AUTHORS

William A. Ambrose � Bureau of EconomicGeology, John A. and Katherine G. JacksonSchool of Geosciences, University of Texasat Austin, Austin, Texas 78713-8924;william.ambrose@beg.utexas.edu

William A. Ambrose is a research scientist atthe Bureau of Economic Geology, John A. andKatherine G. Jackson School of Geosciencesat the University of Texas at Austin. His areas ofinterest include unconventional energy min-erals, clastic depositional systems, and stratig-raphy. Ambrose holds M.A. and B.S. degreesin geosciences from the University of Texas atAustin.

Walter B. Ayers Jr. � Department ofPetroleum Engineering, Texas A&M University,3116 Tamu, College Station, Texas 77843;ayers@pe.tamu.edu

Walter Ayers is a visiting professor of geo-science at Texas A&M University. He teachescourses in reservoir studies, unconventionalreservoirs, and petroleum geology. His on-going research includes CO2 sequestration andenhanced methane production from coalbeds, tight sand reservoirs, and productionoptimization from stripper wells. Ayersholds a Ph.D. from the University of Texasat Austin and M.S. and B.S. degrees fromWest Virginia University.

ACKNOWLEDGEMENTS

This study was funded by the Gas ResearchInstitute under Contract 5087-214-1544. Themanuscript benefited from the reviews ofJack C. Pashin, Gregory C. Nadon, Robert Milici,and Eric C. Potter. Manuscript editing was bySusann Doenges. John Ames prepared theillustrations under the direction of Joel Lardon,manager, Media Information Technology.Publication was authorized by the director,Bureau of Economic Geology, John A. andKatherine G. Jackson School of Geosciences,the University of Texas at Austin. Partial sup-port of this research was received from theJohn A. and Katherine G. Jackson SchoolGeosciences and the Geology Foundation ofthe University of Texas at Austin.

Geologic controls ontransgressive-regressive cyclesin the upper Pictured CliffsSandstone and coal geometry inthe lower Fruitland Formation,northern San Juan Basin,New Mexico and ColoradoWilliam A. Ambrose and Walter B. Ayers Jr.

ABSTRACT

Three upper Pictured Cliffs Sandstone tongues in the northern part

of the San Juan Basin record high-frequency transgressive episodes

during the Late Cretaceous and are inferred to have been caused by

eustatic sea level rise coincident with differential subsidence. Out-

crop and subsurface studies show that each tongue is an amalgam-

ated barrier strand-plain unit up to 100 ft (30 m) thick. Successive

upper Pictured Cliffs tongues display an imbricate relation and are

offset basinward, reflecting net shoreline progradation northeast-

ward. Upper PicturedCliffs barrier strand-plain sandstones underlie

and bound thickest Fruitland coal seams on the seaward side. Con-

trols on Fruitland coal-seam thickness and continuity are a function

of local facies distribution in a coastal-plain setting, shoreline posi-

tions related to transgressive-regressive cycles, and basin subsidence.

During periods of relative sea level rise, the Pictured Cliffs shoreline

was temporarily stabilized, allowing thick, coastal-plain peats to ac-

cumulate. Although some coal seams in the lower Fruitland tongue

override abandoned Pictured Cliffs shoreline deposits, many pinch

out against them. Differences in the degree of continuity of these

coal seams relative to coeval shoreline sandstones are attributed to

GEOLOGIC NOTE

AAPG Bulletin, v. 91, no. 8 (August 2007), pp. 1099–1122 1099

Copyright #2007. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received April 24, 2006; provisional acceptance July 7, 2006; revised manuscript receivedFebruary 22, 2007; final acceptance March 8, 2007.

DOI:10.1306/03080706040

either differential subsidence in the northern part of

the basin, multiple episodes of sea level rise, local varia-

tions in accommodation and progradation, stabilization

of the shoreline by aggrading peat deposits, or a combi-

nation of these factors. Fruitland coalbed methane re-

sources and productivity are partly controlled by coal-

seam thickness; other important factors include thermal

maturity, fracturing, and overpressuring. The dominant

production trend occurs in the northern part of the

basin and is oriented northwestward, coinciding with

the greatest Fruitland net coal thickness. Similar rela-

tionships between trends of thick coal seams of coeval

origin with stacked shoreface sandstones exist in other

Western Interior basins in the United States and serve

as models for coalbed methane exploration in other

basins of the world.

INTRODUCTION

TheFruitlandFormation in theSan JuanBasin (Figure 1)

is a prolific coalbed-methane–producing unit, with

in-situ resources of approximately 50 tcf (Ayers, 2002)

and per-well cumulative production locally exceeding

20 bcf (Meek and Levine, 2006). Several factors ac-

count for this great resource and productivity, includ-

ing coal thermalmaturity (Scott et al., 1994), high coal-

seam permeability (Kaiser and Ayers, 1994), and the

presence of deep basement faults and fractures allowing

upward migration of hydrocarbons (Riese et al., 2005).

Another significant factor is net coal thickness, which

primarily reflects the thick accumulation of Fruitland

peats landward of coeval, northwest-trending shore-

face andwave-dominated deltaic complexes in Pictured

Figure 1. Location ofthe San Juan Basin relativeto the Western Interiorseaway. The San JuanBasin during the intervalof deposition of the LewisShale to the FruitlandFormation (approximate-ly 76–73.5 Ma; Fassett,2000) was located north-eastward of the Mogol-lon Highlands and east-ward of the Sevier thrustbelt. A northeast-trendingfluvial system suppliedsediments to a wave-dominated shoreline inthe San Juan Basin. Modi-fied from Williams andStelck (1975), Irving(1979), Palmer and Scott(1984), Blakey andUmhoefer (2003), andDeCelles (2004).

1100 Geologic Note

Cliffs Sandstone. These trends of greatest net coal thick-

ness occur landward of areas of significant intertonguing

of the Fruitland Formation and Pictured Cliffs Sand-

stone, recording multiple cycles of transgression and

regression.

For this study, three objectives were established

to clarify the influence of transgressive-regressive cycles

on Fruitland coal-seam geometry and distribution of

coalbed methane resources. These objectives were to

(1) provide a sequence-stratigraphic framework that

clarifies the depositional setting of intertonguing cycles

of the Pictured Cliffs Sandstone and lower Fruitland

Formation in the northern part of the San Juan Basin;

(2) assess geologic controls on variations in thickness

and continuity of lower Fruitland coal seams and coeval

upper Pictured Cliffs tongues and briefly review analo-

gous successions in other basins of the United States

Western Interior; and (3) identify relationships between

stratigraphy and Fruitland coalbed methane resources

and productivity in the northern part of the San Juan

Basin.

This study used a sequence-stratigraphic approach

to interpret the interval from the Lewis Shale to the

base of the Ojo Alamo Sandstone, with an emphasis on

characterizing high-frequency transgressive-regressive

cycles in the lower Fruitland Formation and upper Pic-

turedCliffs Sandstone, based on criteria developed from

core and well-log data. This interpretation builds on

previous stratigraphic interpretations of the Lewis Shale

to the base of theOjo Alamo Sandstone by Fassett and

Hinds (1971),Cumella (1981, 1983), Flores andErpen-

beck (1981), Manfrino (1984), Roberts and McCabe

(1992), Ayers et al. (1994), Condon et al. (1997), Pashin

(1998), Fassett (2000), and Ayers (2002). This study

also provides additional details of stratigraphy and fa-

cies analysis of the lower Fruitland Formation and up-

per Pictured Cliffs Sandstone in selected areas in the

northern part of the San Juan Basin.

The core and outcrop data used in this study are

supplemented by maps of selected intervals from the

Pictured Cliffs Sandstone and Fruitland Formation orig-

inally presented byAyers et al. (1994) in a regional study

of the San Juan Basin. In that study, coal was identified

from well logs by its low density, high neutron and den-

sity porosity, low sonic velocity, and/or low neutron

count. Sandstone was identified from gamma-ray curves

calibrated with gross-sandstone values determined from

wells with cores; the formation density curve was used

to differentiate sandstone from coal, where low gamma-

ray values could be misinterpreted to indicate sand-

stone. In well logs without gamma-ray curves, other

curves such as spontaneous potential (SP) and conduc-

tivity were used to differentiate coal from sandstone.

