Clastic Pipes of Probable Solution-Collapse 

Origin in Jurassic Rocks of the 

Southern San Juan Basin, New Mexico

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Chapter L

Clastic Pipes of Probable Solution-Collapse 

Origin in Jurassic Rocks of the 

Southern San Juan Basin, New Mexico

By RALPH E. HUNTER, GUY GELFENBAUM, and 

DAVID M. RUBIN

A multidisciplinary approach to research studies of sedimentary 

rocks and their constituents and the evolution of sedimentary 

basins, both ancient and modern

U.S. GEOLOGICAL SURVEY BULLETIN 1808 

EVOLUTION OF SEDIMENTARY BASINS SAN JUAN BASIN
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government.

UNITED STATES GOVERNMENT PRINTING OFFICE: 1992

For sale by the

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Library of Congress Cataloging-in-Publication Data

Hunter, Ralph Eugene

Clastic pipes of probable solution-collapse origin in Jurassic rocks of the 

southern San Juan Basin, New Mexico / by Ralph E. Hunter, Guy Gelfenbaum, 

and David M. Rubin.

p. cm. (Evolution of sedimentary basins San Juan Basin ; ch. 

L) (U.S. Geological Survey bulletin ; 1808-L)

Includes bibliographical references.

Supt. of Docs, no.: I 19.3:1808-L

1. Clastic pipes San Juan Basin (N.M. and Colo.) 2. Geology, 

Stratigraphic Jurassic. 3. Geology San Juan Basin (N.M. and 

Colo.). I. Gelfenbaum, Guy. II. Rubin, David M. III. Title. IV. Series. 

V. Series: U.S. Geological Survey bulletin ; 1808-L. 

QE75.B9 no. 1808-L 

[QE611.5.U6]

557.3 s dc20 91-4320 

[552'.5] CIP
CONTENTS

Abstract LI 

Introduction LI 

Geologic setting L2 

Description of pipes L3 

Origin of pipes L6

Time of origin L6

A comparison of hypotheses L6

Experiments on collapse structures L9

The problem of excessive downdrop Lll

The problem of complete dissolution L12

Original extent of dissolved evaporite unit L14

Model of pipe origin L14 

Broader implications L16 

Conclusions L16 

References cited LIT

FIGURES

1. Map showing study locations and outcrops of San Rafael Group in the 

southern San Juan Basin L2

2. Stratigraphic chart showing Jurassic rock units in the southern San Juan 

Basin L3

3. Histogram showing distribution of pipe widths in Jurassic rocks of the 

southwestern and south-central parts of the San Juan Basin L4

4. Sketches of large pipe in Horse Mesa Member of Wanakah Formation L4 

5-6. Photographs showing:

5. Oblique section through pipe in which bedding is preserved in Cow 

Springs Sandstone, Red Rock State Park L5

6. Two pipes with preserved, downdropped bedding in Cow Springs 

Sandstone, Red Rock State Park L5

7. Map showing faults associated with a pipe near Coolidge Quarry L6

8. Cross section of several pipes in Beclabito Member just west of Red Rock

State Park L8 

9-11. Photographs showing:

9. Bases of pipes in lower part of Beclabito Member just west of Red 

Rock State Park L8

10. Deformed lower part of Beclabito Member overlying undeformed 

Todilto Limestone Member and Entrada Sandstone, Red Rock State 

Park L9

11. Collapse-induced pipes produced in experiments L10

12. Schematic drawings showing postulated role of lateral spreading in producing 

observed characteristics of pipes L13

13. Photograph showing vugs in salmon-colored sandstone immediately above 

Todilto Limestone Member, Red Rock State Park L14

14. Stratigraphic cross section of Todilto Limestone Member and lower part of 

Beclabito Member in and near Red Rock State Park L15

TABLES

1. Data on collapse experiments Lll

Contents III

EVOLUTION OF SEDIMENTARY BASINS SAN JUAN BASIN

Clastic Pipes of Probable Solution-Collapse Origin in 

Jurassic Rocks of the Southern San Juan Basin, 

New Mexico

By Ralph E. Hunter, Guy Gelfenbaum, and David M. Rubin

Abstract

Pipes that contain downdropped strata are locally 

common in Jurassic rocks above the Todilto Limestone 

Member of the Wanakah Formation in the southwestern part of 

the San Juan Basin, New Mexico. The pipes are elongate, 

vertical, roughly cylindrical structures 0.1-40 m in diameter. 

They are similar in form and stratigraphic position to pipes 

previously studied in the southeastern part of the San Juan 

Basin but differ in that they commonly contain intact strata 

identical to the host strata, whereas pipes in the southeastern 

part of the basin typically consist of breccia and homogenized 

material. Where the sense of movement of material within the 

pipes relative to the host strata can be determined by 

correlation of beds or by drag along the ring faults bounding 

the pipes, material within the pipes has consistently been 

downdropped.

Downdropped but otherwise little disturbed strata are so 

common within pipes in the southwestern part of the basin that 

the fluid-escape or spring-vent origin advocated previously for 

pipes in the southeastern part of the basin seems implausible. 

Gradual collapse following the localized dissolution of evaporites probably is the primary cause of the pipes rather than a 

subsidiary cause as previously suggested. Neither the 

cylindrical form nor the large height-to-width ratios of the 

pipes is evidence against a solution-collapse origin, because 

such features are common in collapse structures of diverse 

origin, including those formed experimentally. The evaporite 

unit at the top of the Todilto in the southeastern part of the 

basin most likely extended farther west originally, and its early 

dissolution at discrete points probably caused localized 

collapse of the overlying sediment. Evaporite dissolution may 

also be the primary cause of pipe formation in the southeastern 

part of the basin and of Jurassic folding and faulting throughout 

the southern part of the basin.

Manuscript approved for publication April 19, 1991.

INTRODUCTION

Clastic pipes are elongate, roughly cylindrical 

structures composed of clastic sedimentary material. Most if 

not all were oriented roughly vertically at the time of their 

formation. Although they are rare or absent in most 

sedimentary rocks, they are locally common, as in several 

Jurassic formations of the southern San Juan Basin, New 

Mexico (fig. 1). Pipes in the southeastern part of the San 

Juan Basin have been studied in considerable detail because 

some of them are sites of uranium mineralization (Hilpert 

and Moench, 1960; Clark and Havenstrite, 1963; Granger 

and Santos, 1963; Schlee, 1963; Wylie, 1963; Megrue and 

Kerr, 1965; Moench and Schlee, 1967; Hilpert, 1969). In 

this paper we describe relatively little studied pipes in 

Jurassic rocks of the southwestern and south-central parts of 

the San Juan Basin.

Clastic pipes can form by the diapiric rise of plastic 

sediment, by fluid (water or gas) escape and associated 

slurry intrusion or foundering, by collapse accompanying or 

following the removal of underlying material, and by the 

filling of pipe-shaped cavities. Water-escape pipes (Alien, 

1961; Lowe, 1975; Bailey and Newman, 1978) include 

spring-vent pipes (Hawley and Hart, 1934; Gabelman, 

1955; Dionne, 1973) and pipes that may underlie 

earthquake-induced sandblows (Sieh, 1978; Obermeier and 

others, 1985). Gas-escape pipes include cryptovolcanic 

pipes (Gabelman, 1957; Wylie, 1963) and pipes that may 

underlie sea-floor pockmarks formed by the escape of 

hydrocarbon gases (Hoviand and Judd, 1988). Collapseinduced pipes include those formed by the dissolution of 

buried evaporites (Landes, 1945; Christiansen, 1971; 

Anderson and others, 1978; Anderson and Kirkland, 1980) 

and carbonate rocks (Dietrich, 1953; Keys and White, 1956; 

Barrington and Kerr, 1963; Hawley and others, 1965;

Clastic Pipes in Jurassic Rocks, New Mexico L1
36' 109 108 107 1

35 l

Window 

Rock

o 

Gallup

Red Rock

State Park Coolidge

UTAH

ARIZONA

COLORADO

AREA SHOWN 

IN DETAIL

'Albuquerque

NEW MEXICO

Mesa Montanosa

Laguna

0 10 20 30 KILOMETERS

Figure 1. Study locations (solid squares) and outcrops of the San Rafael Group and approximate equivalents (screened pattern) 

in the southern San Juan Basin, New Mexico and Arizona.

