TH.ANSGRESSIVE AND REGRESSIVE CRETACEOUS DEPOSITS IN SOUTHERN 

SAN JUAN BASIN, N. MEX. 

By JuLIAN D. SEARS, C. B. HuNT, and T. A. HENDRICKS 

ABSTRACT 

In explanation of the large-scale intertonguing of marine and 

continentnl deposits shown by the Mancos shale and the Mesaverde formntion in the southern part of the San Juan Basin, 

N. Mex., nnd of the marked variations in the stratigraphic 

boundary between those formations, the paper offers views as 

to sedilnentntion processes that are believed not only to account 

1\w tlle features observed in this field but also to be applicable 

in tlle int:erpret·ntion of similar deposits in many other fields. 

~Chree successive series of beds, each comprising a sequence of 

mal'ine, 11enr-shore, coastal, flood-plain, coastal, near-shore, and 

1nn rine deposits, are evidence of three transgressions and regressions of the Upper Cretaceous sea. The writers have 

endertvored to visualize the conditions and processes of land 

nnd sen movement and of sedimentation that would bring about 

such repeated trnnsgressions and regressions and the deposition 

and preservntion of materials in such sequences. 

~ehe paper is divided into three sections: J!"'irst, a presentation 

of the depositional concept held by the writers, with reasons for 

its adoption ; second, a description of the nature, relations, and 

vuriations of the Mnncos and Mesaverde :in the southern part 

of the San Juan Basin; and third, an outline of the inferred 

depositional history of these formations. 

According to the concept presented, deposition took place in 

a broad, shallow trough or geosyncline, the middle,. deeper part 

of which was occupied by a shallow sea, which at times spread 

over the adjacent lowlands and at times withdrew to the midd_le 

zone. Those tmnsgressions nnd regressions of the sea are 

recorded in the altemations of marine and continental deposits 

around the ma1·gins of the trough. The writers accept the 

usual view that the advances of tl1e sea and tl1e development of 

trnnsgressive deposits were brought about by a sinking of the 

trough. 'l'hey do not, however, accept as applicable in this and 

similnr cases the ft·equently expressed view that retreats of 

the sea and tl1e development of regressive deposits were brought 

ubout by a reversal of the movement-that is, by a rising of the 

trough. Instead, they believe that both the regressions and the 

regressive deposits were due to a process of trough filling that 

operated during periods when the rate of sinking of the trough 

wns much reduced and when the supply of debris was sufficient 

to build the near-sho~·e deposits upward to and above the water 

surface nnd also outward, thus forcing the sea to retreat. As 

the result of continued, though slower, subsidence there was 

room beneath tl1e p1·ofile of equilibrium for the quiet, conformable deposition of thick near-shore sands upon the older off-shore 

muds without erosion of the older p1aterial through wave action. 

~~his is shown by the widespread occurrence of transition zones 

between shale and overlying reg1·essive sandstone, and by the 

extent, thickness, and uniformity of such sandstones. 'Vit.h 

continued subsidence there would also be opportunity for the 

development of thick coal-bearing coastal-swamp deposits behind nnd encroaching upon the beach sands as the sands grew 

upward and SC'R ward. On the contrary, a reversal of movement with rise of the trough would bring at least a part:of the 

previous transgressive deposits up into the zone of subaerial 

and subaqueous erosion and would almost surely not permit 

thick and widespread regressive deposits to be laid down on 

them and preserved. Moreover, in a broad structural downfold or trough, the natural tendency of further ep~i~·ogenic 

movement would be in a continuous downward direction, and 

frequently repeated reversals or up-and-down movem€mts of the 

trough would nppear to be so unlikely that they should not be 

postulrtted unless unmistakably required by evidence in the 

rocks. 

In the southwestern _part of the San Juan Basin, near Gallup, 

the Mancos shale is 725. feet thick and consists principally of 

more or less sandy shale with several subordinate sandstones. 

Overlying it is about 1,800 feet of the Mesaverde; the upper 

part of the formation ha·s been removed by erosion. The Mesaverde comprises sandstones, clays, and coal beds, and is pred0minantly of continental origin. In this area it bas been 

divided into five units, named in ascending order the Gallup 

sandstone member, the Dileo coal member, the Bartlett barren 

member, the Gibson coal member, and the Allison ban·en member. 

From the vicinity of Gallup eastward across the southern edge 

of the basin the lower thousand feet of the Mesa verde ( representing the units from the Gallup sandstone member to the Gibson coal member, inclusive) show great progressive changes. At 

first, two new massive -sandstones appear in the section; of 

these, the Dalton replaces the upper part of the Dileo coal member, and the Hosta splits the Gibsop coal member into two parts. 

Moreover, the lower thil:d of the Gallup is divided and in part 

replaced by wedges of Mancos shale. Farther east, a tongue of 

shale, the Mulatto, comes in between the Dileo and the Dalton, 

and a higher tongue of shale, the Satan, splits t·he Hosta sandstone member. These two shale tongues thicken steadily eastwRrd. Concurrently, the intervening units of the Mesaverde 

grow thinner, and some die out; in the southeastern part of the 

basin even their most persistent layers, two thin 'sandstones 

that represent, respectively, the Gallup and a union of the Dalton 

and lower Hosta, become nothing more than sandy zoi1es in the ) 

midst of a great body of shale about 2,000 feet thick. That shale 

body, a union of the thin Mancos farther west with tl{e- greatly 

expanded Mulatto and §atan tongues, forms the Mancos of the 

southeastern and eastern parts of the basin. It is- directly overlnin by the upper part of the Hosta sandstone, whi~h ·in that 

part of the field marks the base of the restricted Mesaverde. 

Thus the Mancos-Mesaverde boundary rises stratigraphically 

about 1,200 feet in a distance of 100 miles from west to east-a 

direction that is basinward but oblique to the trends of the 

aucient shore lines. 

The depositionaJ history of the three transgressive and three 

regressive stages that are recorded in the rocks of the Mancos 

and Mesaverde is outlined in the concluding section or tne 

paper, in accordance with the general concept of depositional 

processes herein presented. 

101 
102 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1938-40 

INTRODUCTION 

P'URPOSE AND SCOPE OF THE PAPER 

Large-scale intertonguing of rnarine and continenta~ 

deposits has been observed at many places and in many 

formations, and its significance as a record of successiv~ 

advances and retreats of· the ancient sea has long been 

recognized. However, the cause of these advances and 

retreats and the conditions under which the material 

.was laid down and preserved have been explained in 

different ways. In general, the transgressive processes 

and deposits have been satisfactorily analyzed by many 

writers, but the regressive processes and the regressive 

deposits seem less thoroughly understood and are believed to merit further study and discussion. 

The Mancos and Mesaverde formations in the southern part of the San Juan Basin, N. Mex., afford excellent examples of such large-scale intertonguing. Within 

the area mapped by the writers 1 in 1929, 1930, and 1931, 

there are three successive series of beds, each comprising a ·stratigraphic sequence of ·marine, near-shore, 

Qoastal, flood-plain, coastal, near-shore, and marine deposits. These three series are clearly a record of three 

transgressions and three regressions of the Upper Cretaceous sea. The problem is : What were the conditions 

and processes of land and sea movement and of sedimentation that would bring about such repeated transaressions and regressions and the deposition and preser-

. ~ation of materials in such sequence~ Study of this 

problem led the writers to a concept of sedimentation 

processes which in their judgment accounts satisfactorily for the features observed in the San Juan Basin. 

and which they believe to be applicable also in the interpretation of similar deposits in many other fields. 

ARRANGEMENT OF THE PAPER 

The main body of the paper is divided into three 

sections-first, a presentation of the concept held by the 

writers, with reasons for its adoption; second, a description of the rock units that make up the Mancos and 

Mesaverde in the southern part of the San Juan Basin; 

and third, an outline of the depositional history of the 

two formations as· interpreted in accordance with the 

writers' concept of the process~s. . · 

DEPOSITIONAL THEORIES 

GENERAL CONDITIONS 

The concept here presented relates to deposition in 

the broad, shallow trough or .geosyncline.that occupied 

the western part of our country -during·Upper Cretaceous time and it is believed' ·to ·lJe applicable tc) other ' ..... ' 

similar troughs. The concept doe~.Jt.ot'_~~l~~ ~o ~eposition on the continental . she~f, facip.g t th,~ ,. qce.a:n. .. deeps, 

. 1 Sears,, J .. D., The coal field . from Gallup. eashva~d. toward Mount 

Taylor: U. S. Geol. Survey Bull. 860, pp. 1-29! .1~34. , :ff,unt, .~. B., The 

Mount Taylor coal field : U. S. Geol. Survey Bull.' 860, pp. 31-80, 1936. 

for there the conditions, the processes, and the final 

results would probably be markedly different. 

The Upper Cretaceous rocks of the western interior 

region were laid down in a trough that extended north 

and south for several thousand n1iles and had an average width of roughly a thousand n1iles. During n1ost 

of the period the middle or deeper part of the trough 

was occupied continuously by a shallow sea, irregular 

in shape and variable in size. At times this sea was 

enlarged and spread far and wide over the gentle slopes 

of the adjoining land, only to withdraw again to the 

deeper middle part. A record of these repeated advances and retreats of the sea is preserved in the variations of the rocks laid down at successive stages and is 

most clearly revealed in the intertonguing of continental and marine deposits found at n1any places around 

the margins of the ancient trough. · 

SIGNIFICANCE OF SHIFTS OF THE STRAND LINE 

Whether or not each of the successive advances and 

withdrawals of the sea was essentially si1nultaneous over 

the entire reaion is a question beyond the scope of this 0 . 

paper-indeed, it could not be answered conclusnrely 

in the present state of knowledge. But the regional 

causes as well as the local modifying influences that 

produced the advances and retreats-the repeated landward and sea ward shifts of the strand line-must be 

sought and considered . 

The explanations usually offered for such shifts of 

the strand line reflect two widely held but conflicting 

views regarding earth movements and sedin1entation. 

One view postulates frequently repeated up-and-down 

movements of the trough in relation to sea level. According to this view, a sinking of the trough (including 

those marginal parts that are above water) and a consequent deepening of the sea bring an advance of the 

sea and transgressive deposition of marine material over 

the land surface; and a rising of the trough and a consequent shallowing of the sea cause a withdrawal of the 

sea, a sea ward migration of the strand line, and regressive deposition of near-shore and continental n1aterial 

on the earlier marine deposits. 

The second view recognizes that a sinking of the 

trough and a consequent deepening of the sea would 

cause advance of the sea and transgressive deposition of 

· marine material' over an older land surface. It seriously questions, however, the concept of frequent vertical oscillations of the trough in relation to sea level; 

and for regressive deposits of the type herein described 

it ·prefers the concept that gradual filling of the trough 

and outward growth of the land by near-shore deposition cause a seaward shifting .of the strand line and a 

forcing back of the sea. 

Thus, by the first view, regressive deposits are the 

result of a seaward migration of the strand line, whereas, by the second view, they are the cattse of the seaward 
'.I~HAN'SGitBS'SIVE AND REGRESSIVE CRETACl~OUS DEPOSITS, S'AN JUAN BASIN, N. MEX. 103 

migration of the strand line. However, the differences 

between the two views are not in general sharply defined, and in 1nany reports in which regressive deposits 

are considered their mode of origin is either ignored or 

only vaguely tliscussed. 

CONDITIONS OF REGRESSIVE DEPOSITION 

The writers wjsh to make clear at the outset their 

belief that the regressive deposits of the Upper Cretaceous described in this paper resulted fr01n a process of 

trough filling-a building up of the near-shore deposits 

to and above sea level by which the strand line was 

gradually forced outward and the sea was restricted 

to narrower limits. They bebeve that this process was 

<:.lfl:ective during periods when the sinking of the trough 

and the consequent tendency of the sea to deepen, 

though continuing, 'vere slowing down, perhaps almost 

to a temporttl'Y standstill. T'hey believe, further, that 

a retreating aHcl shallowing sea. caused by a rising 

trough would. not be likely to permit the formation and 

preserv11tion o:f regressive deposits of the type found in 

the San Juan Basin. 

Some reasons for these beliefs have been outlined 

briefly by the senior author. 2 Others are revealed by 

the nature and relations of the sedimentary rocks described herein. The reasons fall into three principal 

o groups, which are summarized in the following three 

paragraphs. 

First: During periods when the sinking o:f the trqugh, 

though continuing, was at a slower rate the conditions 

would be :favorable to the accmnula,tion and preservation of regressive deposits. With the sea still having a 

general tendency to deepen because of the subsidence, 

room would be provided beneath the profile of equilibrimn 9 for the deposition of the newer near-shore sands 

upon the older offshore n1uds 'without e1·osion of the 

ea:rUe1• material thrO?..tgh 'WO/I)e action or distwrbamce of 

the newm· 1natP/f'ial d~ttring depos-ition. Such conditions 

would be necessary to account for the widely observed 

transition zones o:f alternating thin, regular layers of 

sn.ndstone and shale that mark the change from marine 

shales to overlying regressive sandstones and that are 

evidence o:f undisturbed, conformable deposition. 

