by: J.C. Wynn, R.P. Kucks, and D. J. Grybeck
Excerpts from: U.S. Geological Survey Digital Data Series DDS-56. (For information about how to order this CD-ROM, go to http://mapping.usgs.gov/esic/cdrom/cdlist.html#A2.)
|| Introduction || Geographic and geologic setting of the Craig study area || Previous geophysical work || Gravity data || Aeromagnetic data || Summary || Acknowledgments || References cited ||
The U.S. Geological Survey is required by the Alaska National Interest Lands Conservation Act (ANILCA, Public Law 96-487) to survey certain Federal lands to determine their mineral resource potential. As part of continuing studies designed to fulfill this responsibility, geochemical, geological and geophysical surveys of the Craig and Dixon Entrance 1-degree by 3-degree quadrangles in southeastern Alaska (see Figure 1) were undertaken by the U.S. Geological Survey during the summers of 1983-85, 1989, and 1991. The geochemical data have been reported in Cathrall and others, 1993, and Cathrall, 1994. A simplified geologic map has been compiled especially for this report based largely on the work of Eberlein and others (1983) and Gehrels (1991, 1992). This report presents available magnetic and gravity data and an interpretation thereof.
The Craig-Dixon Entrance study area comprises about 1,400 square miles (3,600 square kilometers) between latitude 54o40' and 56o N and longitude 131o50' and 134o40' W of the southeastern tip of the Alaskan Panhandle. The Craig-Dixon Entrance quadrangles are located west of Ketchikan, and include all but the northern tip of Prince of Wales Island, as well as outlying islands to the west from Dall Island in the south to Coronation Island in the northwest. In order to integrate parts of Prince of Wales Island south of 56oN, parts of the western edges of the Ketchikan and Prince Rupert 1:250,000-scale quadrangles are included in this geophysical compilation. The largest single landmass in the Craig area is Prince of Wales Island, the southernmost major island of the Alexander Archipelago; the study area is entirely in the Tongass National Forest. The Prince of Wales Mountains are moderately glaciated mountains reaching to 3,800 feet (1,160 meters). The Kupreanof Lowlands consists of islands and channels on the western side of Prince of Wales Island; maximum elevation is about 3,000 feet (915 meters). The Coastal Foothills consists of blocks of high mountains on the east of Prince of Wales Island extending to 4,500 feet (1,372 meters) elevation.
The Craig-Dixon Entrance area contains parts of two northwest-trending tectono-stratigraphic terranes separated by a similarly-trending overlap assemblage. From southwest to northeast these are the Alexander Terrane, the Gravina-Nutzotin overlap assemblage, and the Taku terrane (Monger and Berg, 1987). A simplified geology map of the study area compiled by Don Grybeck is included in Digital Data Series DDS-56 as the underlying base for some of the large images representing the geophysical data.
Aeromagnetic surveys were flown in parts of the Craig-Dixon Entrance area as early as 1945 (Rossman and others, 1956; Decker, 1979). These data were flown at 1-mile spacings at elevations ranging from 500 feet to 6,000 feet above mean sea level. Unfortunately, they covered only parts of the eastern Craig quadrangle, and only the southeastern tip of Prince of Wales Island. The 1945 data are not digitally recoverable. A digital aeromagnetic survey was flown in three parts during 1978 (U.S. Geological Survey, 1984), covering all of the land and much of the water of the Craig-Dixon Entrance area, and has been merged and compiled into one map with derivative products as part of this report. A limited-scale helicopter-borne spectral radiometric survey was also carried out in 1978, but covered primarily the previously-identified Bokan Mountain uranium occurrences and surrounding terrane in southern Prince of Wales Island (Burgett and Krause, 1979). Initial gravity data were acquired in 1968-69, using paired altimeters and tide-tables for elevation control (Barnes, 1972; Barnes and others 1972a, 1972b). These data were supplemented with additional helicopter-acquired gravity stations obtained during the summers of 1984-85 and 1991, and are assembled together for the first time in this report. The study area includes all of the Craig and Dixon Entrance 1:250,000-scale quadrangles and a small part of the western edges of the Ketchikan and Prince Rupert 1:250,000-scale quadrangles (Figure 1).