Goraitz and Kerr, 1970; Wenrich, 1985; Krewedl and 

Carisey, 1986; Wenrich and others, 1988), those formed by 

the melting of subsurface ice (McDonald and Shilts, 1975), 

and those formed by other volume-reduction processes 

(Wisser, 1927). Cavities of pipelike form, which maj be 

filled later to form pipes, can form by the localized 

dissolution of an evaporite or carbonate body at the ground 

surface or sea floor, by the erosion of potholes by flowing 

water, and by the decay of tree trunks that were buried while 

standing. Pipe-shaped biogenic structures, which include 

vertical burrows and root structures, should probably not be 

called pipes.

In their studies of the southeastern San Juan Basin, 

Schlee (1963) and Moench and Schlee (1967) interpreted 

the pipes as spring-vent structures, although they 

recognized that collapse due to gypsum dissolution may 

have played a subordinate role in forming the pipes. In this 

paper we present evidence that the pipes in the southwestern 

and south-central parts of the basin formed by solution 

collapse. Although we have not extensively studied the 

pipes in the southeastern part of the basin, we consider a 

solution-collapse origin to be the most reasonable 

explanation for all the pipes in the southern San Juan Basin.

GEOLOGIC SETTING

The stratigraphy of Jurassic rocks in the southern San 

Juan Basin has been discussed in detail by Condon and 

Peterson (1986), and recent changes in stratigraphic 

nomenclature have been discussed by Condon and Huffman 

(1988) and Condon (1989). Stratigraphic units pertinent to 

this study are in the interval from the Wanakah Formation to 

the Morrison Formation (fig. 2). The Wanakah Formation, 

which overlies the Entrada Sandstone and underlies the 

Morrison, consists of the Todilto Limestone Member, 

formerly accorded formational status, the overlying Beclabito Member, formerly called the Summerville Formation, 

and the Horse Mesa Member at the top. The Horse Mesa 

Member, formerly called the lower part of the Bluff 

Sandstone (Moench and Schlee, 1967) or informally called 

the lower part of the sandstone at Mesita (Condon and 

Peterson, 1986), has not been recognized in the 

southwestern San Juan Basin, where the Cow Springs 

Sandstone occupies the same stratigraphic position. The 

Todilto pinches out a short distance west of Red Rock State 

Park, and the Beclabito apparently grades into the Cow 

Springs in the subsurface between Gallup and Lupton. At

L2 Evolution of Sedimentary Basins San Juan Basin
Lupton, the Cow Springs occupies the entire interval from 

the top of the Entrada to the base of the Morrison.

The Todilto Limestone Member of the Wanakah 

Formation consists of a thin (0-9 m), widespread limestone 

unit and an overlying thicker (0-37 m), geographically 

more restricted gypsum-anhydrite unit. The gypsumanhydrite unit, which consists entirely of gypsum at the 

surface and predominantly of anhydrite in the subsurface, 

crops out in the southeastern part of the basin but is absent 

in outcrops farther west. The Todilto has been interpreted as 

probably lacustrine by some workers (Anderson and Kirkland, 1960; Tanner, 1970; Rawson, 1980) and as probably 

marine by others (Harshbarger and others, 1957; Ridgley 

and Goldhaber, 1983; Ridgley, 1984, 1986). In either case, 

the water body was hypersaline by the time the gypsumanhydrite unit was deposited.

The Beclabito Member of the Wanakah Formation is 

9^9 m thick and consists mainly of silty sandstone and 

siltstone with mudstone and claystone partings. Most of the 

beds are reddish in color, although some sandstone beds are 

very light colored or white. The bedding is predominantly 

flat. Lamination and other small-scale sedimentary 

structures are generally absent or indistinct. The Beclabito 

Member was deposited close to the margins of a shallow

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Recapture Member

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Horse Mesa

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Beclabito Member

Todilto

Limestone Member

Entrada Sandstone

Figure 2. Stratigraphy of Jurassic rock units in the southern 

San Juan Basin. The stratigraphy of the interval between the 

Entrada Sandstone and the Morrison Formation differs 

geographically; in the extreme southwestern part of the basin 

(around Lupton, fig. 1) the Cow Springs Sandstone occupies 

the entire interval, whereas at Haystack Mountain and farther 

east the Wanakah Formation occupies the entire interval. In 

the area between Lupton and Haystack Mountain the Cow 

Springs Sandstone overl ies the Beclabito Member of the Wanakah.

water body that, like the water body in which the Todilto 

was deposited, was either shallow marine or lacustrine. 

Some of the Beclabito was probably deposited within the 

water body, but much of the unit was probably deposited on 

a sabkha that bordered the water body (Condon and Peterson, 1986). Swirled vestiges of lamination, which are 

visible on well-exposed outcrops, indicate intense smallscale deformation. The small-scale deformation was 

probably caused by a variety of penecontemporaneous 

processes including bioturbation, physical processes such as 

loading and liquefaction, and chemical processes such as 

intrastratal evaporite precipitation and dissolution. A few 

sandstone beds that have preserved cross lamination can be 

identified as eolian on the basis of structures formed by 

climbing wind ripples (Hunter, 1981).

The Cow Springs Sandstone and the Horse Mesa 

Member of the Wanakah Formation are similar in character, 

thickness, and stratigraphic position. Both are light-colored 

to reddish, fine- to medium-grained sandstones that are 

40-100 m thick along the southern side of the San Juan 

Basin. Cosets of cross-strata are interbedded with sets of 

thin, flat beds. Most of the sets of cross-strata are less than 

1 m thick. The Cow Springs has been interpreted as eolian 

dune and interdune deposits (Condon and Peterson, 1986). 

The Horse Mesa has been interpreted as probably fluvial 

(Moench and Schlee, 1967; Maxwell, 1982) or as eolian 

(Condon, 1989). We concur with an eolian interpretation for 

both the Cow Springs and the Horse Mesa, mainly on the 

basis of structures formed by climbing wind ripples (Hunter, 

1981).

The Morrison Formation is a complex sequence of 

nonmarine sandstone, siltstone, mudstone, and claystone 

that is divided into several members (fig. 2). It is 70-190 m 

thick along the southern margin of the San Juan Basin. 

Depositional environments represented in the formation 

include fluvial, lacustrine, and eolian. Eolian sandstones are 

restricted to the Recapture Member, the lowest member of 

the Morrison (Condon and Peterson, 1986; Condon, 1989). 

The eolian sandstone now assigned to the lower part of the 

Recapture in the southeastern part of the basin (Condon, 

1989) was formerly assigned to the upper part of the Bluff 

Sandstone (Moench and Schlee, 1967).

DESCRIPTION OF PIPES

Pipes are present in outcropping Jurassic rocks along 

the southern margin of the San Juan Basin from near Gallup 

eastward to the Laguna district (fig. 1). Pipes are most 

common in the Beclabito and Horse Mesa Members of the 

Wanakah Formation and in the Cow Springs Sandstone. A 

few pipes are in the Morrison Formation (Hilpert and 

Moench, 1960; Clark and Havenstrite, 1963; Granger and 

Santos, 1963; Schlee, 1963; Wylie, 1963; Megrue and Kerr, 

1965; Moench and Schlee, 1967; Hilpert, 1969); we have 

seen pipes in the Recapture Member of the Morrison as far

Clastic Pipes in Jurassic Rocks, New Mexico L3
west as the Coolidge area. No pipes have been observed 

above the Morrison Formation, in the Todilto Limestone 

Member of the Wanakah Formation, or below the Todilto.

The pipes tend to be clustered geographically. Small 

pipes are abundant in the lower part of the Beclabito in the 

southwestern part of the basin (fig. 3). Fifteen pipes 

averaging 1 m in diameter were observed in an area of 255 

m2 in the lower Beclabito at Red Rock State Park, and such 

concentrations are probably typical of that part of the 

section in an area of at least several square kilometers. Pipes 

are fairly common throughout the Beclabito and Cow 

Springs in the southwestern part of the basin and throughout 

the Beclabito and Horse Mesa in the southeastern part of the 

basin. A cluster of 60 pipes in an area of 0.5 km2 has been 

mapped in the southeastern part of the basin (Schlee and 

Moench, 1963), and we observed a cluster of 34 pipes 

averaging 9 m in diameter in an area of 0.3 km2 north of 

Prewitt in the south-central part of the basin. The clusters of 

pipes in the southeastern part of the basin are associated 

with belts of folds and faults that formed during the Jurassic 

or Early Cretaceous (Moench and Schlee, 1967).

