UGA-Geosite-Boden-Gilsonite.indd Utah Geosites 2019 Utah Geological Association Publication 48 M. Milligan, R.F. Biek, P. Inkenbrandt, and P. Nielsen, editors Taylor Boden Utah Geological Survey, 1594 West North Temple, Suite 3110 P.O. Box 146100, Salt Lake City, Utah 84114 taylorboden@utah.gov Independent Gilsonite Vein, Uintah County Cover Image: Historical open-cut mining on the southeast end of the Cowboy vein. View to the southeast. 2 M. Milligan, R.F. Biek, P. Inkenbrandt, and P. Nielsen, editors 2019 Utah Geological Association Publication 48 Utah Geosites 2019 Utah Geological Association Publication 48 M. Milligan, R.F. Biek, P. Inkenbrandt, and P. Nielsen, editors Utah Geosites showcases some of Utah’s spectacular geology, both little-known localities and sites seen by visitors to Utah’s many national and state parks and monuments. Th e geosites refl ect the interests of the many volunteers who wrote to share some of their favorite geologic sites. Th e list is eclectic and far from complete, and we hope that additional geosites will be added in the coming years. Th e Utah Geological Survey also maintains a list of geosites https://geology.utah.gov/apps/geosights/index.htm. We thank the many authors for their geosite contributions, Utah Geological Association members who make annual UGA publications possible, and the American Association of Petroleum Geologists—Rocky Mountain Section Foundation for a generous grant for desktop publishing of these geosite papers. Design and desktop publishing by Jenny Erickson, Graphic Designer, dutchiedesign.com, Salt Lake City, Utah. Th is is an open-access article in which the Utah Geological Association permits unrestricted use, distribution, and reproduction of text and fi gures that are not noted as copyrighted, provided the original author and source are credited. See the Utah Geological Association website, www.utahgeology.org, and Creative Commons https://creativecommons.org/licenses/by/4.0/ for details. Suggested citation for this geosite: Presidents Message I have had the pleasure of working with many diff erent geologists from all around the world. As I have traveled around Utah for work and pleasure, many times I have observed vehicles parked alongside the road with many people climbing around an outcrop or walking up a trail in a canyon. Whether these people are from Utah or from another state or country, they all are quick to mention to me how wonderful our geology is here in Utah. Utah is at the junction of several diff erent geological provinces. We have the Basin and Range to the west and the Central Utah Hingeline and Th rust Belt down the middle. Th e Uinta Mountains have outcrops of some of the oldest sedimentary rock in Utah. Utah also has its share of young cinder cones and basaltic lava fl ows, and ancient laccoliths, stratovolcanoes, and plutonic rocks. Th e general public comes to Utah to experience our wonderful scenic geology throughout our state and national parks. Driving between our national and state parks is a breathtaking experience. Th e “Utah Geosites” has been a great undertaking by many people. I wanted to involve as many people as we could in preparing this guidebook. We have had great response from authors that visit or work here in the state. Several authors have more than one site that they consider unique and want to share with the rest of us. I wanted to make the guidebook usable by geologists wanting to see outcrops and to the informed general public. Th e articles are well written and the editorial work on this guidebook has been top quality. I would like to personally thank Mark Milligan, Bob Biek, and Paul Inkenbrandt for their editorial work on this guidebook. Th is guidebook could not have happened without their support. I would like to thank Jenny Erickson for doing the great desktop publishing and the many authors and reviewers that helped prepare the articles. Your work has been outstanding and will certainly showcase the many great places and geology of Utah. Last, but not least, Th ank you to the American Association of Petroleum Geologists, Rocky Mountain Section Foundation for their fi nancial support for this publication. Guidebook 48 will hopefully be a dynamic document with the potential to add additional “geosites” in the future. I hope more authors will volunteer articles on their favorite sites. I would like to fi ll the map with locations so that a person or family looking at the map or articles will see a great location to read about and visit. Enjoy Guidebook 48 and enjoy the geology of Utah. Peter J. Nielsen 2019 UGA President T. Boden Independent Gilsonite Vein, Uintah County 3 INTRODUCTION The Uinta Basin of northeastern Utah (figure 1) contains a wide variety of hydrocarbon resources including vast accumulations of crude oil and