Central Brooks Range Research Articles

Water relations of white spruce (Piceaglauca (Moench) Voss) at tree line in north central Alaska

Factors affecting xylem pressure potential and stomatal behavior during the growing season were studied on white spruce (Piceaglauca (Moench) Voss) at tree line in the central Brooks Range of northern Alaska. Leaf conductance and xylem pressure potentials were low during days of moderate and high evaporative demand, even though soil moisture was near field capacity. Boundary line diagrams, regression analysis, and laboratory experiments indicate that stomata are sensitive to changes in vapor pressure deficit and xylem pressure potential. A correlation between resistance to water flow through the tree and soil temperature suggests that soil temperatures less than 9 °C are associated with increased resistance to water flow and may partially explain the position of white spruce tree line in northern Alaska.

Late Wisconsin and Holocene vegetational history of the Upper Koyukuk region, Brooks Range, AK

Three pollen diagrams provide information on late Wisconsin and Holocene vegetation history at high elevation and midelevations (1000 and 600 m) in the east-central Brooks Range. After the retreat of glacial ice and until about 12 000 years before present, the vegetation was a tundra, dominated by Salix, Cyperaceae, Gramineae, Artemisia, and other herbaceous taxa. However, because the early pollen spectra do not have close modern analogs, the detailed composition and ecology of this tundra cannot be inferred. Between ca. 12 000 and ca. 8 000 years before present, Betula nana–glandulosa was the dominant pollen producer at both elevations; a significant ericaceous component was present in the vegetation of the alpine sites, but not at the lower elevation site. During the period 10 500 – 8 500 years before present, Populus balsamifera and Juniperus were probably abundant at the lower site and may also have been present at the higher sites, where they are now absent. Picea glauca, Alnus spp., Picea mariana, and Betula papyrifera became established at the lower site between ca. 8500 and 6000 years before present. None of these species currently grows at the high sites, but their arrival at lower elevations is represented in the regional pollen rain at these sites. The arrival times of Picea glauca and Picea mariana in the study area are consistent with the hypothesis that P. glauca preceded P. mariana into the central Brooks Range.

Meteorological Conditions that Initiate Slushflows in the Central Brooks Range, Alaska

Over 30 individual slushflows were documented and observations were made of both high and low flow rates during 1979, 1980 and 1981 at Atigun Pass, central Brooks Range. Air temperature, precipitation and global radiation were recorded in 1980 and 1981 during the slushflow season. Meteorological conditions that exist in this arctic alpine environment at the time of break-up apparently are factors that encourage flow activity. Persistent low temperature prior to break-up allows the snowpack to remain intact until late spring. An influx of warm air from the interior of Alaska and long hours of radiation input produce an accelerated rate of snow-melt that plays a major role in slushflow occurrence.

Meteorological Conditions that Initiate Slushflows in the Central Brooks Range, Alaska

Over 30 individual slushflows were documented and observations were made of both high and low flow rates during 1979, 1980 and 1981 at Atigun Pass, central Brooks Range. Air temperature, precipitation and global radiation were recorded in 1980 and 1981 during the slushflow season. Meteorological conditions that exist in this arctic alpine environment at the time of break-up apparently are factors that encourage flow activity. Persistent low temperature prior to break-up allows the snowpack to remain intact until late spring. An influx of warm air from the interior of Alaska and long hours of radiation input produce an accelerated rate of snow-melt that plays a major role in slushflow occurrence.

Lateral Continuity of the Blarney Creek Thrust, Doonerak Window, Central Brooks Range, Alaska: ABSTRACT

The contact between Carboniferous and lower Paleozoic rocks, exposed along the northern margin of the Doonerak window in the central Brooks Range, is a major thrust fault called the Blarney Creek thrust (BCT). The BCT has been traced over a distance of 25 km, from Falsoola Mountain to Wien Mountain. The tectonic nature of this contact is demonstrated by: (1) omission of stratigraphic units above and below the BCT; (2) large angular discordance in orientation of first-generation cleavage at the BCT; (3) numerous thrust imbricates developed in the upper-plate Carboniferous section that sole into the BCT; and (4) truncation of an upper-plate graben structure at the BCT. Lack of evidence for pre-Carboniferous deformation in the lower plate casts doubt on the interpretation of the contact as an angular unconformity. However, the localized presence below the BCT of Mississippian Kekiktuk Conglomerate and Kayak Shale, in apparent depositional contact with lower Paleozoic rocks, suggests that the BCT follows an originally disconformable contact between the Carboniferous and lower Paleozoic rocks. The juxtaposition of younger over older rocks at the BCT is explained by calling upon the BCT to act as the upper detachment surface of a duplex structure. Duplex development involves initial imbrication of the Carboniferous section using the BCT as a basal decollement, followed by formation of deeper thrusts in the lower Paleozoic section, which ramp up and merge into the BCT. End_of_Article - Last_Page 678------------