In exposures parallel with bedding, the pipes appear 

roughly circular (fig. 4A). In exposures normal to bedding, 

the pipes appear elongate, roughly parallel walled, and 

oriented roughly normal to bedding (fig. 46). Oblique 

sections across the pipes appear elliptical (fig. 5). Clastic 

pipes in the southwestern and south-central parts of the San 

Juan Basin are 0.1-40 m in diameter (fig. 3). The range in 

diameters is even greater, 0.025-60 m, in the southeastern

10 20 30 

PIPE WIDTH, IN METERS

40

Figure 3. Histogram showing distribution of pipe widths in 

Jurassic rocks of the southwestern and south-central parts of 

the San Juan Basin. Diagonal pattern, pipes in the Beclabito 

member; solid, pipes in other units.

EXPLANATION 

Strike and dip of bedding 

Trace of pipe boundary fault

V

B

0 5 10 METERS

Top of outcrop

 -

Red mudstone bed

Figure 4. Sketches of a large (diameter 42 m) pipe in the 

Horse Mesa Member of the Wanakah Formation. South side of 

Mesa Montanosa (fig. 1), SE 1A SWV* sec. 24, T. 13 N., R. 10 

W., McKinley County, New Mexico. A, Map view. B, Cross 

section; note that red mudstone bed within pipe has dropped 

as much as 17 m relative to its position outside pipe.

part of the basin (Moench and Schlee, 1967). Many more 

large pipes are present in the southeastern part of the basin 

than in the southwestern part.

The full vertical extent of a pipe is nowhere 

indisputably exposed. The original tops of almost all pipes 

have been removed by recent erosion, and almost all pipes 

extend beneath the present outcrop surface. From exposures 

of small pipes on almost vertical outcrops, it can be said 

with certainty that the small pipes are many times higher 

than broad, and we assume that the same is true of larger 

pipes. We have seen no indisputable examples of the 

original tops of pipes, but Hilpert and Moench (1960), 

Schlee (1963), and Moench and Schlee (1967) reported a 

few examples. In these examples, which are interpreted as 

pipes that extended up to the ground surface, the top flares 

and forms a slight depression into which the covering beds 

sag (see, for example, Schlee, 1963, fig. 4D). From 

inaccessible cliff exposures in which some pipes seem to 

fade out upward, we suspect that some pipes never extended

L4 Evolution of Sedimentary Basins San Juan Basin
Figure 5. Oblique section through pipe in which bedding is 

preserved in the Cow Springs Sandstone at Red Rock State 

Park. The pipe is about 4 m wide. A, View looking horizontally 

at outcrop. B, View looking obliquely downward. Although 

the pipe appears in (A) to be truncated by flat-lying beds, it can 

be seen in (B) that the flat-lying beds above the level of the pipe 

outcrop are to one side of the pipe, not directly above it; the 

pipe extended farther upward before erosion of the outcrop.

to the surface. Indisputable examples of pipe bases are as 

rare as pipe tops. The character and stratigraphic level of 

pipe bases are critical for determining pipe origin, and we 

postpone a description of pipe bases until our discussion of 

pipe origin.

Pipe interiors may be structureless sedimentary rock, 

breccia, almost unbroken strata identical to strata 

surrounding the pipe, or mixtures of these three types of 

material. Unbroken strata within pipes are rare in the 

southeastern part of the basin (Schlee, 1963; Moench and 

Schlee, 1967) but are common in the southwestern part, 

where about 20 percent of the pipes have almost completely 

unbroken strata (figs. 5, 6) and only 10 percent have no 

traces of stratification. The material within the pipes is 

well-sorted sandstone to clayey sandy siltstone. Where the

sediment of the pipe is more thoroughly cemented than the 

surrounding rock, the pipe may stand freely as a column. 

Free-standing pipes are common in the southeastern part of 

the basin (Schlee, 1963; Moench and Schlee, 1967) but are 

rare in the southwestern part.

The boundary of a pipe is a vertical ring fault or zone 

of closely spaced ring faults and ring fractures. Other 

roughly concentric ring faults that have relatively small 

displacements may be present within or outside a pipe (fig. 

7). Where the pipe interior is stratified, the sense and 

amount of displacement on the boundary ring fault can be 

determined by correlation of strata within the pipe and 

surrounding strata (figs. 4, 6). Forty-eight determinations in 

the southwestern part of the basin all indicate downdropping 

of the pipe interior. The amount of downdrop is 0.4-15 m. 

There is a poorly defined tendency for the amount of 

downdrop to increase with pipe diameter. On the other 

hand, the ratio of downdrop to pipe diameter decreases with 

increasing pipe diameter; the ratio is greater than 2.0 for 

some pipes less than 2 m wide, less than 1.0 for pipes 2 10 

m wide, and less than 0.5 for pipes greater than 10m wide. 

Displacements on subsidiary ring faults outside the 

principal bounding fault are typically less than 0.3 m, and 

most of the displacements indicate downdropping toward 

the pipe axis (fig. 7).

The sense of displacement across the pipe boundary is 

commonly determinable even where beds within a pipe 

cannot be correlated with those outside because of a lack of 

distinctive beds. Beds within the pipe commonly exhibit 

drag, in which the beds are curved upward toward the 

bounding ring fault (figs. 4-6). Such drag indicates downdropping of the beds within the pipe. Drag in beds 

immediately outside the ring fault is not common but, where 

present, indicates downdropping of the pipe interior (see,

Figure 6. Two pipes with preserved, downdropped bedding 

in the Cow Springs Sandstone at Red Rock State Park. The pipe 

on the right is about 3 m wide. The outcrop slopes steeply, 

whereas the pipes are vertical; the pipes therefore disappear 

into the outcrop in a downward direction and are removed by 

erosion in an upward direction.

Clastic Pipes in Jurassic Rocks, New Mexico L5
5 cm offset

10 cm offset

10cm offset 

10-15 cm offset

N

T

Pipe

0 6 12 METERS

Figure 7. Faults associated with a pipe near Coolidge Quarry 

(fig. 1), SWV4 sec. 28, T. 15 N., R. 14 W., McKinley County, 

New Mexico.

for example, Schlee, 1963, fig. 4B). Sagging of beds toward 

the pipe axis in a wide zone surrounding the pipe is fairly 

common in the south-central and southeastern parts of the 

basin (see, for example, Schlee, 1963, fig. 3A); such 

sagging indicates downdropping toward the pipe axis. In 

some of these pipes the sagging is accompanied by bed 

thickening toward the pipe axis, which indicates that the 

downdropping occurred while the beds were being 

deposited.

Even in the southeastern part of the basin, where 

strata are rarely preserved within the pipes, Schlee (1963), 

Megrue and Kerr (1965), and Moench and Schlee (1967) 

concluded that the material within the pipes had moved 

downward. This conclusion was based partly on drag and 

sagging of beds surrounding the pipes and partly on a 

petrographic comparison between structureless sandstone 

within the pipes and bedded sandstone surrounding the 

pipes. In a few pipes, a vertical change in the composition 

of pipe breccia could be correlated with a stratigraphic 

change outside the pipe, and the correlations indicate 

downdropping of the pipe interiors (Hilpert and Moench, 

1960; Megrue and Kerr, 1965).

ORIGIN OF PIPES 

Time of Origin

Whatever the origin of the pipes, several types of 

evidence indicate a relatively early origin during the Jurassic, before the beds were lithified. The preserved tops of a 

few pipes within Jurassic units indicate that the pipes 

reached the ground surface during the Jurassic (Hilpert and 

Moench, 1960; Schlee, 1963; Moench and Schlee, 1967).

An origin contemporaneous with the pipe-bearing beds is 

also indicated by the bed thickening associated with bed 

sagging around some pipes. We have seen such thickening 

around a pipe in the Recapture Member of the Morrison 

near Haystack Mountain, and Schlee (1963) mentioned 

changes in bed thickness interpreted as indicative of pipe 

formation during deposition of what is now called the Horse 

Mesa Member of the Wanakah.