Moreover, during further gradual sinking there would 

continue to be sufficient 1'00111 beneath the profile of 

equilibrium for the marine part of the regressive sands 

to attain the considerable thickness and wide extent that 

are characteristic of these deposits. Once forn1ed, these 

sands would be preserved by the laying down upon 

D Sears, .T. D., Regt·cssive sandstones: Washington Acad. Sci. Jour., 

vol. 23, pp. 397-398, 1933. 8 '.rhe profile of cqullibrium is a horizon • • • at which, if and 

when attnined, deposition and erosion a·re balanced for the existing conditions. Sea bottoms above the profile of equilibrium are subject to 

scour, 'those below the profile are subject to sedimentation (Eaton, 

J. 111., The by-passing and discontinuous deposition of sedimentary 

materials: Am. Assoc. Petroleum Geologists Bull., vol. 13, pp. 714-715, 

11)20). 

them of still younger deposits. Along the shore the 

excess of sand would result in the building up of 

beaches and barrier reefs not only to and above sea level 

but also outward; and by this outward growth the 

strand line would be 1noved seaward and the sea be 

forced back. Furtherrrwre, in the outwa.rd growth of 

the sand body its upper surface, because of the continuing trough subsidence, would 1·ise sea1v·ard in relation 

to the old land surface and the trough floor ; that is, the 

most recently deposited beach sand would lie at a 

higher altitude than the sand of the earlier beaches 

farther inland, which by that time had been affected 

by the general subsidence. This landward slope or seaward rise of the upper surface of the sand would have 

two effects-( 1) it would contribute to the thickness of 

the regressive sandstone as a whole and (2) it would 

bring about, in the sinking zone landward from the 

beach, conditions that are favorable to the accumulation of thick coal-bearing deposits in swamps whose 

upper surface would remain near sea level. These conditions and the processes suggested in the foregoing 

state1nent are illustrated in plate 25, which shows, according to the writers' concept, the change from trans- . 

gressive to reg1~essive deposition caused by a decrease 

in the rate of trough subsidence. The successive diagranls show the cumulative effect of additional subsidence. The transgression, partly accomplishecl.in diagram A, is continued in diagrams B and 0 where subsidence (drawn arbitrarily as 50 feet per unit period 

and carrying the old land surface down to levels 

marked b and o) predominates over seclitnentation. 

Diagrams D, E, and F (with subsidence reduced to a 

rate of 20 feet per unit period) represent a time when 

sedi,rnentation predominates over subsidence, the result being retreat of the sea through the deposition of 

regressive material. 

Second : A reversal of the movement and a rise of the 

trough would bring about conditions not generally 

favorable to the accumulation and preservation of regressive deposits of the type here discussed; and the 

greater the 1nagnitude or duration of the reversal the 

more unfavorable would be the conditions. In the initial stages of the rise of the trough and the consequent 

shallowing and retreat of the sea the near-shore and 

off-shore deposits of the previous transgression would 

be lifted nearer to the surface of the water or even above 

it, and thus they would .be exposed to subaerial and subaqueous erosion before any regressive material could be 

laid down upon them. (In the Sa~1 Juan Basin there 

was found no evidence of any appreciable erosion of 

that type; on the contrary, there is widespread evidence 

that the newer near-shore sand was laid down confornlably upon the older off-shore mud.) With continuing 

rise of the trough and shallowing of the sea the earlier 

deposits farther a way from the shore would be raised 

higher and higher above the profile of equilibrium and 
104 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 19 3 8-4 0 

into the reach of effective wave action; additional erosioi1 of the earlier ·material by marine scour would 

th~_refore seem more probable than permanent depositio,n ofmore material on it. Moreover, even though a 

. reg~·essive sandstone was being formed, the continuing 

rise· of the trough would bring the land,vard parts of 

the- sandstone farther and farther above sea level and 

thus make difficult if not impossible the accumulation 

upon them of thick coastal-swamp coal-bearing deposits. It is conceded that in certain circumstances 

(for example, during a small slow rise after a trans-. 

gr~?sion in which the supply of d~bris had been insufficient to keep the deposits up to the profile of equilibri:I.1~n) some regressive deposition might accompany or 

re~~{lt from a rise of trough; but such deposits would 

probably be rather thin and local. To the writers it 

seems very unlikely that a rising tro_ugh and shallowing 

sea. could be _the general envirOB:glel"l:t for or explanation. 

of the accumulation and permanent preservation of 

thick regressive sandstones that· are in major part of· 

uiider~ water origin, that stretch in great uniform sheets 

for hundreds of miles along shor~ and extend for 50 to 

100 1niles seaward, and that are· overlain by thick deposits of coastal swamp and estuarine origin. 

· Third: Continued subsidence of the trough seems: 

mechanically more likely than f.requent reversals of the 

direction of its movement. In a broad structural trough 

or geosyncline, such as that in wl~ich the Upper Cretaceous rocks were laid down, the na~ural tendency of. 

epeirogenic movement probably would be in a single 

direction. Once downfolding has begun, further movement probably would follow the downward course, 

whether the cause of the moveip.ent. were isostasy or 

J~teral pressure, and whether the movement were uniform or a differential warping.. True, the subsidence 

l~W9bably would be intermittent ·as stresses grow and are 

relieved. However, it seems illogical to postulate fre-

.que.nt reversals of the direction of "movement unless they. 

~i·e ·iu1inistakably recorded by .. the ~eposits or are re-: 

qi.1ired to explain them-conditiollS not true in the San· 

-J:uan l3asin. ; 

TRANSGRESSIVE AND REGRESSIVE STAGES 

The writers believe that verti:caJ movement continued 

in a single direction; that, with slow subsidence of the 

trough floor and of the nearby land, the sea maintained 

its general tendency to deepen; and that the variations 

consisted oi1ly of variations in .fhe rate of trough subsidence and of supply of debris. According to this concept, the formation of transgressive and regressive de-: 

posits depends upon relative r·ates of sinking and of 

sedimentation. When sinking, predominates, there is 

transgression; when sedimentation prevails, regression 

and regressive deposits are the result. Any increase or 

decrease in the rate of supply of debris merely has an 

effect equivalent to that of some decrease or increase, 

respectively, in the rate of subsidence. For the sake of 

sin1plicity, therefore, in this paper the rate of supply of 

debris is assumed to be constant, and emphasis is put on 

the variations in the rate of subsidence. 

Around the margins of the trough at any one time 

there were four zones of deposition-( 1) an off -shore 

zone, extending far out into the trough but with water 

probably never very deep, in which 1nuds and clays with 

inuch marine organic material were being deposited; (2) 

r.. near-shore zone in which sands and locally some pebbles were being laid down below, at, and just above 

water level; (3) a coastal zone of swamps, lagoons, and 

estuaries in which at many places 1noderately thick coal 

· beds were formed amid drab clays and irregular sand 

bodies containing n1ore or less peaty material; and ( 4) a 

landward zone in which were laid down, and in part 

preserved, flood-plain deposits of highly irregular, 

more or less cross-bedded, and in places conglomeratic 

sand, gray and light-tinted clay la1:gely of the "badlands" type, and only a very :few scattered thin lenses 

of coal. 

During periods when subsidence of the trough was 

fairly rapid, the deepening of the sea was too fast for 

the supply of debris; the water advanced gradually 

over the land, and the position of the four zones shifted 

progressively landward. The material of each zone 

thus came to be overlain by a layer of the material 

characteristic of the zone next toward the sea; a layer 

of the coastal plain deposits, itself being extended landward, became overlain by a growing she~t of near-shore 

sandy deposits, and those in turn became covered by 

1nuds and clays of the advancing off-shore zone. The 

deposits representing any one zone are progressively 

younger and rise across planes of time equivalence in 

a landward direction. (See fig. 2Q.) This was normal transgressive deposition, whicl1, continued as long 

as the controlling conditions remained unchanged. 

The time during which this process c011tinued may be 

referred to as a transgressive stage. 

~ventually, however, the. subside11c~ of the land and 

the deepening of the sea, though still continuing, gradually slowed down to a considerably lower rate. If the 

supply of sediment continued unabated, there came a 

time when its deposition began to gain on the sea, which 

then could no longer encroach on the land, but on the 

contrary was gradually pushed backward by a slow 

outward growth of the near-shore deposits. The several 

zones of deposition began a slow march seaward, each 

zone in turn leaving its typical deposits above those of 

the zone that had retreated. As the ne~r-shore sands, 

built up to and above water level, gradually extended 

seaward, their landward· portions became the sites of 

coastal swamps and lagoons, which superimposed their 

typical deposits, only to be covered in turn by the floodplain materials left by the rivers that extended their 
THAN'SGRES'SIVE AND REGRESSIVE CRETACEOUS DEPOSITS, SAN JUAN BASIN, N. MEX . 105 

.lower courses over the new land reclailned from the sea. 

Under these conditions the deposits representing any 

one zone are progressively younger and rise across 

planes of time equivalence in a seaward direction. (See 

fig. 20.) This, in the writers' view, is the method by 

which regressive deposits of the Upper Cretaceous in 

the San Juan Basin were formed and preserved. The 

Landward 

d 

major types of rocks are logically mapped as the stratigraphic boundaries of these units; and if such changes 

are found at different horizons from place to place, 

because of variation in the sediments, the boundaries are 

free to move correspondingly upward or downward. 

Flexible boundaries of U1is type are characteristic of 

the Mancos and Mesaverde ·strata. vVidespread adSeaward 

Regressive 

stage 

Transgressive 

stage 

Regressive 

stage 

Transgressive· 

stage . 

FJG UHI~ 20.-DlngTnm showing the relations of transgressive and regressive deposits and stages to planes of time equivalence. a, Marine mtltJ~; 

lJ, neat·-shore sands; o, coastal deposits; d, flood-plain deposits: A-A', B-B', etc., planes of time equivalence. 

time during which this process continued may be re-

.ferred to as a regressive stage. It persisted until an 

acceleration of subsidence and water deepening began 

a new ma.rine invasion and a new sequence of tra:nsgressive deposits. 

DESCRIPTION OF MANCOS SHALE AND MESAVERDE 

FORMATION IN SOUTHERN SAN JUAN BASIN 

SIGNIFICANCE OF THE FORMATION NAMES 

The names Mancos shale and Mesaverde ~ormation 

or group have long been applied to two great series of 

beds of Upper Cretaceous age that underlie thousands 

of square miles in vVyoming, Colorado, Utah, Arizona, 

and New Mexico. Recent years, however, have brought 

:increasing realization that the formations thus named, 

though in a general way continuous, are far from uniform over this whole vast region; on the contrary, they 

vary markedly from place to place in age, thickness, 

lithology, and interrelations. In any consideration of 

these rocks it is therefore essential to have clearly in 

mind the precise significance of their names and their 

identity. 

Both by original definition and by most later usage, 

the two formations are differentiated solely on the basis 

of their lithologic character, the Mancos being predominantly fissile clay shale and the Mesaverde an alternating series of irregularly bedded sandstone, clay, and 

con.l. Their definition does not include the criterion of 

precise age limits, nor does it depend specifically upon 

the factor of depositional environment, though the 

Mancos is wholly or mostly a marine dej)osit and the 

1\{esaverde is largely of continental origin. For this 

rcn.son the horizons of sharpest change· between the 

vances and retreats of the Upper Cretaceous ·sea are 

recorded by the many vertical and lateral variations in 

the deposits. Fine muds of the 1\1ancos continued to be 

laid down in some areas long after deposition of rocks 

of the 1\1e.sa verde type had begun in other areas. Thus 

the stra6graphic position of the lithologic change varies 

frmn place to place, and hence the boundary between 

the two formations in a general way cuts obliquely 

across planes of time equivalence. flo,-vever, the 

boundary is not a sip1p~~ plane but is cmnplicated by 1 smaller-scale intertonguing of the two types of rock. 

Such a serrate, risipg- or falling boundary between 

l\1ancos and 1\1esaverde l~as already been observed in 

other areas-for example, in the Rock Springs uplift, 

vVyoming,4 and in the Book Cliffs in Utah 5 a1id 

Colorado. 6 

It should be emphasized, therefore, that the Mancos 

and 1\1esaverde are litholog_ic units-formations whose 

general rock characteristics persist over great areas but 

whose composition, thickn~ss, boundaries, and age vary 

considerably frmn place to place. In earlier years these 

. formations w·ere identified in many separate .fields on 

the basis of their lithologic resemblance and their comparable age, supplemented in son1e instances by reconnaissance tracing bebyeen the fields. This ·sufficed to 

give a broad outline of the nature and distribution of 

'Sears, J. D., Geology of the Baxter Basin gas field, Sweetwater 

County, Wyo. : U. S. Geol. Survey Bull. 781, pp. 20-21, Pls. 4, 5, 1926. 6 Clark, F. R., Economic geology of the Castlegate. Wellington, and 

Sunnyside quadrangles, Carbon County, Utah : U. S. Geol. Survey Bull. 

