stratigraphic relationships and depositional environments and
TRANSCRIPT
STRATIGRAPHIC RELATIONSHIPS AND DEPOSITIONAL ENVIRONMENTS
OF THE UPPER CRETACEOUS PICTURED CLIFFS SANDSTONE
AND FRUITLAND FORMATION, SOUTHWESTERN
SAN JUAN BASIN, NEW MEXICO
by
MICHAEL FRANCIS ERPENBECK, B.S.
A THESIS
IN
GEOSCIENCES
Submitted to the Graduate Faculty of Texas Tech University in
Partial Fulfillment of the Requirements for
the Degree of
MASTER OF SCIENCE
Approved
AccTepted
December, 1979
I c
• / , ' •
i /"or) » e^' /;
ACKNOWLEDGMENTS
I would like to express my sincere gratitude to Dr.
Romeo M. Flores for providing me with assistance, guidance,
and motivation during the course of this study. Special
thanks also to Dr. David K. Davies and Dr. Alonzo D. Jacka
for serving on the Advisory Committee, valuable suggestions,
and critical review of the manuscript. Moral support and
numerous helpful suggestions came from Dr. David Weide,
James Mytton, Bruce Hunter, and Richard Vessel, for whose
assistance I am grateful. Financial support during the course
of the field work in the summer of 1978 was provided by the
Branch of Coal Resources of the U.S. Geological Survey in
Denver, Colorado.
11
TABLE OF CONTENTS
Page
ACKNOWLEDGMENTS ii
LIST OF ILLUSTRATIONS V
I. INTRODUCTION 1
General Statement 1
Ob j ectives 2
Location 2
Previous Work 2
Methods of Study 5
11. GEOLOGIC SETTING 14
III. PICTURED CLIFFS SANDSTONE 20
Descriptive Stratigraphy 20
Environments of Deposition 21
Delta-front Lithofacies 22
Distal Bar Sandstone 22
Distributary Mouth Bar Sandstone 26
Distributary Channel
Sandstone 27
Beach-bar Lithofacies 27
Lower Shoreface Sandstone 29
Middle Shoreface Sandstone 29
Beach-Upper Shoreface Sandstone 32 Tidal-inlet Channel Sandstone 32
111
IV. FRUITLAND FORMATION 37
Descriptive Stratigraphy 37
Environments of Deposition 41
Delta-Plain Lithofacies 41
Distributary Channel
Sandstone 41
Natural Levee Deposits 44
Crevasse Splay Deposits 45
Overbank Deposits 4 7
Backswamp Deposits 51
Back-barrier Lithofacies 55
Tidal Channel Sandstone 56
Back-barrier Marsh Deposits 58
V. PALEOGEOGRAPHIC RECONSTRUCTION 61
Paleocurrent Directions 61
Depositional Model 61
VI. APPLICATION TO COAL EXPLORATION 67
VII. SUMMARY AND CONCLUSIONS 69
Recognition of Deltaic and
Beach-bar Deposits 69
Conclusions 72
REFERENCES CITED 74
IV
LIST OF ILLUSTRATIONS
Figure Page
1. Location of Study Area 3
2. Cross Section Illustrating Delta-plain Deposits of the Fruitland Formation 7
3. Cross Section Illustrating Delta-front and Delta-plain Deposits of the Pictured Cliffs Sandstone and Fruitland Formation 9
4. Cross Section Illustrating Beach-bar and Back-barrier Deposits of the Pictured Cliffs Sandstone and Fruitland Formation Near Black Lake 11
5. Cross Section Illustrating Beach-bar and Back-barrier Deposits of the Pictured Cliffs Sandstone and Fruitland Formation Near Ah-Shi-Sle-Pah Wash 13
6. Upper Cretaceous and Tertiary Stratigraphic Chart Within the San Juan Basin 16
7. Outcrop Pattern of Pictured Cliffs Sandstone and Fruitland Formation 17
8. Distribution of Land and Sea During Late-Campanian Time 18
9. Delta-front Lithofacies of the Pictured Cliffs Sandstone 23
10. Ball-and-Pillow Structure, Pictured Cliffs Sandstone 25
11. Slump Structure, Pictured Cliffs Sandstone 25
12. Erosional Base of Distributary Channel, Pictured Cliffs Sandstone 28
13. Ophiomorpha Burrows in the Pictured Cliffs Sandstone 31
14. Middle Shoreface Parallel Laminations and Ophiomorpha Burrow, Pictured Cliffs Sandstone 33
V
Figure Page
15. Middle Shoreface Trough Cross-Laminations, Pictured Cliffs Sandstone 33
16. Bioturbated Zone of the Middle Shoreface Sandstone, Pictured Cliffs Sandstone 34
17. Tidal-inlet Channel Sandstone of the
Pictured Cliffs Sandstone 36
18. Basal Coal of the Back-barrier Lithofacies... 36
19. Fining Upward Sequence in a Distributary Channel Sandstone, Fruitland Formation 4 3
20. Large Scale Planar and Trough Cross Beds in a Distributary Channel Sandstone, Fruitland Formation 43
21. Crevasse Splay Sandstone in the Fruitland Formation 46
22. Ripple Cross Laminations of a Crevasse Splay Sandstone in the Fruitland Format ion 4 8
23. Interbedded Siltstone, Shale, Carbonaceous Shale, and Coal Comprising Overbank Deposits of the Fruitland Formation 50
24. Siderite Nodules in Overbank Deposits of the Fruitland Formation 50
25. Megafossils in the Delta-plain Lithofacies of the Fruitland Formation 52
26. Split Coal Seams in the Backswamp Subfacies of the Fruitland Formation 54
27. Tonsteins in Thick Basal Coal of the Delta-plain Facies of the Fruitland Formation 54
28. Burrowed Fine-grained, Rippled, and Cross Laminated Tidal Channel Sandstone of the Fruitland Formation 57
29. Rose Diagrams Illustrating Current Directions in Distributary and Tidal Channel Sandstones of the Fruitland Formation 62
vi
Figure Page
30. Depositional Model for Pictured Cliffs Sandstone and Fruitland Formation in the Southwestern San Juan Basin 6 4
31. Generalized Stratigraphic Sections of the Pictured Cliffs Sandstone and Fruitland Formation 71
Vll
INTRODUCTION
General Statement
Coal resources in Cretaceous rocks in the Rocky
Mountain Coal Basins constitute an abundant energy resource.
Faced with the dual problem of increasing energy demands
and limited oil and gas reserves, the evaluation of coal
reserves is becoming an important part of the domestic
energy scenario. The application of depositional models to
exploration and development by the eastern coal industry
demonstrates the important relationship between the deposi
tional setting of coal and its thickness, lateral extent,
quality, and minability. Despite decades of mining activ
ities in the San Juan Basin, little is known about the
depositional environments of the coal deposits. Rather,
coal exploration and description of coal-bearing formations
by workers in the San Juan Basin are, for the most part,
based on 19th century "layer-cake" concepts of William
Smith, as individual coal seams and related lithologic units
are projected over long distances, connecting widely spaced
control points. Stratigraphic position is used as the sole
criterion for correlation, while inherent lateral discon
tinuities resulting from interaction with vertically and
laterally adjacent subenvironments are virtually ignored.
