Semilla Sandstone, a New 

Member of the Mancos Shale 

In the Southeastern Part of the 

San Juan Basin, New Mexico

By CARLE H. DANE, ERLE G. KAUFFMAN, and WILLIAM A. COBBAN

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 12S4-F

Definition, description, and 

paleontology of a new 

stratigraphic unit
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

U.S. GOVERNMENT PRINTING OFFICE WASHINGTON : 1968

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

Washington, D.C. 20402 Price 15 cents
CONTENTS

Page

Abstract___._._._______________________________________ Fl

Occurrence. ___________________________________________________ 1

Lithology _______________________._________________ __ 2

Regional variation.________________________________________________ 7

Faunal content and correlation_____________________-_-____-__-_---_- 9

Conditions of deposition-_-_____--____-___-__---___-------__-------- 10

Lithologic evidence____________________________________________ 10

Paleontologic evidence_________________________________________ 11

Source. ______________________________________________ 12

Problems of nomenclature_______________________________________ 14

Type section.-___________________________________ __ 17

References__ ____________________________________________________ 19

ILLUSTRATIONS

Page 

FIGUBE 1. Map showing location of type section of Semilla Sandstone

Member of Mancos Shale and its outcrop in the nearby area. F3 

2-4. Photographs of:

2. Cuesta ridge of Semilla Sandstone Member of Mancos

Shale and underlying dark-gray shale._________-_-- 4

3. Type section of Semilla Sandstone Member of Mancos

Shale. --__-_.._-_---_-__-___._--__---- 4

4. Concretions weathered out of lower part of Semilla Sandstone Member of Mancos Shale.__________________ 5

in

CONTRIBUTIONS TO STRATIGRAPHY

SEMILLA SANDSTONE, A NEW MEMBER OF THE MANGOS 

SHALE IN THE SOUTHEASTERN PART OF THE SAN 

JUAN BASIN, NEW MEXICO

By CARLE H. DANE, ERLE G. KATiFFMAN,1 and WILLIAM A. COBBAN

ABSTRACT

A sandstone member in the lower part of the Mancos Shale in northwestern 

Sandoval County, N. Mex., is herein named the Semilla (pronounced Say-meeyah) Sandstone Member from outcrops along the west side of the valley of Arroyo 

Semilla in the northern part of the San Ysidro 15-minute quadrangle. It has a 

north-south extent of about 35 miles and a known east-west extent of 7 or 8 miles. 

Its base lies about 200 feet above the top of the Greenhorn Limestone Member of 

the Mancos Shale. The thickest known beds (69 ft., 9 in.) were measured as the 

type section at an easily accessible exposure about 2,000 feet northeast of Holy 

Ghost Spring (Ojo del Espiritu Santo), which is about 1 mile east of New Mexico Highway 44 at a point about 7% miles south of La Ventana store.

The member is composed of a very well sorted fine- to medium-grained sandstone with several persistent zones of calcareous concretions. Slightly more 

than half of the sand grains in the upper part of the member are quartz; 20 

percent are microcline and orthoclase, many with perthitic texture; 20 percent 

are fragments of a variety of rocks; and 10 percent are plagioclase feldspar. 

The sandstone is remarkably free of ferromagnesian minerals.

The Semilla Sandstone Member contains a moderately abundant marine molluscan fauna, chiefly of bottom-dwelling forms, characteristic of the Prionocyclus 

hyatti zone of middle Turonian age. In the upper part, the fauna is closely comparable with that of the Codell Sandstone Member of the Carlile Shale of Kansas, 

southeastern Colorado, and northeastern New Mexico, and in the lower part, 

with that of the upper part of the Blue Hill Shale Member of the Carlile Shade 

of Colorado and Kansas.

The Semilla Sandstone Member was deposited in shallow marine waters from 

rather evenly flowing offshore currents. A nearby source area of plutonic rocks 

or of older sedimentary rocks derived chiefly from plutonic rocks is probable; 

this source area may have been an island undergoing active shoreline erosion.

OCCURRENCE

In the southeastern part of San Juan Basin, New Mexico, a distinctive, locally conspicuous, and moderately persistent sandstone 

unit in the lower part of the Mancos Shale lies about 200 feet above

1 Department of Paleobiology, U.S. National Museum, Washington, B.C.

Fl
F2 CONTRIBUTIONS TO STRATIGRAPHY

the top of the Greenhorn Limestone Member of the Mancos Shale. A 

variable few feet of dark shale separates the top of the sandstone unit 

from the Juana Lopez Member of the Mancos Shale as defined by 

Dane, Cobban, and Kauffman (1966). This sandstone unit, which is as 

much as 70 feet thick, is here named the Semilla Sandstone Member of 

the Mancos Shale. The name is taken from the Arroyo Semilla, where 

the upper part of the sandstone crops out along the northwest side 

for some miles in a continuous low northeast-trending cuesta (figs. 1 

and 2). The type area is near the Holy Ghost Spring Recreation Area 

on lands now belonging to the Jemez Pueblo, which are within the Ojo 

del Espiritu Santo Grant. The type section (fig. 3) is readily accessible and is about 2,000 feet north of Holy Ghost Spring (Ojo del 

Espiritu Santo) and approximately 1 mile east of New Mexico State 

Highway 44 at a point about 71/£ miles south of La Ventana store 

(fig. 1); it is described in detail at the end of this report. The Semilla 

Sandstone Member is known to crop out west of San Ysidro from the 

northern parts of T. 14 N., R. 1 E., and T. 14 N., R. 1 W., and to extend northward across the San Ysidro 15-minute quadrangle and 

thence northward into the steeply dipping beds in the foothills belt 

west of the Nacimiento Mountains, and perhaps intermittently, into 

sec. 11, T. 20 N"., R. 1 W., in the northern part of the La Ventana quadrangle. The known north-south outcrop belt is about 35 miles long 

and the maximum east-west extent of the member is 7-8 miles. The 

sandstone member grades northward into silty shales and loses its 

identity in the northeastern San Juan Basin.