A prelithification origin of the pipes is suggested by 

the fact that drag folding of beds adjacent to the ring faults 

bounding the pipes involves considerable stretching and 

thinning of beds. Convolute folding of beds within a few 

pipes also implies that the sediment was unlithified. 

Although the faulting and brecciation of beds within many 

pipes indicates cohesiveness of the material, no great degree 

of lithification is necessarily indicated. We interpret the 

three types of pipe material (unbroken beds, breccia, and 

homogeneous structureless rock) as stages in a sequence 

that developed with increasing downdrop and deformation 

of relatively unlithified sediment. By this interpretation, the 

pipes in the southwestern part of the basin, which 

commonly contain relatively intact beds, represent a less 

advanced stage of pipe formation than those in the 

southeastern part of the basin, which typically are composed 

of breccia or homogeneous rock.

Although considerable evidence exists for relatively 

early pipe formation, we have seen little evidence to support 

the conclusion of Schlee (1963) and Moench and Schlee 

(1967) that the pipes in what is now called the Wanakah 

Formation formed contemporaneously with Wanakah 

deposition. They cited thickness changes in beds adjacent to 

pipes in the Horse Mesa Member of the Wanakah as 

suggesting deposition in sags around pipes, but the evidence 

of that type that we have seen is restricted to the Morrison 

Formation. Contacts of sets of crossbeds in the Horse Mesa 

Member of the Wanakah Formation and in the Cow Springs 

Sandstone are, for the most part, remarkably even and 

parallel and do not suggest local sagging of the depositional 

surface contemporaneous with pipe growth. J?robably most 

of the pipes formed during Morrison time.

A Comparison of Hypotheses

Most conceivable explanations of clastic pipes are 

easily ruled out for the pipes in the southern San Juan Basin. 

The overwhelming evidence of downward movement 

within the pipes rules out a diapiric origin or an origin by 

slurry intrusion from below. The presence of strata within 

many pipes identical to surrounding strata rules out an 

origin by the infilling of tubular cavities. The presence of 

unbroken, downdropped strata within pipes is incompatible 

with an origin by the sudden or explosive escape of water or 

gas. A cryptovolcanic gas-escape origin, which has been 

favored by a few workers (Gabelman, 1957; Wylie, 1963) 

for clastic pipes in the Colorado Plateau, including those in

L6 Evolution of Sedimentary Basins San Juan Basin
the southeastern part of the San Juan Basin, is further ruled 

out by the absence of pipes in stratigraphic units below the 

Beclabito Member even in areas where pipes are abundant 

in the lower part of the Beclabito Member. Collapse due to 

the melting of ice is ruled out by the low-latitude, warm to 

hot climate of the area during Jurassic time (Kocurek and 

Dott, 1983).

The two remaining possible explanations for the pipes 

are gradual water escape, for example through spring vents, 

and collapse induced by dissolution of underlying soluble 

rocks. Any collapse must have accompanied gradual 

dissolution rather than occurring suddenly by roof failure of 

large, previously formed cavities, or else unbroken beds 

would not be so common in pipes of the southwestern part 

of the basin. In the absence of any evidence of large-scale 

dissolution of the limestone unit of the Todilto, the only 

soluble rock whose dissolution could plausibly be called on 

to explain the pipes is the evaporite unit that forms the upper 

part of the Todilto. An evaporite solution-collapse and 

associated evaporite-flowage origin for the pipes in the 

southeastern part of the basin was proposed and discussed 

briefly by Mirsky (1955), whereas a spring-vent origin for 

the same pipes was favored by Schlee (1963) and Moench 

and Schlee (1967). Water-escape and solution-collapse 

explanations have been favored for most other pipes on the 

Colorado Plateau. Water-escape interpretations have been 

proposed by Gabelman (1955), Phoenix (1958), and Hannum (1980), whereas solution-collapse interpretations have 

been proposed by Keys and White (1956), Weir and others 

(1961), Barrington and Kerr (1963), Hawley and others 

(1965), Gornitz and Kerr (1970), Bowles (1977), Wenrich 

(1985), Krewedl and Carisey (1986), and Wenrich and 

others (1988).

In the spring-vent interpretation proposed by Schlee 

(1963) and Moench and Schlee (1967), the downdropping 

that they recognized in the pipes was explained as the result 

of loss of strength in the column of sediment through which 

water was flowing upward. When the sediment lost its 

strength by dilation of the grain framework, sand could have 

sunk into underlying finer grained, less dense sediment. 

According to this interpretation, room for the downward 

displacement of sand was created by compaction of the 

underlying fine-grained sediment as its pore water escaped 

upward and by mixing of fine-grained sediment into the 

sand. Schlee (1963) and Moench and Schlee (1967) 

recognized that localized dissolution of the Todilto evaporite unit may have helped to create space for downdropping 

in some pipes, but they did not consider such dissolution to 

be essential for pipe formation. Other possible means of 

creating room for downward-displaced sand include the 

upward escape of fine-grained sediment around the edges of 

the descending mass of sand and the lateral pushing aside of 

fine-grained sediment.

A spring-vent interpretation seems implausible for the 

pipes in the southwestern part of the San Juan Basin, which 

commonly have unbroken or only slightly broken strata. We 

cannot conceive how stratification could have remained 

intact during upward water flow intense enough to dilate the 

grain framework and fluidize the sand. Moreover, we doubt 

that mechanisms other than solution-induced collapse could 

have created the space necessary for downdropping in the 

pipes. Compaction of fine-grained material underlying the 

sand would have restored strength to the sediment and 

halted the downdropping. Mixing of sand and underlying 

fine-grained sediment cannot be called on where the sand 

retains its original bedding. Fine-grained sediment could not 

have escaped upward where bedded sand within a pipe is in 

sharp contact with surrounding sand across the ring fault 

that bounds the pipe. Except at the bases of pipes, there is no 

evidence that material within the pipes was squeezed out 

laterally, and, as will be discussed later, the evidence of 

lateral spreading of pipe material at pipe bases can be 

explained by dissolution of an evaporite bed underlying the 

pipe.

Given the difficulties of a spring-vent interpretation, 

further consideration needs to be given to a solutioncollapse origin of the pipes. Schlee (1963) and Moench and 

Schlee (1967) presented several kinds of evidence against 

collapse due to dissolution of the Todilto evaporite unit as 

the primary cause of the pipes in the southeastern part of the 

basin. One such piece of evidence is that pipes are found 

outside the area in which the evaporite unit is presently 

found. The probative value of this argument is controverted, 

however, by the possibility that the evaporite unit originally 

extended beyond its present limit and has been completely 

removed by dissolution in those areas. This possibility was 

admitted by Moench and Schlee (1967).

A stronger argument against the solution-collapse 

hypothesis is the claim of Schlee (1963) and Moench and 

Schlee (1967) that they observed the true bases of several 

pipes at stratigraphic levels within the Beclabito Member, at 

considerable distances above the top of the Todilto. We 

were unable to find downward terminations of pipes above 

the top of the Todilto, and we consider it very difficult to 

determine whether the apparent base of a pipe seen in 

outcrop is the true base. Even if a pipe is seen to terminate 

downward in a vertical exposure, that particular pipe could 

conceivably be one of the atypical pipes that are not 

essentially vertical, in which case the pipe might continue 

downward behind the outcrop surface or might have 

continued downward through now-eroded rock in front of 

the outcrop.

In our studies, we definitely identified the true bases 

of pipes only in a few outcrops 0-2 km west of Red Rock 

State Park. We are confident of those identifications 

because in that area the downward terminations of dozens of 

closely spaced pipes were observed in vertical outcrops. All 

the pipes terminated within a zone no more than 1 m thick

Clastic Pipes in Jurassic Rocks, New Mexico L7
at the top of the Todilto Limestone Member (figs. 8, 9). 

Nowhere else did we find comparable vertical exposures of 

the contact between the Beclabito and the Todilto, but many 

pipes extend to within short distances of the contact in other 

places (fig. 10). These observations are compatible with an 

origin of the pipes by dissolution of the evaporite unit at the 

top of the Todilto, although the precise mechanism of 

collapse remains to be demonstrated.

