Locality 

where chert bed was observed in outcrop or in highest float

Cataclastic deformation bands in sandstone float

Spring

Sinkhole

EXPLANATION OF MAP SYMBOLS

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; in 

cross section, paired arrows indicate relative direction of vertical 

movement. Faults exposed in mines in nearby quadrangles have 

strike-slip displacement; horizontal motions are not shown on this map 

because they cannot be documented in surface exposures

Minor anticline

Minor syncline

Base from U.S. Geological Survey, 1965; photorevision as of 1985 Geology mapped in 2007 and 2009

Polyconic projection. North American Datum of 1927.

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

U.S. DEPARTMENT OF THE INTERIOR SCIENTIFIC INVESTIGATIONS MAP 3320

U.S. GEOLOGICAL SURVEY 

0° MAGNETIC NORTH

TRUE NORTH

APPROXIMATE MEAN

DECLINATION, 2015

MISSOURI

MAP LOCATION

Printed on recycled paper 

GEOLOGIC MAP OF THE MONTAUK QUADRANGLE, DENT, TEXAS, AND SHANNON COUNTIES, MISSOURI

By

David J. Weary

2015

Any use of trade, firm, or product names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government

For sale by U.S. Geological Survey, Box 25286, Denver Federal Center, Denver, CO 80225; 

http://store.usgs.gov; 1–888–ASK–USGS (1–888–275–8747)

Suggested citation: Weary, D.J., 2015, Geologic map of the Montauk quadrangle, Dent, Texas, 

and Shannon Counties, Missouri: U.S. Geological Survey Scientific Investigations Map 3320, 1 

sheet, scale 1:24,000, http://dx.doi.org/10.3133/sim3320.

ISSN 2329-1311 (print)

ISSN 2329-132X (online)

http://dx.doi.org/10.3133/sim3320