793, pp. 14-15, Pl. 4, 1928. Fisher, D. J., TI1e Boo!;: Cliffs coal field in 

Emery and Grand Counties, Utah: U. S. Geol. Survey Bulf. 852, pp. 

ll-16, Pl. 7, 193'6. 6 l~rdmann, c. E., "The Booli: Ciifl's coal fiold in Garfiold and 1\:Iesa 

·Counties, Colo. : U. S. Geol. Survey Bull. 851, pp. 30-44, PI. 4, 1934. 
106 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 19 3 8-4 0 

the formations over a great region and pointed out the 

major differences frmn one. field to another, but it did 

not show how the changes took place. Gradually more 

detailed work has filled in many of the gaps in the picture. Careful tracing has den1onstrated the lateral 

gradations and intertonguing of the sediments, the 

manner in which the boundaries between thmn rise or 

fall, and the n1ore precise age relations of the component 

beds. Though the general picture is still far from complete, each contribution to it has not only been of stratigraphic importance by n1odifying or confirming the 

earlier correlations, but has also provided invaluable 

material for those interested in the broader studies of 

historical geology and sedimentation. 

Recent field work of the Geological Survey has 

thrown new light on the character, variations, and relations of the l\1ancos and Mesaverde in the southern part 

of the San Juan Basin, N. Mex. The inforrnation thus 

acquired· explains some. of the apparent inconsistencies 

in the earlier observations, and places the correlation 

upon a firmer basis. It shows that fron1 \vest to east 

the rise in stratigraphic position and hence in age of 

the Mancos-l\1esaverde boundary is much greater than 

had hitherto been suspected, and also that the boundary 

is complicated by large-scale intertonguing of the two 

formations. Furthermore, it offers valuable evidence 

for interpreting the geologic history of the region, the 

conditions of sedimentation, and the shifting of la11ds 

and seas. 

. FIELD STUDIES 

The first application of the names Mancos and Mesaverde to beds in the southern part of the San J nan Basin 

seems to have been made by Shaler/ who in 1906 carried out a rapid reconnaissance of the west side of th~ 

basin. During the preceding year he had accompanied 

Schrader 8 in an even more rapid trip around the border 

of the entire basin, but their observations were insufficient even for tentative correlation, and Schrader referred to the formations in the south merely as beds of 

Montana and Colorado age. On his second trip Shaler 

was able to demonstrate the general relations; however, 

influenced by fragmentary paleontologic evidence and 

with the lithologic variations not traced in detail, he 

included in his southern Mancos a thick zone of sandstones, clays, arid coal beds which are now placed in the 

Mesa verde formation. 

In 1907 and 1908 Gardner 9 made smnewhat more 

detailed studies of the coal field in the southern part of 

7 Shaler, M. K., A reconnaissance survey of the western part of the 

Durango-Gallup coal field of Colorado and New Mexico : U. S. Geol. 

Survey Bull. 316, pp. 376-426, 19.Q7. 8 Schrader, F. C., The Durango-Gallup coal field of Colorado and 

New Mexico : U. S. Geol. Survey Bull. 285, pp. 241-258, 1906. 0 Gardner, J. H., The coal field between Gallup and San Mateo, N. 

Mex. : U. S. Geol. Survey Bull. 341, pp. 3134-378, 1909. The coal field · 

between San Mateo and1 Cuba1 N, 1\:J~x,; TJ, S, Ge9l, Survey Bull. 381, 

pp, 461-473, 1910. . 

the basin, from Gallup to Cuba, N. Mex. His differentiation of the Mancos and Mesa verde was based on 

Shaler's views. However, because his attention was 

focused on the coal deposits, he did not note the complex 

intertonguing and the great rise of the Mancos-Mesaverde boundary from west to east; hence his correlation 

of the smaller units became confused and the lower zone. 

of sandstones and coal beds was placed in the Mancos in 

some areas and in the 1\!Iesaverde in others. 

Detailed n1apping in the Gallup coal district was 

carried on by the senior author in 1919 and 1920. At 

that time he becan1e convinced that the proper MancosMesa verde boundary in that area was lower than the 

horizon at which it had been placed by Gardner and 

others. In his report 10 this view was st~ted as follows : 

In the Gallup-Zuni Basin the upper limit of the (Mancos) 

formation should be placed at the bottom of the massive sandstone that. forms the crest of the west ridge of the Hogback. 

Thus limited, the formation consistos mainly of dark-gray, somewhat sandy marine shale. * * * Near the top is a transition ~one of sandy shale, shaly sandstone in layers averaging 

1 ineh thick, and a 2Q-foot bed of buff "muddy" sandstone. On 

the west side of the basin and in the anticline at Defiance Switch 

the transition zone is thicker and contains several conspicuous 

sandstones, of which the uppermost has the same "muddy" 

. appearance as that in the Hogback. Some of the earlier workers 

in the field have extended the Mancos shale to include the 

overlying 250 feet of sandstone, shale, and coal, which in this 

report is called the basal member of the Mesaverde formation. 

The Mancos-Mesaverde boundary has never been traced in 

detaii from the type locality in Colorado, and the paleontologic 

evidence is insufficient to decide the question. The writer is of 

the opinion, therefore, that in the Gallup-Zuni Basin the 

boundary should be drawn at the horizon where an abrupt 

change in lithology indicates a change in mode of deposition. 

Such a change occurred with the laying down of the massive 

basal sandstone of the Mesaverde as here defined, and was 

foreshadowed by the deposition of the transition beds in the 

uppe~ part of the Mancos shale. * * * Taking into consideration the thickness, the lithology, and the mode of origin of 

the formations in the Gallup Basin, the writer believes that 

the Mancos shale and the Mesaverde formation as here defined 

are homogenetic equivalents of the formations in Coloradothat is, they were formed under the same conditions and at 

the same relative place in the stratigraphic section; but that the 

Mesaverde rocks were already being laid down in what is now 

the Gallup-Zuni Basin while Mancos shale was still forming 

farther north. 

The infol'lnation upon which the present paper is 

based was gathered during a detailed examination of 

the coal beds in the southern part of the San Juan Basin 

carried out by the authors in the field seasons of 1929, 

1930, and 1931. In 1929 the party was in charge of 

Mr. Sears for the first 3 months and of Mr. Hendricks 

for the succeeding 2· months. In 1930, after a month's 

direction by Mr. Sears, the party was in charge of Mr. 

Hunt, who also continued the examination in 1931. 

10 Sears, J. D., Geology and coal resources of the Gallup-Zuni Basin, 

N. Mex. : U. S. Geol. Survey Bun~· 767, pp, 14-15, 1925. 
GEOLOGICAL SURVEY 

surface 

25 20 

Flood-plain deposits 

Scale in miles 15 10 

EXPLANATION 

Coastal swamp deposits, 

coal-bearing Beach and near-shore sands 

PROFESSIONAL PAPER 193 PLATE 25 

a 

5 0 

Off- shore muds 

DIAGHAMS SHOWING CHANGE Jc.HOM THANSGRESSIVE TO REGRESSIVE DEPOSITION THROUGH DECREASE IN RATE OF TROUGH SUBSIDENCE. 
TU.AN:SGRBf:;'SIVB AND ItEGRESSIVE CRETACEOUS DEPOSITS, SAN JUAN BASIN, N. MEX. 107 

The area _thus ex~unined is shown in figure 21. The 

survey began at the western edge, adjoining the Gallup 

district, and was largely 'concerned with the tracing of 

the formations and their subdivisions ·as defined and 

I 

s 

_) 

A 1 

.\ N 

\~ 't 

\ 

J 

r-···~~:7 ·-- -> ( . 

u 

\ J 3GI----L.::;:..;-i-----~-----·-·--t·--- --------

\1' ) 

(.) ~l'ohachi .? 

<( 

a. 

<( 

v A 

0 Aln.rquo 

10 

A N· 

L E 

0 

the rock mmnbers were 1napped, about 250 stratigraphic 

sections were n1easured, and about 200 fossil collections 

were made. For information as to the areal and structural geology of the field and to its coal deposits and 

··-------···-.. _ _,.....-·· 

0 A 

N C 0 I 

:Acoma 

40Miles 

L E T A 

HILLA 

0 

R B A 

0 Gallinn. 

A L 

li'IGUinJ 21.-Index map showing location nnd extent of the area discussed in tllis paper. 

named in that district. The work was continued eastward across the southern part of the San Juan Basin to 

Grants and La V entana, including also the blockfaulted 

area adjacent to the southeast corner of the basin. 

:Our:ing the course of the survey the boundaries between 

246223-41-2 

possibilities for oil and gas, readers are· referred to two 

other reports 11 and their acc01npanying geologic maps. 

11 Sears, J. D., The coal field from Gallup eastward toward Mount 

Taylor: U. S. Geol. Survey Bull. 860, pp, 1-20, 1934. Hunt, C. B., The 

Mount Taylor coal field: U. S. Geol. Survey Bull. 800, llP· 81-80, l!li:IG. 
108 SHORTER CONTlUBUTIONS TO GENERAL GEOLOGY, 1938-40 

DIAGRAMS OF THE INTERTONGUING RELATIONS 

. The present paper deals chiefly with the beds in the 

upper part of the Mancos shale and the lower part of 

the Mesaverde formati01i. The character of each unit 

is described as it changes laterally from west to east. 

This is the general -direction in which the rocks change 

from predominai1tly continental to predominantly madne types; correspondingly, it is the direction in which 

the :Mesa verde grows thinner and the Mancos thicker 

by intertonguing and lithologic change. For convenience, the succes·sion and general relations of the members and tongu~s ___ are shown in a simple diagram 

(fig. 22). 

SW(Landward) 

these . sections are shown by vertical lines and corresponding numbers; each line is drawn so as to indicate 

the stratigraphic range included in the 1neasured section. 

MANCOS SHALE 

NAME AND TYPE LOCALITY 

The na1ne ·Mancos shale was given by Cross 12 to the 

thick body of shale in the Mancos Valley near the town 

cf Mancos, Colo. In a subsequent report Cross 13 wrote: 

It is an almost hom_o~eneous body of soft, dark gray or nearly 

black, carbonaceous Clay shale. * * * The Mancos is ,therefore a lithologic unit which it is neces~ary to recognize in the 

mapping of this region. It is limited below by the Dakota sandstone and above by the lowest sandstone of the Mesaverde for.ma-

(Seaward) N E. ~c 

(upper)~~ 

- •• --- •• -- (!) 0 

~c;: 

Main body of Mancos shale 

l!"'IGURE 22.-Simple diagram showing the landward-sea ward variations and relations of the tongues and members of the Mancos shale and the 

Mesaverde formation. 

Details of the vertical and lateral changes in lithologic character of phe formations and their members, 

essential to the later discussion of shifting strand lines 

and the methods of deposition, are illustrated by a thre~­

diinensional correlation diagram (pl. 26). The tops of 

the cross-sections or walls in the diagran1 represent the 

lines AB, BC, etc., shown on the inset n1ap, and are 

drawn in true hor·izontal relation to each <;>ther-that is, 

the angle between any two top lines is the same as on the 

map and in nature and is not distorted by perspective. 

All point$ along the tops of the walls are therefore in 

true geographic relation to each other. The walls below these top lines are constructed as a continuous 1,800-

foot stratigraphic.cross-section. The illusion of three 

dimensions in perspective is heightened by the distortion of the patterns in all walls other than the one that 

extends east and west (BC). However, there is· no 

foreshortening, and the vertical scale is the same 

throughout. As measured parallel to the sides of the 

J!aper, the walls are of identical height at all points, and 

therefore all vertical sections can be directly scaled and 

compared. 

The localities. of the measured sections used in constructing the diagram. are shown on tl~e inse~ li;lap by 

short lines and numbers. As. projected on the walls, 

tion of alternating sandstones and shales. * * * This lithologic unit embraces the Colorado group and a part of the Pierre 

division of the Montana group. 

Thus the differentiation of the Mancos from underlying and overlying beds was based on their lithologic 

character, without reference to their marine or continental origin and with only incidental reference to 

. their age. 

RELATION TO ADJACENT FORMATIONS 

In north western New Mexico the Mancos shale rests 

conforn1ably upon a sandstone unit known as the Dakota 

( ? ) sandstone, the basal formation of the Upper Cretaceous series in the region. At many places the contact 

is transitional and no ·sJ1arp plane of separation can be 

chosen. Locally the Dakota ( ? ) sandstone is absent 

and the Mancos rests directly and unconformably upon 

older rocks. 

The upper limit of the Mancos shale-its boundary 

with the Mesaverde formation-is more complex, and 

its changes of stratigraphic position are of considerable 

magnitude. From west to east across the southern 

12 Cross, Whitman, U. S. Geol: Survey Geol. Atlas, Telluride folio 

. (No. 57), p. 4, 1899. 18 Cross, Whitman, U. S. Geol. Survey Geol. Atlas, La Plata folio (No. 

60), ·p. 4, 1899. . 
TRANSGH.ESSIVE AND REGHESSIVE CHETACEOUS DEPOSITS, SAN JUAN BASIN, N. :MEX. 109 

part of the basin, the Mancos-Mesaverde boundary 

rises stratigraphically a thousand feet or more by a 

process of intertonguing in which rocks of the M~saverde type change gradually into marine shale of the 

~1ancos. 

CHAUAC'l'lllU AND VA;RIATIONS OF THE MANCOS 

MAIN BODY OF MANCOS SHALE 

The upper limit of the Mancos shale in the southwestern part of the San Juan Basin, near Gallup, h~s 

been placed u "at the bottom of the massive sandstm1e 

that forms the crest of the west ridge of the Hogback. 

Thus limited, the formation consists mainly of dar)rgray, somewhat sandy marine shale." The upper 

boundary in this locality is shown in plate 27, B. 