Gabelman (1957) presented additional evidence 

against a solution-collapse origin for clastic pipes on the 

Colorado Plateau, including the Jurassic pipes of the San 

Juan Basin. He claimed that the height-to- width ratios of the 

pipes are too great and the geometrical forms too nearly 

perfectly cylindrical for a collapse origin to be plausible. 

However, cylindrical collapse structures of large heightto-width ratio are known to form under a variety of 

conditions, as the following review demonstrates.

One type of collapse structure that is commonly 

cylindrical in form is volcanic ring complexes, including 

cauldron-subsidence structures and calderas (Billings, 1943; 

Macdonald, 1972; Williams and McBirney, 1979); 

however, a large height-to-width ratio can seldom be 

demonstrated for these structures. In structures interpreted 

to be of carbonate solution-collapse origin, cylindrical 

forms have been observed by many workers (for example, 

Dietrich, 1953; Keys and White, 1956; Barrington and Kerr, 

1963; Hawley and others, 1965; Wenrich, 1985; Krewedl 

and Carisey, 1986; Wenrich and others, 1988), and at least

Figure 9. Bases of pipes in the lower part of the Beclabito 

Member just west of Red Rock State Park (part of outcrop 

sketched in figure 8). Jacob's staff for scale is 1.5 m long. 

Bracketed interval comprises units 1-4 of figure 8; arrows 

point to thin red mudstone unit at top of unit 2 of figure 8. Note 

that the deformation associated with the pipes affects the red 

mudstone but does not affect the Todilto Limestone Member 

beneath the bracketed interval.

some of these structures have large height-to-width ratios. 

Cylindrical structures interpreted to have formed by evaporite solution collapse also have been noted (for example,

Cow Springs Sandstone

Entrada Sandstone

10 20 30 

i

Todilto Limestone "'.-. Member"-."'.

40 METERS 

i

No vertical exaggeration

Figure 8. Cross section of several pipes in the Beclabito Member just west of Red Rock State Park (fig. 1), NEV* sec. 10, T. 15 

N., R. 17 W., McKinley County, New Mexico. Sketched from a photograph. Resistant eolian sandstones represented by dotted 

pattern. Numbered units of the Beclabito Member are (1) salmon-colored sandstone, (2) siltstone with thin red mudstone bed at 

top (represented by heavy line), (3) structureless sandstone, (4) eolian cross-laminated sandstone, and (5) silty sandstone and 

siltstone. Note that the deformation associated with the pipes does not affect the Todilto Limestone Member. Also note the flared 

bases of several pipes, which indicate lateral spreading.

L8 Evolution of Sedimentary Basins San Juan Basin
Figure 10. Deformed lower part of Beclabito Member 

overlying undeformed Todilto Limestone Member and Entrada 

Sandstone at Red Rock State Park (fig. 1). Bracketed interval is 

5-6 m thick and comprises the lower part of the Beclabito 

(units 1-4 of fig. 8). The thin, resistant sandstone bed at the top 

of the bracketed interval (unit 4 of fig. 8) is penetrated by 

numerous small pipes and disturbed by small faults and folds 

that do not affect the Todilto. Note especially that the part of 

this bed adjacent to the pipe in the right part of this photograph 

is raised relative to parts of the bed farther from the pipe 

(arrows point to edges of raised area around pipe).

Landes, 1945; Christiansen, 1971; Anderson and others, 

1978; Anderson and Kirkland, 1980), and some of these 

structures have large height-to-width ratios.

Some of the most instructive observations of collapse 

structures come from studies of inadvertent subsidence 

above underground mines (Dunrud, 1984) and artificially 

dissolved salt deposits (Walters, 1977; Ege, 1984) and from 

studies of planned collapse in the caving method of 

underground mining and the flow of granular material from 

outlets at the bottoms of storage bins (Yenge, 1980, 1981; 

Coates, 1981; Kvapil, 1982; Peters, 1984). These studies are 

particularly instructive because the structures are artificially 

induced and, therefore, unequivocally of collapse origin. 

Artificially induced collapse structures can reach heights 

more than twenty times their widths and, where they form 

above an equidimensional cavity or outlet and do not extend 

to the surface, are of highly prolate ellipsoidal (cigarshaped) or elongate cylindrical form (Yenge, 1980, 1981; 

Coates, 1981; Kvapil, 1982; Peters, 1984). Where the 

collapse extends to the surface in unconsolidated material, a 

funnel-shaped top forms by the avalanching of material 

down the sides of the surface depression.

The foregoing comparison of hypotheses for the 

origin of the clastic pipes in the southern San Juan Basin 

suggests that an evaporite solution-collapse interpretation 

cannot be ruled out and is just as capable of accounting for 

the observed phenomena as a spring-vent interpretation, the 

previously most favored interpretation for the pipes. In the

following section we discuss experimental evidence that 

adds further support to a solution-collapse interpretation.

Experiments on Collapse Structures

Previous experiments on the collapse of granular 

material above an actual or potential cavity all resulted in 

highly elongate, vertically oriented structures (Yenge, 1980, 

1981; Coates, 1981; Kvapil, 1982; Peters, 1984), but some 

questions remained unresolved. In particular, it was not 

obvious how the external form and internal structure of the 

collapse structure would vary under different conditions of 

cohesive strength. Some theoretical and experimental work 

suggests that a collapse structure will expand in width 

upward from its base (for example, Yenge, 1981; Kvapil, 

1982). On the other hand, the theoretical analyses and 

experiments of Anderson (1937), Sanford (1959), and 

others suggest that faults on opposite sides of a collapse 

structure will tend to arch over and meet upward, leading to 

upward narrowing and termination of the collapse structure. 

To address these uncertainties we designed a series of 

simple experiments in which the cohesive strength of the 

material was varied by changing its degree of water 

saturation.

The experimental apparatus consisted of a 5-gallon 

paint bucket (30 cm diameter) and a small can (7.5 cm or 

2.5 cm in diameter). The bucket was cut open on one side 

and covered with a removable wall to allow a window for 

viewing the structure at the end of the experiment (fig. 1 IA). 

In the bottom of the bucket a hole was cut just large enough 

to allow the can to slide through. The experiment was begun 

by raising the can approximately 5.5 cm (4 cm for the 

smaller of the two cans) through the hole in the bottom of 

the empty bucket. The bucket was filled loosely with 

alternating thick layers of a light-colored, medium-grained 

sand, similar to that found in pipes in the study area, and 

thin layers of a dark, fine-grained sand to delineate the 

pattern of deformation produced during the experiment (fig. 

1IA). The can was then slowly (over a period of several 

minutes) lowered flush to the bottom of the bucket so as to 

cause gradual collapse of the overlying sand. Finally, the 

bucket was turned on its side while a partial lid was put on 

the top of the bucket to keep sand from spilling out, the 

upward-facing removable wall was taken out, and sand was 

scraped away by a long spatula or machete to allow a view 

of the resulting deformation in a cross section through the 

sand.

Six experiments were performed in which either the 

water content of the sand was varied or the diameter of the 

can was changed. Table 1 gives the values of these 

parameters for the six experiments and a brief description of

Clastic Pipes in Jurassic Rocks, New Mexico L9
Figure 11. Collapse-induced pipes produced in experiments. A, Experiment 2; sand completely dry except for damp, relatively 

dark sand that was placed in bottom of bucket up to initial level of top of can. B, Experiment 6; sand completely water saturated. 

C, Experiment 3; sand damp. D, Experiment 1; sand dry, can diameter 2.5 cm instead of 7.5 cm as in the other experiments.

L10 Evolution of Sedimentary Basins San Juan Basin
Table 1 . Data on collapse experiments

Water content 

Experiment (percent)

1 0 

2 0 

3 3 

4 4 

5 64 

6 100

Cavity diameter 

(centimeters)

2.5 

7.5 

7.5 

7.5 

7.5 

7.5

Results

Collapse did not extend to surface (fig. 1LD) 

Collapse extended to surface, walls vertical (fig. 1L4) 

Collapse extended to surface, narrowed toward top (fig. 11C) 

No collapse, cavity remained open 

No collapse, cavity remained open 

Collapse extended to surface, walls vertical (fig. 11B)

the results. The degree of water saturation was determined 

from the weight percent of water for each run, scaled by run 

6, which was carried out underwater with the sand 

completely saturated.

