Nomenclature of 

Precambrian Rocks 

in Colorado

GEOLOGICAL SURVEY BULLETIN 1422-D

Nomenclature of 

Precambrian Rocks 

in Colorado

By OGDEN TWETO

CONTRIBUTIONS TO STRATIGRAPHY 

GEOLOGICAL SURVEY BULLETIN 1422-D

An appraisal of existing nomenclature 

and measures needed for improvement

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1977
UNITED STATES DEPARTMENT OF THE INTERIOR 

THOMAS S. KLEPPE, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress Cataloging in Publication Data

Tweto, Ogden, 1912-

Nomenclature of Precambrian rocks in Colorado.

(Contributions to stratigraphy) (Geological Survey Bulletin 1422-D)

Bibliography: p.

Supt.ofDocs.no.: I 19.3:1422-D

1. Geology, Stratigraphic Pre-Cambrian. 2. Geology, Stratigraphic Nonmenclature 

Colorado.

I. Title. II. Series. HI. Series: United States Geological Survey Bulletin 1422-D. 

QE75.B9 no. 1422-D [QE653] 557'.3'08s [551.7'l'09788] 76-608373

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

Washington, D.C. 20402 

Stock Number 024-001-02945-6
CONTENTS

Page

Abstract....................................................................................................................... Dl

Introduction................................................................................................................ 1

Pre-l,700-m.y. metamorphic complex........................................................................ 3

Sedimentary rocks younger than 1,700 m.y. .............................................................. H

Age groupings of igneous rocks................................................................................. 11

Igneous rocks of the ~l,700-m.y. age group............................................................. 12

Igneous rocks of the ~l,400-m.y. age group............................................................. 13

Igneous rocks of the ~l,000-m.y. age group............................................................. 13

Discussion................................................................................................................... 15

References cited........................................................................................................... 16

ILLUSTRATION

Page

FIGURE 1. Map of parts of Colorado and adjoining States showing distribution

of Precambrian rocks ............................................................................ D2

TABLE

Page

TABLE 1. Classification of Precambrian rock units in Colorado............................. D4

in

CONTRIBUTIONS TO STRATIGRAPHY

NOMENCLATURE OF PRECAMBRIAN 

ROCKS IN COLORADO

By OGDEN TWETO

ABSTRACT

The Precambrian rocks of Colorado consist of a metamorphic complex more than 

1,700 million years in age, three groups of igneous rocks about 1,700, 1,400, and 1,000 

nillion years in age. here designated informal age groups, and two areally restricted 

units of sedimentary rocks, one between 1,400 and 1,700 million years in age and one 

between 950 and 1,400 million years. A systematic nomenclature for many of the rocks 

does not exist, but the present state of knowledge will permit establishment of a comprehensive nomenclature. Numerous formal names that have been applied to elements of 

the Precambrian assemblage are appraised and classified in a table, and eight names 

are abandoned.

INTRODUCTION

As presently understood, the Precambrian rocks widely exposed in the 

mountains of Colorado (fig. 1) are a composite of six major age or genetic 

classes:

1. A metamorphic complex > 1,700 m.y. (million years) in age.

2. A group of igneous rocks about 1,700 m.y. in age.

3. A unit (Uncompahgre Formation) of moderately metamorphosed 

sedimentary rocks <1,700 m.y. and >1,400 m.y. in age.

4. A group of igneous rocks about 1,400 m.y. in age.

5. A group of igneous rocks about 1,000 m.y. in age.

6. A unit (Uinta Mountain Group) of slightly metamorphosed sedimentary rocks provisionally dated as 950 to 1,400 m.y. in age. 

No system of formal stratigraphic nomenclature exists for many of 

these rocks. Many stratigraphic names have been proposed, but most 

of them apply only to bits and pieces of the total assemblage. Before 

the advent of radiometric dating and widespread detailed mapping, 

many units were defined without knowledge of temporal relationships

Dl
D2 CONTRIBUTIONS TO STRATIGRAPHY

and with little consideration of their relation to rocks in other parts of 

the State or geologic province. Among those correlations that were made, 

many have subsequently proved faulty, and most of the few formal 

groups or "series" that were erected have proved to be based on faulty 

interpretations or assumptions. Thus, by a process of elimination, the 

nomenclature of Precambrian rocks in Colorado has been reduced to 

a rudimentary state. Fortunately, though no action has yet been taken, 

knowledge is now at hand to improve it.