~'he follo·wing section of the Mancos was measured 

near the north end of the Hogback, where the formittion is 725 feet thick : 

Seot·ion of ll{a,ncos s1Htle n:en·r 'IW'I'th end of the Hogba,ck, in sec. 

I, '1'. 15 N., R . .18 W. (app1·oximate location) 

Feet 

Mesa verde formation (base of Gallup sandstone member) : 

Sandstone, buff below, pinkish above; massive. 

1\'l:ancos shale: 

Shale, olive gray to drab, mostly sandy 

(Carlile fauna, 41 feet above base)---- 176 

Sandstone, very shaly, and sandy shale___ 7 

Shale, olive gray to drab_________________ 67 

Sandstone, gray, soft, poorly bedded____ 4 

Shale, olive gray to drab_________________ 16 

Sandstone, gray, soft, poorly bedded____ 3 

Shale, olive gray to drab ________________ 157 

Sandstone, very shaly; concretionary band 

at top -------------------------------- 4 

Shnl~ drab----------------------------- 40 

Shnle, bluish gray to drab----------~---- 101 

Sandstone, calcareous, and marl, containing many shells of G1·yphaea 

newbe'ITyi ----·------------------------ lO 

Shale, sandy, gray and bufL_____________ 6 

Sandstone, bufC, massi.ve_________________ 2G 

Shale, sandy in upper part, bluish gray 

and fissile in lower part_ ______________ 109 

Total thickness of Mancos shale ______ 725 

Dakota ( ? ) sandstone. 

The beds in that portion of the foregoing section 

from the top of the Gryplwea newber'ryi zone up to and 

including the basal sandstone of . the Mesaverde are 

sho,vn in plate 27, A. 

The location of the section is approximately at point 

A, in plate 26. At this locality the :Mancos has its mininuun thickness in the San Juan basin; and, correspondingly, at this place the Mancos-~1esaverde boundary is 

1' Sears, J. D., Geology and coal resources of the Gallup-Zuni Basin, 

N. 1\fex.: U. S. Geol. Survey Bull 767. p. 14, 1925. · · 

at its lowest or oldest stratigraphic position. A Carlile (late Benton) fauna was found 590 feet above the 

. base of the section, so, even if the· uppermost 135 feet 

of the shale (in which no fossils we1:e found) is of 

Niobrara rather than of Carlile age, the Mancos in this 

· vicinity represents only a part of the Colorado group, 

and the .~1ancos-Mesaverde contact here is much older 

than had hitherto been supposed. In contrast, at its 

type locality in southwestern Colorado the Mancos shale 

represents all of Colorado. time and the earlier part of 

· Montana time. 

At point A the Gallup sandstone men1ber (basal 

· ::vresaverde), which lies above the Mancos, consists of 

three massive sandstones with intervening clay and coal 

beds. Tmvard the northeast the lowest sandstone splits 

· into two tong~es, the upper of which, together with the 

, overlying clay zone and a part of the middle sandstone, 

: grades laterally into typical marine shale. As a conse-

. quence, in the vicinity of point B the main body of 

l\1ancos shale is overlain by a massive and thin-bedded 

· t=anclstone 53 feet thick, followed by 116 feet of sandy 

' shale mostly of typical ~1ancos appearance, up to a 

massive cliff-forming sandstone representing the 

, middle and upper sandstones of the Gallup. (See pl. 

· 28, B.) The 53-foot sandstone, known to be a tongue of 

. the lowest sandstone of the Gallup, is at this point 

merely a bench-forming sandstone in the ~1ancos; eastward it becom~s increasingly shaly, and finally it is 

completely replaced by shale so that there is a continu-

: ous shale sequence beneath the upper sandstone of the 

Gallup. Hence the upper shale in the vicinity of point 

B is actually the beginning of a tongue of the Mancos, 

· and in effect the Mancos-Mesaverde boundary is· about 

170 feet higher str~tigraphically there than near 

point A. 

The lithologic change just described, reflecting a 

change fron1 landward· to seaward conditions of deposition, is most rapid in a northeasterly direction, which 

therefore is at ~ight angles to the ancient shore line: 

In marked contrast, ~he Gallup and uppermost Mancos beds show a notable uniformity as their outc~ops 

swing to the east and southeast and approach 'n:iore 

dosely the trend of the old shore line. In the 55 miles 

from point B eastward to C and thence southeastward 

t.o D, and also in the outcrops along the line GH, there 

is no material change in the stratigraphic position of 

the Mancos-Mesa verde boundary or in the character of 

the adjacent beds. Indeed, the only significant variation is the presence or absence of the two tongues of the 

lowest sandstone of the Gallup. 

As. the outcrops swing once more toward the northeast the seaward change of the beds resmnes, though at 

a much slower rate. The first effect is an abrupt vanishing of the sandstone tongues of the Gallup from the 

upper part of th~ Mancos. Thereafter, for a long 

distance northeastward along the lines DEF, GF, and 
110 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 19 3 8-4 0 

]J the Mancos-Mesaverde contact remains at the base 

o/ the restricted Gallup sai1dstone member, but is 

thoucrht to rise slowly by gradation of the basal beds of 

the Gallup into marine shale. Finally, near points F 

and J, the Gallup sandstone thins out and the underlyincr l\1:ancos unites with an overlying shale of appro:rimately eqqal thickness to form a continuous shale 

body. Expanded in this \vay to about 2,000 feet, tl7e 

.. Mancos continues to the north beyond the field of this 

paper. , . . 

The overlying shale thus added to the Mancos IS Itself a union and expansion of two shale tongues, the 

Mulatto and Satan tongues (see pl. 26), which wedge 

into the Mesaverde rocks toward the south ai1d southwest. 

MULATTO TONGUE 

The landward edcre of the lower tongue ·was seen only . b . 

near point B. Where section 5 (see pl. 26) was measured this tongue appears in the lower part of the Mesaverde as two zones of sandy shale and thin sandstones 

separated by more than 70 feet of generally .massive 

sandstone. The lower zone is about 20 feet thick, and 

the upper zone about. 45 feet. The marine origin. of 

the shale is shown by its lithology and by a few mar1ne 

fossils of Niobrara age found in the lower zone. 

Toward the east the intervening sandstone rapidly loses 

its massive character and thins out, and at section 7 the 

shale tongue formed by the union of the two zones is 

nearly 200 feet thick. From point. C southeastwar.d to 

D the thickening of the shale continues, as that duection has a slight seaward component. At section 9 the 

shale is 237 feet thick, at section 14 it is 260 feet (see 

. pl. 28, .A.); and at section 18 it is 375 fe.et ~hick, including 

a prominent 50-foot sandstone lens In Its upper part. 

Near this last-named locality excellent exposures of the 

shale are found at the mouth of Canyon Mulatto. From 

these exposures the unit has been named 15 the Mulatto 

tongue of the Mancos shale. · 

Northeastward from point D the Mulatto tongue soon 

passes under cover. However, in a well drilled in the 

Miguel Creek dome, the log of .which forms the lo~er 

part of ·section 21, the Mulatto 1s about 400 feet thiCk. 

Eastward from point E the Mulatto thickens more 

rapidly. Finally, northeast of point F, the overlying 

and underlying sandstones die out and the Mulatto 

tongue becomes a part of the expanded Mancos shale .. 

Southwest and southeast of Mount 'J'aylor, along lines 

GH and KL the Mulatto tongue is well developed. Its 

appearance, 'thickness, and relations to ~dj acent b~ds are comparable to .. those seen in the outcrops along hne 

CD and there is no trace of its landward edge such as ' that found near point B. Toward the northeast, along 

lines GF and IJ, the Mulatto tongue thickens at the 

expense of the underlying and overlying beds by small1s Hunt, C. B., The Mount Taylor coal field : U. S. Geol. Survey Bull. 

860, p. 44, 1936. . . 

scale intertonguing, and merges with and becomes part 

of the expanded Mancos shale at the northeast corner 

of the field. 

SATAN TONGUE 

The upper tongue of the Mancos does not extend 

as far westward and southwestward as the Mulatto 

tongu·e, therefore it is not s.een in the outcrops along 

lines ABCD, GH, and l{L. At m~ny localities (for 

examples, see the spur ·walls a', b', c', and d' of wall 

CD and walls DE, GF, and IJ in pl. 26), it begins. 

as ~ne or n1ore thin wedges of marine shale splitting 

a thick cliff-forming sandstone in the midst of th~ 

Mesa verde formation about 500 feet above the top of . 

the Mulatto. At each place, the shale tongue thickens 

rapidly toward the northeast, chiefly by the adjacent 

sandstone beds grading northeastward into shale, and 

within a few miles it reaches a thickness of 150 feet or 

more. 

The beginning and growth of the shale tongue and 

its relation to the sandstone that it splits are well shown 

by the exposures in Satan Pass (see pl. 28, 0) , along 

the road frmn Thoreau to Crown Point. The unit has 

therefore been named 16 the Satan tongue of the Mancos 

shale. 

In the western part of the field the Satan tongue 

passes rapidly under cover of younger beds toward the 

north, and hence only its more landward part can be 

observed. In the eastern part of the field it is exposed 

over a much wider area.. Northeastward along the 

lines DEF, GF, and IJ it thickens to more than 300 

feet. This increase seems to be chiefly at the expense 

of the underlying sandstone, from which successive 

wedges separate and . pass laterally into marine shale. 

The upper boundary of the Satan is more regular, 

though the contact with the overlying massive sandstone is thought to rise very gradually across time planes 

toward the northeast. 

Beyond points F and J the Satan tongue merges with 

the expanded· Mancos shale, and the overlying sandstone becmnes the basal unit of the restricted Mesaverde 

formation. Thus the Mancos-Mesaverde boundary rises 

stratigraphically about 1,200 feet from point A to poil~t 

F 100 miles to the east. That distance, however, 1s 

measured ' in a direction oblique to the trend of the 

ancient shores; toward the northeast-that is, directly 

seaward-this stratigraphic rise of 1,200 feet takes place 

within a distance of only 60 miles. 

MESA VERDE FORMATION 

NAME AND TYPE LOCALITY 

The term "Mesa Verde Group" was used by Ho~mes 17 

to describ~ a series of rocks 1,100 to 1,200 feet thick, 

10 Sears, J. D., The coal field from Gallup eastward toward Mount 

Taylor : U. S. Geol. Survey Bull. 860, p. 14, 1934. 

11 Holmes, W. H., Geological report on the San Juan district: U. S. 

Geol. and Geog. Survey Terr. Ninth Ann. Rept., pp. 237-276, 1875. 
TltAN:SGH.:ESSIVE AND H.EGR:ESSIVE CRETACEOUS DEPOSITS, S'AN J'UAN BASIN, N. MEX. 111 

lying above "the great series of Middle Cretaceous 

shales" (J\Ilancos shale) and below the "Sand Shale 

Group" (Le·wis shale). 'I'he name 'vas derived from the 

J\1esa Verde, in southwestern Colorado, where excellent 

exposures are widespread. Three divisions of the group 

were described by I-Iolmes as the "Upper Escarpment'' 

and "Lower Escarpment" sandstones and the "Middle 

Coal Group." 

Fossil evidence 'reported by Cross 18 indicates that the 

Mesaverde in southwestern Colorado is o:fmiddle Pierre 

(Montana) age. 

In a report on coal deposits near Mancos, Colo., 

Collier 10 nmned the three divisions of the Mesaverde 

group, in ascending order, the Point Lookout sandstone, 

250 to 300 feet thick ; the :Menefee formation, consisting 

of 400 feet o:f sandstone, shale, and coal beds; and the 

Cliff House sandstone, several hundred feet thick. 

M:ll:SAVEU.DE NEAU. GALLUP, N.MEX. 

In the Gallup coal district, at the southwest corner of 

the San Juan Basin, the J\1esa verde was originally at 

. least twice as thick as at the type locality in southwestern Colorado. The upper part has been removed by 

erosion but can be seen son1e miles to the northeast. The 

part that remains comprises 1,800 feet of alternating 

sa.ndstone, clay, and coal beds. The three beds of sandstone near the base are massive and persistent; all the 

others are irregular and nonpersistent, so that .stratigraphic sections vary greatly in detail from place 

to place. The coal beds range in thickness from a few 

inches to 10 feet, and the intervals between them differ 

greatly from point to point. · 

Di,l;isions ot the Mesavm·de tonnat-ion in the Gall1tP-Zttni Basin, 

N.Mem. 

Member '!'hick· 

ness 

Feet 

Allison barren member....... 800 

Gibson coal member ....••••. 150-175 

Bartlett barren member ...•.• 330--400 

Dileo coal member ___________ 24Q-300 

Gallup sandstone member ..•. ISQ-250 

Character 

Lenticular sandstones, light-colored clay, and thin irregulat· coal beds, but none of 

commercial importance. Top of member 

eroded; total thickness undetermined. 

Valuable coal beds, lenticular sandstones, 

and light-colored clay. Lenticular sandstones, light-colored clay, and thin, irregular coal beds, but none of 

commercial importance. Valuable coal beds, lenticular sandstones, · and light-colored clay. 3 thick persistent massive sandstones; interbedded clay and coal beds, the upper of 

which are locally of commercial importance. 

