2003
Abandoned tipple at the Suntrana coal mine along Healy Creek, about 125 km southwest of Fairbanks, Alaska. Behind the tipple are north-dipping, coal-bearing strata of the Eocene to Miocene Usibelli Group. Photograph by R.G. Stanley, July 31, 1985.
The Nenana Basin area is a prospective petroleum province in central Alaska, and this geologic and isostatic gravity map is part of a petroleum resource assessment of the area.
The geology was compiled from published sources (Chapman and others, 1971, 1975a, 1975b, 1982; Chapman and Yeend, 1981; Csejtey and others, 1986; Jones and others, 1983; Péwé and others, 1966; Reed, 1961; and Weber and others, 1992), as shown on the index map (map sheet). Map units are organized and presented according to the scheme of lithotectonic terranes proposed by Jones and others (1987) and Silberling and Jones (1984); we recognize, however, that this terrane scheme is controversial and likely to be revised in the future. In some cases, we combined certain terranes because we were unable to match the terrane boundaries given by Jones and others (1987) and Silberling and Jones (1984) with specific faults shown on existing geologic maps. Postaccretion cover deposits represent overlap assemblages that depositionally overlie accreted terranes. Plutonic igneous rocks shown on this map include several plutons that are clearly postaccretionary, based on isotopic ages and (or) field relations. It is possible that some of the plutons predate accretion, but this has not been demonstrated. According to Jones and others (1982), the terranes in the area of our map were assembled during late Mesozoic or earliest Cenozoic time.
The gravity contours are derived from data used in earlier compilations (Barnes, 1961, 1977; Hackett, 1981; Valin and others, 1991; Frost and Stanley, 1991) that are supplemented by some National Oceanic and Atmospheric Administration data along the Alaska Pipeline level line (W.E. Strange, written commun., 1980). The earlier compilations were used for simple Bouguer maps, prepared primarily by non-digital methods, and are superseded by this map. The present map is the result of digital processing that includes the 1967 Geodetic Reference System, the IGSN-71 datum, digital terrain corrections, and conversion to isostatic gravity so that geologic structures on the margin of the Alaska Range are more clearly portrayed (Simpson and others, 1986). Computation procedures are described in part by Barnes (1972, 1984), Jachens and Roberts (1981), and Barnes and others (1994). The calculations used a crustal density of 2.67 g/cm 3 , a density contrast at the base of the isostatic root of 0.4 g/cm 3 , and a root thickness at sea level of 25 km. The distribution of data within the map area is uneven and locally controls the shape of the computer-generated contours. Altimetry was used for most of the elevation control and its inconsistency is responsible for many of the small contour irregularities. Ninety percent of the measurements are estimated to have an accuracy of about 1.5 mgal or about a quarter of the 5 mgal contour interval. Data collection and analysis were assisted by R.V. Allen, R.C. Jachens, M.A. Fisher, T.R. Bruns, J.G. Blank, J.W. Bader, Z.C. Valin, J.W. Cady, R.L. Morin, and P.V. Woodward.
The most promising area for petroleum exploration is a prominent 25 mgal isostatic gravity low north of Nenana (T. 2 S., R. 8 W.). This gravity low probably corresponds to the deepest part of a sedimentary basin filled by Cenozoic strata that includes nonmarine fluvial and lacustrine deposits of the Eocene to Miocene Usibelli Group. Smaller gravity lows are associated with outcrops of these sedimentary rocks north of Suntrana (T. 12 S., R. 6-9 W.) and Sable Pass (T. 16 S., R. 11 W.). A broad low on the north flank of the Alaska Range east of the Wood River (T. 10 S., R. 1 E.) indicates another basin under the Tanana lowland that extends eastward off the map area towards Delta Junction, where its presence was confirmed by both gravity and seismic data (Barnes and others, 1991).
Gravity modelling suggests that the base of the Usibelli Group in the area north of Nenana (T. 2 S., R. 8 W.) is about 3,000 to 3,350 m beneath the ground surface (Barnes, 1961; Hite and Nakayama, 1980; Kirschner, 1988). Organic geochemical studies indicate that mudstones and coals in the Usibelli Group are potential sources of petroleum; calculations based on borehole temperatures suggest that, in the area of the gravity low, these rocks may have been buried deeply enough to generate oil and gas (Stanley and others, 1990). Two exploratory wells, the Union Nenana No. 1 and the ARCO Totek Hills No. 11, were drilled some distance away from the gravity low in areas where the Usibelli Group is thin (Grether and Morgan, 1988; Kirschner, 1988). Mudlogs show that both wells were dry holes that bottomed in schist and had gas shows associated with coal beds in the Usibelli Group, but no reported signs of oil.
Files Available for Download
i2543_sheet_72.pdf (23 MB)
PDF map sheet with raster topographic base at 72 dpi (best for viewing online)
i2543_sheet_300.pdf (31.7 MB)
Portable Document Format (PDF) map sheet with raster topographic base at 300 dpi
i2543_sheet.pdf (2.2 MB)
PDF map sheet with no topographic base (geology & gravity only)
i2543_sheet.eps (15.8 MB)
Encapsulated PostScript (EPS) map sheet with no topographic base (geology & gravity only)
i2543_pam.pdf
PDF file of the 17-page pamphlet (616 KB)
Note:
This map was compiled on a base at 1:250,000 scale and is intended for viewing at that scale. At this scale, the sizes of the map and map sheet are:
Map: 36.5" wide x 34.5" high
Sheet: 55.5" wide x 40.5" high
For questions about the content of this report, contact Rick Stanley
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The URL for this page is: http://pubs.usgs.gov/imap/i2543
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Michael Diggles
Created: February 2, 2003 (cad)
Updated: January 19, 2007 (bwr, mfd)