FIGURE 1. Parts of Colorado and adjoining States showing distribution of Precambrian 

rocks (shaded areas) and localities mentioned in text and in table 1.

A total of 93 formal names applied to Precambrian rocks is recorded 

in the U.S. Geological Survey's computer printout of stratigraphic names
NOMENCLATURE OF PRECAMBRIAN ROCKS IN COLORADO D3

in Colorado. Though it is not the purpose here to denigrate the work 

of the past, the fact is that many of these names were inadequately defined 

or improperly introduced, and many that are technically valid are applicable only in minuscule type areas. A few names have been widely misapplied and have thereby lost whatever stratigraphic value they may have 

had originally. Still others have become obsolete through disuse or reclassification. Of all the names that have been proposed, only about 30 

percent remain valid and in current use, and most of these are for igneous 

rocks.

As an outgrowth of the preparation of a new geologic map of Colorado, 

(Tweto, 1976), the formally named Precambrian stratigraphic units have 

been appraised and classified (table 1). For reasons indicated in the table 

and discussed under following headings, several names applied in the 

past by the U.S. Geological Survey are herein abandoned. Many other 

names,, established by other parties, are obsolete or fail to meet the requirements of Article 13 of the Stratigraphic Code (American Commission on Stratigraphic Nomenclature, 1970). Use of such names without amendment or redefinition is inadvisable.

Though no attempt is made in this report to define new stratigraphic 

units appropriate to the Precambrian rocks, courses of action are considered in the section headed "Discussion."

PRE-1,700-M.Y. METAMORPHIC COMPLEX

A suite of complexly deformed metamorphic rocks of generally high 

rank forms a matrix for the abundant Precambrian igneous bodies in 

Colorado and a basement for the two younger Precambrian sedimentary 

sequences. This suite, herein referred to as the pre-l,700-m.y. metamorphic complex, consists of many varieties of metamorphic rocks, 

reflecting the permutations of varied parent materials, considerable 

range in degree of prograde metamorphism, wide range in degree of penetrative deformation, and wide range in degree of modification resulting 

either from igneous processes or retrograde metamorphism. The parent 

materials were evidently sedimentary, volcanic, and subvolcanic intrusive rocks. These ancestral rocks were in large discrete bodies in some 

places and closely inter layered or intertongued in others. They were a 

part of a huge province of rocks of similar age that are now irregularly 

exposed from southeastern Wyoming through Colorado into New Mexico and Arizona.

A minimum age for the rocks in the metamorphic complex is established by the fact that they were invaded by granites that are about 1,700 

m.y. old. Geologic relations establish that metamorphism and folding 

both preceded and accompanied granitic intrusion. Radiometric age 

determinations indicate that regional metamorphism peaked in the 

period 1,700-1,775 m.y. ago in Colorado (Hedge and others, 1967; Hedge
D4 CONTRIBUTIONS TO STRATIGRAPHY

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Idaho Springs Fm.»................. Cent. Front Range............ Ball (1906) ...........................

Idledale Fm.............................. E.-cent. Front Range......... Lickus and LeRoy (1968)..

Irving Fm."............................ Needle Mountains............. Cross and others (1905).....

Junction Fm............................. E.-cent. Front Range......... Lickus and LeRoy (1968)..

Mitchell Gulch Fm.................. Arkansas River.................. Salotti (1962) .....................

Mount Morrison Fm................ E.-cent. Front Range......... Boos (1954); Boos

	and Boos (1957).

Mount Vernon Canyon Fm..... ...do............................. Lickus and LeRoy (1968)..

Needle Mountains Group........ Needle Mountains............. Cross and others (1905).....

Name originally applied to pred

and gneisses in central Front 

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in favor of lithic units on most m

See Belcher Hill Fm.........................

Sawatch Schist.

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Womack Gneiss...............

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Barker (1969).

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Springs Fm., and older than Pik

that cuts unit is not Pikes Peak b

Name applied to the "quartz m

Lovering and Goddard (1950); in

feldspathic gneiss used on recent 

See Belcher Hill Fm.........................