The J\1esaverde in the Gallup district has been described 20 as ·a single formation rather than a group and 

has been divided into five members, partly because of the 

lithologic differences and partly for convenience in de18 Cross, Whitman, U. S. Geol. Survey Geol. Atlas, La Plata: folio 

(No. 60), p. 5, 1899. 1° Collier, A. J., Coal south of Mancos, Montezuma County; Colo.: 

U. S. Geol. Survey Bull. 691, pp. 296-207, 191.9. 

llO Sears, J. D., Geology nnd coal resources of the. Gallup-Zuni Basin, 

N. Mcx. : U. S. Geol. Survey Bull. 767, pp. 151-18, 1925. 

scription. The basal unit, about 200 feet thick, consisting of the three persistent sandstones and the interbedded shale and coal, is readily separable from the rest 

o:f the formation. (See pl. 27, B.) The overlying 1,600 

feet of irregular sandstone, clay, and coal beds has been 

divided on the basis of the presence or absence of valuable coals. The five men1bers, in order from top to 

bottom, are. briefly described in the preceding table. 

MESA VERDE IN SOUTHERN SAN JUAN BASIN 

The area covered by the present paper extends from 

the Gallup district eastward across the southern part 

of the San Juan Basin and beyond it into the zone of 

block faulting southeast of the basin. "Vithin this 

area the character of the Mesa verde forn1ation changes 

notably toward the east and northeast. The members 

seen in the Gallup district are modified, new n1embers 

appear in the section, and finally all the lower 1,200 

feet of Mesaverde rocks intertongue with and grade 

laterally into marine shale of the J\1ancos. The character and lateral changes of the several members, in 

ascending order, are described in the following pages . 

GALLUP SANDSTONE MEMBER 

The basal sandstone member of the Mesaverde, cmnparatively uniform in character and thickness throughout the Gallup-Zuni Basin, changes rapidly toward the 

northeast frmn the north end of the Hogback (along 

line AB, in pl. 26), as outlined in the description of 

the main body of the Mancos shale. "Vith -these 

changes the appearance of the Gallup sandstone nlember is as shown in plate 28, B. In section 5 the . 

Gallup consists of a lower bench-forming sandstone 

53 feet thick, in thin and thick layers, representing the 

basal part of the lower sandstone of the Hogback section; and an upper sandstone unit 152 feet thick, consisting of thick- and thin-bedded sandstones with two 

thin partings of clay and impure coal and representing 

the combined middle and upper sandstone of the Hogback section. The two sandstone units are separated 

by 116 feet of shale, in small part carbonaceous but 

mostly of marine appearance, representing a tongue of 

the Mancos. 

Eastward from the point where section 5 was measured, the lower bench-forming sandstone is exposed almost continuously to a point west of section 9, where 

it is seen to thin out within a short distance by lateral 

transition to shale in the face of a promontory south of 

Hosta Butte. Near section 7 a second bench-forming 

sandstone appears above tl~e first, and also continues 

southeastward to the promontory near Hosta Butte, 

where it likewise thins out. "Vithin an equally short 

distance the upper bench reappears and continues far 

southeastward to point D. In the cross-section diagrmn 

and on a n1ap of the area the upper bench sandstone 

has the appearance of disconnected lenses. However, 
112 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 19 3 8-4 0 

this sandstone is believed to r~present the layer that In the valley of the Rio Puerco the single, relatively 

splits from the top of the lo1Ver .sandstone of the Gallup thin sandstone representing the Gallup was found to 

between points A and B. Ij so! the gaps in its outcrop be the "Punta de la Mesa s~ndstone" of Herrick and 

and the resultant lens-like appearance are caused chiefly i9hnson, 21 who considered it to be of Fox Hills age. 

by the irregular shape of the li1ie of outcrop, which cuts · Lee 22 at first regarded the ";P.unta de la Mesa" as the 

back and forth across the seaward feather edge of the base of the Mesaverde formation in that vicinity, insandstone. Under this interpretation both the upper eluding in that formation the overlying somewhat sandy 

and .lower bench-forming sa~1dstones formerly were con- shales herein described as the Mulatto and Satan tongues 

nected landward (southwestward) with the lower sand- of the Mancos. Subsequently, because investigations 

stone of the Gallup, and hence rei)resent tongues of that hi:td shown the "Punta de la: Mesa" to be hundreds of 

sandstone between small tongues of the Mancos shale. feet below the top of the Mancos as developed farther 

This interpretation is strepgthened by exposures near north, Lee 23 came to regard it as a member of the l\1ansection14. Here only the upper bench sandstone is pres- cos, although he wrote: "In the Rio Puerco field it is 

ent; it is 22 feet thick and is overlain by 118 feet of not possible, without further investigation, to draw a 

gray sandy marine shale (of the Mancos) up to the definite line between Mancos and Mesaverde." Lee's 

main sandstone of the Gallup. 'Vithin a short distance reconnaissance had not made. clear the intertonguing 

to the south, however, the lower bench sandstone. ap- re1ationship; he thus grouped. a·s part of a single formapears, and within 2 miles both bench sandstones are more ti6!1 a thousand feet of rocks that are actually the interthan 50 feet thick. tonguing patts of two fonnations. Because of this 

An outcrop of what are believed to be the same two grouping and because the "Punta de laMesa" was debench sandstones was found southwest o£ Mount Tay- scribed only in a very li1nited area and its outcrops ar~ 

lor, near Grants (sec. 32, wall GH). ·Because of poor not sh.own on any 1nap, t~1at na1ne is here replaced by 

exposures these sandstones· could not be traced satisfac- the more widely used name, Gallup sandstone member 

torily toward the southeas.t, but on the basis of interval of the .Mesa verde. 

below the main Gallup the lower is thought to be the- DILCO COAL' MEMBER 

one that continues to sections 30, 29, and 28. ;In the eastern part of the Gallup coal district the 

N or.theastward from point D. and from the vicinity Dileo coal member cmnprises 250 to 300 feet of irregular 

of section 28 there is no further trace of either of these I sandstones, light-colored clay, and nine h:nticular coal 

sandstone tongues of the Gallup in the Mancos shale. :.· beds, some of which locally reach a thickness of 5 feet 

With its. lower part lost by lateral transition into or more. 

marine shale, the Gallup·sandstone member throughout j. Northward along the Hogback from the vicinity of 

. most of the area covered by this paper consists only of 1: the old Heaton mine (point A, ·pl. 26) the coals in the 

the upper beds included in the full member at its type ~: upper half of the Dileo grade within a short distance 

locality. At section 5, as already. described, the upper 1 into carbonaceous shale and cla,y; the Black Diamond 

part consists of 152 feet of sandstone with two thin : bed, 'Yhich is the 1nost persistent and valuable coal of 

partings of shale and bony coal.· The member thus 

1

: the meinber in the Gallup district; is the last to disrestricted ·continues far to the east and southeast. : appear. Just below the Black Diamond coal, a sandLocally it shows two or more distinct sandstones with 1: stone, which is thin but somewhat persistent in the 

interbedded clay and smp.e coal, but over the greater j Gallup district, thickens rapidly, and new sandstones 

part of the area it consists of essentially continuous : appear in the zone above. Within a distance of about 

sandstone ranging in th~ckness fron1 60 to 150 feet. : 2:Iniles the upper half of the Dileo is replaced by a body 

Locally (for example, in· the area between sections 14 ; ~- almost continuous sandstone about 180 feet thick. 

and 18) the upper part of the sandstone weathers into ; This sandy zone is sho,vn in section 2, measured where 

~·ounded forms -whose su~_faces are conspicuously bro- : the Hogback ends by a swipg of the rock beds from a 

ken by polygonal jointing .. (See pl. 29, A and B.) ~steep westerly to a gentle riortherly dip. 

After· relative uniformity over a wide area, the mem- 1 __ 'fhe sandf?tone unit thus __ appearing in the section i~ ber show$ a . final sea ward change in the northeast. ~ described on page 113 as the Dalton sandstone member 

corner of the field. For. some miles, because of the ' of· the Mesaverde. : 

dying out of the overlying continental beds and the From the north end of the J:Iogback eastw:ird along 

growth of the Mulatto tongue of the Mancos shale, the · the margin of the basin the lower half of the Dileo conGallup continues 1nerely. a.s a thin sandstone in the 

midst of a thick body· ·o£ marine shale; this sandstone 

becomes gradually thinner and more shaly northeastward,· and finally is an inconspiClJ.OUs sandy zone in the 

midst of the Mancos shale. (See walls EF, GF, 

and IJ.) 

21 Herrick, C. L., and Johnson, D. W., The geology of the Albuquerque 

· sheet: New Mexico 'Univ. Hadley Lab. Bull., vol. 2, pt. 1, p. 15, '1900. 22 Lee, W. T., Stratigraphy of the coal fields of northern central New 

Mexico : · Geol. Soc. Am'erica Bull., vol. 23, pp. 622-625, 1912. 

.. 23 Lee, W. T., Geology ap.d paleontology Qf the Raton Mesa and other 

regions 'in· Colorado and New Mexico: U. S. Geol. Survey Prof. Paper 

101, pp. 174, 194-197, 1917. . 
TltAN!SGR:ES'SIV:E AND RBGRESSIVE CRETACEOUS DEPOSITS, SAN ,JUAN. BASIN, N. MEX. 113 

t:inues with only gradual change. This restricted unit 

forms the Dileo coal member as described in the present 

paper. In the west half of the area (see walls ABCD 

and GH, pl. 26) the member ranges in thickness from 

120 to 200 feet, and includes several lenticular coals 

which only locally are more than 3 feet thick. Northeastward from points D and G the coal beds die out, 

but the clays and irregular sandstones n1aintain their 

continental aspect. The member gradually thins, however, chiefly by gradation of the uppermost beds to 

1narine shale, and in the northeast corner of the area 

the Mulatto tongue of the Mancos shale, thus expanded, 

rests directly on the thin Gallup sandstone (sees. 22, 

23, and 45). 

In the central part of the area a conspicuous bed of 

sandstone that attains a maximum thickness of 75 

feet was found between the typical beds of the Dileo 

coal member and the overlying Mulatto tongue of the 

1\1:ancos shale. Probably its original extent was about 

that of the stippled zone in the inset map, plate 26. 

Because of its limited extent the sandstone has not been 

given a formal stratigraphic name, but for convenience 

it will be referred to in this paper by its field designation, the "stray sandstone." It is shown as a separate 

sandstone in the correlation diagram (in walls CD and 

GH, in the spur wall e', and northeastward in the wall 

GF as far as sec. 2'8), because at places it forms an 

unbroken cliff-forming unit, apparently unrelated to 

the overlying and underlying beds. However, it may 

well be that the greater part of the "stray" belongs to 

the Dileo, and that its uppermost part-a thin but wellmarked sandy layer carrying rounded, generally 

elongate pebbles as much as 3 inches in length-represents local deposition of coarse matedal during the 

early part of the marine transgression that resulted in 

the Mulatto tongue of the Mancos shale. This pebbly 

layer is found in much of the area represented in wall 

CD by the lens between sections 9 and 14. Locally 

it is separated from the main body of the "stray sandstone" by 3 to 25 feet of clay tha,t includes some black 

cn,rbonaceous shale and streaks of coal. 

In the smaller lens 4 miles to the southeast, the "stray" 

includes several conspicuous bodi~s of sandy marl composed almost wholly of Ost1·ea sol&nisrn~Js. (See pl. 

30, A.) Locally this unbedded marl shows striking 

lateral intergrowth with nonfossiliferous well-marked 

layers of the "stray sandstone." (See pl. 30, B and 0.) 

Probably there were estuarine conditions that favored 

the growth of ~xtensive colonies or reefs of these oysters 

despite the rapid deposition of sand virtually surrounding them. 

DALTON SANDSTONE MEMBER 

As alren,dy noted, the upper half of the Dileo emil 

member near the north end of the Hogback is· replaced 

within a short distance by a sand~tqne. ·poCI.y which at 

section 2 is about 180 ·feet thick. and includes only two 

thin beds of softer sandstone and sandy shale. 

A few miles farther east, at section 5, the unit is 

divided into two distinct cliff-forming sandstones, the 

upper of which is 96 feet thick and the lower 72 feet 

thick; these are ,separated by 44 feet of shale. This 

intervening shale and about 20 feet of the shale immediately beneath the lower sandstone are fissile and of 

Mancos type. In the lower zone of shale were found 

a few marine fossils of Niobrara age. These zones of 

· shpJe are the landward end of the Mulatto tongue of 

1\1ancos shale. 

Ea,st of section 6 the lower sandstone rapidly becomes 

thinner and more shaly and wedges out in the midst of 

shale of the Mulatto tongue, which rests directly on the 

Dileo coal member. 

The upper sandstone continues far to the east, with 

a rather uniform thickness of about 100 feet. From 

its. excellent exposures at Dalton Pass,' where it .supports 

the divide and forms a conspicuous bench in the canyon 

draining to the north, the unit has been named the 

Dalton sandstone member of the Mesaverde. 24 In general the Dalton is a clean, white to buff massive 

sa~1dstone, at places containing marine fossils of 

Niobrara age. 

In 'the northeastern part of the area (between sees. 