LITHOLOGY

At the type area the Semilla is divisible into two lithic units: 

(1) a 45-foot-thick lower unit of very fine grained silty poorly bedded 

yellowish-brown sandstone containing six zones of large rounded 

brown calcareous sandstone concretions (figs. 3 and 4); and (2) an 

upper unit, 25 feet thick, consisting of somewhat darker colored and 

coarser grained, massive, ledge-forming sandstone and a few harder, 

brown-weathering sandstone beds. Fossils occur abundantly in the 

concretions and in small lenses of the upper sandstone unit, especially 

near the top.

The sandstone in the Semilla is characteristically well sorted, but 

the sorting is commonly masked throughout by carbonate, ferruginous, 

or siliceous cement, and, in the upper part of the member, by extensive 

secondary overgrowths of quartz on quartz grains. The overgrowths 

develop numerous well-defined crystal faces and, less commonly, 

doubly terminated quartz crystals. In general, the sandstone of the 

lower part of the member contains 70 percent very fine sand to fine sand
SEMILLA SANDSTONE MEMBER, MANCOS SHALE, SAN JUAN BASIN F3

R i w 106°55'

FIGURE 1. Location of the type section of the Semilla Sandstone Member of the 

Mancos Shale and its outcrop in the nearby area. Mesaverde Group stippled, 

Dakota Sandstone and older rocks diagonally lined, Mancos Shale unpatterned. 

Base from parts of San Ysidro and La Ventana 15-minute topographic maps.
F4 CONTRIBUTIONS TO STRATIGRAPHY

FIGURE 2. Cuesta ridge made by the Semilla Sandstone Member of the Mancos 

Shale northeast of type section and by the underlying dark-gray shale of 

Carlile age exposed along a short valley tributary to Arroyo Lopez. Nacimiento 

Mountains on skyline.

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FIGURE 3. Type section of the Semilla Sandstone Member of the Mancos Shale 

showing the upper zones of concretions in a vertical face just east of Arroyo 

Lopez and 2,000 feet north of Holy Ghost Spring (fig. 1).
SEMILLA SANDSTONE MEMBER, MANCOS SHALE, SAN JUAN BASIN F5

fe,Tsa£.>c S«5^ 

;M%t*M*S::;v; ^ ? 

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FIGUEE 4. Concretions weathered out of the lower part of the Semilla Sandstone Member of the Mancos Shale at the type locality. Concretions are 2-3 feet 

in diameter.

(mostly very fine sand), 10 percent silt and clay sized particles, and 

20 percent calcium carbonate. In the middle part of the member, the 

sandstone contains about 80 percent very fine to fine sand (mostly 

fine sand); the remainder is silt, clay, and calcium carbonate. In the 

upper part of the member, the sandstone contains about 80 percent fine 

sand and medium sand sized grains in nearly equal parts; the remainder consists of finer sized material and ferruginous and siliceous 

cement with only a small amount of carbonate. In general, there is thus 

a gradual increase in average grain size upward and a gradual change 

from the dominantly carbonate cement in the lower part to principally 

ferruginous and siliceous cement in the upper part.

The mineralogic composition of the coarser grained upper part of 

the Semilla was examined for the writers in May 1966 by Hoy A. 

Bailey of the U.S. Geological Survey, who reviewed several thin sections of samples taken from two localities. The first of these samples 

was taken near the top of the Semilla about a mile and a half southwest of the type section, just east of New Mexico Highway 44. Mr. 

Bailey described this rock as

a medium-grained, feldspathic sandstone consisting predominantly of subangular 

to rounded grains (0.2-0.7 mm diam) of quartz (more than 50 percent), 

microcline and orthoclase (about 20 percent), plagioclase (about 10 percent),
F6 CONTRIBUTIONS TO STRATIGRAPHY

and a wide variety of sand-size rock fragments (about 20 percent). Small fragments of bone (dahllite and cellophane) are also moderately abundant.

The quartz grains invariably have wavy extinction and contain abundant 

minute inclusions commonly alined in healed fractures. Most of the inclusions 

are too small for identification, but rutile needles and occasional larger crystals 

of apatite, tourmaline, zircon, and epidote are recognizable. All of the quartz 

grains have conspicuous secondary overgrowths of quartz.

Both microcline and orthoclase grains show a wide variety of perthitic textures and some sericitic alteration. Plagioclase shows twinning but seldom any 

zoning and is commonly clouded by a fine-grained, brownish alteration product.

A microgranular variety of chert is the most abundant rock fragment. (Chalcedonic chert is rare.) A particularly distinctive type of chert is a microgranular 

variety containing abundant euhedral grains of magnetite pseudomorphed by 

hematite. Some of the rock fragments appear to be fine-grained altered volcanics, 

but only a few grains of pilotaxitic basalt or andesite could be positively identified as of volcanic origin.

The cementing material is largely authigenic quartz forming overgrowths on 

the quartz grains. Most of these overgrowths (0.02-0.03 mm thick) are optically 

continuous with detrital cores. Overgrowths on adjacent grains are commonly 

grown together and are in irregular contact, but adjacent to interstices the 

overgrowths have well-developed crystal faces. A very minor amount of carbonate 

cement is present also.

The character of the inclusions in the quartz grains, the abundance of 

perthitic feldspars, and the lack of zoning in the plagioclases indicate that the 

bulk of the materials was derived ultimately from a crystalline (igneous and 

(or) metamorphic) terrane. However, the sandstone is remarkably free of 

ferromagnesian detrital grains, which, together with the abundance of chert 

fragments, suggests that it may be a second cycle sediment.

Mr. Bailey also examined thin sections cut from a sample taken 

from the upper 6 feet of the Semilla at the type section. He described 

this rock as

a medium-grained feldspathic sandstone consisting of subangular to subrounded 

grains (0.1-0.5 mm diam) of quartz (about 50 percent), feldspar (about 20 

percent), and rock fragments (about 30 percent) in a ferruginous cement.

