Hydrogeology of Jurassic and Triassic Wetlands 

in the Colorado Plateau and the Origin of 

Tabular Sandstone Uranium Deposits

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1548
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Hydrogeology of Jurassic and Triassic Wetlands 

in the Colorado Plateau and the Origin of 

Tabular Sandstone Uranium Deposits

By Richard F. Sanford

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1548

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1994
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

Published in the Central Region, Denver, Colorado 

Manuscript approved for publication March 10, 1994 

Edited by Judith Stoeser

Graphics prepared by R.F. Sanford and Carol A. Quesenberry 

Photocomposition by Carol A. Quesenberry

For Sale by U.S. Geological Survey, Map Distribution

Box 25286, MS 306, Federal Center

Denver, CO 80225

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

Library of Congress Cataloging-in-Publication Data

Sanford, Richard F.

Hydrogeology of Jurassic and Triassic wetlands in the Colorado Plateau and the origin 

of tabular sandstone uranium deposits / by Richard F. Sanford.

p. cm.—(U.S. Geological Survey professional paper ; 1548) 

Includes bibliographical references. 

Supt.ofDocs.no. : I 19.16: 1548

1. Paleohydrology—Colorado Plateau. 2. Geology, Stratigraphic—Jurassic. 

3. Geology, Stratigraphic—Triassic. 4. Geology—Colorado Plateau. I. Title. 

II. Series

QE39.5.P27S26 1995

553.4'932'097913—dc20 94-15974

CIP
CONTENTS

Abstract................................................................................................................................. 1

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

Description of Tabular Sandstone Uranium Deposits.................................................. 1

Hydrogeological Principles Used in Reconstruction............................................................ 2

Controls on Ground-Water Flow.................................................................................. 2

Topography........................................................................................................... 2

Geology................................................................................................................ 2

Aquifers and Confining Units............................................................................... 3

Composition......................................................................................................... 3

Features of Ground-Water Flow................................................................................... 4

Scale of Ground-Water-Flow Systems................................................................. 4

Compositional Variations..................................................................................... 4

Recharge and Discharge in Fluvial-Lacustrine Environments............................. 4

Transient Effects and Water-Table Variations..................................................... 4

Paleohydrogeology in the Colorado Plateau ........................................................................ 5

Major Controls on Late Jurassic Ground Water........................................................... 5

Topography........................................................................................................... 5

Geology................................................................................................................ 5

Composition......................................................................................................... 7

Salt Wash Sandstone Member of the Morrison Formation.......................................... 7

Recapture Member of the Morrison Formation............................................................ 11

Westwater Canyon Member of the Morrison Formation............................................. 13

Brushy Basin Member of the Morrison Formation...................................................... 15

Jackpile Sandstone Member of the Morrison Formation............................................. 17

Lower Part of the Chinle Formation............................................................................. 19

Topographic Controls on Ground-Water Flow .................................................... 23

Geologic Controls on Ground-Water Flow.......................................................... 23

Diagenesis and Uranium Deposition.................................................................... 25

Origin of Tabular Sandstone Uranium Deposits.................................................................. 26

Role of Humate............................................................................................................. 26

One or Two Solutions?................................................................................................. 27

Seepage, Compaction, and Density.............................................................................. 27

Transient, Depression-Focused Ground-Water Recharge............................................ 29

Summary and Conclusions................................................................................................... 31

References Cited................................................................................................................... 34

FIGURES

1. Diagram showing classification of topographic, geologic, and compositional controls on ground water

showing conditions favorable for discharge................................................................................................................ 3

2. Map showing major geologic features that influenced ground-water flow in the Late Jurassic in the

Colorado Plateau region.............................................................................................................................................. 6

3. Simplified map and cross section of the Colorado Plateau for Late Jurassic time at end of deposition

of the Salt Wash Sandstone Member of the Morrison Formation............................................................................... 8

4. Generalized map showing source rocks for solutes in ground water in the Colorado Plateau region........................ 9

5. Schematic cross section showing influences on ground-water flow in the Colorado Plateau region during the

Late Jurassic at the end of deposition of the Salt Wash Sandstone Member of the Morrison Formation................... 10

III
IV CONTENTS

6. Map showing areas of alteration in mudstone underlying uranium-bearing sandstone and location of

uranium deposits in the northern end of the Uravan mineral belt................................................................................ 11

7. Simplified map and cross section of the San Juan Basin for Late Jurassic time at the end of deposition

of the Recapture Member of the Morrison Formation................................................................................................. 12

8. Map showing contours of sandstone to mudstone thickness ratio in the Westwater Canyon Member of

the Morrison Formation ............................................................................................................................................... 14

9. Simplified map and cross section of the San Juan Basin for Late Jurassic time at the end of deposition

of the Westwater Canyon Member of the Morrison Formation................................................................................... 16

10. Detailed schematic cross section of the Westwater Canyon Member of the Morrison Formation and

underlying units for Late Jurassic time at the end of Westwater Canyon deposition .................................................. 18

11. Map showing contours of the interval of total ilmenite-magnetite dissolution in the upper part of the

Westwater Canyon Member of the Morrison Formation ............................................................................................. 19

12. Maps showing interpretations of the diagenetic patterns in the Brushy Basin Member of the

Morrison Formation..................................................................................................................................................... 20

13. Schematic map of Colorado Plateau area showing diagenetic alteration in Brushy Basin Member of the

Morrison Formation, zone of mixed local and regional discharge at distal edge of alluvial plain, and location

of tabular sandstone uranium deposit clusters in the Morrison Formation.................................................................. 21

14. Detailed schematic cross section of Brushy Basin Member and underlying units for Late Jurassic time

at the end of Brushy Basin deposition in the Colorado Plateau area........................................................................... 22

15. Isopach map of the thickness of the Jackpile Sandstone Member of the Morrison Formation showing facies

of underlying Brushy Basin Member, location of uranium deposits, and directions of paleoflow.............................. 23

16. Map showing relationship between uranium deposit clusters and ground-water discharge in the

Colorado Plateau during deposition of the lower part of the Triassic Chinle Formation ............................................ 24

17. Map showing relationship between uranium deposits, syndepositional synclines, and organic-rich

lacustrine mudstone in part of the Chinle Formation in southeast Utah...................................................................... 25

18. Schematic cross section across a topographic depression occupied by a channel or lake showing

relationships of hydrology and humate........................................................................................................................ 30
Hydrogeology of Jurassic and Triassic Wetlands

in the Colorado Plateau and the 

Origin of Tabular Sandstone Uranium Deposits

By Richard F. Sanford

ABSTRACT INTRODUCTION

During parts of the Jurassic and Triassic Periods, fluviallacustrine sediments were deposited in the area of the Colorado Plateau, and tabular sandstone (tabular-type) uranium 

deposits formed at a density-stratified ground-water interface 

in areas of regional ground-water discharge. The typical 

effects of topographic, geologic, and compositional controls 

on ground-water flow, together with modern ground-water 

analogs, can be used to reconstruct ground-water flow during 

and shortly after sedimentation. Diagenetic effects of the passage of ground water can also provide further constraints. In 

the Upper Jurassic Morrison Formation and lower part of the 

Upper Triassic Chinle Formation, tabular sandstone uranium 

deposits are in lenticular, arkosic, fluvial, channel sandstone 

overlain by tuffaceous overbank and lacustrine mudstone and 

underlain by marine rocks, especially evaporites. The thin, 

subhorizontal, tabular-shaped bodies float in the sandstone 

with no apparent lithologic control. They are in reduced sandstone within dominantly redbed sequences of sandstone and 

mudstone. The deposits favor transitional facies of interbedded sandstone and mudstone between dominantly fluvial 

sandstone and dominantly overbank and lacustrine mudstone. 

They tend to be concentrated in syndepositional synclines. 

Organic matter, either as detrital coalified plant fragments or 

structureless impregnations, is closely associated with uranium ore. Decreases in paleotopographic slope are indicated 

by lithofacies changes, typically from distal alluvial plain to 

mudflat or floodplain. Geologic controls focused discharge in 

zones of abrupt thinning and pinching out of Lower Jurassic 

and upper Paleozoic aquifer systems. Tabular sandstone uranium deposits formed where topographic controls favored 

mixed local and regional discharge. Transient, depressionfocused recharge of humic-acid-bearing ground water at wetlands in paleotopographic depressions may have provided a 

mechanism for downward and downdip transport of humic 

acid that precipitated at a subhorizontal, density-stabilized 

interface between relatively fresh, shallow ground water and 

discharging, saline regional flow. Uranium was precipitated 

during and after humate precipitation.

Previous studies suggest that tabular sandstone (tabulartype) uranium deposits formed in areas of ground-water discharge (Sanford, 1982, 1988, 1990a, b, 1992), and quantitative models for selected areas support the hypothesis 

(Sanford, 1982, 1990a, 1994). In this study, I reconstruct the 

hydrogeology of the Colorado Plateau during parts of the 

Jurassic and Triassic Periods when fluvial-lacustrine sediments and tabular bodies of uranium ore were being deposited. I examine additional evidence pertaining to groundwater flow, extend the earlier studies to other areas of uranium deposits on the Colorado Plateau, and reconcile evidence for ground-water paleodischarge and evidence of 

ground-water paleorecharge at the site of uranium deposition.

Hydrologic principles, observation of modern groundwater-flow systems, and results of quantitative modeling 

help predict the flow of ancient ground water. Hydrologic 

controls are then combined with stratigraphic, sedimentologic, paleoenvironmental, and structural data from the Colorado Plateau to construct conceptual models. The effects of 

the passage of ground water based on diagenetic evidence 

further constrain the model.

Previous hydrogeologic work on the Colorado Plateau 

typically focused on surface water or shallow ground water 

but neglected the large-scale flow of ground water in the 

basin as a whole. Paleotopography was not fully utilized in 

paleohydrogeologic reconstruction. In this paper, I incorporate concepts previously proposed to provide an integrated 

model for basin- and local-scale ground-water flow in the 

Colorado Plateau as a whole.

DESCRIPTION OF TABULAR SANDSTONE 

URANIUM DEPOSITS

Tabular sandstone uranium deposits are tabular, originally subhorizontal bodies entirely within reduced fluvial 

sandstone of Late Silurian age or younger. Tabular sandstone uranium deposits constitute the largest uranium
JURASSIC AND TRIASSIC WETLANDS AND ORIGIN OF URANIUM DEPOSITS

resource type in the United States. They account for approximately 65 percent of the total United States production and 

reserves of uranium (Chenoweth and McLemore, 1989). 

Throughout this paper, the terms "reduced" and "oxidized" 

refer to iron unless otherwise noted.

The general features of tabular sandstone uranium 

deposits are summarized elsewhere (Finch, 1967; Fischer, 

1974; Adams and Saucier, 1981; Nash and others, 1981; 

Thamm and others, 1981; Finch and Davis, 1985; Granger 

and Finch, 1988; Shawe and others, 1991; Sanford, 1992). 

Typically, tabular sandstone uranium deposits are in lenticular, arkosic, fluvial, channel sandstone overlain by tuffaceous overbank and lacustrine mudstone and underlain by 

marine rocks, especially evaporites. The thin, subhorizontal, 

tabular-shaped bodies "float" in the sandstone with no apparent lithologic control. They are in reduced sandstone within 

dominantly redbed sequences of sandstone and mudstone. 

The deposits favor transitional facies of interbedded sandstone and mudstone between dominantly fluvial sandstone 

and dominantly overbank and lacustrine mudstone. They 

tend to be concentrated in syndepositional synclines. 

Organic matter, either as detrital coalified plant fragments or 

structureless impregnations, is closely associated with uranium ore. The age of the host rocks ranges from Late Silurian 

to Tertiary, the older age limit corresponding to the first 

appearance of woody land plants. The deposits probably 

formed shortly after deposition of the host sandstone. In the 

Colorado Plateau, the largest tabular sandstone uranium 

deposits are in sandstone of the Upper Jurassic Morrison 

Formation and Upper Triassic Chinle Formation.

HYDROGEOLOGICAL PRINCIPLES 

USED IN RECONSTRUCTION

Here I consider both the controls that govern the flow of 

ground water and the effects of the passage of ground water. 

The controls are described by the general principles of physical hydrogeology, and they lead to predictions that can be 

tested by looking for evidence of the effects of the passage 

of ground water.

CONTROLS ON GROUND-WATER FLOW

Controls on ground water flow are topographic, geologic, and compositional. Topographic controls are imposed 

by the configuration of the water table and, indirectly, the 

ground surface. Geologic controls are imposed by the distribution of permeability and thickness of units in the saturated 

zone. Compositional controls are imposed by masses of 

water having different density due to salinity. Darcy's Law 

provides a framework for classifying these controls (fig. 1) 

(Hitchon, 1969; Winter, 1988). The volumetric flow rate (g)

in saturated porous media is the product of three parameters: 

hydraulic conductivity (K), cross-sectional area normal to 

flow (A), and gradient in hydraulic potential (d/z/d/) (Hubbert, 1940, 1953, 1956).

Although distinguishable conceptually, ground-water 

controls may be correlated. Finer sediments commonly are 

associated with both lower topographic slope and lower permeability. Thinning of aquifers may be accompanied by a 

decrease in permeability owing to an increase in the proportion of fine-grained sediments.

TOPOGRAPHY

In mature, topographically uplifted basins, such as 

those of the Colorado Plateau, gravity-driven flow predominates; recharge occurs at higher elevations, whereas discharge occurs at lower elevations (Kreitler, 1989). For an 

unconfined aquifer at the surface, a decrease in the slope of 

the water table results in a lower potential gradient and 

smaller flow rate (0 through the downstream end of the 

medium, which causes discharge at the decrease in slope. 

The most common expression of this case is at decreases or 

breaks in slope, typically where an alluvial fan or plain 

meets an almost flat valley bottom (Freeze and Witherspoon, 1967; McLean, 1970; Habermehl, 1980; Allison and 

Barnes, 1985; Duffy and Al-Hassan, 1988; Straw and others, 1990; Winter and Woo, 1990). A special case of a concave break in slope that focuses discharge is at the shoreline 

where the water table meets a standing body of water (see, 

for example, Pfannkuch and Winter, 1985; Cherkauer and 

Nader, 1989). Wetlands typically are present in areas of 

perennial discharge.

In this study, I assume that paleotopographic highs 

were recharge areas, that paleotopographic depressions were 

discharge areas, and that discharge occurred at major 

decreases in slope due to a decrease in dh/dl (fig. 1). Thus, 

the largest scale topographic controls on ground-water flow 

are indicated by the major highs in orogenie zones and the 

major depressions in sedimentary basins. Following the theory of Toth (1962, 1963), it is assumed that the deepest 

regional flow discharges at the lowest elevations and that 

shallower flow discharges farther upslope. The distal edge of 

the alluvial plain, or transition from alluvial plain to mudflat, 

is expected to be a zone of discharge and mixing of shallow 

local and deeper regional ground water (McLean, 1970; 

Duffy and Al-Hassan, 1988; Straw and others, 1990).

GEOLOGY

Both a facies change resulting in lower permeability 

and a thinning of hydrostratigraphic units in the downstream 

direction favor discharge of ground water and the presence 

of wetlands (fig. 1) (Freeze and Witherspoon, 1966, 1967; 

Garven and Freeze, 1984a, b; Bethke, 1989; Kreitler, 1989). 

The thinning may involve just one aquifer within a basin or
HYDROGEOLOGICAL PRINCIPLES USED IN RECONSTRUCTION

GROUND-WATER 

CONTROLS

FAVORABLE FOR 

DISCHARGE

DARCY'S LAW: Q = KA dh/dl

Topography

Geology 

Permeability 

Thickness

Composition

Concave change in slope 

Decrease in transmissivity 

Decrease in permeability 

Thinning of aquifer 

Thinning of fresh-water lens

Decrease in: 

dh/dl

K 

A 

A

Figure 1. Classification of topographic, geologic, and compositional controls on ground water showing 

conditions favorable for discharge. Q is volumetric flow rate; Kis hydraulic conductivity; A is cross-sectional 

area normal to flow; dh/dl is gradient of hydraulic potential (head) with distance.

the basin as a whole. For the Colorado Plateau in Mesozoic 

time, geologic controls on ground-water flow are reconstructed from permeability data and stratigraphic evidence.

AQUIFERS AND CONFINING UNITS

Ground water in horizontally stratified sedimentary 

basins tends to flow laterally through aquifers and vertically 

through confining units (Hubbert, 1940; Toth, 1978; Kreitler, 

1989). The notion that fine-grained or clay-rich units are 

impermeable continues to be expressed by some geologists, 

even though it was understood more than 90 years ago to be 

a "time-honored delusion" (Munn, 1909). Flux across confining units can be significant; the lower permeability (K, fig. 

1) of a confining unit is compensated by the cross-sectional 

area (A), which is orders of magnitude greater when flow is 

perpendicular to bedding, and by a higher potential gradient 

(dh/dl) that may exist across the confining unit. For example, 

upward flux through the bed of Lake Frome, Australia, is 

170-180 mm/yr (Allison and Barnes, 1985).

