Dakota Group in 

Northern Front Range 

Foothills, Colorado

GEOLOGICAL SURVEY PROFESSIONAL PAPER 274-B
Dakota Group in 

Northern Front Range 

Foothills, Colorado

By KARL M. WAAGE

A SHORTER CONTRIBUTION TO GENERAL GEOLOGY

GEOLOGICAL SURVEY PROFESSIONAL PAPER 274-B

A revised subdivision and terminology 

for the Dakota group and local details 

of its stratigraphy

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1955
UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington 25, D. C. - Price 45 cents (paper cover)
CONTENTS

Abstract._ ________________________________________

Introduction ______________________________________

Dakota terminology and subdivision_. _______________

Stratigraphy. _______ ________________________

Lytle formation _________________________ _____

Definition, ______________________

Type locality of the Lytle. _____________

Lateral variation____________________

Contact of Lytle and Morrison formations 

Eldridge type locality of the Morrison. __ 

Alameda Parkway type section. _______

Post-Morrison warping _______________

Summary. ___________________________„

Contact of Lytle and South Platte formations. 

Age_ _____________________________________

South Platte formation.________________________

Definition.. _____________________

Lithology_ ____________________________

Southern nonmarine phase__________________

Plain view sandstone member_ _______

Page Page

15 Stratigraphy—Continued

15 South Platte formation—Continued

16 Southern nonmarine phase—Continued

18 Beds between the Plainview and Kassler

19 sandstone members _________________ 30

19 Kassler sandstone member. _____________ 31

20 Van Bibber shale member._______________ 31

20 First sandstone subunit and Benton con23 tact_________________________________ 33

24 Lateral variation. ____________________ 33

25 Type locality_________ _________________ 34

25 Intermediate phase—Boulder County..-------- 36

25 Northern marine phase______________________ 38

26 Age____ ______________________________ 40

26 Relationship of the phases_ __________________ 42

27 Status of the term Dakota group__-______---_-_--__--_ 44

27 Correlation of the Lytle and South Platte formations.--- 45

27 Evaluation of unconformities.________________________ 47

28 Literature cited___________________________________ 48

28 Index__.___________________________________________ 51

ILLUSTRATIONS

FIOIITKE 4. Distribution of the Dakota group__________________________________________

5. Lateral variation in the Lytle formation.___________________________________

6. Basal sandstone lens in the Lytle formation on Turkey Creek-________________

7. Interpretations of the Morrison-Dakota contact _____________________________

8. Contact of the Lytle and South Platte formations at Spring Canyon.__________

9. Contact of the Lytle and South Platte formations at Eldorado Springs. _______

10. Composite section of the South Platte formation-____________________________

11. Plainview member of the South Platte formation at Plainview-________________

12. Second shale subunit of the South Platte formation _________________________

13. Base of main clay bed of the Van Bibber member of the South Platte formation.

14. Nonmarine phase of the South Platte formation on Willow Creek-_____________

15. Intermediate phase of the South Platte formation on Little Thompson Creek____

16. Double hogback formed by marine phase of the South Platte formation. _______

17. Lateral changes in the South Platte formation_________________________._____

18. Characteristic exposure of the Dakota group in the Denver area_______________

19. Correlation of the Lytle and South Platte formations.________________________

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A SHORTER CONTRIBUTION TO GENERAL GEOLOGY

DAKOTA GROUP IN NORTHERN FRONT RANGE FOOTHILLS, COLORADO

KARL M. WAAGE

ABSTRACT

Pre-Benton Cretaceous strata throughout eastern Colorado 

and adjacent areas are divisible into a lower part which consists 

of sandstone, conglomeratic sandstone, and variegated claystone, 

and an upper part which consists of dark-gray shale interbedded 

with units of brown-weathering sandstone. A sharp disconformity—the most pronounced lithogenetic break in the sequence 

of beds between the Morrison formation and the Benton shale— 

separates these two parts. For the northern Front Range 

foothills of Colorado the name Lytle formation is applied to 

the lower part of the pre-Benton sequence; the upper part is 

named the South Platte formation. The term Dakota group is 

retained to include these two formations.

Lytle sediments were deposited on flood plains and are more 

closely related lithogenetically to the Morrison formation than 

to the overlying South Plafcte formation. The Morrison-Lytle 

contact is indefinite in many places because, (1) conglomeratic 

beds are not persistently present in the base of the Lytle and 

(2) the rocks in the upper part of the Morrison are similar to 

those in the Lytle. Both the old and new type sections of the 

Morrison have indefinite upper contacts. The Morrison-Lytle 

contact probably corresponds closely to Eldridge's MorrisonDakota contact in the Denver basin area; the prevalent belief 

that Eldridge included plant-bearing Dakota beds in the type 

locality of the Morrison is a misinterpretation of his original 

description.

South Platte sediments were deposited in deltaic, estuarine, 

littoral, and neritic environments around the spreading Cretaceous sea. The formation is largely in a nonmarine clastic 

phase in the southern part of the northern foothills but changes 

northward into a marine shale phase. Thin beds of altered 

volcanic ash and a single persistent marine zone facilitate correlation of the phases. Prominent local subunits are distinguished 

as members. All the South Platte formation except the beds 

at the top and base are in the zone of Inoceramus comancheanus 

Cragin, and the age of the entire unit is presumed to be Early 

Cretaceous (Albian).

In the northern Front Range foothills three distinct physical 

breaks occur in the sequence of beds between the undoubted 

Jurassic (Portlandian) part of the Morrison formation and 

beds of Early Cretaceous (Albian) age in the South Platte formation. Not enough fossil evidence is available to permit 

evaluation of these unconformities or to assign a greater time 

value to a particular unconformity.

The Lytle and South Platte formations can be readily correlated with their lithic equivalents in adjacent areas. The

disconformity that separates them is a regional feature that lies 

between the Lytle sandstone and Glencairn shale members of 

the Purgatoire formation in southeastern Colorado. In eastern 

Wyoming this disconformity lies within the Cloverly formation, 

at or near the base of the upper sandstone member.

INTRODUCTION

Correlation and description of pre-Benton Cretaceous (Dakota) rocks in eastern Colorado is made 

difficult by the multiplicity of names that have been 

applied to them. The problem is not entirely one of 

confused nomenclature that can be resolved by submitting the different names to tests of precedence or 

usage; a basic difficulty is that none of the terminologies 

now in use accurately express the principal natural 

lithogenetic subdivisions and stratigraphic breaks that 

occur in the pre-Benton Cretaceous sequence throughout 

eastern Colorado. This report is a response to the need 

for a revised terminology which is suited to the broad 

natural subdivisions of the beds in question and yet is 

flexible enough to be adjusted to their local variations 

as revealed by detailed stratigraphic study.

Pre-Benton Cretaceous strata exposed in the Dakota 

hogback of the northern Front Range foothills were 

studied in 1951-52 during investigations of refractoryclay deposits for the U. S. Geological Survey. Previous field studies of the Dakota that contributed 

supplementary material include an investigation of 

refractory clays in the Purgatoire and Dakota formations of south-central Colorado by the Survey (Waage, 

1953) and a reconnaissance study of critical Dakota 

localities in Colorado and adjacent states. The reconnaissance was supported by a grant from the Shell 

Research Fund of the Department of Geology, Yale 

University.

The area of study here referred to as the northern 

Front Range foothills includes a narrow belt of preBenton Cretaceous exposures extending southward 

along the east flank of the Front Range. These expo15
16 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

sures extend from 2 miles south of the Wyoming State 

line, in Larimer County, where the beds emerge from 

under Cenozoic cover, to the fault-terminated end of 

the Dakota hogback just south of Indian Creek in the 

Kassler quadrangle, Douglas County. The belt of 

outcrop (fig. 4) is continuous except for a few short

Northern ^ 

Front Range ^ 

foothills V,

COLOR ADO

INDEX MAP

EXPLANATION 

Outcrop of the Dakota group 

LOCATION OF SECTIONS

1. Boxelder Creek

2. Bellvue

3. Spring Canyon

4. Handy Ditch

5. Little Thompson Creek

6. Eldorado Springs

7. Plainview

8. Van Bibber Creek

9. Alameda Parkway

10. Morrison

11. Turkey Creek

12. Deer Creek

13. South Platte River

14. Helmer Ranch

15. Willow Creek

16. Rainbow Creek

G L A S

10 Miles 

i

FIGURE 4.—Distribution of the Dakota group in the northern Front Range foothills,

Colorado.

gaps in which the beds are eliminated by faulting; the 

largest of these, a gap of about 3.5 miles, is at Golden. 

South of the end of the hogback at Indian Creek the 

pre-Benton Cretaceous rocks disappear by faulting 

for about 8 miles along the strike, and reappear in a 

short segment of hogback at Perry Park in the Castle 

Rock quadrangle.

The northern foothills area of study corresponds 

with the area in which Lee (1923) applied the name 

Dakota group to the pre-Benton Cretaceous rocks of 

Colorado and divided it into five informal subdivisions. 

The terminology presented in this report is a revision 

of Lee's Dakota group; consequently it is intended to 

apply only to the northern foothills area.

DAKOTA TERMINOLOGY AND SUBDIVISION

Pre-Benton Cretaceous strata in eastern Colorado 

were originally included in the Dakota group. Reports of the Hayden Survey, published between 1869 

and 1878, used this term in the same sense in which it 

was first applied to the basal sandy beds of the Cretaceous rocks (formation no. 1) in the type area in 

eastern Nebraska (Meek and Hayden, 1862, p. 419- 

420). King (1878, p. 298) also adopted the term 

Dakota group for the reports of the 40th parallel 

survey, but only after Hague (1877, p. 39) had used 

the terms Dakota Cretaceous, Dakota formation, 

Dakota sandstone, and Dakota group interchangeably.

After the organization of the U. S. Geological Survey 

in 1879 successive attempts to standardize stratigraphic 

terminology in Surve}^ reports (Powell, 1882, p. XL— 

XLVII; 1890, p. 63) fed to the selection of the formation as the "grand unit" for mapping. The term 

group gradually lost favor as a synonym for formation 

and was eventually reclaimed and defined (Walcott, 

1903, p. 21-27) in the broader sense in which it is 

understood today. Consequently the original Dakota 

group in publications on eastern Colorado properly 

became the Dakota formation, or Dakota sandstone, 

a change that was purely taxonomic and in no way 

altered the meaning of Meek and Hayden's original 

definition.

The first attempts to subdivide the Dakota formation 

in eastern Colorado were informal. Gilbert (1897) 

distinguished an upper, fire-clay-bearing part on the 

economic geology sheet of the Pueblo folio. Hills 

was the first to emphasize a threefold division of the 

formation. In both the El Moro folio (Hills, 1899) 

and the Walsenburg folio (Hills, 1900) he notes that 

the Dakota formation consists of a lower porous, 

conglomeratic sandstone, a middle shale bed, and an 

upper fine-grained sandstone containing some shale. 

Because the middle shale was locally refractory it was
DAKOTA GROUP IN NORTHERN FRONT RANGE FOOTHILLS, COLORADO 17

commonly referred to as fire clay. Hills mapped this 

fire clay on the economic geology sheet of both folios.

Subsequent to Hills' work the threefold division of 

the Dakota formation in the Arkansas River valley 

area and elsewhere in southeastern Colorado was 

noted by a number of geologists. In 1905 fossils of 

Early Cretaceous age were discovered in the middle 

shale in Prowers County by Darton (1905, p. 120) 

and along the Purgatoire River, south of La Junta, 

and on Oil Creek near Canon City by Stanton (1905, 

p. 661-663, 666-667). Up to this time the Dakota 

formation in Colorado had been assumed to be Late 

Cretaceous (Cenomanian) in age on the basis of its 

fossil plants, of which the greater number were collected from sandstone in the upper part of the Dakota 

strata in eastern Kansas and Nebraska. Previous 

discovery of marine Early Cretaceous beds in Kansas, 

where the Dakota group underwent a taxonomic overhaul and subdivision completely separate from that in 

Colorado, apparently had not prepared the Colorado 

investigators to deal with a similar discovery. Possibly this was because the Colorado work was largely 

confined to quadrangles adjacent to the Rocky 

Mountain front and did not include Dakota terrain in 

southeastern Colorado, where the beds are more 

obviously similar to the Kansas sequence.

The recognition of the middle and, consequently, 

the lower part of the Dakota formation as Early Cretaceous and the continued acceptance of the top sandstone as Late Cretaceous did not bring about an immediate change in the formal terminology in Colorado, 

but subdivision of the Dakota on the basis of this age 

difference was informally expressed wherever Early 

Cretaceous fossils could be found in the sequence. 

The Early Cretaceous part of the sequence was commonly discussed separately under the heading Comanche series, or Comanche formation, (Darton, 1906, 

p. 25; Henderson, 1909, p. 172), in spite of the fact 

that it was included as a part of the "Dakota" formation or "Dakota" sandstone. The quotation marks 

probably indicate the dissatisfaction of the authors 

with this ambiguity. The first formal subdivision of 

the Dakota formation was made by Stose (1912) in 

the Apishapa quadrangle. He named the Early 

Cretaceous lower sandstone and middle shale of the 

threefold sequence the Purgatoire formation, and retained the name Dakota sandstone for the upper sandstone. Subsequently Finlay (1916), in the Colorado 

Springs quadrangle, named the sandstone and shale 

units in the Purgatoire formation the Lytle sandstone 

member and Glencairn shale member respectively. 

With the exception of the addition to the Dakota 

sandstone of a local refractory shale unit, the Dry 

Creek Canyon member (Waage, 1953, p. 12-17), the

subdivision and nomenclature have remained as 

designated by Stose and Finlay. The names Dakota 

sandstone, in its restricted sense, and Purgatoire 

formation have been used throughout southeastern 

and south-central Colorado and as far north along 

the Front Range foothills as Perry Park in the Castle 

Rock quadrangle (Richardson, 1915).

In the northern Front Range foothills very little 

work was done on the pre-Benton Cretaceous strata between 1912 and 1923. No attempt was made to extend 

Stose's subdivisions of the original Dakota formation 

north of the Castle Rock quadrangle, although the 

presence of a similar sequence had previously been 

recognized in the foothills north of Boulder (Henderson, 

1909, p. 174). Lee's work in the northern foothills 

(Lee, 1923, p. 1-20) led to a fivefold rather than 

threefold subdivision of the Dakota formation in this 

area. His stratigraphic study of the Dakota was based 

on a section exposed in the Poudre Valley and Reservoir 

Company ditch along the north side of the Cache La 

Poudre River, 2 miles north of Bellvue, Larimer 

County. Lee called the Dakota formation the Dakota 

group and divided it into five informal subunits: the 

lower sandstone, lower shale, middle sandstone, middle 

shale—later renamed upper shale (Lee, 1927)—and 

upper sandstone. Lee used this terminology throughout 

the Front Range foothills north of the Castle Rock 

quadrangle, identifying his subunits in many measured 

sections and correlating them with the subdivisions of 

Stose and Finlay to the south and with the Wyoming 

terminology to the north. Later Lee (1927) elaborated 

on this study and extended his correlations northward 

into Montana. Fossils collected by Lee, and others, 

from the middle shale (upper shale of Lee's 1927 

report) of his Dakota group were described by Reeside 

(1923) who recognized that the fauna was an impoverished representation of the Early Cretaceous (Washita) fauna found in the Purgatoire formation of southcentral and southeastern Colorado. No formal changes 

have been made in nomenclature or subdivision of Lee's 

Dakota group since it was proposed, and it has existed 

side by side with the southern classification of Stose 

and Finlay.

Terminologies other than that of Lee have been used 

in the northern Front Range foothills presumably 

because Lee's Dakota group is too broad a unit for 

detailed work and because some of its subdivisions lack 

lateral continuity and are difficult to apply. The Stose 

and Finlay terminology was used in the Denver-Golden 

area by Waage (1952) but subsequent work leading to 

the present report has shown that it was incorrectly 

applied and that it is equally as unsatisfactory as Lee's 

in providing a logical subdivision of the pre-Benton 

Cretaceous strata. George (1927, p. 63-64) called the
18 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Early Cretaceous rocks throughout eastern Colorado 

the Purgatoire and incorrectly used the names Lakota 

and Fuson in the northern foothills for members of the 

Purgatoire that he considered equivalent, respectively, 

to Finlay's Lytle and Glencairn members of the Purgatoire in the Colorado Springs area. This unfortunate 

usage, based on incorrect correlation with the Black 

Hills area (fig. 19), is still used in Colorado by some 

workers. Other workers have extended the CloverlyThermopolis-Muddy terminology southward from 

Wyoming, and some still use the term Dakota formation 

in the sense in which it was used in early reports of the 

Geological Survey.

Inasmuch as Lee's terminology is the only one based 

on a careful stratigraphic study, it is certainly the most 

acceptable and the most useful of the existing possibilities. The Geological Survey (Wilmarth, 1938, p. 

566) provisionally accepted Lee's Dakota group for 

northern Colorado with the condition that

If the Purgatoire proves to be present in the Bellevue section it 

will be removed from the Dakota group of W. T. Lee.

Several sources of confusion in the usaj;e of preBenton Cretaceous terminologj^ are apparent in the 

history of subdivision and nomenclature outlined in the 

preceding paragraphs. The first source was the taxonomic change from the use of the terms formation and 

group as synonyms to their use as terms for separate 

ranks of rock units. A more critical source was the 

tendency to separate the Early Cretaceous from the 

Late Cretaceous parts of the pre-Benton Cretaceous 

sequence, a tendency that influenced, and is reflected in, 

Stose's formal subdivision of the old Dakota formation 

in southeastern Colorado. The use of age as a criterion 

for the subdivision of rock units, a practice at the root 

of many nomenclatural problems, can lead only to 

confusion in correlation and to ambiguity in terminology 

when applied to complex transgressive deposits like 

the pre-Benton Cretaceous sequence. Much of the 

confusion associated with the name Dakota stems from 

this cause. An unfortunate nomenclatural practice, 

that of retaining the name Dakota for the Late Cretaceous part of the sequence in those areas where Early 

Cretaceous rocks can be identified and separated, has 

served to crystallize the confusion. This practice has 

been applied in Kansas and in Colorado, making it 

impossible to use the name Dakota in a single sense for 

physical correlation between areas in which Early 

Cretaceous strata are separated from the Dakota and 

areas in which the occurrence of Early Cretaceous rocks 

has not been sufficiently well established to permit 

their separation from the Dakota.