This sandstone is similar to the one first examined but is slightly finer 

grained, and the grains tend to be somewhat less rounded. The quartz oVergrowths go prominent in the first sample are lacking, but the quartz grains 

otherwise are similar, having wavy extinction and abundant inclusions. Microoline, orthoclase, and plagioclase are present in proportions similar to th'e first 

section examined, and they exhibit similar textural and alteration characteristics.

iSand-size fragments of chert, microgranular vein quartz, aplite, and variety 

of microcrystalline volanic(?) rocks are present, but the distinctive magnetite- 

(hematite-) bearing chert of the first sample is absent and bone fragments are 

less abundant.

This rock is also clearly derived from a crystalline source area and like the 

first sample, it contains almost no ferromagnesian grains.
SEMILLA SANDSTONE MEMBER, MANGOS SHALE, SAN JUAN BASIN F7 

REGIONAL VARIATION

The type section of the Semilla Sandstone Member is the thickest 

sequence (69 ft, 9 in) that has been observed. The member varies 

rather erratically in thickness throughout its geographic extent.

About 4 miles northeast of the type locality the member is represented by about 17 feet of soft muddy white sandstone with a few hard 

brown calcareous concretions containing Inoceramus howelli White 

and Cardium, sp. About 5 miles farther north, the Semilla is represented by a few feet of siltstone. At the equivalent horizon 8 miles still 

farther north, however, there is nearly 30 feet of siltstone and very 

fine grained light-colored sandstone containing Pinna sp., oysters, 

and indetermine small pelecypods; in the lower part are large hard 

brown sandy calcareous concretions. This locality, in the NE% sec. 

11, T. 20 N., R. 1. W., on the north side of Senorito Arroyo, La 

Ventana quadrangle, is the farthest north the Semilla Sandstone Member has been observed by the authors. Southward from the type locality, however, its outcrop extends almost continuously for more than 20 

miles in the western part of the San Ysidro 15-minute quadrangle 

and tho adjoining area to the south.

South of the type locality, the Semilla exhibits the same irregularity 

in thickness and character as it does north of the type locality. Its 

outcrop extends for more than 2 miles southwestward from the type 

section in a well-defined low escarpment crossing New Mexico Highway 44:, where there are good exposures in the highway cut and 

north of the highway. It diminishes considerably in thickness, however, until it is no longer topographically expressed about a mile 

southwest of the highway where only a few siltstone beds crop out. 

About a mile and a half southwest of the highway the member is represented only by a few brown concretions in a unit of gray and brown 

sandstone beds that are a fraction of an inch in thickness. For about 

9 miles in the southwestern part of San Ysidro quadrangle and from 

1-2 miles east of the western edge of the quadrangle, the Semilla is 

represented by 25-50 feet of soft silty sand or siltstone with large hard 

brown sandy calcareous concretions. At one locality (about 6 miles 

north of the southern edge of the quadrangle, 1.1 miles southwest of 

Ojito Spring) the lowest zone of large rusty-brown concretions contains very large numbers of Spathites puercoensis (Herrick and 

Johnson).

Near the southern edge of the San Ysidro quadrangle in the western 

part of sec. 17 and the northeastern part of sec. 18, T. 15 N., R 1 W.,
F8 CONTRIBUTIONS TO STRATIGRAPHY

bedded sandstone 3 or 4 feet thick again overlies the soft muddy siltstone or sandstone with brown concretions. As at the type locality, 

the contact with the overlying shales appears to be quite sharp.

Although the Semilla Sandstone Member has not been traced south 

of the San Ysidro quadrangle, similar sandstones have been noted 

at several localities to the south and east of the type area. At this time of 

writing, this sandstone is not known to be present on the south side 

of Mount Taylor north of U.S. Highway 66 except probably at one 

locality. In the SW^ sec. 4, T. 10 N., B. 7 W., on the west side of San 

Jose Canyon, a massive to weakly bedded sandstone 8-10 feet thick 

lies within a few feet below the basal part of the Juana Lopez Member 

of the Mancos Shale. The sandstone contains Callianassa borings but 

no other fossils; it is sharply defined at the top but at the base grades 

into dark-gray silty shale. The Semilla is also probably equivalent to an 

unnamed sandstone in the Galisteo-Tonque area (Stearns, 1953, p. 968) 

which is described as "thin hard sandstone forms a persistent hogback 

below the Juana Lopez Member in the Galisteo Lowland. The sandstone is generally only 1 or 2 feet thick, but at one locality in the southeast part of the Galisteo Lowland, soft massive sandstone below the 

principal bed increases the thickness to 30 feet."

The Semilla is not present north of Cerrillos at the type locality 

of the Juana Lopez Member although 28 feet of silty shale, bearing 

very thin lenticular sandstones, siltstones, and five septarian concretion zones occurs 90 feet below the Juana Lopez Member and probably represents a shale facies equivalent to the Semilla. Concretions 

in shale about 150 feet below the base of the Juana Lopez contain a 

fauna of the Prionocyclus hyatti zone. A few miles to the north, at a 

locality about 12 miles southwest of Santa Fe along U.S. Highway 

85, approximately 40 feet of silty shale lies about 40 feet below the 

base of the Juana Lopez; this silty shale contains warty septarian 

concretions at several levels and a few thin siltstone beds with a fauna 

of the P, hyatti zone. Although these silty units should not be considered a part of the Semilla Sandstone member, they lie within 

or proximal to the hyatti fauna and indicate at least local deposition 

of finely clastic material at an equivalent level. Similarly, far to the 

northwest in the type area of the Mancos Shale in southwestern Colorado, about 10 feet of silty shale and thin siltstone beds containing 

the P. hyatti fauna lies about 50 feet below the base of the Juana 

Lopez Member. The basal sandstone of the Straight Cliffs Formation of southwestern Utah is also age equivalent, but neither the 

P. hyatti zone nor a sandstone unit equivalent to the Semilla is recog-
SEMILLA SANDSTONE MEMBER, MANGOS SHALE, SAN JUAN BASIN F9

nized in the intervening Black Mesa, Arizona area (Repenning and 

Page, 1956, p. 268).