COMPOSITION

A gravitationally stable body of saline water may force 

fresher water flowing over it to discharge (fig. 1). (By definition, saline water has 1,000-35,000 mg/L total dissolved 

solids, and brine has >35,000 mg/L total dissolved solids 

(Robinove and others, 1958).) Some surface water and much 

ground water is saline (Hanor, 1983; Hammer, 1986; 

Kreitler, 1989). Saline bodies of water, including salt lakes 

and the oceans, typically receive inflow from more dilute 

surface and ground water. Buoyancy forces cause less dense 

fresher water to overlie more dense salt water in the subsurface. At equilibrium, the fresh water-salt water interface is 

horizontal. When the upper fluid is moving, the interface 

slopes upward in the direction of movement (Cooper and 

others, 1964). The salt-water wedge acts as a barrier to the

flow of relatively fresh water, which tends to discharge at the 

surface more or less at a shoreline or other break in slope 

(McLean, 1970; Duffy and Al-Hassan, 1988; Button and 

others, 1989; Fee and others, 1992; Herczeg and others, 

1992; Hines and others, 1992; Long and others, 1992; 

Macumber, 1992). In topographic depressions where the 

water table slopes inward and ground-water flow converges, 

the hydraulic potential for shallow ground water is least at 

the depression (Hubbert, 1940). The salt water-fresh water 

interface must therefore rise toward the depression, even if 

salt water fails to discharge at the surface.

Compositional controls on ground-water flow cannot 

be reconstructed in ancient basins with as much certainty as 

topographic and geologic controls because the original fluids 

have disappeared. The chemical evolution of the ground 

water can be inferred, however, from the composition of the 

rocks through which the water passes. Because evaporite 

minerals dissolve rapidly, ground water that passes through 

evaporites or evaporite-cemented sediments becomes saline, 

as demonstrated by salinities as high as 439,000 ppm in modern basins, including the Paradox Basin (Mayhew and Heylman, 1965; Hanor, 1983; Kreitler, 1989). The contribution of 

evaporites to saline ground water is well documented (Gallaher and Price, 1966; Boswell and others, 1968; van Everdingen, 1971; Anderson and Kirkland, 1980; Johnson, 1981; 

Fisher and Kreitler, 1987; Banner and others, 1989; Button 

and others, 1989). Evaporites are not required, however, for 

the presence of brines; brines can evolve by interaction of 

rock and normal or evaporatively concentrated sea water 

originally trapped during sedimentation (Fisher and Kreitler, 

1987; Dutton, 1987) or by reverse chemical osmosis (Bredehoeft and others, 1963; Coplen and Hanshaw, 1973; Graf, 

1982). Thus, in reconstructing ancient ground-water flow, 

the fact that ground water flowed through or around evaporitic horizons is strong evidence for saline ground water, but 

flow through marginal-marine rocks deposited in an arid or 

sabkha environment also suggests saline water. A close
JURASSIC AND TRIASSIC WETLANDS AND ORIGIN OF URANIUM DEPOSITS

modern analog to ancient saline water movement is the dissolution of the Middle Jurassic Carmel Formation evaporites, transport of saline water downdip in the underlying 

Lower Jurassic Navajo Sandstone, and upward discharge 

from the Navajo (Taylor and Hood, 1988).

FEATURES OF GROUND-WATER FLOW

The features discussed here are commonly observed 

characteristics of ground-water flow in sedimentary basins 

analogous to ancient basins such as those of the Colorado 

Plateau.

SCALE OF GROUND-WATER-FLOW SYSTEMS

The features of ground-water-flow systems vary with 

the scale of the system, which is generally classified as 

regional, intermediate, or local (Toth, 1962,1963). Regional 

flow recharges at major drainage divides and discharges at 

basin floors. Intermediate flow recharges and discharges in 

areas that are separated by one or more local topographic 

highs and lows.

COMPOSITIONAL VARIATIONS

Toth (1962, 1963) predicted that chemical contrasts 

would be greatest across the boundaries between flow systems of different magnitude as a result of the different flow 

paths. In fact, numerous modern examples demonstrate that 

diverse types of ground water from local, intermediate, and 

regional flow systems converge in mixed discharge zones 

(Counts, 1957; Toth, 1963; Freeze and Witherspoon, 1966; 

Gallaher and Price, 1966; Boswell and others, 1968; van 

Everdingen, 1971; Foreman and Sharp, 1981; Mono Basin 

Ecosystem Study Committee, 1987; Sharp, 1988; Swanson 

and others, 1988; Banner and others, 1989; Dutton and others, 1989; Whittemore and others, 1989; Huff, 1990; Fee 

and others, 1992; Herczeg and others, 1992; Hines and others, 1992; Long and others, 1992; Macumber, 1992; Strobel, 

1992). The local flow system typically discharges dilute 

water, whereas the intermediate and regional flow systems 

discharge saline water of diverse compositions, commonly 

within a relatively small area. For example, in the Cascade 

Range, north-central Oregon, two springs only meters apart 

vary from less than 100 to 8,000 mg/L total dissolved solids 

(Ingebritsen and others, in press). Both springs are in a 

regional discharge area, but the saline spring is fed by the 

regional flow, and the fresh-water spring is fed by local 

flow. Such compositional contrasts at the surface clearly 

indicate the presence of steep compositional gradients, or 

ground-water interfaces, in the subsurface. Such interfaces 

may be widespread in the shallow subsurface (Runnells, 

1969) and are observed in modern coastal environments 

(Cooper and others, 1964), in geothermal areas (Williams

and McKibben, 1989) in unconsolidated alluvium beneath 

large rivers (Counts, 1957; Gallaher and Price, 1966; 

Boswell and others, 1968; Foreman and Sharp, 1981; Sharp, 

1988), and where rivers erode exposed evaporites (Warner 

and others, 1985).

A variety of diagenetic reactions occur at such interfaces, including precipitation of humate (Swanson and Palacas, 1965; Hair and Bassett, 1973; Sholkovitz, 1976; Ortiz 

and others, 1980; Fox, 1983; Thurman, 1985, p. 26ff and 

394ff), precipitation of dolomite (Ward and Halley, 1985), 

precipitation of calcite (Mono Basin Ecosystem Study Committee, 1987), dissolution of calcite (Plummer, 1975), and 

other reactions (Magaritz and Luzier, 1985; Randazzo and 

Bloom, 1985; Hines and others, 1992). In the Colorado Plateau, tabular layers of humate, uranium, and dolomite suggest former interfaces (Fischer, 1947, 1974; Shawe, 1955, 

1962, 1966,1976; Granger and others, 1961; Granger, 1968; 

Wood, 1968; Melvin, 1976; Sanford, 1982, 1990a, b, 1992; 

Granger and Santos, 1986; Northrop and Goldhaber, 1990; 

Wanty and others, 1990; Hansley and Spirakis, 1992).

RECHARGE AND DISCHARGE IN 

FLUVIAL-LACUSTRINE ENVIRONMENTS

Recharge and discharge in complex present-day lakegroundwater systems (Meyboom, 1967; Lissey, 1971; 

Stephenson, 1971; Winter, 1976, 1978, 1986, 1988) are 

likely analogs for such systems in the ancient Colorado Plateau. Lakes always are in topographic depressions, and the 

water table normally slopes toward the lake, causing discharge of ground water to the lake. Lakes at higher elevation may recharge the ground-water system, but the lake at 

the lowest elevation typically only receives ground-water 

discharge and contributes no recharge (Winter, 1976). A 

flow-through lake may have discharge on the upgradient 

side and recharge on the downgradient side. Greater width 

of the lake relative to the thickness of the underlying aquifer 

favors increased focusing of ground-water discharge around 

the lake margins (Pfannkuch and Winter, 1984). At play a 

lakes, relatively fresh water discharges around the shoreline, 

and saline water recharges or discharges in the center 

(Friedman and others, 1982; Allison and Barnes, 1985; 

Spencer and others, 1985; Duffy and Al-Hassan, 1988; 

Winter and Woo, 1990).

TRANSIENT EFFECTS AND WATER-TABLE 

VARIATIONS

The water-table level fluctuates as a result of fluctuations in precipitation (Lissey, 1971; Winter, 1983). Three 

zones are distinguishable: (1) the permanently saturated zone 

beneath the steady-state position of the water table, (2) the 

transiently saturated zone above the steady-state water-table 

level but below the permanently unsaturated zone, and (3) 

the permanently unsaturated zone. Water-table fluctuations
PALEOHYDROGEOLOGY IN THE COLORADO PLATEAU

leave traces in the authigenic mineralogy of the sediments. 

The most conspicuous and easily mappable feature in ancient 

sedimentary rocks is color variation due to the oxidation state 

of iron. Red, yellow, and buff indicate oxidized iron, whereas 

green, gray, and black indicate reduced iron. Oxidized and 

reduced sedimentary rocks have been related to paleo-watertable level in the Colorado Plateau and elsewhere (Walker, 

1967; Reading, 1978, p. 48-49; Dodson and others, 1980; 

Huber, 1980; Dubiel, 1983, 1989; Davis, 1988; Ghiorse and 

Wilson, 1988; Dubiel and others, 1991). Water-table level 

has been related to redox conditions in analogous modern 

systems (Jackson and Paterson, 1982; Fee and others, 1992; 

Hines and others, 1992; Long and others, 1992; Macumber, 

1992). In this study, I assume by analogy that most of the permanently saturated zone tends to be reducing where organic 

matter was deposited, the transiently saturated zone and 

upper part of the permanently saturated zone have mottled or 

variegated sediments, and the permanently unsaturated zone 

has oxidized sediments.

Transient, depression-focused recharge to the groundwater system may occur during high-water periods and may 

constitute a major source of shallow, relatively fresh ground 

water at topographic depressions (Gallaher and Price, 1966; 

Lissey, 1971; Winter, 1983; Wood and Petraitis, 1984; Fetter, 1988, p. 45ff; Logan and Rudolph, 1992). During periods 

of high water, topographic depressions fill with surface 

water that then migrates into unsaturated, porous sediments 

of the stream bank or lake shore. This newly infiltrated 

ground water is slowly released to the ground-water system. 

In the Southern High Plains of Texas, for example, most of 

the recharge is focused at such topographic depressions 

(Wood and Petraitis, 1984). In the coastal plain of Argentina, 

fresh water recharges in marshes that are only 0.5-1.0 m 

below the surrounding land surface and overlies regional 

saline ground water (Logan and Rudolph, 1992). This phenomenon provides an explanation for apparent downward 

flow of ground water in areas of regional discharge in the 

ancient Colorado Plateau, as discussed following.

PALEOHYDROGEOLOGY IN THE 

COLORADO PLATEAU

MAJOR CONTROLS ON LATE JURASSIC 

GROUND WATER

TOPOGRAPHY

The drainage divides marking the southern and western 

margins of the drainage basin (Mogollon and Elko highlands, respectively, fig. 2) (Peterson, 1984, 1986, 1988a, in 

press; Thorman and others, 1990, 1991) during deposition of 

the Morrison Formation must have been beyond the presentday margins of the Colorado Plateau, perhaps in southern 

Arizona or northern Mexico (Billodeau, 1986) and Nevada.

Paleocurrent directions of streams in the Morrison Formation (Craig and others, 1955; Mullens and Freeman, 1957; 

Young, 1978; Dodson and others, 1980; Tyler and Ethridge, 

1983; Peterson, 1984,1986, in press; Turner-Peterson, 1986) 

indicate that the regional topographic slope, and probably 

the regional ground-water flow, was mainly northeastward. 

Intermediate and local flow systems were undoubtedly 

affected by tectonically active structures within the Colorado 

Plateau. Throughout deposition of the Morrison Formation, 

the Elko and Mogollon highlands probably were regional 

recharge areas (fig. 2).

GEOLOGY

The most important geologic controls on ground-water 

flow in the Late Jurassic were buried Precambrian blocks 

and Phanerozoic eolian sandstone and carbonate aquifers 

(fig. 3) (Jobin, 1962; Sanford, 1982, 1990a; Stone and others, 1983; Freethey and Cordy, 1991; Geldon, in press). 

Northeastward from the middle of the Colorado Plateau, in 

the direction of ground-water flow, sedimentary rocks 

beneath the Morrison Formation thin from as much as 5,000 

m (15,000 ft) to a thin veneer over the Uncompahgre and San 

Luis blocks, remnants of the Pennsylvanian-Permian ancestral Rocky Mountains, which formed a hydrologic barrier. 

Northeast of these blocks, sediments again thicken in the 

Eagle and Piceance Basins. The ancestral Front Range block 

formed a second barrier behind these basins. Ground water 

must have been forced around the ends and over the top of 

the barriers where it discharged. Similar flow of ground 

water around the Uncompahgre block takes place today in 

Devonian and Mississippian carbonate rocks (Taylor and 

Hood, 1988).

Major aquifer systems in the Colorado Plateau are the 

lower Paleozoic, upper Paleozoic, and Lower Jurassic aquifer systems (fig. 3) (Jobin, 1962, 1986; Sanford, 1982, 

1990a; Taylor and others, 1986; Freethey and Cordy, 1991; 

Geldon, in press). The lower Paleozoic aquifer system 

includes the karstic Mississippian Leadville Limestone and 

correlative Redwall Limestone. The upper Paleozoic aquifer 

system includes the eolian Cedar Mesa Sandstone Member 

of the Cutler Formation, Coconino Sandstone, De Chelly 

Sandstone, Glorieta Sandstone, Meseta Blanca Sandstone 

Member of the Yeso Formation, Weber Sandstone, and 

White Rim Sandstone Member of the Cutler Formation. The 

Cedar Mesa Sandstone Member is the most transmissive of 

these units, and the arkosic Cutler Formation, with which the 

eolian sandstone units intertongue, is only an aquifer locally 

(Geldon, in press). The Lower Jurassic aquifer system 

includes the eolian Lower Jurassic Wingate and Navajo 

Sandstones. In general, the eolian sandstone aquifers thin 

and the percentage and thickness of confining units thicken 

to the northeast. Of the eolian sandstones, the thickest parts 

of Lower Jurassic Navajo Sandstone were the most transmissive and most variable hydrostratigraphic unit on the
JURASSIC AND TRIASSIC WETLANDS AND ORIGIN OF URANIUM DEPOSITS

109°

WYOMING

37°

PICEANCE AND 

EAGLE BASINS

fy~~~ SAN RAFAEL 

^-""TOPOGRAPHIC HIGH

100 KILOMETERS

Figure 2. Map showing major geologic features, both active tectonic structures and topographic nonstructural features, that influenced ground-water flow in the Late Jurassic in the Colorado Plateau region. Shaded area is area of present-day Colorado Plateau. 

Outward-pointing arrows are anticlines, inward-pointing arrows are synclines, and dashed arrows indicate generalized direction of surface- and ground-water flow. Modified from Craig and others (1955), Mallory (1972), Peterson (1984, 1986, 1988a, in press), and 

Turner-Peterson (1986).
PALEOHYDROGEOLOGY IN THE COLORADO PLATEAU

Colorado Plateau. The decrease in thickness of the Navajo 

Sandstone from more than 600 m (2,000 ft) to a feather edge 

across the Colorado Plateau (Jobin, 1962; Blakey and others, 

1988) probably determined the major discharge zone during 

Late Jurassic time. The pinchout of the Navajo Sandstone, 

from 150 m (500 ft) to zero (Jobin, 1962; Blakey and others, 

1988), forms an arcuate band with its main axis oriented 

north-south and bowed to the east (fig. 3). The underlying 

Wingate Sandstone and overlying Entrada Sandstone also 

tend to thin to the northeast; however, the change in their 

thicknesses is much less than that of the Navajo Sandstone 

(Jobin, 1962; Blakey and others, 1988). The upper Paleozoic 

aquifer system pinches out west of the Navajo pinchout in 

the Paradox Basin area but not in the San Juan Basin area, 

where the Navajo is absent. The highly transmissive Leadville and Redwall Limestones probably were important for 

regional flow but may have been too deeply buried under 

confining units to influence near-surface ground-water flow 

in Triassic and Jurassic time.

Thickness variations suggest that northeast-flowing 

regional ground water generally flowed from the upper Paleozoic aquifer system upward into the base of the Lower Jurassic aquifer system (fig. 3). This flow then discharged upward 

from the Lower Jurassic aquifer system where it thinned to 

the northeast. Because the Navajo Sandstone thins so dramatically, it probably was a major source of upward groundwater discharge. Thinning of the Wingate and Entrada Sandstones, as well as thinning of the whole sedimentary section 

over the buried Uncompahgre block, contributed to discharge 

beyond the pinchout of the Navajo Sandstone. North and east 

of the Uncompahgre block, the eolian Middle and Upper 

Pennsylvanian and Lower Permian Weber Sandstone of the 

upper Paleozoic aquifer system favored recharge and focusing of ground water in a northerly direction around the Front 

Range block into central Wyoming.