Abandoned herein; included metase

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Gneiss, and granites of ~ 1,700-m

Invalid; incidental mention in an a

Invalid; incidental mention withou

See Belcher Hill Fm.........................

Abandoned herein; impractically 

Schist; not recognized in recent m

Abandoned herein; not used sin

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Invalid; name preoccupied; obsolete

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stratigraphic positions in me

application has given term a p

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See Belcher Hill Fm.........................

Originally, Twilight Granite; red

origin established by Barker (1969

Originally was assigned with Unc

Group; later was established by 

Irving Fm. and Twilight Gneiss

unconformably.

Uncompahgre Plateau...... Keyes (1924)............................ Invalid; incidental mention of nam

Phantom Canyon Quartzite..... S. Front Range.................. Martinez (1959).................,

Ralston Fm.............................. E.-cent. Front Range......... Boos and Boos (1934)........

Ralston Buttes Fm................... ...do............................. Lickus and LeRoy (1968).

River Portal Mica Schist.......... Gunnison River................ Hunter (1925)...................

Salida Schists........................... Arkansas River.................. Cross (1893) ......................

Sawatch Range.................. Stark and Barnes (1935).

Cent. Front Range............ Lovering (1935).............. Swandyke Hornblende 

Gneiss.

Turkey Creek Canyon Fm....... E.-cent. Front Range......... Lickus and LeRoy (1968)..

Twilight Gneiss" ................... Needle Mountains............. Cross and others (1905).....

Vallecito Conglomerate"........ ...do............................. ...do.............................

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(1906).

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DIG CONTRIBUTIONS TO STRATIGRAPHY

and others, 1968; Hansen and Peterman, 1968; Silver and Barker, 1968; 

Stern and others, 1971). Age of the rocks parent to the metamorphic rocks 

is not firmly established but apparently does not greatly exceed the age 

of metamorphism. Age in the range 1,750-1,820 m.y. has been determined 

from U-Pb isotopic ratios in zircon in metaigneous rocks in the complex 

in southwestern Colorado (Silver and Barker, 1968), northern New Mexico (L. T. Silver, in Barker and Friedman, 1974), and central Arizona 

(Anderson and others, 1971). Rb-Sr ratios indicate that metasedimentary 

rocks in the complex were not derived in any substantial part from an 

older crustal source such as the >2,500-m.y. Precambrian age province 

in Wyoming but were derived from rocks that were of mantle origin 

no more than 2,000 m.y. ago (Z. E. Peterman, oral commun., 1970). The 

one exception to these generalizations is the Red Creek Quartzite, which 

in Colorado occupies a very small area at the Utah border in the Uinta 

Mountains (fig. 1). This unit has a minimum age of 2,320 m.y. (Hansen, 

1965), and it probably is part of the >2,500-m.y. age province of Wyoming. 

Many names have been applied to elements of the pre-l,700-m.y. metamorphic complex in Colorado (table 1), but only a few have survived 

the test of usage after introduction. In the Needle Mountains isolated 

from other exposures of Precambrian rocks the Vallecito Conglomerate, Irving Formation, and Twilight Gneiss are in stratigraphic sequence 

upward (Barker, 1969), and these long-established names remain in use. 

Elsewhere, the tendency in recent decades has been to forego formal 

names and to map on the basis of lithic units in the metamorphic complex. Two names that were widely applied in the past, Idaho Springs 

Formation and Swandyke Hornblende Gneiss, are falling or have fallen 

into disuse for two principal reasons: (1) The names have been applied 

so indiscriminately that they no longer convey any stratigraphic relationships but are only ambiguous synonyms for, respectively, the biotitic 

and the hornblendic gneisses, and (2) neither unit is of the proper rank 

either to serve any useful purpose in mapping or to serve as a collective 

term for diverse rocks that are untold miles in thickness. Additionally, 

neither unit embraces the great bodies of felsic gneiss now recognized 

to be metavolcanic and subvolcanic intrusive and to be interbedded with 

the other gneisses. The felsic gneiss was formerly thought to be plutonic 

quartz monzonite intruded into Idaho Springs and Swandyke.

