Late Jurassic Ecosystem Reconstruction in the
Western Interior of the United States


Christine E. Turner and Fred Peterson

U.S. Geological Survey, Box 25046, MS-939, Denver, CO 80225.

And the Morrison Research Team (contact through above address):
D.J. Chure, T.M. Demko, S.P. Dunagan, D.D. Ekart, G.F. Engelmann,
E. Evanoff, A. Fiorillo, S.C. Good, S.T. Hasiotis, B.J. Kowallis, R.J. Litwin,
D.L. Newell, M.E. Schudack, and G.L. Skipp.


Abstract—Interdisciplinary studies of the Late Jurassic Morrison Formation throughout the Western Interior have resulted in reconstruction of the extinct ecosystem inhabited by the largest herbivores that ever roamed the earth. The ecosystem within the Morrison depositional basin was governed in large part by a rain shadow that developed in the lee of the mountainous uplands to the west, which greatly influenced the availability of fresh surface and nearsurface water in the depositional basin. Isotopic analyses of fossil soil nodules in the Morrison depositional basin confirm the rain-shadow effect of the uplands to the west. The upland regions captured moisture from the westerly winds and some of this precipitation fed streams that flowed eastward across the alluvial plain and also fed underground aquifers that controlled the water table beneath the alluvial plain. Wind-blown sand, evaporites, development of a large saline, alkaline lake, together with evidence from the flora, fauna, and trace fossils suggest that fresh surface water may have been scarce during Morrison deposition.

Local presence of unionid clams in some stream beds indicate that at least these streams were perennial in nature because the larval stage of unionid clams attaches onto the gills of fish, which only live in perennial streams. Crayfish burrows that occur in beds adjacent to stream channels offer clues to the nature of some of the streams as well, because crayfish must burrow down to the water table to survive. Crayfish burrows that extend downward into sandstone near the stream channels in the Morrison indicate that the water table was below the level of the stream, a condition consistent with effluent streams where the streams are feeding the water table. This situation is consistent with streams that develop in a semi-arid to arid climate.

Taken together, the evidence from the sedimentology, isotopic studies, body fossils, and trace fossils suggests that some of the Morrison streams were probably perennial but that many were probably ephemeral and may have experienced substream flow, resulting in the availability of surface water in the form of water holes much of the time. Streams that were perennial in nature, such as the deposits that contain the abundant dinosaur remains in the quarry sandstone bed at Dinosaur National Monument, Utah, may thus have been the exception rather than the rule, accounting for the concentration of dinosaur skeletons. The dinosaurs may have been congregating close to the last reliable stream during a major drought.



Introduction

The Morrison Extinct Ecosystem Project is a joint NPS-USGS-funded interdisciplinary study to reconstruct the Late Jurassic predominantly terrestrial ecosystem throughout the Western Interior during deposition of the Morrison Formation. This colorful formation is known worldwide for the skeletons of large dinosaurs, especially the giant sauropods, that have been recovered from it and displayed in many museums throughout the world. The formation is exposed in many NPS units including Arches NP, Bighorn Canyon NRA, Black Canyon of the Gunnison NM, Capitol Reef NP, Colorado NM, Curecanti NRA, Devils Tower NM, Dinosaur NM, Glacier NP, Glen Canyon NRA, Hovenweep NM, Wind Cave NP, and Yellowstone NP, as well as the newly designated Grand Staircase-Escalante NM managed by the BLM.

The goals of the project were to (1) apply modern research techniques that would yield an improved understanding of the habitat that existed when the Late Jurassic dinosaurs roamed the western U.S., which, in turn, would (2) help land managers make science-based decisions in resource management, and (3) improve NPS interpretive programs. Most of the results are included in technical reports published in the scientific literature and in administrative reports submitted to the NPS. In addition, under the auspices of the NPS-GIP (Geolo
gist-in-the-Park) program, one of the principal investigators (Fred Peterson) will distill the scientific findings and prepare a less technical publication for the lay public.

The multidisciplinary approach allowed us to study various aspects of the rock and biostratigraphic record for the Morrison Formation, with various lines of evidence leading to an integrated picture. The investigations included studies of regional tectonics, regional stratigraphic framework, radiometric and paleontologic dating, sedimentology, paleosols (fossil soils), dinosaur biostratigraphy, trace fossils, taphonomy (processes that occur between the death of an organism and discovery as a fossil or trace fossil), microfossils, invertebrates, smaller vertebrates, and isotopic analysis of teeth and paleosol nodules. Integration of data from the various studies is resulting in one of the most complete understandings of an ancient continental ecosystem.