This study presents core data from the Blackwood

and Nichols Northeast Blanco Unit (NEBU) 403 well

(Figure 2). Data from other cores, included in previous

studies (Ayers and Ambrose, 1990), are not presented

here because the Blackwood and Nichols NEBU 403

core offers a continuous section in the Pictured Cliffs-

Fruitland succession. In addition, this core has a variety

of lithofacies types where the PicturedCliffs Sandstone

intertongues with the Fruitland Formation. Outcrops

near Durango, Colorado (Figure 2), were examined to

further clarify stratigraphic relationships.

GEOLOGIC SETTING

Tectonic Setting and Climate

During the Late Cretaceous (late Campanian and early

Maastrichtian), the region of the present San Juan Ba-

sin was on the western margin of the Western Interior

seaway, eastward of the Sevier thrust belt (DeCelles,

2004) (Figure 1). The San Juan Basin is a Laramide tec-

tonic feature with major structures having formed from

73 to 30 Ma (Fassett, 2000). Structural flexures devel-

oped in the area of the San Juan Basin, eastward of the

Sevier thrust belt. Mitrovica et al. (1989) and Pang and

Nummedal (1995) inferred from Campanian isopach

maps by DeCelles (2004) that dynamic subsidence be-

gan to exert the dominant control on regional sediment

distribution in basins east of the thrust belt. The San

Juan Basin also contains several coeval, synthetic, down-

to-the-north normal faults that were reactivated inter-

mittently through the Paleozoic to Mesozoic (Kelly,

1951, 1955; Kelly and Clinton, 1960; Laubach and Tre-

main, 1994). Continuing fault movement created low-

amplitude, monoclinal flexures, providing accommoda-

tion space for Cretaceous shoreface andmarginal-marine

sediments (Silver, 1957). PicturedCliffs paleoshorelines

are oblique to subparallel of basement gravity trends and

fault zones in the northern part of the basin (Fassett,

2000; Riese et al., 2005). Where fault displacements

did not propagate upward into the overlying section,

fractures developed that may have influenced the coal-

bed methane hydrocarbon system (Riese et al., 2005).

A fluvial system transported sediments frommeta-

morphic and volcanic source terrainswithin theMogol-

lon Highlands to the northwest-trending Campanian

shoreline (Williams and Stelck, 1975; Irving, 1979;

Palmer and Scott, 1984; Blakey and Umhoefer, 2003)

Ambrose and Ayers 1101

Figure 2. Structure map of the San Juan Basin, contoured on top of the Huerfanito Bentonite Bed. Previous study of Ayers and Zellers(1988) (Navajo Lake area) is shown in rectangle in the northern part of the basin. Type log is shown in Figure 3. Stratigraphic dip sectionAA0 is shown in Figure 4. Core description of the Blackwood and Nichols NEBU 403 well is shown in Figure 7. Interpretation of outcropsnear Durango, Colorado, described by the authors, is presented in Figure 9. Modified from Ayers et al. (1994).

1102 Geologic Note

(Figure 1). The coastline migrated to the northeast,

depositing a vertical succession of shelf through coal-

bearing continental sediments. The PicturedCliffs Sand-

stone (Figures 3, 4) is a coastal facies thatwas deposited as

the Late Cretaceous shoreline prograded northeastward

into theWestern Interior seaway. Pictured Cliffs Sand-

stones are the depositional platforms uponwhich Fruit-

land peat (coal) accumulated, and ultimately, Pictured

Cliffs shoreline sandstones bound coal seams in the ba-

sinward direction. Progradation of the Pictured Cliffs

shoreline, dependent on complex interactions of sed-

iment supply, basin subsidence, and eustasy, was in-

termittent andwas punctuated by shoreline stillstands

(Kaufman, 1977;Weimer, 1986). Intertonguing of the

Pictured Cliffs Sandstone and the lower Fruitland For-

mation resulted from temporary landward shifts of the

shoreline during overall regression of the Late Creta-

ceous shoreline (Ayers et al., 1994). Three major up-

per Pictured Cliffs tongues occur in the northern part

of the basin. The oldest is referred to as UP1 (Figure 3)

(Ambrose andAyers, 1994), and theother twoare called

UP2 and UP3 (Figure 4).

The Lewis-Fruitland succession, deposited in the

period spanning approximately 76–73.5 Ma (Fassett,

2000), was deposited in one of the greatest extents of

theWestern Interior seaway andduring a peak period of

greenhouse climate (Barron, 1983; Huber, 1998; Sell-

wood andValdes, 2006).Much ofwesternNorthAmeri-

cawas covered by a shallow seaway connected to boreal

regions (Huber, 1998). Coastal onlap in theCampanian

andMaastrichtian was significant, although slightly less

than that of the Turonian, and it diminished in the late

Maastrichtian (Haq et al., 1987).

Stratigraphy

Stratigraphic Nomenclature and Previous Studies

In geophysical well logs, cores, and outcrops, the Pic-

turedCliffs Sandstone grades into the underlying Lewis

Shale, interpreted as shelf and shoreface mudstone and

sandstone interbeds (Flores and Erpenbeck, 1981). The

Pictured Cliffs Sandstone typically has a blocky well-

log response and is composed of amalgamated sand-

stone bodies having a composite thickness of 40–120 ft

(12–36 m); it is inferred to be the framework facies of

prograding barrier strand-plain orwave-dominated delta

depositional systems (Fassett and Hinds, 1971; Erpen-

beck, 1979;Cumella, 1981, 1983; Flores andErpenbeck,

1981;Manfrino, 1984; Ambrose andAyers, 1990; Ayers

et al., 1994).

Conventionally, the base of the PicturedCliffs Sand-

stone is placed at the base of coarsening-upward units

(Figure 3). However, this boundary is time transgres-

sive because the Pictured Cliffs Sandstone intertongues

with themarineLewis Shale (Figure 4). The Lewis Shale

contains several bentonite beds that are excellent cor-

relation marker beds. Contemporaneity of the coastal

(blocky) and shoreface and shelf (coarsening-upward)

units is documentedby low-resistivity shalemarker beds

that cross the proximal shelf and shoreface to intersect

and terminate in the coastal Pictured Cliffs Sandstone

(Figure 4). ThePicturedCliffs Sandstone and equivalent

marine units thicken basinward above the Huerfanito

Bentonite Bed. In addition, progradation of the Pictured

Cliffs shoreline resulted in the basinward offset of the

landward pinch-outs of these marker beds.

The three upper Pictured Cliffs Sandstones that in-

tertongue with the Fruitland Formation in the northern

part of thebasin are composedofwave-dominateddeltaic

or northwest-trending, amalgamated barrier strand-plain

sandstones (Figure 5a) that are individually as thick as

100 ft (30m).Anetmajor (defined as a sandstone >40 ft

[>12 m] thick) sandstone map of the undivided upper

Pictured Cliffs tongues exhibits northwest-trending,

strike-elongate patterns representing a complex of off-

lapping shoreface andwave-dominated coastline deposits

(Figure 5b). However, both dip-elongate and strike-

elongate (northeast- and northwest-trending, respective-

ly) sandstone bodies occur in the northwestern part of

the basin, inferred by Ambrose and Ayers (1991) to rep-

resent awave-dominateddelta complex (CedarHill delta

system) (Figure 5b).

The Fruitland Formation (Figures 3, 4), the pri-

mary coal-bearing formation in the San Juan Basin, is

the continental facies deposited landward of the Pic-

tured Cliffs Sandstone shoreline. It is composed of sand-

stone, mudstone, and coal interbeds. In the northwest

part of the San Juan Basin, thick coal deposits in the

Fruitland Formation are associated with the landward

margins of the Pictured Cliffs shoreline deposits (Rob-

erts and McCabe, 1992).