For reasons detailed in table 1, the following names applied in places 

to parts of the pre-l,700-m.y. metamorphic complex are herein abanoned:

Gunnison River Series Salida Schists

Needle Mountains Group Swandyke Hornblende Gneiss

River Portal Mica Schist Womack Gneiss
NOMENCLATURE OF PRECAMBRIAN ROCKS IN COLORADO Dl 1

SEDIMENTARY ROCKS YOUNGER THAN 1,700 M.Y.

The two units of sedimentary rocks younger than 1,700 m.y. the Uncompahgre Formation and the Uinta Mountain Group are straightforward and require little comment. The Uncompahgre Formation 

(quartzite, slate, and phyllite) is removed from the Needle Mountain 

Group, which is herein abandoned. The Uncompahgre is bracketed in 

age between granites dated at 1,460 and 1,720 m.y. (Barker, 1969). It has 

no known counterpart in Colorado or in New Mexico outside the Needle 

Mountains. The nearest sedimentary and volcanic unit with which it 

might be partly contemporaneous is the Texas Gulch Formation in central Arizona (Anderson and others, 1971).

The Uinta Mountain Group is limited in Colorado to the Uinta Mountains and, largely in the subsurface, to a belt extending eastward to about 

long 107° (Edwards, 1966). The group has not been subdivided in Colorado, where it consists of quartzite and sandstone as much as 24,000 ft 

(7.3 km) thick (Hansen, 1965). On the basis of studies in Utah, the group 

is provisionally dated at 950 m.y. near the top and 1,400 m.y. at the base, 

and it is thus largely contemporaneous with the Belt Supergroup in 

Montana and Idaho (Crittenden and Peterman, 1975).

AGE GROUPINGS OF IGNEOUS ROCKS

Radiometric dates are now available for all the major types of Precambrian igneous rocks in Colorado and from all parts of the Precambrian 

terranes except the Sangre de Cristo Range. The dates fall statistically 

into three populations centered near 1,700, 1,400, and 1,000 m.y. The 

rocks of each of these age categories have generally distinctive characteristics, and though age determinations serve to corroborate their assignments, most of them are distinguishable on other grounds. In brief, the 

rocks that are about 1,700 m.y. in age are generally syntectonic in habit, 

concordant, commonly foliated, and, among the granitoid facies, mainly 

granodioritic. Rocks that are about 1,400 m.y. in age are in general posttectonic in habit, discordant in some part, unfoliated or only locally 

foliated, and, among the granitoid facies, mainly quartz monzonitic to 

granitic in composition. Rocks that are about 1,000 m.y. in age are 

anorogenic, discordant, unfoliated, and highly differentiated as indicated by the combination of siliceous granites, various syenites, and 

minor mafic facies.

Formal collective names for the rocks in each of the three age categories do not exist at this time. In the absence of such names, the informal 

term "age group" is used as a temporary means of identification. If Precambrian time were more finely divided and named than it is at present,
D12 CONTRIBUTIONS TO STRATIGRAPHY

an age-connoting name (comparable, for example, to "Jurassic rocks") 

could be substituted for "age group." The rocks of each age group were 

emplaced and crystallized at intervals through certain periods of time, 

but they do not define geologic time through those periods. Therefore, 

they do not meet the requirements of chronostratigraphic units.

IGNEOUS ROCKS OF THE ~1,700-M.Y. AGE GROUP

Syntectonic igneous rocks ranging in age from about 1,650 to 1,730 

m.y. are present in all the Precambrian terranes of Colorado except the 

Uinta Mountains. They range in composition from granite to gabbro. 

Granitoid rocks, principally granodiorite and quartz monzonite, are by 

far the most abundant. They occur in several batholiths, numerous 

smaller plutons, and countless small lenticular bodies. The rocks of some 

of the batholiths and smaller plutons bear formal names; those of many 

other bodies do not. The mafic rocks, principally gabbro, occur in widely 

scattered plutons, the largest of which has a diameter of 10 mi (16 km). 

None of the mafic rocks bear formal names.