Recent advances in the analysis of coal environments
in studies by Ferm (1976) , Home and Ferm (1976) , and
Home et al. (1976, 1978b) in Appalachian Carboniferous 1
2 coal fields, and by Weimer (1978) and Flores (1978b)in the
U.S. Western Interior coal fields demonstrate the importance
of recognizing depositional environments in the exploration
and development of coal.
Objectives
The objectives of this study are as follows:
(1) To map vertical and lateral variations of rock types in
the Pictured Cliffs Sandstone and Fruitland Formation ex
posed in surface outcrops.
(2) To explain these variations in terms of the interaction
of depositional environments and subenvironments.
(3) To utilize these environments in developing a viable
model of Pictured Cliffs and Fruitland deposition.
(4) To determine how this depositional model can be utilized
in the effective exploration and mining of coal deposits
in the Fruitland Formation.
Location
The study area (Fig. 1) is located about 30 miles south
of Farmington, in northwest New Mexico. In the study area
the Pictured Cliffs Sandstone and Fruitland Formation are
exposed in four areas within an outcrop belt that averages
about five miles wide between Hunter's Wash and Ah-Shi-Sle-
Pah Wash. These outcrops are all located within T23N,
R12 and 13W, and T22N Rll and 12W.
Previous Work
Despite vast amounts of stratigraphic work on Upper
•Formington
\ ^ STUDY ^ AREA
NEW MEXICO
loelis'
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lOMI
rs KM
Fig. 1. Location of study area and cross sections constructed from measured sections in 4 outcrop localities.
Cretaceous rocks in the San Juan Basin, studies explaining
their depositional environments are scarce, particularly
with coal-bearing rocks. Previous works have recognized
environments in the broad, theoretical sense, describing
them in terms of strandline, paludal, nearshore, marine,
or fluvial depositional settings, but specific facies and
subfacies were seldom described. Even in studies assoc
iating stratigraphic units with specific depositional
environments, criteria for their recognition were not
included.
Several studies on Upper Cretaceous deposition in the
San Juan Basin are important in obtaining a general
overview of the depositional setting. Early Fruitland
coal investigations were made by Bauer and Reeside (1921) ,
Reeside (1924), and Dane (1936). Transgressive and
regressive cycles of Upper Cretaceous deposits in the San
Juan Basin were first described in a classic paper by
Sears, Hunt, and Hendricks (1941). Broad depositional
concepts related to epeiric sea regression during late
Cretaceous time were presented in early studies by Weimer
(1960) and Hollenshead and Pritchard (1961). An excellent
sumiTiary of the depositional and tectonic history of the
San Juan Basin during Pictured Cliffs and Fruitland depo
sition can be found in Baltz (1967). Fasset (1977) related
formation of coals and their orientation to stillstands of
sea level during late Cretaceous time.
Methods of Study
One hundred thirty closely-spaced stratigraphic
sections were measured and described in the study area.
Lithologic units were correlated by physically tracing
marker beds between outcrops. The thickness, lithology,
internal structures, morphology, vertical textural changes,
and nature of the contacts of the rock types were described
in detail. Closely-spaced stratigraphic sections provided
preliminary assessment of the continuity of the rock types.
In the four localities, many thick (30 ft.) sandstones
could be traced for up to 1/4 miles. Thus, lateral spacing
on the order of 1/4 mile or less was considered an ac
ceptable spacing. Cross sections from the four outcrop
belts were constructed (Figs. 2 through 5) by "hanging"
each measured section on multiple marker beds and adjacent
rock units, matched with adjoining sections according to
the best geometric fit. Facies and subfacies were delin
eated in the cross sections based on homogeneity of the
rock types. The specific measured locations, detailed
description of each section, and large scale versions of the
four cross sections are available as a Miscellaneous
Field Studies Map through the U.S. Geological Survey, Denver,
Colorado.
A total of 159 crossbedding measurements in channel
sandstones of the Fruitland Formation were made to determine
paleocurrent direction.
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GEOLOGIC SETTING
The vast epeiric seaway that occupied the western-
central part of the North American continent during much
of the Cretaceous Period covered the present San Juan Basin
during the Late Cretaceous Epoch. The basin is a nearly
circular, northwest-southeast-trending structural de
pression.
Upper Cretaceous rocks in the basin total more than
6000 feet in thickness and consist of intertonguing marine
and nonmarine deposits representing numerous transgressions
and regressions of the sea (Fig. 6). Pictured Cliffs Sand
stone and Fruitland Formation were deposited during the
final episode of regression of the sea from the basin in the
late Campanian Stage. Their present vertical superposition
along a northwest-southeast-trending outcrop belt within
the study area (Fig. 7) is a reflection of their prograd-
ation northeastward, as the sea retreated in that direction.
The seaway as it existed during this time is shown in
Figure 8.
The general source of the Pictured Cliffs and Fruitland
sediments within the study area was from the southwest. In
later phases of this regression, a major shift in the shore
line occurred as these sediments in more eastern parts of
the basin were supplied from the north and west (Beaumont,
1971) or from the north and northeast (Baltz, 1967).
Uplift in the southeastern part of the basin resulted 14
15
16
17
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^\.
I
KpcA Kf \
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^ P - - - ^
K f - ^
i<f?^$f^w"^
Fig. 7. Outcrop pattern of Pictured Cliffs Sandstone (Kpc) and Fruitland Formation (Kf) in the San Juan Basin. Study area is on southwestern flank of the basin. (Modified from Fasset and Hinds, 1971).
18
Studx Area
Fig. 8. Distribution of land and sea in North America during late-Campanian time. After Obradovich and Cobban, 1975.
19 m either non-deposition or post-depositional erosion of
the Pictured Cliffs Sandstone, and the Fruitland Formation
directly overlies the Lewis Shale (Fasset, 1977).
PICTURED CLIFFS SANDSTONE
Descriptive Stratigraphy
The Pictured Cliffs Sandstone was named by Holmes
(1877) for massive cliffs of yellowish-gray marine sand
stone exposed north of the San Juan River west of the town
of Fruitland. Reeside (1924) redefined the formation to
include the sequences of interbedded shale and sandstone
beneath the massive sandstone and above the Lewis Shale.
It is generally described throughout the basin as having
an upper part consisting of massive medium-grained sand
stone interbedded with a few thin discontinuous shale beds
up to 2 feet thick. The lower part of the Pictured Cliffs
Sandstone is composed of interbedded shale and fine
grained sandstone. The shale is physically similar to the
underlying Lewis Shale, and the sandstone is also physical
ly similar to the overlying upper part of the formation
(Fasset and Hindis, 1971). Within the lower part of the
Pictured Cliffs Sandstone, the sandstone beds increase up
ward in number and thickness. The lower contact of the
formation is conformable and gradational with the Lewis
Shale, and it has been arbitrarily placed to separate pre
dominant shale interbeds in the Lewis Shale from predominant
sandstone beds in the Pictured Cliffs Sandstone. The upper
contact of the formation with the overlying Fruitland
Formation is placed at the base of the lowest coal of the
Fruitland Formation. 20
21 The Pictured Cliffs Sandstone crops out in a fairly
continuous band around the north, west, and south sides of
the central basin, while it is conspicuously absent for
considerable distances along the east flank of the basin
(Fig. 7). Throughout the basin the Pictured Cliffs Sand
stone ranges in thickness from 0 feet on the east side to
40 0 feet in the north-central part. In the study area, the
Pictured Cliffs Sandstone averages about 90 feet in thick
ness. Because the Pictured Cliffs Sandstone is moderately
resistant to weathering, it is usually exposed in a line of
low-lying cliffs between the subdued topography of the under
lying Lewis Shale and the overlying Fruitland Formation.