In early regional subsurface studies, the contact be-

tween the coal-bearing Fruitland Formation (Campa-

nian) and the barren, overlying Kirtland Shale (Cam-

panian andMaastrichtian) was placed at the top of the

highest coal bed or carbonaceous shale bed (Fassett

and Hinds, 1971). However, coal and carbonaceous

shale occur locally within the Kirtland, and thus, the

boundary is erratic. Therefore, the Fruitland-Kirtland

contact was placed by Ayers et al. (1994) at a high-

conductivity peak (Figure 4), corresponding to the base

Ambrose and Ayers 1103

Figure 3. Type log(Blackwood and NicholsNEBU 77) showing Up-per Cretaceous strati-graphic units and inferredfourth-order sequence-stratigraphic cycles. Up-per Pictured Cliffs tonguesUP2 and UP3 are notpresent in this well, butare depicted in strati-graphic dip section AA0

(Figure 4). See Figure 2for well location. Modi-fied from Ayers andAmbrose (1990) andAyers et al. (1994).

1104 Geologic Note

of regionally extensive shale that contains foraminifera

and which formed as a consequence of a short-lived

marine transgression over the Fruitland coastal plain

(Dilworth, 1960). Thismarker is at approximately the

same stratigraphic position as the boundary chosen by

earlier workers (Fassett andHinds, 1971; Fassett, 1987;

Molenaar and Baird, 1989), and it has more genetic sig-

nificance and less variability than laterally discontinu-

ous upper Fruitland coal seams and carbonaceous shale

beds, as it is inferred to be a marine flooding surface.

Thick Fruitland coal seams in the southern part of

the San Juan Basin occur in the lower half (150–200 ft;

45–60m) of the formation (Figure 4). However, thick-

est lower Fruitland coal seams in the northern part of

the basin are stratigraphically equivalent to the thin up-

per Fruitland coal seams in the south. The lower Fruit-

land Formation and coal seams of the southern part of

the basin are absent in the north because they pinch out

between Pictured Cliffs tongues UP1, UP2, and UP3

(Figure 4). Most of the coal seams in the north are

interpreted to pinch out against upper Pictured Cliffs

tongues (Fassett, 2000), although some locally override

the tongues (Figure 4).

Thick, aggradational, sandy successions in the Pic-

turedCliffs Sandstone, landward pinch-out of the upper

Pictured Cliffs tongue UP1, and thick lower Fruitland

coal seams (Figure 4)may have resulted from increased

accommodation in the northern part of the basin. This

increased accommodation has been interpreted as ei-

ther causedby differential subsidence across a fold hinge

(Ayers et al., 1994), episodes of eustatic sea level rise

during periods of highstand during a sustained period of

peak greenhouse climate in theLateCretaceous (Huber,

1998; Sellwood and Valdes, 2006), or shoreline stabi-

lization associated with aggradation of peat swamps

(McCabe and Shanley, 1992; Roberts and McCabe,

Figure 4. Stratigraphic dip section AA0, located in Figure 2. Conductivity curves are shown for wells in this section; datum is the baseof the Kirtland Shale, defined as a high-conductivity zone at the top of a fining-upward sequence in the upper Fruitland Formation.Dates of the Huerfanito Bentonite Bed and the base of the upper Pictured Cliffs tongue UP3 are from Fassett (2000). Modified fromAyers and Ambrose (1990) and Ayers et al. (1994).

Ambrose and Ayers 1105

1992). Abrupt, increased thickness of stacked, aggra-

dational Pictured Cliffs sandstones in the northern part

of the basin are termed ‘‘stratigraphic rise’’ by Fassett

and Hinds (1971). This stratigraphic architecture was in-

terpreted by Fassett andHinds (1971) to reflect periods

of transgressive maxima. These varying interpretations

Figure 5. (a) Depositional model for the Pictured Cliffs Sandstone, Fruitland Formation, and upper Pictured Cliffs tongues in thenorthern part of the San Juan Basin. Aggradation of shore-zone deposits in the Pictured Cliffs Sandstone may have been the result ofincreased subsidence along a basin homocline northeast of a hinge line or may reflect cycles of marine transgression during rates ofincrease of coastal onlap (Pashin, 1998). Peat accumulations may have also helped stabilize Pictured Cliffs barrier sandstones(McCabe and Shanley, 1992). (b) Net thickness of major sandstones in all three upper Pictured Cliffs Sandstone tongues (UP1, UP2,and UP3) in the northern part of the San Juan Basin. The Cedar Hill deltaic complex, inferred from irregular contours in the northwesternpart of the basin, is interpreted to have provided sediments that were reworked along strike (southeastward) in a barrier-strand-plaincomplex. Modified from Ayers et al. (1994).

1106 Geologic Note

and their implications for Fruitland coal-seam thickness,

continuity, and distribution (Figure 6), as well coalbed

methane productivity, are addressed in the Discussion

section.

Sequence Stratigraphy

The stratigraphic section from the Lewis Shale to the

Ojo Alamo Sandstone in this study is divided into a suc-

cession of inferred tracts (lowstand systems tracts [LST],

transgressive systems tracts [TST], and highstand sys-

tems tracts [HST]) (Figure 3). These sequence tracts

are bounded by significant stratigraphic surfaces, includ-

ing nonmarine regressive surfaces (NMRS), transgres-

sive surfaces of erosion (TSE), and marine flooding sur-

faces (FS). These stratigraphic surfaces and tracts are

inferred primarily from lithologic data (Figures 7, 8).

Where lithologic data were absent, these features were

inferred from log profiles (Figures 3, 4). Recognition

criteria for these significant stratigraphic surfaces and

sequence tracts, summarized inTables 1 and2, are briefly

reviewed here. Full descriptions of Pictured Cliffs and

Fruitland stratigraphic intervals containing these fea-

tures are presented in the section on Lithofacies.

Nonmarine regressive surfaces in the Fruitland For-

mation, also representing lowstand surfaces of erosion

and sequence boundaries, are interpreted to be erosional

contacts that separate coarsening-upward successions of

burrowed sandstone or sparsely burrowed, cross-bedded

sandstone with planar to inclined laminations from het-

erolithic, commonly fining-upward, organic-rich, and

coal-bearing successions (Figures 7, 8a). In well logs of

the Fruitland Formation (for example, Figure 3), NMRS

are postulated to bound LSTs at the base, occurring near

the base of high-resistivity, low-density intervals inferred

to be coal seams above coarsening-upward intervals.

Difficult to correlate in logs, these NMRS may simply

record superposition of lower-coastal-plain deposits over

beach deposits during episodes of coastal offlap and

progradation instead of eustatic sea level fall.

Transgressive surfaces of erosion in the Pictured

Cliffs Sandstone and Fruitland Formation define the

bases of TSTs. These surfaces are inferred to be ero-

sional horizons underlying thin (typically 4-in. [10-cm])

zones of detrital coal and organic material (Figures 7,

8b). The zone of detrital material in the Blackwood

and Nichols NEBU 403 core (located in Figures 2, 5) is

overlain by a 4.7-in. (12-cm) interval composed of low-

angle, inclined beds with crosscutting scour surfaces

(Figure 8b). This section of inclined beds grades up-

ward into coarsening-upward, sparsely burrowed, and

plane-bedded sandstone, recording continued transgres-

sion and rise in base level (Figure 7). In logs, TSEs are

inferred to occur below the base of upper PicturedCliffs

tongues (Figures 3, 7). TSEs are interpreted in this study

to represent the initial phase peat-swamp destruction

and of flooding bymarine processes. The top of theTST

is marked by a regional shale inferred to contain a flood-

ing surface; these flooding surfaces are interpreted to

overlie Pictured Cliffs tongues (Figures 3, 4, 7) and are

inferred to mark the maximum degree of transgression

in the northern part of the basin associated with the

deposition of each upper Pictured Cliffs tongue.