Geological Setting

Eleven named members are currently recognized in the Morrison (Szigeti and Fox, 1981; Peterson, 1994; O'Sullivan, 1997), and all but two are restricted to the Colorado Plateau. Another closely related formation that correlates with the lower part of the Morrison is the Ralston Creek Formation in the Front Range foothills west of Denver (Peterson and Turner, in press). For simplicity the formation is here divided into upper and lower parts that are separated by a conspicuous difference in clay mineralogy, although other lithologies are also present and may predominate in each part. Clay minerals in the lower part consist dominantly of non-swelling types whereas clay minerals in the upper part consist dominantly of swelling (smectitic) types. The change in clay mineralogy reflects the dramatic increase in altered volcanic ash that was incorporated in the sediments (Turner and Fishman, 1991). The increased volcanic component indicates increased volcanism in the volcanic arc that lay off to the west. The change in clay mineralogy occurs as far north as northern Wyoming but is not present in Montana or the Black Hills of northeastern Wyoming and western South Dakota where all the clays in the formation are of the non-swelling type. Where present, the change in clay mineralogy constitutes a convenient marker horizon that is of considerably value for correlation purposes.

About 6-15 m (20-50 ft) below the clay change is a fairly persistent paleosol (or closely spaced series of paleosols) that also is fairly widespread and ultimately may prove to be another excellent marker horizon near the middle of the formation (Demko and others, 1996). Interestingly, although dinosaur bones and skeletons have been recovered in many parts of the Western Interior and from much of the vertical thickness of the formation, notable changes in the dinosaur fauna occurred near the middle of the formation and correlate with the distinct paleosol zone and the change in clay mineralogy (Turner and Peterson, in preparation).

Radiometric dating shows that the Morrison was deposited 155_147 million years ago (Kowallis and others, in press ). Deposition stopped some 6 million years before the close of the Jurassic Period, which ended approximately 141 million years ago.

Because of continental drift, the Western Interior depositional basin was about 650 km (400 mi) farther south than today. This places the present-day Four Corners near the latitude of the southern border of Arizona (Parrish and others, 1982). Data from other workers who deal with climate on a global scale suggest that the Earth was warmer than today (for example, polar ice caps probably were absent; Hallam, 1982). Stable isotopes in carbonate nodules from Morrison paleosols indicate a significantly higher carbon dioxide content in the atmosphere than at present (Cerling and others, 1996; Ekart and Cerling, 1997). Because carbon dioxide is a significant "greenhouse" gas, this, as well as the more southerly latitude of the region, suggests that the climate in the Western Interior was appreciably warmer than today.

During the Late Jurassic, a volcanic mountain chain similar to the present-day Andes existed along the west coast of North America more or less along California's border with Arizona and Nevada. Another highland or possibly mountainous range lay farther inland roughly along the Nevada_Utah state line. The nature of the terrain between these two areas is unclear but it probably included a small number of scattered volcanoes.

Farther east lay the vast Western Interior lowland plain on which the Morrison Formation was deposited. The inland plain extended from Arizona and New Mexico northward to Montana and on into Alberta, Canada, and it may have originally extended much farther east, as some beds of possible Late Jurassic age in Iowa (Cody and others, 1996) and Michigan suggest. Streams originating in the highlands flowed eastward, carrying their bedload of sand and gravel onto the aggrading Morrison alluvial plain (Turner-Peterson, 1986, Peterson, 1994).

Westerly to southwesterly winds (Peterson, 1988) impinged on the mountain range that lay to the west of the Morrison depositional basin, which left much of the basin in a rain shadow, as shown by isotopic analyses of carbonate soil nodules (Ekart and Cerling, 1997). The rain-shadow effect was responsible for the dry climate that prevailed throughout most of Morrison deposition. For most of the time, the climate in this area was semiarid or perhaps even arid in places, as indicated by deposits of bedded gypsum, which forms under highly evaporative conditions; windblown sandstone deposits; magadi-type chert (indicative of highly alkaline lake waters; Dunagan and others, 1997); and saline, alkaline lake beds (Turner and Fishman, 1991). There may have been somewhat wetter time intervals that occurred seasonally or intermittently.