Gravity data acquisition in the Craig-Dixon Entrance area began in 1968, using early 0.1 milligal sensitivity LaCoste-Romberg gravimeters. Elevation control was obtained using paired altimeters and tide-tables (Barnes, 1972; Barnes and others, 1972b, 1972c). Most of these stations were around the coastal margins of Prince of Wales and some outlying islands. Denser station spacing was acquired during 1984-85 and 1991 using skiffs and helicopter support to fill in the interior of Prince of Wales Island and some of the other outlying islands. These data were reduced to the Simple Bouguer Anomaly using elevation control from three Wallace and Tiernan1 altimeters calibrated against known elevations at the beginning and at the end of each base-station loop. Barometric pressures obtained from the National Weather Service for Ketchikan were used to improve interpolated elevations between base station readings. The Complete Bouguer Anomaly (CBA) for each measurement was calculated using edited digital elevation data acquired from the Department of Defense to remove the effects of topographic relief. Data and further information on global topography at 30-minute grid spacing can be obtained at several World Wide Web sites (U.S. Geological Survey, 1997). The CBA map is viewable in the "\images" directory of Digital Data Series DDS-56. The Isostatic Residual Anomaly (ISO) was calculated using U.S. Geological Survey digital topographic data and digital bathymetry obtained from Oregon State University (Simpson and others, 1983). Data and more information on available sea floor bathymetry can be obtained from Smith, W.H.F., and Sandwell, D.T., 1997. The ISO gravity should theoretically remove the effects of isostatic disequilibrium associated with the continental-crust-oceanic-margin transition; it's effect is to shift anomalies so they more precisely overlay their causative source bodies. The reasons and methodology behind isostatic corrections to gravity data can be found in Blakely (1995). The ISO map is viewable in the "\images" directory of Digital Data Series DDS-56 in several different forms, including draped as color over a shaded-relief topographic image, and as contours superimposed on the simplified geologic map.
We present two different versions of the gravity data in Digital Data Series DDS-56: the CBA and the ISO. The CBA represents the observed gravity field for a given station, with corrections for instrument drift, diurnal variation caused by the Sun and the Moon, corrections for latitude (the oblateness of the Earth's surface), and for the density of the rock lying between the station and sea-level added in. These in aggregate give the "Bouguer Anomaly". Finally, corrections for topographic effects are also added, which then gives the CBA. These topographic corrections are made by using a digital topographic model to calculate the effects of mass contributions (from nearby mountains) or lack thereof (from voids caused by nearby valleys) due to the physical relief that surrounds the gravity station. These calculations are carried out to 167 kilometers away from the station, and a compensation is added to the corrected gravity value. These corrections and adjustments are explained in greater detail in Blakely (1995).
Continents and ocean basins represent mass concentrations and deficiencies, respectively, and typically have large lateral dimensions. There is a natural compensation for topographic loads by deeper compensating masses underlying them, not unlike ice-cubes floating in a glass of water; this is referred to as isostatic compensation. The Craig and Dixon Entrance quadrangles are located on a continental-oceanic margin, and there is ample reason to believe that isostatic compensation effects in these regions would be non-trivial - simply compare the CBA versus the ISO anomaly maps. Usually, the anomalies caused by the compensating masses are generally long in wavelength and correlate inversely with long-wavelength attributes of topography. In general large mass concentrations and mass deficiencies are compensated for at depth to first order, so that total mass in each vertical section of a gravity profile is laterally uniform. Small higher-order transition effects, however, are apparent when we compare the CBA to the ISO gravity maps in the Craig and Dixon Entrance quadrangles: gravity anomalies shift and stretch perpendicular to the direction of the ocean-margin axis. These two different maps are shown in several different forms in the "\imagery" directory of Digital Data Series DDS-56.