Introduction of terminologies from other areas is a 

recent source of confusion in the Colorado pre-Benton

Cretaceous classification. It is understandable, when 

one considers the confused state of the local terminology 

and the reluctance of workers to use the name Dakota 

in any sense because of its ambiguity, but the practice 

is untrustworthy in light of our lack of knowledge 

about the details of regional Dakota stratigraphy. 

The continued use of the rather obvious incorrect 

correlation of the Glencairn member of the Purgatoire 

formation with the Fuson shale illustrates the hazard 

of using a foreign nomenclature to circumvent the 

inadequacies of the native nomenclature. It has become increasingly obvious, as work on the Dakota has 

progressed, that neither the local classifications proposed for the pre-Benton Cretaceous sequence in eastern 

Colorado nor the foreign classifications are adapted to 

the natural lithogenetic subdivision that characterizes 

the sequence throughout much of the western interior. 

This natural subdivision either has been overlooked or 

has been effectively obscured by the unquestioned 

acceptance of previous classifications.

STRATIGRAPHY

Inasmuch as none of the classifications applied to 

the pre-Benton Cretaceous sequence in Colorado 

accurately express its principal lithogenetic subunits 

and stratigraphic breaks, all previous systems of nomenclature are excluded from the following discussion in 

order to present the general stratigraphic features of 

the sequence without the bias that such systems 

impose.

The most noticeable stratigraphic feature in the 

sequence of Dakota strata lying between the Morrison 

formation and the Benton, or Graneros, shale in eastern 

Colorado is its divisibility into two distinctive parts. 

The lower part consists of irregular lenses of light-gray 

to gray-white, commonly porous sandstone, and conglomeratic sandstone which are irregularly interbedded 

with variegated claystone similar to that in the underlying Morrison formation. Weathered outcrops of 

the sandstone are highly variable in color but light 

values of buff and gray predominate, and pink and 

yellow staining is common. This lower part of the 

sequence is typified by its variability, both in the 

relative amounts of sandstone to variegated claystone 

and in the irregular thickness and distribution of the 

sandstone lenses.

Contrary to common belief, the base of the lower 

part of the sequence is not consistently marked by a 

conglomerate bed. Where the lower part is a single 

massive unit composed chiefly of sandstone and conglomerate, an obvious physical break occurs between 

it and the Morrison formation. At other localities the 

lower part of the sequence consists of alternating lenses
DAKOTA GROUP IN NORTHERN FRONT RANGE FOOTHILLS, COLORADO 19

of sandstone and variegated claystone similar to the 

upper part of the Morrison and there is no obvious 

unconformity between the units.

The upper part of the sequence consists of sandstone 

units that weather buff and brown and alternate with 

units of dark-gray to black shale and siltstone. The 

sandstone beds are dominantly fine grained, tabular to 

massive, cross laminated, and less porous and more 

resistant than those in the lower part of the sequence. 

The intervening dark-colored shale units are laminated, 

commonly silty, and contain thin layers of altered 

volcanic ash. Although somewhat variable in lithology 

and thickness, the changes in the sandstone and shale 

units are gradual, lateral changes in facies, and the 

upper part of the sequence as a whole presents a regularity in its succession and a continuity of its parts 

that contrasts markedly with the lenticularity of the 

lower part of the sequence.

The contact between the upper and lower parts of 

the sequence is a persistent disconformity marking the 

abrupt change from the variegated claystone and graywhite sandstone below to the black shale and brownweathering sandstone above. The disconformity is a 

planed surface on which rests, in many places, a thin 

conglomerate or conglomeratic sandstone. Beneath the 

disconformity the upper few feet of the lower part of 

the sequence commonly consists of weathered sandy clay 

or claystone containing local concentrations of iron 

oxide. The disconformity is the most obvious physical 

break in the section between the Morrison formation 

and the Ben ton shale; it can be traced throughout 

eastern Colorado and into adjacent States.

The interpretation of the gross twofold lithic division within the pre-Benton Cretaceous sequence is 

fairly evident. The lower part of the sequence consists 

of flood-plain deposits more closely related lithogenetically to the underlying Morrison formation than to the 

upper part of the pre-Bentoii sequence. Probably 

much of the sediment in the lower part of the sequence 

is locally derived from the Morrison. The disconformity separating the lower from the upper part of 

the sequence is a transgressive feature affording the 

first evidence of an invading Cretaceous sea. The 

initial deposits above the disconformity are chiefly 

fresh- and brackish-water beds presumably deposited 

in local deltas or in bodies of water impounded along 

the coastal plain by the rise in sea level. The time 

value of the disconformity cannot be accurately assessed 

at any given locality because of the lack of diagnostic 

fossils, but it was sufficiently long in many places to 

allow leaching of the top of the lower part of the 

sequence.

The upper part of the sequence is a record of both

332778—55———2

local and regional fluctuations in strand line. In some 

places, presumably the larger deltaic areas, the sequence 

is predominantly nonmarine, in other places it is predominantly marine or estuarine: everywhere it is a 

complex unit recording local conditions at or near the 

strand line from the initial impounding of coastal-plain 

drainage until marine conditions prevailed.

The upper and lower parts of the pre-Benton Cretaceous sequence and the disconformity that separates 

them are regional features useful for physical correlation throughout much of the interior region. In the 

northern Front Range foothills of Colorado, this twofold 

lithogenetic division affords a logical basis for separating 

the sequence into formational units. The name Lytle 

formation is here applied to the lower unit of sandstone, 

conglomeratic sandstone, and variegated clay, and the 

name South Platte formation is given to the upper 

unit of alternating sandstone and dark-colored shale 

beds. The term Dakota group is retained to include 

these two formations.

LYTLE FORMATION

DEFINITION

The beds of sandstone, conglomeratic sandstone, and 

variegated claystone that make up the lower part of 

the pre-Benton Cretaceous sequence are correlated 

here with the Lytle sandstone member of the Purgatoire formation in south-central Colorado and the 

name Lytle, raised to the rank of formation, is applied 

to them throughout the northern Front Range foothills.

Finlay (1916, p. 7-8) gave the name Lytle sandstone 

member to the lower sandy part of the Purgatoire 

formation in the Colorado Springs quadrangle, noting 

that it consists of light-colored sandstone, pebbly beds, 

and "scattered lenses of greenish or reddish clay near 

the top;" he also mentions that the overlying Glencairn shale member is "rather sharply separated from 

the Lytle member." Finlay did not recognize the 

disconformity separating the two lithogenetically dis 

tinct parts of the pre-Benton sequence but it is obvious 

from his descriptions that he intended to separate his 

Glencairn and Lytle members at this lithic change. 

Waage (1953, p. 7-9, 27) traced the members southward 

into south-central Colorado and used the disconformity 

as the contact between them. Correlation of the 

Lytle sandstone member of the Colorado Springs area 

with the lower part of the pre-Benton Cretaceous 

sequence in the northern foothills is clear cut inasmuch 

as the disconformity occurs in both the northern and 

southern Front Range foothills. Furthermore the 

beds beneath it are lithologically identical and show 

the same stratigraphic relations with the underlying 

Morrison formation.
20 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

TYPE LOCALITY OF THE LYTLE

Finlay took the name Lytle from a small settlement 

of a few homes along Turkey Creek in the southwestern 

part of the old Colorado Springs 15-minute quadrangle. 

Today the locality is marked by the Lytle School, situated along Lytle Road just south of Turkey Creek in 

the SWJ.NWtf sec. 2, T. 17 S., R. 67 W., Timber 

Mountain quadrangle. The following section was 

measured at the typical exposure northeast of the 

school on the west-facing scarp of the hogback between 

Turkey Creek and Little Turkey Creek.

Description

Purgatoire formation (in part).

Glencairn shale member (in part): Feet 

Sandstone, fine- to coarse-grained, tabular,

cross-laminated; weathers brown_ ________ 3. 0-7. 0

Disconformity.

Lytle sandstone member:

Sandstone, fine-grained, argillaceous, soft; 

weathers white, with local yellow stain; 

grades into unit below___________________ 1. 5-6. 0

Sandstone, fine- to medium-grained, massive, 

cross-laminated, with scattered irregular 

layers of conglomeratic sandstone; weathers 

gray, light gray, and yellowish gray__^___ 47. 0-? 

Remainder of section obscured by slope wash.

Dinosaur bone fragments found in the slope wash 

90-100 feet below the top of the type section suggest 

that the Lytle sandstone member is less than 100 feet 

thick at this place. Finlay gives an average thickness 

of 145 feet for the Lytle in the Colorado Springs area 

but this figure is excessive. No clear-cut contact 

between the Lytle member and the Morrison formation 

was seen in the type area, but conglomeratic sandstone 

ledges that presumably mark the base of the member 

crop out locally between 50 and 70 feet below the top 

of the unit.

LATERAL VARIATION

In the northern foothills abrupt lateral variation in 

the local dominance of its sandstone or claystone fraction is characteristic of the Lytle formation. At one 

extreme the Lytle consists almost entirely of sandstone 

and conglomerate and resembles its more consistently 

arenaceous southern equivalent, the Lytle sandstone 

member of the Purgatoire formation. At the other 

extreme the Lytle lacks conglomeratic beds and consists of approximately equal amounts of fine- to 

medium-grained sandstone and variegated claystone 

distributed in alternating lenses. Generalized graphic 

sections of the Lytle formation illustrating its variable 

lithic content and thickness are shown in figure 5. 

Two of these sections are given in more detail.

A dominantly sandy section of the Lytle formation is 

exposed in the road cut along Colorado Route 186 on

the south side of the dam across Spring Canyon, 

Horsetooth Reservoir, SE# sec. 32, T. 7 N., R. 69 W., 

Larimer County.

Description

South Platte formation.

Disconformity.

Lytle formation: Feet

8. Claystone, light-gray with pink and yellow 

stain, sandy; grades imperceptibly into 

unit below_ —---_--____-_----_--------- 1. 0-4. 0

7. Sandstone, fine-grained, massive, cross-laminated; weathers yellowish gray to buff 

with local pink and light-purple staining, _ 24. 0

6. Claystone, grayish-green with red and yellow

stain in uppermost foot, silty____________ 2. 5

5. Sandstone, argillaceous, and claystone, sandy;

weathers with red, pink, and yellow stain __ 1. 5

4. Sandstone, light-gray with pink to yellow cast, 

fine-grained, cross-laminated, with thin 

irregular interbeds and lenses of greenishgray silty claystone. Clay-pellet conglomeratic layers throughout, thickest at base.. 22. 0

3. Sandstone, as in unit 4 but fine- to coarsegrained with a few thin lenses of small 

chert and quartzite pebbles. Few lenses 

of claystone and a 2.2-foot bed of claypellet conglomerate 8 feet from top_---__- 14. 0

2. Sandstone and conglomerate. Upper 3-4 

feet are chert and quartzite-pebble conglomerate; remainder partially obscured by 

wash, apparently massive, cross-laminated, 

friable sandstone and conglomeratic sandstone_______________________--__--__-_ 13. 0

Unconformity. 

Morrison formation:

1. Clay and claystone, variegated, silty, upper 

2 feet yellow, underlain by 4-foot zone of 

purplish-red claystone which grades downward through about 3-foot zone of maroon 

and bluish-purple to typical greenish-gray 

claystones of the Morrison- _____________ 12. 0±

In the scarp face of the Dakota hogback north of 

Deer Creek in SE^NE/4SEK sec. 5, T. 6 S, R., 69 W., 

Indian Hills quadrangle, the Lytle consists of alternating units of sandstone and claystone.

Description

South Platte formation. 

Disconformity.

Lytle formation: Feet 

9. Claystone, gray containing local pink stain,

sandy, some zones argillaceous sandstone-___ 0. 3—1. 0 

8. Sandstone, medium- to coarse-grained and conglomeratic, massive, lenticular, locally cliff 

forming; weathers light gray and light buff. 

Conglomerate chiefly in lower 10 feet_ _______ 24. 0

Unconformity.

7. Siltstone, light-gray and greenish-gray, red 

mottling in lower 5 feet, some scattered lenses 

of silty claystone and some argillaceous sandstone_____---___--_--__--_--_----------- 8. 0
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22 SHOETER CONTRIBUTIONS TO GENERAL GEOLOGY

Lytle formation—Continued

6. Sandstone, fine- to medium-grained, massive, 

ferruginous specks throughout; weathers 

orange brown; 0.4-foot zone of argillaceous feet 

siltstone 3.0 feet from top_________________ 6. 5

Unconformity.

5. Claystone, silty, and argillaceous siltstone, 

chiefly red, some gray and greenish gray. 

Contains thin bed of hard light-gray sandstone 2.0 feet above base__________________ 5. 0

4. Sandstone, fine- to medium-grained, massive, 

ranges from 4 to 15 feet in thickness within 

100 feet along strike; weathers light gray; 

local silty zones at base and top___________ 12. 0

Unconformity.

3. Claystone, red, silty, some green mottling at 

top and base. Contains rare polished chert 

pebbles ("gastroliths") as much as 3 inches 

in long diameter- ________________________ 6. 5

2. Sandstone, medium- to coarse-grained; weathers 

light gray to buff; some conglomeratic zones 

of small chert and quartzite pebbles. 8. 5 

Unconformity. . o 

Morrison formation (in part):

1. Claystone, silty with interbeds of argillaceous siltstone, chiefly green and gray 

with silty zones weathering yellowish 

gray, minor red mottling in lower part _ 25. 0

Distribution of conglomeratic lenses along the 

Lytle outcrop in the northern foothills is erratic and,

contrary to common belief, there is no persistent basal 

conglomerate marking the Morrison-Lytle contact. 

Less than half the Lytle outcrop between Indian Creek 

and Boulder, where the formation was mapped at a 

scale of 1:10,000, contains prominent conglomeratic 

units and as much as a third of this outcrop lacks 

conglomeratic beds (fig. 6).

No persistent mappable subdivisions of the Lytle 

formation are recognized in the northern foothills. In 

parts of Larimer County and at a few scattered localities 

to the south, the beds in the upper 5-15 feet of the 

Lytle are commonly variegated claystone and the 

beds below are sandstone and conglomeratic sandstone. 

The Bellvue section, chosen by Lee (1923, p. 4) as the 

type for his Dakota group, shows this arrangement of 

beds, and Lee recognized two separate subdivisions, his 

lower sandstone and lower shale, to include them. 

This arrangement is not persistent along the Colorado 

Front Range, where it is interrupted too repeatedly 

along the strike to afford a valid or practical basis for 

subdivision. Moreover, Lee did not notice the disconformity and included black shale above it as well as 

variegated claystone below it in his lower shale unit. 

The variegated shale at the top of the Lytle formation 

in the Bellvue section is 15 feet thick, about its maximum thickness for the northern foothills. Elsewhere

FIGURE 6.—Basal sandstone lens (L) in the Lytle formation thins out along strike on north wall of Turkey Creek gap, Morrison quadrangle. D, contact of Lytle and 

South Platte formations; Ku, combined Kassler sandstone member and upper sandstone subunit.
DAKOTA GROUP IN NORTHERN FRONT RANGE FOOTHILLS, COLORADO 23

it is rarely more than 10 feet thick and at many places 

it is absent.

CONTACT OF LYTLE AND MORRISON FORMATIONS

The position of the Morrison-Lytle contact cannot 

be located at many places because of the lithic similarity 

between the Lytle formation and the uppermost beds 

of the Morrison formation. Where the Lytle formation 

lacks a conglomeratic basal lens, or where it consists 

of several weakly conglomeratic lenses separated by 

variegated claystone, no single, obvious physical break 

separates the two formations. Even where the Lytle 

is clominantly conglomeratic the position of the contact may be in question because of the presence of 

similar conglomeratic lenses in the upper part of the 

Morrision (fig. 5). A thorough discussion of this 

problem is not within the scope of this paper, but a 

brief review of it is pertinent for an understanding of 

what the Lytle formation is meant to include.

Leroy (1946, p. 59) has recognized the following 

six informal subdivisions in the new type section of the 

Morrison formation (Waldschmidt and Leroy, 1944) 

along West Alameda Parkway 2 miles north of the 

original type section at Morrison.

Units

Sandstone and shale 

Red shale

Gray shale and sandstone 

Gray clay and limestone 

Gray and red shale 

Basal sandstone

Not all these units can be identified away from the 

Alameda Parkway section, and a more generalized 

subdivision is desirable for discussion of the Morrisoa 

in the northern Front Range foothills. Leroy's two 

middle units make up the most characteristic part of 

the Morrison, a light-gray to greenish-gray marl and 

claystone with intercalated thin limestone beds and a 

few thin calcareous sandstone lenses. This zone occupies 100 feet or more of the Morrison throughout 

the Front Range foothills and includes the famous 

dinosaur-bearing beds as well as beds containing the 

Morrison fresh-water molluscan fauna (Yen, 1952, 

p. 28). Above these undoubted Morrison strata the 

sequence locally consists of dominantly red, variegated, 

silty claystone with lenses of sandstone, the whole too 

variable laterally to permit division into useful subunits. At many localities discontinuous lenses of 

conglomerate occur approximately at the contact of 

the dominantly red variegated beds and the underlying greenish-gray beds. The Lytle formation is 

similar to the upper dominantly red variegated zone of 

the Morrison and differs only in its local predominance 

of sandstone and conglomeratic sandstone, in the

somewhat lighter hues of its variegated beds, and in the 

composition of its conglomerates. Conglomerates of 

the Lytle are dominantly chert and quartzite-pebble 

conglomerates with interstitial sand; locally they are 

cemented by chert. Conglomerates at the base of the 

upper Morrison are also made up of chert and quartzite 

pebbles but have considerable interstitial clay commonly 

peppered with ferruginous specks.