Deposition of the Morrison Formation

During the earliest stages of deposition of the lower Morrison (Windy Hill Member and correlative strata), a seaway that was an arm of the ancestral Pacific Ocean extended east across Wyoming and into adjacent parts of Montana, the Dakotas, Nebraska, northern Colorado and northern Utah. Farther south in southeastern Utah and in western and eastern Colorado, gypsum in the Tidwell Member and correlative Ralston Creek Formation was precipitated as evaporite deposits in hypersaline lagoons at the margin of the seaway (Peterson and Turner, in press).

Subsequently, the seaway retreated to the northwest into Canada and streams that drained upland regions west of the Western Interior carried gravel, sand, and mud (represented largely by the Salt Wash Member) into the depositional basin, building up an extensive alluvial plain. In central Colorado, scattered low hills that were remnants of the ancestral Rockies were sufficiently high to support small streams that furnished local stream deposits unrelated to the Salt Wash fluvial system. Small lakes and ponds also developed locally on the alluvial plain as well as in the most distal regions in eastern Colorado and eastern Wyoming. The more distal lake deposits yield charophytes, stromatolites, oncolites, sponge spicules, mollusks, and rare fish remains (Dunagan, 1997; Dunagan and others, 1996).

During times when the streams dried up in the Colorado Plateau region, winds from the west and southwest removed sand-sized material from the dry stream beds and deposited it farther downwind in extensive dune fields that covered large parts of the Four Corners area. These deposits are represented today by the Bluff Sandstone and Junction Creek Sandstone Members as well as the eolian sandstone facies of the Recapture Member. Smaller dune fields also were established farther north in northern Utah, northwestern Colorado, Wyoming, and South Dakota (Unkpapa Sandstone Member) by deflation of previously deposited shallow marine sands.

During deposition of the upper part of the Morrison Formation, a large stream complex in the Colorado Plateau region (Westwater Canyon and Fiftymile Members) gave way to a large shallow saline, alkaline lake called Lake T'oo'dichi' that covered parts of northwestern New Mexico, northeastern Arizona, southeastern Utah, and southwestern Colorado during deposition of much of the Brushy Basin Member and, although much shallower, had about the same areal extent as Lake Michigan (Turner and Fishman, 1991). Judging from similar modern saline, alkaline lakes, the alkalinity of the water would have been high enough to cause alkaline burns to human skin. Development of the lake attests to the aridity of the time, as evaporation must greatly exceed precipitation and runoff to achieve the alkalinities and salinities recorded in the deposits of the ancient lake. The lake was fed by surface water from intermittent and perennial streams, but ground water was also an important component of lake hydrology. At times when the lake dried out to form a large pan or salina, flash floods carried sand well out into the lake basin. Throughout most of Morrison time, shallow carbonate-dominated lakes developed east and north of the present-day Front Range of the Rocky Mountains (Dunagan and others, 1996, 1997).

Toward the end of Morrison deposition, large fluvial complexes including the Jackpile Sandstone Member were locally established because of renewed uplift in the highlands west and southwest of the Western Interior. Increased precipitation, especially in the highlands (Bassett and Busby, 1997), probably was responsible for the renewed stream activity at this time.

Wetter conditions toward the end of Morrison deposition is supported by scattered black mudstone beds near the top of the formation in scattered localities from the Colorado Front Range foothills to Montana. Abundant carbonaceous mudstone and extensive coal beds in the upper part of the Morrison in central Montana suggest greater precipitation and a temperate climate in the northern part of the Western Interior plain. The northward or latitudinal temperature gradient is also supported by a northward increase in charophytes (lacustrine green algae) that prefer cool waters and a corresponding decrease in charophytes that prefer warm waters (Schudack, 1996).

Morrison deposition ended with thick soil development although the soil was partly or entirely removed in many places during the succeeding erosion event or by scour that accompanied deposition of lowermost Cretaceous fluvial strata (T.M. Demko, oral commun., 1996).

Paleoecology

A variety of life forms lived in the Morrison ecosystem, from the giant herbivorous sauropods to small lacustrine algae. These include dinosaurs, small mammals, reptiles, amphibians, fish, sponges, arthropods, mollusks, and a variety of vegetation from large trees to algae. The diversity of life forms at first suggests that an equable climate prevailed during Morrison time, but reconstruction of the ecosystem suggests that, instead, the life forms were well adapted to a relatively dry and perhaps somewhat hostile environment.