Environments of Deposition
In order to evaluate the depositional setting of the
coals in the Fruitland Formation, it was essential that
the environments of deposition of the underlying Pictured
Cliffs Sandstone be determined. Thus, the Pictured Cliffs
Sandstone and Fruitland Formation were studied as a deposi
tional package. Depositional facies and subfacies within
the Pictured Cliffs Sandstone were recognized by the size,
shape, lateral and vertical association, and sedimentary
structures of the sandstone, and comparing these character
istics to studies made on Holocene depositional environments
Within the study area the Pictured Cliffs Sandstone
grades from a delta-front deposit in the northwest (Hunter
Wash to De-Na-Zin Wash) to a beach-bar deposit in the
22
southwest (exposures west of Black Lake and along Ah-Shi-
Sle-Pah Wash). The delta-front facies includes distal bar,
distributary mouth bar, and distributary channel subfacies
(Fig. 3); the beach-bar facies consist of lower, middle, and
upper shoreface and tidal-inlet channel subfacies (Figs.
4 and 5).
Delta-front Lithofacies
The Pictured Cliffs Sandstone was deposited in a delta-
front environment in the northwest part of the study area
(Fig. 3). Although the Pictured Cliffs Sandstone was not
exposed in Cross Section 1, (Fig. 2), a study in an adjacent
area to the northwest by Flores (1979) indicates a delta-
front mode of deposition of the Pictured Cliffs Sandstone in
that area. Flores (1979) recognized distal bar, intervening
contorted distributary mouth bar, and distributary channel
sandstones in the Pictured Cliffs Sandstone based on differ
ences in sedimentary structures. Sedimentary structures
observed in the delta-front lithofacies are similar to those
reported by Fisher et al. (1969); Reineck and Singh (1973);
Home et al. (1978a) ; and Saxena and Klein (1974) .
Distal Bar Sandstone
The lower part of the Pictured Cliffs Sandstone and the
upper part of the Lewis Shale consist of alternating beds of
shale and distal bar silty sandstone (Fig. 9). The distal
bar sandstone was deposited landward of the prodelta shales
and was the seaward-sloping margin of the delta-front
23
Fig. 9. Delta-front lithofacies of the Pictured Cliffs Sandstone. Sub-facies seen are distal bar (a); distributary mouth bar (b); and distributary channel (c).
24 deposits. The distal bar sand probably formed sporadically
during flood conditions followed by settling of silt and
clay out of suspension. The sandstone beds range in thick
ness from several inches to five feet and increase in thick
ness and frequency upwards.
Common sedimentary structures observed are horizontal
and low angle planar cross-laminations and ripple cross-
laminations. Some unidentifiable burrows are observed.
A characteristic feature of the Pictured Cliffs delta-
front deposits usually found between the upper distal bar
sandstone and distributary mouth bar sandstone, is ball-and-
pillow structure (or flow rolls) and slump structure.
The ball-and-pillow structures (Fig. 10) probably formed
by the partial liquifaction of fine-grained sand when over
lying sediments were rapidly deposited during delta prograd-
ation. Slump structures (Fig. 11), disrupting continuity
of beds by thickening and thinning of distorted and over
turned laminae, probably resulted from downslope movement
when the sediment interface steepened beyond the angle of
repose (Baganz et al. , 1975; Home et al. , 1978a, 1978b).
Similar structures have been well documented by Howard and
Lohrengell (1969) , Hubert et al. (1972) , Coleman and Wright
(1973, 1975), and Weimer and Land (1975) in modern and
ancient delta-front deposits.
25
*-* sV"
."l
Fig. 10
#> - • ^ ( * T "
f i i i e ^ ' ' * ^ ^
» *
Bali-and-piliow structure between the distal bar and distributary mouth bar subfacies, Pictured Cliffs Sandstone.
^-^.
Fig. 11. Slump structure between the distal bar and distributary mouth bar subfacies. Pictured Cliffs Sandstone.
26 Distributary Mouth Bar Sandstone
Distributary mouth bar sandstone is formed at the sea
ward front of the distributary channel as a result of the
decrease of water velocity as the fluvial flow enters the
sea. The sediment settles out of suspension and is distrib
uted by marine processes, forming a sandy shoal. Within
the Pictured Cliffs Sandstone, these sandstone bars are
elongate in cross section parallel to depositional strike.
The distributary mouth bar sandstone ranges in thickness
from 70 to 80 feet. In thin section it consists of 49%
quartz and 12% matrix.
The distributary mouth bar sandstone consists of multi
directional planar and trough cross-laminations usually
with low to moderate angles formed by fanning out of seaward-
flowing distributary currents and the counteracting effect
of wave and tidal currents. Minor occurrence of ripple
cross-laminations and lenticular shaly and silty intervals
attest to occasional low energy periods, probably in areas
that occupied the lateral extremity of a distributary mouth
bar that were not reworked by marine processes. Thin lam
inae of transported carbonaceous material are numerous,
especially near the top of the distributary mouth bar sand
stone. The lower contact of the distributary mouth bar
sandstone is usually sharp, accounted for by large amounts
of coarser material which were suddenly deposited over finer
grained distal bar deposits. In most places, the distributary
mouth bar sandstone is overlain by basally scoured distrib-
27 utary channel sandstone (Fig. 9), or less frequently by
coals, siltstones, and shales of the delta-plain lithofacies
of the Fruitland Formation.
Distributary Channel Sandstone
In most places, the distributary mouth bar sandstone
is dissected by distributary channel sandstone. The channel
sandstone is characterized by an erosional base (Fig. 12),
and contains moderate to high angle trough cross beds, con
volute laminations, and ripple cross-laminations. Discon
tinuous shaly intervals, usually less than 2 feet thick,
represent fine-grained sediment deposited during low river
stages. The coal and fine-grained deposits that overlie the
distributary channel sandstone (or in some places, the dis
tributary mouth bar sandstone) represent marsh deposits that
delineate the contact between the Pictured Cliffs Sandstone
and the Fruitland Formation.
Beach-bar Lithofacies
Along the paleoshoreline, to the southeast of the delta-
front Pictured Cliffs Sandstone, waves and longshore currents
redistributed sediments forming a beach-bar deposit. This
is seen in Figures 4 and 5. As no dune deposit could be
identified overlying the beach-bar Pictured Cliffs Sandstone,
this lithofacies is more properly termed "beach-bar" rather
than "barrier island". Sedimentary structures and subenviron
ments within both facies are generally similar (Reineck and
Singh, 1973). The absence of eolian deposits indicates that
28
1 ^ • ^ t :
Fig. 12. Erosional base of the distributary channel sandstone, Pictured Cliffs Sandstone. Arrows indicate erosional base of the channel sandstone.