Flooding surfaces are interpreted to occur within

5–10-ft (1.5–3.0-m) intervals of marine shale above

TSTs (Figure 7). They are also inferred from logs as high-

conductivity zones above high-frequency, coarsening-

upwardparasequences ofmarine origin (Figure3). These

high-frequency parasequences are interpreted to rep-

resent episodes of coastal offlap and progradation, punc-

tuated by brief periods of transgression. They reflect

either short-term (�100-k.y.) eustatic transgressive-

regressive cycles, pulses of tectonic activity, or possibly

a combination of these factors. Although the Late Cre-

taceous was in a period of peak greenhouse climate

(Barron, 1983; Huber, 1998; Sellwood and Valdes,

2006), glacioeustasy, associated with small (<106-km3),

ephemeral ice sheets in Antarctica, could have provided

the mechanism for the origin of transgressive-regressive

cycles on a global scale (Miller et al., 2003). Moreover,

400-k.y. cyclicity related to theMilankovitch long eccen-

tricity band, described by Gale et al. (2002), could pos-

sibly account for major offlapping cycles (third-order?)

in the Lewis Shale-Fruitland succession (Figure 3).

However, the origin of high-frequency, transgressive-

regressive cycles in the Lewis Shale-Fruitland interval

may also reflect intermittent tectonic activity, similar

to that described by Heller et al. (1993), Zaleha et al.

(2001), and Vakarelov et al. (2006) for other Western

Interior basins. For example, episodes of coastal off-

lap and accommodation for Cretaceous shoreface and

marginal-marine sediments in the San Juan Basin may

have been strongly influencedby intermittent faultmove-

ment associated with low-amplitude, monoclinal flex-

ures (Silver, 1957;Kelly andClinton, 1960; Laubach and

Tremain, 1994).

In log cross sections, the downward-stepping stratal

architecture in Pictured Cliffs Sandstone units that in-

terfinger with the Lewis Shale (labeled DS in Figure 3)

may indicate falling-stage sedimentation associated

with lowstand episodes. Similar stratal stacking patterns

have been documented by Nummedal et al. (1993) in

Ambrose and Ayers 1107

Figure 6. Fruitland net coal map. The greatest net coal thickness occurs in a northwest-trending belt in the north-central part of thebasin. Narrow, dip-oriented coal trends occur southwestward from this belt and are inferred to have formed in a flood-plain setting.Cross section AA0 is shown in Figure 4. Modified from Ayers et al. (1994).

1108 Geologic Note

Figure 7. Core description of the Pictured Cliffs Sandstone, lower Fruitland tongue, upper Pictured Cliffs tongue (UP1), and lowerFruitland Formation in the Blackwood and Nichols NEBU 403 well, located in Figures 2 and 5. Core photographs of selected intervalsin the Pictured Cliffs Sandstone, the basal part of the upper Pictured Cliffs tongue (UP1), and the Fruitland Formation above UP1 areshown in Figure 8.

Ambrose and Ayers 1109

Figure 8. Core photographs of the lower Fruitland tongue and the basal part of the upper Pictured Cliffs tongue (UP1) in theBlackwood and Nichols NEBU 403 well, located in Figures 2 and 5. (a) Upper part of the Pictured Cliffs Sandstone at 3196 ft (974.4 m),inferred to be stained from overlying organic-rich deposits in a regressive interval in the lower Fruitland tongue. (b) Inferredtransgressive surface of erosion above a coal seam in the lower Fruitland tongue at 3161 ft (963.7 m), showing a 4-in. (10-cm) section ofdetrital coal, overlain by finely interbedded siltstone, thin (millimeter-scale) coaly laminae, and carbonaceous shales. (c) Fine-grained,clay-clast–rich, fining-upward sandstone bed in the Fruitland Formation, 3 ft (0.9 m) above an inferred nonmarine regressive surfacethat overlies burrowed coarsening-upward sandstone of an inferred HST. Core description is shown in Figure 7.

Table 1. Summary of Significant Stratigraphic Surfaces in the Pictured Cliffs and Fruitland Formation Interpreted in This Study*

Surface Type

Position in

Sequence Description

Typical

Underlying Facies

Typical Overlying

Facies Figures

NMRS** Erosional Base of LST Scour surface commonly

overlain by clay clasts

and organic fragments

Highstand

shoreface

Delta-plain; swamp 3, 4, 7, 8a

TSE Erosional Base of TST Zones of detrital coal

grading upward into

burrowed sandstone

Lower delta

plain

Destructional barrier;

transgressed swamp

3, 4, 7, 8b

FS Nondepositional Base of HST Dark mudstone in shaly

section overlying upper

Pictured Cliffs tongues

Transgressive

barrier/shoreface

Lower shoreface;

delta front

3, 4, 7

*With inferred position within sequences, lithologic description, and facies association.**NMRS = nonmarine regressive surface (lowstand surface of erosion).

1110 Geologic Note

downward-stepping Cretaceous units of the Western

Interior.

Lowstand systems tracts in the Pictured Cliffs

Sandstone and Fruitland Formation are bounded at the

base by NMRS surfaces and at the top by TSE surfaces

(Figures 3; 7; 8a, b; Table 2). The lower Fruitland tongue

in the Blackwood and Nichols NEBU 403 well is in-

terpreted to represent an LST interval approximately

35 ft (10.7 m) thick, including a coal-bearing, fining-

upward section above an NMRS surface (depth of

3196 ft [974.4 m]; Figure 7) inferred to have truncated

the PicturedCliffs Sandstone (Figure 8a) and extending

upward to a TSE surface at the top of a thin coal seam

(depth of 3161 ft [963.7 m]; Figures 7, 8b).

Transgressive systems tracts in the Pictured Cliffs

and Fruitland Formation overlie TSE surfaces and are

bound at the top by FS (Figures 3, 7). Representing

episodes of transgression and destruction of Fruitland

coastal-plain deposits by floods and storm-related wash-

over-fan deposits, these TSTs are typically thin (~30 ft;

~9m) and consist of sparsely burrowed, planar-laminated

and cross-bedded, coarsening-upward sandstones over-

lying thin (commonly <1-ft [<0.3-m]) zones of detrital

coal beds and organic siltstone (Figures 7, 8b).

Highstand systems tracts in theLewis Shale-Fruitland

succession are bounded at the base by flooding sur-

faces and truncated byNMRS surfaces (Figure 7). These

HST intervals are composed primarily of burrowed,

coarsening-upward sandstones, representing prograda-

tional episodes during periods of high relative sea level;

they range from sparsely burrowed, planar-laminated,

and cross-bedded sandstone (Pictured Cliffs Sandstone

from 3196 to 3214 ft [974.4 to 979.9 m] in Figure 7)

to heterolithic, intensely burrowed, slightly coarsening-

upward silty sandstone, above the upper Pictured Cliffs

tongueUP1(3105–3130 ft [946.6–954.3m] inFigure7).

LITHOFACIES

Core Data: Pictured Cliffs Sandstone andLower Fruitland Tongue

Description

The 21-ft (6.4-m) section of Pictured Cliffs Sandstone

in the Blackwood and Nichols NEBU 403 core con-

sists of a slightly coarsening-upward section of plane-

bedded and cross-bedded, sparsely burrowed, fine- to

medium-grained sandstone (Figure 7). The Pictured

Cliffs Sandstone in this core is overlain by organically

stained, fine-grained sandstone with inclined bedding

and an irregular base (Figure 8a).

The lower Fruitland tongue in the Blackwood and

Nichols NEBU 403 core consists of a basal, 7-ft (2.1-m)

fining-upward section of organic-rich, very fine- to fine-

grained sandstone interbedded with siltstone (Figure 7).