COMPOSITION

Judging from the widespread distribution of evaporites 

and evaporitic clastic rocks throughout the Colorado Plateau 

(fig. 4), regional and intermediate ground water was probably saline toward the distal ends of flow paths. The deepest 

regional flow penetrated Pennsylvanian and Permian 

evaporites. This ground water probably approached halite 

saturation and contained significant Na+, K+, Ca2+, CF, and 

SO42~ from the dissolution of halite, sylvite, anhydrite, and 

gypsum. Analysis of modern ground water whose solutes 

came from these evaporites indicates that the ancient brines 

were probably Na+-CP type (Hanshaw and Hill, 1969; 

Thackston and others, 1981; Sanford, 1990a). Sulfur isotopes from authigenic barite cement are consistent with the 

transport of Pennsylvanian sulfate up to the Upper Jurassic 

Morrison Formation (Breit and others, 1990). Ground water 

at intermediate depths flowed through Jurassic and Triassic 

evaporites (fig. 4). Gypsum in the Lower and Middle(?)

Triassic Moenkopi Formation (Blakey, 1 974), Middle Jurassic Curtis Formation (Peterson, 1988a), Middle Jurassic 

Todilto Limestone Member of the Wanakah Formation 

(Ridgley, 1989), and Upper Jurassic Tidwell Member of the 

Morrison Formation (Peterson, 1988a) suggests that intermediate ground water below and in the lower part of the 

Morrison Formation was saline.

Local ground water that remained in the uppermost 

layer of alluvium was probably a dilute Na+-Ca2+-HCO3~ 

solution, judging from analysis of modern ground water in 

the Colorado Plateau and elsewhere (Hanshaw and Hill, 

1969; Thackston and others, 1981; Davis, 1988; Sanford, 

1990a). This relatively fresh water probably rested on the 

saline water formed by passage through Mesozoic and older 

evaporites and marine sediments.

SALT WASH SANDSTONE MEMBER OF THE 

MORRISON FORMATION

Owing mainly to local paleotopographic variations, the 

different members of the Morrison Formation exhibited distinct differences in ground-water flow. The Salt Wash Sandstone Member was deposited on a broad fan-shaped alluvial 

plain that included, from southwest to northeast, dominantly 

low sinuosity, sand-dominated stream deposits; high-sinuosity, mud-dominated, floodplain deposits; and lacustrine 

mudstone-limestone deposits in the undifferentiated Morrison Formation (fig. 3) (Craig and others, 1955, Mullens and 

Freeman, 1957; Young, 1978; Galloway, 1979; Dodson and 

others, 1980; L.C. Craig, U.S. Geological Survey, unpublished data, 1982; Tyler and Ethridge, 1983; Peterson, 1984, 

1986, in press; Peterson and Tyler, 1985; Peterson and 

Turner-Peterson, 1987). L.C. Craig (unpublished data, 

1982) estimated the slope of the alluvial plain at from 

1:1,000 to 1:2,000.

During deposition of the Salt Wash Sandstone Member, 

ground water probably discharged at wetlands in topographic depressions in the Henry Basin, Uravan mineral belt, 

and Carrizo Mountains area (figs. 3, 5). A variety of evidence, including the transition from low- to high-sinuosity 

channels, decrease in thickness of sandstone, decrease in 

sandstone as a percent of total thickness, increase in mudstone thickness and percentage, transition from sandstonemudstone to claystone-limestone facies, and transition from 

fluvial sheet gravels to mudflat-lake muds (Craig and others, 

1955; Mullens and Freeman, 1957; Peterson and Tyler, 

1985), suggests a major decrease in topographic slope at the 

distal edge of the alluvial plain. The greatest concentration of 

uranium deposits in the Salt Wash Member is where stream 

deposits in the Salt Wash Member thin from 60 to 30 m 

(200-90 ft) (Craig and others, 1955) and where the Navajo 

Sandstone thins from 150 m (500 ft) to zero (fig. 3).
NEVADA_ 

UTAH

on on

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D H

C/3

on 

OHr>2;

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on

D

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on
PALEOHYDROGEOLOGY IN THE COLORADO PLATEAU

EXPLANATION

Salt Wash Sandstone Member 

Navajo Sandstone 

Precambrian block 

Zone of mixed discharge

Uranium deposit cluster

Thickness of channel deposits in Salt Wash Member (in feet)

Thickness of Navajo Sandstone (in feet)

Pinchout of Wingate Sandstone (east edge)

— - — - — - Edge of Precambrian block

Figure 3 (above and facing page). Simplified map and cross section of the Colorado Plateau for Late Jurassic time at end of deposition of the Salt Wash Sandstone Member of the Morrison Formation. 

Thickness of stream deposits in the Salt Wash Member, total thickness of the underlying Lower Jurassic Navajo Sandstone, and location of buried Precambrian blocks indicate areas of ground-water 

discharge. Arrows on map and cross section show direction of 

ground-water flow. Areas in cross section delineated by boxes are 

shown in detail in figure 5. Dotted line in cross section is dilute-saline 

interface. The Lower Jurassic aquifer system is composed of the 

Wingate and Navajo Sandstone aquifers. Modified from Craig and 

others (1955), Baars (1962), Jobin (1962), Mallory (1972), Peterson 

(1984, 1986, 1988a, in press), and Blakey and others (1988).

The Henry Basin was an area of decreased topographic 

slope as indicated by thicker sediments, more channel sandstone, higher sinuosity channels, increased upper flow 

regime horizontal laminations, and lacustrine muds tone 

(Peterson, 1984, 1986). Topographic depressions within the 

Henry Basin are suggested by thick sediments, evaporite 

deposits, lacustrine deposits, repetition of facies, and coincidence of synclines with present-day synclines (F. Peterson, 

1980, 1984). Tabular sandstone uranium-vanadium deposits 

are associated with topographic depressions, actively subsiding synclines, and reduced, carbon-bearing lake deposits, and 

they slope upward stratigraphically in the downstream flow 

direction of surface water (F. Peterson, 1980; Northrop and 

Goldhaber, 1990). The tabular ore layers and diagenetic 

alterations have been interpreted as suggesting two fluids 

with a mixing zone between them (Northrop and Goldhaber, 

1990; Wanty and others, 1990). Although some of the specific reactions proposed by Northrop and Goldhaber (1990) 

and Wanty and others (1990) are questionable (Spirakis, 

1991; Hansley and Spirakis, 1992), the tabular nature of the 

uranium-vanadium bodies strongly suggests a densitystabilized interface. The tabular shape, upward slope, and 

association with paleotopographic depressions are consistent 

with a saline water interface that arches upward at the topographic depression (fig. 5A).

100 MILES

_I

50

I 

100 KILOMETERS

EXPLANATION

.-..--- Area and boundary of 

Jurassic source rocks

• — - — - Area and boundary of 

Triassic source rocks

- — — — Area and boundary of 

Pennsylvanian and 

Permian source rocks

Figure 4. Generalized map showing source rocks for solutes in 

ground water in the Colorado Plateau region. Source rocks include 

halite-sylvite-bearing evaporites, anhydrite-gypsum evaporites, and 

clastic rocks containing anhydrite-gypsum beds. Pennsylvanian and 

Permian source rocks include Paradox Formation, Supai Formation, 

Cedar Mesa Sandstone Member of the Cutler Formation, San Andres Limestone, and Yeso Formation. Triassic source rocks include 

Moenkopi Formation. Jurassic source rocks include Carmel Formation, Todilto Limestone Member of the Wanakah Formation, and 

Tidwell Member of the Morrison Formation. Data from Baars 

(1962), Mallory (1972), Blakey (1974, 1980), J.A. Peterson (1980), 

F. Peterson (1984, 1988a), and Ridgley (1989).
10 

SOUTHWEST

JURASSIC AND TRIASSIC WETLANDS AND ORIGIN OF URANIUM DEPOSITS

NORTHEAST

Summerville Formation

.• •." . -' •''. -. '. .'. Navajo Sandstone'/'.': •.' •.'• •'' .' .'! •.''. •/'.' : •; ''••^-^

EXPLANATION

Aquifer Anhydrite-gypsum rock 

Steady-state water table 

Flow system divide (interface) 

Stagnation point

Oxidized (red-yellow) mudstone 

and siltstone _ _ _

Reduced (gray-green) mudstone < 

and siltstone

Tabular uraniun-vanadium 

deposit

Figure 5. Schematic cross section showing influences on ground-water flow in the Colorado Plateau region during the Late Jurassic at the 

end of deposition of the Salt Wash Sandstone Member of the Morrison Formation (not to scale). Location of areas are delineated by boxes 

on cross section of figure 3. No vertical scale implied. Flow lines, flow-system divides, and stagnation points drawn according to the theory 

of Toth (1962), Hitchon (1969), Winter (1976, 1983, 1988), and others, as constrained by oxidized and reduced rocks suggestive of recharge 

and discharge. Principle sources of dissolved solids are evaporitic rocks. A, Henry Basin. B, Paradox Basin.

In the Uravan mineral belt, tabular sandstone uranium 

deposits are associated with distributary channels, thick 

host sandstone, higher sandstone to mudstone ratio, lenticular rather than flatbedded sandstone, scour-and-fill bedding, 

and overlying conglomeratic sandstone (Weir, 1952; Phoenix, 1958; Shawe, 1962; Motica, 1968; Thamm and others, 

1981), all of which suggest topographic depressions. The 

deposits typically are within gray reduced sandstone that 

contains carbonized wood and overlies a green reduced 

mudstone bed (fig. 6) (McKay, 1955). As in the Henry 

Basin, the shape, upward slope, and associated reduced

rocks are consistent with a saline water-fresh water interface inclined upward toward an area of perennial discharge 

(fig- 5B).

An absence of uranium deposits in the Four Corners 

area between the Uravan mineral belt and the Carrizo Mountains deposits coincides with an inferred local recharge area 

(fig. 3). This area, characterized by an abundance of eolian 

sandstone (Bluff Sandstone Member of the Morrison Formation) and a lack of channel sandstone, has been interpreted as 

a local topographic high (Peterson and Tyler, 1985; Blakey 

and others, 1988; Peterson, 1988b, in press).
PALEOHYDROGEOLOGY IN THE COLORADO PLATEAU 11

South of the recharge area, in the Carrizo Mountains 

and vicinity (fig. 3), uranium deposits are in a relatively 

thick part of the Salt Wash Sandstone Member where the 

sedimentary structures and thickness data indicate an eastward-trending, syndepositional structural depression (Hilpert, 1969). This area also is characterized by an abrupt 

decrease in the sandstone to mudstone ratio and by abrupt 

changes in rock color (Chenoweth and Malan, 1973).

Modeling of regional ground-water flow in the Colorado Plateau during Late Jurassic-Early Cretaceous time 

indicates that local dilute ground water and regional saline 

fluids converged at the topographically and geologically 

controlled discharge zones (Sanford, 1982, 1994). 

Recharge was in highland areas to the southwest, and deep 

regional flow gained solutes through interaction with 

evaporites and other rocks along the flow path (fig. 3). At 

the more local scale, hydrologic theory and modern analogs 

suggest the presence of an interface that would have been 

near the surface in topographic depressions (fig. 5). In 

some areas, such as the Henry Basin, the saline water interface may have remained at shallow depth and not reached 

the surface (fig. 5A). In other areas, such as the Uravan 

mineral belt, the interface may have reached the surface, 

allowing relatively fresh water and saline water to discharge together (fig. 5B).

RECAPTURE MEMBER OF THE 

MORRISON FORMATION

Similar to the Salt Wash Sandstone Member, the 

Recapture Member of the Morrison Formation was deposited on a broad, roughly fan shaped alluvial plain sloping to 

the northeast (fig. 7) (Craig and others, 1955; L.C. Craig, 

unpublished data, 1982; Peterson and Turner-Peterson, 

1987; Peterson, in press).

A sandstone facies between conglomeratic and claystone-sandstone facies (Craig and others, 1955; L.C. Craig, 

unpublished data, 1982) marks a decrease in paleotopographic slope where local and regional ground-water systems probably discharged. Syndepositional subsidence 

(Hilpert, 1969) favored local and regional ground-water 

discharge.

Geologic controls favoring discharge from the Recapture Member include thinning of the Lower Jurassic and 

upper Paleozoic aquifer systems. Dramatic thinning of the 

Navajo Sandstone favored discharge beneath the northwestern part of the Recapture. The Wingate Sandstone, which is 

the major aquifer east of the pinchout of the Navajo Sandstone, is thickest in the Four Corners area and thins from 

there toward the northeast (Jobin, 1962; Blakey and others, 

1988). Thinning of the Wingate to the northeast also 

favored discharge. The Entrada Sandstone has a relatively 

constant thickness (Jobin, 1962; Blakey and others, 1988).

EXPLANATION 

Greenish-gray alteration 

Mottled alteration 

Red-brown alteration

Uranium deposit discov 

after 1950

Uranium deposit discov 

through 1950

Figure 6. Map showing areas of alteration in mudstone underlying uranium-bearing sandstone and location of uranium deposits in 

the northern end of the Uravan mineral belt. Uranium deposits are 

closely associated with reduced mudstone in areas of inferred 

ground-water discharge. Mudstone alteration and uranium deposits 

discovered through 1950 from McKay (1955); deposits discovered 

after 1950 from Umetco Company (unpublished map, 1976).

The Lower Permian Glorieta Sandstone, Meseta Blanca 

Sandstone Member of the Yeso Formation, San Andres 

Limestone, and De Chelly Sandstone are today the major 

upper Paleozoic aquifers in the San Juan Basin (Stone and 

others, 1983). The sandstone aquifers thin and pinch out 

toward the north and east (Baars and Stevenson, 1977; 

Blakey and others, 1988), but, because the present transmissivity of the San Andres Limestone is related to dissolution near the outcrop, it may not have been an aquifer in 

Jurassic time.

Tabular sandstone uranium deposits in the Recapture 

Member in northwest New Mexico (Hilpert, 1969) are at 

the transition between the conglomeratic and sandstone 

faces and near the pinchout of the Navajo and Wingate 

Sandstones. An eastward-trending troughlike structure 

traverses the main part of the mining district (Hilpert, 

1969). Anhydrite cement in the Recapture Member (Hansley, 1990) and the upper part of the underlying Middle 

Jurassic Wanakah Formation (Ridgley, 1989) suggests a 

source of saline water in the underlying Middle Jurassic 

Todilto Limestone Member of the Wanakah Formation or 

the Permian evaporitic rocks of the Black Mesa uplift 

(figs. 2, 4).
12 JURASSIC AND TRIASSIC WETLANDS AND ORIGIN OF URANIUM DEPOSITS

109°

37°

Limit of recognizable 

Recapture Member

^COLORADO 

NEW'MEXICO

SAN LUIS 

PRECAMBRIAN

BLOCK 

(hydrologic barrier)

100 MILES

50 100 KILOMETERS

EXPLANATION

Conglomeratic fades 

Sandstone facies 

Claystone-sandstone facies 

Zone of mixed discharge

0 Uranium deposit cluster

— • — ' —' Thickness of Navajo Sandstone 

(in feet)

Thickness of Wingate Sandstone 

(in feet)

SOUTHWEST

zone OT mixed discharge discharge Nava'° Sandstone 

and wingate Sandstone

aquifers NORTHEAST

Evaporites of the 

lower Cutler Formation

EXPLANATION

Major eolian aquifers 

Evaporitic rocks 

Other rocks

Evaporites of the 

Todilto Limestone

Member of the 

Wanekah Formation

FEET METERS 

3000-~ 10°°

1500- -500

Figure 7. Simplified map and cross section of the San Juan Basin for Late Jurassic time at the 

end of deposition of the Recapture Member of the Morrison Formation. Discharge is favored by 

decrease in topographic slope in sandstone facies and by pinchout of the Navajo Sandstone and 

Wingate Sandstone aquifers. The De Chelly Sandstone (not shown) also thins abruptly just east of 

the pinchout of the Wingate Sandstone. Arrows on map and cross section show direction of 

ground-water flow. Dotted line in cross section is dilute-saline interface. Data from Craig and others (1955), Baars (1962), Jobin (1962), Mallory (1972), Baars and Stevenson (1977), and Blakey 

and others (1988).
PALEOHYDROGEOLOGY IN THE COLORADO PLATEAU 13

WESTWATER CANYON MEMBER OF THE 

MORRISON FORMATION

The Westwater Canyon Member was deposited on a 

fan-shaped alluvial plain traversed by braided streams flowing generally toward the northeast and locally to the east and 

southeast (Craig and others, 1955; Hilpert, 1969; L.C. Craig, 

unpublished data, 1982; Condon and Huffman, 1984; Condon and Peterson, 1986; Turner-Peterson, 1986; Peterson 

and Turner-Peterson, 1987).