29
the beach-bar complex was built up to near sea level, result
ing in a deposit similar to that of Hoyt and Henry (1967) ,
Weimer and Land (1975), and Flores (1977). In thin section,
the beach-bar sandstones consist of 56% quartz and 9% matrix;
thus, these sands are cleaner compared to the distributary
mouth bar sands. Subfacies within the beach-bar lithofacies
are lower, middle, and beach-upper shoreface and tidal-inlet
channel; structures observed are similar to those of Davies
et al. (1971) and Flores (1978a, 1979).
Lower Shoreface Sandstone
The offshore muds of the Lewis Shale grade upward into
the lower shoreface, which consists of fine grained sand
stone intercalated with siltstone and shale layers. The
sandstone is fine-grained and ranges in thickness from 20
to 30 feet. Characteristic structures of the sandstone are
horizontal laminations and minor ripple laminations, with
sparse burrowing by Ophiomorpha (Fig. 13). The siltstone and
shale are commonly burrowed. These sediments represent
deposition farthest from shore, seaward of the break in off
shore slope (Davies et al., 1971).
Middle Shoreface Sandstone
The middle shoreface deposits were formed landward of
and grade vertically upward from the lower shoreface deposits
They consist mainly of fine-grained sandstone which contains
sub-parallel and low angle planar cross-laminations, low
angle trough cross-laminations, horizontal laminations, and
30
31
(a)
(b)
>i?C ^ ^
. ^ c • "^ •• -^^stf
32 ripple laminations sparsely burrowed in the lower part
(Figs. 14 and 15), and heavily burrowed in the upper part
(Fig. 16). Shaly units in the middle shoreface are rare
indistinct and discontinuous layers less than 2 feet thick,
which are extensively burrowed, and contain occasional
silty laminae. The thickness of this subfacies ranges from
30 to 40 feet.
Beach-Upper Shoreface Sandstone
The beach-upper shoreface sandstones were seldom pre
served, as overlying tidal-inlet channels eroded down to the
middle shoreface in all but a few localities. Where observed,
the beach-upper shoreface sandstone grades vertically down
ward to the middle shoreface sandstone. Small remnants of
upper shoreface sandstone were interpreted where parallel
laminated or low angle planar laminated sandstone with oc
casional Ophiomorpha burrows graded upward from middle shore-
face. Sparse burrows probably indicate deposition of the
beach-upper shoreface sands in high-energy wave conditions.
Tidal-inlet Channel Sandstone
Tidal-inlet channel sandstone of the Pictured Cliffs
Sandstone ranges from 30 to 50 feet in thickness, with a
distinct erosional base. The tidal-inlet sandstone in
cross section consists of multiple elongate-shaped sandstone
bodies. Each sandstone body represents a bar which was de
posited in response to the lateral migration of the channel
in the direction of longshore drift. In both areas where
33
F i g . 14.
F i g . 15
tioiur.a Cliff. s,„a.t„„r °
34
Sandstone.
35 the tidal-inlet channel is observed, modern erosion has
produced an outcrop oriented nearly parallel to the axis of
each channel, allowing only a longitudinal view of the
sandstones.
The tidal-inlet channel sandstone is fine to medium
grained and is interbedded with siltstone and shale (Fig. 17)
Tidal currents may have transported and deposited offshore-
derived suspended clay in the tidal inlets. In addition to
the siltstone and shale lenses, thin coals and carbonaceous
shale lenses accumulated as rafted organic material in the
tidal-inlet channel sandstone.
Internal structures associates with the tidal-inlet
channel sandstone are low to high angle bidirectional trough
cross-laminations and moderate-angle planar cross-laminations
that occupy the lower part. The upper part of the channel
sandstone contains low angle planar cross-laminations, with
occasional ripple laminations, indicating a general upward
decrease in current energy. Zones containing high-angle
trough cross beds, however, are also found in the upper part
of the channel sandstone. Minor clay rip-up clasts occur
at the base, while extensive burrowing by Ophiomorpha and
smooth-walled burrows is evident throughout the channel
sandstone.
The tidal-inlet channel sandstone is overlain in many
places by a carbonaceous shale or discontinuous coal inter
bedded with carbonaceous shale, which delineates the base
of the Fruitland Formation (Fig. 18)-
36
Fig. 17. Tidal-inlet channel sandstone of the Pictured Cliffs Sandstone. Arrows indicate erosional base of the channel sandstone.
^ - • ^
Fig. 18 Basal coal of the back-barrier lithofacies, the base of which delineates the contact between the Fruitland Formation and the Pictured Cliffs Sandstone. The coal bed contains abundant carbonaceous shale interbeds.
FRUITLAND FORMATION
Descriptive Stratigraphy
The Fruitland Formation was first defined by Bauer
(1916) after the town of Fruitland, New Mexico, where out
crops are well exposed. It is primarily composed of inter
bedded nonmarine sandstone, siltstone, shale, carbonaceous
shale, and coal beds. Carbonized plant remains, silicified
logs and wood fragments, and iron-rich concretions are also
common in the formation. Coal and sandstone beds occur in
the lower part of the Fruitland Formation, while siltstone
and shale beds predominate in the part (Fasset and Hines,
1971). The Fruitland Formation is the most commercially
important coal-bearing formation in the San Juan Basin.
The contact of the Fruitland Formation with the under
lying Pictured Cliffs Sandstone is placed at the top of the
uppermost massive sandstone bed of the Pictured Cliffs Sand
stone, or the lowermost coal bed of the formation, except
where intertonguing occurs. The contact of the Fruitland
Formation with the overlying Kirtland Shale is arbitrarily
placed at the top of the uppermost coal in the Fruitland
Formation.
Throughout the San Juan Basin, the thickness of the
Fruitland Formation is variable because of an indefinite
contact with the overlying Kirtland Shale, but ranges in
thickness from 0 to 500 feet as reported by Fasset and Hines
(1971). In the study area, the Fruitland Formation averages 37
38 about 170 feet thick.
The Fruitland Formation crops out continuously around
the San Juan Basin except for two areas on the eastern part
of the basin (see Fig. 7). The age of the Fruitland Forma
tion within the study area corresponds to the Late Campanian
Stage of Western Europe (Peterson and Kirk, 1977; Molenaar,
1977) .
The vertical and lateral lithologic variations in the
Fruitland Formation are illustrated in Figures 2 through 5.
In the Bisti Badlands area (Fig. 2), distributary channel
sandstone makes up about 20% of the Fruitland Formation and
occurs in lense-shaped bodies that range in thickness from
5 to 30 feet and extend laterally for 0.1 to 1 mile. The
lowermost coal bed is thick (11 feet), while overlying coals
generally decrease in thickness from 7 to 1 feet upward.
Two thick coal beds about 70 feet above the lowermost coal
bed extend across the entire length (3.5 miles) of the line
of cross section and remain constant in thickness (average
7 feet). Other coal beds range in thickness from 1 to 3 feet
and extend laterally for less than 1 mile along the extent of
the cross section, merging and splitting in most places with
adjacent coal beds.
South of De-Na-Zin Wash (Fig. 3), the Fruitland Forma
tion consists of numerous (greater than 50% of the rock vol
ume) distributary channel sandstones, which are arranged
en-echelon. Distributary channel sandstones range from 5
to 60 feet in thickness and extend laterally from 0.2 to 1
39 mile. The coal beds in the lowermost Fruitland Formation
are numerous and thicken from west to east, varying from 1
to 9 feet thick and extending laterally from 0.8 to 3 miles.