In experiments 4 and 5, which involved damp, 

cohesive sand, the strength of the sand was not exceeded; 

therefore no deformation occurred above the cavity formed 

by the lowered can. In experiments 2 (dry sand) and 6 

(completely saturated sand), the sand was essentially noncohesive, and the results were almost identical. In both of 

these experiments the walls of the collapsed material were 

almost vertical, extending from the top of the can to the top 

of the bucket, where a small avalanche funnel formed (figs. 

1 IA, B). The center of the collapsed material was always 

downdropped the most, giving a broad U-shaped 

appearance to the stratification. Upon close examination it 

was observed that the walls of the collapse structure were 

zones of finite thickness (approximately 0.75 cm) and that 

the sand within the wall zone was strongly sheared, as 

indicated by drag of the originally horizontal layers within 

the wall zone.

In experiment 3, in which the sand was slightly 

cohesive because of its low water content, the collapse 

extended to the surface but narrowed toward the top (fig. 

11C). The downward widening of the collapse structure 

caused the material near the top of the structure to drop a 

greater distance than the can was lowered. In experiment 1, 

in which the sand was dry and the smaller can was used, the 

collapse did not extend all the way to the surface. The walls 

of the collapse were almost vertical for approximately 15 

cm from the base; above that level the collapse narrowed 

and disappeared at a point 22 cm from the base (fig. 11£>). 

In this experiment, the amount of downdrop was greatest at 

the base of the bucket and decreased upward.

Our experiments were of such small scale that a 

relatively small cohesive strength prevented collapse; 

however, scaling principles suggest that a relatively small 

cohesive strength in our experiments would correspond to a 

greater cohesive strength in a structure of larger scale. 

Within the limitation of the small range of cohesive 

strengths investigated, it can be stated that, in both noncohesive and cohesive sand, the collapse structures were pipes

that had large height-to-width ratios and almost vertical 

walls. Layers within the collapse structures were deformed 

by drag but were not broken.

Altogether, the experimental collapse structures 

closely simulated the pipes of the southwestern part of the 

San Juan Basin. Although this similarity does not prove a 

collapse origin for the natural pipes, the experimental 

evidence adds support to a collapse interpretation.

The Problem of Excessive Downdrop

The hypothesis that the pipes originated by collapse 

resulting from local dissolution of a relatively thin evaporite 

bed presents problems not yet discussed. One such problem 

is finding an explanation for amounts of downdropping in 

the pipes that are excessive in the sense of being greater 

than the original thickness of evaporites that were later 

dissolved. The amount of downdroping in pipes in the 

southwestern part of the basin is as much as 15 m, and 

evaporites at the top of the Todilto Limestone Member were 

probably no more than 1 or 2 m thick in that part of the 

basin.

Keys and White (1956) and Hawley and others (1965) 

noted an amount of downdropping greater than the 

thickness of underlying dissolved rock within pipes in Utah 

interpreted to be of solution-collapse origin. They 

speculated that the excessive amount of downdropping 

might be due to lateral spreading of downdropped material 

at the base of a pipe as the underlying bed of soluble rock 

was progressively dissolved outward from a central point 

beneath the pipe. Similar lateral spreading of collapsed 

material into underground mine workings was called on by 

Dunrud (1984) to explain mine subsidence pits deeper than 

the thickness of the underlying material removed by mining. 

Excessive amounts of downdrop can be caused by gradual 

downward widening of a pipe, such as occurred in our 

experiment 3 (fig. 11C), as well as by widening due to 

lateral spread at the very base.

Another possible cause of downdropping greater than 

the thickness of underlying dissolved rock is dissolution of 

part of the material in the pipe itself. For example, Keys and

Clastic Pipes in Jurassic Rocks, New Mexico L11
White (1956) and Hawley and others (1965) mentioned 

dissolution of carbonate cement, carbonate-rock beds, and 

siliciclastic grains within the pipes as possible contributing 

causes of the excessive downdropping that they observed; 

however, significant amounts of carbonate are not available 

for dissolution in the pipe-bearing parts of the stratigraphic 

section in the southern San Juan Basin. In addition, although 

mineralogical comparisons of pipe rock and surrounding 

rock suggest that some feldspar grains in the pipes were 

altered to clay minerals (Megrue and Kerr, 1965), we doubt 

that the alteration caused significant volume changes. 

Certainly no changes in thickness of preserved beds within 

the pipes can be demonstrated by comparison with the same 

beds outside the pipes.

Yet another possible cause of excessive downdropping is the flushing of material from pipe bottoms by 

underground streams that flowed through caverns in the 

underlying soluble rock. There is no evidence, however, of 

caverns or underground stream deposits in or at the top of 

the Todilto Limestone Member in the southern San Juan 

Basin.

We have not observed systematic downward 

widening of pipes, although the vertical extent of pipe 

outcrops may not be great enough to rule out such widening. 

On the other hand, we have observed evidence of lateral 

spreading at the bases of pipes. In the small pipes whose 

bases we observed west of Red Rock State Park, a small 

amount of lateral spreading is indicated by basal flaring of 

the pipes (fig. 8). Additional lateral spreading is suggested 

by the fact that a distinctive salmon-colored sandstone bed 

(unit 1, fig. 8) that generally rests on the Todilto Limestone 

Member is absent beneath the pipes; deformed bedding in 

the salmon-colored sandstone adjacent to the pipes suggests 

that sand in this bed was squeezed to the sides as the pipe 

material descended. Other abrupt lateral variations in bed 

thickness in the lower few meters of section above the 

Todilto (units 1, 2, and 3, fig. 8) suggest significant lateral 

redistributions of material in this part of the section; 

however, we cannot demonstrate that these variations are 

the result of lateral spreading from the bases of pipes.

Material that spread laterally would support the 

overlying sediment after complete dissolution of the evaporite bed from areas around the pipes; therefore, it might be 

expected that the elevation of individual beds in the host 

rock would be highest adjacent to the pipes, where the beds 

would be supported by the greatest thickness of injected 

pipe material (fig. 12). Although sagging or downfaulting of 

beds toward the pipe axis was observed adjacent to many 

pipes, in several pipes the beds adjacent to the pipes are 

higher than those farther away (fig. 10). Beds adjacent to 

some pipes have a relative vertical displacement on opposite 

sides of pipes that amounts to tens of centimeters (fig. 8). 

This displacement suggests that material from the pipes 

spread differentially in opposite directions.

The evidence of lateral spreading helps to explain the 

large amounts of downdrop observed in some of the pipes. 

We cannot claim, however, to have demonstrated that the 

amount of lateral spreading is sufficient to account for 

amounts of downdrop that almost certainly exceed the 

original thickness of evaporites later dissolved. Although 

the solution-collapse hypothesis thus cannot be regarded as 

completely satisfactory at present, we regard it as being 

better able to account for downdropping than the alternative 

spring-vent hypothesis.

The Problem of Complete Dissolution

In addition to the problem of excessive downdropping, a serious problem facing the hypothesis of pipe 

origin by solution collapse is what will be called the 

problem of complete dissolution. Complete dissolution of 

the evaporite bed following the localized dissolution that 

gave rise to the pipes would cause the beds adjacent to the 

pipes to settle to the same level as the beds filling the pipes, 

thereby eliminating the relative downdrop of the pipe fill 

(fig- 12).

The problem of spatially discrete collapse structures 

in an area where an evaporite bed has been completely 

dissolved pertains primarily to the southwestern part of the 

San Juan Basin. In the southeastern part of the basin, the 

gypsum-anhydrite unit has been thinned or removed by 

dissolution only locally, and many pipes are concentrated in 

areas of appreciable gypsum-anhydrite dissolution (Schlee, 

1963; Moench and Schlee, 1967), as would be expected if 

they were evaporite solution-collapse structures. Even in 

that part of the basin, however, discrete pipes are present in 

large areas where the gypsum-anhydrite unit is now absent. 

Why did recognizable collapse structures not form between 

the pipes in areas where the solution-collapse hypothesis 

requires that the evaporite unit be completely dissolved?

We can offer several partial answers to the question. 