FAUNAL, CONTENT AND CORRELATION

The Semilla Sandstone Member at the type section lies entirely 

within the ammonite zone of Prionocyclus hyatti (Stanton) as defined 

by Cobban and Reeside (1952, chart lOb, p. 1018). In south-central 

Colorado and Huerf ano Park, this zone is divisible into two subzones 

on the basis of unnamed species of Turritella, subspecies of Lopha 

~bellaplicata (Shumard), and species of Inoceranws (Kauffman, 1961, 

1965).

The lower subzone occurs in the upper part of the Blue Hill Shale 

Member of the Carlile Shale, which contains septarian limestone 

concretions; it is characterized in south-central Colorado and in parts 

of Kansas (Hattin, 1962) by Lopha bellaplicata novamexicana Kauffman, Inocemmus cuvieri Sowerby and forms transitional between 

this species and /. howelli White, and a new turritellid which we here 

designate Turritella n. sp. A. This subzone also contains a diverse 

molluscan fauna similar to that of tiie overlying Oodell Sandstone 

Member ("Pugnellus Sandstone" of Stanton, 1893).

The upper subzone in south-central Colorado contains the diverse 

"Pugnellus Sandstone" fauna described by Stanton in 1893 and is 

characterized by the presence of Lopha ~bettaplicata ~bellaplicata (Shumard) , Inoceramus howelli f .t., a new turritellid here designated Turritella n. sp. B, 'and the absence of /. cuvieri. CoUopoceras colleti Hyatt 

is also found in this subzone.

The upper resistant sandstone unit of the Semilla Sandstone Member includes the upper part (upper subzone) of the P. hyatti zone and 

is thus the age equivalent of the Codell Sandstone Member of southcentral Colorado. This is confirmed by the presence in the resistant 

sandstone unit of the Semilla of Lopha bettaplicata ~bellaplwata, Inocerarrws howelli without /. cuvieri, Turritella n. sp. B, and possibly 

also by the presence of a new species of oyster known previously only 

from the younger Juana Lopez Member.

The soft concretion-bearing sandstone unit that forms the lower part 

of the Semilla Sandstone Member seems to be equivalent to the upper 

part of the Blue Hill Shale Member of Colorado and Kansas (commonly silty and sandy, with several zones of septarian limestone 

concretions), and it represents the upper part of the lower P. hyatti 

subzone. Hie presence of Inoceramus howelli subsp. transitional to 

/. cuvieri and possibly the occurrence of Spathites puercoensis and of 

Coilopoceras springeri instead of 0. coUeti suggest this correlation.

289-085 O 68
F10 CONTRIBUTIONS TO STRATIGRAPHY

Despite their lithologic and, in part, age equivalence, the Codell 

Sandstone Member of Colorado and the Semilla Sandstone Member 

of New Mexico do not form a continuous sand body; instead, they are 

separated by nearly 100 miles of shale without sand that is of equivalent age; therefore, the introduction of a new name for the New 

Mexico unit seems justified.

CONDITIONS OP DEPOSITION

LITHOLOGIC EVIDENCE

Many sections of the Semilla Sandstone Member that have been 

examined, including the type section, display the general habit of the 

shallow marine sublittoral sandstones of the western interior Cretaceous as described by Spieker (1949, p. 62, 64) ; that is, a gradation upward from shales into increasingly coarser siltstones and sandstones, 

with a clear-cut upper boundary that normally has a hard brown ferruginous carbonate-cemented sandstone bed at the top at places where 

the sandstone unit is overlain by marine shale. The transitional base 

of such marine sandstones represents the transition from mud to sand 

in the zone of shallow shelf deposition beyond the marginal sand sheet, 

and owes its origin to either a gradual influx of clastic material or a 

retreat of the sea. Although the clean-cut upper boundary of some such 

marine sandstones represents a subaerial gradation plane (Spieker, 

1949, p. 64), for others, such as the Semilla, the sharp top results from 

a buildup of clastic material into shallower and shallower waters in 

which agitation by waves and transport by offshore currents created a 

bypass sedimentation condition and prevented the further accumulation of sand. Thus, submarine erosion resulted in a planar upper surface 

bevelling crossbedding planes in the sand deposit, as in the type section 

of the Semilla, where the upper two units (22 and 23, p. 25) 

thicken markedly in a northwesterly direction. As in other regressive 

marine sandstones, granules and pebbles characteristically occur 

locally in the upper part of the Semilla. This coarse material has been 

variously attributed to rapid accumulation and poor sorting; to winnowing; and, in beach deposits, to the rapid lodgement of gravel on 

beach surfaces by quick wave pulses with sudden acceleration of 

velocity transporting coarser material than was returnable by slower 

back flowing of ebbing wave water (Barrell, 1925, p. 295). The coarse 

material observed in the Semilla has been found in poorly sorted crossbedded units associated with fragmented oyster shells, and their 

presence suggests episodes of winnowing during rapid accumulation.

Throughout the Semilla, the grains are well sorted within a narrow 

range of particle sizes, and this indicates that they were deposited by
SEMILLA SANDSTONE MEMBER, MANGOS SHALE, SAN JUAN BASIN Fl 1

marine currents flowing at a rather steady rate. A relatively sudden 

change in depositional environment probably caused the rather abrupt 

gradation from shale at the base of the member to dominantly very fine 

grained sandstone above. The gradual coarsening of grain size upward 

in the member indicates the increasing velocity of the currents that introduced and distributed the sediments, and this condition, presumably though not necessarily, reflects a closer approach of the shoreline 

to the outcrop area. The upward decrease in the proportion of clay 

matrix to sand can be related to increasingly shallow depth, increasing 

wave turbulence, and winnowing of the sediment, as Sabins (1963, 

p. 220) has noted for the bar sands of the Bisti area in the northwestern 

part of the San Juan Basin.

The angularity of the quartz grains in some of the coarser parts of 

the Semilla has been greatly overemphasized because secondary overgrowths have produced remarkably sharp crystal faces on originally 

rounded grains, nevertheless, there are many subangular to angular 

original grains of quartz, feldspar prisms, and other rock fragments in 

the sandstone. The general angularity of these various grains in the 

Semilla suggest the possibility that the source area was not far away; 

however, there is another possible explanation for the preservation of 

the angularity of the fine grains, that the currents which transported 

them over a long distance were either of uniform moderate velocity or 

of sufficiently high velocity to cause the grains to saltate or remain 

suspended.