A thin coal bed immediately above the Pictured Cliffs Sand
stone at the west is laterally equivalent to numerous thick
coal beds at the east, their continuity broken by erosion of
distributary channel sandstone. It is suggested that at the
time of deposition of the lowermost Fruitland Formation,
well developed backswamps were at the eastern part where
thick peat coals formed. At the same time, distributary
channels formed to the west which migrated to the low-lying
backswamp areas to the east. Thus, the laterally shifting
behavior of the distributary channel sandstone controlled
the development of adjacent coal-forming backswamps.
Fruitland Formation outcrops near Black Lake (Fig. 4)
differ with previous cross sections in the volume of channel
sandstone and the nature of the coal beds. The channel
sandstones in this cross section are few, making up less
than 10% of the rock types, and they range in thickness from
10 to 35 feet and extend laterally for 0.3 to 1.3 miles. The
lowermost coal bed is lenticular, varying from a few inches
to 7.5 feet thick and is laterally continuous for up to 1.5
miles. Stratigraphically higher coal beds range in thickness
from 2 to 12 feet and extend laterally from 0.8 to greater
than 2.5 miles, thus, also showing lenticular shape. These
coal beds overlie, underlie, or are laterally adjacent to
channel sandstones, which do not show any erosional effect
40 on the underlying coal beds.
In the Ah-Shi-Sle-Pah Wash area (Fig. 5) the Fruitland
Formation consists of very few channel sandstones (less than
5% of the total rock volume), which range in thickness from
5 to 20 feet and extend laterally for 0.3 to about 0.8 miles.
The lowermost coal bed is variable in thickness, ranging
from 0 to 6 feet, and extends for 2 miles. The stratigraph
ically higher coal beds are thick (2.5 to 11 feet) and ex
tend laterally for greater than 2 miles.
The differences in thickness and lateral continuity of
the coal beds in the Fruitland Formation appear to be best
illustrated by the lowermost coal deposits between the four
cross sections. The cross sections in Bisti and south of
De-Na-Zin Wash show thick but laterally variable coal beds.
In addition, the coal beds in these areas are usually eroded
and split by channel sandstone and overbank detritus. The
characteristics of the coal beds in these areas probably re
flect their deltaic environment of deposition. In the delta
plain, the channel sandstone and accompanying overbank de
tritus were commonly formed, which generally affected the
thickness and continuity of organic accumulation in adjacent
backswamps. Maximum peat accumulation probably occurred in
interchannel backswamps where the channels had not interrupted
organic accumulation by lateral migration. During flood
stages, these channels probably provided splay detritus which
interrupted organic accumulation in adjacent backswamps.
In contrast, the lowermost Fruitland coal beds in the
41
Black Lake and Ah-Shi-Sle-Pah Wash areas are thin and later
ally continuous compared to those of Bisti and De-Na-Zin
Wash areas. The insignificant amount (less than 10% of the
rock volume) of channel sandstone in the interval suggest
that these areas were not as active areas of sedimentation
as the Bisti and De-Na-Zin Wash areas. This condition may
have provided widespread swamps relatively uninterrupted by
channel migration, thus forming laterally extensive peat
coals. The thin nature of the coal beds in these areas prob
ably reflects inhospitable conditions that prevented abundant
plant growth, which would normally have developed if the
swamps had been influenced by fresh water conditions. Thus,
the process of sedimentation in the Black Lake and Ah-Shi-
Sle-Pah Wash areas may indicate back-barrier conditions ad
jacent to the deltaic plain to the northwest in the Bisti and
De-Na-Zin Wash areas.
Environments of Deposition
Delta-plain Lithofacies
The Fruitland Formation consists of delta-plain deposits
in the northwest part (Bisti and De-Na-Zin Wash) of the study,
where it formed landward of the delta-front Pictured Cliffs
Sandstone. The delta-plain deposits (Figs. 2 and 3) consist
of distributary channel, levee, crevasse splay, and inter-
distributary backswamp deposits.
Distributary Channel Sandstone
The distributary channel sandstone makes up the major
42 lithofacies type of the delta-plain deposits. In thin
section, the distributary channel sandstone contains 37%
quartz and 18% matrix. In cross section, the channel sand
stones are lenticular-shaped, up to 50 feet thick, and up to
several thousands of feet wide. The channel sandstone is a
fining upward unit (Fig. 19) and has an erosional base that
displays some clay rip-up clasts. The en-echelon arrange
ment of the channel sandstone, as shown in Figure 3, prob
ably indicates lateral migration of channels into adjacent
backswamps.
The sedimentary structures of the channel sandstone
consist of trough, planar, and ripple cross-laminations.
The vertical change in grain size and internal structures
of the channel sandstones suggest that the channels were
filled in the deepest parts with coarse detritus by high
energy water flow and in the shallow or shoreline parts with
fine detritus by low energy water flow; these flow conditions
and detrital filling were maintained as channel migration
developed. Large scale planar and trough cross-laminations
(Fig. 20) occur at the lower part, grading upward into small
scale trough cross-laminations, convolute laminations, and
ripple cross-laminations in the channel sandstone. In modern
distributary channel environments, the large scale bedforms
(i.e. trough and planar cross-laminations) are developed in
the deeper part of the channel, and the small scale bedforms
(i.e. ripples) are formed in the shallow part of the channel.
In several distributary channel sandstones, point bar
43
1 • . -^l-r Vi- -^ f'' • r'
•f^ri-^'-^
r.
• -' . # - - * i . • • ^ * ^ • ^ - - • •
Fig. 19. Fining-upward sequence in a distributary channel, Fruitland Formation.
1
X
Fig. 20 Large-scale planar and trough cross beds in a distributary channel sandstone of the Fruitland Formation.
44 sequences developed, separated in some cases by siltstone
lenses.
Associated with the distributary channel sandstone
are slump structures, probably from cut-bank erosion. Thin
clay drapes and thin laminae of carbonaceous material prob
ably resulted from filling of mud in swales and rafting of
organic material into the channels, respectively.
In the Carboniferous lower delta-plain facies in the
Appalachian region. Home et al. (1978a, 1978b) and Saxena
and Ferm (1976) reported that point bar development was not
extensive, and low sinuousity of the channels was inferred.
Associated with these nearly straight channels were poorly
developed natural levees which were easily breached, result
ing in deposition of numerous crevasse splays into adjacent
interdistributary bays. Within the Fruitland Formation,
neither point bar nor crevasse sandstone development showed
any preferential upper or lower stratigraphic association;
thus, it is suggested that the delta-plain, characterized by
extensive channel migration and point bar and natural levee
development, cannot be subdivided into lower and upper delta
plains. This environmental condition is consistent with
concepts of meandering in moderately sand-rich deltas pro
posed by Scott and Fisher (1969).
Natural Levee Deposits
The natural levee deposits consist of silty sandstone,
siltstone, and shale. These deposits are mainly preserved
45 above the channel sandstones. The deposits formed when the
levee, or elevated areas adjacent to the channels were
flooded during the high flood water stages, when suspended
fine sand, silt, and clay were deposited in these areas.