Local and regional ground-water discharge in the Westwater Canyon Member probably occurred along a northwest-trending zone where a transition from an alluvial-plain 

facies to a mudflat facies suggests a major change in slope 

(figs. 8, 9). A map of sandstone to mudstone ratio indicates 

the regional paleotopographic slope (fig. 8). Sandstone to 

mudstone ratios vary from 300:1 in the southwest to 0.5:1 in 

the northeast (Robert Lupe, U.S. Geological Survey, unpublished data, 1982; Kirk and Condon, 1986). Discharge probably occurred in a zone of major decrease in topographic 

slope, approximately from the >10:1 to 4:1 contours of sandstone to mudstone ratio (fig. 8). Tectonic activity affected 

the distribution of facies and the paleoslope on a more local 

scale, as indicated by locally thick sediments, high sandstone 

to mudstone ratios, and smaller numbers of sandstone-mudstone interbeds (Kirk and Condon, 1986). Stream channels 

were concentrated in subsiding structures, whereas overbank 

muds were deposited on paleohighs. Thus, local groundwater flow tended to recharge in the local highs dominated 

by overbank mudstone and discharge in the depressions 

dominated by channel sandstone.

The most significant geologic control on ground-water 

flow in the San Juan Basin probably was the thinning of the 

De Chelly Sandstone and the Meseta Blanca Member of the 

Yeso Formation (fig. 9). The De Chelly Sandstone thins 

abruptly from 240 to 100 m (800-300 ft) and then pinches 

out gradually toward the east (Baars and Stevenson, 1977). 

The Meseta Blanca Member thins abruptly from 150 to 100 

m (500-300 ft) and then pinches out gradually toward the 

north (Baars and Stevenson, 1977). Because the change in 

thickness (rather than the thickness itself) is the critical geological control on ground-water flow, the region of abrupt 

thinning (rather than the pinchout) was most likely the zone 

of maximum geologically controlled discharge for regional 

ground water.

Tabular sandstone uranium-humate deposits in the 

western part of the Grants uranium region were a major 

source of uranium (Kelley, 1963; Hilpert, 1969; Rautman, 

1980; Adams and Saucier, 1981; Turner-Peterson, Santos, 

and Fishman, 1986; Finch and McLemore, 1989). The 

deposits are in the Westwater Canyon Member and cluster 

in a belt from just upslope of the 10:1 contour of sandstone 

to mudstone ratio to the 4: 1 contour, in the zone of abrupt

thinning of the Meseta Blanca Member (fig. 9). The uranium region itself consists locally of two or more local belts 

of deposits that are 3-5 km (2-3 mi) apart (Granger and others, 1961). The main deposits tend to be in the centers of the 

thickest sandstone masses, which are in syndepositional 

synclines (Hilpert and Moench, 1960; Hilpert, 1969; Kirk 

and Condon, 1986). The general position of the Grants uranium region probably was controlled by ground-water discharge at the maximum change in topographic slope, 

indicated by the major facies change, and by the zone of 

maximum thinning of the Meseta Blanca Member. Local 

clusters of uranium deposits within the region were probably controlled by local paleotopographic depressions.

Discharging regional ground water was probably 

saline owing to dissolution of anhydrite-bearing evaporites 

of the Middle Jurassic Todilto Limestone Member of the 

Wanakah Formation (figs. 4, 10). Dissolution is shown by 

the numerous breccia pipes that terminate downward in the 

Todilto (Moench and Schlee, 1967; Hilpert, 1969; Hunter 

and others, 1992).

The oxidation state of iron in the sedimentary rocks 

suggests local recharge and discharge areas within the general area favorable for ground-water discharge in the Grants 

uranium region. For example, a syndepositional syncline 

contains reduced rock and uranium deposits, whereas the 

adjacent paleohighs contain oxidized rock and lack uranium deposits (Turner-Peterson, 1985). The combination of 

paleotopographic lows and reduced rock suggests that high 

water table, ground-water discharge, and preservation of 

organic matter are associated with the deposits.

Detrital ilmenite and titaniferous magnetite in sandstone 

of the Westwater Canyon Member show variable degrees of 

alteration to anatase, leucoxene, hematite, and pyrite (Adams 

and others, 1974; Adams and Saucier, 1981; Reynolds and 

others, 1986). Alteration probably occurred by incongruent 

dissolution of detrital iron-titanium oxide minerals by ground 

water that was reducing, neutral to weakly acid, and humic 

acid rich (Adams and others, 1974). Dissolution of iron-titanium oxide minerals is diagenetic evidence for shallow, relatively fresh, ground water, and the pattern of alteration 

indicates the extent of the shallow ground-water system (figs. 

10, 11). In general, the intensity of iron-titanium oxide dissolution decreases downward and from southwest to northeast (Adams and others, 1974; Adams and Saucier, 1981; 

Turner-Peterson, 1985, 1986; Reynolds and others, 1986). 

Some authors (Adams and Saucier, 1981; Turner-Peterson, 

1985; Turner-Peterson and Fishman, 1986) have plotted the 

total thickness of the zone of iron-titanium oxide dissolution. 

This measure does not truly represent the intensity of irontitanium oxide mineral alteration because the thickness of the 

host Westwater Canyon Member also decreases to the southwest. Plotting the same data in terms of the percentage of the 

Westwater Canyon Member that is altered better indicates
14 JURASSIC AND TRIASSIC WETLANDS AND ORIGIN OF URANIUM DEPOSITS

109°

Westwater Canyon Member

EXPLANATION

Westwater Canyon Member of the 

Morrison Formation

Topographically controlled discharge—

Sandstone/mudstone decreases from 

>10to4

Sandstone/mudstone data point

Figure 8. Map showing contours of sandstone to mudstone thickness ratio in the Westwater Canyon Member of the Morrison Formation. Decrease in ratio from southwest to northeast indicates decrease in topographic slope. Ground-water discharge and mixing is favored where sandstone to mudstone ratio is from >10 to 4. Data from Kirk and Condon (1986; area 

in New Mexico outlined by dotted curve) and from Robert Lupe (U.S. Geological Survey, unpublished data, 1982; data points 

shown by solid circles).
PALEOHYDROGEOLOGY IN THE COLORADO PLATEAU 15

the intensity of alteration in the Westwater Canyon (fig. 11). 

The contours indicate a wedge-shaped lens of alteration that 

includes the entire Westwater Canyon Member in parts of the 

south and west (100 percent alteration) and thins to a feather 

edge toward the northeast. Pockets of less alteration are 

present in the middle of the southern edge of the Westwater 

Canyon Member but probably are secondary effects superimposed on the regional trend. Although considerable latitude is possible in contouring the data, there is no reason to 

interpret a pinching-out of the alteration zone to the south, as 

indicated by previous authors (Adams and Saucier, 1981; 

Turner-Peterson, 1985; Turner-Peterson and Fishman, 

1986). The pattern of iron-titanium oxide alteration has been 

interpreted to indicate downward flow of fluid and is a major 

argument in support of the "lacustrine-humate model" of uranium ore formation (Turner-Peterson, 1985; Turner-Peterson 

and Fishman, 1986). This evidence alone is not compelling 

because the alteration pattern can equally well be interpreted 

as a result of downdip flow of dilute ground water that tended 

to be confined to the upper part of the sandstone by saline 

ground water.

BRUSHY BASIN MEMBER OF THE 

MORRISON FORMATION

The Brushy Basin Member and the lithologically similar undifferentiated Morrison Formation consist of discontinuous sandstone beds in a predominantly mudstone and 

claystone matrix with limestone beds and nodules (Craig and 

others, 1955; Dodson and others, 1980; Bell, 1983, 1986; 

Turner-Peterson, 1985, 1987; Lockley and others, 1986; 

Peterson and Turner-Peterson, 1987; Turner and Fishman, 

1991). Lithofacies suggest a low-gradient alluvial plain and 

scattered lakes or playas. A large lake having well-developed diagenetic facies similar to those in modern playas such 

as Searles Lake (Smith, 1979) is thought to have covered 

150,000 km2 mainly in northwest New Mexico and southwest Colorado (figs. 12-14) (Bell, 1983,1986; Turner-Peterson, 1985, 1987; Turner and Fishman, 1991). The general 

direction of streams was northeastward.

The generally fine grain size of the Brushy Basin Member suggests a low and uniform topographic slope that would 

not have favored significant recharge or discharge over 

much of the area. Lithofacies vary from continuous sandstone (sandstone to mudstone >1.0), to discontinuous sandstone (sandstone to mudstone <1.0 and >0.6), to dominantly 

mudstone (sandstone to mudstone <0.6) (fig. 12A) (Bell, 

1983, 1986). The main change in topographic slope was 

probably in the southern part of the depositional area of the 

Brushy Basin Member in northwest New Mexico where 

mostly sandstone was deposited (continuous sandstone, fig. 

12A, and sandstone, fig. 12E).

Quantitative modeling of ground-water flow during 

deposition of the Brushy Basin Member indicates that discharge occurred throughout a large topographic depression 

(Sanford, 1990b, 1994). A lake in the depression would have 

focused ground-water discharge at the shoreline. Most of 

this discharge presumably took place on the upstream or 

southwest side of the lake because of the regional hydraulic 

gradient (fig. 13). Both regional and local ground-water systems probably discharged around the playa. In the absence 

of a lake, ground water probably discharged in the topographically lowest parts of the closed depression.

The low permeability of the Brushy Basin Member, 

compared to the Westwater Canyon Member, suggests that 

ground-water flow through the Brushy Basin Member may 

have been mainly vertical. Geologic controls on groundwater flow during deposition of the Brushy Basin Member 

were similar to those for the rest of the Morrison Formation, 

as discussed above (figs. 3, 7, 9). Deep, saline ground water 

discharged where Jurassic and upper Paleozoic aquifer systems thinned abruptly in the subsurface. The Uncompahgre 

and San Luis blocks forced most of the remaining ground 

water to discharge or flow around the blocks.

Diagenetic studies of the Brushy Basin Member have 

focused on the alteration associated with a large alkalinesaline playa lake (figs. 12, 13) (Bell, 1983, 1986; TurnerPeterson, 1985; Turner-Peterson and Fishman, 1986; Turner 

and Fishman, 1991). Turner and Fishman proposed welldefined facies boundaries. T.E. Bell (oral commun., 1989) 

considered the facies boundaries too poorly defined to allow 

identification of discrete boundaries. A calcite facies (Bell, 

1983, 1986) is important evidence for fluid mixing. The marginal mudflat of the Brushy Basin playa corresponds to the 

smectite-discontinuous sandstone and calcite facies (figs. 

12A, 14) (Bell, 1983, 1986), alternatively termed the smectite facies (fig. 12B) (Turner-Peterson, 1985; Turner-Peterson and Fishman, 1986; Turner and Fishman, 1991). The 

smectite-discontinuous sandstone facies contains isolated 

limestone nodules, and the calcite facies contains abundant 

laterally continuous limestone beds (Bell, 1983, 1986). By 

analogy with modern playas and saline lakes (Eugster and 

Hardie, 1978; Mono Basin Ecosystem Study Committee, 

1987; Duffy and Al-Hassan, 1988), this carbonate formed as 

a result of discharge of bicarbonate-bearing ground water at 

the edge of the playa. The increasing amount of carbonate 

going from smectite to calcite facies suggests increasing carbonate concentration and deeper flow depth of the discharging ground water.

Diagenetic minerals such as analcime, zeolites, and 

potassium feldspar occupy the central part of the playa and 

are interpreted as evidence for an alkaline-saline brine (fig. 

12) (Bell, 1983, 1986; Turner-Peterson, 1985; TurnerPeterson and Fishman, 1986; Turner and Fishman, 1991). 

Other diagenetic indicators are more difficult to interpret.
16 JURASSIC AND TRIASSIC WETLANDS AND ORIGIN OF URANIUM DEPOSITS

109°

•..-••.' • SAN LUIS

..'. PRECAMBRIAN 

: .'•. • BLOCK •

SOUTHWEST

Dilute

Area of detail NORTHEAST

tntraaa

Sandstone

aquifer Evaporites

of the 

Todilto Limestone

Member of the 

Wanakah Formation Meseta Blanca 

Evaporites of Member aquifer the 

Yeso Formation

EXPLANATION

Major eolian aquifers 

Evaporitic rocks 

Other rocks
PALEOHYDROGEOLOGY IN THE COLORADO PLATEAU 17

For example, the alteration of smectite to illite, which 

increases downdip toward the center of the basin, has been 

interpreted as evidence both for upward flow of deep 

ground water (Whitney and Northrop, 1987) and for downward flow of playa lake water (Owen and others, 1989; 

Turner and Fishman, 1991).

Modern playa analogs (Allison and Bames, 1985; 

Duffy and Al-Hassan, 1988) and computer modeling (Sanford, 1990b, 1994) show that flow can be downward or 

upward depending on hydrodynamic conditions. If the 

ground water had a constant density throughout, it would 

discharge at a topographic depression; however, in a playa 

setting where surface water is highly concentrated, buoyancy may cause ground water to sink beneath the playa and 

recirculate by convection back updip to discharge near the 

shoreline or margin of the playa (Duffy and Al-Hassan, 

1988). Downward flow of playa lake water is suggested by 

zeolite alteration in the Westwater Canyon Member beneath 

the Brushy Basin Member (Hansley, 1986, 1990). One possible interpretation that is consistent with hydrologic theory, 

modern analogs, model predictions, and diagenetic alteration is shown in figure 14. Although the flow of shallow 

ground water in the alluvial plain probably was consistently 

downdip, the flow of ground water in the playa probably

EXPLANATION

Westwater Canyon Member of Morrison Formation 

Upper Paleozoic aquifers >300 feet 

Precambrian block 

Zone of mixed discharge

Uranium deposit cluster 

— — — — — Thickness of De Chelly Sandstone (in feet)

Thickness of Meseta Blanca Member of Yeso Formation (in 

feet)

Contour of sandstone to mudstone ratio in Westwater 

Canyon Member of Morrison Formation

Figure 9 (above and facing page). Simplified map and cross section of the San Juan Basin for Late Jurassic time at the end of deposition of the Westwater Canyon Member of the Morrison Formation. 

Transition in sandstone to mudstone ratio from >10 to 4 indicates 

topographically controlled discharge. Abrupt thinning of the De 

Chelly Sandstone and Meseta Blanca Sandstone Member aquifers 

indicates geologically controlled discharge. Where both coincide is 

an inferred zone of mixed local and regional discharge. Arrows on 

map and cross section show principle direction of ground-water 

flow. Area in cross section delineated by box is shown in more detail 

in figure 10. Dotted line in cross section is dilute-saline interface. 

Data from Craig and others (1955), Baars (1962), Jobin (1962), 

Peterson and others (1965), Mallory (1972), Baars and Stevenson 

(1977), and Blakey and others (1988).

varied with time, alternately moving downward or upward 

as the density of the lake and pore fluid varied.

According to one version of the lacustrine-humate 

model, uranium deposits in the Westwater Canyon Member 

and Jackpile Sandstone Member are genetically related to 

the mudflat (smectite) facies of the Brushy Basin Member 

(Turner-Peterson, 1985; Turner-Peterson and Fishman, 

1986). Although a fair correspondence between deposits 

and the mudflat facies exists in the San Juan Basin, the correspondence completely breaks down for other parts of the 

Colorado Plateau (fig. 13). In the Uravan mineral belt, most 

deposits in the Salt Wash Member are beneath the analcime-potassium feldspar facies of the Brushy Basin Member. Furthermore, the Salt Wash Sandstone Member may be 

separated from the upper part of the Brushy Basin Member 

by an unmineralized lower part that has been interpreted as 

equivalent to the Recapture Member (Turner and Fishman, 

1991). In the Henry Basin, uranium deposits are beneath the 

alluvial or continuous-sandstone facies. Thus, the correspondence of Brushy Basin mudflat with tabular sandstone 

uranium deposits is probably a fortuitous coincidence limited to the San Juan Basin rather than a general characteristic having genetic significance.

JACKPILE SANDSTONE MEMBER OF THE 

MORRISON FORMATION

The Jackpile Sandstone Member overlies and interfingers with the Brushy Basin Member in the southeastern part 

of the San Juan Basin (Moench and Schlee, 1967; Owen and 

others, 1984). It consists mostly of fluvial sandstone deposited in an active northeast-trending syncline. The upper part 

of the Jackpile Member was removed in places by erosion 

prior to deposition of the Upper Cretaceous Dakota Sandstone (Moench and Schlee, 1967; Adams and others, 1978).

The presence of zeolite facies mudstone of the Brushy 

Basin Member beneath the thickest part of the Jackpile 

Sandstone Member indicates that a low area existed prior to 

Jackpile deposition (Bell, 1983). Isopachs and interfingering 

relations with the Brushy Basin Member indicate active subsidence during deposition (Moench and Schlee, 1967). Preservation of the Jackpile Member suggests that low 

topography persisted while surrounding areas were uplifted 

and eroded.

Paleocurrent directions and isopachs of the Jackpile 

Sandstone Member (Moench and Schlee, 1967; Adams and 

others, 1978) suggest that shallow, relatively fresh ground 

water flowed northeast along the axis of the syndepositional 

syncline (fig. 15). Because the area was a topographic 

depression, deeper ground water probably discharged into 

the base of the unit (Sanford, 1990b, 1994).