The rarity of ferromagnesian minerals may be due, as suggested 

by Bailey, (p. F6), to the fact that the Semilla represents a second 

cycle sediment. Alternative possibilities are that the relatively evenly 

flowing currents, from which the sand was deposited, moved for such 

considerable distances that the heavier minerals dropped out by gravity 

separation before reaching the rather limited site of deposition to 

which the outcrop of the Semilla is now restricted, or that the source 

area contained an unusually low amount of ferromagnesian minerals. 

Locally in the lower part of the member, small niuscovite flakes and 

a very few flakes of oxidized biotite are present with coarser quartz, 

feldspar, and rock grains. These mica flakes, because of their shape, 

would be the most buoyant and transportable of the ferromagnesian 

minerals.

PALEONTOLOGIC EVIDENCE

Insofar as the ecologic significance of the faunas of the Semilla is 

interpretable, it accords with and supports the physical evidence of 

conditions of deposition. The abundance of mollusks and arthropods 

that have living representatives inhabiting nearshore sandy substrate 

is evidence of an active shallow-water marine environment. Among
F12 CONTRIBUTIONS TO STRATIGRAPHY

the fauna are plicate oysters, Pinna, Cymbophora (a mactrid), Turritella, Trigona/rca, Telling Cardium, and others. The beds are riddled to within 15 feet of the top of the member at several levels by the 

burrowings of marine worms and callianassid crabs. The presence of 

a crab carapace in the collections is accordant. The distribution of 

species of ammonites in the Semilla is also apparently accordant. The 

paleoecological significance of the ammonites has been briefly summarized by Birkelund (1965), and although apparently no consensus 

has been reached, it seems likely that Hattin's suggestion (1962, p. 

121) that Prionovyclus Jiyatti and Scaphites are bottom dwelling 

ammonites is correct and that the smooth discoidal ammonites such as 

Placenticeras and Coilopoceras are nektonic or planktonic forms. 

Prionocyclus hyatti is found nearly throughout the Semilla but thus 

far Coilopoceras springeri has been found only in the lower part. If 

further collections confirm this difference in occurrence of the ammonites, then the difference in assemblages could be interpreted to represent an upward change of sediments deposited from moderate depths to 

shallower depths with possible inshore conditions where shells of 

nektonic ammonites would be less likely to be found. The unique abundance of adult specimens of the otherwise rare species Spathites puercoensis (Herrick and Johnson), a smooth-shelled presumably nektonic 

tissotid ammonite, at a locality 1.1 miles S. 10° W. of Ojito Spring in 

the basal part of the Semilla is of special interest. Birkelund (1965, p. 

145) has suggested that such concentrations represent the remains of 

breeding swarms of a nektonic species migrating en masse and especially sensitive to changes in salinity, temperature, and (or) other controls. It is clear that an abrupt change in physical conditions would 

likely be encountered marginal to or with the relatively abrupt onset of 

a sediment-laden marine current such as the one postulated as the transporting vehicle for the Semilla Sandstone Member.

SOURCE

Although conclusions about the environment of deposition seem 

rather firmly established, inferences as to source are speculative and 

will remain so until more detailed petrographic and sedimentary structural studies have been made of the Semilla and other sandstones in 

the Cretaceous sequence.

The general northwesterly inclination of the crossbedding and the 

general southeast onlap of beds in the upper part of the Semilla near 

the type section clearly indicate deposition from northwesterly directed 

currents. It is highly probable that these currents were directed seaward from a source of supply that lay to the southeast inasmuch as the 

unit terminates on the outcrop both northeast and southwest of the type 

locality. This interpretation is substantiated by the geographic loca-
SEMILLA SANDSTONE MEMBER, MANGOS SHALE, SAN JUAN BASIN F13

tion of the deposit as well as by the fact that it is thickest and contains 

most clastic material in the central part of its area of outcrop.

Rocks that are time equivalent to the Semilla are found to the southwest in the Cebolleta Mesa area and the Datil Mountain coal field 

area; they thicken and become coarser grained to the southwest and 

were obviously derived from the southwestern source common to most 

of the Upper Cretaceous elastics of northwestern New Mexico. A considerable geographic gap separates these time equivalent rocks from 

the Semilla, which has a different pattern of distribution. The writers 

know of no evidence of equivalent clastic rocks on the outcrop or in 

the subsurface of the San Juan Basin to the northwest, and there are 

none for a very long distance to the northwest. Therefore, there must 

have been a local source not far to the southeast for the Semilla sediments, although there is no other evidence of uplift in that area at that 

time. Muehlberger, Adams, Longgood, and St. John (1960, p. 95) have 

evidence to support the theory of a local source to the northeast in the 

Brazos uplift for the Dakota Sandstone in the Chama area. Such a 

source would indicate that even at the beginning of the Late Cretaceous 

local sources were adding to the supply of clastic detritus from southwestern New Mexico.

As Bailey has pointed out (p. F6), the petrographic character of the 

Semilla shows that much of it was derived, at least ultimately, from 

plutonic rocks. To the writers, the abundance of feldspars of several 

plutonic types and their lack of deep alteration suggests that most 

of them were derived directly from rocks of an igneous or metamorphic 

terrane rather than from older sandstone beds. The chert grains of various kinds, on the other hand, bespeak a source from older Paleozoic 

rocks, which quite likely also provided some of the rounded quartz 

grains. Some of the Semilla contains an estimated 20-30 percent grains 

of feldspar, they are subangular to angular, and include some cleavage fragments of feldspar. This seems to require a special explanation 

in view of the humid climatic conditions that prevailed during Late 

Cretaceous time in the central and western interior of the United 

States, where paleogeographic conditions permitted accumulation 

of materials such as carbonaceous detritus, leaf and stem impressions, 

and coal beds. As Hattin (1962, p. 117) has pointed out in an analysis 

of the possible source area for the Codell, the source area could not have 

been one of low relief because under the humid climatic conditions the 

feldspar would have been thoroughly decomposed.