As the levee grew upward it became topographically higher
than the adjacent backswamp and the channel.
The levee deposits are primarily composed of interbedded
sandstone, siltstone and shale found immediately above and
interfingering with the channel sandstones. The fine-grained
sandstone and siltstone display horizontal laminations and
ripple cross-laminations. The levee deposits, which support
ed vegetation, contain root penetrations and numerous
silicified tree stumps in upright position. Zones of iron
carbonate concretions are common, as are thin carbonaceous
stringers.
Crevasse Splay Deposits
The crevasse splay deposits consist of sandstone con
formably underlain by siltstone and shale that represent
episodic diversion of the distributary channel sediments
into adjacent interdistributary areas through a break in
the natural levee. The crevasse splay sandstone is fine
grained and varies from several inches to five feet thick
(Fig. 21). The crevasse splay sandstones decrease in thick
ness away from the levee, although exposures within the
study area were not complete enough to permit the complete
tracing of individual splays from levee breach to distal
46
Fig. 21. Crevasse splay sandstone underlain by siltstone and shale in the Fruitland Formation.
47 areas. The distal end of the crevasse splay sandstone grades
into siltstone and shale, and the lower contact is grad
ational.
The crevasse splay sandstone is characterized by
abundant ripple cross-laminations (Fig. 22) and some horizon
tal laminations. Laminae of macerated plant debris are
common, as are root bioturbations, suggesting rapid re-estab
lishment of vegetation on the splay. Iron carbonate occurs
in narrow bands or as scarce nodules.
Crevasse splay deposits are found in equal abundance
throughout the Fruitland Formation. This suggests that the
levees were probably low and narrow, allowing frequent
breaching. In comparison to the thick crevasse deposits of
the Mississippi delta, the Fruitland crevasse splays are
considerably thin. In the Mississippi delta the stage of
development of crevasse building is longer because of a rel
atively high gradient between distributary channel and inter
distributary bay (Home et al. , 1978a). However, the
gradient between the distributary channel and shallow inter
distributary marsh of the Fruitland delta-plain was probably
low. Under this condition, the marsh filled more rapidly,
causing early plugging of the levee breach, which led to
abandonment of the system and thin deposits.
Overbank Deposits
Interbedded fine-grained sediments that gradationally
overlie and intertongue with the channel sandstone and levee
48
Fig. 22 Ripple cross-laminations of the crevasse splay sandstone in the Fruitland Formation.
49 deposits represent overbank detritus of the delta-plain
complex (Fig. 23). These deposits formed in flat, low-lying
interdistributary areas where fine-grained sediment settled
after the coarser sediment was deposited on levees. The
predominant lithology is light to medium gray blocky shale
with some carbonaceous shale and fissile dark gray shale.
The upper contact of the overbank sediments is sharp where
overlain by distributary channel sandstone, while contacts
with overlying and underlying backswamp deposits are grada
tional. Lateral contacts with the channel sandstone are
also sharp in places, or gradational where levee sandstone
and siltstone are well developed. Intercalations with
crevasse splay sandstones are numerous.
Intense root mottling has destroyed much of the original
bedding, but fine horizontal laminations are observed where
churning of the sediments by vegetation did not occur.
Scarce small-scale ripple bedding occurs in some silty layers.
Whole plant remains, and stems and twigs are sometimes seen,
but preserved vegetative matter usually occurs as macerated
plant material distributed throughout the shaly or silty
layers. Abundant silicified wood fragments occur in distinct
zones, while numerous silicified tree stumps in growth posi
tion suggest rapid burial by clastic sediments in the overbank
areas.
Distinct zones of siderite nodules (Fig. 24), some with
veins of barite, are associated with the overbank deposits.
Although their specific mode of formation is not known.
50
mm&Bs^'e^
•''". .*-<-•«•.:• i'.'i.-i^: - ^ - ^ - C ••?»*--..>'••.-, •>•- -.- -, •,
Fig. 23. Interbedded siltstone, shale, carbonaceous shale, and coal comprising overbank deposits of the Fruitland Formation.
Fig. 24. Siderite nodules in overbank deposits of the Fruitland Formation.
51 several explanations have been suggested. Weimer (1978)
related them to oxidation in well-drained swamps, resulting
in the alteration of pyrite to siderite. Early formation
and lithification of these concretions was suggested by
Home et al. (1978b) because of compaction of the enclosing
sediments around them. Iron carbonate nodules have also been
proposed as suggestive of cessation of detrital influx (Ferm
et al., 1971), high rates of evaporation (Reineck and Singh,
1973), and precipitation of calcium carbonate from ground
water in permeable zones (Monzolillo, 1976).
Burrowing and brackish water-megafauna are sparse in
the overbank deposits. Some fossils of brackish-water origin
are found in three localities near the base of the Fruitland
Formation (Fig. 25). In addition, carapace remains of fresh
water turtles are found associated with brackish-water bi
valves suggesting that the carapace fragments were transported
rather than accumulated in place.
The predominant occurrence of plant roots over burrow
structures and brackish-water fossils, and light gray shales
over dark gray to black shales suggests that the overbank
areas were occupied primarily by marshes than open bays.
The presence of brackish-water pelecypods in some of the
overbank deposits, however, suggests that bodies of water
may have encroached over some of the overbank areas.
Backswamp Deposits
Backswamp deposits in the Fruitland Formation consist
52
Fig. 25 Megafossils (pelecypods and carapace of turtle) found in the lower part of the delta-plain facies of the Fruitland Formation.
53 of coal and carbonaceous shale, with some medium to dark
gray organic-rich shales. Coal beds of the Fruitland Forma
tion are up to 11 feet thick in the lower part and up to 1
foot thick in the upper part. Throughout the outcroppings,
the coal beds split (Fig. 26) or grade laterally into
carbonaceous shale. Siltstones and shales are common partings
of the coal beds. In some places coal beds are truncated by
distributary channel sandstone. Tonsteins (Fig. 27) up to
an inch in thickness also make up some of the coal partings.
They probably represent airfall volcanic ash deposits, and,
because of their extensive distribution, they were sometimes
used as an aid in correlating coal beds between outcrops
where the beds were unable to be physically traced.
The role of groundwater levels in the backswamps was
probably significant in the accumulation of peat coal.
Groundwater table in the backswamp areas probably became
higher or lower in response to flooding events in the dis
tributary channel. A toptgraphically high well-drained swamp
formed as accumulation of overbank deposits raised the depo
sitional interface, allowing subaerial exposure of the swamp
on occasion, and oxidizing conditions prevented the preserva
tion of peat (Weimer, 1978). Well-drained swamps in the
Fruitland Formation are characterized by light to medium gray
shales (indicative of oxidation), root bioturbations, scarce
carbonized plant fragments, and numerous zones of iron
carbonate concretions.
Transition from a well-drained to a poorly-drained swamp
54
-<^s.
Fig. 26. Split coal seams in the backswamp subfacies of the Fruitland Formation.
Fig. 27. Tonsteins (thin, light colored layers) in thick basal coal of the delta-plain facies of the Fruitland Formation.
55 is represented by a change of the light gray non-carbonaceous
shale to black shale, carbonaceous shale, and coal. In poor
ly-drained swamps, the groundwater table was continuously
higher than the depositional interface. Peat layers accumu
lated in this chemically reducing environment. The poorly-
drained swamps are characterized by interbedded layers of
dark gray to black shale, carbonaceous shale, and coal that
contains numerous plant fragments and imprints, and abundant
root marks.