The ecosystem of the Morrison depositional system was governed in large part by the rain shadow that developed in the lee of the highland areas to the west and was largely influenced by the availability of fresh surface and near-surface water. Wind-blown dunes, evaporites, the nature of the stream and lake deposits, together with evidence from the flora, fauna, and trace fossils suggest that fresh surface water may have been scarce during Morrison deposition.

Mountainous uplands to the west captured moisture from the westerly winds and some of this precipitation fed streams that flowed eastward across the alluvial plain and also fed underground aquifers that controlled the water table beneath the alluvial plain. Large alluvial complexes in the Morrison (Westwater Canyon and Salt Wash Members) attest to the development of major eastward-flowing streams. Subsequently (after burial), these alluvial complexes probably also served as major aquifers within the depositional basin. Whether these streams and other more isolated stream channels in the Morrison were perennial or ephemeral is an important aspect of ecosystem reconstruction. Local presence of unionid clams in some stream beds indicate that at least these streams were perennial in nature because the larval stage of unionid clams attaches onto the gills of fish, which only live in perennial streams (S.C. Good, oral commun., 1997). Crayfish burrows that occur in beds adjacent to stream channels offer clues to the nature of the streams as well, because crayfish must burrow down to the water table to survive. Crayfish burrows that extend downward into sandstone near the stream channels in the Morrison indicate that the water table was below the level of the stream, a condition consistent with effluent streams where the streams are feeding the water table (S.T. Hasiotis, oral commun., 1998). This situation is consistent with streams that develop in a semi-arid to arid climate.

Another indication of the semi-arid to arid climate is the association of large dune fields adjacent to the Salt Wash fluvial complex. The interfingering of fluvial and eolian deposits suggests that at times the streams were dry, and winds carried the sand from the exposed stream beds and deposited it in adjacent dune fields. Locally, termite nests occur in the eolian deposits. Because termites burrow down to the down to capillary fringe above the water table, the vertical extent of the nests is an indicator of the depth to the water table. Some of these nests extend as much as 40 m (130 ft) below the paleoland surface in the lower part of the formation in the southern San Juan Basin of northwestern New Mexico (Hasiotis, 1997). This observation requires that the water table there was at least 40 m (130 ft) beneath the surface.

Another clue to the nature of the streams derives from the nature of lake deposits in the Morrison. Development of Lake T'oo'dichi' (Turner and Fishman, 1991), an extensive saline, alkaline lake, required that evaporation far exceeded precipitation and runoff. This suggests that fresh-water replenishment of the lake by streams (either perennial and/or ephemeral) was not enough to dilute the lake brines. Lack of diversity but high abundance of trace fossils in these lake sediments (S.T. Hasiotis, oral commun., 1998), which is typical of a harsh or highly stressed environment, confirms the harsh conditions implied by the high salinities and alkalinities of Lake T'oo'dichi'.

Taken together, the evidence from the sedimentology; saline, alkaline lake geochemistry; isotopic data; body fossils; and trace fossils suggests that some of the Morrison streams were probably perennial but that many were probably ephemeral and may have experienced substream flow, resulting in the availability of surface water in the form of water holes much of the time. Streams that were perennial in nature, such as the deposits that contain the abundant dinosaur remains in the quarry sandstone bed at Dinosaur National Monument, Utah, may thus have been the exception rather than the rule, accounting for the concentration of dinosaur skeletons. The dinosaurs may have been congregating close to the last reliable stream during a major drought.

Conclusions

The Morrison landscape was a reflection of the interaction of surface water, subsurface water, and the moisture content of the overlying air mass. All of these were dramatically affected by creation of a rain shadow in the lee of mountainous highlands that lay to the west of the depositional basin. The Morrison fauna and flora adapted to the availability of moisture (or lack thereof).

We envision an environment similar in some respects to the depositional plain that contains Lake Eyre in Australia. The Lake Eyre basin may be more extreme than what we envision for the Morrison basin, but interesting parallels exist. In southern Australia, a coastal mountainous upland captures much of the moisture from winds that derive their moisture from the ocean, leaving the Lake Eyre basin in a severe rain-shadow region, where surface water is scarce and only furnished by infrequent storms that enter the region several years to several decades apart. The life forms in Lake Eyre are well adapted to the dryness and proliferate during times when moisture is temporarily abundant. Some surface and subsurface water did enter the Late Jurassic Western Interior basin, in the form surface runoff from precipitation in the highlands farther west and through underground aquifers that were recharged from source areas in the highlands. Surface runoff fed some perennial streams, but many of the streams may only have flowed intermittently.