Three principal types of stratigraphic relationships 

are found at the Morrison-Lytle contact along the Front 

Range. The most obvious contact is formed where a 

thick, conglomeratic sandstone lens of the Lytle rests 

unconformably on the greenish-gray claystone and marl 

of undoubted Morrison strata. Contacts of this kind 

are most common north of the latitude of Lyons; the 

Spring Canyon section (fig. 5) exposed in the road cut 

at the Spring Canyon dam site on Horsetooth Reservoir 

is an example. At most exposures where conglomeratic 

sandstone of the Lytle rests on the greenish-gray Morrison a thin zone of purple or deep-maroon clay from 

1 to 3 feet thick underlies the contact.

A second type of contact is formed by a basal conglomeratic sandstone lens in the Lytle resting with 

obvious unconformity on some part of the dominantly 

red variegated clay and sandstone in the upper Morrison. Exposures of this type of contact can be found 

on the scarp of the hogback south of Eldorado Springs 

(fig. 5), in Turkey Creek gap south of Morrison (fig. 5), 

on the scarp of the hogback north of Deer Creek (fig. 5) 

and at other places throughout the hogback between 

Boulder and Indian Creek.

The third type of contact is a variation of the second 

type in which the contact itself is obscure because the 

entire sequence of beds between the disconformity at 

the top of the Lytle and the greenish-gray claystones of 

undoubted Morrison consists of alternating lenses of 

sandstone, or conglomeratic sandstone, and variegated 

claystone. Sequences of this type occur where the upper 

Morrison is sandier than usual and the Lytle is more 

argillaceous than usual and lacks a conspicuous conglomeratic lens at its base. Indefinite contacts ca.n be 

seen in the Denver and Salt Lake Railroad cut at 

Plainview (fig. 5), in Eldridge's type Morrison exposure 

at Morrison (fig. 5), and in the new type Morrison exposure in the Alameda Parkway road cut (fig. 5). At 

Plainview, five massive sandstone units, four of them 

conglomeratic to varying degrees, lie between the disconformity at the top of the Lytle and the greenish-gray 

Morrison. Lateral tracing from the Eldorado Springs 

section, where the base of the Lytle is clear cut, indicates that the second lens from the top in the Plainview 

cut is in the correct relative position to be the basal lens 

of the Lytle.
24 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

ELD RIDGE TYPE LOCALITY OF THE MORRISON

An indefinite contact between the Morrison and Lytle 

formations in both the old and new type sections of the 

Morrison raises the question of where its upper contact 

was originally placed. Eldridge's (1896, p. 60-62) 

definition of the Morrison did not include a measured 

section, and his description of the upper limit of the 

formation has been interpreted in different ways.

Lee's (1920) idea that Eldridge included plantbearing beds of the Dakota in his original definition of 

the Morrison has been accepted by many geologists 

(Stokes, 1944, p. 954-956; Keeside, 1952, p. 22); others 

(Waldschmidt and LeRoy, 1944, p. 1100) contend that 

Lee misinterpreted Eldridge and that the MorrisonDakota contact as defined by Eldridge is acceptable. 

Lee's (1920, p. 183-184) interpretation of Eldridge's 

Morrison-Dakota contact was based on the word of 

G. L. Cannon, a Colorado geologist who assisted in the 

fieldwork for the Denver basin monograph. According 

to Lee, Cannon intimated that Eldridge placed the 

contact in question at the base of the ledge-forming 

sandstone capping the Dakota hogback at Morrison, a 

horizon above the middle of the gray shale-bearing part 

of the Dakota group in this area. In terms of the 

nomenclature introduced in the present report this horizon is the base of the Kassler sandstone member of 

the South Platte formation (fig. 7).

Eldridge's original description of the Morrison not 

only fails to offer evidence in favor of Lee's interpretaFIGUKE 7.—Several interpretations of the contact between the Morrison formation 

and the Dakota group in the Denver area.

tion, but contains lithic descriptions and thickness 

figures that serve to refute Lee's interpretation. Eldridge states (1896, p. 61) that the "upper third" of the 

Morrison is "a succession of sandstones and marls" 

which includes a conglomeratic sandstone at the base 

and, above it, sandstone interbedded with shale. He 

specifically describes the shale as "similar to those 

comprising the bulk of the Jura," in other words, variegated shale, not gray or black shale. A comparison 

of the relative thicknesses of the units described by 

Eldridge further demonstrates that he could not have 

included any of the plant-bearing Dakota with his 

type Morrison. Eldridge gives an average thickness 

of 200 feet for the Morrison; consequently the part of 

the formation he refers to as the "upper third" can be 

expected to be about 60 or 70 feet thick. In the section 

measured by the writer at Morrison, 65 feet of variegated clay and sandstone, including a weakly conglomeratic sandstone, lie between the dominantly greenish-gray Morrison claystones and the probable base 

of the Lytle formation. Actually the Morrison is 

about 270 feet thick at the type locality, not 200, but 

even a third of this larger figure would not make 

the upper part of the Morrison more than 100 feet 

thick. Yet, if Lee's interpretation is applied to the 

section measured at Morrison, the "upper third" of the 

Morrison is 228 feet thick, and the formation as a 

whole is between 350 and 400 feet thick, or about double 

Eldridge's figure. By Lee's interpretation the overlying Dakota, said by Eldridge to be between 225 and 

300 feet thick, is only 78 feet thick. Certainly Eldridge would have mentioned local deviations of this 

magnitude in the thickness of his formations had they 

existed.

Lee (1920) revised Eldridge's type Morrison section, 

placing the upper contact under the so-called Saurian 

sandstone which marked the base of the upper third 

of Eldridge's Morrison. His stated reason for this 

revision was that "No obvious break was found in the 

section between the plant horizon and the Saurian 

conglomerate" (Lee, 1920, p. 185). In the Turkey 

Creek section 1.8 miles south of the type Morrison, 

Lee (1927, p. 28) placed his Morrison-Dakota contact 

at the base of the Lytle, apparently unaware that the 

break he had chosen as the contact at Morrison l&y 

some 98 feet stratigraphically below. Throughout the 

foothills region Lee (1927) confused the two conglomeratic zones in question and consistently placed his 

Morrison-Dakota contact at the base of the most 

obvious conglomerate zone, apparently unaware that 

more than one such zone existed. This is understandable when one recalls that Lee's study of the Dakota 

group along the Front Range was made with reference 

to the "type section" of his Dakota group at Bellvue
DAKOTA GROUP IN NORTHERN FRONT RANGE FOOTHILLS, COLORADO 25

where the entire upper variegated part of the Morrison 

is absent and the Lytle rests directly on the greenishgray marl and claystone of the Morrison.

An examination of the Morrison type section with 

its lack of conglomeratic beds and indefinite MorrisonLytle contact indicates that Eldridge based his description of the Morrison on its aspect throughout the 

Denver basin area rather than at Morrison, Colo. 

Had he presented a measured section of the type it 

would have been difficult for him to indicate his 

Morrison-Dakota contact because of the absence or 

obscurity of the basal conglomerate of the Dakota. 

Two clues to Eldridge's conception of the position of 

the Morrison-Dakota contact are implicit in his description of these two formations. The first is his recognition of two horizons at which conglomeratic lenses are 

commonly located, one at the base of his Dakota and 

one at the base of the upper third of his Morrison. 

The second is the presence of variegated clay, rather 

than black shale, in the sandy strata between these 

two conglomeratic zones. The pattern of two conglomeratic zones separated by beds containing variegated clay corresponds closely to the sequence including 

the basal conglomerate of the Lytle and the underlying variegated beds of the upper part of the Morrison 

with their discontinuous basal conglomeratic zone. 

Consequently, it is most likely that the Morrison-Lytle 

contact of the present report corresponds to Eldridge's 

Morrison-Dakota contact.

ALAMEDA PARKWAY TYPE SECTION

Waldschmidt and LeRoy (1944, p. 1100) in recommending a new type section for the Morrison formation 

recognize that Lee (1920) misinterpreted Eldridge's 

Morrison-Dakota boundary. Nevertheless they believe that Lee corrected this error in a subsequent paper 

(Lee, 1927), for they state that

. . . the Morrison lies with apparent disconformity below the 

conglomeratic phase of the Dakota as it was originally defined 

by Fldridge (1896, p. 60-62) and Lee (1927, p. 28) ...

Lee's 1927 work offers a somewhat more accurate section of the type Morrison than the estimated section 

presented in his 1920 work, but the two sections are 

obviously much the same and can be matched to show 

that Lee did not change the position of his MorrisonDakota contact in his 1927 work. Consequently the 

top of the Morrison as defined by Eldridge (1896, p. 

60-62) and as redefined by Lee (1927. p. 28) is in different stratigraphic positions, Lee's being at the base of 

the conglomeratic zone in the upper third of Eldridge's 

Morrison formation.

The new type section of the Morrison formation 

along west Alameda Parkway is better exposed than 

Eldridge's type section at Morrison but the upper

contact is equally obscure. None of the many thin 

sandstone lenses between the disconformity at the top 

of the Lytle formation and the greenish-gray claystones 

of the Morrison contain pebbles of chert and quartzite. 

Waldschmidt and LeRoy have chosen the base of the 

uppermost massive sandstone lens in the Lytle as the 

top of their Morrison, but this ledge thins out into 

variegated claystone a short distance north of the road 

cut. The statement (Waldschmidt and LeRoy, 1944, 

p. 1100) that this sandstone is conglomeratic in the 

Alameda exposure should be modified inasmuch as the 

conglomerate consists of scattered clay pellets but lacks 

the chert and quartzite pebbles so typical of the 

"conglomeratic phase of the Dakota." Chert and 

quartzite pebble conglomeratic sandstones, presumably 

at the base of the Lytle, crop out locally along the 

hogback north of the Parkway and, when followed 

laterally toward the road cut, apparently grade into 

nonconglomeratic sandstone that lies between 35 and 

55 feet below the Morrison-Dakota contact chosen by 

Waldschmidt and LeRoy. Slope wash prevents continuous lateral tracing from the conglomeratic outcrops 

so that the exact contact of the Morrison and Lytle 

formations in the Alameda type section remains in 

doubt.

POST-MOREISON WARPING

The contact of the Lytle formation with both the 

greenish-gray part of the Morrison formation and its 

upper variegated sandy part suggests either local, deep, 

pre-Lytle channeling, slight angular unconformity, or a 

combination of the two. Detection of discordance in 

bedding at the contact is impossible because the sandstone bodies on either side are irregularly lenticular and 

crossbedded. In the Canon City area and to the south 

in the Wet Mountains the overlap of older formations 

by the Lytle sandstone member of the Purgatoire 

formation (or Dakota) has been noted by a number of 

authors. Although some of these occurrences need to 

be corroborated by further study, there is evidence 

that the Morrison is locally overlapped by the Lytle. 

In western Oklahoma Stovall (1943, p. 60) has demonstrated gentle angular unconformity between the 

Morrison and Purgatoire. For the northern foothills 

of the Front Range all that can be said with certainty 

is that the Lytle rests unconformably on different parts 

of the Morrison which are stratigraphically as much as 

100 feet apart. The fact that this crosscutting of the 

underlying Morrison takes place gradually along the 

strike rather than in abrupt local channels strongly 

suggests slight angular unconformity.

SUMMARY

The beds between the disconformity that marks the 

top of the Lytle formation and the greenish-gray clay-
26 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

stones of undoubted Morrison are highly lenticular and 

variable in thickness and lithology. They contain two 

horizons at which conglomeratic lenses are locally 

common. The upper one of these has the thicker and 

more persistent conglomeratic lenses and the unconformity at their base is taken as the Morrisoii-Lytle 

contact. As far as can be inferred from the original 

description of the Morrison formation, this contact 

corresponds to the Morrison-Dakota contact of Eldridge. Slight angular unconformity is indicated at the 

base of the Lytle formation in additon to local channeling. The conglomeratic lenses of the Lytle are generally the more resistant and consequently are more 

prominent on the outcrop. Where they are absent, or 

iioiiresistant, or separated into many benches by variegated claystone, the Morrisoii-Lytle contact is indefinite. Throughout most of the northern foothills the 

local conglomeratic lenses at the base of the Lytle crop 

out near enough to one another so that the position of 

the Morrisoii-Lytle contact can be interpolated between 

them. Where it is not possible to recognize the contact 

over a large area, it is expedient to map the Lytle with 

the Morrison as an undifferentiatecl unit.

CONTACT OF LYTLE AND SOUTH PLATTE FORMATIONS

Beneath the disconformity that marks the top of the 

Lytle formation the beds appear to be leached. Yellow 

to white, soft clay, argillaceous siltstone, and finegrained sandstone with ferruginous panlike concretionary 

layers form a distinctive zone that is helpful in locating 

the disconformity on the outcrop. Tests made of this 

material where it is chiefly clay or claystone show that 

the alumina content is high just beneath the contact 

but decreases abruptly downward. Locally the clay 

just beneath the contact qualifies as a high heat duty 

refractory clay, but the clay bodies are too thin and 

discontinuous to be mined. Clay below this zone is 

generally light shades of red and green and is well below 

refractory grade. At a very few places the leached 

zone is thin or entirely lacking, but the sharp contact is 

nonetheless evident because of the contrasting rock 

types of the beds above and below it (figs. 8, 9).

AGE

The only organic remains found to date in the Lytle 

formation in the northern Front Range foothills are 

chertified fragments of coniferous wood. No fossils

FIGURE 8.—Tabular sandstone of the Plainview sandstone member of the South Platte formation (above) in disconformable contact (D) with massive light-colored sandstone and claystone of the Lytle formation at Spring Canyon dam site, Fort Collins quadrangle.
DAKOTA GROUP IN NORTHERN FRONT RANGE FOOTHILLS, COLORADO 27

FIGURE 9.—Silty shale and basal sandstone of the Plainview sandstone member of the South Platte formation in disconformable contact (at man's hand) with argillaceous sandstone of the underlying Lytle formation on hogback south of Eldorado Springs.

have been described from equivalents of the Lytle in 

eastern Colorado, but beds in a similar stratigraphic 

position in Wyoming (Cleverly formation in part) 

locally contain a sparse assemblage of fresh-water 

mollusks and charophytes which are considered (Yen, 

1946 and 1951; Peck, 1941) Early Cretaceous in age. 

The mollusks and the charophytes are distinct from 

similar fossils in undoubted Morrison strata. On the 

basis of its physical correlation with fossiliferous beds 

in Wyoming the Lytle formation is classified as Early 

Cretaceous.

SOUTH PLATTE FORMATION 

DEFINITION

The upper part of the pre-Benton Cretaceous sequence, lying between the Lytle formation and the 

Benton shale, is here treated as a single unit in the 

northern Front Range foothills and named the South 

Platte formation. It is equivalent to the upper sandstone, upper shale, middle sandstone, and uppermost 

black shale part of the lower shale of Lee's Dakota 

group. The South Platte formation grades laterally 

northward from a dominantly nonmarine clastic phase 

in the Kassler quadrangle to a dominantly marine shale

332778—55———3

phase in Larmier County, consequently no single 

exposure is typical for the unit over its entire area of 

outcrop, and three localities rather than a single locality 

are necessary as standards for reference. The formation is named for the South Platte River inasmuch 

as the southernmost of the three standard sections, 

selected as the type because it shows the maximum 

number of subunits, is exposed on the north side of the 

gap made by the river through the Dakota hogback 

in the Kassler quadrangle, Jefferson County. In 

addition, the entire outcrop area of the formation in 

the northern foothills lies within the drainage basin of 

the South Platte River.

LITHOLOGY

The South Platte formation consists of 200-350 feet 

of alternating units of gray to black shale and brownweathering sandstone. The sandstone is chiefly fine 

grained but coarser fractions are common locally in the 

nonmarine phase. Individual sandstone units have 

considerable lateral extent and vary laterally in thickness, nature of bedding, and amount of contained argillaceous matter. Quartzitic sandstone, in the sense that 

siliceous cement causes the rock to break across the
28 SHOETER CONTRIBUTIONS TO GENERAL GEOLOGY

grains, is rare and is confined largely to the top of the 

uppermost massive sandstone bed.

The shale units in the nonmarine phase consist of 

hard, laminated, noncalcareous, silty shale that is 

commonly interlaminated or thinly interbedded with 

siltstone and fine-grained sandstone. Differential thermal analyses of these shale units show that kaolinite is 

the dominant clay mineral. Some of the shale is relatively silt free and highly refractory. The transition 

into the marine phase is marked by (1) a change of the 

dominant clay mineral in the shale units to illite, (2) a 

decrease in the sand content of most shale units, and 

(3) the appearance of zones of calcareous shale contain 

thin beds of fossiliferous silty limestone.

Thin beds of white, yellow, or light-gray claystone 

presumed to be altered volcanic ash are common in the 

shale. These beds hold fixed positions relative to other 

lithic units and some are very useful as key beds inasmuch as they are laterally persistent and can be traced 

from the nonmarine phase into the marine phase. In 

the nonmarine phase they are dominantly kaolinitic and 

rarely exceed 3 inches in thickness; locally they are 

hard enough to be classed as porcellanite. One bed 

(the second key marker in fig. 10.) was sampled at 

intervals from the nonmarine into the marine phase; its 

dominant clay mineral changed along the strike from 

kaolinite to montmorillonite.

One thin zone of slightly fossiliferous marine siltstone extends through most of the nonmarine phase 

and is useful, along with the larger shale and sandstone 

units and the thin beds of altered volcanic ash, for 

correlating between the phases of the South Platte 

formation.

SOUTHERN NONMARINE PHASE

Exposures of the nonmarine phase of the South Platte 

formation extend from the south end of the northern 

foothills in the Kassler quadrangle, Douglas County, 

northward to about Coal Creek in the Ralston Buttes 

quadrangle, Jefferson County. Within this area the 

formation exhibits its maximum number of subunits. 