The granitic rock generally recognized as most typical of the ~ 1,700- 

m.y. age group is the Boulder Creek Granite (or Granodiorite), which 

has been studied and mapped in considerable detail in the type batholith 

west of Boulder, Colo. (Levering and Tweto, 1953; Wells, 1967; Wrucke 

and Wilson, 1967; Gable, 1972). The rock is also firmly dated at about 

1,720 m.y. by both Rb-Sr and U-Pb (zircon) methods (Peterman and 

others, 1968; Stern and others, 1971; Phair and others, 1971). The name 

Boulder Creek has been extended beyond the type batholith and its satellites to plutons scattered through the 200-mi (320-km) length of the Front 

Range (Levering and Goddard, 1950, pi. 1), and, in informal usage at 

least, much farther. Considering the uncertainties inherent in longdistance correlation of igneous rocks, the recent practice of referring to 

rocks far removed from the type batholith as "of Boulder Creek type" 

or "of Boulder Creek age" is preferable.

The Cross Creek Granite of the Sawatch and Gore Ranges is dated 

at 1,715 m.y. (C. E. Hedge, written commun., 1973). The Denny Creek 

Granodiorite of the Sawatch and Mosquito Ranges probably is of a similar age as it is older than the Kroenke Granodiorite, which is dated at 

1,700 m.y. (Barker and others, 1974). Granitic plutons on the southwest 

flank of the Sawatch Range have been dated at 1,650±35 m.y. (Wetherill 

and Bickford, 1965). This age may be too young; all the rocks are cataclasized and no completely fresh material has yet been found. Gneissic granodiorite in the Uncompahgre Plateau of extreme western Colorado is 

dated at 1,670 m.y. (Hedge and others, 1968), but this figure is also suspected to be too low (C. E. Hedge, written commun., 1976). However, 

dates somewhat below 1,700 m.y. are recorded to the south in New Mexico 

(Fullagar and Shiver, 1973) and to the southwest in Arizona (L. T. Silver, 

oral commun., 1974).
NOMENCLATURE OF PRECAMBRIAN ROCKS IN COLORADO D13

In the past, granites of the ~l,700-m.y. age group were widely miscorrelated with the much younger Pikes Peak Granite, as discussed in the 

section "Igneous rocks of ~l,000-m.y. age group."

IGNEOUS ROCKS OF THE ~1,400-M.Y. AGE GROUP

Posttectonic igneous rocks ranging in age from about 1,350 to 1,480 

m.y. are widely distributed in Colorado and are especially abundant in 

the Front Range. They are overwhelmingly of granitic or quartz monzonitic compositions, but alkalic and mafic compositions are also represented. The granitic rocks are in several batholiths and countless smaller 

bodies. Rocks of several of the batholiths bear formal names (table 1). 

Only one mafic rock, Electra Lake Gabbro, is formally named.

The rock generally recognized as most typical of the ~l,400-m.y. age 

group is Silver Plume Granite, the type body of which is an irregular 

batholith northwest of Silver Plume in the central Front Range. The 

name Silver Plume has been widely extended from the type batholith, 

not only in the Front Range but in ranges to the west. However, the 

very considerable spread in ages of granites of this age group over the 

State as a whole dictates caution in extending the name beyond the central and northern Front Range.

The Silver Plume Granite is dated in different bodies at 1,390 to 1,450 

m.y. (Peterman and others, 1968; Stern and others, 1971). The Sherman 

Granite, in a large batholith that extends into Colorado from Wyoming, 

is dated at 1,435 m.y. (C. E. Hedge, written commun., 1976) and is cut 

by Silver Plume Granite of the Log Cabin batholith (Peterman and 

others, 1968). The St. Kevin Granite in the Sawatch Range is of about 

the same age as the Silver Plume. The St. Kevin is dated at 1,470±65 m.y. 

by the Rb-Sr method and 1,410±40 m.y. by the U-Pb method (Doe and 

Pearson, 1969). The Vernal Mesa Quartz Monzonite of western Colorado 

may be somewhat older, being dated at l,480±40m.y. by the Rb-Sr method 

(Hansen and Peterman, 1968). The Eolus Granite, the principal granite 

of the ~l,400-m.y. age group in the Needle Mountains, is closely dated 

at 1,466±27 m.y. by the Rb-Sr method (Bickford and others, 1969) and 

1,460±20 m.y. by the U-Pb method (Silver and Barker, 1968).