natural gas deposits, one of the largest oil shale re- sources in the world, and the largest tar sand deposit in the United States. In addition, unique solid hydrocarbons, including gilsonite, wurtzilite, tabbyite, and ozokerite, have a long and colorful history of exploration and/or production in the region. The most abun- dant of these, gilsonite, occurs in distinctive swarms of subparallel, northwest-trending veins. The lateral continuity of the veins is im- pressive, with relatively long, straight ribbons stretching across the hills of the eastern Uinta Basin. The veins are also vertically contin- uous, extending hundreds to more than 3000 feet (900 m) below the ground, commonly having only small variations in width. The Uinta Basin contains the world’s largest deposit of gilsonite and is the only place in the world where this unique resource is economically produced. Gilsonite is remarkable for its unusual geologic origin, chemical and physical properties, and industrial uses. Industry pioneers are noted for creating innovative uses for their product and for over 100 years have solved mining, process- ing, transportation, marketing, and other challenges to supply gilsonite to world markets. Accordingly, gilsonite has been studied and described in a large body of research dating back to the 1880s. Most recently, the Utah Geological Survey (UGS) published Special Study 141 (Boden and Tripp, 2012), which presents the latest mapping of gilsonite deposits and a compilation of existing data. To date, over 70 significant veins and vein systems, having a total combined vein length of over 170 miles (270 km), have been mapped by UGS geologists. BACKGROUND Definitions Gilsonite is classified as a member of the asphaltite group of hydrocarbon bitumens. This group of naturally occurring solid hydrocarbons are somewhat similar in appearance, occurrence, and properties. Gilsonite occurs in dikes (veins), sills, fractures, and disseminated blebs, commonly in association with Tertia- ry-age Green River Formation oil shale. Gilsonite has a dull black appearance on weathered surfaces and a shiny black appearance on fresh surfaces. Fractures vary from conchoidal to columnar (pencillated) to flaky or scaly (figure 2). Pencillated textures form at right angles to vein walls and penetrate about 6 inches into the ore (Verbeek and Grout, 1993). Historically, industry defined three major subdivisions of gilsonite based on appearance and melting temperature: selects, seconds, and jet. Select material is very shiny, melts from 300° to 334°F, and tends to occur in the center of the veins. Seconds are somewhat duller, melt from 306° to 361°F, and tend to occur along vein margins, sometimes having pencillated texture. Jet gilsonite has a brilliantly shiny surface, a bluish-black color, and melts from 390° to 446°F (Abraham, 1960). To date, jet has only been found in the Cowboy vein (figure 3). Figure 1. The eastern Uinta Basin in northeast Utah is host to numerous gilsonite veins (purple lines). The red box is the area of focus and is shown on figure 3. Figure 1. The eastern Uinta Basin in northeast Utah is host to numerous gilsonite veins (purple lines). The red box is the area of focus and is shown on figure 3. Figure 2. Gilsonite hand samples: A - gilsonite with columnar “pencillated” structure, and B - “select” gilsonite from underground workings on the Indepen- dent vein showing conchoidal fracturing. T. Boden Independent Gilsonite Vein, Uintah County 4 Qaf Qal Tgp Tua Qat Qal Tgp Qal Tgp Qac Qal Tua Tua Tgp Tgp Qaf Little Emma Wagon Hound Cowboy Tabor Chepetta Independent Little Bonanza Caldwell Little Chepetta Augustine »» »»»» »»» » »»»»»» » »» » »»» » »»» » » »»» »» » » » »»» Tub Tub Tub Tua Tgp Tuc Qaf Qae Tub Qac Tua Qac Tub #* 0 1 2 30.5 Miles EXPLANATION Geologic Units (modified from Sprinkel 2007, 2009) Figure 3. Independent vein geosite area mining activity and geology. See figure 1 for location. Ü _̂! Qac Qac T8S R25E T10S T9S R24ER23E 45 45 0 1 2 3 40.5 Kilometers Stream Alluvium (Holocene)Qal Stream Terrace Deposits (Holocene and Pleistocene[?])Qat Member B of Uinta Formation (Eocene)Tub Mixed Alluvium and Colluvium (Holocene and Pleistocene[?])Qac Alluvial-Fan Deposits (Holocene and Pleistocene[?])Qaf Member A of Uinta Formation (Eocene)Tua Mixed Alluvium and Eolian Deposits (Holocene)Qae Member C of Uinta Formation (Eocene)Tuc Parachute Creek Member of Green River Formation (Eocene)Tgp Gilsonite vein Mahogany zone outcrop Gilsonite vein with permitted mining _̂ Independent vein geosight overlook #* American Gilsonite Company mill » Shaft ! Bonanza, Utah Figure 3. Independent vein area mining activity and geology. T. Boden Independent Gilsonite Vein, Uintah County 5 Fractures filled by gilsonite veins are large-scale hydraulic exten- sion