Sedimentary structures in this basal Fruitland section

are partially obscured by burrows. The dominant strati-

fication consists of millimeter-scale laminae of very fine-

grained sandstone interbedded with siltstone. Thicker

Table 2. Summary of Sequence Tracts in the Pictured Cliffs and Fruitland Formation Interpreted in This Study*

Sequence

Lower

Bounding

Surface

Upper

Bounding

Surface Description Typical Facies

Significant

Coal-Bearing? (Interval) Figures

LST NMRS TSE Nonmarine section composed

of thin, silty sandstones,

mudstones, and coal beds

Delta plain;

swamp

Yes (lower Fruitland

tongue; upper

Fruitland Formation)

3, 7, 8a–c

TST TSE FS Commonly coarsening-upward

section of burrowed,

planar-laminated and

cross-bedded marine

sandstones

Washover fan;

transgressive

barrier

No (upper Pictured

Cliffs tongues)

3, 7, 8b

HST FS NMRS Commonly coarsening-upward

section of burrowed,

marine sandstones

Shoreface;

delta front

No** (Pictured

Cliffs Sandstone)

3, 7, 8a

*With bounding, significant stratigraphic surfaces, lithologic description, facies association, and indication of being a significant coal-bearing interval.**Although coal is not shown in HSTs in cored intervals in Figures 7 and 8, this sequence tract is inferred to be coal bearing because of available accommodation space

in the lower coastal plain in periods of relatively high sea level.

Ambrose and Ayers 1111

(1–2-cm; 0.4–0.8-in.) beds of fine-grained sandstone

are weakly contorted, discontinuous, and occur as len-

ticular beds less than 5 cm (2 in.) across. This basal sec-

tion is overlain by a 6-ft (1.8-m) coal bed. A 4-ft (1.2-m)

shale bed is above a 12-ft (3.7-m) section of missing

strata. This shale bed is overlain by a thin (1-ft [0.3-m])

coal bed at the top of the lower Fruitland tongue.

Interpretation

The Pictured Cliffs Sandstone in the lower part of this

core is interpreted to be highstand, upper-shoreface de-

posits overlain by lower-coastal-plain bay fill and peat

swamp deposits in the lower Fruitland tongue. These

strata are also typical of shore-zone successions in the

CretaceousWestern Interior basin (e.g., Asquith, 1970;

McCubbin, 1982; Ryer, 1984). A modern analog for

these upper shoreface deposits in the Pictured Cliffs

Sandstone includes Galveston Island, Texas (Bernard

et al., 1962). Galveston Island has a net coarsening-

upward profile, reflecting progradation in a period of

relatively high sea level. The upper shoreface to the

beach at Galveston Island is composed of fine-grained,

coarsening-upward sandstone with planar, low-angle

bedding and trough cross-lamination. The upper section

at Galveston Island is overlain by an eolian-reworked

section of trough cross-bedded, or rooted, structureless

sandstone. However, emergent, subaerial dune depos-

its, such as those at Galveston Island, are commonly

not preserved because they are unlikely to be buried

before subsequent transgression (Davis, 1983, p. 414)

or eroded during major storms and flood events. Al-

ternatively, they are poorly preserved because of the ac-

tion of roots and vadose groundwaters in the overlying

nonmarine section (Blatt et al., 1980, p. 661).

The coal seam overlying the basal section of the

lower Fruitland tongue records development of bay-

fill or lacustrine-fill deposits overlain by emergent peat-

swamp deposits associated with a major regression or

equivalent lowstand cycle. Bay or lagoonal infill processes

in microtidal settings are commonly characterized by

lacustrine deltaic sedimentation, where heterolithic

successions of siltstones and sandstones provide the

platform for peat accumulation (Belt, 1975; Tye and

Coleman, 1989). The introduction of these lower Fruit-

land peats is inferred to reflect emergent conditions,

recording the onset of terrestrialization caused by re-

gression and decreasing accommodation in the lower

coastal plain, a process described by Frenzel (1983),

Boron et al. (1987), and,more recently, byDiessel et al.

(2000). Peat accumulation is favored after the relative

sea level rise has decelerated to a rate comparable to

that of the growth rate of peat-forming plants. The

beginning of peat formation is part of the terrestri-

alization process that corresponds to decreasing ac-

commodation (vanWagoner et al., 1987, 1990), which

facilitates paludification and peat accumulation, there-

by leading to regressive coals (Diessel, 1992).

Core Data: UP1

Description

The lower Fruitland tongue is overlain by a 30-ft (9-m)

section of coarsening-upward sandstone in the UP1 in-

terval (Figure 7). However, there is a transitional zone

above the upper coal seam in the lower Fruitland tongue

to UP1, marked by a 7.1-in. (18-cm) section of finely

interbedded siltstone, thin (millimeter-scale) detrital coal

beds, and carbonaceous shales (Figure 8b). This transi-

tional section is, in turn, overlain by a sparsely burrowed,

coarsening-upward sectionof fine-grained, cross-bedded

sandstone that grades upward into fine- to medium-

grained and plane-bedded sandstone with numerous,

thin, organic-rich silty layers (Figure 7).

Interpretation

The base of the UP1 interval in this core (Figure 8b)

records a transgression over lower-coastal-plain deposits

in the lower Fruitland tongue. The 7.1-in. (18-cm) sec-

tion of finely interbedded siltstone, thin (millimeter-

scale) coaly beds, and carbonaceous shales above the

upper coal seam in the lower Fruitland tongue repre-

sents the invasion and destruction of lower Fruitland peat

swamps by floods and storm-related washover-fan de-

posits. This basal UP1 section is overlain by a coarsening-

upward interval composed of fine- to medium-grained,

sparsely burrowed, and dominantly planar-bedded sand-

stone that represents upper-shorefacedeposits recording

continued transgression associated with sea level rise.

Modern analogs for peat-forming environments in

transgressive sequences include the Snuggedy swamp

and Cape Romain in South Carolina. These areas occur

along an upper microtidal to lower mesotidal coast-

line where peat and marsh environments are being en-

croached by relative sea level rise. Areas in the Snuggedy

Swamp exist where freshwater peat is currently under

salt-marsh vegetation or lagoonal clay and silt (Staub and

Cohen, 1979), corresponding to an early deepening and

submergence pretransgressive phase in the backbarrier.

Cape Romain illustrates active transgression and

destruction of the lower coastal plain. Cape Romain is

a cuspate, transgressive barrier headland on the South

1112 Geologic Note

Carolina coast, south of the Santee delta. It fronts a

7.2-mi (12-km)-widemarsh and tidal-flat complex.Cape

Romain is retreating 32.8–49.2 ft/yr (10–15 m/yr),

although hurricanes and tropical storms have caused

coastal retreat as great as 82 ft (25 m) in some years

(Ruby, 1981). The vertical succession consists of a

49.2-ft (15-m) upper Pleistocene, regressive sandy es-

tuarine section overlain by a 39.4-ft (12-m) Holocene

transgressive-barrier section. Current transgression and

ravinement have exhumed older peat deposits dated

between 11 and 7.5 ka (Ruby, 1981). Both the peats

and lagoonal clays are currently being eroded and are

overlain by shelly washover-fan deposits. In the Black-

wood and Nichols NEBU 403 core, this ravinement

surface is expressed as the base of the UP1 sandstone

above the zone ofmultiple thin (centimeter-scale) coaly

detrital layers (Figure 8b).With continued transgression

and water deepening, washover-fan deposits at Cape

Romain will be overlain by upper-shoreface sands, pro-

ducing a profile inferred to be similar to that of the

upper part of the lower Fruitland tongue and UP1.

Core Data: Fruitland Formation above UP1

Description

The upper part of the Fruitland Formation present in the

core of the Blackwood and Nichols NEBU 403 well is a

47-ft (14.3-m) heterogeneous section of very fine- to

fine-grained sandstones and silty mudstones (Figure 7).

The lower one-half of the section (from3105 to 3130 ft

[946.6 to 954.3 m]) consists of burrowed, slightly

coarsening-upward, very fine- to fine-grained sandstones

above a 9-ft (2.7-m) section of predominantly siltymud-

stone. Stratification in these sandstones, although ob-

scured by burrows, consistsmostly of ripples and small-

scale trough cross-bedding. In contrast, the upper one

half of the section (from 3083 to 3105 ft [939.9 to

946.6 m]) is an extremely heterolithic interval of thin

(commonly <2-ft [<0.6-m]), very fine-grained sand-

stone beds interbedded with muddy siltstone. Sand-

stone beds in the basal part of the upper section (from

3100.5 to 3105 ft [945.3 to 946.6 m]) are erosionally

based, organic-rich, and feature climbing ripple cross-

stratification, as well as abundant clay rip-up clasts

(Figures 7, 8c).