No single exposure of the formation shows each subunit 

in its entirety, either because of stratigraphic variation 

such as the local thickening of sandstone subunits at 

the expense of underlying shale subunits, or because of 

masking by talus and slope wash. The type section of 

the formation, which is also the standard for the nonmarine phase, is given on a later page. This type 

section contains all the subdivisions but some of the 

shale subunits are incomplete. Consequently an annotated composite section, figure 10, is used for reference 

in describing the subdivisions of the formation. The 

descriptions and the annotated graphic section give 

only those details pertinent to the aims of this paper.

c

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Platte formatioi

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Lithology of subunits

First sandstone: brown-weathering, finegrained sandstone; interbeds of gray silty 

shale in upper half. Lower half chiefly 

massive, cross-laminated sandstone

First shale: dark -gray refractory shale and 

First silty shale with interbeds of fine-grained

marker at base of main refractory shale bed

• 10

- 20 Second sandstone: brown-weathering

massive fine-grained sandstone 

•30 

Second

marker

Second shale: gray shale, minor siltstone

Marine

Third sandstone: silty, fine-grained

sandstone, nonresistant

Third shale: two zones of silty gray shale

separated by siltstone and sandstone

—————————————————— DISCONfOf>Min(?) ————————————————

Fourth sandstone: brown-weathering tabular, 

cross-laminated sandstone

^X^Fourth shale: gray silty shale, locally absent 

Basal conglomeratic sandstone

FIGURE 10.—Composite section of the dominantly nonmarine phase of the South 

Platte formation.

PIAINVIEW SANDSTONE MEMBER

The fourth sandstone subunit (fig. 10) at or just above 

the base of the South Platte formation is a persistent, 

distinctive unit throughout all phases of the formation 

in the northern Front Range foothills. This subunit 

forms a characteristic brown- to rusty brown-weathering 

ledge of markedly tabular, cross-laminated sandstone. 

In some places it rests directly on the disconformity at 

the base of the South Platte formation but commonly 

it is underlain by a unit of black silty shale (fourth shale 

of fig. 10) which in turn rests on the thin conglomeratic
DAKOTA GROUP IN NORTHERN FRONT RANGE FOOTHILLS, COLORADO 29

sandstone at the base of the formation. The fourth 

sandstone subunit and, to simplify mapping, the black 

shale and basal conglomerate between it and the disconformity at the base of the South Platte formation, 

are named the Plainview sandstone member for exposures (fig. 11) which occur in the cut of the Denver and 

Salt Lake Railroad (Moffat Tunnel Route) through the 

Dakota hogback at Plainview, NE^NEK sec. 12, 

T. 2 S., R. 71 W., Jefferson County. The foUowing 

section was measured.

Description

South Platte formation (in part).

8. Sandstone, fine-grained; weathers light 

gray; and sandy siltstone, thin-bedded, 

with interbeds of gray plastic silty shale 

0.1-0.6 foot thick. Bedding surfaces Feet 

with ripple marks and worm? castings. _ 5. 0 

Plainview sandstone member:

7. Sandstone, fine- to medium-grained, tabular cross-laminated to crossbedded; 

weathers red brown. Some bedding surfaces ripple-marked. Ridge-forming_ _ _ _ 24. 0

6. Sandstone, fine-grained, irregularly thin 

bedded locally crossbedded, in layers as 

much as 0.3 foot thick; ferruginous layers 

weather rusty brown; argillaceous layers 

weather light gray. Lower 2.2 feet has

South Platte formation (in part)—Continued 

Plainview sandstone member—Continued

interbeds of fissile gray clay shale. Feet 

Worm? castings on bedding surfaces____ 3. 2

5. Sandstone, fine- to medium-grained, tabular cross-laminated and crossbedded in 

uppermost 2 feet, remainder massive 

cross-laminated; weathers buff with reddish stain_ _________________________ 5. 8

4. Siltstone, gray argillaceous, thin-bedded, 

with interbeds of shaly siltstone and 1 

bed of dark-gray shale 1.5-2.1 feet from 

top_______________________ 3. 0

3. Shale, dark-gray to black, silty with thin 

interbeds and laminae of light-gray argillaceous siltstone. Local thin concretionary lenses of pyritic silt. Basal 0.8 foot 

ferruginous sandy siltstone. Worm? 

castings on silt bedding planes_________ 6. 2

2. Sandstone, light-gray, medium- to coarsegrained and conglomeratic, with scattered 

chert and quartzite pebbles as much as 

0.4 foot in long diameter._____________ 1. 0-4. 0

Disconformity.

Lytle formation (in part):

1. Siltstone and silty clay stone, greenish-gray 

to gray-white with local reddish stain, 

soft, contains irregular, thin lenses of 

red-weathering, ferruginous, fine-grained 

sandstone. _______________ ____________ 4. 0

FIGUBE 11.—Plainview sandstone member of the South Platte formation (right) and upper part of the Lytle formation (left) in railroad cut atPlainview. Man stands at 

basal conglomeratic sandstone of Plainview member. D, Lytle-South Platte disconformity.
30 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Units 5-7 comprise the characteristic sandstone part 

of the Plainview sandstone member. In the type section an 11-foot bed of similar sandstone lies about 5 

feet above the top of the member and appears as if it 

should be included with it. As far as can be determined from outcrops to the north and south, this overlying sandstone is probably part of a locally thickened 

lens of the third sandstone subunit. Within a distance 

of 2.3 miles north of Plainview, at Eldorado Springs, 

this lens reaches a maximum thickness of 65 feet and 

occupies the position of both the third shale and third 

sandstone in the area between Coal Creek and Boulder; 

elsewhere in the northern foothills the distinctive sandstone of the underlying Plainview member is the only 

ledge-forming unit in the lower part of the South Platte 

formation.

At many places the shale part of the Plainview sandstone (fig. 8) member is absent and at a few places the 

basal conglomeratic sandstone is absent. In the road 

cut along Colorado Route 186 at Spring Camion dam 

site, Horsetooth Reservoir, Larimer County, the sandstone portion of the Plainview member rests directly 

on the surface of disconformity on the Lytle formation.

The Plainview sandstone member is the only subdivision that extends essentially unchanged, except for 

variation in thickness and shale content, throughout all

phases of the South Platte formation in the northern 

foothills. The Plainview is from 10 to 70 feet thick in 

the nonmarine phase and has an average thickness of 

about 30 feet in the northern marine phase, where it 

forms the western ridge of the familiar double hogback 

of the Dakota group.

BEDS BETWEEN THE PLAINVIEW AND KASSIER SANDSTONE MEMBERS

In the nonmarine phase the sequence of beds between 

the Plainview and the Kassler sandstone members possesses a number of small subunits many of which are 

laterally persistent. Complete exposures of these beds 

are rare as they contain few resistant layers and generally underlie wash and grass-covered slopes along the 

scarp face of the hogback. The single outstanding exception to this is the lens of tabular, cross-laminated 

sandstone that overlies the Plainview member between 

Coal Creek and Boulder. For the northern foothills 

area as a whole, only the details of the uppermost beds, 

the second shale of figure 10, are pertinent to this report.

The second shale is commonly exposed, at least in part, 

under the overhanging ledge formed by the Kassler 

sandstone member. The shale is dark gray to black 

and contains in its upper part a prominent bed of altered 

volcanic ash which is locally as much as 4 inches thick 

(fig. 12). Thisbed, thesecond key marker, is the most perFi> '.H,K iz.- ~>econd shale subunit of tne »outn riatte formation, north side of Ralston Reservoir, Ralston Buttes quadrangle. K, base Kassler sandstone member;

Mb, second key marker bed; Mz, platy siltstone of marine zone.
DAKOTA GROUP IN NORTHERN FRONT RANGE FOOTHILLS. COLORADO 31

unit lying between the Kassler sandstone member and 

the first sandstone subunit. This shale unit, which has 

been the chief source of clay for the refractories industry 

of the Denver-Golden area since 1865, warrants formal 

recognition as a unit of economic value and is here 

named the Van Bibber shale member for exposures in 

the vicinity of Van Bibber Creek, Golden quadrangle, 

Jefferson County. No single exposure in this area 

shows the entire member clearly. The type section 

is a composite of adjacent exposures on the scarp of the 

hogback, between 200 and 500 yards north of Van 

Bibber Creek; the southernmost of these exposures, 

showing the main clay bed, is in the airhole of the 

Golden Fire Brick Co. mine.

sistent of all such layers in the South Platte formation 

and can be followed throughout the northern foothills 

except in local areas where it was removed by channeling before the deposition of the Kassler member. The 

lower half of the second shale contains a zone of silty 

shale interbedded with platy siltstone and fine-grained 

sandstone which weathers a characteristic orange brown. 

These platy beds are from 2 to 10 feet thick and locally 

contain, in about the upper 4 to 10 inches, a sparse 

scattering of marine fossils the most common of which 

are Inoceramus comancheanus Cragin and Pteria salinensis White. This is the only marine zone known in the 

dominantly nonmarine phase of the South Platte formation; it has been found along the hogback as far as 

6 miles south of the South Platte River in the Kassler . .

quadrangle. North from Boulder the fossiliferous platy _, „ _,, ,, , x . ,. A .

, i , in i i-i South Platte formation (in part): Feet beds become gradually more calcareous and, m the ma- 22 SandBtoM| fine.grained, tabular, in beds

rine phase, they form a conspicuous ledge of platy, fossil- 0.5-2.0 feet thick, dense with promiferous, silty limestone that is locally petroliferous inent vertical jointing; weathers buff

(fig. 17). t° brown. Local current ripple marks

KASSLER SANDSTONE MEMBER °' 5 f°ot between crests on basal surface. 5. 0-8. 0

	Unconformity.

Throughout most of the nonmarine phase in the Van Bibber shale member:

southern part of the northern foothills area the first 21. Shale, dark-gray, silty, hard, with intersandstone subunit at the top of the South Platte forma- beds of ^-grained shaly sandstone 

,. ,. -ivi f .1 i i i i ^i ^ and platy siltstone in uppermost foot; tion lies on the dip slope of the hogback and the crest weatherg light bluigh gray __________ 3. 5

of the hogback is formed by the second sandstone sub- 2a siltstone and fine-grained sandstone,

unit. The latter is the dominant ledge-forming sand- weather, rusty brown, argillaceous._'___ 1.2

stone of the South Platte formation from the Golden 19. Shale, dark-gray, hard, silty, locally less

quadrangle, where it is about 30 feet thick, south to the silty> semiplastic; weathers light bluish

fr | , , i -i-Ui >-rr 1-1 f\(\ gray. Contains thin kaolinitic gray- Kassler quadrangle, where it is between 75 and 100 ,:, , , , . .,,, * OK J1 & ' white marker bed m middle part-_____ 2.5

feet thick. As it thickens southward, scattered chert 18 Sandstone, fine-grained, argillaceous, irand quartzite pebbles appear in its basal part. North regularly bedded; weathers light gray 

of the Golden quadrangle it is too thin to be mapped, with rusty stain; upper half with interbut can be traced as far as the Eldorado Springs quad- beds argillaceous siltstone. ___________ 2. 8

i i -j. • i ,-,nj!j_,i--i,j_-i ii-j. 17. Shale, dark-gray, finely silty, hard; rangle, where it is about 10 feet thick; to the north it ' ,-v, 1, • , i c & ' . • -i weathers light bluish gray------------ 1.8

appears to grade gradually into marine siltstone and 16 Sandstone, fine-grained, tabular, crossshale, laminated, in beds 0.4-2.0 feet thick; 

The second sandstone subunit is here named the weathers buff to brown. Bedding surKassler sandstone member for exposures in the Kassler faces ripple-marked———________ ____ 5. 5

quadrangle. Its type section, at the end of the hogback 15' Sha^' as in uf'' 17' with thin , intfrbeds'

?• _ ., ° i r i n i T-II T,- T^ T • siltstone and fine-grained sandstone in 0.5 mile north of the bouth Platte River at Kassler, is lower part________________ 4. 0

a part (units 23-25) of the type section of the South 14. Sandstone, as in unit 16, in beds as much

Platte formation given on a subsequent page. Formal as l foot thick_____ — ______________ 2. 0

recognition of the Kassler sandstone member is war- I 3- Shale- 8ray> silty in lower Part» refractory,

ranted, (1) for purposes of correlation with south- 10 suboonchoidal fracture. —- 1.0

, i 11 ,1 ^ i i i • • • 12- Sandstone, rusty-brown, fine-grained, lo- central and southeastern Colorado where it is persistent call ferruginous______________ .3

and widespread, (2) as a map unit of value in delimiting n. shale, gray, semiplastic, refractory, suboptimum areas for prospecting refractory clay. It is conchoidal fracture; lower 1.5 feet pepalso of value in locating the thin, persistent zone of pered with minute white to yellowish 

platy fossiliferous beds a few feet below its base. ferruginous ooliths that are locally con-

	centrated m bedding laminae-_--_,--- 4. U 

VAN BIBBER SHAIE MEMBER 10- Shale, gray, stained red and rusty brown;

	contains concentration of ferruginous

In the nonmarine phase of the South Platte formation ooliths that locally form concretionary

most of the refractory shale is contained in a mappable structures.________________--------_ 0. 5-1. 0
32 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

South Platte formation (in part)—Continued 

Van Bibber shale member—Continued

9. Shale, gray with rusty stain, semiplastic, 

refractory, with scattered ferruginous 

eoliths, oolitic laminae and local concretions as much as 3 feet in diameter Feet 

composed largely of the eoliths______ 3. 0

8 Shale, dark-gray, semiplastic, refractory,

with scattered silt laminae__________ 6. 0

7. Shale, dark-gray, silty with interbeds of 

rusty brown-stained siltstone in lower 

part-______________________________ 1. 5

6. Kaolinitic clay, yellowish-white, first key

marker bed_________________________ .1

5. Siltstone and fine-grained sandstone, gray, 

weathers light gray with local brown 

stain. Some interbedded silty claystone, sandstone in upper part, top 

surface locally ripple-marked_________ 2. 3

4. Shale, dark-gray, silty with interbeds of

hard siltstone_______________________ 4. 0

3. Siltstone, irregularly thin bedded with 

sandy and argillaceous layers, hard, 

locally resistant; weathers light gray 

with rusty stain____________________ 3. 5

South Platte formation (in part)—Continued 

Van Bibber shale member—Continued

2. Siltstone, probably soft argillaceous, 

scured _________________________

ObTotal thickness Van Bibber shale member- _____________________________

Feet

. 5

50

Kassler sandstone member:

1. Sandstone, fine-grained, locally silty, 

chiefly irregularly thin bedded to crossbedded, beds 0.3-0.5 foot thick, locally 

massive, cross-laminated; weathers gray 

to brown__________________________ 22. 0

Most of the parts of the Van Bibber shale member 

persist throughout its area of outcrop but vary in thickness. The main shale bed (units 6-15) and, generally, 

the three thin shale beds above it (units 17, 19, and 21) 

are refractory; the lower bed (unit 4), here called the 

lower split, is only locally refractory. The most distinctive beds are the first key marker (unit 6), a persistent white, yellow or light-gray kaolinitic ash bed 

from 2 to 4 inches thick, and the ripple-marked sandFIGURE 13.—Characteristic exposure of base of main clay bed in the Van Bibber shale member of the South Platte formation. First key marker (Mb) rests on ripplemarked surface.
DAKOTA GROUP IN NORTHERN FRONT RANGE FOOTHILLS, COLORADO 33

stone (unit 5), a bed of fine-grained sandstone whose 

indurated upper surface is covered with ripple marks. 

The first key marker commonly rests directly on the 

ripple-marked surface, or is separated from it by a few 

inches of silty shale (fig. 13). The ripple-marked bed 

and the overlying ash bed serve to identify the main 

shale bed from the northernmost mines near Coal 

Creek in the Ralston Buttes quadrangle to the end of 

the hogback in the Kassler quadrangle. The first key 

marker can be traced north of Coal Creek and for some 

distance into the marine phase of the South Platte 

formation (fig. 17).

The type section of the Van Bibber shale member is 

the thickest of the many sections measured; more 

commonly an entire section of the member ranges from 

35 to 45 feet thick. The lower contact with the 

Kassler member is sharp where the lower split or the 

silty beds beneath it rest directly on the massive sandstone of the Kassler. In areas where the Kassler 

member is thick the lower split of the Van Bibber 

member is commonly absent and the ripple-marked 

sandstone is in contact with the Kassler sandstone.

The upper contact of the Van Bibber member is an 

unconformity of considerable local relief. Channeling 

before the deposition of the overlying first sandstone 

subunit has removed part or all the Van Bibber member 

over large areas; consequently complete sections of the 

member are rare.

FIRST SANDSTONE SUBUNIT AND THE BENTON CONTACT

The first sandstone subunit is the most variable part 

the South Platte formation in the southern normarine phase. In the Kassler quadrangle this unit is 

dominantly sandstone and has a fairly persistent shale 

bed a short distance above the middle. Northward 

more shale lenses come into the unit but no persistent 

pattern of parts is discernible other than a general 

tendency for the thickest sandstone bed to be at the 

base of the unit. Where this basal sandstone bed is 

incised deeply into the underlying beds it contains 

scattered chert and quartzite pebbles in its lower few 

inches.

Throughout the southern part of the northern foothills area, a zone of platy siltstone and interbedded 

silty shale from 5 to 20 feet thick lies between the black 

shale of the Benton and the first bed of brown-weathering, fine-grained sandstone typical of the South Platte 

formation. . The top of the South Platte formation in 

the nonmarine phase is drawn at the top of this uppermost bed of fine-grained brown-weathering sandstone, 

and the overlying platy beds are included in the base 

of the Benton shale. At a number of exposures in the 

Kassler quadrangle the contact between the platy beds 

and the underlying sandstone is sharp, and the upper

inch of the sandstone bed contains small pebbles and 

granules of quartzite and chert, which may have been 

winnowed by current action on a surface of disconformity. This contact, though locally sharp, is not as 

suggestive of disconformity in other exposures of the 

nonmarine phase, and on cursory inspection the platy 

beds appear to be a transition zone in a gradation of 

beds between the South Platte and Benton formations. 

It remains to be demonstrated whether a persistent 

disconformity is actually present at the base of the 

platy beds throughout the area.