Diagenetic evidence indicates the timing, relative 

extent, and movement of the shallow and deeper ground 

water. Iron-titanium oxide minerals were extensively
18 

SOUTHWEST

JURASSIC AND TRIASSIC WETLANDS AND ORIGIN OF URANIUM DEPOSITS

NORTHEAST

Tabular uraniumhumate deposit

Sandstone to mudstone ratio 10:1 4:1 1:1

Zone of complete

iron-titanium oxide

dissolution

Figure 10. Detailed schematic cross section of the Westwater Canyon Member of the Morrison Formation and underlying units for Late 

Jurassic time at the end of Westwater Canyon deposition. Location of area of cross section is delineated by box on cross section of figure 

9. No vertical scale implied. Arrows show direction of ground-water flow. Dotted line is dilute-saline interface. Flow is primarily gravity 

driven and downdip. Discharge occurs where topographic surface has decrease in slope. Breccia pipes, formed by dissolution of anhydrite-bearing Todilto Limestone Member of the Wanakah Formation, were conduits for upward flow of saline water. Shallow fresh water 

forms a lens resting upon deeper regional saline water. Unit abbreviations: Je, Entrada Sandstone; Jwt, Todilto Limestone Member of the 

Wanakah Formation; Jwb, Beclabito Member of the Wanakah Formation; Jcs, Cow Springs Sandstone; Jmr, Recapture Member of the 

Morrison Formation; Jmw, Westwater Canyon Member of the Morrison Formation.

dissolved in "hydrologically continuous" sandstone of the 

Jackpile Sandstone Member (Adams and Saucier, 1981). 

This alteration was paragenetically very early and probably 

occurred during or shortly after deposition (Adams and others, 1978). As discussed earlier for the Westwater Canyon 

Member, iron-titanium oxide dissolution suggests alteration 

by dilute, reducing, humic-acid-bearing ground water.

Other alteration suggests later incursion of brackish or 

saline ground water from the underlying Brushy Basin 

Member or lower units into the deepest parts of the syncline (Adams and others, 1978; Bell, 1983). Albitization of 

feldspar, which decreases in intensity upward, suggests 

that the later ground water was sodium-rich fluid similar to 

that inferred for pore water in the zeolite facies of the 

Brushy Basin Member directly beneath the Jackpile Member (Bell, 1983). Alternatively, saline ground water may 

have discharged upward from units such as the Todilto 

Limestone Member of the Wanakah Formation. Tabular

uranium-humate deposits suggest deposition at an interface 

between relatively fresh and saline ground water within the 

Jackpile Sandstone Member. The presence of the deposits 

in the thickest part of the sandstone above the deepest part 

of the structure is consistent with ponding of saline water 

at depressions within the sandbody (Bell, 1983).

Breccia pipes that extend up from the Todilto Limestone Member and to the Jackpile Sandstone Member 

(Moench and Schlee, 1967; Hilpert, 1969; Hunter and others, 1992) may have been conduits through the Brushy 

Basin Member for the upward-moving, deep regional 

ground water. Although such conduits may have aided 

upward ground-water flow, they would not have been necessary for significant upward flow. Discharge through 

modern playa lakebeds is a common phenomenon (Meyboom, 1967; Lissey, 1971; Stephenson, 1971; Winter, 

1976, 1978, 1986, 1988; Friedman and others, 1982; Allison and Barnes, 1985; Spencer and others, 1985; Duffy
PALEOHYDROGEOLOGY IN THE COLORADO PLATEAU 19

i i i 

10 20 30 40 50 KILOMETERS

SOURCES OF DATA

• Adams and Saucier (1981)

• Reynolds and others (1986)

^ N.S. Fishman (written commun., 1989)

Figure 11. Map showing contours of the interval of total ilmenite-magnetite dissolution in the upper part of the Westwater Canyon 

Member of the Morrison Formation. The thickness of the altered interval is plotted as a percentage normalized to the total thickness of the 

Westwater Canyon Member. The normalized thickness indicates the intensity of alteration. Compare with maps of the unnormalized thickness of the same interval shown in Adams and Saucier (1981), Turner-Peterson (1985), and Turner-Peterson and Fishman (1986).

and Al-Hassan, 1988; Winter and Woo, 1990), and computer modeling of Late Jurassic-Early Cretaceous groundwater flow in the San Juan Basin shows that Brushy Basin 

Member mudstone could have been sufficiently permeable 

to yield significant discharge, approximately 15+10 

mm/yr (Sanford, 1994).

Thus, the diagenetic and paragenetic relations indicate 

that the Jackpile Sandstone Member was altered mostly by 

shallow, relatively dilute ground water early in its history 

and, later in its history, by alkaline-saline ground water moving upward from the underlying Brushy Basin Member and 

(or) by saline water moving upward from deeper units such 

as the Todilto Limestone Member. Alkaline-saline and (or)

saline ground water probably ponded in deeper parts of the 

basin and formed an interface with fresher water above along 

which tabular sandstone uranium deposits formed.

LOWER PART OF THE CHINLE FORMATION

The lower part of the Upper Trias sic Chinle Formation 

consists of continental deposits of fluvial and lacustrine origin (Stewart and others, 1972; Blakey and Gubitosa, 1983; 

Dubiel, 1987, 1989; Dubiel and others, 1991). Clastic sediments were shed from the Uncompahgre highlands on the 

northeast and from the Mogollon highlands on the southwest.
20 JURASSIC AND TRIASSIC WETLANDS AND ORIGIN OF URANIUM DEPOSITS

109°

- Txs

s

\

\

\ 

\

\

1

o

N

\

0 \ .

Cuba o

0

<?„

,

O

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1

'<?. \ ''•-. Analcime and

heulandite- ; • O s "• •. chnop 

3 \ •'• <£ 2 0Gallup^^ \ \ •-... C^ \

1 £

^ u

Z

McKINLEY COUNT

1

tilolite :

^ x '. <>• •-. tv

'o.S'^N '•. •**.. • "•• *®

&//^^ '' ^On f .

ty ^^^.'- ^l/ • '

1

V<<?Grants ••-... •...-•'

j \ " 0 10 20 30 40 50 MILES

( , , , , I III

1 \ " ' ' I 1 II

1 \0 10 20 30 40 bO KILOMETERS

\

s

35°

109°

35°

Analcime

and

potassium 

feldspar

TTTT1 I I T 

\0 \ 10 20 30 40 50 KILOMETERS

B

Figure 12. Maps showing interpretations of the diagenetic patterns in the Brushy Basin Member of the Morrison Formation. Both show transition from alluvial plain to playa, but they differ on choice of diagnostic minerals and nature of facies 

boundaries. A, Drawn from data and interpretation of Bell (1983, 1986). Dotted lines are contours of sandstone to mudstone 

ratio. Facies boundaries in A are dotted to indicate Bell's view that facies boundaries cannot be clearly identified. B, Drawn 

from Turner-Peterson (1985) and Turner-Peterson and Fishman (1986). Note that calcite facies in A are omitted from B.
PALEOHYDROGEOLOGY IN THE COLORADO PLATEAU

109°

21

37°

ELKO 

I HIGHLANDS 

I (recharge)

MOGOLLON

HIGHLANDS

(recharge)

100 MILES

50 100 KILOMETERS

EXPLANATION

Brushy Basin lake—Playa facies 

Brushy Basin lake—Mudflat facies

;> Zone of mixed discharge

Uranium deposit cluster by host 

member of Morrison Formation

Jackpile Sandstone Member 

Westwater Canyon Member 

Recapture Member 

Salt Wash Member

Figure 13. Schematic map of Colorado Plateau area showing diagenetic alteration in the Brushy Basin 

Member of the Morrison Formation, zone of mixed local and regional discharge at distal edge of alluvial 

plain, and location of tabular sandstone uranium deposit clusters in the Morrison Formation. Facies: S, 

smectite mudflat, C, clinoptilolite; A-K, analcime-potassium feldspar; Ab, albite. Arrows show general 

direction of surface- and ground-water flow. Brushy Basin facies from Turner and Fishman (1991); 

uranium deposit clusters from Finch (1991).
22 

SOUTHWEST

JURASSIC AND TRIASSIC WETLANDS AND ORIGIN OF URANIUM DEPOSITS

NORTHEAST

Sandstone to 

mudstone ratio

ALLUVIAL PLAIN

(continuous 

sandstone fades)

0.6

MUDFLAT 

(discontinuous

sandstonesmectite facies)

0.4

CARBONATE MUDFLAT 

(calcite facies)

0.23

PLAYA LAKE

(zeolite-potassium

feldspar facies)

Fossil mixing zone and 

-Springs .^Mixing zone uranium-humate deposits

Figure 14. Detailed schematic cross section of the Brushy Basin Member of the Morrison Formation and underlying units for Late Jurassic 

time at the end of Brushy Basin deposition in the Colorado Plateau area along line of section shown in figure 13. Arrows show general direction of ground-water flow. Gravity-driven downdip flow from southwest meets updip density-driven local flow from playa and regional 

flow from deep basin. Discharge occurs where the topographic surface has decrease in slope, as indicated by transition from alluvial plain to 

mudflat. Facies and average sandstone to mudstone ratios for the Brushy Basin Member are from Bell (1983, 1986). Unit abbreviations: Je, 

Entrada Sandstone; Jwt, Todilto Limestone Member of the Wanakah Formation; Jwb, Beclabito Member of the Wanakah Formation; Jcs, 

Cow Springs Sandstone; Jmr, Recapture Member of the Morrison Formation; Jmw, Westwater Canyon Member of the Morrison Formation; 

Jmb, Brushy Basin Member of the Morrison Formation.

Major rivers flowed northwestward from northwestern New 

Mexico to northwestern Utah, and tributaries flowed northward from the Mogollon highlands and westward from the 

Uncompahgre highlands (figs. 2, 16).

The lower part of the Chinle Formation consists of the 

Shinarump, Monitor Butte, Moss Back, and Petrified Forest 

Members (Stewart and others, 1972; Blakey and Gubitosa, 

1983; Dubiel, 1987, 1989). The basal Shinarump Member, 

characterized by conglomerate and sandstone deposited in 

braided stream channels, fills valleys and scours in the 

underlying marine Kaibab Limestone, the marine-marginal 

Moenkopi Formation, and the eolian De Chelly and 

Coconino Sandstones. The overlying Monitor Butte Member shows the same transport directions as the Shinarump

Member but consists of tuffaceous, bentonitic mudstone, 

siltstone, sandstone, and limestone deposited in lacustrine 

and fluvial environments. The next higher unit, the Moss 

Back Member, similar to the Shinarump Member, consists 

mainly of fluvial sandstone and conglomerate. Locally, the 

Moss Back Member is the basal unit and rests on the Permian Cutler Formation (Wood, 1968). At the top of the 

Chinle, the Petrified Forest Member consists of variegated 

mudstone and lenticular sandstone deposited on a low-gradient alluvial plain having high- and low-sinuosity fluvial 

channel systems, overbank and floodplain environments, 

and scattered lakes and marshes. Lithofacies and fossils 

indicate that precipitation and surface water were abundant, 

streams and lakes were fresh and perennial, and the water
PALEOHYDROGEOLOGY IN THE COLORADO PLATEAU 23

table was typically high; however, paleosols and ichnofossils indicate that water tables and lake levels fluctuated episodically owing to a tropical monsoonal climate (Dubiel and 

others, 1989, 1991).

TOPOGRAPHIC CONTROLS ON 

GROUND-WATER FLOW

Based on lithofacies data (Stewart and others, 1972; 

Blakey and Gubitosa, 1983; Dubiel, 1987, 1989), topographic control caused ground water to flow dominantly 

northwestward following the main channel systems of the 

Shinarump and Moss Back Members (fig. 16). Flow in the 

margins of the depositional area was along the tributary 

streams toward the main northwest-trending fluvial axis. 

Active structural features affected ground-water flow at 

regional and local scales. A large, actively rising highland 

apparently controlled the distribution of lithofacies in the 

White Canyon and Monument Valley areas (Young, 1964; 

Malan, 1968; Fisher, 1972). In the Paradox Basin, particularly in the Lisbon Valley area, active anticlines were associated with salt diapirism (Wood, 1968; Fisher, 1972).

The topographic gradient and probably the rates of 

ground-water flow probably changed systematically with 

time, as shown by the variations in mean grain size of the different members. The coarse-grained, fluvial channel sandstone of the Shinarump Member suggests a relatively steep 

gradient. The change to fine-grained rocks of the Monitor 

Butte Member that were deposited in lacustrine, deltaic, and 

marsh environments implies a shallower gradient and slower 

ground-water flow. Low-gradient conditions were locally 

interrupted during deposition of the dominantly coarser 

grained Moss Back Member but returned during deposition 

of the fine-grained Petrified Forest Member.

GEOLOGIC CONTROLS ON 

GROUND-WATER FLOW

Geologic control on ground water was dominated by 

complex variations in thickness and facies of the upper Paleozoic aquifers (fig. 16). Southwestward along the flow path 

from the ancient Uncompahgre highlands, the upper Paleozoic aquifer system thickens abruptly into the Paradox Basin 

and from there it thins (Baars, 1962; Jobin, 1962; Blakey and 

others, 1988; Geldon, in press and unpublished data). Also in 

a southwest direction, arkosic rocks decrease in thickness, 

whereas eolian sandstone, particularly the Cedar Mesa Sandstone and White Rim Sandstone Members of the Cutler Formation, increases in thickness. Regional thinning of the 

Paleozoic section westward from the axis of the Paradox 

Basin suggests decreasing transmissivity and therefore discharge, but more eolian sandstone relative to arkose favors 

recharge because of increased transmissivity. Farther from 

the Paradox Basin, depositional and tectonic variations are 

important locally. The net effect of the various influences is 

unclear, at least locally; however, in the area between the 

Uncompahgre highlands and the main lower Chinle fluvial

107°15'

Figure 15. Isopach map of the thickness (in feet) of the Jackpile 

Sandstone Member of the Morrison Formation showing facies of 

underlying Brushy Basin Member, location of uranium deposits 

(solid black), and directions of paleoflow. Area of calcareous facies 

of Brushy Basin Member is unpatterned; area of zeolite facies of 

Brushy Basin Member is diagonal-lined. Modified from Moench 

and Schlee (1967), Adams and others (1978), and Bell (1983).

channel, there may have been three general ground-water 

zones. Next to the uplift, where the topographic slope was 

greatest and the upper Paleozoic sediments were dominantly 

arkosic, recharge probably was dominant. Southwestward 

from this zone, where the slope was more gradual and thinner 

sediments (favoring discharge) competed with thicker eolian 

sandstone (favoring recharge), recharge gradually gave way 

to discharge. Finally, as ground water neared the main Chinle 

channel axis, where the topographic slope was minimal, 

ground water probably discharged.

A similar zonation may have been present northward 

from the Mogollon highlands, although thickness and facies 

changes were less extreme. From the Mogollon highlands 

northward almost to the Utah State line, the upper Paleozoic 

section gradually thins, whereas eolian sandstone units, particularly the De Chelly (Blakey and others, 1988), gradually 

thicken. Based on the isopachs of total sandstone thickness, 

the net effect was probably a slight increase in transmissivity. In the vicinity of the Arizona-Utah State line, the thickness of Permian sandstone abruptly decreases mainly due to 

thinning of the De Chelly Sandstone, but, farther to the north, 

Permian sandstone units thicken again owing to the Cedar 

Mesa Sandstone and White Rim Sandstone Members 

(Blakey and others, 1988).
24 JURASSIC AND TRIASSIC WETLANDS AND ORIGIN OF URANIUM DEPOSITS

109°

37° ;•»* v--'tf 2000 i * *» \ w i

j

NEW MEXICO

50 100 KILOMETERS

EXPLANATION

Area of topographic lows in deltaic, 

————— lacustrine, and marsh environments 

(topographically controlled discharge)

---..-.-. Contour of combined thickness of upper 

Paleozoic aquifers (in feet)

• Uranium deposit cluster

Figure 16. Map showing relationship between uranium deposit clusters and ground-water discharge in 

the Colorado Plateau during deposition of the lower part of the Triassic Chinle Formation. Channels in 

Shinarump and Moss Back Members indicate north-, west-, and northwestward surface- and ground-water 

flow (indicated by arrows). Discharge of local and regional ground water was in topographically low areas 

marked by deltaic, lacustrine, and marsh environments of the Monitor Butte Member. Paleogeographic 

data simplified from Dubiel (1989); thickness of upper Paleozoic aquifers from A.L. Gelden (unpublished 

data, 1992); uranium deposit clusters from Finch (1991).
PALEOHYDROGEOLOGY IN THE COLORADO PLATEAU 25

Thus, the overall pattern of geologic control on groundwater flow suggests a transition from dominantly recharge to 

dominantly discharge from basin margin to central fluvial 

channels. Unlike the Morrison Formation, geologic and 

topographic controls in the Chinle Formation did not clearly 

combine to favor discharge, but, instead, topographic control 

of discharge was partly counteracted by geologic controls 

that favored recharge.