features that resulted from the over pressurization of pore- space fluid in the organic-rich source beds in the Green River Formation (Verbeek and Grout, 1993). Regional stress fields from early-stage, post-Laramide, regional tectonic extension is also be- lieved to be a factor in allowing fracture formation. Fractures were first forcefully propagated by over pressurized formational water in pore spaces. Subsequently, fractures were widened as viscous, liquid asphaltite generated from deeply buried kerogen was in- jected under high pressure. The gilsonite later solidified, probably through cooling and polymerization. Based on laboratory and field evidence it has been proposed that the liquid asphaltite was sourced from kerogen-rich oil shale beds in the Parachute Creek Member of the upper Green River Formation, possibly beds that are deeply buried on the west side of the basin. The Mahogany oil shale zone, which is a sequence of organic-rich (kerogen-rich) dolomitic marlstone in the up- per Parachute Creek Member, is a probable major source for the gilsonite (Pruitt, 1961; Cashion, 1967). This interval reaches up to 40% total organic carbon and was deeply buried, especially to the northwest (down to 10,000+ feet), bringing these deposits into the upper ranges of the oil generation window. History and Mining Uintahite, the formal scientific name for gilsonite, was first dis- covered in the 1860s. The material was later informally named gil- sonite, after Samuel H. Gilson, due to his enthusiastic development and promotional efforts (Kretchman, 1957; Covington, 1964). The name gilsonite was further solidified in common usage when an early mining company adopted and trademarked the name. Gilsonite has been mined since the late 1880s in the Uinta Basin. The first regular shipments began in 1888 from veins in the Fort Duchesne area. Early mining was predominantly by open-cut ex- cavating (figure 5) with picks, shovels, and horse-powered hoists. Industrial uses of gilsonite can be grouped into five major catego- ries: 1) asphalt paving mixes and coatings, 2) oil and gas well drill- ing and completions, 3) inks and paints, 4) chemical products, and 5) metal foundry (Boden and Tripp, 2012). Physical and chemical characteristics of gilsonite are important for determining possible industrial applications. Gilsonite from different veins or different parts of veins can be mixed to achieve a product with a specific melting temperature range. Origins of Gilsonite Gilsonite deposits primarily occur as long, mostly vertical dikes (veins) (figure 4) that predominantly trend northwest to southeast and can range in width from less than an inch to more than 20 feet (6 m). Horizontal gilsonite sills are occasionally associated with the gilsonite dikes. The source of the gilsonite veins and the mech- anism of their emplacement have long been debated and the vari- ous theories are well summarized by Verbeek and Grout (1993). Gilsonite float is very light and may wash far down hill from an outcropping vein. Weathered gilsonite vein near the surface has a typical “pencillated” fracture structure. This ore generally has a higher melting point and is called “seconds.” Unweathered massive gilsonite generally has a lower melting point and is called “select.” Sandstone walls are very firm and in clean contact with the gilsonite, with very little penetration. Uinta Formation Green River Formation The typical gilsonite vein reaches its maximum width in the vicinity of the Uinta-Green River formation contact. Debris tends to collect at the bottom of a vein where it begins to split upon entering the organic-rich shale beds, making clean mining at this level difficult. Most veins gradually thin upward until they thin to extinction in the upper Uinta or Duchesne River Formations. oil shale Most veins have their “roots” in the organic-rich oil shale beds of the Green River Formation. A few veins extend downward into the underlying Wasatch Formation. Figure 4. Cross section of a typical gilsonite vein (modified from Eldridge, 1901). Figure 5. Historical open-cut mining on the southeast end of the Cowboy vein. View to the southeast. T. Boden Independent Gilsonite Vein, Uintah County 6 The gilsonite veins actively being mined are near the Bonanza area and were discovered by early prospectors who located surface exposures. Permitted mining currently occurs in the Cowboy, Independent, Little Bonanza, Wagon Hound, and Little Emma veins (figure 3) via underground mining methods. Mining consists of two major phases: (1) shafts are sunk at regular intervals along the veins, and (2) drifts and stopes are then extended laterally from the shafts. The top 30 feet (9 m) of the gilsonite is left intact for safety and reclamation reasons. Gilsonite mining is labor intensive because of its unusual