21 and 22 in wall DEF and between sees. 24 and 23 

in wall GF), the base of the sandstone moves stratigraphically upward in a serrate pattern through the 

separation of small tongues that grade laterally into 

shale of the expn,nding Mulatto tongue. Sjmultaneously the overlying continental beds become increasingly sandy, and the sandstone above them loses its 

upper part by the separation of tongue,s that die out 

laterally into shale of the Satan tongue. By this process the 300 to 400 feet of Mesa verde rocks between the 

Mulatto and Satan tongues of the Mancos are, within 

a few miles, represented only by a single, relatively 

thin sandstone in the midst of a great body of marine 

s?ale. This .sandstone becomes softer and thinner and, 

hke the Gallup sandstone member, is a mere sandy zone 

in the midst of the ·Mancos shale near the. northeast 

corner of the area. At that point the Mulatto and 

Satan tongues merge with the. lower shale to form 

an expanded 1\1ancos about 2,000 feet thick. 

This change in the Dalton and overlying beds is seen 

a1so, but less clearly, in the outcrops along the line IJ. 

BARTLETT BARREN MEMBER ' 

In the Gallup district the Bartlett. bar.ren. member, 

330 to 400 feet thick, is lithologically similar to the 

under lying Dileo and overlying Gibson coal members. 

It is differentiated from those units, however, because 

its feweoal beds are less than 14 inches thick, except in 

24 Sears, J. D., The coal field from Gallup eastward toward· Mount 

Taylor: U. S. Geol. Survey Bull. 860, p. 17, 1934. (For 1ocation of 

Dalton Pass, see map, pl. 1.) 
114 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 19 3 8-4 0 

small lenses, and are of no commercial importance. 

The member has relatively small extent in the area 

described in the present paper. For some miles eastward from the Hogback (wall ABC) it continues as a 

thick barren zone, its base resting either on the Dileo 

coal member or the Dalton sandstone member. However, begi1ming near section 6 and continuing far to the 

east and southeast, this zone includes more numerous 

and thicker coal beds. The Bartlett barren member is 

therefore considered as e'nding near section 6 by lateral 

gradation into coal-bearing rocks that are assigned to 

and hence expand the overlying coal member. 

GIBSON COAL MEMBER 

At the north end of the Hogback and for some miles 

to the east (wall AB) the Gibson coal member .comprises. about 250 feet of clay, irregular sandstones, and 

coals. (See pl. 29, 0.) Locally it contains nearly a 

dozen coal beds ranging in thickness from 14 inches to 

12 feet. In the southeast part of the area (see sees. 31 

and 49) the apparent equivalent of the Gibson member 

is GOnsiderably thicker, but exposures there are not 

very satisfactory. 

A sandstone, wedging into the midst of the member 

near point B, thickens greatly in short distances toward the north and east, attaining a thickness of 150 

feet within a .few miles. This sandstone, a separate 

1nember of the Mesaverde, divides the Gibson coal member into two parts throughout most of the area covered 

by the present report. The upper part of the Gibson 

continues above the sandstone without material change 

to and beyond the northeast corner of the area and will 

not be described further. 

The lower part of the Gibson coal member, expanded 

downward near section 6 by the inclusion of the zone 

that to the west forms the Bartlett member, continues 

eastward for several miles with a thickness of about 

400 feet. Near section 7, at the south end of Dalton 

Pass, several wedges of sandstone that appear in its 

upper 100 feet thicken in short distances and coalesce 

with the overlying massive sandstone unit. From that 

vicinity the lower part of the Gibson continues as a 

coal-bearing zone far to the east and southeast (wall 

CD), generally ranging in thickness from 275 to 300 

feet. Its contact with 'the underlying Dalton sandstone 

is irregular at many places, and the lenticular sandstones near the base of the Gibson locally touch the 

Dalton and make it difficult to determine a consistent 

dividing line. 

N orthea.stward from point D the lower part of tho 

Gibson continues without change for a few miles. 

Farther along the .line DE the zone is under cover of 

higher beds. It evidently undergoes a material change, 

however, for in a well drilled in the J\1iguel Creek dome 

(the log of which forms ~he lower part of sec. 21.) the 

zone is represented .chiefly by sandstones. The inferred 

manner in which the .sandstones replace the clays and 

coal beds is ·indicated diagrammatically in the crosssection, plate 26. As already discussed in the description of the Dalton, these sandstones, together with the 

underlying Dalton and the sandstone above the lower 

Gibson, unite to form a thick sandy zone, which in turn 

thins eastward and northeastward by gradation into 

the marine shale of the J\1ancos. 

Simihir changes of the lower part of the Gibson coal 

1nember, were observed along lines GF and IJ, where 

the unit grades northeastward into sandstones near sect)on 24 and between sections 45 and 46. 

HOSTA SANDSTONE MEMBER 

The sandstone that splits the Gibson coal n1en1ber has 

been named 25 the Hosta sandstone n1ember of the Mesaverde, because it caps I-Iosta Butte, a prominent landmark in T. 16 N., R. 13 W. 

The land·ward edge of the Hosta was observed only in 

the vicinity of point B, where it begins in the midst of 

the Gibson as two thin sandstones separated by a few 

feet of clay and coal. The two sandstones join within 

n short distance east,vard, and theteafter the unit thickens rapidly. Near section 6 it is 150 feet thick, and near 

section 7 it increases to 200 feet or more by the addition 

of basal sandstone beds that are fanned by lateral 

transition of the upper beds of the lower Gibson. (See 

fig. 23, left-hand part.) From the locality of section 7 

the unit continues far to the east and southeast along 

line CD with rather uniform thickness and character. 

It consists almost wholly of sandstone, the upper part 

weathering into ledges and at places into beehive forms 

(see pl. 31, 0), and the lower part being in general 

very massive and forming a great and nearly continuous 

rim or escarpment. Locally its contact with the underlying Gibson is somewhat irregular, and there are occasional thin clays and small coal lenses in the lower 25 

or 50 feet of the sandstone. 

In striking contrast to the uniformity of the Hosta 

along line CD, there is a marked and abrupt change 

northeastward from that line. Within a few miles the 

member splits into two distinct sandstone units, separated by the Satan tongue of the Mancos shale, which 

thickens toward the northeast. This splitting of the 

Hosta sandstone member is shown graphically in plate 

26 (in spur walls a', b', c', and d' of wall CD, and 

in wall DE); and the details of the transition near 

Dalton Pass are expressed in figure 23. The appearance of· the upper and lower parts of the Hosta and 

the intervening Satan tongue where the splitting is 

complete is shown in plate 28, 0. 

The landward edge of the Hosta cannot be recognized near J\1ount Taylor or in the southeastern part 

of the field. The Hosta is a thick, fully developed 

2a Sears, J. D., op. cit., p. 18. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 193 PLA'rlil 'tl 

A. UPPER PART OF MANCOS SHALE AND BASAL SANDSTONES OF MESAVERDE FORMATION, NEAR NORTH END OF THE HOGBACK NORTHEAST 

OF GALLUP. 

The top of the Gryphaea neu:berryi zone is seen at the right. 

-- . M ESAV EROE FORMATION ------· ---- .... ' , 

.:-.Gall.up sandstonemember --

-.. ... 

-· 

B. LOWER MEMBERS OF MESA VERDE FORMATION IN EASTERN PART OF GALLUP COAL DISTlUCT, AS NAMED IN BULLETIN 767. 

View of the Hogback 2)6 miles northeast of Gallup. Photograph by N. H. Darton. 
GEOLOGICAL SURVEY 

A . MULATTO TONGUE OF MANCOS SHALE OVERLAIN BY DALTON 

SANDSTONE MEMBEU OF MESAVERDE FORMATION; IN FOREGROUND, THE TOP OF TliE "STRAY SANDSTONE." 

View in sec. 30, '1'. 15 N., 1{. 10 ,V, (:..bout 2 miles west of the locaUty of sec. 14 in the diagram, pl. 26). At this locality the Mulallo is about 250 feet 

thick. Through the combined effects of topographic sloi>C, dip of beds, and foreshortcnint, the photograph greatly exaggerates the apparent 

thickness of the lower, rather sa ndy portion of the 1\IIu lauo, which extends 

as far up as the tree-covercO bench a L the left. as contrasted with the partly hidden upper, less sa ndy portion of the l\1ulallo. 

PROFESSIONAL PAPER 193 PLATE 28 

B. INTERTONGUING OF MANCOS SHALE AND GALLUP SANDSTONE MEMBER OF MESA VERDE FORMATION. 

Foreground and lower ope~ main bod y of Mancos; bench, a sandstone wedge from tbe Ga llup ~ thinning eastward (toward the dght); 

upper slope, a tongue of 1\'Iancos thinning westward; cliff, main body of Gallup sandstone member. {Between point B and sec. 5 in the 

diagram, pl. 26.) 

C. liOSTA SANDSTONE MEMBER OF MESAVERDE FORMATION SPLIT BY SATAN TONGUE OJ,' MANCOS SHALE. 

Upper two ledges of sandstone are the Hosta; intervening shale is the Satan tongue; intermediate slope_ lower part of Gibson coal member; sa nd stones io foreground, the D alton sandstone member. \ iew looking west a t oorlh end of Satan Pass. sou theast of Crown Point (near 

point a' on the spur from wall CD in the diagram, pl. 26). 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 193 PLATE 29 

A. GALLUP SANDSTONE MEMBER OF MESAVERDE l:' OHMATION, SHOWING MODE OF WEATHERING. 

Sec. 16, 'I'. 14 N., R. 9 W. 

Closer view at the locality shown in plate 29, A. 

C. GIBSON COAL MEMBER OF MESAVERDE FORMATION WEST OF (LANDWARD FROM) AREA IN WHICH IT IS SPLIT BY THE HOSTA SANDSTO E 

MEMBER. 

View in T. 17 N., H. 17 W. (northwest of point Bin the diagram, plate 26). 
OEOLOCICAL SURVEY PROFESSIONAL PAPER 193 P!.ATE 30 

B. CLOSER VIEW OF THE OUTCROP SHOW IN PLATE 30. C. The indica ted specimen is 18 inches long. 

'I'he marl is composed largf'ily of Ostrea soleniscus; the bedded sandstone contains only scattered remains of that elongated oyster. View in sec. 9, T. 14 N., R. 9 W. 
GEOLOGICAL SURVEY 

A. SATA TONGUE 0}' MANCOS SHALE AND OVERLYI G UPPER PART 

OF BOSTA SA 'DSTONE MEMBER OF M.ESAVERDE FORMATIO . 

Note the Lransitionaluatur(' of tbe contact. Tn sec. 36, T. ]8 N .• R. 2 \V. Photogc~1 JI1 

by W. G. Pierce. 

PROFESSIONAL PAPER 193 PLATE 31 

B . TRANSITIO ZONE MARKING CONTACT BETWEEN MULATTO 

TONGUE 01' MANCOS SHALE A ' D OVERLY! G DALTON SANDSTONE 

MEMBER OF MESAVERDE FOHMATION. 

View on cast side of Satan Pass in sec. 20, T. 16 N., H.. 12 \V. (ncar locality of sec tion 9 in the diagram, plate 26). 

C. BOSTA SA 'DSTONE MEMBER OF MESAVERDE FORMATION, SHOWING MODE OF WEATHERING OF UPPER PART. 

View in sec. 4, T. 17 N., R. 16 W. 
TRANiSGRES'SIVE AND REGRESSIVE CRETACEOUS DEPOStlTS, SAN JUAN BASIN, N. MEX. 115 

sandstone at section 27 in line GF and at point I of 

the line IJ. On the other hand it seems to be absent 

:dong the line IIGKL, some 20 miles to the southwest, 

for no sn.ndstone identifiable as the Hosta. was found 

:in f·.he several hundred feet of the Gibson exposed at 

~ect:ion 31 (in spur wall e') and section 49. The Hosta 

:is therefore believed to have once extended landward 

to a line a few miles northeast of and roughly parallel 

to the line I-IGKL. It thickens seaward, or northeastward, and continues as a prominent sandstone until, 

near section 26 on line GF and section 43 on line IJ, 

it is split by the landward edge of the Satan tongue 

of the Maricos. 

Feet 

·150 

100 

50 

Host a 

sandstone 

member 

S.27°W. (Landward) 

. 1::::.>:'/l Sandstone D Shale 

0 

ALLISON BARREN MEMBER 

Within the Gallup district the upper part of the . 

Mesa verde formation has been removed by erosion so 

that only about 800 feet of beds remain above the 

Gibson coal1nember. These beds of continental origin, 

comprising lenticular sandstones and light-colored clay 

with little or no coal, were there named the Allison 

barren member. No upper stratigraphie limit of the 

member was indicated, and the lower limit varies from 

place to place with. the variation :in the horizon at 

which is found the highest coal bed of the Gibson. 

The lower beds of the Allison are exposed along the 

northern edge of the field here discussed and also as a 

-Coal-bearing beds 

2Miles 

(Seeward) N.27°E. 

.ll'IOuum 23.-Landward-seuwtu·d variations of the Hosta sandstone member of the Mesaverde formation and its relation to the Satan tongue of t!1e 

Mancos shale. The diagram begins at the left with conditions representing those seen in the cliffs on the west side of Dalton Pass (at the loGality of section 7 in wall BC, pl. 26), and extends N. 27° E., directly seaward. 