With our digital terrain model (meshed with a digital bathymetry model), it is a relatively straightforward procedure to estimate the shape of the crust-mantle interface consistent with the Airy model for isostatic compensation, and calculate at each observation point the gravitational effect of the volume (Jachens and Roberts, 1981; Simpson, Jachens, and Blakely, 1983).
The significant difference observable between the CBA anomaly map and the ISO anomaly map makes it clear that in the Craig and Dixon Entrance area, isostatic compensation is significant, and therefore the ISO map is the gravity map of choice to work with. In such a transition zone, we can reasonably expect that there will be a higher correlation between mapped geology and gravity anomalies if we work with the isostatic gravity anomaly map and not the CBA map. In the ISO map the longer wavelength anomalies (the larger, smoother features) generally represent large bodies at depth - typically, large blind intrusives and plutons. The shorter wavelength anomalies (the smaller, sharper features) tend to correlate best with surface geology, especially in the isostatic gravity map.
Sedimentary rocks have relatively low densities (typically 1.5 to 2.5 grams/ centimeter cubed), while igneous rocks generally have greater densities (typically 2.5 to 3.5 grams/centimeter cubed). This means that igneous rocks have a relatively higher "weighting" in these maps, and this can be seen in the rather high degree of correlation we can find between mapped intrusive bodies and the isostatic gravity anomalies. The overall effect is that we see relatively little correlation between anomalies and offsets in the isostatic anomaly map and any sedimentary rock units, unless there is a substantial contrast between, say, a dense shale and a relatively porous sandstone. Another example is when we are trying to distinguish Descon flows versus clastics - the flows will generally be denser than the clastics and should show up as an increase in the strength of the ISO field in an area where there is a transition from flows to clastics. There is a significant limitation to this claim, however: we can distinguish transitions only if there are sufficient data to clearly show a transition in the first place. With few exceptions, the gravity data coverage in the Craig-Dixon Entrance area is sparse, and further stations along profiles would have to be acquired to allow us to map such transitions.
The character of the gravity map is somewhat different from the magnetic map, in large part because the resolution of the data is coarser due to limits on helicopter-access during our surveys. Nevertheless, there are some excellent correlations between the magnetic and gravity maps, and several differences that allow us to break up certain intrusives into discrete and substantially different components that in the magnetic data seem to be a single feature. The following discussion is keyed to the isostatic gravity map.
The Union Bay complex and the small islands to the northwest of it must be dense bodies, based on the gravity amplitude, something to be expected for a zoned mafic-ultramafic body (though serpentinization often reduces the aggregate density); the correlation with the magnetic image is very good considering the lack of gravity stations in Clarence Strait (the deep channel between Union Bay and Prince of Wales Island.
There is also a good correlation between the magnetic and gravity data for the Kasaan Peninsula and the rocks to the northwest of the peninsula. The magnetic data would seem to imply that they are separate, while the gravity would seem to imply they are continuous. This apparent continuity is probably an artifact of the coarser gravity station density and consequently poorer resolution of the gravity map. Interestingly, the Salt Chuck ore body lies on the southeastern edge of the highest part of the gravity anomaly and the coincident magnetic anomaly. This would imply that the rest of the margins of this coincident geophysical anomaly on the northwestern end of the Kasaan Peninsula hold significant potential for similar deposits.
The cryptic Twin Mountain-Staney Creek magnetic anomaly (model shown in figure 3) shows up in the gravity map as a broad gravity high. But for the magnetic data, the source would otherwise be interpreted as a large buried and homogeneous pluton; instead we feel it is a composite pluton composed of several different intrusives.
The higher isostatic gravity values west of Hecate Island are probably a direct consequence of an underlying oceanic crust; station density does not permit a more detailed characterization, and the intrusives seen in the magnetic data over Warren and Coronation Islands cannot be resolved in the coarser gravity data.