occurrence in narrow (minable widths down to 18 inches), deep, vertical veins, and the explosive hazards associ- ated with gilsonite dust. Mining is still done by hand using air-pow- ered chipping hammers to break the gilsonite while avoiding contaminating the ore with broken wall rock, since product purity is important to customers. The broken ore enters a vacuum tube at the bottom of the underground mine and is air lifted to the surface, where it is dropped into a holding container next to the shaft head- frame before being trucked to the processing plant in Bonanza. Over the past decade, gilsonite production from the Uinta Basin has ranged between 20,000 and 85,000 short tons per year, de- pending on market conditions, mainly associated with the boom and bust cycles of the oil and gas industry, currently one of the largest markets. The American Gilsonite Company has been the major producer for many years. Cashion (1967) estimated that the original gilsonite resource was approximately 45 million short tons (40.8 million mt); though mining has occurred for many years, millions of tons of resource are estimated to remain. Current resources are becoming more difficult and more expensive to mine as they are in deeper, thinner, and more remote veins. GILSONITE VEIN OVERLOOK – INDEPENDENT VEIN The gilsonite veins are in a remote and rugged region and access to some areas is difficult. The Independent (Bonanza), Tabor, and Little Bonanza vein system is exposed near Bonanza, Utah, along Utah State Highway 45, and is one of the easiest and most dramat- ic places to view the remnants of a gilsonite vein and historical mine workings (figure 3). The Independent and Tabor veins are a single vein, but are called by different names northwest and south- east, respectively, of the junction with the Little Bonanza vein. The remnants of the Independent (Bonanza) vein at the overlook lo- cation are exposed in a wide, open-cut trench that stretches across the landscape (figure 6). The Independent (Bonanza) vein is the second widest gilsonite vein in the Uinta Basin and has support- ed mining operations since the late 1800s (Pruitt, 1961). In the Bonanza area, these veins are exposed in the Eocene-age Member A and B sandstone of the Uinta Formation (figure 7). The Inde- pendent/Tabor vein is reported to be more than 7.5 miles (12 km) long, generally strikes from N. 55°–62° W., has a maximum width of 14 feet (4 m), and has an estimated maximum vertical extent of 1100 feet (335 m) (Cashion, 1967). This area is accessed by traveling south on Utah State Highway 45 out of Vernal for about 42 miles (68 km). Gilsonite veins and mine workings around the Bonanza area can be observed along public roads but all mining takes place on private property having no pub- lic access. The gilsonite mine workings are dangerous and should not be approached; they should always be viewed from a safe distance. Figure 6. Overlook of the Independent (Bonanza), Tabor, and Little Bonanza vein system exposed along Utah State Highway 45 at Bonanza, Utah. View to the northwest. GPS coordinates of overlook are: Easting 655864, Northing 4431585. Figure 7. Stratigraphic column of the Eocene section that hosts gilsonite veins in the Independent vein area (modified from Hintze and Kowallis, 2009). T. Boden Independent Gilsonite Vein, Uintah County 7 GEOLOGIC SETTING The Uinta Basin is an asymmetric, intermontane basin along the northern edge of the Colorado Plateau. The basin is bordered by the Uinta Mountains to the north, Douglas Creek arch to the east, Uncompahgre uplift to the southeast, San Rafael Swell to the southwest, and Wasatch Range to the west. Structural features in the Uinta Basin and Uinta Mountains region have a development history that is long and complex; some structures like the Uinta rift basin formed during Proterozoic time and other structural ac- tivity possibly occurred during the Pennsylvanian-Permian ances- tral Rocky Mountains uplift (Stone, 1993). The Uinta Basin, Uinta Mountains, and associated folds were formed by west-southwest to east-northeast compression during the Cretaceous-early Tertiary Laramide orogeny (Erslev, 1993; Stone, 1993). On the eastern side of the Uinta Basin in the Bonanza area, gilsonite veins are hosted in the gently dipping Eocene-age strata of the Green River and Uinta Formations (figures 3 and 7). These formations were deposited in lacustrine (Green River Formation) and fluvial (Uinta Formation) environments and range in compo- sition from carbonate rocks to clastic rocks. Contacts are grada- tional with complex intertonguing relationships and abrupt facies changes that reflect fluctuating paleo-lake levels. The extensive vertical and horizontal continuity of the gilsonite veins is related to the stratigraphy and lithology of the host