The first sandstone subunit is between 75 and 100 

feet thick throughout most of the southern part of the 

northern foothills. North of Golden this subunit thins 

appreciably, decreasing to little more than 25 feet in 

thickness at Eldorado Springs. Here the uppermost of 

three thin beds of sandstone that compose the subunit 

is quartzitic and filled with straight and U-shaped 

tubelike structures probably made by burrowing organisms. The overlying platy transition beds are about 

20 feet thick and grade imperceptibly into the Benton 

shale.

LATERAL VARIATION •

Within the nonmarine phase of the South Platte 

formation the sandstone subunits, in particular the 

Kassler sandstone member, thicken southward at the 

expense of shale subunits, and at the south end of the 

area, between Willow Creek and Indian Creek in the 

Kassler quadrangle, the formation consists largely of 

sandstone and conglomeratic sandstone (fig. 14). 

Superimposed on this progressive southward thickening 

and coarsening of the sandstone subunits are many 

local stratigraphic variations, the most common of 

which involves the first sandstone subunit and the 

unconformity at its base. At many places along the 

hogback, local thick lenses of sandstone in the base of 

the first sandstone subunit lie in channels cut to different depths in the underlying Van Bibber shale 

member. This feature accounts in large part for the 

erratic distribution of refractory shale bodies in the 

Van Bibber member. Northward from a point slightly 

more than 1 mile north of the Alameda Parkway the 

first sandstone subunit rarely thickens at the expense 

of the Van Bibber member and the refractory shale 

beds are relatively continuous. South of this point, 

the first sandstone subunit is characteri ed by massive 

basal sandstone lenses incised deeply into or through 

the Van Bibber member and, except for a small area 

south of the South Platte River, only local pockets of 

the refractory shale beds remain.

The greatest depth of incision of the channels at the 

base of the first sandstone unit is in the MorrisonTurkey Creek area. Here the South Platte formation
34 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

FIGXJEE 14.—Sandy, nonmarine phase of the South Platte formation south of Willow 

Creek gap, Kassler quadrangle. Upper ledge is Kassler sandstone member, lower 

ledge contains Plainview sandstone member (P) and Lytle formation (£); saddle 

between ledges is chiefly in friable third sandstone subunit.

is between 180 and 195 feet thick, its minimum thickness for in the northern foothills. South from the 

Alameda Parkway the unconformity at the base of the 

first sandstone subunit gradually transects the Van 

Bibber member bringing the basal massive sandstone 

of the first sandstone subunit in contact with the 

Kassler sandstone member. Within the same distance 

south from Alameda Parkway, the base of the Kassler 

member also cuts across underlying beds, consequently 

much of the underlying second shale subunit is missing. 

The massive cliff-forming sandstone that forms the 

crest of the hogback in the Morrison-Turkey Creek 

area is made up of the coalesced first sandstone subunit 

and the Kassler member. The reduction in the thickness of the formation is accounted for by the loss of the 

Van Bibber member and probably all of the second 

shale subunit. Elsewhere along the hogback to the 

south, the first sandstone subunit locally coalesces with 

the Kassler member, but at no place outside the Morrison-Turkey Creek area is the coalesced unit so deeply 

incised in the underlying beds.

TYPE LOCALITY

The type exposures of the South Platte formation 

are in the south end of the hogback on the north side 

of the gap made by the South Platte River, about 0.5 

mile north of Kassler, in the NEXNWXSEK sec. 27, 

T. 6 S., R. 69 W. A complete pre-Benton Cretaceous 

section, down to the top of the Morrison, was measured 

in offset exposures easily joined by tracing key horizons 

northward along the strike.

Description

Benton shale not exposed. 

South Platte formation:

35. Sandstone, fine-grained, thinly interbedded 

with siltstone, light-gray with sandy 

ledges stained rusty brown, platy, lo- Feet 

cally argillaceous.___________________ 15. 3

34. Sandstone, fine-grained, massive, soft, 

locally pitted surface; weathers light 

gray with local rusty stain__________ 5. 0

33. Shale, brown to gray, silty, grading to

light-gray argillaceous siltstone,_____ 3. 0

32. Sandstone, fine-grained, massive; weathers

brown. ____________________________ 3. 7

31. Shale, gray and brownish-gray, silty; contains 1 foot argillaceous siltstone at 

base_______________________________ 4. 2

30. Sandstone, fine-grained, massive- to thinbedded, laminated to cross-laminated, 

interbedded with light-gray friable siltstone; weathers brown_______________ 8.2

29. Sandstone, fine- to medium-grained, tabular, cross-laminated to crossbedded; 

weathers gray; basal 3 or 4 feet contain 

scattered clay pellets._______________ 45. 7

Uncomformity.

Van Bibber shale member:

28. Shale, gray, plastic. ___________________ . 4

27. Shale, gray, silty, interbedded with platy 

to irregularly bedded siltstone and finegrained sandstone_----____-----_____ 2. 5

26. Shale, gray, plastic with 0.1-0.4 foot of

ferruginous clay at top_______________ 0. 4-0. 7

25. Sandstone, fine-grained, platy, locally 

silty, grades to argillaceous siltstone; 

weathers light gray with brown stain__ 1. I

24. Kaolinitic marker bed, porcellanitic, locally ferruginous in upper 0.3 foot_ ___ . 6

23. Sandstone, fine-grained, massive, with 

rusty-brown, ferruginous cap and ripple-marked upper surface; weathers 

gray to brown______________________ 2. 5

22. Sandstone, fine-grained, even-bedded, in 

beds 0.1-2.0 feet thick, locally laminated; weathers light gray; contains 

partings and thin interbeds of gray 

shaly siltstone chiefly in upper 2 feet_ 12. 0
DAKOTA GROUP IN NORTHERN FRONT RANGE FOOTHILLS, COLORADO 35

South Platte formation—Continued 

Disconf ormity ? 

Kassler sandstone member:

21. Sandstone, fine- to medium-grained, irregularly thin bedded, cross-laminated to 

cross bedded; weathers gray; some zones 

intraformational, clay-pellet conglom- Feet 

erate in lower part_________________ 44. 0

20. Sandstone, medium- to coarse-grained, 

massive, cross-laminated, friable; weathers gray to light gray. Contains many 

clay pellets and few small chert pebbles 

throughout_________________________ 10. 6

19. Sandstone, fine- to coarse-grained, thinbedded, crossbedded: weathers gray. 

Contains 0.2 foot of chert and quartzite 

pebble conglomerate at base __________ 17. 2

Base Kassler member. 

Unconformity.

18. Sandstone, fine-grained, thinly interbedded with argillaceous siltstone; 

weathers brown, platy hard; contains 

some silty gray shale at top__________ 2. 8

17. Siltstone, gray, argillaceous; and silty

shale_______ _________________ 1. 9

16. Sandstone as in unit 18, contains basal 

brown-weathering bed about 1 foot 

thick. Rarely contains fossils.

Inoceramus bellvuensis Reeside______ 4. 9

15. Shale, dark-gray, silty, with interbeds of 

argillaceous siltstone. Upper 1.5 feet 

is siltstone_________________________ 5. 4

14. Sandstone, fine-grained, irregularly thin 

bedded, becoming progressively more 

friable and silty downward; weathers 

light gray with rusty stain in upper 

part. Upper 1-2 feet indurated ferruginous cap__________________________ 14. 8

13. Sandstone, fine-grained irregularly bedded, 

locally ledge-forming; weathers light 

gray..________________ — __________ 1. 5

12. Kaolinitic marker bed, light-gray to graywhite, clay, locally shaly, silty in lower 

0.7 foot. Thin layer ferruginous siltstone 0.1 foot from top_____________ 1. 5

11. Siltstone and fine-grained sandstone, soft, 

friable, locally argillaceous; top 0.6-0.8 

foot is resistant, locally ledge-forming. _ 11. 4

10. Sandstone, fine-grained, massive, surface

pitted; weathers rusty brown.________ 2. 5

9. Siltstone, argillaceous and carbonaceous 

laminae, soft; weathers mottled gray 

and brown_________________________ 8. 3

8. Shale, gray to black, silty, with thin interbeds of rusty-stained silt, partially obscured. Basal foot is chiefly siltstone 

with shale partings__________________ 8. 7

Plainview sandstone member:

7. Sandstone, fine-grained, tabular, crosslaminated, silty partings in upper 6.0 

feet; weathers light brown with local 

rusty stain_________________________ 17. 2

6. Sandstone, fine-grained, irregularly bedded, 

friable; weathers light gray with rustybrown streaks_____________________ 6. 4

South Platte formation—Continued

Plainview sandstone member—Continued

5. Siltstone, gray, argillaceous, shaly in

lower part containing 1 foot plastic Feet 

paper-thin shale at base______________ 5. 8

4. Sandstone, fine-grained, tabular, crosslaminated to crossbedded; weathers 

light gray to yellow brown. Basal 2 

feet contain scattered small clay fragments____________________ 20. 8

3. Siltstone and fine-grained sandstone, gray, 

platy to shaly, locally argillaceous, interbedded with silty gray shale_______ 4. 7

2. Sandstone, fine- to coarse-grained, massive, with chert, quartzite and claystone pebbles throughout; weathers 

light gray— ——————— ____ —— _____ .8

Total thickness South Platte formation. 296

Disconformity.

Lytle formation (in part):

1. Siltstone grading downward to fine-grained 

sandstone, yellow with red stain in 

sandy part, argillaceous._____________ 1.7

The type section does not show the Benton shale 

contact clearly, but this contact can be seen on the south 

side of the South Platte River near the adit to an old 

clay mine on the east side of the hogback. Missing 

from the type section is about the upper half to twothirds of the Van Bibber shale member, including the 

main refractory shale. The kaolinitic marker bed 

(unit 24) and the ripple-marked sandstone beneath it 

(unit 23) are clearly exposed and serve to identify what 

remains of the member. The Kassler sandstone member, for which this exposure is also the type, is 72 feet 

thick. The intraformational clay-pellet conglomerate 

zones in units 20-21 are typical of the member throughout the Kassler quadrangle and are also common as far 

north as the south half of the Indian Hills quadrangle. 

The base of the Kassler sandstone member is marked 

not only by the thin chert and quartzite pebble conglomerate of unit 19, but also by peculiar vermiform 

ridges on the undersurface of the bed formed by the 

filling of irregular depressions in the surface of the 

underlying beds.

Beneath the Kassler sandstone member the upper 

part of the second shale subunit, containing the second 

marker, is missing and the Kassler member rests 

directly on the platy beds in the lower part of the 

subunit. Fossils in the platy beds are rare and fragmental.

Unit 14 marks the top of the third sandstone subunit; 

it grades downward into the third shale subunit and, 

because the latter is becoming increasingly sandy 

southward, is not locally separable from it.

The Plainview sandstone member is about 56 feet 

thick and consists of two benches of the typical rusty-
36 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

weathering, tabular sandstone and a thin basal conglomeratic sandstone in sharp disconformable contact 

with the underlying Lytle formation.

INTERMEDIATE PHASE—BOULDER COUNTY

The pattern of change in the South Platte formation 

from the nonmarine phase to the marine phase along 

its narrow band of outcrop is not a simple one. The 

nonmarine phase extends from the Kassler quadrangle 

northward through the Ralston Buttes quadrangle to 

about Coal Creek in the Eldorado Springs quadrangle. 

Here the aspect of the formation as a whole changes 

appreciably, owing to the thinning of both the Kassler 

sandstone member and the first sandstone subunit. 

In addition there is an abrupt local thickening of both 

the Plainview sandstone member and the third sandstone subunit north of Coal Creek (fig. 17). As a 

result of these combined changes the outcrop pattern 

of the South Platte formation is completely altered in 

the Eldorado Springs quadrangle; here the Kassler 

member, and locally the first sandstone subunit, 

instead of forming the principal hogback, lie in the 

flat, or form low ridges east of a prominent hogback 

made by the greatly expanded third sandstone subunit 

and Plainview member. The thickening of the lower 

sandstone subunits in the South Platte formation is a

local feature, and both subunits regain their typical 

character and thickness in the vicinity of Boulder.

Between Boulder and the latitude of Lyons the South 

Platte formation is very poorly exposed, owing to an 

extreme local thickening of shale units and the lack of 

resistant sandstone units above the Plainview sandstone member. This sandstone member supports the 

serrate, uneven Dakota hogback north of Boulder, and 

the remainder of the South Platte formation lies in the 

float-covered eastern slope of the hogback. A thin, 

commonly quartzitic, sandstone at the top of the formation, presumably a continuation of part of the first 

sandstone subunit of the nonmarine phase, locally 

forms a low ridge east of the hogback formed by the 

Plainview member, but throughout much of the outcrop 

between Boulder and Lyons this quartzitic sandstone 

is too thin to support more than a low prominence at 

the foot of the slope.

The nature of the South Platte formation in the 

Boulder-Lyons area is revealed northeast of Lyons in 

an exposure on the east flank of Rabbit Mountain just 

south of Little Thompson Creek. This exposure is 

selected as the standard section for the South Platte 

formation in Boulder County, where the formation is 

in an abnormally thick intermediate phase between the 

nonmarine phase to the south and the typical marine

FIGURE 15.—Part of the South Platte formation in the intermediate phase, south of Little Thompson Creek, Loveland quadrangle. Us, beds equivalent to upper sandstone unit and Ts, beds equivalent to third sandstone subunit. Beds below Ts are locally thickened equivalents of third shale subunit. The Plainview sandstone member is just out of sight at base of shale slope.
DAKOTA GROUP IN NORTHERN FRONT RANGE FOOTHILLS, COLORADO 37

phase to the north (fig. 15). All the section except the 

Plainview sandstone member was measured on the 

west-facing scarp of a local prominence just south of 

Little Thompson Creek in the SWtfNWJ. sec. 2, 

T. 3 N., R. 70 W. The Plainview member and underlying beds were measured just west of this locality in 

exposures near the entrance to the Bureau of Reclamation tunnel through Rabbit Mountain.

Description

Benton shale, not measured. 

South Platte formation: feet

34. Sandstone, fine- to medium-grained, 

tabular cross-laminated to crossbedded, quartzitic; weathers brown. 

Caps prominence above tributary to 

Little Thompson Creek. 

Fish remains. 

Halymenites sp_________________ 17. 0

Unconformity.

33. Sandstone, platy, irregularly bedded, 

and gray silty sandstone with interbeds of shaly siltstone and argillaceous sandstone; weathers brown_____ 6. 0-10. 0

32. Siltstone, gray, argillaceous, irregularly bedded, local carbonaceous 

and shaly partings, uppermost foot 

fine-grained sandstone.___________ 7. 0

31. Siltstone, argillaceous, and dark-gray 

silty claystone. At base is 0.1 foot 

of mixed light-brown silt and bentonitelike clay___________________ 1.0

30. Sandstone, fine-grained, and argillaceous siltstone; massive to irregularly 

bedded with shaly carbonaceous 

partings; weathers to a brownishgray, crumbly, blocky ledge.______ 19. 0

129. Siltstone, gray, argillaceous, becoming 

sandy in upper 8 feet and grading 

into unit 30. Lower 15 feet contain 

interbedded shaly siltstone-_______ 23. 0

28. Siltstone, argillaceous and fine-grained 

sandstone, weathers orange brown, 

thin-bedded, locally calcareous. 

Ledge-forming.__________________ 2. 4

27. Siltstone, argillaceous, hard, with

blocky fracture._________________ 2. 0

26. Bentonitelike clay, gray, waxy, containing 0.1 foot of dark-gray shale 

at top__________________________ .3

25. Siltstone, as in unit 27; upper 0.6 foot

forms local ledge_________________ 5. 3

24. Bentonitelike clay, gray, waxy, containing 0.1 foot of dark-gray shale 

at top __________________________ .2

'23. Siltstone, gray, argillaceous, thinbedded________________________ 9. 6

22. Sandstone, gray to brownish-gray, 

fine-grained, thin-bedded, laminated 

to cross-laminated, interbedded with 

shaly siltstone and some silty shale. 

Locally calcareous. Fossils rare... 14.5

South Platte formation—Continued

21. Shale, dark-gray to black, becoming 

silty in upper part. Scattered thin 

beds siltstone and fine-grained sand- Feet 

stone-____---__---____--_--____- 9. 0

20. Siltstone and fine-grained sandstone, 

argillaceous, upper 2 feet ledgeforming- ________________________ 8. 0

19. Sandstone, brownish-gray, silty, irregularly thin-bedded, with argillaceous 

partings contorted as if disturbed 

when soft. Forms crumbly ledge __ 10.0

18. Siltstone, gray, argillaceous, soft, massive becoming harder and sandier in 

upper 10 feet. Irregularly bedded, 

with irregular argillaceous partings 

in upper part-___________________ 30. 0

17. Siltstone, argillaceous, with ledges of 

hard, thin-bedded, calcareous siltstone in lower 1.9 feet; weathers gray 

to brown. Calcareous concretions 

weather orange-brown in zones at 

top and base of unit-_____________ 9. 5

16. Siltstone, gray, argillaceous, and silty,

gray shale_______________________ 2. 0

15. Bentonitelike clay, greenish-gray_____ . 2

14. Siltstone in thin, hard beds, slightly 

calcareous, locally argillaceous; 

weathers brown__________________ 3. 4

13. Siltstone, gray, argillaceous, soft, irregularly bedded with interbeds 

dark-gray silty shale in upper part- 8. 0

12. Sandstone, fine-grained, and siltstone, 

brown-weathering, even-bedded, 

laminated to cross-laminated, forms 

blocky ledges broken by softer intercalated argillaceous siltstone and 

silty shale. Calcareous throughout- 16.5

11. Shale, gray, silty, with interbeds, hard, 

platy, laminated to cross-laminated, 

tan-weathering, locally calcareous, 

siltstone increasing in number upward. Some calcareous concretions 

in upper part. Fossils.

Inoceramus comancheanus Cragin

Ostrea noctuensis Reeside*

Pteria salinensis White

Ammonite fragment ____________ 25. 0

10. Shale, gray, silty with thin beds and 

laminae of argillaceous siltstone. 