Interpretation

The Fruitland Formation above UP1 is interpreted to

be a lower section of highstand, burrowed, lower shore-

face deposits that are abruptly overlain by a nonmarine,

lowstand section of sparsely burrowed, organic-rich

crevasse-splay, splay-channel, and overbank deposits

in an emergent, lower-coastal-plain setting. The lack

of coal seams in the Fruitland Formation in the upper

interval from 3083 to 3131 ft (939.9 to 954.6 m) may

be caused by unfavorable conditions for peat to accu-

mulate because of sporadic marine influence or rela-

tively rapid deposition.An inferrednonmarine regressive

surface occurs at 3105 ft (946.6 m), marking an abrupt

transition from burrowed, shallow-marine deposits to

relatively unburrowed, nonmarine carbonaceous depos-

its. Alternatively, the transition from the coarsening-

upward, burrowed sandstones to the section of thinly

bedded, poorly burrowed sandstones could simplymark

the superposition of lower-coastal-plain deposits above

shallow-marine shorezone deposits during an episode

of progradationwith insignificant change in base level.

This alternative interpretation is possible, in view of

the lack of an unambiguous unconformity that can be

traced in well-log correlations at this stratigraphic level

in the Fruitland Formation.

Outcrop Data

Description

Outcrops of the Pictured Cliffs Sandstone and the

Fruitland Formation in the northwestern part of the San

Juan Basin commonly contain multiple transgressive-

regressive cycles anddisplay intertonguing facies relation-

ships (Manfrino, 1984; Condon et al., 1997). Outcrop

data and interpretations presented in this section are

from a transect between the Carbon Junction gas seep

area andFloridaRiver, nearDurango,Colorado (Figure 9,

located in Figures 2, 5b). A 5.7-mi (9.1-km) stratigraphic

cross section, parallel to paleodepositional dip, displays

southwestward thinning of UP2 above a thin (5–15-ft

[1.5–4.5-m]) coal seam in the lower Fruitland tongue

(Figure 9). UP2 is approximately 50 ft (15 m) thick

at the Florida River (measured section 6, located in

Figure 9), where it consists of a lower section of rippled,

very fine-grained sandstone interbeddedwith siltstone,

overlain by a 20–25-ft (6–7.6-m) section of heavily

burrowed, very fine- to fine-grained sandstone. In con-

trast,UP2 atmeasured section 2 is a 23-ft (7.0-m) fining-

upward, unburrowed, and organic-rich cross-bedded

sandstone that abruptly overlies the lower Fruitland

coal seam. This fining-upward sandstone pinches out

southwestward less than 150 ft (45.7m) into a section

composed predominantly of coal and thin interbeds of

very fine-grained sandstone. The lower part of the Fruit-

land Formation at measured section 1 (Carbonero coal

seam alongColorado StateHighway 3) contains a thick

Ambrose and Ayers 1113

Figure 9. (a) Strati-graphic section BB0 fromoutcrops from the CarbonJunction gas seep area tothe Florida River, east ofDurango, Colorado, in LaPlata County. Datum is thetop of the Pictured CliffsSandstone. Locations ofthese stations are (1) swnw1/4 sec 4 T34N R9W;(2) ne nw1/4 sec 4 T34NR9W, (3) sec 33/33 T341/2/35N R9W, (4) w1/2 sec34 T35N R9W, (5) sec26/27 T35N R9W, and(6) sec 19 T35N R8W/sec24 T35N R9W. (b) Locationof outcrop section BB0.Area of cross section in theSan Juan Basin is shownin Figure 2.

1114 Geologic Note

(almost 100-ft [30.5-m]) coal seam overlain by a 10–

15-ft (3–4.5-m) coarsening-upward section of thin,

rippled, very fine- and fine-grained sandstones inter-

bedded with siltstone.

Interpretation

The cross section in Figure 9 illustrates complex facies

variability and coal-seam geometry in a major Pictured

Cliffs-Fruitland transgressive-regressive cycle in the

northwest part of the basin. UP2, interpreted to be a

succession of predominantly shoreface deposits, inter-

tongues with lower-coastal-plain deposits of the lower

Fruitland Formation. UP2 thickens and becomes finer

grained toward the northeast (seaward) direction. The

southwest-thinning UP2 tongue records the updip or

landward limit of a transgression over lower-coastal-

plain deposits of the lower Fruitland tongue, interpreted

between measured sections 2 and 3. The transition be-

tween these measured sections is abrupt, where shore-

face deposits inUP2 are inferred to pinch out seaward of

stratigraphically equivalent lower Fruitland distributary-

channel deposits.

Continuity and thickness of individual coal seams

vary in this cross section. For example, the basal Fruitland

coal seam pinches out between measured sections 4 and

5 (Figure 9). Its continuity between measured sections 1

and 2 is unclear; it may merge with the thick coal seam

in measured section 1 or pinch out against the basal

Fruitland sandstone bed at that locale. In contrast, the

coal seam overlying UP2 is continuous. At measured

section 2, this seam is inferred to be split from the thick

coal seam in measured section 1 and to override UP2

throughout, although it is inferred to terminate between

measured sections 5 and 6, where it either was eroded

by an upper Fruitland fluvial-channel complex or was

never deposited.

Condon et al. (1997, their plate 7) also described

facies and coal-seam variability in the Carbon Junction

to Florida River area, presentingmeasured sections from

BasinCreek,west of theCarbon Junctionmine, toTexas

Creek, farther east of the Florida River. They also in-

ferred that the thick Fruitland coal seam southwest of

theCarbon Junctionmine splits northeastward and over-

rides a northeast-thickening UP2, although the lower

part of the coal-bearing section above UP2 is inferred

in an area of cover at their Ewing Mesa section, not far

from measured section 5 in Figure 9. Condon et al.

(1997) also showed southwestward thinning of UP2,

where it may either pinch out into lower Fruitland coal

seams or be the equivalent of thin sandstone at the top

of the Pictured Cliffs Sandstone at Carbon Junction.

DISCUSSION: COAL THICKNESS,CONTINUITY, AND IMPLICATIONS FORCOALBED METHANE RESERVOIRS

Controls on Coal-Seam Thickness and Continuity

Documenting geologic controls on coal-seam thickness

and continuity, as well as intertonguing relationships be-

tween coal seams and sandstones in the transition from

lower-coastal-plain to shoreline successions, is vital in

understanding coalbed methane reservoir characteris-

tics, not only in the Fruitland Formation, but also in

other coalbed-methane–producing basins. Knowledge

of coalbed reservoir characteristics, including continuity

and geometry, influence well spacing and design of pro-

duction facilities. Additionally, estimates of coalbed

methane reservoir volume and extent are partially de-

pendent on models of coal-seam continuity; discontin-

uous, lower-coastal-plain coal seams that pinch out

against coeval, sandy shoreface successions are inferred

to contain lesser volumes of coalbedmethane than those

interpreted to override these shoreface complexes.More-

over, draping of compactable coal seams over relatively

less compactable, subjacent sandstones may cause frac-

tures that could enhance coalbed permeability and pro-

ducibility (Donaldson, 1979; Houseknecht and Iannac-

chione, 1982; Tyler et al., 1991; Laubach et al., 1998,

2000). Finally, laterally continuous coal seams that over-

ride shoreline complexes better explain the artesian

overpressure that is present and greatly affects coalbed

reservoir performance in the northern San Juan Basin

(Kaiser et al., 1994).