formations (Pruitt, 1961; Cashion, 1967). Eocene Green River Formation lacustrine deposits host gilsonite veins in both the basal and upper members. All significant known gilsonite veins are located above the Mahogany oil shale zone in the upper Green River Formation, except two veins that are in the basal member in the southeastern part of the basin (Boden and Tripp, 2012). Gilsonite veins are known to achieve their greatest thickness in the lower Uinta Formation. The Eocene Uinta For- mation overlies the Parachute Creek Member of the Green River Formation and consists of marginal lacustrine deposits in the lower members, mixed fluvial and marginal lacustrine deposits in the middle part, and entirely fluvial deposits in the upper part. Margin- al lacustrine deposits in the lower part consist primarily of thick, laterally continuous, medium-bedded to massive sandstone con- taining interbedded siltstone and thin intervals of marlstone and tuff. Fluvial beds in the middle and upper members are composed of channel-form sandstone, variegated mudstone, and minor con- glomerate that tend to be laterally discontinuous and thinner than marginal lacustrine deposits down section (Pruitt, 1961; Cashion, 1967). Gilsonite veins commonly split, become discontinuous, and/ or pinch out in the mudstone-rich upper Uinta Formation. SUMMARY Gilsonite is a unique material that has been mined for well over a hundred years and its geological occurrence and mining histo- ry have been studied by numerous researchers in a large body of work. Even though significant amounts of the approximately 45-million-short-ton original gilsonite resource have been mined, millions of tons of the valuable resource remain. Additional resources are likely to be found in the deeper parts of the Bonanza area veins and in thinner, more remote veins that will likely be more expensive to mine. Gilsonite will continue to be mined in the Uinta Basin for decades, ensuring a steady supply to world markets of this unique and valuable Utah resource. REFERENCES Abraham, H., 1960, Asphalts and allied substances—their occur- rence, modes of production, uses in the arts and methods of testing (6th edition), Volume 1, Historical review and natural raw materials: Princeton, New York, D. Van Nostrand Com- pany, Inc., p. 1–302. Boden, T., and Tripp, B.T., 2012, Gilsonite veins of the Uinta Basin, Utah: Utah Geological Survey Special Study 141, 50 p., 1 plate, CD. Cashion, W.B., 1967, Geology and fuel resources of the Green Riv- er Formation, southeastern Uinta Basin, Utah and Colorado: U.S. Geological Survey Professional Paper 548, 48 p. Covington, R.E., 1964, A brief history of early mineral exploita- tion in the Uinta Basin, in Sabatka, E.F., editor, Geology and mineral resources of the Uinta Basin, Utah’s hydrocarbon storehouse: Intermountain Association of Petroleum Geolo- gists Guidebook, 13th Annual Field Conference, p. 1–16. Eldridge, G.H., 1901, The asphalt and bituminous rock deposits of the United States, in Walcott, C.D., director: U.S. Geological Survey Twenty-second Annual Report of the United States Geological Survey to the Secretary of the Interior, pt. 1, p. 209–364. Erslev, E.A., 1993, Laramide basement tectonics, in Schmidt, C.J., Chase, R.B., and Erslev, E.A., editors, Laramide basement deformation in the Rocky Mountains foreland of the western United States: Geological Society of America Special Paper 280, p. 339–358. Hintze, L.F., and Kowallis, B.J., 2009, Geologic history of Utah: Provo, Brigham Young University Geology Studies, Special Publication 9, 225 p. Kretchman, H.F., 1957, The story of gilsonite: Salt Lake City, Utah, American Gilsonite Company, 96 p. Pruitt, R.G., Jr., 1961, The mineral resources of Uintah County: Utah Geological and Mineralogical Survey Bulletin 71, 101 p. 8 M. Milligan, R.F. Biek, P. Inkenbrandt, and P. Nielsen, editors 2019 Utah Geological Association Publication 48 Sprinkel, D.A., 2007, Interim geologic map of the Vernal 30' x 60' quadrangle, Uintah and Duchesne Counties, Utah, and Moffat and Rio Blanco Counties, Colorado: Utah Geological Survey Open-File Report 506DM, 3 plates, scale 1:100,000, CD. Sprinkel, D.A., 2009, Interim geologic map of the Seep Ridge 30' x 60' quadrangle, Uintah, Duchesne, and Carbon Counties, Utah, and Garfield and Rio Blanco Counties, Colorado: Utah Geological Survey Open-File Report 549DM, 3 plates, scale 1:100,000, CD. Stone, D.S., 1993, Tectonic evolution of the Uinta Mountainspalin- spastic restoration of a structural cross section along longi- tude 109°15', Utah: Utah Geological Survey Miscellaneous Publication 93-8, 19 p. Verbeek, E.R., and Grout, M.A., 1993, Geometry and structural evolution of gilsonite dikes in the eastern Uinta Basin, Utah: U.S. Geological Survey Bulletin 1787-HH, 42 p., 1 plate, scale 1:250,000.