Fossils rare______________________ 19. 0

9. Siltstone, gray, shaly and thin interbeds laminated, silty, fine-grained 

sandstone. Inoceramus fragments- 2. 6 

8. Shale, dark-gray to black, finely silty, 

with laminae and thin beds of shaly 

siltstone in lower half____________ 14. 0

7. Shale, black, hard, fissile, with 0.4-foot 

bed hard, orange-brown stained siltstone at top___-________-_-______ 4. 0

6. Siltstone, hard, ferruginous, irregularly 

thin bedded, ledge-forming; weathers 

orange brown____________________ 1. 6
38 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

South Platte formation—Continued

5. Shale, dark-gray to black, finely silty 

to very silty, some interbeds shaly 

siltstone. Upper 3 feet irregularly 

interbedded carbonaceous shale and 

siltstone. ________________________

4. Chiefly shale with thin, hard beds siltstone. Obscured by wash_ ______

3. Siltstone and sandstone, gray, argillaceous irregularly bedded, with interbeds of platy gray fine-grained 

sandstone and gray silty shale. Partially obscured.__________________

Plainview sandstone member:

2. Sandstone, fine-grained, even-bedded, 

laminated to cross-laminated, in beds 

rarely more than 1.5 feet thick; 

weathers brown. Vertical borings 

and worm? trails on bedding planes. 

Some shaly partings and thin Beds. 

Forms east slope of Rabbit Mountain in vicinity_________________

Feet 

15. 5

7.0

11.0

25. 0±

Total thickness South Platte formation____________________ 330

Disconformity. 

Lytle formation:

1. Siltstone, argillaceous, weathers yellow 2. 0-3. 0

Only the Plainview sandstone member and the 

quartzitic sandstone at the top of the South Platte 

formation crop out with sufficient regularity along the 

strike to be traced from the Little Thompson Creek 

section south through Boulder County to the next 

complete exposures at Eldorado Springs. The shale 

and soft, argillaceous sandstone and siltstone beds lying 

between these resistant sandstones at Little Thompson 

Creek are difficult to correlate with either the Eldorado 

Springs exposures to the south or the exposures of the 

typical marine phase to the north because the lack of 

outcrop prevents lateral tracing of individual subunits. 

Nevertheless both the general succession of subunits 

and the recognition of a few key beds afford a basis 

for correlating the larger part of the Little Thompson 

Creek section. The two beds of bentonitelike clay, 

units 24 and 26, represent the second key marker, which 

is commonly split into two or more parts by shale partings throughout the northern foothills. The calcareous, 

platy sandstone of unit 22, about 10 feet beneath the 

bentonitelike clays, is equivalent to the persistent marine 7 one of the dominantly nonmarine phase. In the 

Little Thompson Creek section fossil remains in this 

bed are fragmental, and the preservation of the specimens found in it is too poor to permit specific identification; they include fragments of Inoceramus and a single 

unidentifiable ammonite fragment. Other recognizable

lithic equivalents of subunits in the nonmarine phase 

are units 18 to 20 inclusive which are correlated with 

the third sandstone subunit, and units 3 to 18 inclusive 

which are correlated with the third shale subunit. 

Within units 3-18 the three parts of the third shale 

subunits can be identified.

Above the bentonitelike clays correlated with the 

second key marker bed, correlation of individual subunits in the Little Thompson Creek section is only 

tentative. The uppermost quartzitic sandstone, unit 

34, and the sandstone beneath it, unit 33, probably 

represent the first sandstone subunit of the nonmarine 

phase. The argillaceous siltstone of units 31-32 may 

represent the Van Bibber shale member.

The thickening of the South Platte formation in the 

Little Thompson Creek section, and, presumably, 

throughout the intermediate phase, takes place beneath 

the second key marker in beds equivalent to the third 

sandstone and third shale subunits. A trend toward 

this is observed in the northern part of the nonmarine 

phase in which the third sandstone subunit thickens 

abruptly northward in the Eldorado Springs quadrangle, 

reaching 65 feet in thickness at Eldorado Springs. Presumably this sandstone becomes argillaceous and silty 

northward in Boulder County, and decreases somewhat 

in thickness. Consequently the thickening of the third 

shale subunit, which is little more than 10 feet thick at 

Eldorado Springs where the sandstone subunits above 

and below it almost coalesce, is largely responsible for 

the abrupt local thickening of the formation in Boulder 

County. Accompanying this abrupt northward thickening of the third shale subunit is its change to a marine 

shale bearing the Inoceramus comancheanus fauna.

That part of the South Platte formation above the 

second key marker is much thinner in Boulder County 

than in the nonmarine phase to the south, but again 

this change is gradual and begins in the northern part 

of the nonmarine phase with the northward thinning of 

the Kassler sandstone member and the first sandstone 

subunit.

NORTHERN MARINE PHASE

Between the exposure on Little Thompson Creek and 

an outcrop 9 miles, airline, to the north in Handy Ditch 

on the south side of Dry Creek, Loveland quadrangle, 

the South Platte formation decreases 100 feet in thickness. This decrease takes place in the shaly beds below 

the second key marker bed; the beds above this marker 

increase 30 feet in thickness between the two localities.

From Handy Ditch north to the Wyoming State line 

the South Platte formation consistently ranges from 230 

to 250 feet in thickness and is in its typical marine 

phase. Exposures north of Boxelder Creek in Larimer
DAKOTA GROUP IN NORTHERN FRONT RANGE FOOTHILLS, COLORADO 39

County are selected as the standard for the marine 

phase of the formation. The measured section given 

below was pieced from exposures 0.25-0.5 mile north 

of Boxelder Creek in the W5£NE# sec. 9, T. 10 N., 

R. 69 W., Larimer County. This is approximately the 

same locality described by Lee (1927, p. 39) about 2 

miles east of Greenacre Ranch along Boxelder Creek.

Description

Bentori shale, obscured by slope wash.

South Platte formation: pett 

33. Sandstone, fine-grained, quartzitic, hard;

weathers brown_____________________ 9. 7

32. Sandstone, light-gray with local rustybrown stain, fine-grained, in massive 

beds 0.6-2.0 feet thick, cross-laminated, 

soft to friable_________________ 26. 0

31. Sandstone, as in unit 32, but hard and

resistant, forms hogback _____________ 36. 0

Unconformity.

30. Siltstone, crumbly argillaceous, locally 

capped by lens of hard fine-grained 

sandstone; weathering olive brown. 

Partially obscured___________________ 8. 5

29. Sandstone interbedded with siltstone, 

platy to shaly, locally calcareous; some 

sandy shale, weathers light olive brown. 12. 7 

28. Siltstone, platy, to shaly, calcareous, with 

thin interbeds of silty to sandy, petroliferous, crystalline limestone and partings 

of silty shale. Fossiliferous.

Inoceramus comancheanus Cragin

Ostrea larimerensis Reeside

Fish scales and bones______________ 8. 3

27. Shale, gray to dark-gray, silty; with two 

bentonite layers 0.2 foot thick, 2.4 feet 

and 3.2 feet above base._____________ 4. 1

26. Bentonite________________________ .9

25. Shale, gray, silty with many thin beds and 

laminae of shaly siltstone. Bentonite 

bed 0.1 foot thick 4 feet from top_ _ _ _ _ 7. 8

24. Limestone, gray, silty to sandy, crystalline, petroliferous; wTeathers tan; contains interbeds of dark-gray to black 

shale. Fossiliferous.

Inoceramus comancheanus Cragin

bellvuensis Reeside 

Ostrea larimerensis Reeside

noctuensis Reeside____________ 2. 0

23. Shale, dark-gray to black_____________ 3. 0

22. Limestone and shale, fossiliferous, as in

unit 24____._______-_________ 2. 0

21. Shale, dark-gray to black, hard, with 

scattered thin layers of siltstone that 

weather with yellow-brown to red ferruginous stain. Bentonite 0.2 foot 

thick, 1.2 feet above base____________

20. Bentonite- ___________________________

19. Shale, dark-gray to black with a few silt 

laminae-___________________________

12.4 

. 3

4,5

South Platte formation—Continued

18. Siltstone, gray, argillaceous and silty gray 

shale, irregularly thin bedded, weathers 

light gray and brown. Zone of large, 

barren ironstone concretions 9 feet 

above base_________________________

17. Shale, dark-gray to black, fissile, scattered thin beds and laminae of silt in 

upper part; weathers bluish gray______

16. Shale, dark-gray to black, weathers fissile; 

thin zones with interlaminated silt, 

weathers rusty brown; argillaceous siltstone 1 foot thick at top__-________-_

15. Siltstone, shaly, ferruginous; weathers 

rusty brown, contains local ironstone 

concretions.________________________

14. Shale, as in unit 16, with scattered silt 

laminae and a thin bed of siltstone 4 feet 

above base_________________________

13. Ironstone concretionary layer___________

12. Shale, gray to dark-gray with scattered 

interbeds of olive-gray siltstone_______

11. Siltstone, olive-gray and silty, gray to 

dark-gray shale. Zones of ironstone 

concretions in lowTer 1.5 feet and 3 feet 

above base. Bones in siltstone at top 

of bed.

Plesiosaur vertebrae _______________

10. Siltstone, platy to shaly; weathers buff to 

dark browTn, locally ledge-forming-____

9. Siltstone, platy to shaly and interbedded 

silty shale; weathers light gray and buff _ 

Plainview sandstone member:

8. Sandstone, fine-grained, medium- to thiiibedded, cross-laminated; wTeathers 

browT n. Contains some thin zones and 

partings of sandy shale. Forms second 

Dakota hogback. Measurement, on 

dip slope, is approximate-____________

7. Argillaceous wash, sandy. Obscured-___ 

6. Sandstone, light-gray, fine- to mediumgrained, massive, cross-laminated; 

weathers buff to brown______________

5. Shale, black, carbonaceous, silty________ 0.

4. Sandstone, as in unit 6, varies in thickness 

at expense of containing shale beds____ 0.

3. Shale, gray, sandy___________________ 2.

2. Sandstone, as in unit 6, locally conglomeratic, contains pebbles of chert and 

quartzite_ _________________________

Feet 

23. 5

4.5

15.7 

. 6

2. 2

4. 3 

1.5 

2. 9

19. 0 

2. 7

1.0 

5-2. 0

5-1.5 

5-4.0

2. 8

Total thickness South Platte formation __ 250

Disco nformity.

Lytle formation (in part):

1. Clay stone, sandy, with zones of argillaceous sandstone; weathers gray white 

and light gray mottled pink and yellow. 

Partially wash-covered. Sandstone 

layers commonly ferruginous and 

weather brick red_ __________________ 7.0
40 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Us

FIGURE 16.—Typical double hogback formed by marine phase of the South Platte formation near Boxelder Creek, Larimer County. Upper sandstone (Us) forms hogback to right, Plainview sandstone member (P) holds dip slope of hogback to left, marine shale (Ms) lies between. Light-colored sandstone lens in Lytle formation lies 

in slope below Lytie-South Platte contact (Z>).

The Boxelder Creek section is typical of the marine 

phase of the South Platte formation throughout most 

of Larimer County (fig. 16). The chief characteristics 

are the hogback-forming sandstone subunits at the top 

and base of the formation and the thick intermediate 

shale which contains, in its upper third, two benches of 

silty fossiliferous limestone, both bearing the Inoceramus 

comancheanus fauna, and which generally lacks invertebrate macrofossils in the lower two-thirds. Scattered 

foraminifera in the lower part of the shale and the 

plesiosaur bones at its base indicate that the entire 

shaly part of the formation is marine. Bentonite beds 

(units 25, 26, and 27) between the fossiliferous limestone 

benches are equivalent to the second key marker, and 

the lower limestone bench (units 22-24) is equivalent 

to the platy fossiliferous siltstone zone of the nonmarine 

phase.

The Plainview sandstone member of the Boxelder 

Creek section is somewhat atypical for most of the marine phase in that it contains some shale and a basal 

conglomerate beneath the tabular sandstone, which is 

its chief constituent. Elsewhere in the marine phase 

the tabular sandstone commonly rests directly on the 

plane of disconformity at the top of the underlying 

Lytle formation. This type of contact can be seen in 

Lee's Bellvoie section, at the exposures in Handy Ditch, 

and at the Spring Canyon dam site on Horsetooth 

Reservoir.

The equivalent of the third shale subunit (units 9-17) 

is thinned appreciably, compared with its thickness in 

the intermediate phase at the Little Thompson Creek 

exposures; together with a silty zone (unit 18) probably 

equivalent to the third sandstone subunit, it constitutes 

a little less than one-half the middle shaly part of the 

formation.

The fossiliferous limestone layers are found only in 

the marine phase. Southward in Larimer County they 

grade into calcareous siltstone at about the latitude of 

Loveland. The limestone beds are characterized by 

their coarse texture, abundant disseminated fish remains 

and oily odor on fresh breaks. Some beds contain 

abundant remains of the fossils of the Inoceramus 

comancheanus fauna.

The sandstone at the top of the marine phase of the 

South Platte formation consists of more than one bed. 

The uppermost bed, unit 33, is quartzitic and is apparently continuous with the uppermost sandstone of the 

intermediate phase. The massive cross-laminated sandstone beds below it (units 31-32), thicken northward in 

Larimer County; they rest unconformably on a zone of 

interbedded siltstone and sandstone containing the 

Inoceramus comancheanus fauna.

AGE

The most valuable fossils for dating the South 

Platte formation are the marine mollusks. With one
41

exception all of these are associated with the zonal 

index Inoceramus comancheanus. This small fauna, 

described by Reeside (1923), consists of the species 

listed below:

Inoceramiis comancheanus Cragin

bellvuensis Reeside 

Pteria salincnsis White 

Ostrea larimerensis Reeside

noctuensis Reeside 

Anchura kiowana Cragin?

Of these the two species of Inoceramus and Pteria 

salinensis are the most widespread and are the only 

species known in the thin marine zone of the dominantly nonmarine phase of the formation. In the 

marine phase the two oysters are also common. A 

few ammonite fragments have been found associated 

with the other mollusks but none are identifiable.

Vertical distribution of the mollusks of the Inoceramus comancheanus fauna in the marine phase of the 

South Platte formation is controlled by the distribution of the platy siltstone and silty limestone beds to 

which they apparently are restricted. In the marine 

phase in northern Larimer County the siltstone and 

silty limestone layers are confined to the upper third 

of the argillaceous strata that lie between the Plainview sandstone member and the sandstone beds at the 

top of the formation. The /. comancheanus fauna is 

correspondingly distributed and thus apparently is restricted to the upper third of the shaly portion of the 

formation. On the other hand, platy siltstone and 

calcareous siltstone are more common in the Little 

Thompson Creek section, occurring as far down in the 

South Platte formation as beds equivalent to the 

lower shale part of the third shale subunit. The /. 

comancheanus fauna occurs in these lower platy beds 

as well as in beds higher in the Little Thompson Creek 

section. Consequently the entire interval between the 

Plainview sandstone member and the uppermost sandstone beds of the South Platte formation lies within 

the /. comancheanus zone.

No diagnostic fossils have been found in either the 

uppermost sandstone of the formation or the Plainview 

sandstone member. From the upper sandstone near the 

Boxelder Creek section in Larimer County, Reeside 

(1923, p. 205) reported a poorly preserved ammonite 

which he identified as Pachydiscus? sp., stating that

It might well be a specimen of P. brazoensis (Shumard), but it 

might equally well belong to some later species, and even a 

certain generic assignment must await better material.

The Plainview member locally contains poorly preserved linguloid brachiopods in its lower shaly part. 

Plant remains are fairly abundant in some of the 

subunits of the nonmarine phase of the South Platte 

formation. Knowlton (1896, p. 466-471) summarized what is known of the Dakota flora from the 

Denver basin and gives a distribution table of the 28 

species, most of which were described by Lesquereux 

(1883, p. 2-107). The table shows that only 11 of the 

28 species are present in the Kansas-Nebraska area 

where the bulk of the fossil plants comprising the 

Dakota flora were found. The Dakota plants from 

the Denver area came chiefly from "... the hard, 

white sandstone at Morrison." (Knowlton, 1896, 

p. 469); this is the Kassler sandstone member of the 

South Platte formation. The Kassler member contains the most abundant remains of plants in the nonmarine phase but similar plant fossils are also locally 

common in the shale lenses and, at a few localities, in 

the sandstone beds of the first sandstone subunit. 

Inasmuch as the Kassler sandstone member grades 

laterally into marine shales with Inoceramus comancheanus, its plant remains must be Early Cretaceous in 

age, and the low correspondence in species between its 

flora and the supposedly Late Cretaceous flora of 

Kansas and Nebraska may be indicative of an age 

difference. Whatever the relationship, too little of 

an exact nature is known about the Dakota flora to 

make it of value even for distinguishing between Upper 

and Lower Cretaceous rocks.

Vertebrate remains from the South Platte formation 

consist of a few dinosaur tracks, abundant fragmental 

remains of fish in the silty limestone of the marine 

phase, and the plesiosaur vertebrae from the Boxelder 

Creek section. None of these are of value for age 

determination.

The Inoceramus comancheanus fauna is Early Cretaceous (late Albian) in age; Reeside (1923, p. 196- 

200) points out that it is a depauperate Washita fauna. 

As 110 diagnostic fossils have yet been found either in the 

upper sandstone, above the beds with /. comancheanus, 

or in the Plainview sandstone member below these beds, 

it is not possible to place the entire formation within 

this zone. The upper sandstone beds of the marine 

phase, and the presumably equivalent first sandstone 

subunit of the nonmarine phase, could be either Early 

or Late Cretaceous in age. The lowest beds of undoubted 

Late Cretaceous age carry the early Cenomaniari 

ammonite genus Calycoceras, and lie from 40 to 60 feet 

above the base of the Benton shale along the east 

flanks of the Front Range and Wet Mountains. 

Between the Calycoceras and /. comancheanus zones 

no diagnostic marine invertebrates have been reported 

from eastern Colorado.