On the contrary, if the source area had been one of high relief, there 

would be evidence of steep stream gradients such as, at least close to the 

source, an abundant supply of variably coarse clastic detritus. It seems 

unlikely that such a relatively fine-grained uniformly sized deposit as 

the Semilla could have been derived from such a source. (Hattin also
F14 CONTRIBUTIONS TO STRATIGRAPHY

showed that the kaolin found in the Blue Hill could not have originated 

under conditions of high relief.) As Petti John (1957, p. 125) has 

pointed out, feldspar can survive long transport in large streams, but if 

the source area for the Semilla was, as we have tentatively postulated, 

not many miles to the southeast of its present outcrop area, then a likely 

source area might have been a small island mass within the Cretaceous 

sea; however, there is no indication that such an island mass existed 

at the time of deposition of the Upper Cretaceous rocks that were 

deposited before the Semilla, If there had been such an island source 

area, the angularity and relative abundance of comparatively fresh 

feldspar in the Semilla could be explained as derived from shoreline 

erosion and submarine planation rather than from subaerial erosion 

followed by fluviatile transport. The foregoing speculations serve 

primarily to indicate the need for more intensive and also more extensive mineralogic, petrographic, and petrologic studies of these rocks 

to be coordinated with stratigraphic and paleontologic studies.

PROBLEMS OP NOMENCLATURE

The writers of this paper are in complete accord on the definition, 

lithologic characterization, faunal content, and correlation of the 

Semilla Sandstone Member. They do differ in their present opinions as 

to the way in which these should be expressed within the necessarily 

arbitrary framework of stratigraphic classification, and it seems desirable to consider these differences of opinion briefly at this time 

because further analyses of the stratigraphic relations of the rocks of 

Colorado age in the San Juan Basin by us and by other authors are 

already in preparation.

The details of the physical and age relationships between the section in the San Juan Basin and the sections in Kansas and eastern 

Colorado have not been clearly defined in papers published thus far. 

In Kansas and eastern Colorado the stratigraphic concepts of the 

Colorado Group were first formed and have since been extensively 

studied. The sequence in those areas is consistently divisible into extensive thin units, for they were sites of marine deposition that was 

relatively far offshore. Three formations are recognized in the lower 

(pre-Niobrara) part of the Colorado Group; in ascending order, they 

are the Graneros Shale, Greenhorn Limestone, and Carlile Shale. Detailed discussions of these units, as currently used, may be found in 

Rubey and Bass (1925, p. 32-53), Bass (1926, p. 26-35), Hattin (1962, 

1965 a, b), Eicher (1965, p. 875-879), Dane, Pierce, and Reeside (1937), 

Scott (1962, 1963, 1964), Kauffman (1965 p. 4-6), and Kauffman and 

Pope (1961, p. 1004-1009, text-fig. 2).

In general, the Graneros consists of dark noncalcareous clay shale 

containing numerous bentonites; the Greenhorn Limestone consists of
SEMILLA SANDSTONE MEMBER, MANGOS SHALE, SAN JUAN BASIN F15

gray chalky and calcareous clay shales with gray calcarenite bands in 

the lower part (Lincoln Limestone Member), gray to brown chalky 

shales with a few chalky limestones and calcarenites in the middle 

(Hartland Shale Member), and massive sublithographic limestone 

beds with intercalated chalky shale units at the top (Bridge Creek 

Limestone Member of Colorado). (The Bridge Creek Member is the 

original "Greenhorn Limestone" of Gilbert, 1897.)

The Carlile Shale consists of (in ascending order) a basal marly 

to chalky shale containing thin calcarenite bands and concretions 

(Fairport Chalky Shale Member); a middle dark-gray clay shale 

that is silty at the top and that contains numerous septarian limestone 

concretions in the upper half (Blue Hill Shale Member) ; a variable 

massive to slabby sandstone and sandy shale unit (Codell Sandstone 

Member); a thin rusty-brown calcarenite (Inoceramite)-bearing 

unit (Juana Lopez Member); and locally, a thin calcareous shale 

and a dark-clay shale member at the top (unnamed member). These 

units have been defined as far south as the Huerfano Park-Walsenburg areas of south-central Colorado (Johnson, 1959; Kauffman, 

1961, 1965; Kauffman and Pope, 1961 2 ) and are clearly recognizable 

(except locally for the Codell Sandstone Member) in northern and 

northeastern New Mexico (Kankin, 1944; Kauffman, unpub. data).

K-ankin (1944), who first introduced the nomenclature of the 

Colorado Group from Colorado and Kansas into this area, clearly 

recognized the Graneros, Greenhorn, and Carlile as formations in 

northeastern New Mexico; however, his definitions of the Graneros 

and Greenhorn were those of Gilbert (1897) instead of those bf Kubey 

and Bass (1925) and Bass (1926), whose definitions are in current use. 

In the northeastern part of the San Juan Basin, because the 

Graneros, Greenhorn, and Carlile seemed sufficiently distinct as 

lithologic units compared to their counterparts in Colorado, Dane 

and Bryson (1938) were led to recognize them as members of the 

Mancos Shale and subsequent workers have followed this lead (Muehlberger and others, 1960, p. 96; Smith and others, 1961, p. 17).

It is now known that the upper boundary of the rocks of Carlile age 

in the northeastern part of the basin falls at an as yet undetermined 

level in a shale unit above the Juana Lopez Member, although very 

probably only a few feet above the top of the Juana Lopez (Landis 

and Dane, 1968). Farther south in the southeastern part of the basin, 

however, a considerably greater thickness of beds of late Carlile age 

succeeds the Juana Lopez Member, and only about 10 miles southwest of the type locality of the Semilla Sandstone Member the boundary between rocks of Carlile and Niobrara age lies at the base of the

2 Juana Lopez Member and the overlying unnamed shale were included in the Codell 

in this paper.
F16 CONTRIBUTIONS TO STRATIGRAPHY

northeasterly tapering wedge edge of the Gallup Sandstone. There the 

rocks exposed are within that part of the San Juan Basin where complex marginal marine sand bodies, ranging from earliest Late 

Cretaceous to Montana in age, intertongue with the marine sequence. 

