n Alas - Alaska Division of Geological & Geophysical Surveys - State ...
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n Alas - Alaska Division of Geological & Geophysical Surveys - State ...

n Alas - Alaska Division of Geological & Geophysical Surveys - State ... n Alas - Alaska Division of Geological & Geophysical Surveys - State ...

dggs.alaska.gov
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s -,------- Medial moraine with number . ................... Ice divide Trident Glacier , Sample traverse (A-A') Geochemical anomaly with number F@me (n.-Rldent Glseier, showing mdid morajnes sampled on traverse A-A' and geochemically anomalous exploration-target arms inferred from samples (see fig. BOB). Key moraines are numbered progressively from west to east side of glacier system. Same moraine are denignated as right limit (8) or left limit (L) where they origimte from a cornnlon source arm.

Smaller placer mines ere active on Valdez Creek itseJf and several of its tributaries. Values in th$ Valdez Creek drainage range from $4.00 to $15.00/m (at $400.00/troy oz of gold). Gold from the Valdex Creek area is typically oatmeal size, smaller, and commonly much flattened with rounded edges. Colors range from lemon yeUow to a rich golden brown and reddish brown (iron stained). Gold was panned as far east as Pass Creek on the extreme east boundary of the Healy quadrangle. Several small lode-gold mines are located in the surrounding Clearwater Mountains. These lodes and, possibly, other undiscovered lode-gold deposits in the area are most likely the source for the placer gold in the Valdez Creek drainage. An area of the old Bs~lfield milling district, described by Cap@ (1912), lies at the north edge of the Healy quadrangle. Mining operations on Totatlanika and Platt Creeks, near the north-central boundary of the quadrartgle, and on Portege Creek in the northwest corner of the quadrangle are small scale and generally employ 2 to 5 people per opration. Capps (1912, p. 43) pointed out that the workable plscers in this northern district have all been found in areas that drain terrain presently or originally covered by the upper Tertiary Nenana Gravel, and he postu- lated that rw * * doubtless a reconcentration of the gold from these gravels has produced most of the pres- ent placer deposits, although sorne gold may have been contained In the Iower Tertiary beds and have been re- concentrated * * *." This would seem to be a correct assessment of the gold source in this area, substanti- ated by the fact that I panned gold from the Lignite Creek Formation-Nenana Gravel (Tertiary formations) contact near where the main highway to Fairbanks crosses Panguingue Creek. Ln the Mount Hayes quad- rangle, directly east of the Healy quadrangle, gold is present in conglomerate of early Tertiary(?) age (Yeend, 1981). It seems increasingly clear that gold has been recycled through the Tertiary continental deposits before its incorporation into the Holocene placers throughout much of interior Alaska. REFERENCES ClTBD Capps, S. R., 1912, The Bonnifidd region, ALaska: U.S. Geological Survey Bulletin 501, 64 p. Smith, T. E., 1970, Gold resource potential of the Denali bench gavels, Valdez Creek mining dis- trict, Alaska, & Geological Survey research 19 7 0: U.S. Geological Survey Professional Paper 700-D, p. D146-D152. -1981, Geolcgy of the Clearwater Mountains, south- central Ahka: Alaska Division of Gmlogical and Geophysicd Surveys Geologic Report 60, 72 P. Yeend, Warren, 1981, Placer gold deposits, Mount Hayes quadrangle, Alaska, Albert, N. R. D., and Hudson, Travis, eds., The United States Geo- logical Survey in Alaska: Accomplishments during 2979: U.S. GeoIogical Survey Circular 853-13, p. B68. Pmmian plant megafoe& fmm the eongbmerate of MoantDen,aenfral~~ Only a few occurrences of late Palemoic plant megafossils ere known in Alaska. Before the discovery of Permian pplant fossils in the co lomerate of Mount Dall in the central Alaska Range"$reviously assigned to the Middle Pennsylvanian by Reed and Nelson, 1977, 19801, no credible Pennsylvarian or Permian plants had been reported. The sole report of a Pennsylvanian Calamites arnbwus fragment from the Alaska Penlnm w a l d , 1871, pl. 4, fig. 9) has never been propedy corroborated. Sparse but slgnlficant Devonian plants have been documented (Mamay, 1963; Churkin and others, 1969), and smdl but stratigraphically informative collections of Early Mississjppian plants are made occasionally, especially in the Brmks Range. The paleotmtany of the Pennsylvanian and Permian of Alaska, however, has been enigmatic until recently. This report describes a small collection of plant fossils that provides a limited but intriguing insight into that late Paleozoic hiatus. These fossils were colleoted in 1976 by B. L. Reed, B. L. Detterman and S. W. Nelson while investigating the geology of the Talkeetna quadrangle in the central Alaska Range (area 12, fig. 44). The fossiliferous shale and siltstone are exposed south of the Denall fault, about 5.2 krn N. 26' E. from Mount Dall (at lat 63°25r52u N., long 152 11'40" W., in the SE1/4NW1/4 sec. 23, T. 29 N., R. 16 W., Talkeetnn C-5 quadrangle) at about 1,525-m elevation. They occur within a thick (min 1,500 m) sequence of chiefly terrestrial conglomerate, sandstone, ~Lltstone, and shale. The unit consists largely of massive lenticular beds of conglomerate and sandstone, with numerous cut-and-fill channels; the top is not exposed. This unit bas been informally called the conglomerate of Mount Dall by Reed and Nelson (1977). The lower part is believed to be gradational downward into a depositionally and structurally complex terrane of mostly marine flyschoid sedimentary rocks containing fossil marine invertebrates of Devonian, Mississippian, and Pennsylvanian age ( ~eed and Nelson, 1977). Before the 1976 investigntions in the Talkeetna quadrangle, the sedimentary rocks south of the Denali fault in the Mount Dall area were regarded 8s part of a sequence of upper Mesozoic graywacke and qillite, and any unit of conglomerate was assurn& to be Late Creteceous or Tertiary. The fossil plants discussed here, however, establish a late Paleozoic age and a continental origin for at least parts of the conglomerate of Mount Dall. The conglomerate and flyschoid sediment are part of the Mystic tectonostratigraphic terrane (Jones and others, 1981). The fossils, which were found in the upper half of the conglomerate, are most abundant in a hard finegrained dark-gray irregularly bedded matrix. About 20 small to mediumsize slabs Iapprox 4-30 crn in maximum dimension) containing discernible plant parts constitute the collectjon. Some of these slabs contain only n single leaf fragment; others hove sufficiently abundant interbedded plant parts to suggest the original presence of a fairly luxuriant growth of vegetation near the site of deposition. All the specimens are fragmentary; the largest Is 28 cm long by 2 cm wide.

s -,-------<br />

Medial moraine with number<br />

.<br />

................... Ice divide Trident Glacier<br />

, Sample traverse (A-A')<br />

Geochemical anomaly with number<br />

F@me (n.-Rldent Glseier, showing mdid morajnes sampled on traverse A-A' and geochemically anomalous<br />

exploration-target arms inferred from samples (see fig. BOB). Key moraines are numbered progressively from<br />

west to east side <strong>of</strong> glacier system. Same moraine are denignated as right limit (8) or left limit (L) where they<br />

origimte from a cornnlon source arm.

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