DIAGENESIS AND URANIUM DEPOSITION

The distribution of oxidized and reduced rocks aids in 

identifying recharge and discharge zones and accumulations 

of organic matter. Green and gray mudstone and sandstone 

coincide with Shinarump and Moss Back channels (Wood, 

1968; Dubiel, 1989), supporting the inference that the Chinle 

fluvial channels were zones of persistent ground-water discharge throughout deposition of the lower part of the Chinle 

Formation. The green mudstone fades of the Monitor Butte 

Member (Dubiel, 1989) also suggests reducing conditions, a 

constant high water table, and perennial ground-water discharge. Preserved organic carbon and reduced sandstone and 

mudstone elsewhere in the Chinle Formation (Dubiel, 1 989) 

further indicate areas of persistent ground-water discharge.

The inferred ground-water-flow pattern would have 

caused zones of mixing between relatively fresh, local 

ground water and more saline, regional ground water in areas 

of discharge (fig. 16). The gypsiferous Moenkopi Formation 

(Blakey, 1974) and evaporites in Pennsylvanian and Permian 

rocks probably contributed to the salinity of deeper ground 

water that tended to discharge upward into the Chinle Formation in areas marginal to and within floodplain, lacustrine, 

and marsh environments.

Tabular sandstone uranium-vanadium and uraniumcopper deposits in the Chinle Formation are associated with 

transitional lithofacies, channel sandstone, syndepositional 

synclines, and reduced lacustrine mudstone in the Shinarump and Moss Back Members (Johnson, 1957; Finch, 

1959; Witkind and Thaden, 1963; Thaden and others, 1964; 

Malan, 1968; Wood, 1968; Lupe, 1977; Huber, 1980; 

Dubiel, 198?). Almost all of the deposits are in areas of 

inferred topographically controlled discharge, as shown by 

deltaic, marsh, and lacustrine deposits of the Monitor Butter 

Member (fig. 16). Ore deposits are further localized at transitional lithofacies in the Shinarump and Moss Back Members where fluvial channel sandstone pinches out and where 

distal braided stream deposits grade into floodplain deposits 

(Finch, 1959; Lupe, 1977). Many of the Chinle-hosted 

deposits are in a belt 4.8-19 km (3-12 mi) wide and 209 km 

(130 mi) long that corresponds to a major facies change from 

dominantly sandstone to dominantly mudstone on the flanks 

of a large uplift approximately in the area of the present 

Monument uplift (Young, 1964; Malan, 1968; Fisher, 1972).

110*30' now

oo1

10 MILES

I ' ' ' ' I 1

0 5 10 KILOMETERS

o

37° 

30'

Figure 17. Map showing relationship between uranium deposits 

(solid circles), syndepositional synclines, and organic-rich lacustrine mudstone (shaded areas) in part of the Chinle Formation in 

southeast Utah. Modified from Dubiel (1983).

In the Lisbon Valley area, on a smaller scale, uranium 

deposits are on the flanks of a rising salt-cored anticline 

(Wood, 1968; Fisher, 1972). In addition to transitional lithofacies, uranium deposits are closely associated with black, 

organic-rich, lacustrine-marsh mudstone that was deposited 

where the water table was high (fig. 17) (Dubiel, 1983). 

Generally, the uranium deposits are below or, less commonly, beside the lacustrine mudstone (R.F. Dubiel, oral 

commun., 1992). In addition, approximately 90 percent of 

uranium deposits in this area are within 3.2 km (2 mi) of the 

axis of a syndepositional syncline (fig. 17). In the Lisbon 

Valley area, a density-stratified interface was proposed as a 

mechanism for ore-deposit formation, based on the occurrence of the deposits at a particular horizon within the Chinle 

Formation (Wood, 1968; Fischer, 1974; Huber, 1980). 

Minor uranium deposits and concentrations are in the Petrified Forest Member of the Chinle Formation. These are 

associated with thicker than normal sediments that are interpreted to indicate greater subsidence and a high water table 

that helped preserve organic matter (Spirakis, 1980).
26 JURASSIC AND TRIASSIC WETLANDS AND ORIGIN OF URANIUM DEPOSITS

ORIGIN OF TABULAR SANDSTONE 

URANIUM DEPOSITS

In summary, the tabular shape, transitional sedimentary 

lithofacies, syndepositional synclines, and reduced channel 

sandstone and mudstone associated with tabular sandstone 

uranium deposits in the Morrison and Chinle Formations on 

the Colorado Plateau suggest that deposits formed" where 

decreasing topographic slope and topographic depressions 

caused shallow, relatively fresh ground water and deeper, 

more saline ground water to discharge and mix at a densitystratified interface. The next step is to integrate these conclusions with the other types of evidence for this ore-deposit 

type and to reconcile divergent opinions on diagenesis and 

ore formation.

ROLE OF HUMATE

The location of tabular sandstone uranium deposits in 

the San Juan Basin likely is closely controlled by the prior 

deposition of pore-filling amorphous organic matter that is 

generally thought to be humate precipitated as a gel from a 

humic-acid-bearing solution (Granger and others, 1961; 

Moench and Schlee, 1967; Granger, 1968; Nash, 1968; 

Schmidt-Collerus, 1969; Squyres, 1970, 1980; Haji-Vassiliou and Kerr, 1973; Hatcher and others, 1986; Turner-Peterson, Fishman, and others, 1986). Uranium-vanadium 

deposits elsewhere in the Morrison Formation are poor in 

pore-filling organic matter, but humate may have been 

essential in uranium ore-formation and subsequently 

destroyed (Hansley and Spirakis, 1992). Humate in uranium 

deposits in the Chinle Formation has been postulated to exist 

(Huber, 1980), but studies necessary to determine its presence have not been performed. Thus, the role of humate is 

critical to the origin of the uranium deposits in the San Juan 

Basin and possibly elsewhere.

Previous workers have suggested that humate came (1) 

from the overlying Upper Cretaceous Dakota Sandstone and 

moved downdip in the Westwater Canyon Member ("Dakota 

source hypothesis") (Granger and others, 1961), (2) from the 

host sandstone and moved laterally downdip ("internal 

source hypothesis") (Granger and others, 1961; Squyres, 

1970, 1980), (3) from the mudstone beds overlying and 

interbedded with sandstone and moved downward and 

upward perpendicular to bedding or laterally outward 

("lacustrine-humate model") (Turner-Peterson, 1985; Fishman and Turner-Peterson, 1986; Turner-Peterson and Fishman, 1986), or (4) from surface water flowing downdip 

during deposition of the host sands ("syngenetic source 

hypothesis") (Granger and others, 1961). None of these 

interpretations can be ruled out based on availability of 

organic material. Organic "trash" is available in the host 

sandstone, and the former presence of organic matter in

mudstone is suggested by the fact that the mudstone is 

reduced today. Alteration patterns of iron-titanium oxide 

mineral and feldspar dissolution are interpreted as indicating 

downward flow from the mudflat of the Brushy Basin Member (Turner-Peterson, 1985; Fishman and Turner-Peterson, 

1986; Turner-Peterson and Fishman, 1986); however, replotting of the iron-titanium oxide data shows that downdip flow 

was significant (fig. 11). The downdip flow only has to 

remain in the upper part of the Westwater Canyon Member, 

which is normal for a dilute ground water overlying a 

slightly more saline ground water. The lacustrine-humate 

model would predict symmetric humate layers above and 

below mudstone layers; however, lacustrine mudstone beds 

generally are above the host sandstone, and humate layers do 

not wrap around the ends of mudstone beds. The Dakota 

source hypothesis would require that humate move from 

shoreline marshes downdip and seaward, which would only 

allow humate to accumulate within a short distance downdip 

from the Late Cretaceous outcrop of the Westwater Canyon 

Member. Deposits far downdip in the Westwater Canyon 

Member are difficult to explain by this mechanism.

Both the Dakota source hypothesis and lacustrinehumate model imply that humate was introduced significantly later than sedimentation. Paragenetic relations suggest that humate precipitation and associated iron-titanium 

oxide dissolution were early, but a variety of interpretations 

are possible (Adams and others, 1974; Adams and Saucier, 

1981; Hansley, 1986). Channels that scoured into slightly 

older humate- and uranium-impregnated channels (Fitch, 

1980; Squyres, 1980) suggest humate precipitation almost 

immediately after sedimentation; however, humate impregnation may have been later, controlled by permeability variations. These types of evidence for a very early age favor the 

syngenetic source hypothesis.

Spatial and chemical evidence tends to favor a syngenetic source in muds deposited very shortly after the host 

sand. Perhaps the strongest geologic evidence for a specific 

source is the close spatial association of organic-rich lacustrine mudstone typically above the uranium deposits (F. 

Peterson, 1980; Peterson and Turner-Peterson, 1980; 

Dubiel, 1983; Turner-Peterson, 1985). In addition, analyses 

of modern dissolved organic carbon suggest that lakes and 

wetlands were a more likely source of humic acid than 

ground water. Natural ground water has low dissolved 

organic carbon (median concentration, 0.7 mg/L), almost 

equal to that of sea water (mean, 0.5 mg/L); in contrast, oligotrophic and eutrophic lakes have mean dissolved organic 

carbon of 2.2 and 12 mg/L, respectively, and bogs have a 

mean dissolved organic carbon of 33 mg/L (Thurman, 1985, 

p. 8ff). The low dissolved organic carbon values in ground 

water suggest that dissolved organic carbon cannot be transported in large amounts for great distances and that nearby 

sources of dissolved organic carbon, especially lakes and 

bogs, are more likely.
ORIGIN OF TABULAR SANDSTONE URANIUM DEPOSITS 27

ONE OR TWO SOLUTIONS?

Some workers have suggested that tabular humate layers formed by the action of one solution (Squyres, 1980; 

Turner-Peterson, 1985; Fishman and Turner-Peterson, 1986; 

Turner-Peterson and Fishman, 1986). According to one version of the lacustrine-humate model (Turner-Peterson, 1985; 

Fishman and Turner-Peterson, 1986; Turner-Peterson and 

Fishman, 1986), humic acid from pore water in mudstone is 

expelled vertically upward or downward into the adjacent 

sandstone. The expelled fluid dissolves iron, vanadium, and 

aluminum from detrital grains and clays. As these cations 

increase in concentration along the flow path, they cause 

humate to precipitate. Once precipitation begins, flocculation, van der Waals forces, "herding instinct," or "the affinity 

of organics for each other" (Turner-Peterson, 1985; Fishman 

and Turner-Peterson, 1986) is supposed to account for the 

tabular shape.

This mechanism is unrealistic on spatial, chemical, and 

hydrologic grounds and is not supported by a plausible 

mechanism, modern analogs, or experimental evidence. 

Flocculation and van der Waals forces act on a very small 

scale, not over the thousands of meters typical of tabular 

sandstone uranium deposits. They result in small clumps 

because the forces act radially from numerous centers. How 

flocculation or van der Waals forces would yield an extensive, subhorizontal layer is not explained. Chemically, the 

proposed mechanism is implausible because the amount of 

water from compaction is too small to dissolve and transport 

the observed amounts of humate precipitated or cations 

leached. For example, humic acid in ground water is typically 0.5-1.0 mg/L and locally as much as 15 mg/L (Thurman, 1985); however, humate can constitute more than 5 

percent of the rock in a uranium deposit (Levanthal, 1980). 

To achieve such high concentrations, a much higher water to 

rock ratio is required than can be provided by pore water 

from compaction (Hilpert, 1969). In addition, it is improbable that the pore water in the mudstone would be so different 

from that in the sandstone that humic acid would dissolve 

from the first and precipitate in the second. Hydrologically, 

the model neglects gravity-driven flow, which would have 

been orders of magnitude greater than compaction-driven 

flow. Flow in transmissive fluvial sandstone is normally parallel with stratification, not perpendicular as depicted by the 

lacustrine-humate model. Downdip flow of one fluid would 

not result in the observed tabular layers but would probably 

create podlike or roll-shaped deposits as, for example, in the 

roll sandstone uranium deposits of Wyoming (Harshman, 

1972). The proposed mechanism appears to be loosely based 

on the process of podzolization; however, podzolization 

occurs today at the ground surface under boreal forests in 

areas of high rainfall and ground-water recharge (Petersen, 

1976; Mokma and Buurman, 1982; Birkeland, 1984, p. 

120ff), an environment very different from the arid to semiarid conditions during deposition of the Morrison Formation.

Further, "M. Thurman, 1981, oral communication," is the 

only reference to experiments showing the formation of tabular humate layers from one solution. Thurman today states 

that there are no experiments showing the formation of tabular humate layers from one solution (Michael Thurman, 

U.S. Geological Survey, oral commun., 1992).

On the other hand, field, theoretical, and laboratory evidence for formation of tabular humate layers at a saline water 

interface between two solutions is abundant. The subhorizontal attitude, gently crosscutting relationships, and lack of 

small-scale lithologic control are strong evidence for a density-stratified interface (Fischer, 1947, 1974; Shawe, 1955, 

1962, 1966; Granger, 1968; Wood, 1968; Melvin, 1976; 

Sanford, 1982, 1990a, b; Granger and Santos, 1986; 

Northrop and Goldhaber, 1990). On theoretical grounds, a 

density-stratified interface provides a laterally extensive, 

subhorizontal site for humate deposition and other reactions. 

The solutes in the lower fluid also provide a mechanism for 

humate precipitation, as demonstrated by experimental and 

field studies (Swanson and Palacas, 1965; Hair and Bassett, 

1973; Sholkovitz, 1976; Ortiz and others, 1980; Fox, 1983; 

Thurman, 1985, p. 26ff and 394ff). The maximum amount of 

humate removal is at salinities between 15,000 and 20,000 

mg/L. Therefore, although models of uranium and humate 

precipitation based on a density-stratified interface are commonly called "brine-interface" models, the term "brine" is 

misleading because the denser ground water need only be 

saline. As discussed above, topographic depressions associated with the uranium deposits are favorable for discharge of 

saline ground water, and the abundant evaporitic rocks in the 

subsurface are likely sources for saline water. Modern analogs of mixing between discharging, saline ground water 

overlain by relatively fresh water are common (Counts, 

1957; Toth, 1963; Freeze and Witherspoon, 1966; Gallaher 

and Price, 1966; Boswell and others, 1968; van Everdingen, 

1971; Foreman and Sharp, 1981; Mono Basin Ecosystem 

Study Committee, 1987; Sharp, 1988; Swanson and others, 

1988; Banner and others, 1989; Dutton and others, 1989; 

Huff, 1990; Fee and others, 1992; Herczeg and others, 1992; 

Hines and others, 1992; Long and others, 1992; Macumber, 

1992; Strobel, 1992). Thus, the saline water interface or twosolution model is the only plausible explanation for the precipitation of tabular humate layers.

SEEPAGE, COMPACTION, AND DENSITY

Seepage, compaction, and density have been proposed 

to account for the movement of humic-acid-bearing ground 

water from the presumed source (lacustrine mudstone) to the 

presumed site of humate deposition (adjacent sandstone). An 

early version of the lacustrine-humate model calls on downward seepage or compaction to transport humic acid from 

lacustrine mudstone to sites of deposition in adjacent sandstone (Peterson and Turner-Peterson, 1980). Later versions
28 JURASSIC AND TRIASSIC WETLANDS AND ORIGIN OF URANIUM DEPOSITS

of the model rely on compaction and density for moving 

humic acid (Turner-Peterson, 1985; Fishman and TurnerPeterson, 1986; Turner-Peterson and Fishman, 1986). All the 

proposed mechanisms have serious difficulties.

Downward seepage is proposed as a mechanism; however, no hydrologic reason is given for downward seepage as 

opposed to upward seepage. As discussed above, each of the 

regions of uranium ore formation in the Colorado Plateau is 

dominated by evidence for upward flow of ground water. 

Syndepositional synclines, decreases in slope, topographic 

depressions, lacustrine sediments, and reduced rocks suggest 

a high water table, preservation of organic matter in the saturated zone, and the discharge of ground water from local 

and regional flow systems. Under these conditions, seepage 

is normally upward into lakes and wetlands (Meyboom, 

1967; Lissey, 1971; Stephenson, 1971; Winter, 1976, 1978, 

1986, 1988; Friedman and others, 1982; Allison and Barnes, 

1985; Spencer and others, 1985; Duffy and Al-Hassan, 1988; 

Winter and Woo, 1990). Transient, depression-focused 

recharge has not previously been mentioned as having a role 

in uranium ore formation.

Compaction has been suggested as a significant mechanism for downward or lateral flow of ground water in the 

Morrison Formation (Shawe, 1976; Wood, 1980; Adams and 

Saucier, 1981; Turner-Peterson, 1985; Adams, 1986; Hansley, 1986; Turner-Peterson and Fishman, 1986; Northrop 

and Goldhaber, 1990; Wanty and others, 1990). Compaction 

is unlikely, however, to cause flow in the proper direction, is 

a minor ground-water control compared to gravity-driven 

flow, and cannot account for significant humate transport or 

diagenetic alteration (Hilpert, 1969; Sanford, 1990a, 1994). 