This is a very abbreviated summary of the gravity anomalies encountered in the Craig-Dixon Entrance area, and a number of important issues have not been addressed, including the depth to the crust below Prince of Wales Island. Because of the sparcity of gravity stations and time-limits on the release of these data, further analysis and modeling are probably not warranted at this time. The authors are aware of density data collected by Dave Barnes in the late 1960's and early 1970's, however. These densities might be compared to the new geologic map included in Digital Data Series DDS-56 to determine ranges of densities for given lithologies. Normally, there is quite a bit of variability in the densities and magnetic susceptibilities of rock units, since there are many variables involved (including local alteration) that can modify these values. Nevertheless, if additional, more closely-spaced gravity stations are acquired along profiles, these densities could prove useful in more detailed modeling. In the "/Data" folder of Digital Data Series DDS-56 there is a simplified version of the sample-density database assembled by Dave Barnes, called CDE-Densities.txt (Dave Barnes, USGS, Written Communication, 1984).
During August and September 1982 three airborne magnetic surveys were flown draped over the topography in the Craig-Dixon Entrance area (the data are summarized in Table 1, where 1000' AG means the aircraft draped the topography at 1000 feet (305 meters) mean terrain-clearance) by Diversified Technology Corporation under contract to the U.S. Geological Survey (U.S. Geological Survey, 1984). These datasets were combined, then processed to remove the International Geomagnetic Reference Field (IGRF) for 1982. The resulting residual magnetic data ("Resid") are shown in the "\images" directory of Digital Data Series DDS-56. Several derivative and filtered products were developed digitally in order to enhance the data to aid in geologic interpretation; one of these steps was Reduction-to-the-Pole ("RedPole"), whereby asymmetry caused by non-polar geomagnetic latitude were removed. The result is an image where anomalies should lie over their sources. This RedPole map is shown in the "\images" in several different forms, including draped as color over a shaded-relief topographic image, and as contours superimposed on the simplified geologic map. More information and data about magnetic data in Alaska can be found in Saltus and Simmons, 1997.
| No. | Name | Spacing-mile/kilometer | Direction | Altitude | Maximum latitude | Minimum latitude | Maximum longitude | Minimum longitude | Line-Km | IGRF model |
|---|---|---|---|---|---|---|---|---|---|---|
| 1 | Craig | 1.0/1.6 | N-S | 1000' AG | 56.03 | 54.63 | 133.83 | 131.83 | 5730 | IGRF75 |
| 2 | NW Craig | 1.0/1.6 | E-W | 1000' AG | 56.00 | 55.83 | 134.50 | 133.25 | 630 | IGRF75 |
| 3 | S Craig | 1.0/1.6 | N86E | 1000' AG | 54.95 | 54.67 | 132.75 | 132.20 | 350 | IGRF75 |
Similar to the distinction made between CBA and ISO gravity, there have also been shape- shifting compensations performed on the magnetic data. The raw magnetic data was delivered to the U. S. Geological Survey by the contractor after correcting and compensating for several forms of time-varying noise. One of these is instrumental drift; another is diurnal drift, and a third is a tie-line or leveling correction. A fourth correction compensates for the fact that there is a slow, steady westward drift of a second-order component of the Earth's dipolar field. The time-scale here is measured in years. Instrumental drift is self-explanatory; diurnal (daily) drift is caused by the Earth's rotation within a complex geomagnetic field, some of whose components are asymmetric with respect to the direction of the Sun (caused by the buffetings of the solar wind). The leveling or tie-line correction simply forces the field to be consistent for a given elevation above ground, despite the fact that the acquisition aircraft inevitably varies its altitude during the survey. The fourth correction compensates for the shifting nature of the Earth's magnetic field; the actual correction process is described as "removal of the IGRF", where IGRF stands for International Geomagnetic Reference Field. The resulting residual magnetic field after all these corrections can thus be compared with confidence to other magnetic surveys acquired in different years. We call this the Residual Magnetic Field, because it is the residual after all these drift effects are taken out.