First, lateral spreading of pipe material into potential 

cavities around the bases of the pipes, such as we observed 

west of Red Rock State Park (fig. 8), reduces the amount of 

downdrop required in areas between the pipes. Second, if 

small-diameter pipes that extend only short distances 

upward from the Todilto are taken into account, the areal 

density of pipes is much greater than is suggested by the 

relatively small numbers of pipes at higher stratigraphic 

levels. Third, the later stages of evaporite dissolution may 

have occurred when the sediments were more lithified and 

rigid than they were when the pipes formed, and the 

downdropping accompanying this later dissolution may 

have taken a form other than pipes.

An example of later downdropping in a form other 

than pipes is faulting, which might not be easily recognized 

as being caused by solution collapse. Many faults having 

small offsets (less than 0.4 m) are present in the Beclabito 

Member and Cow Springs Sandstone in and near Red Rock

LI 2 Evolution of Sedimentary Basins San Juan Basin
WITHOUT LATERAL SPREADING 

AT BASE OF PIPE

WITH LATERAL SPREADING 

AT BASE OF PIPE

Before dissolution. Bed containing 

evaporites

Before dissolution.

Local dissolution of evaporites causes 

cylindrical failure. Downdropping of beds 

within pipe can not exceed thickness of 

evaporite bed.

Local dissolution of evaporites causes 

cylindrical failure. Sediment from pipe 

spreads laterally into bed that contained 

evaporites. Downdropping can exceed 

thickness of evaporite bed.

Continued dissolution of evaporite bed 

causes host rock to settle.

Host rock adjacent to pipe is supported 

by sediment that flowed out of base of 

pipe.

Figure 12. Schematic drawings showing postulated role of lateral spreading at pipe base in producing observed characteristics 

of pipes. Without lateral spreading or other methods of reducing bulk volume of material within pipe (left), initial downdrop within 

pipes cannot exceed thickness of evaporite bed; downdrop is eventually eliminated as the evaporite bed continues to dissolve and 

rock surrounding the pipe drops. Lateral spreading from bottom of pipe (right) enables initial downdrop within pipe to exceed 

thickness of evaporite bed. Rock adjacent to pipe is supported by sediment that spreads laterally from base of pipe, thereby 

preserving the relative downdrop after remainder of evaporite bed dissolves and rock farther away from pipe settles.

State Park. These faults do not extend downward into the 

Todilto Limestone Member, and at least many of the faults 

terminate upward in the Cow Springs Sandstone, though not 

in a manner that suggests faulting contemporaneous with 

deposition of the Cow Springs. The faults are vertical to 

high-angle reverse and are spaced 5-20 m apart within a 

given set of faults. All faults in a given set have similar 

orientation and displacement. The faults in one set curve in 

plan view as much as 50° in a distance of 300 m; such a

degree of curvature suggests that the faults are not of 

tectonic origin. Possibly these faults are the result of 

solution collapse, predominantly at a stage following pipe 

formation.

The evidence presented in this section helps to 

explain the fact that the pipes preserve a record of localized 

downdropping even though the hypothesized evaporite bed 

has been dissolved not only under the pipes but also in areas 

between the pipes. As in the problem of excessive downClastic Pipes in Jurassic Rocks, New Mexico L13
drop, however, we cannot claim to have demonstrated 

conclusively that the processes called on here are sufficient 

to account for the observed features.

Original Extent of Dissolved Evaporite Unit

According to the solution-collapse interpretation of 

pipes in the southwestern part of the San Juan Basin, the 

gypsum-anhydrite unit that forms the upper part of the 

Todilto Limestone Member originally extended far beyond 

its present limits. In this section we attempt to reconstruct its 

former distribution along the southern margin of the San 

Juan Basin and its precise stratigraphic position in this area.

The gypsum-anhydrite unit that forms the upper part 

of the Todilto crops out only in the southeastern part of the 

San Juan Basin. In the subsurface just a few kilometers 

north of the outcrop belt, however, the unit extends as far 

west as Prewitt (Hilpert, 1969). Local dissolution near the 

present edges of the evaporite unit is indicated by great 

variations in evaporite thickness over short distances 

(Moench and Schlee, 1967; Stapor, 1972; Tanner, 1972). 

Another feature that has been attributed to the dissolution of 

evaporites from a sequence of interbedded limestone and 

evaporites in the Todilto is brecciation and recrystallization 

in the upper part of the limestone unit of the Todilto 

Limestone Member beyond the present edges of the 

gypsum-anhydrite unit (Rawson, 1980; Ridgley, 1986). 

Evidence (other than the presence of pipes) that evaporite 

minerals were present as far west as the western pinchout of 

the Todilto Limestone Member, a few kilometers west of 

Red Rock State Park, includes (1) recrystallization of the 

upper 0.1-0.2 m of the limestone as far west as its pinchout 

and (2) the presence of abundant vugs, which are probably 

molds of gypsum nodules and crystals, in the salmoncolored calcareous sandstone (unit 1, fig. 8) that 

immediately overlies the limestone from the Coolidge area 

to the western pinchout of the limestone. The inference that 

the vugs were originally filled by gypsum is based on the 

rhombic shapes of some of the vugs (fig. 13).

The stratigraphic position of the hypothesized former 

evaporite in the southwestern part of the basin can be 

inferred within narrow limits. In and near Red Rock State 

Park, the Todilto Limestone Member is overlain 

sequentially by (1) 0.5-1.0 m of fine- to medium-grained, 

moderately to very calcareous, vuggy, salmon-colored 

(typically 10R6/4 in the Munsell notation) sandstone, 

commonly with swirled bedding; (2) 0.8-2.0 m of thinbedded to structureless, dark-grayish-red mudstone and 

reddish-brown siltstone; (3) 1.5-2.3 m of fine- to mediumgrained, reddish-orange to brown, structureless sandstone; 

(4) 1.0 1.2 m of fine-grained, light-grayish-pink, crosslaminated, resistant eolian sandstone; and (5) a thick 

sequence of very fine to fine grained, thick-bedded, 

indistinctly bedded, reddish-brown silty sandstone and siltstone typical of the Beclabito Member (fig. 14). A lower

Figure 13. Vugs in salmon-colored sandstone bed (unit 1 of 

fig. 8) immediately above the Todilto Limestone Member in 

Red Rock State Park (fig. 1). Coin for scale is 18 mm in 

diameter. Rhombic shape of vug to right of coin suggests origin 

by dissolution of a gypsum crystal; rounded vugs probably 

originated by dissolution of gypsum nodules. .

limit for the stratigraphic position of the former evaporite is 

the base of the recrystallized upper 0.1-0.2 m of the Todilto 

Limestone Member, as indicated by the absence of 

deformation in the underlying thin-bedded limestone. An 

upper limit for the stratigraphic position of the evaporite is 

the base of the red mudstone in unit (2) of the Beclabito 

Member, as indicated by the fact that the red mudstone is 

the lowermost bed in pipes with preserved, downdropped 

bedding just west of Red Rock State Park (figs. 8, 9). Units 

(1) and (2) pinch out westward at about the same point that 

the Todilto Limestone Member pinches out, and the western 

limit of pipes that are easily visible in the resistant unit (4) 

coincides with these pinchouts (fig. 14). We infer that the 

western limit of the former evaporite unit coincided with the 

western limit of the limestone unit of the Todilto Limestone 

Member.

Model of Pipe Origin

The evidence for an evaporite solution-collapse origin 

of the pipes in Jurassic rocks of the southwestern part of the 

San Juan Basin is strong, although not conclusive, and no 

other mode of origin seems plausible. Moreover, we 

consider the evidence from the southeastern part of the 

basin to be compatible with a solution-collapse origin for 

the pipes in that area as well, and we see no reason to favor 

different origins for pipes in the two parts of the basin. We 

now propose a scenario for the development of pipes 

throughout the southern San Juan Basin.