The best developed coals formed in areas that were iso
lated from the influence of distributary channels which
contained high sediment load. Poorly developed "boney" coals
and carbonaceous shales formed when muddy waters of the dis
tributary channels flowed into the backswamps. Carbonaceous
shales also formed by the accumulation of rafted plant frag
ments into backswamps in which the groundwater table was too
high to support vegetation. Shales and siltstones directly
underlying such deposits show no root bioturbations. Car
bonaceous shales directly overlying, and interbedded with
coal seams may indicate low energy reworking of an upper peat
surface during a drowning phase of a swamp (Home et al. ,
1978b).
Back-barrier Lithofacies
In the southeastern part of the study area, the Fruit
land Formation represents back-barrier deposits. These
deposits were formed landward of, and prograded over the
56 beach-bar deposits of the Pictured Cliffs Sandstone.
Precise environmental evaluation of back-barrier subenviron
ments was difficult because of a high degree of modern ero
sion; however, noticeable differences between back-barrier
deposits in this area and the delta-plain deposits to the
northwest can be seen. In the back-barrier lithofacies tidal
channel and back-barrier marsh deposits can be delineated
(Figs. 4 and 5). Characteristics of these deposits are sim
ilar to those described by Weimer and Land (1975) , Flores
(1977, 1978a), and Home et al. (1978b).
Tidal Channel Sandstone
Tidal channels transected fine-grained material in the
back-barrier in a rather similar manner as did the distribu
tary channels in the delta-plain. However, back-barrier
tidal channels are fewer in number, thinner, and finer
grained in contrast to the distributary channel sandstones.
High amounts of shale and siltstone in the interval, as well
as occasional burrowing (Fig. 28) in the channel sandstone,
suggest the influence of tidal or estuarine processes. The
tidal channel sandstone, in thin section, consists of 42%
quartz and 21% matrix.
The tidal channel sandstone grades seaward into tidal-
inlet channel sandstones. The tidal-inlet channel sandstones
contain siltstone and shale which probably represent off
shore clay and silt that were carried in suspension and
deposited during tidal flood flow conditions, resulting in
57
Fig. 28. Burrowed, fine-grained, rippled, and small-scale cross-laminated tidal channel sandstone of the Fruitland Formation.
58 a generally finer-grained deposit. Tidal channel sandstones
deposited further inland were filled by landward-derived
sediment and less influenced by tidal processes. These
channel deposits contain fewer shaly intervals and less fine
grained matrix.
Internal structures in the tidal channel sandstone are
similar to those of the distributary channel sandstones, a
feature noted by Davies (1969) and Oomkens (1974). Large
and small scale trough cross bedding, with a variable, but
predominant northwest orientation, are the primary structures
Low angle planar cross-laminations and minor ripples are
also seen. Basal contacts are erosional, while upper con
tacts, and lateral contacts interfingering with back-barrier
marsh sediments, are usually gradational.
Back-barrier Marsh Deposits
The back-barrier marsh deposits consist of thin carbon
aceous shales and coals separated by fine-grained sandstone,
siltstone, and shale that are extensively rooted. The coal
deposits are interlaminated with carbonaceous shale and are
thin in the lower part and thick in the upper part of the
Fruitland Formation. Interbedded light gray shale and silt
stone are parallel-laminated, wavy-bedded, rooted, and con
tain disseminated plant fragments.
The absence of•megafauna, predominance of rooting over
burrowing, and light gray color of the shales indicate that
the back-barrier marsh was characterized by oxidizing con-
59 ditions (Weimer, 1978) and sparse pond development. This
condition may explain the thin coal beds in the lower part
of the Fruitland Formation in the southeastern part of the
study area. Rapid detrital filling of the back-barrier was
probably effected by deposition of both offshore and land-
derived fine-grained material by tidal-inlets and tidal
channels. In order for such rapid infilling to occur, land
ward sediment probably predominated over the marine con
tribution.
The poor quality of the back-barrier coal seams at the
base of the Fruitland Formation is indicated by the abundant
carbonaceous shale intercalations. These coal beds are
locally thin, discontinuous, and grade laterally into carbon
aceous shale (Fig. 18). The high carbonaceous shale content
of the back-barrier coals probably formed by the influx of
salt water and flocculated clays in the tidally-influenced
low-lying marsh adjacent to the beach-bar (Cohen and Staub,
1977; Flores, 1978a; and Home et al. , 1978b). In addition,
discontinuous carbonaceous shale associated with the tidal-
inlet channel sandstones may represent rafted plant material.
In contrast to the back-barrier coals in the lowermost part
of the Fruitland Formation, the stratigraphically higher
coals are thick and laterally persistent. These coal de
posits probably represent fresh water peat that developed
further inland and on topographically higher swamps, where
there was less influence of tidal processes and salt water.
These coal beds contain relatively few carbonaceous shale
60
or boney coal interbeds. This suggests formation in poorly-
drained swamps that were isolated from sediment-laden waters
of tidal channels; thus, levees may have been formed adjacent
to the tidal channels at the more landward areas. Splitting,
merging, and truncation of the back-barrier coals of the
upper part of the Fruitland Formation are seldom observed.
This suggests that energy and flooding conditions associated
with the tidal channels were not as effective and common
compared to the distributary channels in the delta-plain.
PALEOGEOGRAPHIC RECONSTRUCTION
Paleocurrent Directions
Measurements of crossbed attitudes of the Fruitland
channel sandstones revealed a difference in dominant current
flow directions between channels in the delta-plain and
those in the back-barrier environment (Fig. 29). Delta-
plain distributary channels show roughly a unimodal pattern
with a dominant northeast orientation (Fig. 29a). In con
trast, the crossbed measurements of the tidal channel sand
stone show a polymodal distribution (Fig. 29b) suggesting
a more variable current flow in the tidal channel complex.
Thus, the characteristic bimodal pattern, indicating ebb
and flood tidal flow, is not exhibited by the crossbeds in
the tidal channel sandstones. The distributary channels
appeared to have meandered along a predominantly northeast
paleoslope, while the tidal channels generally flowed parallel
to shoreline, as shown by the predominant northwesterly cur
rent direction. Both sets of measurements lend support to
the theory of a northwest-trending shoreline in this area
during late-Campanian time (see Fig. 8). However, a highly
irregular shoreline was probably developed at this time.
Depositional Model
As the late-Campanian seaway retreated to the northeast,
the deltaic and back-barrier deposits of the Fruitland Forma
tion prograded over the delta-front and beach-bar deposits
of the Pictured Cliffs Sandstone. Figure 30 summarizes the 61
62
Fig. 29. Rose diagrams showing current directions in Fruitland (a) distributary and (b) tidal channel sandstones. Diagrams were plotted from a total of 159 trough cross beds in the channel sandstones. Sections represent 50°, etc.
,o 20^ of arc plotted at 10 o 30 o
63
64
65 depositional model of this prograded sequence.