Some of these sand bodies are lenticular; some are demonstrably time 

transgressive. The lower ones have thus far been assigned to the Dakota 

Sandstone or to the sandstone members in the lower part of the 

Mancos Shale (Hunt, 1936, p. 40-42; Owen, 1966, and many others). 

The upper ones have been designated formations, some with component members, of the Mesaverde Group (Beaumont and others, 

1956). In this southeastern part of the San Juan Basin, Carlile and 

Niobrara are not useful or practicable as rock stratigraphic names, 

although they will continue to have value in a time stratigraphic sense.

Dane and Cobban would prefer to retain, for the entire San Juan 

Basin of New Mexico and Colorado, the presently accepted classification of Dakota Sandstone, Mancos Shale, and Mesaverde Group with 

named formations and members of each as appropriate; and to indicate 

the equivalence in age, paleontology, and lithology with the KansasColorado sequence insofar 'as this can be done. Kauffman, on the other 

hand, would prefer to extend each of the Kansas-Colorado formation 

and member subdivisions as far as possible into the San Juan Basin. 

He would prefer to recognize Graneros, Greenhorn, and Oarlile as 

formations within a Mancos Group as far as possible, with the top of 

the Carlile at the top of the Juana Lopez Member and above it an as 

yet unnamed and unmapped formation of late Carlile and early 

Niobrara age. Dane and Cobban believe that this approach, although 

theoretically preferable, imposes difficulties of stratigraphic treatment 

in regional papers that can be conveniently avoided by maintaining the 

formational status of the Mancos Shale throughout the San Juan 

Basin.

The Semilla Sandstone Member is herein established, for the present, 

as a member of the Mancos Shale rather than of the Carlile Shale.
SEMILLA SANDSTONE MEMBER, MANGOS SHALE, SAN JUAN BASIN F17

TYPE SECTION

Type Section of Semilla Sandstone Member of Mancos Shale

[From 2,000-3,000 feet north of Ojo del Espiritu Santo (Holy Ghost Spring) and about 1 mile east of New 

Mexico State Highway 44 on Ojo del Espiritu Santo Grant, San Ysidro quadrangle, Sandoval County, 

N. Mex. Measured by Carle H. Dane and Edward John, November 1962, and by Carle H. Dane and 

E. K. Landis, May 1963. Fossil collections by the foregoing and by W. A. Cobban, August 1963, and by 

Erie G. Kauffman and Gerald R. Paulson, June 1964]

Shale of late Carlile age: ft in. 

24. Shale, dark to black, paper-bedded, with small gray septarian carbonate concretions. 

Semilla Sandstone Member of Mancos Shale:

23. Sandstone, soft; weathering pale to dark yellowish orange; 

in rounded weathering beds 6-19 in. thick. Unit thickens 

northwestward-_____________________________________ 6 0

22. Sandstone, medium-grained, hard; weathers yellowish 

brown (10 YR5/2); thickens northwestward to as much 

as2ft___________-__________._____ ___ 6

USGS 28875:

Prionocyclui hyatti (Stanton) 

Inoceramus howelli White 

Lopha bellaplicata bellaplicata (Shumard) 

Ostrea malachitensis Stanton 

Trigonarca sp. aff. T. obliqua Meek 

Pinna sp. indet. 

Tettinal sp. indet. 

Syncyclonemal sp. indet. 

Turritella n. sp. B 

Anatina sp. indet. 

21. Sandstone, soft, bedded, poorly exposed-_-______-_____--_ 1 6

20. Sandstone; somewhat harder than beds above and below;

irregular in thickness._-_-_________________-___------ _-___- 5

19. Sandstone, soft, indistinctly bedded, very fine grained;

riddled with borings.--_---______________--.___---_-- 1 9

18. Siltstone, parting-____________________________________ _____ 1

17. Sandstone, light-gray; coarser grained than beds above and 

below, with some black siliceous pebbles up to % m- in 

diameter; abundant oyster shell fragments; ranges to as 

much as 1 ft 6 in. in thickness.

USGS 28874 from this bed and overlying three units: 

Ostrea malachitensis Stanton 

Inoceramus n. sp. aff. /. dimidius White (Common

in Codell Sandstone Member of Colorado) 

Lopha bellaplicata bellaplicata (Shumard) 

"Ostrea" sp. aff. "0." congesta Conrad 

Ostrea n. sp. (Known from lower Juana Lopez Member of Huerfano Park, Colorado 

Trigonarca aff. T. obliqua Meek

Collections from about this level about 150 yd westsouthwest from type section contain Prionocyclus hyatti 

(Stanton), Pinna petrina White, and Ostrea malachitensis 

Stanton________________________________________ _____ 6
F18 CONTRIBUTIONS TO STRATIGRAPHY

Type section of Semilla Sandstone Member of Mancos Shale Continued!

Semilla Sandstone Member of Mancos Shale Continued Ft in. 

16. Sandstone, soft, lighter gray than units below, massive to

poorly bedded, rounded weathering__________________ 5 0

15. Sandstone; somewhat harder than adjacent beds__________ _____ 4

14. Sandstone, very soft, fine-grained, yellowish-gray.________ 2 8

13. Siltstone, with thin streaks of gray shale_________________ 6 0

12. Sandstone, silty, soft, very fine grained, yellowish-gray 

(5 Y7/2), poorly bedded; contains roundish pale-yellowishbrown-weathering limy sandstone concretions from 5 in.-

1 ft in diameter. This and underlying concretion zones 

have a marked tendency to be alined in nearly continuous 

beds. Fossil collections (USGS 28873) made from this 

and the underlying five concretion zones are listed from 

the lowest concretion zone. Most specimens were collected 

from talus slope, in part below the Semilla Sandstone 

Member, and are questionably in place. Those species 

which occur in the upper three concretion zones in place 

are followed with a "U"; those from the lower three 

zones are followed with an "L." The fauna seems to be 

consistently the same from all the concretion zones.____ 1 0

11. Sandstone, silty, soft, very fine grained; like second bed

above_______________________-___-_____--__-______- 9 0

10. Sandstone; like beds above; contains rounded to ellipsoidal 

pale-yellowish-brown (10 YRQ/2) weathering limy concretions as much as 2 ft thick_______________________ 2 0