Northward, in eastern Wyoming and the Black Hills 

region, the Mowry shale overlies the lithic equivalents 

of the upper sandstone of the South Platte formation, 

the so-called Muddy sandstone and the Newcastle 

sandstone. The Newcastle sandstone of the Black
42 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Hills region contains an Early Cretaceous microfauna 

(Crowley, 1951) and the Mowry contains the very late 

Early Cretaceous ammonite genus Neogastroplites 

(Cobban and Reeside, 1952, p. 1015). The Mowry 

shale is a northern element in the interior Cretaceous 

sequence of strata and its exact relation to the section 

along the Colorado Front Range is not known. It is 

probably equivalent in part to the interbedded platy 

siltstone and shale that immediately overlie the South 

Platte formation and mark the base of the Benton in 

the northern foothills. That part of the Benton shale 

between these basal platy beds and the Calycoceras 

zone may also be equivalent to part of the Mowry; 

it is largely hard, noncalcareous shale with bentonite 

beds and, except for undiagnostic pelagic foraminifera, 

has not yielded marine inventebrates.

Although the dominantly transgressive nature of the 

South Platte formation and its lithogenetic equivalents 

elsewhere in the interior region implies a difference in 

age from place to place, it does not seem likely that 

the upper sandstone of the formation is appreciably 

younger than its nearby equivalents in eastern Wyoming. Consequently the entire South Platte formation 

is here considered to be of Early Cretaceous age.

RELATIONSHIP OF THE PHASES

Figure 17 shows the correlation of representative 

sections of the South Platte formation and serves to 

summarize the changes that take place between its 

different phases. Correlation from the nonmarine to 

the marine phase is relatively clear cut between the 

second key marker bed and the Lytie-South Platte 

contact. The major changes in this part of the formation have already been mentioned and need no elaboration. Above the second key marker bed it is more 

difficult to trace individual units because of, (1) the 

lateral change in facies of the Kassler and Van Bibber 

members, (2) the variable lithology and thickness of 

the overlying beds equivalent to the first sandstone 

subunit, and (3) the variable depths of incision of the 

channels in which the latter beds were deposited.

The disappearance of the prominent Kassler sandstone member northward is the most conspicuous 

change in the beds above the second key marker and, 

for that matter, in the formation as a whole. The 

gradual northward thinning of the Kassler member 

continues to about Eldorado Springs, from which point 

the member becomes increasingly silty and grades, in 

Boulder County, into a massive argillaceous siltstone 

between 10 and 25 feet thick. North of Boulder County 

the siltstone gradually becomes platy and fossiliferous 

and is interbedded with shale. These platy beds grade, 

in turn, into calcareous siltstone and silty limestone in

northern Larimer County. The overlying Van Bibber 

shale member loses its identity as its refractory kaolinitic shale changes northward to silty, illitic shale 

intercalated with platy siltstone, and as the limiting 

Kassler sandstone member thins out into similar silty 

and shaly beds.

North of Boulder County the beds equivalent to the 

first sandstone subunit bear the same relationship to 

underlying beds as they do in the nonmarine phase, 

thickening locally at the expense of the underlying 

beds. Consequently some exposures, like that in Handy 

Ditch (fig. 17), contain the marine equivalents of the 

Kassler sandstone and Van Bibber shale members, 

whereas at other exposures, like that at Boxelder 

Creek (fig. 17), these beds were eroded before the 

deposition of the overlying sandstone. The unconformity at the base of the first sandstone subunit and 

its equivalents is the only unconformity within the 

South Platte formation that obviously persists throughout the northern foothills.

The narrow band of outcrop of the South Platte 

formation in the northern foothills is only a single 

section across the different phases and so is insufficient 

evidence on which to base more than generalities about 

the local paleogeography. The gross distribution of the 

phases of the formation does indicate a positive area 

south of the center of the Colorado Front Range from 

which sediments spread northward and northeastward 

to form the nonmarine phase in Jefferson and Douglas 

Counties.

A broader perspective is afforded by examining the 

regional distribution of the sandstone subunits of the 

South Platte formation. The Plain view sandstone 

member extends northward from Colorado and is 

included as part of the Cloverly formation in Wyoming; 

it is also equivalent, at least in part, to the Fall River 

sandstone of the Black Hills region. South of the 

northern Front Range foothills its most conspicuous 

part, the tabular sandstone that forms the bulk of the 

member, has not been distinguished in south-central 

and southeastern Colorado and is either absent or has 

thinned and become shaly. Apparently the Plainview 

member is a northern element in the sequence.

The third sandstone subunit probably is a local 

unit in the nonmarine phase of the South Platte formation, lensing out northward into marine shale and 

siltstone in Larimer County. Its thickest and most 

resistant exposures are between Boulder and Coat 

Creek; it is also more prominent than usual where the 

South Platte formation as a whole thickens and coarseens in the Kassler quadrangle. In south-central Colorado it may be represented by interbedded silty shale 

and platy sandstone but it is not a conspicuous unit.
NORTH SOU

Boxelder 

Creek

Handy 

Ditch

Little Thompson 

Creek

Eldorado 

Springs

Alameda 

Parkway

South 

Platte River

Rainbow 

Creek 

=1= 

Sandstone Conglomeratic sandstone Gray to black shale and 

silty shale

Interbedded siltstone 

silty shale

Silty 

Ash beds, kaolinitic to 

montmorillonitic

1 

First key marker beds Second key marker bed Marine fossil

FIGURE 17.—Lateral changes in the South Platte formation along the northern Front Range foothills.
44 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

The Kassler sandstone member is a southern element 

that lenses out northward in the northern foothills. 

It is widely distributed in south-central and southeastern Colorado where it forms prominent cliffs.

Lithic equivalents of the first sandstone subunit of 

the South Platte formation are widespread in the interior region (Muddy sandstone member, Dakota sandstone, Newcastle sandstone) where its great variability 

suggests complications beyond our present knowledge 

of the stratigraphy. In the southern plains, and foothills of the Rocky Mountains from east-central Colorado south and southeastward, this sandstone subunit 

is a resistant and conspicuous unit. Commonly it 

coalesces with the equivalent of the Kassler sandstone 

member below and together these two units form Stose's 

Dakota sandstone. Locally they are separated by a 

refractory shale subunit correlative with the Van Bibber 

shale member.

The change in phase within the South Platte formation in the northern foothills reflects both local and 

regional events. After an initial sandy transgressive 

phase (Plainview sandstone member), a marine environment prevailed in much of northeastern Colorado and 

eastern Wyoming until the withdrawal of the sea 

marked by the regressive, uppermost sandstone of 

the sequence. In east-central and southeastern Colorado, however, the sea withdrew earlier as deltas 

spread eastward and northeastward from local positive 

areas to form an irregular deltaic plain along what is 

now the southern Front Range and Wet Mountains. 

The Kassler sandstone and Van Bibber shale members, 

and their equivalents, form these deposits. Locally, 

near the positive areas, nonmarine sedimentation was 

almost continuous. The nonmarine phase in the 

northern foothills marks such an area; preliminary 

studies have revealed other similar nonmarine sequences 

along the north half of the Wet Mountains. Following 

the deposition of the marine beds to the north and the 

sandstone and overlying refractory shales and clays 

to the south there was a widespread break in sedimentation, presumably caused by slight uplift over a large 

part of the interior. Subsequently the regressive upper 

sandstone, which is largely continental in the south 

and varies locally between marine and continental in 

the north, was deposited. The overlying platy siltstone and shale at the base of the Benton were deposited on the upper sandstone as the sea readvanced 

throughout the interior region.

The generalized summary above is based on exposed 

rocks and so pertains only to the outcrop areas along 

the mountains in eastern Colorado and Wyoming. 

Many more details of pre-Benton Cretaceous stratigraphy have been revealed by the subsurface work in the

Denver-Julesburg basin area but little of this information is available. Fogarty 1 and Haves 2 have compiled 

and interpreted much well data in this area, and their 

work reveals that the pre-Benton Cretaceous beds 

thicken eastward from the mountain front into the 

deeper part of the basin of Early Cretaceous sedimentation. This basin is irregular in shape and lies obliquely northwest across the northeastern part of 

Colorado. In the deeper part of the basin the subunits increase in number and the stratigraphy is complicated by the addition to the sequence of subunits of 

eastern provenance, at least one of which apparently 

lies stratigraphically above the equivalent of the first 

sandstone subunit of the South Platte formation. The 

study of the subunits that outcrop in the northern 

foothills supports the belief generally held that subunits 

of eastern provenance do not extend to the outcrop 

along the Front Range in Colorado.

STATUS OF THE TERM DAKOTA GROUP

Lee's (1923) usage of the term Dakota group in the 

northern foothills is retained here to include the Lytle 

and South Platte formations because it is a useful map 

unit for reconnaissance, or for quadrangle maps of small 

fractional scale. The relatively thin Lytle formation is 

difficult to map separately at a scale less than 1:24,000 

(fig. 18).

The usage follows the original definition of the Dakota 

group (Meek and Hayden, 1862, p. 419-420) inasmuch 

as it includes all pre-Benton Cretaceous strata. In the 

type area along the Missouri River in Nebraska the 

Dakota rests on Paleozoic rocks so its lower contact is 

unequivocal. The characteristic twofold lithogenetic 

division is present in the type area where a sharp break 

separates sandstone with variegated clay below from 

sandstone with dark-gray clay, carbonaceous clay, and 

lignite above. In light of previous attempts to make 

the name Dakota reflect opinion on the age of the rocks 

it includes, it must be emphasized that Dakota group, 

as used here, is strictly a rock term and whether or not 

the group contains both Lower and Upper Cretaceous 

rocks, is entirely Lower Cretaceous, or varies in age 

from region to region, is irrelevant to this definition.

Because of the indefinite nature of the MorrisonLytle contact in parts of the northern foothills it may 

not be convenient in some places to use the Dakota 

group as a unit for small-scale mapping; in such places 

the best map units would be the Morrison and Lytle 

formations imdifferentiated, and the South Platte 

formation.

1 Fogarty, C. H., 1952, Subsurface geology of the Denver basin. Unpublished 

doctorate thesis in flies of Colo. School of Mines.

2 Hayes, J. R., 1950, Crataceous stratigraphy of eastern Colerado. Unpublished 

doctorate thesis in flies of Colo. Univ.
DAKOTA GROUP IN NORTHERN FRONT RANGE FOOTHILLS, COLORADO 45

CORRELATION OF THE LYTLE AND SOUTH PLATTE 

FORMATIONS

The correlation of the pre-Benton Cretaceous strata 

in the northern Front Range foothills with equivalent 

strata in adjacent parts of the interior region is indicated in the chart, figure 19. Correlation with southcentral and southeastern Colorado is based on field 

work in those areas; for other regions it is based largely 

on the literature.

The Dakota sandstone of south-central and southeastern Colorado is equivalent to the combined first 

sandstone subunit, Van Bibber shale member, and 

Kassler sandstone member of the South Platte formation. The Glencairn shale member of the Purgatoire 

formation is presumably equivalent to that part of the 

South Platte formation belo\A the Kassler sandstone 

member, although it is considerably thinner and the 

possibility that beds equivalent to the Plainview sandstone member have been at least partially overlapped 

by the beds in the Tnoceramus comancheanus zone cannot 

be discounted. In and southeast of the Canon City 

embavment area the 7. comancheanus fauna is locallv

present in the top of the Glencairn member. Farther 

to the southeast on the plains it occurs in silty layers 

throughout the black shaly part of the Purgatoire. 

The Lytle formation of the northern foothills is equivalent, by definition, to the Lytle sandstone member of 

the Purgatoire formation.

The Purgatoire and Dakota terminology is objectionable because it does not conform to the major lithogcnetic units within the pre-Benton Cretaceous sequence, 

and because of its misuse of the name Dakota. The 

separation of the Dakota sandstone from the Purgatoire 

formation is not at the most significant break in the 

sequence, as was supposed when these units were 

defined, but at a relatively local unconformity. The 

old idea, based originally on the supposed Late Cretaceous age of the Dakota flora, that this unconformity 

is the break between Lower and Upper Cretaceous 

rocks is no longer admissible inasmuch as the unconformity vanishes northward, in the northern foothills, 

within the 7. comancheanus zone. The regional disconformity in the sequence lies within the Purgatoire 

formation, separating it into two lithogenetically distinct parts.

Fi«rRE 18.—Characteristic exposure of the Dakota group in the Denver area, looking north in Dutch Creek gap, Indian Hills quadrangle. Us, upper sandstone 

subunit; K, Kassler sandstone member; and P, Plainview sandstone member of South Platte formation. D, disconformity between Lytle (L) and South Platte 

formations. U, approximate Morrison-Lytle contact.
Oi

NE. Wyoming Central Wyoming Northern Front Range foothills, Colorado South-central Colorado

Northern 

Larimer County W. T. Lee, 1923 This report Feet

Mowry shale Mowry shale

Newcastle 

sandstone

Muddy 

sandstone

Skull 

Creek 

shale

Thermopolis

shale 

(restricted usage)

Fall River 

sandstone

Fuson shale

Lakota 

sandstone

Upper sandstone 

member

Shale 

member

Conglomeratic 

sandstone 

member

Morrison formation

Benton shale Benton shale Graneros shale

>Upper sandstone

Upper shale

-Middle sandstonerLower shale — __ •Lower sandstone

Morrison formation 

EXPLANATION

/Dry Creek 

Canyon 

member

Van Bibber

shale 

member

Kassler 

sandstone 

member

Glencairn

shale 

member

Lytle 

sandstone 

member

Morrison formation

Plainview 

sandstone 

member

Lytle 

formation

'Morrison-formation

Conglomeratic sandstone

Shale, gray to black

Shale, refractory

Interbedded siltstone, 

shale, and sandstone

Variegated claystone Ash beds, kaolinitic 

and montmorillonitic

Limestone, silty

Marine fossils

0-

50-

100-

200-

FIGCBE 19.—Correlation of the Lytle and South Platte formations in eastern Colorado and eastern Wyoming.
DAKOTA GROUP IN NORTHERN FRONT RANGE FOOTHILLS, COLORADO 47

Correlation with the pre-Benton Cretaceous strata of 

eastern Wyoming largely repeats Lee's work (1923, 

1927) and follows the correlation of Lee's Dakota group 

in the recent Cretaceous correlation chart (Cobban and 

Reeside, 1952) except for minor adjustments at the 

position of the disconformity separating the Lytle and 

South Platte formations. In southeastern Wyoming 

this disconformity probably lies within the shale member of the Cloverly formation where the latter contains 

gray or black shale in its upper part. Where the Greybull sandstone member of the Cloverly, or its equivalent 

sandstone, rests directly on variegated beds or other 

rocks similar to the Lytle, the disconformity would be 

at the base of the Greybull. In like manner the disconformity in northeastern Wyoming is probably in 

the upper part of the Fuson shale where the latter 

contains gray or black shale in its upper part; in other 

places it may be at the base of the Fall River sandstone.

EVALUATION OF UNCONFORMITIES

The upper contact of the Morrison formation is generally accepted as marking the boundary between 

Jurassic and Cretaceous rocks in the western interior. 

Recently, Reeside (1952, p. 22-26) summari ed what is 

known regarding the Morrison and its age in relation 

to the overlying Lower Cretaceous rocks, concluding 

that

... in the United States there is reason to infer a time interval 

between the Morrison formation and the succeeding beds that 

represents possibly part of the Portlandian, probably the Purbeckian, the Berriasian, the Valanginian, the Hauteriviau, and 

possibly the Barremian of the standard European sequence.

However, the interpretation of the magnitude of the 

hiatus rests largely on the interpretation of the freshwater mollusks, charophytes, and plants that occur in 

the continental beds lying between the Jurassic (Portlandian) dinosaur-bearing part of the Morrison formation, and the Lower Cretaceous marine beds, such as 

the South Platte formation, containing the Albian 

Inoceramus comancheanus fauna. Neither the paleobotanists nor the students of the fresh-water mollusks 

agree on the interpretation of their separate kinds of 

fossils, and although most favor an Early Cretaceous 

age for the beds in question, there is no substantial 

evidence to indicate what part or parts of the Lower 

Cretaceous is represented.

The study of the pre-Benton Cretaceous beds in the 

northern Front Range foothills can add little to the 

problem of the Jurassic-Cretaceous boundary other than 

to point out that the gap in time between the Jurassic 

(Portlandian?) dinosaur-bearing part of the Morrison 

formation and the Lower Cretaceous (Albian) beds 

of the South Platte formation is taken up by more than 

one physical break in the sequence. Three unconformities are present in the interval in question: one 

is at the base of the dominantly red, variegated upper 

part of the Morrison, the second marks the MomsonLytle contact, and the third is the disconformity marking the Lytie-South Platte contact. Neither the uppermost part of the Morrison nor the Lytle formation 

contains diagnostic fossils in the area.

The fresh-water molluscan faunas described from 

Lower Cretaceous rocks elsewhere in the western interior 

(Yen, 1949) come from beds that occupy the same stratigraphic position and are lithologically similar to both 

the upper part of the Morrison formation and the 

Lytle formation along the Colorado Front Range. 

Whether these faunas are restricted to beds equivalent 

to the Lytle part of the sequence or whether they also 

occur in beds equivalent to the top part of the Morrison 

is not known. Most authors have considered what is 

here called the lower part of the Lytle as the basal 

Cretaceous conglomerate; if rock units so designated 

could be correlated on the strength of such designation 

alone, the beds containing the fresh-water mollusks 

elsewhere would indeed be equivalent to the Lytle 

along the Front Range. Inasmuch as a single, welldefined, and persistent conglomerate is lacking at the 

base of the Dakota group along the Front Range, and 

in other regions as well, no such generalised correlation 

can be made. Consequently the unconformity at the 

base of the Lytle could mark either the Jurassic and 

Cretaceous boundary or a hiatus of unknown magnitude 

within Lower Cretaceous rocks.

Evidence is equivocal as to what part of the Lower 

Cretaceous the fresh-water molluscan fauna in question 

represents. Yen's (1949, p. 469-470) conclusion that 

the mollusks are older than the British Wealden mollusks 

and younger than the British Purbeck is not convincing 

in light of the lack of a physical break between these 

British units (Arkell, 1933, p. 543). Moreover it has 

not yet been satisfactorily demonstrated that freshwater mollusks can be used for such precise long-range 

correlation. Charophytes and ostracods from the beds 

in question (Peck, 1941) contain elements similar to 

both Purbeck and Wealden microfossils and have corroborated their Early Cretaceous age. Peck does not 

attempt a more precise age determination than Early 

Cretaceous on the basis of these fossils. Dinosaur 

remains in the same beds have not been studied. 