In preparing this CD-ROM we have added a fifth correction, called "reduction-to-the- pole" (sometimes abbreviated "Redpole"). The magnetic anomaly over a given source body is asymmetric with respect to that body because of the non-vertical direction of the Earth's magnetic field vector; in North America this field vector has an inclination ranging between 40 and 60 degrees from the horizontal, and a declination (i.e., the difference in compass direction between magnetic field lines and true north) of up to 15 degrees. Consequently, hand-held compasses do not generally line up pointing towards true north, and contour plots of magnetic anomalies do not line up over their source or causative bodies. This can be mathematically corrected for, however (see for example Blakeley, 1996), and we have done this. We call the result the Reduced-to-the-Pole Magnetic Field, or RedPole for short. The RedPole field now has magnetic highs (and in some cases lows) mostly positioned directly over the source or causative bodies. We can use either the Residual Magnetic Field or the RedPole field to model the source bodies in three dimensions, but in general, the Redpole map correlates more closely to mapped geology. For the same reasons that we generally prefer not to use the CBA gravity map, we generally avoid using the residual magnetic anomaly map when we have a significant inclination and declination in the Earth's magnetic field vector. Simply put, the Redpole map gives a much better representation of the geology to casual examination than the Residual map. In cases where sources are buried, or where there is extensive vegetative or water cover, the RedPole magnetic map (and sometimes the ISO gravity map) allow us to confidently extend specific map units beneath sediment and water cover.
Euler deconvolution is one method for calculating depth-to-source from magnetic data (Blakely, 1995). On an experimental basis, Euler deconvolution calculations were made from the Craig-Dixon Entrance magnetic data. Results are shown in Figure 3 and in two files in the "\images" directory of Digital Data Series DDS-56 for two different structural indexes. The example in Figure 3 shows an example for a Structural Index = 1 (explained below). In this representation the smaller circles represent shallower sources and larger circles represent deeper sources; dips and strikes of 3-dimensional features can be readily observed by how the circles lie one atop another. The features utilized for the calculations are gradients and offsets in the magnetic data. For this calculation, a Structural Index of 1 (shown here) emphasizes intrusions, dikes, and fault-caused features; for this reason not all of the magnetic gradients in the RedPole image are strongly represented. An SI of 2 emphasizes cylindrical source objects; this figure is also included in the "\images" directory of Digital Data Series DDS-56 and is significantly different from Figure 3. In this other image the major intrusions in the Craig and Dixon Entrance quadrangles stand out much more sharply. From figure 3, on the other hand, a geologist can readily identify lithologic contacts and get a sense of how deep the causative rocks are - in some cases we can even see the dip of the contact. Because of the limited data-window we cannot use Euler deconvolution to obtain from these data the thickness of the crust. The isostatic correction algorithm discussed earlier in the gravity section utilizes gravity data, bathymetry, and topography from the Craig-Dixon Entrance and well beyond to compensate for the changing thickness of the crust in the narrower Craig-Dixon Entrance area.
A brief overview of the magnetic map follows. This discussion is keyed to the RedPole map (see figure 6, or "CDE-AnnotatedMagAnomalyMap.jpg" in the "\images" directory of Digital Data Series DDS-56), beginning in the northeast. Coordinates in the following text are UTM, keyed to the linked figure above. In Union Bay, there is a strong, vertical-cylinder magnetic anomaly overlying the Union Bay Intrusive. This anomaly is unusually strong; in fact, airline navigation maps even warn pilots to expect large deviations in their compass headings in the vicinity. While acquiring a gravity station in this location, two of us (J.C.W. and D.J.G.) observed 5-centimeter-thick veins of magnetite in several exposures.