Pipe formation by evaporite solution collapse requires 

that dissolution begins in small areas beneath the future 

pipes. The cause of such localization of dissolution is 

unknown, but upward water flow through spring vents, as

L14 Evolution of Sedimentary Basins San Juan Basin
w

Sec. 9

Western limit of pipes

Sec. 10

Todilto Limestone Member

Entrada Sandstone

2 KILOMETERS

VERTICAL EXAGGERATON x 312

Figure 14. Stratigraphic cross section of the Todilto Limestone Member and lower part of Beclabito Member in and near Red 

Rock State Park (fig. 1). The line of section is near the north edge of sections 9, 10, 11, and 12, T. 15 N., R. 17 W., McKinley 

County, New Mexico. Vertical lines represent measured sections. Numbered units correspond to those in figure 8.

hypothesized by Schlee (1963), may have played a role in 

the localization, although we doubt its direct role in pipe 

formation. A solutional role for upward water flow was 

suggested by Barrington and Kerr (1963) for other solutioncollapse pipes on the Colorado Plateau. On the other hand, 

Anderson and Kirkland (1980) proposed a model for 

localized evaporite dissolution by brine density flow 

through permeable rock beneath an evaporite bed; this 

process eliminates the need for upward water flow through 

sediment overlying the evaporite bed. Whether or not 

localized upward water flow was involved in the initiation 

of the evaporite dissolution that led to pipe formation, the 

pipes probably served as conduits for water movement after 

they formed, and such water movement was undoubtedly 

important in pipe mineralization both in the San Juan Basin 

and elsewhere (Keys and White, 1956; Barrington and Kerr, 

1963; Hawley and others, 1965; Megrue and Kerr, 1965; 

Gornitz and Kerr, 1970; Bowles, 1977; Wenrich, 1985).

As the evaporite gradually dissolved locally, the 

overlying unconsolidated or poorly consolidated sediment 

probably subsided equally gradually because the sediment 

was too weak to allow open cavities to form or to prevent 

upward propagation of the subsidence. The gradual nature 

of the subsidence probably inhibited fracturing of the 

downdropped sediment and thus helped to preserve

stratification of the sediment. Gradual subsidence of unconsolidated sediment simultaneous with dissolution has been 

called on to explain other solution-collapse structures that 

have bedding preserved (Dietrich, 1953).

The degree to which bedding in the downdropped 

sediment was disrupted must have depended in part on the 

distance of downdropping. A larger amount of downdropping in the southeastern part of the basin, where the 

thickness of dissolved evaporite was much greater than it 

was farther west, is probably a major factor accounting for 

the greater rarity of preserved bedding in pipes in the 

southeastern part of the basin. Disturbance of the grain 

framework during downdropping would have caused the 

sediment within the pipe to become more loosely packed, 

and this increase in bulk volume could have compensated 

for the volume lost by evaporite dissolution and led to the 

end of growth of some pipes before they reached the 

surface, as in our experiment 1 (fig. 1 ID).

As the area of evaporite dissolution expanded 

outward from a point directly beneath a pipe, the downdropped material at the base of the pipe tended to spread 

laterally into the potential cavity, producing an amount of 

downdrop greater than the original thickness of the evaporite bed. In the southeastern part of the basin, where the 

evaporite unit was thick, dissolution and pipe formation

Clastic Pipes in Jurassic Rocks, New Mexico L15
generally ceased before the evaporite unit was completely 

removed, but not before a few pipes extended into beds as 

high as the upper part of the Morrison Formation. In the 

southwestern part of the basin, where the evaporite unit is 

interpreted to have been thin and is now absent, the 

processes at work during the final stages of evaporite 

dissolution are not clear. Lateral spreading at the bases of 

pipes probably played some part in preserving the downdrop within the pipes relative to the surrounding beds. In 

addition, the final stages of evaporite dissolution probably 

occurred when the overlying beds were sufficiently lithified 

and rigid for their collapse to take the form of faults that 

were less tightly curved and of much greater lateral extent 

than the faults that bound the pipes.

BROADER IMPLICATIONS

We suspect that some other large pipes previously 

interpreted as water-escape or slurry-injection structures 

may be of solution-collapse origin. In particular, a solutioncollapse origin should be considered for pipes in southern 

Utah studied by Hannum (1980). These pipes have a 

stratigraphic range from the Carmel Formation into the 

overlying Entrada Sandstone and possibly higher. Two 

kinds of evidence that suggest a solution-collapse origin for 

these pipes are (1) the probable occurrence of downdropped 

material within the pipes, as noted by Hannum (1980), and 

(2) the occurrence of gypsum within the Carmel Formation 

in the area of pipe occurrence.

Jurassic rocks of the San Juan Basin contain deformational features other than pipes that may be the result of 

subsidence due to dissolution of the gypsum-anhydrite unit 

that forms the upper part of the Todilto Limestone Member. 

Among such features are (1) broad sags between remnants 

of gypsum in the eastern part of the San Juan Basin 

described by Stapor (1972) and Tanner (1972) and (2) folds 

and associated faults of Jurassic age in the southeastern part 

of the basin described by Moench and Schlee (1967) and 

Hilpert (1969). The folds are in rocks as young as the 

Morrison Formation and are truncated by the unconformity 

at the top of the Morrison. The folds are mapped as 

extending only short distances into the Entrada Sandstone 

and have not been recognized in older formations; they 

probably do not extend down to basement. Many of the 

folds are closely associated with belts of pipes and with 

variations in thickness of the gypsum-anhydrite unit.

The Jurassic folding and faulting affected 

sedimentation in parts of the stratigraphic section from the 

limestone unit of the Todilto Limestone Member to the 

Morrison Formation (Moench and Schlee, 1967). We 

suspect that much of the folding and associated phenomena 

was caused by evaporite dissolution. Such dissolution 

cannot account, however, for folds at the Todilto-Entrada 

contact or for variations in thickness of the Todilto 

limestone unit associated with the folds, because these

features are beneath the evaporite unit. Rather than being 

due to folding, these features may be expressions or effects 

of depositional topography at the top of the Entrada 

Sandstone, similar to the preserved dune topography 

mapped at the top of the Entrada in the subsurface by 

Vincelette and Chittum (1981). Other intraformational 

folding in the limestone unit of the Todilto may be the result 

of differential loading by overlying sediments (Green, 

1982).

CONCLUSIONS

New observations on clastic pipes in Jurassic rocks of 

the southern San Juan Basin indicate that the pipes are 

present as far west as the pinchout of the Todilto Limestone 

Member of the Wanakah Formation in the vicinity of 

Gallup, New Mexico. The pipes in the southwestern part of 

the basin commonly contain downdropped, almost 

unbroken strata identical to strata surrounding the pipes, 

whereas previously studied pipes in the southeastern part of 

the basin typically contain brecciated or homogeneous rock. 

This difference in character probably reflects a difference in 

the amount of downdrop of the pipe-filling material less in 

the southwestern part of the basin and more in the 

southeastern part rather than a difference in pipe origin. 

The less advanced state of pipe development in the 

southwestern part of the basin more readily permits 

interpretation of pipe origin than does the relatively 

advanced stage of pipe development in the southeastern 

part.

The presence of downdropped but otherwise little 

disturbed strata within many pipes in the southwestern part 

of the basin is not easily explained by a spring-vent 

interpretation, the explanation proposed in the most detailed 

study of pipes in the southeastern part of the basin (Schlee, 

1963). We interpret the clastic pipes in the southwestern 

part of the basin to be solution-collapse structures resulting 

from the dissolution of evaporites originally present at the 

top of the Todilto Limestone Member. Some aspects of the 

pipes amounts of downdrop greater than the probable 

original thickness of dissolved evaporites and the restriction 

of easily recognizable downdropping to discrete pipes in 

areas where any original evaporite bed has been entirely 

dissolved remain difficult to explain by a solutioncollapse mechanism. Nevertheless, we regard the overall 

body of evidence as favoring a solution-collapse origin for 

the pipes in the southwestern part of the basin. Moreover, 

we favor the solution-collapse interpretation for the pipes in 

the southeastern part of the basin, and we regard subsidence 

that accompanied dissolution of evaporites as a probable 

cause of folding and faulting of Jurassic rocks in the 

southern San Juan Basin.

L16 Evolution of Sedimentary Basins San Juan Basin
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L18 Evolution of Sedimentary Basins San Juan Basin
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*U.S. GOVERNMENT PRINTING OFFICE: 1992- 673-049/46020 Clastic PipCS in JuraSSk Rocks, NCW Mexico L19

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Geological Survey" should write to the U.S. Geological Survey, 582 National Center, Reston, VA 22092.

Note.-Prices of Government publications listed in older catalogs, 

announcements, and publications may be incorrect Therefore, the prices 

charged may differ from the prices in catalogs, announcements, and publications.