Controls on Fruitland coal-seam thickness and con-

tinuity are a function of several factors, including local

facies distribution in coastal-plain settings, the position

of the Pictured Cliffs shoreline through time caused by

multiple episodes of regression and transgression, and

basin subsidence. For example, in an early regional study

of the San Juan Basin, Fassett and Hinds (1971) inter-

preted that many thick, lower Fruitland coal seams

pinch out landward (southwestward) of thick, aggra-

dational Pictured Cliffs Sandstone successions that ex-

hibit stratigraphic rise, coincidingwith periods of trans-

gressive maxima. In a study of the Navajo Lake area,

located in Figure 2, Ayers and Zellers (1988) used

closely spaced wells (approximately 400 well logs in a

215-mi2 [557-km2] area) to show that geologic con-

ditions that affect the occurrence and producibility of

coalbed methane are more complex than inferred from

regional studies conducted with sparse data. Using a da-

tum above instead of below the coal-bearing Fruitland

Ambrose and Ayers 1115

Formation, as had been done by previous researchers,

they inferred that basin subsidence indirectly controlled

the occurrence of thick coal seams by causing reversals

in the direction of shoreline migration and interfinger-

ing of Fruitland coal seams with upper Pictured Cliffs

Sandstones (Figure 4). They also concluded that although

lower Fruitland coal seams pinch out behind Pictured

Cliffs shoreline sandstones, some upper Fruitland seams

override abandoned-shoreline sandstones, thereby form-

ing extensive coalbed methane reservoirs.

Possible controls on both types of coal-seam conti-

nuity (those seams that pinch out against the landward

margin of shoreface sandstones and those that override

them) include relative rates of aggradation versus pro-

gradation, rate of change in eustatic sea level, and peat

type (raised versus low-lyingmires). For those coal seams

inferred to pinch out against the landward margins of

shoreface sandstones, aggradation is inferred to have ex-

ceeded progradation during periods of shoreline stabi-

lization and vertical peat accumulation from increased

accommodation space, because of either differential sub-

sidence or eustatic sea level rise. Slow tomoderate rates

of relative sea level rise may be optimum for formation

and preservation of coal seams that pinch out against

these shoreface successions; rapid rates of sea level rise

would result in the inundation and destruction of peat

swamps, whereas low rates of sea level rise associated

with lesser accommodation space increase the possibility

of sediment supply and progradation exceeding aggra-

dation, resulting in peat-forming environments migrat-

ing seaward and being superimposed over older shore-

face sandstones (Cross, 1988).

In a study of the various factors that control coal

distribution in several United States Western Interior

basins, Cross (1988) demonstrated that many Creta-

ceous coals accumulated in lower-coastal-plain settings

at the top of, and landward of, shoreline trends, and that

the most continuous and thickest coals were associated

with aggradational, vertically stacked progradational

successions reflecting transgressive and regressive max-

ima. These thickest coals reflect both high accommo-

dation potential andmajor sediment availability, which

typically occur with vertically stacked progradational

events of third-order cycles.Many of these thickest coal

seams developed along the landward pinch-out of the

main progradational shoreface and deltaic sandstones;

althoughmost coal seams pinch out along the landward

terminus of these marine progradational successions,

some override them, extending seaward 2–10mi (3.2–

16 km). Examples include the Mesaverde Group in

northwest New Mexico (Beaumont et al., 1971), the

Ferron Sandstone Member in Utah (Ryer, 1981, 1984),

the Adaville Formation in southwestern Wyoming

(Lawrence, 1982), and the Rock Springs Formation in

the Greater Green River Basin (Levey, 1985).

Controls on thickest, high-quality peats in recent

and modern deposits in the Mississippi delta plain are

shown by Kosters and Suter (1993) to have been in

part caused by rising sea level that provided increased

accommodation space and promoted optimum ground-

water levels and nutrients that fostered accumulation

of abundant, high-quality organic material. In contrast,

relatively stable sea level conditions associated with

subsequent progradational settings resulted in less ac-

commodation space landward of the shoreline and sub-

sequent migration of peat-forming environments. How-

ever, in a study of the coastal plain of the Netherlands,

McCabe and Shanley (1994) report that from 14 ka to

the present, low-lying, thinmires developed as sea level

rise exceeded peat accumulation rates. As the rate of

sea level rise decreased, regressive conditions prevailed,

and raised mires developed as the rate of peat accu-

mulation outstripped base level rise.

Controls on Accommodation Space in the NorthernSan Juan Basin

Possible factors associated with increased accommo-

dation in the northern part of the San Juan Basin in-

clude increased subsidence northeastward of an inferred

fold hinge (Ayers et al., 1994), multiple episodes of

sea level rise during a period of peak greenhouse climate

in the LateCretaceous (Huber, 1998), or shoreline stabi-

lization and aggradation of peats in raidedmires (Roberts

andMcCabe, 1992; McCabe and Shanley, 1992). These

varying interpretations and their implications for coal-

seam continuity and thickness, as well coalbed meth-

ane productivity, are discussed here.

Fold Hinge

Ayers et al. (1994) inferred a fold hinge (structural

hinge line) in the northern part of the San Juan Basin

(Figure 2). Its presence was inferred from the coinci-

dence of several geologic anomalies, including (1) change

in structural attitude from northeast-dipping beds on

the southwest to nearly flat-lying beds on the northeast

(Figure 2, this study); (2) faults identified in well logs

in this area (Ambrose and Ayers, 1994); (3) highly or-

ganized gravity values that trend northwestward, par-

allel to the hinge line, and that are closely spaced (Scott

et al., 1994, their figure 9.4); and (4) increased net coal

1116 Geologic Note

thickness in the Fruitland Formation along the fold

hinge (Figure 6, this study). Additionally, northwest-

trending normal faults, having as much as 70 ft (21 m)

of displacement (Roberts and Uptegrove, 1991), are

present where the fold hinge projects to outcrop along

the western margin of the basin. Studies of reflection

seismic data by the U.S. Geological Survey indicate

northwest-trending faults affecting basement rock in

this area (Huffman and Taylor, 1991). Sporadic move-

ment along northwest-trending faults in the northern

part of the basin in the Late Cretaceous may have been

associatedwith a low-amplitude flexure in the northern

part of the basin, resulting in increased subsidence and

accommodation space for shoreface complexes (Silver,

1957). However, the hinge line is approximately 10 mi

(16 km) southwestward of the updip pinch-out ofUP1,

and stacking of the upper Pictured Cliffs tongues (UP1,

UP2, and UP3) is 15–25 mi (24–40 km) downdip of

the hinge line (Figure 4). Moreover, individual strati-

graphic units display no increased thickness across the

fold hinge, suggesting that this feature had very little

local control on depositional topographywhere present-

day faults are located. However, the fact that the up-

per Pictured Cliffs tongues are not coincident with the

postulated hinge line may not argue against its exis-

tence and importance. For example, in the southern part

of theSan JuanBasin, sediment supply is inferred to have

outstripped accommodation space (restricted accommo-

dation), and thus, the shoreline prograded rapidly across

a gentle shelf. However, when the shoreline encoun-

tered the hinge line, the balance between accommo-

dation and sediment supply would have been closer.

Therefore, north of the hinge line where accommoda-

tion was greater, eustasy and sediment supply would

have waxed and waned, resulting in stillstands, verti-

cal barrier stacking, and shoreline oscillation.

Eustatic Sea Level Rise

Relative sea level was high in the late Campanian and

early Maastrichtian during deposition of the Pictured

Cliffs Sandstone and Fruitland Formation (Haq et al.,

1987). The Lewis-Fruitland succession was deposited

in the period spanning approximately 76–73.5Ma (Fas-

sett, 2000), during one of the greatest extents of the

Western Interior seaway and during a period of green-

house climate (Barron, 1983; Huber, 1998; Sellwood

and Valdes, 2006). Coastal onlap associated with sea

level rise could have provided additional accommo-

dation for stacking of shallow-marine parasequences

in the San Juan Basin, with each upper Pictured Cliffs

tongue representing a fourth-order transgressive cycle.