If Yen's age determination based on the mollusks is 

correct and the Lytle formation and its genetic equivalents are basal Lower Cretaceous, the disconformity at 

the top of the Lytle is probably the larger of the three 

breaks and represents most of Early Cretaceous time, 

from early Neocomian to about middle Albian.

The occurrence of more than one obvious physical 

break in the beds between the Morrison and South
48 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Platte formations along the northern Front Range in 

Colorado and the possibility of similar multiple breaks 

in correlative beds elsewhere in the western interior 

suggest that more than one Early Cretaceous freshwater molluscan fauna may be present. Yen (1951, 

p. 3) points out that certain molluscan faunas do suggest at least slight age differences between the Kootenai 

formation in Montana, the Cleverly formation in 

Wyoming, and the Peterson limestone in Wyoming 

and Idaho.

Obviously there is insufficient evidence for evaluating 

any of the three unconformities between undoubted 

Jurassic strata and the marine Lower Cretaceous rocks 

in the northern foothills. If a clear-cut break exists 

between Jurassic and Cretaceous rocks it could be 

either the unconformity at the base of the Lytle formation or that at the base of the upper part of the 

Morrison formation. On the strength of Yen's interpretation of the fresh-water mollusks it is also possible 

that the disconformity between the Lytle and South 

Platte formations has the greatest time value of any 

of the three breaks. In the absence of definitive 

paleontologic evidence the slight but widespread 

angular unconformity between the Lytle and Morrison 

formations seems the most reliable indication of a 

major time break in the sequence, but this unconformity 

does not necessarily mark the Jurassic-Cretaceous 

boundary.

LITERATURE CITED

Arkell, W. J., 1933, The Jurassic system in Great Britain: 

681 p., Oxford, The Clarendon Press.

Cobban, W. A., and Reeside, J. B., Jr., 1952, Correlation of the 

Cretaceous formations of the western interior of the United 

States: Geol. Soc. America Bull., v. 63, p. 1011-1044.

Crowley, A. J., 1951, Possible Lower Cretaceous uplifting of 

Black Hills, Wyoming, and South Dakota: Am. Assoc. 

Petroleum Geologists Bull., v. 35, no. 1, p. 83-90.

Darton, N. H., 1905, Discovery of the Comaiiche formation in 

southeastern Colorado: Science, new ser., v. 22, p. 120.

———— 1906, Geology and underground waters of the Arkansas 

Valley in eastern Colorado: U. S. Geol. Survey Prof. Paper 

52.

Eldridge, G. H., 1896, Mesozoic geology, in Emmons, S. F., 

Cross, Whitman, and Eldridge, G. H., Geology of the 

Denver basin in Colorado: U. S. Geol. Survey Mon. 27, 

p. 51-150.

Finlay, G. I., 1916, Description of the Colorado Springs quadrangle, Colorado: U. S. Geol. Survey Geol. Atlas, folio 203.

George, R. D., 1927, Geology and natural resources of Colorado: 

Colo. Univ. Semicentennial Pub., p. 63-64.

Gilbert, G. K., 1897, Description of the Pueblo quadrangle, 

Colorado: U. S. Geol. Survey Geol. Atlas, folio 36.

Hague, Arnold, 1877, Colorado Range, in Hague, Arnold, and 

Emmons, S. F., Descriptive geology, U. S. geological 

exploration of the fortieth parallel (King): Prof. Papers 

Eng. Dept. U. S. Army, no. 18, v. 2, p. 39-41.

Henderson, Junius, 1909, The foothill formations of northcentral Colorado: Colo. Geol. Survey, 1st Rept., 1908, 

p. 172-176.

Hills, R. C., 1899, Description of the Elmoro quadrangle, Colorado: U. S. Geol. Survey Geol. Atlas, folio 58.

———— 1900, Description of the Walsenburg quadrangle, Colorado: U. S. Geol. Survey Geol. Atlas, folio 68.

King, Clarence, 1878, Systematic geology, U. S. geological 

exploration of the fortieth parallel: Prof. Papers Eng. Dept. 

U. S. Army, no. 18, v. 1, p. 278-305.

Knowlton, F. H., 1896, The fossil plants of the Denver basin, in 

Emmons, S. F., Cross, Whitman, and Eldridge, G. H., 

Geology of the Denver basin in Colorado: U. S. Geol. 

Survey Mon. 27, p. 466-472.

———— 1920, A dicotyledonous flora in the type section of the 

Morrison formation: Am. Jour. Sci., 4th ser., v. 49, 

p. 189-194.

Lee, W. T., 1920, Type section of the Morrison formation: Am. 

Jour. Sci., 4th ser., v. 49, p. 183-188.

———— 1923, Continuity of some oil-bearing sands of Colorado 

and Wyoming: U. S. Geol. Survey Bull. 751-A, p. 1-20. 

1927, Correlation of geologic formations between eastcentral Colorado, central Wyoming, and southern Montana:

U. S. Geol. Survey Prof. Paper 149. 

LeRoy, L. W., 1946, Stratigraphy of the Golden-Morrison

area, Jefferson County, Colo.: Colo. School Mines Quart.,

v. 41, no. 2, p. 69-75. 

Lesquereux, Leo, 1883, Contributions to the fossil flora of the

western territories, pt. 3, The Cretaceous and Tertiary

floras: U. S. Geol. Survey Terr., rept, 8, 283 p. 

Meek, F. B., and Hayden, F. V., 1862, Descriptions of new

Lower Silurian, (Primordial), Jurassic, Cretaceous, and

Tertiary fossils, * * * : Acad. Nat. Sci. Phila., Proc., v. 13,

p. 419-420. 

Peck, R. E., 1941, Lower Cretaceous Rocky Mountain nonmarine

microfossils: Jour. Paleontology, v. 15, p. 285-304. 

Powell, J. W., 1882, Plan of publication: U. S. Geol. Survey

2d Ann. Rept., p. XL-XLVII.

———— 1890, Conference on map publication: U. S. Geol. Survey 

10th Ann. Rept., pt. 1, p. 63-79.

Richardson, G. B., 1915, Description of the Castle Rock quadrangle, Colorado: U. S. Geol. Survey Geol. Atlas, folio 198.

Reeside, J. B., Jr., 1923, The fauna of the so-called Dakota 

formation of north-central Colorado and its equivalent in 

southeastern Wyoming: IT. S. Geol. Suvey Prof. Paper 

131-H, p. 199-208.

———— 1952, Summary of the stratigraphy of the Morrison

formation: in Yen, Teng-Chien, Molluscaii fauna of the

Morrison formation: U. S. Geol. Survey Prof. Paper 233-B,

p. 21-51. 

Stanton, T. W., 1905, The Morrison formation and its relations

with the Comanche series and the Dakota formation: Jour.

Geology, v. 13, p. 654-669. 

Stovall, J. W., 1943, Mesozoic stratigraphy, in Schoff, S. L.,

Geology and ground water resources of Cimarron County,

Okla.: Okla. Geol. Survey Bull. 64, p. 43-132. 

Stokes, W. L., 1944, Morrison formation and related deposits

in and adjacent to the Colorado Plateau: Geol. Soc.

America Bull. v. 55, p. 951-992. 

Stose, G. W., 1912, Description of the Apishapa quadrangle,

Colorado: U. S. Geol. Survey Geol. Atlas, folio 186. 

Waage, K. M., 1952, Clay deposits of the Denver-Golden area,

Colo.: Colo. Sci. Soc., Proc., v. 15, no. 9, p. 375-381.
DAKOTA GEOTJP IN NORTHERN FRONT RANGE FOOTHILLS, COLORADO 49

Waage, K. M., 1953, Refractory clay deposits of south-central

Colorado: IT. S. Geol. Survey Bull. 993. 

Walcott, C. D., 1903, Nomenclature and classification for the

Geologic Atlas of the United States: U. S. Geol. Survey

24th Ann. Rept., p. 21-27. 

Waldschmidt, W. A., and LeRoy, L. W., 1944, Reconsideration

of the Morrison formation in the type area, Jefferson

County, Colo.: Geol. Soc. America Bull. v. 55, p. 1097-

1114. 

Wilmarth, M. G., 1938, Lexicon of geologic names of the United

States: U. S. Geol. Survey Bull. 896.

Yen, Teng-Chien, 1946, On Lower Cretaceous fresh-water 

mollusks of Sage Creek, Wyoming: Acad. Nat. Sci. Phila., 

Notulae Naturae, no. 166.

———— 1949, Review of the Lower Cretaceous fresh-water 

molluscan faunas of North America: Jour. Paleontology, 

v. 23, no. 5, p. 465-472.

———— 1951, Fresh-water mollusks of Cretaceous age from 

Montana and Wyoming with a summary by J. B. Reeside, 

Jr.: U. S. Geol. Survey Prof. Paper 233-A, p. 1-20.

———— 1952, Molluscan fauna of the Morrison formation:

U. S. Geol. Survey Prof. Paper 233-B, p. 21-51.

INDEX

Page 

Abstract_— _._.__.........._____...._.......................... 15

Acknowledgments________.._____,___......._.-.-.-...---. 15

Alameda Parkway, six subdivisions of Morrison along___ - _________ 23 

type section of Morrison.___.___...._____...__..__...... 23,25

unconformity at base of first sandstone subunit.___.----.---.----.-.-- 34

Anchura kiowana....-...--...-.....__--...-..._.__..................... 41

Area of study_--_-___-______.__-_...___._.________ 15,16

Bellvue section, type locality of Lee's Dakota group._——————__.... 24,25

variegated shale,---------_--.----.-----.-........-..................... 22

bellvuensis, Inoceramus...___._..-----...-.-..._._...._......__.... 41

Benton shale._--_---------------------..-_--. — .- 18,27,33,35,42

Bentonitelike clay-._...._..._....__-_-._.__.-__-_______-____-____ 38

Boxelder Creek, stratigraphic section measured &t............................ 39

typical marine phase of South Platte at_..................___-__ 40

Calycoceras.----------------- _-__--__-_______-_-..__....

Oharophytes.---_------------_---------.__ ___________

Cleverly formation...—— — — — ———---.-_ — - — - — _- ——_.

Oomanche formation...._-----------------------_ ......

comancheanus, Inoceramus...--. ........ __._. ...

Coniferous wood in Lytle... . . — —. ———— ..:........

Correlation of nonmarine and marine phases, South Platte.

Dakota group, history of terminology.. . . - __ __ ___ ___.___ 16,17,18

in northern Front Range foothills_-----__....................... 19, 44,45

Dakota hogback. . . __——__...._______________.__.. 16,24,36

Deer Creek, alternating units of sandstone and claystone at____ -------- 20

stratigraphic section measured at...—————.__._.. _-------------- 20,21

Dinosaur-bearing beds—._..__-.—.-_-..._-------------________ 20,23,47

Disconformity within pre-Benton Cretaceous sequence._-----__---------- 19

between Lytle and South Platte—___________ .....___.-... 26

Distribution of conglomeratic lenses in Lytle formation-._---------- — ___-_ 22

Dry Creek Cany on member..___-------_...._..____..._-------- 17

Eldorado Springs, hogback_________-______ ....__-_...__. 23,36 

stratigraphic section measured at... — --------------___-_-.__-_-___._ 23

First sandstone subunit of South Platte, lithic equivalents_......__._.. 44

source of fire clay.-----,-.---- — --------- —............................. 31

thickness__._-______..____-_.__....________..._„ 30,33

Fossils.—--------------------------------------.-..-.--------.------------ 17,40,41

Fusonshale---..._______......._...._--.-..._...... ._...-_-_._-----_ 18

Glencairn shale.--..-..__---_-__-_--_-___---__......_... _____........ 17,19

Golden Fire Brick Co. mine------.------.-- --.....-.----..- — -.—-.--.. 31

Graneros shale____.--___-___-_---_--_----.----.-__._._._._............. 18

Horsetooth Reservoir, stratigraphic section measured at. 23

Indian Creek hogback-.-____.-_---_------__......................._.... 16

Inoceramus bellvuensis.--------------...-------------_-... — ___--_ — ------ 41

comancheanus_........_....._..._..._.__.-_ — ___-..... 31,37,38,40

distribution---.----..-.- —.__-__-_-.-_-__--_. — _ —.._ — ——... — _. 41

Jurassic-Cretaceous boundary. 47

Kassler sandstone, composition--------...--------______.---.-.----._-. 28,30,31

distribution.-..---------- -------- —.. -------- ——.. ——. — ------ ——.. 44

thickness.------____-._-__-- —— —.-------._....._.-._..- —— ---------- 31

kiowana, Anchura-—..--------..-.............. ...... ——. — ----------------- 41

Lakota sandstone___...--._.---------. —.. ...___.-.-...-- — -- .... 18, 46

larimerensis, Ostrea.-- ----------.-. ................... —.. —... — -------- 41

Lateral tracing, Morrison-Lytle contact——.... —.......... — -----------— 23

Lateral variation in Lytle formation--------------__.. ——.-.. ——.. ——- 20,21

Lee, W. T., quoted___ ———— ————— — 

Lytle formation, composition of conglomerate...

correlation with the pre-Benton Cretaceous.

thickness in Bell vue section. ..-. ——.--...- 

Colorado Springs area...—. —————..

typical locality.. ———. — —— —— — - — - 

Lytle-South Platte disconformity———————

Page 

Molluscanfauna_.......____....................._______.. 23,27,40,48

Morrison formation, angular unconformity between the Purgatoire and_.... 25

composition of conglomerate-.-------------------.---.---..------.-...-.-. 23

lateral variation........___.___.._...__..._................. 21

six subdivisions along west Alameda Parkway....__....__------------ 23

Morrison-Dakota contact_-.__..___..____.__......__.._..... 24,25

Morrison-Lytle contact, interpretation of Eldridge's description of-.-—___ 24,25

Morrison-Turkey Creek area, base of first sandstone unit_____ „ _____ 33

contact of Kassler and first sandstone subunit----------------------._---. 34

Newcastle sandstone. 

noctwensis, Ostrea.....

Ostrea larimerensis-... 

noctuensis-- __.

Pachydiscus..... ..._...__-.-----.-.---.-.-...-.........-..........._...

Plainview sandstone, composition..------------------------------------------

fourth sandstone subunit---------_____..._----------------------

stratigraphic section of typical exposure.— -_----.---.-..--_---.---.-.-...

stratigraphic section near tunnel entrance, Rabbit Mountain.------------

thickness,.--.___._.....................__........_......_...

Plant-bearing beds----------- — ---------------------------------.--- — ------

Porcellanite - - —————— -------------- —...-..--...-...-...-.-.............

Post-Morrison warping.---..,----—- ——— --------------- —... — ----------

Pre-Benton Cretaceous strata, extent of exposures --------------- _ ----------

faulting.—-__------,,---- —— — -————— ——-—--_----_- — — — — -—_

fivefold division.... ——.. —.-.-.---.-_,____,_.._-_-------- ——..........

interpretation of twofold lithic division_._-___-_-_-_--.--- —.........

lithogenetic subunits-.... ——.. —. — — — --------— ------------------

subdivision in northern Front Range foothills.--.-- —...... ——... —...

threefold division...-- — ---------- —.. —— ------- —— -___-_-___-_--__..

Pteria salinensis-.............................................................

Purgatoire formation..------------------- — ------------- — ----------- 17,18,

41

41

41

41

41

28,29

28

29

37

30,35

24,41

28

25

15,16

16

16,17

18

18

19

16

31

19,25

Reeside, J. B., Jr., quoted-.—_- _ - - ——— —— --————- — -—— ——— —— 41 

Refractories industry, chief source of clay for—-------- —— — — ——— -_-_-— 31

Refractory clay-.-...--.--.-...----..-.-------------------.- ------------- 15,26,31

shale, erratic distribution----- —— — — — — — ——............—.. —..— 33

in Dakota sandstone—— — --------------------- —. — ---------- — _-.-- 17

salinensis, Pteria.......---------- ... ------- — --------------------------——— 31

Second shale subunit----------------------------------------------------- 28,30,35

Shell Research Fund grant.-.----------.------------ — ------ — _ —— — — —— 15

South Platte formation, composition of units -------------------------- — -.. 27,28

extent of nonmarine phase__-------------------- ——.... —. — — -._. 28

first sandstone—------ —— ------- — ----------------- — — -... — ——... 28

pattern change.--------- — —...- — ----------- —.... —— -------------- 36

standard section on Rabbit Mountain..———_-_-————-. — -- — - —.... 36,37

thickness. —.............——.—..— ——. ..—.. —— ...— --.. 27,38

transition of nonmarine phase to marine phase..---------- ——... —...— 28

type section....------ —— __.—.- — —.. —. —... —.... —... —.. ——.. 27,34

typical marine phase-.-... — —— - ——.. — —... —— —— --- — —— -....- 40

Spring C anyon, stratigraphic section measured at.-. — —... — - — —... — -20,21

Stratigraphic section, across Spring Canyon, Horsetooth Reservoir.._———— 20,23

between Turkey Creek and Little Turkey Creek. ——.————— ——— —— 20

Boxelder Creek—..--------------------------------------------------- 39

Dakota hogback north of Deer Creek- — _ —. —. — — —— ---------------- 20

Eldorado Springs.———_ — ——— —— ——— — —————- ——— ——— —— 23

Lytle..——— — — — — —————— — — - .-.——————— 20

railroad cut at Plainview. —— ————————— — ——————— ----—— 29

Third sandstone subunit-_- — --- — ——————— ———— —— —— 28,30,38,40,42

Third shale subunit——— ————————————————————— 28,38,40

Turkey Creek, stratigraphic section measured at———. — — — _. —-.. — — 20,21

Van Bibber shale, composition.------ ——.. —— --------- ——— _- —— -------- 32

economic value of-—------------------------------------------------------ 31

first key marker.--_-—-—-- — - — —. —— -. ——...- ——- — — — -— 30,32 

thickness.-. — - — — — --- —— — - ——— — - - —— ------ —— ----- — —— 32,33

Vertebrate fossils. ———— ———————— -————————————————————— 41

51