We suspect that life concentrated around perennial streams and near water holes in streams where substream flow occurred. This may explain the concentration of dinosaur bones in the quarry sandstone bed at Dinosaur National Monument, a stream deposit that we interpret as perennial in nature. Wind blown sands; evaporite deposits; saline, alkaline lake deposits; and evidence from the trace and body fossils are consistent with a much drier interpretation than previously envisioned for the Morrison ecosystem. This raises questions about the amount of vegetation available for the large herbivores, and we can only surmise that they were able to range the basin and find enough vegetation to satisfy their food requirements. The vegetation was probably mostly, but not entirely riparian. The death assemblages of the dinosaurs probably tell us more about the conditions of stress and drought than about the normal course of events, but in our new interpretation, drought may not have been that uncommon.

Planned Research and Remaining Questions

A popular publication is planned that attempts to capture and bring to life for the general public the extinct ecosystems of the giant herbivorous dinosaurs in the Late Jurassic. This will be prepared from the scientific synthesis in preparation for the Morrison Extinct Ecosystems Project. The scientific synthesis is underway and the popular publication will be prepared by Fred Peterson, one of the Principal Investigators of the project, under the auspices of the NPS GIP (Geologist-in-the-Park) Program in Dinosaur National Monument. Work on the popular publication will begin this summer. Preparation of a geologic history interpretive trail, and recommendations to improve exhibits at the Monument are also planned. In the future, it will be possible to prepare the information for other parks in the study area that have Morrison exposures. Additional summary scientific papers are in preparation by project members for publication in the technical literature, in addition to the ones already published by project members.

Remaining research areas that need to be addressed in the Morrison Formation include the plant taphonomy, and a better understanding about the distribution of vegetation. Preliminary studies of the plant taphonomy will begin this summer by Judy Parrish, a professor of paleoclimatology at the University of Arizona. Additional detailed work on the paleosols would also contribute to our understanding of the paleoclimate, especially when tied to the regional sedimentology and trace fossil data. We also discovered in the course of our studies that the beginning and end of some range zones of the dinosaurs as well as of the microfossils (charophytes, ostracodes, spores and pollen) correspond with significant changes in the sedimentary rock record, such as major hiatuses or changes in depositional style. It is possible that both the rock record and the biota were changing in response to climatic signals, but further work would be needed to refine these ideas. Additional age determinations (both radiometric and paleontologic) of the Morrison in Wyoming would contribute considerably toward our correlations. Paleoclimatic interpretations of the spores and pollen would also be an important contribution to constraining the climatic interpretations for the ecosystems studies.

References

Bassett, K., and C. Busby. 1997. Intra-arc strike-slip basins in the Late Jurassic southern Cordillera: Structural and climatic controls on deposition: Geological Society of America, Abstracts With Programs, v. 29, no. 6, p. A-201.

Cerling, T. E., C. L. Mora, and D. D. Ekart. 1996. Soils, paleosols, and the history of atmospheric CO2: Geological Society of America, Abstracts With Programs, v. 28, no. 7, p. A-180.

Cody, R. D., R. R. Anderson, and R. M. McKay. 1996. Geology of the Fort Dodge Formation (Upper Jurassic), Webster County, Iowa: Iowa Department of Natural Resources, Geological Survey Bureau, Guidebook Series No. 19, 74 p.

Demko, T. M., B. S. Currie, and K. A. Nicoll. 1996. Paleosols at sequence boundaries in the Upper Jurassic Morrison Formation, Colorado Plateau and Rocky Mountain regions, USA: Geological Society of America, Abstracts With Programs, v. 28, no. 7, p. A-185.

Dunagan, S. P. 1997. Jurassic freshwater sponges from the Morrison Formation, USA: Geological Society of America, Abstracts With Programs, v. 29, no. 3, p. 14-15.

———, T. M. Demko, and K. R. Walker. 1996. Lacustrine and palustrine carbonate facies of the Morrison Formation (Upper Jurassic): Implications for paleoenvironmental reconstructions: Geological Society of America, Abstracts With Programs, v. 28, no. 7, p. A-336. .