There is a continuous high magnetic anomaly covering all of the Kasaan peninsula, but also extending across Kasaan Bay to include Clover Mountain to the south. The character of the anomaly, however, changes as it crosses Kasaan Bay. In the north, over the Kasaan Peninsula, the mag anomaly displays relatively long wavelengths, whereas south of the bay this changes to higher amplitudes and somewhat shorter wavelengths, suggesting that the anomalies from two different sources are merging. The Kasaan Peninsula is mapped primarily as gabbros and monzonite intrusives. The rocks underlying the Clover Mountain magnetic anomaly are Descon (SOg) sediments, implying that the source rocks for the magnetic anomaly must lie - relatively shallowly - buried beneath a Descon layer. Note the previous discussion of the relationship of the economically important Salt Chuck mine with the edge of a coincident magnetic and gravity high located in the northwestern corner of the Kasaan Peninsula.
There is an unexpected magnetic high underlying Twin Mountain (see figure 4) at roughly 615000E by 618000N, trending approximately east-west. It is unexpected because the underlying geology is sedimentary. The magnetic anomaly extends from central Hecate Island in the west to Staney Creek in the east. The area is underlain by Hecate limestone, Karheen Formation (Dk) sediments, and sedimentary rocks of the Staney Creek and Tuxekan Passage area (DSs). U. S. Geological Survey geologists have seen scattered rhyolite and andesite flows, and even small plugs of andesite, dacite, and diorite elsewhere in the Karheen Formation. This magnetic anomaly appears to map the presence of a major blind intrusive (see computed model in Figure 4). The modeled intrusive doesn't quite breach the surface, but must reach to within a kilometer. These data are unusual in that the shorter-wavelength shape of the magnetic anomaly suggests a much shallower source than the broader shape of the gravity anomaly (they are both modeled simultaneously here). In order to model the two together, we were forced to invoke a two-phase intrusive body with a shallow, more magnetic component and a deeper, more dense part of the intrusive below about 13,500 meters. This intrusive is significant in that it may imply an undiscovered Cu-Pd mineral resource potential due to the spatial connection of this anomaly to the magnetic anomaly over the Salt Chuck deposit to the immediate east (Gault and Wahrhaftig,1992; Loney and Himmelberg, 1992; Watkinson and Melling, 1989).
Another magnetic high underlies Warren Island, but this is no surprise, since the island is mapped as Descon sediments intruded by Cretaceous intermediate plutonic rocks, some of which outcrop and have moderate to strong magnetic susceptibility as hand-samples. Not surprisingly, the areal extent of the pluton beneath the island and the surrounding sea is more than twice the island exposure.
Coronation Island is mapped as roughly 95% Descon sediments and Hecate limestone, but two tiny exposures of Cretaceous intrusive rock in the north and central part of the island suggest that most of the island is underlain by intrusive rock. It is possible that there are up to four separate but probably related intrusive bodies underpinning the island (based on the overlapping but apparently discrete anomalies observed).
San Juan Batista Island correlates closely with a magnetic anomaly similar to Warren Island, but in this case it appears to be connected to the west and southwest to a long arcuate anomaly beneath Baker Island, most of which is covered by Descon sediments. In the southeast and on the southern tip of Baker Island more Cretaceous intrusive rocks are exposed, and the magnetic data suggest that a sub-surface pluton is continuous from San Juan Batista to the southwestern tip of the island (Cape Bartolome). A (probably) separate anomaly lies over the southern part of the immediately-adjacent westward island (Lulu Island), centered where another Cretaceous stock is partially exposed, but the anomaly extends under the sea to the west.
A strong magnetic high lies over Suemez Island. Where it is strongest in the southeastern part of the island, it is underlain by another Cretaceous pluton. The magnetic signature suggests two things here: the entire island is underlain by a small pluton, and this pluton is distinct and different, despite its proximity, from the huge pluton underlying all of Dall Island.
A major surprise in the magnetic data is the extensive magnetic anomaly associated with Dall Island. This island is covered by Hecate limestone, Descon sediments, and magnetically-inert Wales Group metasedimentary rocks, with tiny bits and pieces of intrusive rock (Kg) exposed. We modeled a profile across the Dall Island magnetic data (see figure 9) and it suggests that the Cretaceous pluton causing the anomaly may be substantial and rises to within a kilometer of the ground surface beneath and slightly to the northeast of the topographic high marking the island ridge.