In the northeastern part of the field, along lines EF, 

GF, and IJ, the lower stuidstone of the Hosta gradually loses its identity. By lateral gradation this 

sandstone and the Dalton sandstone member, together 

with the intervening coal-bearing beds of the lower 

Gibson, bec01ne_ represented by a single sandstone between the expa.nded Mulatto and Satan tongues. 

vVhere this sandstone dies out the Satan and Mulatto 

tongues merge with the main body of Mancos shale, 

and the upper sandstone of the Hosta, of lower Montana age, becomes the basal member of the Mesaverde 

formation. 

Mr. Pike, an associate of the authors in the work 

here described, has subsequently, as part of a project 

sponsored by the Geological Society of America, traced 

the l\1ancos andl\1esaverde formations from the Gallup 

district northward along the west side of the San Juan 

Basin into Colorado. He reports 20 that the upper 

sandstone of the Hosta is essentially the stratigraphic 

equivalent of the Point Lookout sandstone, which, at 

the type locality in southwestern Colorado, is the basal 

formation of the l\1esn,verde group. 

Do Pike, William S., Jr., personal communication. 

few local remnants within the field. However, they 

are of only slight and indirect significance in connection with the problems presented in this paper and 

need not be described. It is sufficient here to emphasize that the Hosta sandstone n1ember is .overlain by 

many hundreds of feet of continental beds, which are 

coal-bearing in the lower part (Gibson coal n1ember) 

and barren above. 

DEPOSITIONAL HISTORY OF MANCOS AND MESA VERDE 

The variations and intertonguing of the Mancos 

shale and the Mesaverde formation in the southern part 

of the San Juan Basin and elsewhere are evidence of 

alternating processes of transgression and regression 

that were repeated several times during the Upper 

Cretaceous. The history, relations, and conditions of 

deposition of the various rock units, as set forth in the 

following pages in accordance with the depositional 

theories already outlined, are grouped for convenience 

under the several transgressive and regressive stages, 

each of which is designated by the names of the stratigraphic units that are related to it. 
116 SHORTER. CONTRIBUTIONS' TO GENERAL 'GEOLOGY, 1938-40 

DAKOTA ( 1) -MANCOS TRANSGRESSIVE STAGE 

With the first deepening of the Upper CretaceOtlS 

trough, the sea waters that occupied it spread gradually 

over the sinking marginal lands. Upon the gently sloping surfaces of those lands were laid down the first 

material of the new series. In southern San Juan 

Basin, as in many other areas, it consisted of irregular, 

coarse, and in places conglomeratic sandstones, with 

associated layers of clay, carbonaceous shale, and locally some coal. From its lithologic character and its 

stratigraphic position this material in the New Mexico 

region has been called the Dakota ( ? ) sandstone, in 

tentative correlation with the accepted Dakota of other 

regions. ·It seems evidently a record of sea beaches and 

of coastal lagoons and swamps. 

As subsidence continued and the sea encroached 

farther upon .the land, these zones of swamp and beach 

deposition gradually moved landward, their material 

being laid down transgressively over the old land surface. In turn their outer, earlier portions, now under 

deepening water, were slowly buried under off-shore 

muds that became the first beds of the tran$gressive 

Mancos shale. In the San Juan Basin the transgression 

was probably toward the southwest or west, and the 

strand line 1noved slowly in that directio~ across the 

San J nan Basin and on beyond it for an undetermined 

distai1ce. vVith progressive . subsidence, even ·though 

deposition may have been rapid enough to prevent th~ 

sea from ever attaining ll10re 'than a .relatively shal-. 

low depth, the duration and amount of the sinking were 

sufficient. to permit the accumulation, in the area dis-. 

cussed in this report, of hundreds of feet of Mancos shale 

above the Dakota ( ? ) . 

MANCOS-TO-DILCO REGRESSIVE STAGE 

After a considerable time, however, subsidence of the 

. trough and deepening of the water became slower and 

slower, and the deb~·is, brought jn by rivers from the 

southwest and moved along the shore by waves and 

currents, filled in the shallow, near-shore zone of the sea 

to and above water level and pushed the strand line 

· outward-that is, northeastward. Farther seaward th~ 

deposition of marine 1nud continued, but along shore, 

the sand was slowly built upward and outward over. 

the earlier mud. Thus there came to be a regressive 

deposition of near-shore material upon off-shore material-sand upon marine mud. 

Within this area the newer, coarser material appeared 

·first near the southwest corner (see pl. 26, point A) as 

the basal beds of the Gallup sandstone member. The 

change from marine shale to overlying sandstone seems 

to have taken place gradually, for at that locality and at 

many other places the transition .is marked by 10 to 20 

feet of alternating layers of sandstone· and:shale ea9h 

an inch or two thick_. · Thus the first sand? ·of. the new 

regressive deposits must have come to rest on the earlier 

muds under water of appreciable depth, in which wave 

action was not strong enough to disturb the even, thin 

bedding of the alternating sand and mud. Above this 

transition zone, the main body of sand grew to a consider~ble thickness; the lower, major part apparently 

accumulated under water, and the upper part, which 

contains interbedded thin layers of clay and coal, accumulated just below and above the water surface. 

As . regression proceeded, the strand line was pushed 

gradually northeastward and a growing sheet of sand 

was spread out in that direction over the muds of the 

Mancos. Some minor interruptions to the regressive 

process are recorded by the local intertonguing of the 

lower part of the Gallup sandstone with corresponding 

small wedges of Mancos shale (for example, between 

points A and B. in tl~e diagram, pl. 26). However, in 

general the regression went on steadily, and the Gallup 

sandstone was extended northeastward across the. area 

here discussed until it finally reached the appr~ximat~ 

position of the line 1:-1'. shown on the inset map, plate 

26. The shale-sandstone contact cuts obliquely upward 

across time planes toward the northeast~that is, the 

uppermost shale and ~he lowest sands are progressively 

younger in that direction. 

In the meantime, as the strand -line moved sea ward, 

its earlier positions,· marked l;>y the accurp.ulation of 

nea~-s~ore and beach sa~d, beca~e the site of lagoons 

and coastal swamps. In these were laid down irregular 

bodies of 'sarid and day and of organic material. These 

deposits form the Dileo coal member, of evident continental origin. Their contact with the underlying Gallup .sandstone is irregular from place to place, as might 

be expected; for the earlier- sands, moved and built up 

by. the action of .waves, currents, and winds, naturally 

did not attain an e~en upper surface. 

The Dileo deposits were formed in the third or 

coastal zone. As regression continued they in turn 

grew northeastward, extending out over the sands of thP. 

second or n·ear~shore zone. Eventually they reached 

the approximate position shown by the line 2-2' in tlie 

inset ·map, .plate 26, falling short of the line reachPd 

by the Gallup sandstone. 

DILCO-MULATTO TRANSGRESSIVE STAGE 

By that time, however, regression was beginni:ng to 

falter. ,There was a quickening of the rate of subsidence, and the sea, regaining its supremacy over the 

forces of deposition, began a new advance on the land. 

In this advance it moved in over the near-shore and 

continental .deposits of the preceding regressive stage. 

Those deposits, whose outer (northeastern) portions had 

by that .time reached only partial development, were· 

interrupted in their growth by the new advance-a 

fact recorded by the marked northeastward thinning, 

practically to a feather edge, of the Gallup sandstone 

i 

.. 

~· 
TRANSGRESSIVE AND REGRESSIVE CRETACEOUS DEPOSITS, S'AN JUAN BASIN, N. lHEX. 117 

and D.ilco coal members, as shown in the threedimensional diag1;am, plate 26. 

In the deeper waters seaward from the line 1-1' the 

deposition of 1\1ancos shale had gone on continuously 

during the regressive stage: 'Vith the new advance ·of 

the sea, marine muds of the Mancos, the Mulatto tongue, 

were laid clown transgressively over the earlier, coarser 

materials. The first muds, coming to rest in the a·rea 

between the lines 1-1' and 2'-2', lay directly on the thin 

Gn.llu p sandstone. :As transgression proceeded, southwestward from the line 2-2' the basal muds of the 

1\1ulatto invasion were deposited at most places on the 

wedge of the Dileo coal member, which thickens gradua1ly in that direction. I-Iowever, in parts of the area 

shown by stippling on the inset map, the transgression 

was first marked by sandy and pebbly deposits, which 

in turn became overlain by the -growing body of mud. 

These sandy and pebbly deposits, constituting the top 

part of the unit described on page 113 as the "stray 

sandstone," were evidently the coarser beach and near-. 

shore debris laid clown before the sea reached any considerable depth. As the deposition of marine muds 

advanced southwestward, the coastal zone in which 

deposition of the D.ilco was continuing moved ahead 

of it. 

The marii1e transgression just depcribed continued 

for a much shorter time and reached southwestward a 

much shorter distance than did the earlier Dakota 

(~)-Mancos transgression. Water deep enough to 

receive and preserve offshore muds reached only the 

approximate position of the line 3-3' (in.set map, plate 

26), which thus forms the landward limit of the 1\1ulatto tongue of the 1\1ancos shale. Farther landward 

the deposition of the Dileo member-its irregular sands, 

clays, and coals-had gone on unabated. 

Thus far two transgressive stage.s and an intervening 

regressive stage have been described. As the product 

of these three stages, the Gallup sandstone and the 

Dileo coal member formed a great tongue of near-shore 

and continental deposits wedging northeastward into a 

mass of the Mancos shale. At any one place a vertical 

stratigraphic section through the deposits records a 

completed cycle of sedimentation, from marine transgression through regression back to 1narine tran8gression. Beginning at the base, the rock sequence is ( 1) 

transgressive offshore n1uds (1\1ancos) ; (2) regressive 

offshore 1nuds (1\1ancos) ; ( 3) regressive near-shore and 

beach sands (Gallup) ; ( 4) regres.sive continental 

deposits (Dileo) ; ( 5) transgressive· continental deposits 

(D.ilco); (6) transgressive near-shore and beach sands 

and pebble.s (top of the "stray sandstone," where present); and (7) transgressive offshore muds (1\1ancos). 

This cycle, according to the view that the writers have 

presented, was the result of trough subsidence that was 

continuous, but with varying rate, and was not cau.sed 

by an alternately sinking and rising trough. 

MULATTO-TO-LOWER GIBSON REGRESSIVE STAGE 

With a second slowing down of the rate of sqbsidence there began a new regressive stage which, though 

somewhat comparable to the first, proceeded to a more 

complete and soi:newhaf more. complex ·development 

within the southern part of the San J nan Basin. 

The period of change from tran,sgression to regression was marked in the southwestern part of the area 

(near sec. 2 in the three-din1ensional diagran1, pl. 

26) by the for1nation of a sandstone body about 180 

feet thick. Toward the southwest this sandstone 

grades laterally into the upper part of the Dileo near 

point A; and toward the east and northeast the lower 

part of the .sandstone grades into the Mulatto tongue 

of the 1\1ancos shale (between sees. 5 and 6). This thick 

sandstone deposit was formed near a relatively stationary strand line, during a period when subsidence 

and deposition were approximately balanced and 

growth was upward rather than land ward or sea ward. 

At one time, however, represented by the intertonguing 

of sandstone and shale bet\veen sees. 5 and 6, there was 

an intermediate minor regression and transgression. 

From the vicinity of section 5 the upper or main 

part of the Dalton sandstone continues far to the east 

with a generally uniform thickness of about 100 feet. 

The change from 1\iulatto to the overlying Dalton, just 

as fr01n Mancos to the overlying Gallup, is marked at 

many places by a transition zone of alternating thin 

layers of sandstone and shale. .(See pl. 31, B.) The 

presence of marine fossils in the lower part of the 

Dalton, their absence fr01n its upper part, the more 

variable composition of that upper part, and the very 

irregular upper surface of the sandstone, are additional 

evidence of its mode of origin, the first sand being laid 

down on earlier n1ud in near-shore but relatively quiet 

water, and the later sand being built up to and above 

the water level. 

As the body of sand grew upward and northeastward, pushing the strand line seaward before it, the 

landward parts of its irregular upper surface became 

covered progressively toward the northeast by beds 0f 

the continental type. Eastward fron1 section 6 (see 

pl. 26) the Dalton is overlain by the lower part of the 

Gibson coal member, formed in the coastal zone. 

Eventually the Dalton sandstone reached the approximate position of line 4-::1:', and the lower part of the 

overlying Gibson coal n1mnber reached about to the 

line 5-5', as shown on the inset map, pl. 26. The attainment of that growth brought to a close the second 

principal regression herein discussed. 

At the conclusion of that second regression the shore 

line lay near the position of line 5-5' for a considerable 

period, 'vith only minor oscillations landward and. se~tward. During that period the rate of subsidence was 

more rapid so that it almost exactly balanced the rate 

of deposition. of ·sand ·in the near-shore ·.~one: The 
118 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 19 3 8-4 0 

existence of such a balance between subsidence and 

depo~ition brought about in the near-~ore zone the 

formation of a thick body of sand. This sand intertongued landward with the Gibson coal member, which 

was growing in the swamps behind it, and also intertween the uppermost Dileo to the west and the lower 

part of the Gibson east ·of section 6, as might be expected with regular seaward movement of the several 

zones. The mapped and hypothetical conditions in 

this vicinity are contrasted graphically 1n figure 24. 