However, eustatic sea level rise may not be the only

process accounting for the stacking of upper Pictured

Cliffs tongues. For example, UP1, UP2, and UP3 ac-

count for greater than 75% of the total thickness of all

Pictured Cliffs stratigraphic units that intertongue with

the Fruitland Formation. The distance overwhich these

upper PicturedCliffs tongues are in stacking or shingled

relationships is only about 25 mi (40 km), compared

to the 90-mi (145-km) northeast-southwest extent of

the basin (Figures 3, 4), suggesting local basin subsi-

dence could be a factor in controlling deposition. How-

ever,modest relative sea level rise accompanying steady

tectonic subsidence and strong sediment supply could

explain a stabilized shoreline position during upper Pic-

turedCliffs deposition. Similar stratal architectures have

been documented by Ryer (1981, 1984) in Ferron strata

in Utah, where aggradational shoreface successions oc-

cur regardless of local subsidence.

Stabilization of the Pictured Cliffs Shoreline and

Peat Swamp Aggradation

The Pictured Cliffs shoreline may have been stabilized

by aggrading peat swamps in raised mires (Roberts and

McCabe, 1992). Because peat accumulation may keep

pacewith sea level rise, the development of raisedmires

may reduce the extent of marine transgressions and sta-

bilize the shoreline (McCabe and Shanley, 1994).Raised

mires, protected from clastic influx, typically result in

low-ash peats (coals) and are present closely landward

ofmany vertically stacked parasequences in Cretaceous

strata in the Western Interior (Ryer, 1981; McCabe,

1987, 1991). Roberts andMcCabe (1992) suggest that

Fruitland coals are analogous to high-ash peats of the

Dismal swamp in the southeasternUnited States. Peats

in the Dismal swamp are accumulating in raised mires

approximately 12.4mi (20 km) thick. This is consistent

with their interpretation of lower Fruitland coals hav-

ing formed 12.4–15.5 mi (20–25 km) inland of coeval

Pictured Cliffs shorelines, based on their own coal-

isopach maps. However, the high ash content (17–

34%) and numerous ash partings in basal Fruitland coals

(Roberts and McCabe, 1992) are inconsistent with a

raised mire model and suggest that coals in the lower

part of the Fruitland Formation accumulated in mires

that were transitional from low lying to raised. More-

over, even low-lying mires can be effective sediment

baffles and traps, thereby helping to retard erosion and

to stabilize barriers (Knutson et al., 1982; Knutson,

1988; Kadlec, 1990; Leonard and Luther, 1995).

Ambrose and Ayers 1117

Controls on Fruitland Coalbed MethaneResources and Productivity

The Fruitland Formation is the most prolific coalbed

methane play in the world; it has produced more than

1 tcf of gas since 1980 (Ayers, 2002). Fruitland coalbed

gas production is greatest from the northwest-trending

fairway (Figure 10), where cumulative gas production

exceeds 20 bcf/well, and daily production commonly

exceeds 1 mmcf/well (Kaiser and Ayers, 1994; Meek

and Levine, 2006). Controls on Fruitland coalbed meth-

ane productivity are related to several factors, including

thickness, coal rank, hydrologic overpressuring caused

by coal-seam discontinuities related either to offset by

faults or stratigraphic pinch-outs, local structural com-

plexity, and regional faults and fractures. For example,

Fruitland coalbed methane resources and cumulative

production are related to net coal thickness (Ayers

et al., 1994; Kaiser and Ayers, 1994; Kaiser et al., 1994;

Meek and Levine, 2006). The greatest Fruitland net

coal thickness (>70 ft; >21. 3 m) and cumulative pro-

duction values occur in a coalbed methane fairway

(Ayers, 2002) that corresponds to a northwest-trending

belt along the updip (southwestward) pinch-out line of

the upper Pictured Cliffs tongue UP1 (Figures 6, 10).

The northwest-trending belt of great cumulative pro-

duction does not correspond perfectly to theUP1 pinch-

out line, but instead crosses it eastward, suggesting that

other controls such as natural fractures or hydrologic

overpressuring may exist on cumulative coalbed meth-

ane production, such as coal-seam discontinuities and

structure. The northwest-trending belt also corresponds

to areas containing methane resources greater than

25 bcf/mi2 (Ayers et al., 1994). Secondary, northeast-

oriented trends of high initial potential were also noted

by Ayers et al. (1994); these trends were interpreted to

Figure 10. Cumulative, per-well coalbedmethane production in the Fruitland Formation, showing four type producing areas (TPA 1,TPA 2,TPA 3, and TPA 4) and superimposed updip pinch-out limits of the upper Pictured Cliffs tongues UP1, UP2, and UP3 and a structural foldhinge, inferred from Ayers et al. (1994). A northwest-trending fairway of very high cumulative gas production corresponds to areas ofthickest Fruitland net coal (see Figure 6, this study), where Fruitland peat swamps are inferred to have accumulated landward of stabilizedPictured Cliffs shorezone complexes. Modified from Meek and Levine (2006).

1118 Geologic Note

correspond to northeast-oriented belts of thick Fruit-

land coal seams of fluvial origin.

In addition, coal reservoir overpressuring and in-

creased values in potentiometric surface in the northern

part of the basin, although caused by a variety of hydro-

logic factors and not required for production of coal-

bed methane, are partly related to pinch-outs of thick

coal seams southwest of the UP1 and UP2 pinch-outs

(Kaiser and Ayers, 1994). Southwestward pinch-outs

and significant thinning in coal seams southwest of UP1

and UP2 pinch-outs reflect a transition from amarginal-

marine to coastal-plain setting and accommodation.

However, coal reservoir pressure discontinuities (over-

pressure) along the southwestward pinch-outs of UP1

may also be caused by structural factors, where normal

faults commonly offset strata 40 ft (12m) (Ambrose and

Ayers, 1994), sufficient to introduce additional discon-

tinuities in most Fruitland coal seams.

SUMMARY AND CONCLUSIONS

� We infer that the upper PicturedCliffs tonguesUP1–

UP3 are high-frequency, possibly fourth-order TSTs

associatedwith three episodes of relative sea level rise

coincident with differential subsidence in the north-

ern part of the San Juan Basin.� Each upper Pictured Cliffs tongue is an amalgam-

ated barrier and strand-plain unit up to 100 ft (30m)

thick that trends northwestward, parallel to deposi-

tional strike.� Upper Pictured Cliffs tongues are imbricated, and

successive tongues are offset basinward, reflecting

net shoreline progradation.� Temporary stabilization of the Pictured Cliffs shore-

line during periods of highstand allowed thick,

northwest-trending belts of coastal-plain peats to

accumulate landward of the Pictured Cliffs tongues.

Possible causes of the HSTs are a fold hinge, eustatic

sea level rise, and shoreline stabilization via aggra-

dation of peats in raisedmires. However, many lower

Fruitland coals have high ash contents, suggesting de-

position in low-lying mires unprotected from clastic

influx.� Although individual stratigraphic units do not pinch

out against the postulated basin hinge line, signifi-

cant stratigraphic rise and amalgamation of shore-

face complexes occur in a geographically restricted

area, suggesting that barrier stacking was influenced

by differential subsidence. We conclude that many

basal Fruitland coal seams pinch out against the land-

wardmargin of shoreface sandstones, but others over-

ride them, reflecting progradation and basinward

migration of peat-forming environments.� Fruitland coalbed methane resources and produc-

tivity are related to coal-seam thickness, geometry,

and extent; other important factors include thermal

maturity, fracturing, and overpressuring. The high-

productivity fairway trend occurs in the northern

part of the basin, where it (1) is oriented northwest-

ward; (2) coincides with the greatest Fruitland net

coal thickness; and (3) is located landward of coeval,

aggradational shoreface deposits of three TSTs.� Similar relationships between thick coal seams and

coeval stacked shoreface sandstones exist in other

Western Interior basins in theUnited States, and thus,

the San Juan Basin can serve as models for coalbed

methane exploration in other western and, possibly,

international basins.

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