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There are inadequate gravity data in this southwestern area for us to model the gravity component also, and in fact the magnetic coverage is truncated to the immediate west of Dall Island, meaning that the western boundaries of the model are poorly constrained. Tonalite and other components of the Kg unit are commonly magnetic, suggesting to us that Dall Island is underlain by a large pluton. The upper sedimentary rocks are exposed because they were uplifted by the emplacement of this pluton. The depth to the bottom of the pluton cannot be defined with available data. Normally the bottom of a magnetic source is poorly defined because of the dominance of the upper surface of the source object on the data. The Curie temperature for magnetite is about 580oC (Blakeley, 1995). At depths below where this temperature is reached (the Curie isotherm, anywhere from 10 to 25 kilometers, depending on the geothermal regime; Stacey, 1969), there is no further contribution by the source body to the magnetic anomaly, even though the body may reach deeper. Because we have only limited gravity data around Dall Island, the bottom of the intrusive in figure 6 cannot be defined and is drawn arbitrarily.
The large magnetic anomaly underlying Dall Island (see figure 9 substantially increases the possibility of a significant mineral resource potential for this island, in part because fluids associated with the the later intrusive event have invaded and altered the sediments. We have also mapped mineralized quartz veins at the southern tip of Dall Island (at a locality called McCloud Bay), and believe these veins are pluton-related. There is also mineralization reported at several places along the western side of the island, for instance at Grace Mountain (see the Dixon Entrance digital topographic map included in the "\images" directory of Digital Data Series DDS-56) farther north. There may be other sites where mineralizing fluids have escaped into overlying sediments, giving rise to undiscovered skarns and massive sulfide/epithermal vein deposits, among other things.
A roughly north-oriented magnetic anomaly at Sukkwan Island suggests that the exposed syenite in the southeastern part of the island may underlie the entire island, and also may extend across Sukkwan Strait to Eek Lake all the way north to Deer Bay on the main body of Prince of Wales Island. In the magnetic data, it's not clear if the strong Jumbo Mountain magnetic anomaly is just an extension of the Sukkwan syenite, but the gravity data (discussed below) make it clear that they are separate, restricting any economic potential associated with this particular magnetic anomaly to the Jumbo Mountain side of the Strait.
A large magnetic anomaly encompasses Jumbo Mountain, extending north across Sulzer Portage to Beaver Mountain. Cretaceous plutons are mapped on both mountains, and in the magnetic data they seem to be continuous. The gravity data show this not to be true; agravity high at Beaver Mountain contrasts with a gravity low at Jumbo Mountain, with a sharp gradient at Sulzer Portage-Cholmondely Sound. This gradient is important in any mineral resource estimate, because the Jumbo Mine has produced substantial amounts of copper and gold. The gravity data are consistent with a major east-west fault separating the two plutons; more likely they are different intrusive events with different compositions. Because of this, Beaver Mountain may not have the same economic potential.
With Digital Data Series DDS-56, we are releasing U. S. Geological Survey-acquired geophysical data for the Craig and Dixon Entrance 1:250,000 quadrangles of southeastern Alaska. Much of the data have never appeared before in the public domain, including a simplified geologic map, the raw gravity and magnetic data, derivative products including depth-to-source images, various grids of the data, imaging representations and overlays of the data. In addition, we have added a limited interpretation of what the gravity and magnetic data indicate about the underlying geology, structures, and their relationship to known and potential hidden mineral resources. This is necessarily limited in the case of the gravity data by the general sparcity of stations. For convenience we have also included here various representations of the digital topography as well as U. S. Geological Survey topographic images, including the raw, original form of the digital topography and the DRG images of the topographic maps also available on a U. S. Geological Survey Web-site.