FIGURE 24.-Diagram showing actual and hypothetical relations of the Dileo, Dalton, and Bartlett members. a, Bartlett barren member resting 

directly on the Dalton sandstone member for several miles (part of the d·iagram near point Bin pl. 26) ; b, expectable continuation of regressive 

coal-bearing beds above the Dalton sandstone. 

tongued seaward with marine shales of the Mancos. 

(See beds in the vicinity of sees .. 21 and 22 in the wall 

D EF and of section 24 in the wall G F.) This balance 

persisted until further acceleration of subsidence in 

relation to deposition initiated the succeeding transgressive movement. 

In the meantime, in the Gallup district and near 

point A in the diagram, plate 26, a further effect of 

the regression was being recorded. As the coastal zone 

moved sea ward, its last deposits at the west, the uppermost beds of the Dileo coal member, became covered 

gradually by the first beds <;>f the next landward or 

flood-plain zone in which the Bartlett barren member 

was laid down. Inasmuch as the two zones merged 

irregularly and transitionally, the lithologic changes 

in a sequence of their deposits are gradual, and a 

boundary between the zones is difficult to determine. 

Thus, although the Bartlett throughout is essentially 

lacking in coal, its marginal beds otherwise closely resemble the materials of the Dileo. Consequently, at 

places in the coastal zone where coal may not have 

been formed and preserved, some beds rna y have been 

mapped as Bartlett rather than Dileo, because for mapping purposes the members were differentiated arbitr·arily by the presence or absence of workable coals. 

Thus, in general, the mapped boundary between the 

members marks only approximately the boundary between the third and fourth zones of deposition and in 

places may miss it by a stratigraphic interval of as 

much as a hundred feet. Though tracing of individual 

beds did not so indicate, this is possibly the reason 

why for a few miles west of section 6 the Bartlett, as 

mapped, rests directly upon the Dalton sandstone 

rather than upon an intervening sheet of coal-bearing 

n1aterial, which would have formed a connection beIf coal-bearing beds had continued unbroken between 

the Dalton and the Bartlett in this vicinity, the .sequence would have resembled that of the upper Hosta, 

upper Gibson, and Allison strata shown near point B 

in the diagram, plate .26. 

LOWER GIBSON-TO-SATAN TRANSGRESSIVE STAGE 

The next transgression and the subsequent regression 

reached a fuller developn1ent in the southern part of 

the San Juan Basin and were recorded by a more complete sedimentary sequence than those of the earlier 

stages. As shown in the several main and spur walls . 

of the cross-section diagram in plate 26, the materials 

were laid down and preserved with striking uniformity 

throughout the area. 

During the preceding regressive stage, deposition of 

the Mancos shale had gone on steadily in the deeper 

waters northeast of the line 4--4'. The change from 

regression to transgression was not abrupt; instead, as 

already pointed' out, there was a considerable intervening period of appro~imate balance between subsidence and deposition. 'During that period of balance 

the strand line remained relatively stationary, and there 

was formed in the near-shore zone the great body of 

sandstone that is represented between point E and section 22 in the wall EF, near section 24 in the wall G F, 

and near section 46 in the wall IJ, in plate 26. · Seaward, the deposition of shale' continued. Inland, 

through a broad area, there was further accumulation 

of the coal-bearing beds of the lower Gibson; and far 

to the west, near point B, the latest beds of the Bartlett 

barren n1ember were being formed. 

As trough subsidence became· still faster and the 

balance was overcome, actual transgression began and 

the deposits of each zone spread southwestward over 
TUAN:SGH.ESSIVE AND REGltESSIVE CRETACEOUS DEPOStl'.rS, S'AN J"UAN BASIN, N. MEX. 119 

Hte 1naterin.l o:f. the next landward zone. The coalbearing beds of the lower Gibson extended inland over 

the Bartlett to and beyond the limits of the area at 

point A. Above the coal-bearing beds spread a growing sheet of sand, which formed the lower part of the 

I-Iosta sandstone member and which eventually reached 

the approximate position of the line 6-6' (inset map, 

pl. 26). As a final phase of the transgression, with 

deepening water the muds of the Satan tongue of the 

:Mancos grew inward upon the earlier sands, finally 

reaching as far as the line 7-7', some 10 to 15 miles short 

of the line reached by those sands. The straightness of 

line 7-7' for so many miles is noteworthy; it seems 

to reflect a very straight strand line caused by even subsidence and the advance of the sea over a nearly flat, 

featureless plain of the earlier sand. If any rivers 

entered the sea along that strand line, their small size, 

their relative freedom from debris, or· the power of 

waves and currents to spread materials evenly along 

shore must have prevented the formation of deltas or 

other breaks in the regular deposition. Despite the apparently even subsidence at that stage, however, it 

should be observed that the line 7-7' is not parallel with 

the line 3-3', which marks the extreme landward extent of the next earlier transgression of marine mudthe Mulatto tongue. Indeed, the gradual change in 

direction of the series of lines 1-1' to 7-7' from about 

N. 34° W. to about N. 63° ,V. would seem to indicate 

that the strand line, in its landward and seaward shifts, 

gradually assumed a more westerly direction through a 

slow regional differential warping. · 

SATAN-TO-ALLISON REGRESSIVE STAGE 

After tr~nsgression of the Satan tongue of the Mancos 

shale, there began the last regression recorded by the 

rocks now preserved in this southermnost part of the 

San Juan Basin. The processes and sequence of events 

were essentially the san1e as in the earlier regressive· 

stages. The muds of the Mancos, growing upward ai1d 

·northeastward, completed the wedge of shale, the Satan 

tongue, that thickens steadily in that direction. The 

Hosta sandstone, which between the lines 6-6' and 7-7' 

forms a single thick sand body, likewise grew upward 

and northeastward, forcing the sea waters outward before it; its upper part, thus deposited with transitional 

contact above the shale of the Satan tongue (see pl. 

31, A), gradually extended to and far beyond the northeastern limits of the area herein discussed. In turn, 

its upper surface became the site of lagoons and coastal 

swamps in which were laid down and gradually extended outward those coal-bearing beds that represent 

the upper part of the Gibson coal member. Finally, 

these upper coal-bearing beds also extended to and beyond the northeastern limits of the field, and the whole 

n1ass gradually beca1ne buried beneath many hundreds 

of feet of continental, predominantly flood-plain debris 

of the Allison barren member, which n1arked the last 

phase of this major regression. 

INDEX 

Page 

A h~;truct: ------------------------------------------------- 101 

All-Ison bunen member of Mesaverde J'ot·mation, character and occurrence of----------------------------- 108, 111, 115 

conditions of deposition oL---------------------------- 11\J 

Bartlett barren member of Mesaverde formation, character and 

occurrence of--------------------------------- 10~ 

111, 113-114, pl. 27, B 

conditions of deposition oL----------------------------- 118 

lntcnll variations oL---------------~-113~114, pl. 26 (in pocket) 

Book Cliffs, Utnh-Colo., character of Mancos-Mesaverde boundary in----------------------------- 7 ---------- 105 

Carlile fossils, occurrence oL------------------------------- 109 

Constal zone of deposition, defined---------------------------- 104 

Ct·oss, Whitman, quoted------------------------------------- 108 

Cycles of sedimentation, sequence of formations in------------ 102, 117 

Dakota ( ?) sandstone, stratigmphic relations of_ ____________ 108, 116 

Du.lwtn (?)-Mancos transgressive stage, conditions of deposition . during________________________________________ 116 

Dalton Puss, formations aL-------------------------·· 113, 114, 115 

Dulton snndstone member of Mesaverde formation, character 

and occ~rrence of----------------------------- 108, 

112, 113, pis. 28, A, 31, B 

conditions of deposition of---------------------------- 117-118 

extent of---------------------------- 117-118, pl. 26 (in pocl{et) lateral variations of __________________ 113, 115t, pl. 26 (in pocket') 

Debris, rate of supply oL----------------------------------- 104 

Depositional histot·y, Mancos and Mesaverde----------------- 116-119 

Depositional theory, discussed _______________________ 102-105, pl. 25 

DJlco coal member of Mesaverde formation, character and oc- cm·rence of-_________________________ 108, 111, 112-113 

conditions of deposition oL---------------------- 116-117, 118 

extent oL-------------------------- 116, !17, pl. 26 (in pocket) ' lateral vnriattons oL _________________ 112-113, pl. 26 (in pocket) 

Dilco-Mulntto tr3:nsgressive stage, conditions of deposition 

during-------------------------------------- 116-117 

ll'leld studies in the area----------------------------------- 106-107 

Gallup sandstone member of Mesaverde formation, character 

and occurrence of_____________________________ 10~ 

109, 111-112, pls. 27, B, 28, 29, A, B 

conditions of deposition oL------------------------------ 116 

correlation of----------------------------------------- 112 

extent oL----------------------------- 116, pl. 26 (in pocket) Intent I vat·iations oL __________ 111-112, pis. 26 (in pocl,et), 28, B 

Gibson coal member of Mesaverde. fot·mation, character and occurrence of------------------------------------ 108, 

111, 114, pis. 28, a; 29, a 

conditions of deposition of------------------------ 117-118, 119 

extent o·L--------------------- 117-118, 119, pl. 26 ('in pocket) lnteral varlntions of-________________ 114, 115, pl. 26 (in poclwt) 

Gt'V1Jhaea ne'wberryi zone, position oL----------------- 109, pl. 27, A 

Hogback, fot·mations of------------------ 109, 111, 112, 113, 114, pl. 27 

Hosta Butte, formation capping______________________________ 114 

Hosta sandstone member of Mesaverde formation, character and occurrence of-________ 108, 114-115, pis. 28, a, 31, A., a 

conditions of deposition of------------------------------ 119 

correlntlon oL---------------------------------------- 115 extent of-_________________________ .:. _____ 119, pl. 26 (in pocket) 

lateral variations of _________________ 114-115, pl. 26 (in pocket) 

Page 

Intertonguing of madne and continental deposits--------------- 102, 

105, 106, 108, ·110, 111, 112, 113, 114-115, 117 

Landward zone of deposition, defined-------------------------- 104 

Lower Gibson-to-Satan transgressive stage, conditions of deposi- tion during __________________________________ 118-119 

See also Gibson coal member of Mesaverde formation. 

Mancos shale, character, age, and boundaries oL------------- 105-106, 

108, 110, pis. 27, 28, 31, A, B 

name and type locality oL------------------------------- 108 

section of--------------------------------------------- 109 

Mancos-Mesaverde boundary, significance and relations of---- 105-106, 

108-110, 112, 11_5, pls. 26 (in pocket), 27, A, B 

Mancos-to-Dilco regressive stage, conditions of deposition during_ 116 

Mesaverde formation, character, age, and boundaries of-------- 105-106, 

108, 110-111, 115, pis. 26 (in pocket), 27 

divisions of--------------------------- 111-115, pis. 27, B, 28-31 

name and type locality oL--------------------------- 110-111 

Miguel Creek dome, well in--------------------------------- 114 

Mulatto tongue of Mancos shale, character and occurrence oL- 108, 

110, 113, pis. 28, A, 31, B 

conditions of deposition oL----------------------------- 117 

' extent and thickness of--------------- 110, 117, pl. 26 (in pocket) 

Mulatto-to-lower Gibson regressive stage, conditions of deposition during--------------------------------- 117-118 

Near-shore zone of deposition, defined------------------------- 104 

Niobrara fossils, occurrence oL--------------------------- 110, 113 

Off-shore zone of deposition, defined--------------------------- 104 

Ostrea soleniscus, occurrence-of--------------------------- 113, pl. 30 

Profile of equilibrium, defined-------------------------------- 103 

"Punta ·de In Mesa sandstone," correlation oL_________________ 112 

Regressive stages of deposition, conditions governing ___ 103-i05, pl. 25 

defined---------------------------------------------- 104-105 

examples of-------------------------------- 116, 117-118, 119 

Rock Springs uplift, Wyo., character of Mancos-Mesaverde boundary in---------------------------------------- 105 

Satan tongue of Mancos shale, character and occurrence oL_____ 108, 

. 110, 113, 114, pis. 26 (in pocket), 28, a, 31, A 

conditions of deposition oL----------------------------- 119 

extent and thickness of --------------- 110, 119, pl. 26 (in pocket) 

Satan--to-Allison regressive stage, conditions of deposition during_ 119 

Strand-line migrations, examples oL---------------------- 116-119 

significance oL-------------------------------- 102-103, pl. 25 

"Stray sandstone" of Mesaverde formation, character and oc-

. cm·rence oL----------- 113, pis. 26 (in pocket), 28, A, 30 

conditions of deposition oL----------------------------- 117 

exte!lt oL----------------------------- 117, pl. 26 (in pocl{et) 

Transgressive stages of deposition, conditions governing_ 104-105, pl. 25 

defined----------------------------------------------- 104 

examples of--------------------------------- 116-117, 118-119 

'l'rough, 'rising------------------------------.--------- 102, 103, 104 

sinking or subsiding _____________________ 102, 103, 104, 116-119 

Upper Cretaceous, features oL--------------------------- 102 

Upper Cretaceous geosyncline, features oL----------------·--- . 102 

Zones of deposition, defined--------------------------------- 104 

movements of, described------------------------------ 116-119 

relation of, to planes of time equivalence _________________ 104-105 

121