9. Sandstone, silty, soft, very fine grained._________________ 2 0

8. Sandstone; like beds above; contains sandy limestone concretions as much as 1 ft thick._______________________ 1 0

7. Sandstone, silty, soft, very fine grained._________________ 6 6

6. Sandstone; like beds above; contains sandy limestone concretions as much as 1 ft 6 in. thick._____________ ______ 1 6

5. Sandstone, silty, soft, very fine grained, up to 8 ft 6 in.

thick-_-__________________________________________. 6 6

4. Sandstone; like beds above; contains concretions up to 2 ft 

thick and as much as 3 ft along bedding with a few elongated across bedding._______________________________ 2 0

3. Sandstone, silty, soft, very fine grained, yellowish-gray (5 

F7/2), poorly bedded; some tendency to be subparallel 

to bedding; contains numerous "burrowings" of Callianassat with elliptical cross sections as much as 1^4 by

2 in., flared ends, and treelike branches at 6-inch intervals;

some tendency to be subparallel to bedding ____________ 5 6
SEMILLA SANDSTONE MEMBER, MANGOS SHALE, SAN JUAN BASIN F19

Type section of Semilla- Sandstone Member of Mancos Shale Continued

Semilla Sandstone Member of Mancos Shale Continued Ft in. 

2. Sandstone; like beds above; contains sandy limeatone concretions as much as 2 ft thick.________________________ 2 0

USGS 28873:

Prionocydus hyatti (Stanton) U L

Coilopoceras springeri Hyatt; common in upper part, 

rare in lower part.

Inoceramus howelli White U L

Inoceramus howelli n. subsp., transitional to I. cuvieri 

Sowerby U L

Inoceramus n. sp. aff. I. dimidius White U L (common in Codell Sandstone Member of Colorado).

Ostrea malachitensis Stanton U L

Syncyclonema? sp. indet.

Cardium sp. aff. C. pauperculum Meek L

"Cyrena" securis Meek

"Cyrena" n. sp.

Cymbophora? emmonsi Meek

Trigonarca sp. aff. T. obliqua Meek U

Sinonia n. sp. aff. S. levis Stephenson U

Anatina n. sp. aff. A. lineata Stanton U

"Ostrea" sannionis White, n. subsp.

Tellina n. sp. U

Tellina sp. immature

Xenophora simpsoni Stanton U L

Pyropsis? sp. indet. U

Crab carapace cf. Callianassa

"Burrowings" of Callianassaf 

1. Sandstone, very fine grained, soft; grades downward into

silty shale________________________-______________--_ 6 0

Total thickness of Semilla Sandstone Member__________-__ 69 9

Shale of Blue Hill (Carlile) age.

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Beaumont, E. C., Dane, C. H., and Sears, J. D., 1956, Revised nomenclature of 

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v. 63, no. 10, p. 1011-1044.
F20 CONTRIBUTIONS TO STRATIGRAPHY

Dane, C. H., and Bryson, R. P., 1938, Preliminary map showing geologic structure of part of Bio Arriba County, New Mexico: U.S. Geol. Survey General 

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Dane, C. H., Cobban, W. A., and Kauffrnan, E. G., 1966, Stratigraphy and regional relationships of a reference section for the Juana Lopez Member, 

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Bull. 1224-H, 15 p.

Dane, C. H., Pierce, W. G., and Reeside, J. B.( Jr., 1937, The stratigraphy of the 

Upper Cretaceous rocks north of the Arkansas River in eastern Colorado: 

U.S. Geol. Survey Prof. Paper 186-K, p. 207-232.

Eicher, D. L., 1965, Foraminifera and biostratigraphy of the Graneros Shale: 

Jour. Paleontology, v. 39, no. 5, p. 875-909,4 figs., 4 pis.

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Survey Geol. Atlas, Folio 36.

Hattin, D. E., 1962, Stratigraphy of the Carlile Shale (Upper Cretaceous) in 

Kansas : Kansas State Geol. Survey Bull. 156,155 p.

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western Kansas, in Geol. Soc. America Field Conf. Guidebook, Kansas City, 

Missouri, 1965: p. 1-69.

 1965b, Stratigraphy of the Graneros Shale (Upper Cretaceous) in central Kansas: Kansas Geol. Survey Bull. 178, 83 p. 

Hunt, C. B., 1936, Geology and fuel resources of the southern part of the San

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Survey Bull. 860-B, p. 31-80. 

Johnson, R. B., 1959, Geology of the Huerfano Park area, Huerfano and Custer

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SEMILLA SANDSTONE MEMBER, MANGOS SHALE, SAN JUAN BASIN F21

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CONTRIBUTIONS TO STRATIGRAPHY

"Contributions to stratigraphy" in the Geological Survey's Bulletin series, consists of reports dealing primarily with stratigraphy, including those defining changes in stratigraphic nomenclature in reports of the Geological Survey. About 40 short reports 

pertaining to one or several stratigraphic units have been published 

or are in press. Like other Survey publications, these are announced in the monthly "New publications of the U.S. Geological 

Survey." In addition to reports on specific problems, an annual 

report within the series lists all changes in stratigraphic nomenclature such as (1) new names, (2) previously used names now 

adopted, (3) revised names, (4) changes in age designations, and 

(5) abandoned names. The age of the unit, the area in which the 

name is employed, the title of the pertinent report, and the publication in which the change is described are given. Thus far, the 

following reports dealing with annual compilations of changes in 

stratigraphic nomenclature have been published:

Bulletin 1194-A (for 1963). 20c 

Bulletin 1224-A (for 1964). 30c 

Bulletin 1244-A (for 1965). 25c 

Bulletin 1254-A (for 1966). 20c

U. S. GOVERNMENT PRINTING OFFICE : 1968 O - 289-085