The magnetic and gravity data released here cover all of the Craig and Dixon Entrance quadrangles, as well as the westernmost portions of the Ketchikan and Prince Rupert quadrangles, up to 56 degrees north latitude. The combination of magnetic and gravity data together allow us to resolve ambiguities in subsurface structural interpretation, ambiguities that would be unresolvable if only one or the other dataset was used by itself. Several "blind" intrusives and plutons have been recognized, and two of these have been modeled. Known mineral deposits and prospects are now placed in the context of the subsurface geology influencing and moderating their emplacement.
The simplified geologic map was compiled by Don Grybeck and digitized by Kelly Brunt under the initiative and supervision of Nora Shew, all from the Anchorage, Alaska office of the U. S. Geological Survey. Anne McCafferty (U. S. Geological Survey, Denver) was instrumental in guiding the assembly of the DEMs into a useable grid.
Barnes, D.F., 1972, Sixteen 1:250,000 simple Bouguer gravity anomaly maps of southeastern Alaska showing station locations, anomaly values, and generalized 10-milligal contours: U.S. Geological Survey Open-File maps, 16 sheets.
Barnes, D.F., Olson, R.C., Holden, K.D., Morin, R.L., and Erwin, M.J., 1972, Tabulated gravity data from southeastern Alaska obtained during the 1968 field season: U.S. Geological Survey Open-File Report, 76 p.
Barnes, D.F., Popenoe, Peter, Olson, R.C., McKenzie, M.V., and Morin, R.L., 1972, Tabulated gravity data from southeastern Alaska obtained during the 1969 field season: U.S. Geological Survey Open-File Report, 75 p.
Barnes, D.F., 1977, Bouguer gravity map of Alaska: U.S. Geological Survey Geophysical Investigations Map GP-913, scale 1:2,500,000
Blakely, R.J., 1995, Potential theory in gravity and magnetic applications: New York, Cambridge University Press, 441 p. (Reprinted in 1996.)
Brew, D.A., (Compiler), 1996, Geologic map of the Craig, Dixon Entrance, and parts of the Ketchikan and Prince Rupert quadrangles, southeastern Alaska: U.S. Geological Survey Map MF-2319, scale: 1:250,000, 2 sheets, 58 page pamphlet.
Burgett, W.A., and Krause, K.J., 1979, Helicopter-assisted radiometric survey of the Dixon Entrance quadrangle, Alaska: Bendix Field Engineering Corp., U.S. Department of Energy, Grand Junction Office Report GJBX-019 (79), 6 p.
Cathrall, J. B., Arbogast, B. F., VanTrump, G., Jr., and McDanal, S. K., 1993, Geochemical maps showing the distribution of selected elements in stream-sediment samples for the Craig, Dixon Entrance and western edges of the Ketchikan and Prince Rupert quadrangles, southeast Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-2217-A, 2 sheets, scale 1:250,000.
Cathrall, J.B., 1994, Geochemical survey of the Craig area Craig and Dixon Entrance quadrangles and the western edges of the Ketchikan and Prince Rupert quadrangles, southeast Alaska: U.S. Geological Survey Bulletin 2082, 52 pp., 1 plate, and 7 tables.
Decker, John, 1979, Preliminary aeromagnetic map of southeastern Alaska, U.S. Geological Survey Open-File Report 79-1694, 1:1,000,000 scale.
Eberlein, G. D., Churkin, Michael, Jr., Carter, Claire, Berg, H. C., and Ovenshine, A. T., 1983, Geology of the Craig quadrangle, Alaska: U.S. Geological Survey Open-File Report 83-91, 52 p.
Gault, H. R., and Wahrhaftig, C., 1992, The Salt Chuck copper-palladium mine, Prince of Wales Island, southeastern Alaska. U.S. Geological Survey Open-File Report 92-293, 5 maps, 24 p.
Gehrels, G. E., 1991, Geologic map of Long Island and southern and central Dall Island, southeastern Alaska: U.S. Geological Survey Map MF-2146, 1 sheet, scale 1:63,360.
Gehrels, G. E., 1992, Geologic map of southern Prince of Wales Island, southeastern Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-2169, 1 sheet, scale 1:63,360, 23 p.
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