IGNEOUS ROCKS OF THE ~1,000-M.Y. AGE GROUP

Rocks of the Pikes Peak batholith in the southern Front Range compose a ~l,000-m.y. age group. The batholith is composite, and several 

distinct intrusive units and rock types are distinguished within it. Four 

of the units or types have formal names (table 1), and several others are 

distinguished by rock names. Most of the batholith is referred to the Pikes 

Peak Granite, a term that in past usage has encompassed not only the 

coarse-grained pink to pale-orange biotite granite characteristic of the 

batholith but several other varieties of granite, syenite, and granodiorite 

(Barker and others, 1975; Hutchinson, 1960). The Mount Rosa, Windy
D14 CONTRIBUTIONS TO STRATIGRAPHY

Point, and Redskin Granites are intrusive into the Pikes Peak Granite.

Age of the Pikes Peak Granite and several of its associated rocks is 

1,041±13 m.y. as established from a whole-rock Rb-Sr isochron (Hedge, 

1970). As shown by intrusive relations, certain rocks of the batholith 

are older than those represented in the isochron (Barker and others, 1975, 

fig. 3), and the Mount Rosa, Windy Point, and Redskin Granites are 

all younger. However, the entire assemblage is interpreted to be cogenetic 

(Barker and others, 1975) and may have been emplaced within a time 

span of no more than 20 m.y. (Hedge, 1970).

In the past, the name Pikes Peak was widely misapplied to granites 

of Boulder Creek type and age, with the result that the Pikes Peak Granite 

was once classed as the oldest granite in the Front Range (Levering, 

1929). At its south end, the Pikes Peak batholith is in contact with a 

granodiorite of the Boulder Creek type. For many years, and as late as 

1950 (Levering and Goddard, 1950, pi. 1), the granodiorite was assumed 

to be a part of the Pikes Peak batholith and, therefore, to be a variety 

of Pikes Peak Granite. Similarly, north of a satellitic pluton called the 

Rosalie lobe of the Pikes Peak batholith, granite of Boulder Creek type 

was assigned by Lovering (1929) to the Pikes Peak. These misassignments 

led to further misassignments in ranges to the west, where, in several 

localities, rocks of the ~l,700-m.y. age group were dubbed Pikes Peak 

Granite (Stark and Barnes, 1935; Stark and others, 1949; Behre, 1953; 

Dings and Robinson, 1957).

Confusion over the age of the Pikes Peak Granite relative to granites 

of the ~l,400-m.y. age group began at an early date. Because the Cripple 

Creek Granite of the ~l,400-m.y. age group is finer grained than the 

Pikes Peak, it was judged to be the younger by Ma thews (1900). On the 

grounds of being less sheared than granodiorite that was erroneously 

assumed to be Pikes Peak, the Cripple Creek was also judged to be the 

younger by L. C. Graton (in Lindgren and Ransome, 1906). Further, 

Finlay (1916) erroneously assigned fine-grained dike rocks that cut Pikes 

Peak Granite to the Cripple Creek. In the central and northern Front 

Range, similar confusion prevailed with respect to the Silver Plume 

Granite. Because of the misassignment of granitic rocks of the Boulder 

Creek type to Pikes Peak Granite, the Silver Plume Granite was long 

interpreted to be younger than the Pikes Peak Granite (Lovering, 1929; 

Lovering and Goddard, 1939, 1950; Boos and Aberdeen, 1940; Lovering 

and Tweto, 1953). This interpretation was supported by yet another miscorrelation: On the basis of a general resemblance, the Sherman Granite 

was correlated with the Pikes Peak, and the Sherman is observed to be 

cut by Silver Plume Granite as noted under the preceding heading.

For reasons indicated in table 1, the names Spring Creek Granite and 

Front Range Granite Group are herein abandoned.
NOMENCLATURE OF PRECAMBRIAN ROCKS IN COLORADO D15

DISCUSSION

A need clearly exists for a systematic nomenclature for Precambrian 

rocks in Colorado. The state of knowledge is now well enough advanced 

to begin establishment of such a nomenclature. In particular, the three 

age groups of igneous rocks warrant formal designation. However, a 

problem exists in adapting them to the Stratigraphic Code as now worded 

(American Commission on Stratigraphic Nomenclature, 1970). The age 

groups would logically become formal rock-stratigraphic groups or, 

conceivably, supergroups. The problem lies in the fact that "a group 

consists wholly of divisions defined as formations" and a supergroup 

is "a formal assemblage of related groups or of formations and groups" 

(Stratigraphic Code, Article 9). The igneous age groups, on the other 

hand, consist not only of named units (formations) but of numerous 

unnamed rocks.