In the northwest part of the study area, distal bar,
distributary mouth bar, and distributary channel detritus
of the Pictured Cliffs Sandstone was deposited landward of
the prodelta sediments of the Lewis Shale. Rapid deposition
into the shallow, slowly subsiding basin resulted in the
formation of a delta lobe of the Fruitland Formation, which
was characterized by numerous weakly meandering or linear
distributary channels and thin crevasse splays. Overbank
areas were rapidly filled by extensive overflowing and
breaching of low-lying narrow levees from flood waters that
contained abundant suspended sediment load. The fringes of
the delta lobe may have been innundated by brackish-marine
waters to form few interdistributary embayments. Well-
drained and poorly-drained swamps were associated with the
overbank deposits, depending on the relationship of ground
water table to depositional interface. Thick coals formed
in poorly-drained backswamps that were isolated by levees
from sediment-laden waters of the distributary channels.
Redistribution of sediments to the southeast of the
deltaic and delta-front areas by wave and tide-generated
longshore currents resulted in the development of a beach-
bar deposit there. Seaward to the beach deposit, lower,
middle, and upper shoreface sediments were deposited. In
places, tidal-inlet channels dissected the beach-bar com
plex down to the middle shoreface deposits. In the back-
barrier facies an intertwined network of tidal channels
66
developed, many of which flowed parallel to the shoreline.
The cancelling effect of ebb and flood tidal flows resulted
in the deposition of abundant fine-grained material in the
channels. Rapid filling of the back-barrier by both land-
derived and offshore-derived elastics resulted in the forma
tion of a marsh platform devoid of ponds. Thick coals
formed on the landward side where the influence of fresh
water was greatest, while in the marsh immediately at the
back side of the beach-bar, the influence of salt water re
sulted in the formation of abundant carbonaceous shale-rich
coal beds.
APPLICATION TO COAL EXPLORATION
Recent studies by Ferm (1976) , Home and Ferm (1976) ,
Weimer (1978), Home et al. (1976, 1978b), and Flores
(1978b, 1979) demonstrate the role of depositional environ
ments in the formation of coal beds and how they affect
thickness, aerial extent, and quality of the coals. By
applying concepts presented by these studies to deltaic
and back-barrier coals of the Fruitland Formation, perhaps
their exploration and development can be economically
maximized.
The thickness and aerial extent of coal deposits are
related to their environments and subenvironments of
deposition. In the delta-plain facies, the coals are thick
in the lower part of the Fruitland Formation, while in the
back-barrier facies the coals are thick in the upper part
of the Fruitland Formation. It is suggested that thick or
ganic accumulation in interchannel coals was well developed
in the lower part of the delta-plain. As fluvio-deltaic
progradation followed, coal-forming environments in the lower
delta-plain were succeeded by fluvial environments that con
tained insignificant amounts of organic accumulation. On
the other hand, in the back-barrier environment, the coal-
forming subenvironments were formed on the landward side of
the beach-bar complex. Progradation of the complex placed
the thick peat coals from the landward side stratigraphically
over the carbonaceous shale-rich coals formed at the seaward
67
68
side immediately at the back of the beach-bar complex. Re
ducing backswamp conditions probably existed in topographic
ally high landward areas of the back-barrier marsh, removed
from the influence of marine water brought in through tidal
inlets. Poorly developed coals and carbonaceous shales are
common in lower parts of the back-barrier Fruitland Forma
tion probably because of their development in a salt water
marsh adjacent to the beach-bar.
More extensive coals can be found in the upper parts
of the Fruitland back-barrier deposits, where fluvial in
fluence was more evident, than in the lower parts, where a
complex network of tidal channels formed. In the delta-
plain Fruitland deposits, lenticular coal beds formed be
cause numerous channel meanders and sometimes coalescing
distributary channels restricted the lateral extent of the
coals and truncated the upper surface of the coal seams.
These coals, therefore, can be expected to be more contin
uous in the direction of paleoslope and discontinuous par
allel to the depositional strike. Numerous splitting and
merging of these coals in the delta-plain are accounted for
by crevasse deposits.
SUMMARY AND CONCLUSIONS
Recognition of Deltaic and Beach-bar Deposits
Although it is often difficult to distinguish between
ancient deltaic and beach-bar deposits, numerous distinguish
ing characteristics facilitated their recognition in the
study area. These differences are illustrated in a composite
stratigraphic section in Figure 31.
Within the Pictured Cliffs Sandstone, extensive burrow
ing, especially by Ophiomorpha, and complete absence of
penecontemporaneous deformation structures is noted in the
shoreface lithofacies. Where represented by delta-front
deposition, on the other hand, the Pictured Cliffs Sandstone
exhibits slump and ball-and-pillow structures, while burrow
ing by Ophiomorpha is sparse. These sedimentary structures
indicate rapid deposition of sediments in the delta-front
environment.
Within the Fruitland Formation, the presence of numerous
distributary channels in the delta-plain is a stark contrast
to the few tidal channels in the back-barrier. Paleocurrent
analysis indicates that distributary channel flow was pre
dominantly parallel to paleoslope, while back-barrier tidal
channels have a variable orientation, but a large component
of flow parallel to paleoshoreline. Coals in the back-bar
rier facies are thick and continuous in the upper part of
the Fruitland Formation, while the coals are thin, discon
tinuous, and predominantly interbedded with, and laterally
69
70
7 1
a z <
cc
<
S cc O
Q LU CC
CO L J .
o 0. r o
LU z o I -(/)
a z < CO
LU
< I CO
CO
LU
^
o < QQ
CC LU
CC CC < OQ
(J < LU CQ
CC < OQ
<
(/) I k
o - « <N < I-
l i i
>
O - I
72
grade to carbonaceous shale in the lower part of the Fruit
land Formation. Deltaic coals of the Fruitland Formation
are thickest at the lowermost part, decrease in thickness
stratigraphically upward, and are frequently truncated and/or
split by channel and crevasse splay deposits.
Conclusions
1. Thoroughly describing and correlating lithologic
units in closely spaced stratigraphic sections allowed the
recognition of lithofacies types in the Pictured Cliffs
Sandstone and Fruitland Formation in the southwest San Juan
Basin. Where deposited in a deltaic/delta-front, litho
facies recognized were distal bar, distributary mouth bar,
distributary channel, levee, crevasse splay, overbank, and
backswamp deposits; whereas lower, middle, and upper shore-
face, tidal-inlet channel, and back-barrier marsh deposits
were observed in the beach-bar/back-barrier lithofacies.
2. The final regression of the late-Cretaceous sea
resulted in progradation of delta-plain sediments over
delta-front sediments in the northwest part of the study
area. To the southeast of the delta, back-barrier sediments
prograded over the beach-bar deposits.
3. The close relationship between coal environments
and subenvironments of deposition and its thickness, con
tinuity, and quality can be demonstrated within the Fruit
land Formation. Thus, the exploration and development of
the Fruitland coals should include specific environmental
73 determinations.
4. Recognition of depositional environments and sub-
environments can also help clarify problems in strati
graphic subdivisions. The criterion for formational con
tact placement is often based on the erroneous concept of
completely tabular units, such as "the basal coal" of the
Fruitland Formation. Determining facies within the deltaic
facies of the Fruitland Formation permits recognition of
the upper limit of the delta-front facies of the Pictured
Cliffs Sandstone, even in places where a basal coal of the
Fruitland Formation does not exist.
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