Compaction-driven flow is generally directed upward, especially near the top of a thick section of sediments (Bethke, 

1986), as was present below the Morrison Formation. In the 

center of a large alkaline-saline playa, downward flow is 

possible, but more as a result of density than compaction. 

The marginal mudflat of a large playa lake, which is inferred 

to have downward ground-water flow (Turner-Peterson, 

1985; Fishman and Turner-Peterson, 1986; Turner-Peterson 

and Fishman, 1986), is the least likely area for downward 

flow. The mudflat is at a lower elevation than the surrounding alluvial plain, so gravity-driven flow is directed upward 

and discharges in the mudflat. Ground water in the mudflat 

is less dense than that in the central part of the playa, so density drives ground water from the central playa outward 

toward the mudflat, where it discharges, as shown for modern examples (Allison and Barnes, 1985; Spencer and others, 

1985; Duffy and Al-Hassan, 1988; Winter and Woo, 1990). 

The mudflat of a large playa lake can therefore be expected 

typically to have ground-water discharge.

Second, simple calculations show that compaction is a 

minor influence as compared to gravity-driven flow (Sanford, 1990a, 1994). The Brushy Basin Member would contribute an average of 0.0035 mm/yr of ground-water flow

from expulsion of pore water in 5 m.y., whereas gravitydriven flow would contribute approximately 15 mm/yr. The 

dominance of gravity-driven flow over compaction-driven 

flow in this type of basin is further demonstrated by more 

elaborate calculations (Bethke, 1986; Person and others, 

1992; Garven and others, 1993). For example, Bethke (1986) 

estimated that maximum compaction-driven flow in the Illinois Basin is 2 mm/yr, whereas the maximum gravity-driven 

flow is 11,000 mm/year. Thus, available evidence indicates 

that gravity-driven flow in these settings exceeds compaction-driven flow by some two to four orders of magnitude.

Finally, compaction cannot account for the observed 

humate accumulation and diagenetic alteration of detrital 

minerals. As discussed above, humate accumulation requires 

a higher water to rock ratio than can be achieved by expulsion of pore water. Iron-titanium oxide minerals are locally 

100 percent altered from the top to the base of the Westwater 

Canyon Member. It is doubtful that enough pore water 

would have been expelled from the Brushy Basin Member to 

displace all the pore water in the Westwater Canyon Member, much less cause the intense alteration. Again, a higher 

water to rock ratio would be required than was available 

from compaction of the Brushy Basin Member. Further, 

there is no petrographic evidence for compaction. In the 

playa facies of the Brushy Basin, relict shard textures are 

perfectly preserved by replacement minerals such as chalcedony and clinoptilolite; in the mudflat facies, original shard 

textures are destroyed (Turner and Fishman, 1991).

Density of ground water has also been suggested as a 

mechanism for downward movement of ground water 

(Turner-Peterson, 1985; Fishman and Turner-Peterson, 

1986; Turner-Peterson and Fishman, 1986). The references 

cited by the authors of the lacustrine-humate model for 

downward flow of ground water pertain to playa lake water 

in the center of the playa where solutes are highly concentrated and abundant evidence shows that dense brines can 

move downward. The lacustrine-humate model depends, 

however, on ground water descending in the mudflat, not in 

the central playa. An argument that demonstrates that ground 

water can descend in the central part of a playa does not demonstrate that it descends in the mudflat. As shown by diagenetic facies in the Brushy Basin Member, the mudflat has 

relatively fresh water, and the concentration of solutes 

increases toward the center of the lake. The density is therefore less in the mudflat than in the playa center. All other factors being equal, density would cause ground water to flow 

downward in the playa center, then outward, and finally 

upward in the mudflat. Modeling of modern environments 

confirms this pattern (Duffy and Al-Hassan, 1988).

Another difficulty with the density argument is that it is 

incompatible with transport of humic acid. Humic acid precipitates in concentrated solutions (Swanson and Palacas, 

1965; Hair and Bassett, 1973; Sholkovitz, 1976; Ortiz and 

others, 1980; Fox, 1983; Thurman, 1985, p. 26ff and 394ff), 

a fact acknowledged by the authors of the lacustrine-humate
ORIGIN OF TABULAR SANDSTONE URANIUM DEPOSITS 29

model. To rely on a dense solution to move humic-acid-bearing ground water is to contradict the assumed chemistry of 

the humic-acid-transporting solution, which is thought to be 

dilute and mildly alkaline. The solution cannot be both dense 

and dilute.

A third difficulty is that a dense solution cannot explain 

the upward movement of ground water, which the authors of 

the lacustrine-humate model consider necessary for ore 

deposits in sandstone above mudstone, such as in the Jackpile Sandstone Member and locally in the Westwater Canyon Member. Much emphasis is placed on an apparent 

"mirror image" alteration pattern, where the intensity of 

alteration in sandstone decreases away from the Brushy 

Basin Member, both down into the Westwater Canyon 

Member and up into the Jackpile Member. This pattern cannot be explained, however, by density-driven flow. Although 

density-driven flow probably was a significant mechanism 

in certain situations, it is incompatible with the lacustrinehumate model.

TRANSIENT, DEPRESSION-FOCUSED 

GROUND-WATER RECHARGE

The above discussion raises two critical questions. If 

there was a saline water interface, why was humate only precipitated in certain places along the interface? If humic acid 

came from lacustrine muds, how did it move downward, 

apparently against the prevailing hydrologic gradient?

Transient, depression-focused recharge during periods 

of high runoff is a common phenomenon in modern wetland 

environments and appears to be the only solution to the 

dilemma (fig. 18). A stream that normally gains water from 

ground-water discharge may, during flood stage, lose water 

to the banks of the channel (Gallaher and Price, 1966; Speer 

and others, 1966; Fetter, 1988, p. 47; Jacobson and others, 

1989). Ground-water flow, which normally is toward the 

channel, is reversed away from the channel. Similarly, during periods of high runoff, lakes that normally have groundwater discharge into them may recharge the ground-water 

system (Meyboom, 1967; Lissey, 1971; Winter, 1976, 1983, 

1986; Wood and Petraitis, 1984; Allison and Barnes, 1985; 

Fetter, 1988, p. 45ff; Logan and Rudolph, 1992). Watertable "mounds" build up temporarily in the ground surrounding the lake, and the direction of ground-water flow is 

reversed. More water infiltrates the ground around the temporarily deepened lakes than infiltrates the ground over the 

intervening hills. Thus, the wetland temporarily becomes an 

area of ground-water recharge. The time period between 

such events may range from days to decades.

For the ancient environment relevant to uranium ore 

formation, transient, depression-focused recharge in wetlands can explain the apparent downward transport of humic 

matter in zones of normally upward discharge (fig. 18). Dissolved organic matter probably was concentrated in the

bottom water of lakes and in the pore water of lacustrine and 

fluvial sediments that were deposited at topographic depressions. A perennially high water table, which favors the preservation of organic matter, accounts for the observed 

reduced rocks. During baseline or steady-state flow, ground 

water discharged upward into the channel or lake sediments. 

Depending on the prevailing climate, discharge consisted of 

saline ground water (fig. 18A, baseline stage-dry) or saline 

ground water overlain by fresher ground water (fig. 186, 

baseline stage-wet). During periods of high water, surface 

water probably rose and ponded in topographic depressions. 

The organic-acid-bearing pore water in the sediments may 

have been flushed downward and outward along the channel. Judging from analyses of modern water (Thurman, 

1985), dissolved organic carbon in the descending pore 

water may have been 2-33 mg/L, in contrast to 0.5-1.0 

mg/L in the underlying saline ground water. The relatively 

fresh water formed a lens that spread out on top of the underlying saline ground water. The underlying saline fluid was 

displaced slightly downward owing to the increased pressure 

of the relatively fresh water on top. The fresh-water lens 

occupied the upper part of the zone formerly occupied by 

saline ground water. The less soluble, humic part of the dissolved organic carbon precipitated in the mixing zone.

The localization of humate and uranium only at certain 

places along the interface can be explained by the localization of humate sources and depression-focused recharge. 

The humic-acid-rich water descended from specific locations on the surface where organic-rich sediments preferentially accumulated and transient recharge was focused; that 

is, at topographic depressions. Wetlands may have been the 

only sites of appreciable accumulation of organic matter 

because grasses had not yet evolved and the major lowgrowing plants were ferns and related vegetation that require 

an abundant and constant water supply. Further, topographic 

depressions were favorable for accumulation of detrital 

organic matter, which is widely associated with the uranium 

deposits. Although an interface between shallow fresher 

water and deeper more saline water may have been widespread, precipitation of humate apparently only occurred at 

the interface hydrologically downgradient from sources of 

humic acid at local topographic depressions. Ground-water 

flow downdip along the channels would account for the 

commonly observed elongation of deposits.

Fluctuations in the position of the interface due to 

intermittent, seasonal, climatic, and tectonic variations can 

explain the thickness and multiple positions of uraniumhumate bodies. Small fluctuations in the position of the 

interface (on the scale of meters) can explain thickness 

variations by means of dispersion. During periods of high 

water, shallow, relatively fresh ground water displaced 

saline water below. In the zone of displacement, the humic 

acids would have been transported in the interconnected 

pores, whereas saline water would have remained in the 

less connected pores. Humate precipitation may have taken
30 JURASSIC AND TRIASSIC WETLANDS AND ORIGIN OF URANIUM DEPOSITS

FLOOD STAGE OR WET PERIOD

::. /JT' :..••.•-••::. •i '-jj^-j'=-"-•••

EXPLANATION

Saline water ^^^^H Organic-rich mud

Fresh water 

Unsaturated zone

Humate layer

Figure 18. Schematic cross section across a topographic depression occupied by a channel or lake showing relationships of hydrology and 

humate. Ground water normally discharges at topographic depressions, but, during high-water conditions, recharge of relatively fresh water 

can take place. Gravity-driven, transient, downward-flowing ground water carries humic acids from wetlands, and humate precipitates in an 

interface zone of mixing between relatively fresh water and displaced saline water. A, Possible baseline or steady-state discharge of saline 

ground water under dry conditions, no fresh-water discharge. B, Possible baseline or steady-state discharge under wetter conditions, relatively fresh and saline ground water discharge. Modified from Dutton and others (1989). C, Transient recharge during high-water stage or 

period. Modified from Winter (1976, 1986) and Fetter (1988, p. 47).
SUMMARY AND CONCLUSIONS 31

place by dispersive mixing of humic-acid-bearing ground 

water in the interconnected pores with saline ground water 

in the less connected pores. Multiple positions of tabular 

humate bodies are explained by larger scale variations of 

the interface, which may have been controlled by longer 

term fluctuations of climate or by tectonic adjustments that 

affected the steady-state or baseline position of the water 

table. Because humate can be redissolved and reprecipitated by dilute water (Swanson and Palacas, 1965), downward movement of the interface could have moved the 

humate downward, but upward movement of the interface 

would have left the humate in place. Thus, a single layer of 

humate could be concentrated by repeated downward 

excursions of the interface.

Whereas compaction is a one-time occurrence involving small amounts of ground water, transient flushing can 

involve large ground-water fluxes and thus account for 

intense alteration. The widespread alteration of iron-titanium oxide minerals probably can only be explained by 

many pore volumes passing through the sediments. The pattern of dissolution of iron-titanium oxide minerals in the San 

Juan Basin and the elongation of tabular uranium bodies 

along the channels throughout the Colorado Plateau are consistent with a combination of transient, depression-focused 

recharge and lateral flow down the channels.

Variability in precipitation is necessary for reversals in 

ground-water flow. In Triassic and Jurassic times, precipitation fluctuated in time periods that varied from single-event 

storms to seasonal and longer term climatic cycles. Paleoclimatic, sedimentologic, and, paleontologic evidence (Dubiel, 

1989; Dubiel and others, 1991) indicates a monsoonal climate in the Late Triassic. Generally abundant water supply 

was punctuated by periods of dry conditions. A generally 

drier climate prevailed during the Late Jurassic (Peterson 

and Turner-Peterson, 1987). Lithofacies in the Brushy Basin 

Member are similar to those of the Lake Eyre region of central Australia, where major flooding occurs every 5-10 years 

(Bell, 1986). Conglomeratic layers in the otherwise finegrained Brushy Basin Member (Phoenix, 1958) are evidence 

for episodic flooding. During both Late Triassic and Late 

Jurassic time, variegated red and green mudstone suggests 

fluctuating water tables.

SUMMARY AND CONCLUSIONS

Reconstruction of ground-water-flow directions in 

Jurassic and Triassic fluvial-lacustrine rocks of the Colorado 

Plateau strengthens the conclusions of previous studies (Sanford, 1982, 1990b, 1992, 1994) that tabular sandstone uranium deposits formed in zones of inferred regional groundwater discharge. On the scale of the Colorado Plateau, the 

deposits are commonly associated with transitional lithofacies that indicate topographic controls on ground-water flow.

Commonly arcuate zones of transitional lithofacies indicate 

a decrease in paleotopographic slope and favor discharge 

and mixing of local and regional ground-water-flow systems 

in wetland environments. For example, such facies changes 

in the Salt Wash Sandstone Member of the Jurassic Morrison 

Formation include the sandstone-mudstone facies, transitional between conglomeratic sandstone facies and claystone-limestone facies (Craig and others, 1955), and the 

fluvial sheet-sand facies, transitional between the fluvial 

sheet gravels and mudflat-lake muds (Peterson and Tyler,

1985). In the Triassic Chinle Formation, decreasing topographic slope is indicated by a transition from distal braided 

stream to floodplain deposits (Finch, 1959; Young, 1964; 

Malan, 1968; Lupe, 1977). Tabular sandstone uranium 

deposits are closely associated with these facies changes, 

typically where the distal edge of the alluvial plain merges 

into mudflat. Measurable parameters that indicate a decrease 

in slope on a regional scale include an increase from low- to 

high-sinuosity channels, a decrease in thickness of sandstone, a decrease in sandstone as a percent of total thickness, 

and an increase in mudstone thickness and percentage (Craig 

and others, 1955; Mullens and Freeman, 1957; Peterson and 

Tyler, 1985). On a more local scale, the deposits are commonly associated with syndepositional synclines, possibly 

controlled by basement faulting. Such structural control has 

been inferred for the Henry Basin (Peterson, 1984, 1986), 

Uravan mineral belt. Grants uranium region (Kirk and Condon, 1986), Jackpile Sandstone Member of the Morrison 

(Moench and Schlee, 1967), and Chinle strata (Spirakis, 

1980; Dubiel, 1983). The topographic depressions associated with these structural downwarps were favorable for 

ground-water discharge and wetlands. Sedimentologic evidence for structural downwarps includes, in the Henry Basin, 

thicker sediments, more channel sandstone, higher sinuosity 

channels, increased upper flow regime horizontal laminations, and lacustrine mudstone (Peterson, 1984, 1986); in the 

Uravan mineral belt, distributary channels, thick host sandstone, higher sandstone to mudstone ratios, lenticular rather 

than flatbedded sandstone, scour-and-fill bedding, and overlying conglomeratic sandstone (Weir, 1952; Phoenix, 1958; 

Shawe, 1962; Motica, 1968; Thamm and others, 1981); and, 

in the Grants uranium region, locally thick sediments, high 

sandstone to mudstone ratios, and smaller numbers of sandstone-mudstone interbeds (Kirk and Condon, 1986). Lacustrine mudstone is associated with uranium deposits in the 

Henry Basin (Peterson, 1984, 1986), Grants uranium region 

(Turner-Peterson, 1985; Turner-Peterson and Fishman,

1986), and Chinle strata (Dubiel, 1983). As evidence for 

closed topographic depressions, lacustrine deposits strongly 

suggest low hydraulic potential and ground-water discharge 

during sedimentation.

The only generally consistent association between uranium deposits and lithofacies is between lithofacies in the 

host sandstone and uranium deposits. Lithofacies in overlying units are not consistently associated with uranium
32 JURASSIC AND TRIASSIC WETLANDS AND ORIGIN OF URANIUM DEPOSITS

deposits. For example, a proposed association between tabular sandstone uranium deposits and the mudflat facies of the 

Brushy Basin Member of the Morrison Formation (TurnerPeterson and Fishman, 1986) breaks down for deposits in 

the Uravan mineral belt and Henry Basin, where uranium 

deposits are associated with analcime-potassium feldspar 

and alluvial-plain facies of the Brushy Basin Member, 

respectively, not the mudflat facies (fig. 13). Other workers 

noted the relationship between tabular sandstone uranium 

deposits and lithofacies of the host sandstone but failed to 

explain the relationship. For example, Craig and others 

(1955) noted that deposits cluster where the Salt Wash 

Sandstone Member of the Morrison is more than 73 m (240 

ft) thick and stream channel sandstone makes up 40-55 percent of the thickness of the member; the deposits are 

between 27 and 60 m (90 and 200 ft) thick and have a percentage mean deviation of between 5 and 18 percent. They 

noted that the permeability of these rocks is intermediate 

between more sandstone rich facies and more mudstone rich 

facies, and they hypothesized that intermediate flow rates 

were optimum for uranium and vanadium supply. I suggest 

that the intermediate lithologic facies represents paleotopographic conditions that are hydrologically favorable for 

humate and uranium deposition. The facies changes are concomitant with hydrologic changes that controlled the transport and deposition of organic matter and uranium.