The mechanism of assigning group rank to a name used in one area 

and formation rank to the same name in another area (permitted by the 

Code, Article 9) would not be a satisfactory solution to the problem because the named and unnamed igneous rocks are intersprinkled throughout the Precambrian terranes. An area as small as a 7^-minute quadrangle 

might then require two ranks for the same name, and this obviously 

is not practical.

One of three courses of action would be necessary to fit the age groups 

into the existing Code:

1. Apply formal names to the unnamed rocks. Considering the great 

number of names that would be required, this course is hardly practical and is certainly undesirable.

2. Classify all the unnamed rocks of an age group into one of the named 

units or, possibly, into one of a few newly defined units. This 

course, akin to hammering a square peg into a round hole, is undesirable because it would equate rocks that might be of different 

character, mode of occurrence, position in intrusive sequence, or 

many millions of years apart in age. Much of the confusion in 

nomenclature and rock sequence that has existed in the past resulted from unwise extension of names.

3. Establish the age groups as formations and reduce the presently 

named units to rank of members. (The Stratigraphic Code, Article 

9, allows a mixture of named and unnamed members in a formation.) This course would not befit the magnitude and diversity of 

the age groups. Units such as Boulder Creek, Silver Plume, and 

Pikes Peak are each divisible into units that are logical members 

or even formations.
D16 CONTRIBUTIONS TO STRATIGRAPHY

A committee within the American Commission on Stratigraphic 

Nomenclature is currently (1976) examining the question of whether 

the Stratigraphic Code should be amended to provide more realistically 

for the Stratigraphic problems peculiar to plutonic and metamorphic 

rocks and characteristic of many Precambrian terranes. Pending the outcome, establishment of a formal nomenclature for the igneous age groups 

should be deferred. In the interim, it is possible to classify rocks and 

communicate unambiguously by referring rocks to the informal age 

groups used here.

The problems with the pre-l,700-m.y. metamorphic complex differ 

from those with the igneous rocks. Radiometric dating has not been 

applicable thus far to subdivision of the complex because of the effects 

of metamoiphism and because the analytical uncertainties in age determinations are of the same order as the time span in which the parent rocks 

of the complex apparently were deposited (~ 50-75 m.y.). Stratigraphic relations in the metamorphic complex are obscured by multiple 

folding, partial melting and flowage to the point of structural disorganization in some places, differences in metamorphic grade in different 

areas, large displacements on Precambrian faults of both premetamorphic and postmetamorphic age, and losses of continuity owing to batholithic intrusion. A large amount of the detailed mapping necessary to 

overcome these obstacles has been accomplished in the last two decades. 

Though additional mapping is needed, enough has been done to allow 

the beginning of regional studies to establish a Stratigraphic framework 

within the metamorphic complex. Until such a framework is established 

in at least major areas of the Precambrian terranes, there is little point 

in defining additional formal Stratigraphic units in the complex. Meanwhile, informal lithic units can continue to serve the needs in mapping 

and other studies.

Whatever nomenclatures may be established, they are not limited to 

Colorado. Various units of the Precambrian rocks of Colorado extend 

across State lines into New Mexico, Wyoming, and Utah.

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D18 CONTRIBUTIONS TO STRATIGRAPHY

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NOMENCLATURE OF PRECAMBRIAN ROCKS IN COLORADO D21

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____1975, Reconnaissance geologic map of the Buena Vista quadrangle, Chaffee and

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Scott, G. R., and Taylor, R. B., 1974, Reconnaissance geologic map of the Electric

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D22 CONTRIBUTIONS TO STRATIGRAPHY

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Tweto, Ogden, Moench, R. H., and Reed, J. C., Jr., 1976, Preliminary geologic map

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U.S. GOVERNMENT PRINTING OFFICE 1976-777-034/33