———, S. G. Driese, and K. R. Walker. 1997. Paleolimnological implications of magadi-type cherts from lacustrine carbonates in the Morrison Formation (Upper Jurassic), Colorado, U.S.A.: Geological Society of America, Abstracts With Programs, v. 29, no. 6, p. A-270.

Ekart, D. D., and T. E. Cerling. 1997. A 400 million year record of atmospheric carbon dioxide: Results from a paleosol stable isotope paleobarometer: Geological Society of America, Abstracts With Programs, v. 29, no. 6, p. A-96.

Hallam, A. 1982. The Jurassic climate: Climate in Earth history, Studies in Geophysics, National Academy Press, Washington, D.C., p. 159-163.

Hasiotis, S. T. 1997. Gigantic termite (Insecta: Isoptera) nests from the Upper Jurassic Morrison Formation, northwestern New Mexico: New implications to isopteran evolution and environmental settings: Geological Society of America,
Abstracts With Programs, v. 29, no. 6, p. A-461.

Kowallis, B. J., E. H. Christiansen, A. L. Deino, F. Peterson, C. Turner, M. J. Kunk, and J. D. Obradovich. In press. The age of the Morrison Formation: Modern Geology.

O'Sullivan, R. B. 1997. Correlation of the Middle Jurassic San Rafael Group from Bluff, Utah, to Cortez, Colorado: U.S. Geological Survey, Geologic Investigations Series I-2616.

Parrish, J. T., A. M. Ziegler, and C. R. Scotese. 1982. Rainfall patterns and the distribution of coals and evaporites in the Mesozoic and Cenozoic: Palaeogeography, Palaeoclimatology, Palaeoecology: v. 40, p. 67-101.

Peterson, F. 1988. Pennsylvanian to Jurassic eolian transportation
systems in the western United States: Sedimentary Geology, v. 56, p. 207-260.

———. 1994. Sand dunes, sabkhas, streams, and shallow seas: Jurassic paleogeography in the southern part of the Western Interior Basin; In, Caputo, M. V., J. A. Peterson, and K. J. Franczyk (eds.), Mesozoic systems of the Rocky Mountain Region, USA: Rocky Mountain Section of SEPM (Society for Sedimentary Geology), p. 233-272.

———, and C. E. Turner. In press. Stratigraphy of the Ralston Creek and Morrison Formations (Upper Jurassic) near Denver, Colorado: Modern Geology.

Schudack, M. E. 1996. Ostracode and charophyte biogeography in the continental Upper Jurassic of Europe and North America as influenced by plate tectonics and paleoclimate; in, Morales, M., (ed.), The Continental Jurassic: Museum of Northern Arizona Bulletin 60, p. 333-341.

———, C. E. Turner, and F. Peterson. In press. Biostratigraphy, paleoecology, and biogeography of charophytes and ostracodes from the Upper Jurassic Morrison Formation, Western Interior, U.S.A.: Modern Geology.

Szigeti, G. J., and J. E. Fox. 1981. Unkpapa Sandstone (Jurassic), Black Hills, South Dakota: An eolian facies of the Morrison Formation; In, Ethridge, F. G., and R. M. Flores (eds.), Recent and Ancient Nonmarine Depositional Environments: Models for Exploration: Society of Economic Paleontologists and Mineralogists, Special Publication No. 31, p. 331-349.

Turner, C. E., and N. S. Fishman. 1991. Jurassic Lake T'oo'dichi': A large alkaline, saline lake, Morrison Formation, eastern Colorado Plateau: Geological Society of America Bulletin, v. 103, no. 4, p. 538-558.

———, and F. Peterson. In preparation. Biostratigraphy of dinosaurs in the Upper Jurassic Morrison Formation of the Western Interior, USA: Utah Geological Survey.

Turner-Peterson, C. E. 1986. Fluvial sedimentology of a major uranium-bearing sandstone—A study of the Westwater Canyon Member of the Morrison Formation, San Juan Basin, New Mexico; in, Turner-Peterson, C.E., Santos, E.S., and Fishman, N.S., (eds.), A Basin Analysis Case Study: The Morrison Formation, Grants Uranium Region, New Mexico: American Association of Petroleum Geologists, Studies in Geology No. 22, p. 47-75.