The scale of the observation and the influence of local 

tectonic structures must be accounted for in interpreting the 

paleotopographic slope. On the scale of the Colorado Plateau, sedimentary facies, total thickness, sandstone to mudstone ratio, and other parameters change systematically in 

the direction of flow. For example, facies of the Salt Wash 

Sandstone Member of the Morrison Formation change from 

conglomeratic sandstone to sandstone and mudstone to claystone (Craig and others, 1955) or from fluvial sheet gravels 

to fluvial sheet sands to mudflat and lake muds (Peterson and 

Tyler, 1985). In addition, there are regional changes such as 

decrease in total thickness and sandstone to mudstone ratio. 

At this regional scale, coarser facies, thicker sandstone, and 

higher sandstone to mudstone ratio suggest greater topographic slope closer to the source. Local tectonic activity 

can, however, significantly affect these overall trends (Peterson, 1984; Kirk and Condon, 1986). On a more local scale, 

for example in the Henry Basin, local tectonic activity 

affected stream gradients, and the deposition of more channel sandstone in synclines may have resulted from combing 

back and forth of streams and winnowing out of finer material (Peterson, 1984). At this scale, coarser facies, greater 

thickness, and higher sandstone to mudstone ratio are associated with streams that were slightly lower in elevation than 

the surrounding overbank deposits.

Geologic controls on ground-water flow favored discharge of generally northeast flowing ground water everywhere in the Morrison Formation and especially in arcuate

zones of abrupt aquifer thinning. Thinning and pinching out 

of the Lower Jurassic and upper Paleozoic aquifer systems 

favored geologically controlled discharge. Within these 

aquifer systems, northward and eastward thinning of the 

Navajo Sandstone, Wingate Sandstone, Cedar Mesa Sandstone Member of the Cutler Formation, De Chelly Sandstone, and Meseta Blanca Sandstone Member of the Yeso 

Formation most favored deep ground-water discharge. Thinning of the section over the Uncompahgre and San Luis Precambrian basement blocks probably contributed to deep 

ground-water discharge. For deposits in the Chinle Formation, westward and northward flow was in the direction of a 

thinner Paleozoic section but a thicker eolian aquifer, and the 

net effect of geologic controls is unclear.

Within the major discharge zones where topographic 

and geologic controls generally favored local and regional 

ground-water discharge, specific areas of discharge are 

marked by reduced rocks such as green and gray mudstone. 

A close association between reduced mudstone and channel 

sandstone and tabular sandstone uranium deposits has been 

noted for the Henry Basin (Peterson, 1984, 1986), Paradox 

Basin (McKay, 1955), San Juan Basin (Turner-Peterson, 

1985; Turner-Peterson and Fishman, 1986), and Chinle Formation (Wood, 1968; Dubiel, 1983). Reduced rocks in dominantly redbed sequences suggest perennially high water 

table conditions (Walker, 1967; Reading, 1978, p. 48-49; 

Dodson and others, 1980; Dubiel, 1983, 1989; Davis, 1988; 

Ghiorse and Wilson, 1988; Dubiel and others, 1991) and 

reducing conditions owing to bacterial degradation of 

organic matter, both of which are most likely in areas of 

perennial ground-water discharge.

The composition of the discharging ground water may 

be inferred from the types of rocks through which the ground 

water passed. Shallow, local ground water was probably 

dilute meteoric water. As it passed downdip through the 

aquifer, it reacted with detrital material and any bacterially 

degraded organic matter. Judging from modern ground water 

in shallow aquifers in arid environments, the water was probably a dilute Na+-Ca2+-HCO3- type (Hanshaw and Hill, 

1969; Thackston and others, 1981; Davis, 1988; Sanford, 

1990a). Recharging, depression-focused ground water was 

probably relatively fresh, neutral to slightly acidic, reducing, 

and humic acid rich.

In contrast to the dilute, local ground water, deeper, 

regional ground water was probably saline owing to the 

widespread presence of evaporites and evaporitic clastic 

rocks. Shallow source rocks included the Curtis Formation, 

Todilto Limestone Member of the Wanakah Formation, and 

Tidwell Member of the Morrison Formation. Deeper 

sources included gypsum beds in the Triassic marginalmarine Moenkopi Formation. Still deeper Pennsylvanian 

and Permian sources of saline water were especially abundant and included all or parts of the Paradox Formation,
SUMMARY AND CONCLUSIONS 33

Kaibab Limestone, Cutler Formation, and Yeso Formation. 

Dissolution of salts and migration of saline water has modern-day analogs. For example, Carmel Formation evaporites 

are dissolved by descending relatively fresh water, which 

then becomes saline; this saline water moves downdip in the 

underlying Navajo Sandstone and finally discharges upward 

from the Navajo (Taylor and Hood, 1988). Modern ground 

water in the Colorado Plateau today is locally highly saline 

owing to dissolution of evaporites (Mayhew and Heylman, 

1965; Hanshaw and Hill, 1969; Thackston and others, 1981; 

Sanford, 1990a). Evidence for an interface between regional 

saline and local fresher ground water is shown by tabular 

layers of humate, uranium, and dolomite in the Colorado 

Plateau (Fischer, 1947, 1974; Shawe, 1955, 1962, 1966; 

Granger, 1968; Wood, 1968; Melvin, 1976; Sanford, 1982, 

1990a, b; Granger and Santos, 1986; Northrop and Goldhaber, 1990) and by experimental evidence and modem 

analogs, as discussed above. Many uranium-humate and 

uranium-vanadium deposits are elongated along the channels and rise stratigraphically toward the basin (Reinhardt, 

1963; Moench and Schlee, 1967; Granger, 1968; Hilpert, 

1969; Fischer, 1974; Melvin, 1976; Northrop and Goldhaber, 1990), which would be expected at an interface 

where an upper dilute ground water flowed over a lower 

brine and discharged.

Thus, abundant evidence exists for an association 

between tabular sandstone uranium deposits and regional 

ground-water discharge during ore formation. Further, the 

discharging ground water included dilute local and saline 

regional fluids that probably interacted at a density-stabilized interface.

The fact that a consistent set of characteristics is associated with uranium deposits in Jurassic and Triassic deposits throughout the Colorado Plateau indicates that the 

deposit type can be described by one general genetic 

model. The recurrent deposit characteristics including 

geometry, channel sandstone host rock, elongation and 

stratigraphic rise in the direction of paleoflow, association 

with reduced rocks, and presence in structurally controlled 

paleotopographic depressions suggest common physical 

and chemical controls in which wetlands played a critical 

role. Humate-rich and vanadium-rich deposits probably are 

variations on a general theme rather than distinct deposit 

types (Sanford, 1992). Chemical changes among these variations can be attributed to post-ore diagenesis (Hansley and 

Spirakis, 1992).

The conclusion that tabular sandstone uranium deposits 

are closely associated with a particular hydrologic environment implies that the ore, or at least the humate, was deposited very soon after deposition of the sediments, perhaps 

after only meters or tens of meters of burial. Many prior estimates could narrow the age only to "shortly after deposition" of the host sediments, which could mean anything

from days to tens of millions of years. For example, ore 

deposits in the Westwater Canyon Member of the Morrison 

Formation are thought to have formed during compaction of 

the overlying Brushy Basin Member (Turner-Peterson, 

1985; Turner-Peterson and Fishman, 1986). The present 

analysis suggests that ore formed even earlier. In a zone of 

regional ground-water discharge, depression-focused 

recharge would only displace the uppermost ground water. 

Humic acid from wetlands would only be transported to 

shallow depths. Once the humate layer was deposited, uranium may have accumulated later as a result of the reducing 

conditions generated by bacterial degradation of humate. A 

very early age for humate precipitation is supported by 

channel scours into humate-impregnated sandstone (Fitch, 

1980; Squyres, 1980).

The associations among channel sandstone, transitional lithofacies, decrease in topographic slope, thinning of 

aquifers, syndepositional synclines, paleotopographic 

depressions, interbedded sandstone and mudstone, underlying marine rocks especially evaporites, reduced mudstone, 

and tabular sandstone uranium deposits are so significant 

that any model of uranium deposition must account for 

them. The only unifying phenomenon yet proposed that is 

consistent with these observations is the regional discharge 

of deep gravity-driven ground water that mixed with shallow dilute ground water during and shortly after deposition 

of the host sediments in a wetland environment. Depression-focused transient recharge can explain the apparent 

downward transport of humic matter in areas of normally 

discharging, saline, regional ground water.

The present analysis suggests that exploration for new 

districts and belts of tabular sandstone uranium deposits 

should be guided by features that indicate areas of regional 

ground-water discharge and transient, depression-focused, 

local recharge during and shortly after sedimentation. 

Major uranium belts can be expected where coarsegrained, alluvial-plain facies merge into finer grained, 

lower gradient facies. Areas where aquifers also thin in the 

direction of flow are still more favorable. Within these 

broad areas, local structural, sedimentologic, and diagenetic features such as syndepositional synclines, fluvial 

channels, and reducing conditions that indicate ancient 

wetlands are most favorable.

Acknowledgments.—Discussions with Warren I. 

Finch, Harry C. Granger, Paula L. Hansley, Fred Peterson, 

and Charles S. Spirakis were constructive. Thomas E. Bell, 

Steven M. Condon, Russell F. Dubiel, Christopher J. Duffy, 

Warren I. Finch, Arthur L. Geldon, Harry C. Granger, 

Steven E. Ingebritson, and Charles S. Spirakis reviewed the 

paper. I, of course, take sole responsibility for the interpretations expressed.
34 JURASSIC AND TRIASSIC WETLANDS AND ORIGIN OF URANIUM DEPOSITS

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Whittemore, D.O., McGregor, K.M., and Marotz, G.A., 1989, 

Effects of variations in recharge on groundwater quality: Journal of Hydrology, v. 106, p. 131-145.

Williams, A.E., and McKibben, M.A., 1989, A brine interface in 

the Salton Sea geothermal system, California—Fluid 

geochemical and isotopic characteristics: Geochimica et Cosmochimica Acta, v. 53, p. 1905-1920.

Winter, T.C., 1976, Numerical simulation analysis of the interaction of lakes and ground water: U.S. Geological Survey Professional Paper 1001, 45 p.

————1978, Numerical simulation of steady state three-dimensional groundwater flow near lakes: Water Resources 

Research, v. 14, p. 245-254.

————1983, The interaction of lakes with variably saturated 

porous media: Water Resources Research, v. 19, p. 

1203-1218.

————1986, Effect of ground-water recharge on configuration of 

the water table beneath sand dunes and on seepage lakes in the 

sandhills of Nebraska, U.S.A.: Journal of Hydrology, v. 86, p. 

221-237.

————1988, A conceptual framework for assessing cumulative 

impacts on the hydrology of nontidal wetlands: Environmental 

Management, v. 12, p. 605-620.

Winter, T.C., and Woo, M.-K., 1990, Hydrogeology of lakes and 

wetlands, in Moss, M.E., Wolman, M.G., and Riggs, H.C., 

eds., Surface water hydrology: Geological Society of America, 

The Geology of North America, v. O-l, p. 159-187.

Witkind, I.J., and Thaden, R.E., 1963, Geology and uranium-vanadium deposits of the Monument Valley area Apache and Navajo Counties, Arizona: U.S. Geological Survey Bulletin 1103, 

171 p.

Wood, H.B., 1968, Geology and exploitation of uranium deposits in 

the Lisbon Valley area, Utah, in Ridge, J.D., ed., Ore deposits 

of the United States, 1933-1967: New York, American Institute of Mining, Metallurgical, and Petroleum Engineers, p. 

771-789.

Wood, W.W., and Petraitis, M.J., 1984, Origin and distribution of 

carbon dioxide in the unsaturated zone of the Southern High 

Plains of Texas: Water Resources Research, v. 20, p. 

1193-1208.

Young, R.G., 1964, Distribution of uranium deposits in the White 

Canyon-Monument Valley district, Utah-Arizona: Economic 

Geology, v. 59, p. 850-873.

————1978, Depositional systems and dispersal patterns in uraniferous sandstones of the Colorado Plateau: Utah Geology, v. 5, 

p. 85-102.

*U.S. GOVERNMENT PRINTING OFFICE: 1994-673-046/86050
SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS

Periodicals

Earthquakes & Volcanoes (issued bimonthly). 

Preliminary Determination of Epicenters (issued monthly).

Technical Books and Reports

Professional Papers are mainly comprehensive scientific reports 

of wide and lasting interest and importance to professional scientists and 

engineers. Included are reports on the results of resource studies and of 

topographic, hydrologic, and geologic investigations. They also include 

collections of related papers addressing different aspects of a single scientific topic.

Bulletins contain significant data and interpretations that are of 

lasting scientific interest but are generally more limited in scope or geographic coverage than Professional Papers. They include the results of 

resource studies and of geologic and topographic investigations; as well 

as collections of short papers related to a specific topic.

Water-Supply Papers are comprehensive reports that present significant interpretive results of hydrologic investigations of wide interest 

to professional geologists, hydrologists, and engineers. The series covers 

investigations in all phases of hydrology, including hydrology, availability of water, quality of water, and use of water.

Circulars present administrative information or important scientific information of wide popular interest in a format designed for distribution at no cost to the public. Information is usually of short-term 

interest.

Water-Resources Investigations Reports are papers of an interpretive nature made available to the public outside the formal USGS 

publications series. Copies are reproduced on request unlike formal 

USGS publications, and they are also available for public inspection at 

depositories indicated in USGS catalogs.

Open-File Reports include unpublished manuscript reports, maps, 

and other material that are made available for public consultation at depositories. They are a nonpermanent form of publication that may be cited in other publications as sources of information.

Maps

Geologic Quadrangle Maps are multicolor geologic maps on topographic bases in 7 1/2- or 15-minute quadrangle formats (scales mainly 1:24,000 or 1:62,500) showing bedrock, surficial, or engineering 

geology. Maps generally include brief texts; some maps include structure 

and columnar sections only.

Geophysical Investigations Maps are on topographic or planimetric bases at various scales, they show results of surveys using geophysical techniques, such as gravity, magnetic, seismic, or radioactivity, 

which reflect subsurface structures that are of economic or geologic significance. Many maps include correlations with the geology.

Miscellaneous Investigations Series Maps are on planimetric or 

topographic bases of regular and irregular areas at various scales; they 

present a wide variety of format and subject matter. The series also includes 7 1/2-minute quadrangle photogeologic maps on planimetric 

bases which show geology as interpreted from aerial photographs. The 

series also includes maps of Mars and the Moon.

Coal Investigations Maps are geologic maps on topographic or 

planimetric bases at various scales showing bedrock or surficial geology, 

stratigraphy, and structural relations in certain coal-resource areas.

Oil and Gas Investigations Charts show stratigraphic information for certain oil and gas fields and other areas having petroleum potential.

Miscellaneous Field Studies Maps are multicolor or black-andwhite maps on topographic or planimetric bases on quadrangle or irregular areas at various scales. Pre-1971 maps show bedrock geology in relation to specific mining or mineral-deposit problems; post-1971 maps 

are primarily black-and-white maps on various subjects such as environmental studies or wilderness mineral investigations.

Hydrologic Investigations Atlases are multicolored or black-andwhite maps on topographic or planimetric bases presenting a wide range 

of geohydrologic data of both regular and irregular areas; the principal 

scale is 1:24,000, and regional studies are at 1:250,000 scale or smaller.

Catalogs

Permanent catalogs, as well as some others, giving comprehensive 

listings of U.S. Geological Survey publications are available under the 

conditions indicated below from USGS Map Distribution, Box 25286, 

Building 810, Denver Federal Center, Denver, CO 80225. (See latest 

Price and Availability List.)

"Publications of the Geological Survey, 1879-1961" may be purchased by mail and over the counter in paperback book form and as a set 

microfiche.

"Publications of the Geological Survey, 1962-1970" may be purchased by mail and over the counter in paperback book form and as a set 

of microfiche.

"Publications of the U.S. Geological Survey, 1971-1981" may be 

purchased by mail and over the counter in paperback book form (two 

volumes, publications listing and index) and as a set of microfiche.

Supplements for 1982, 1983, 1984, 1985, 1986, and for subsequent years since the last permanent catalog may be purchased by mail 

and over the counter in paperback book form.

State catalogs, "List of U.S. Geological Survey Geologic and Water-Supply Reports and Maps For (State)," may be purchased by mail 

and over the counter in paperback booklet form only.

"Price and Availability List of U.S. Geological Survey Publications," issued annually, is available free of charge in paperback booklet 

form only.

Selected copies of a monthly catalog "New Publications of the 

U.S. Geological Survey" is available free of charge by mail or may be 

obtained over the counter in paperback booklet form only. Those wishing a free subscription to the monthly catalog "New Publications of the 

U.S. 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.