Cambrian Trilobites with Siberian Affinities, Southwestern Alaska: ABSTRACT

Cambian trilobites occur in two levels (about 7 m apart) in the core of a large, complex anticlinal structure in the area between the Taylor Mountains and the Hoholitna River in southwestern Alaska. The lower collection contains Erbia, Macannaia (a species close to Soviet forms described as Pagetia ferox Lermontova), two species of Kootenia (including one perhaps conspecific with forms from the central Brooks Range), and several species of ptychoparioid trilobites. It is clear that biogeographic affinities are with the transitional facies of the eastern Siberian platform and the south Siberian foldbelt. In Soviet terms, the age of the collection falls in a disputed interval called latest Early Cambrian (Tojonian) by some authors, and earliest Middle Cambrian (Amgan) by ot ers. In North American terms, Macannaia is known only from early Middle Cambrian beds. The younger collection contains abundant agnostids, a variety of conocoryphids, Paradoxides, and several species of ptychoparioid trilobites. This is an assemblage of undoubted late Middle Cambrian age, comparable to faunas described from the Maya Stage of the Siberian platform and the Paradoxides paradoxissimus Stage of the Baltic region. Both faunas are from ocean-facing or outer shelf environments. None of the key non-agnostid or non-pagetiid elements have been seen previously in deposits of Cambrian North America. End_of_Article - Last_Page 295------------

Phosphatic Glauconitic Sandstone and Oncolite Deposition at the Upper Paleozoic Base of Etivluk Group, North-Central Brooks Range, Alaska: ABSTRACT

Carboniferous stratigraphy of the Picnic Creek allochthon in the central Brooks Range is dominated by bedded cherts and shales. In the Killik River quadrangle, bedded black cherts of the Lisburne Group are overlain by a thin diagnostic clastic unit composed of sandstone and conglomerate. The sandstone is a thin (0.35-m), laterally extensive, planar, laminated litharenite with an average Q:F:L of 40:17:43 and a Qp:Lv:Ls of 12:8:80. The provenance is interpreted to be a recycled orogen dominated by uplifted sedimentary sequences with minor plutonic, metamorphic, and volcanic sources. The presence of glauconite (7%) and authigenic phosphate (18%) indicates deposition in a shelf environment. This phosphatic sandstone forms part of the matrix in a conglomerate at one locality. The conglomerate is lenticular (2 m × 10 m), crudely graded, and very poorly sorted, and it contains black chert ripups. Clasts are composed of oncoids (70%), chert (22%), shale (5%), and limestone (3%). Barite preferentially replaces all clasts except chert and part of the matrix. The oncolites are SS-type mode C hemispheroids, indicating formation in a continuously agitated shallow to intertidal marine environment. Sedimentologic and petrographic observations suggest that the phosphatic glauconitic sandstone developed in a shelf environment, and the oncolitic conglomerate is a debris flow off a nearby carbonate platform that transported shallow-water material out onto the shelf. Preservation of unaltered echinoderm fragments and calcareous algal oncolites clearly indicates deposition above the CCD. Radiolarians from immediately above the clastics include spongy tetrahedral Latentifistulidea, which suggests that sandstone and conglomerate deposition probably occurred in the Morrowan (Early Pennsylvanian). End_of_Article - Last_Page 678------------

Pre-Mississippian Accreted Terranes of Northeastern Brooks Range, Alaska: ABSTRACT

Low-grade metamorphic rocks (Neruokpuk Formation, sensu lato) underlying a regional angular unconformity in the northeastern Brooks Range have been interpreted as a conformable Precambrian to Devonian stratigraphic succession. The pre-Mississippian rocks include not only miogeoclinal quartzites and carbonates but also a variety of eugeoclinal lithologies such as radiolarian cherts, argillites, and graptolitic shales; mafic to intermediate volcanic rocks; and volcanogenic gray-wackes. Fossils of Cambrian, Ordovician, and Silurian age have been identified in these lithologies. Many of these units are fault-bounded and may be interpreted as pre-Mississippian tectono-stratigraphic terranes. Several of the terranes are depositionally overlapped by Middle(?) Devonian clasti rocks and intruded by plutonic rocks depositionally overlain by Mississippian and younger rocks, but which yield equivocal middle Paleozoic age dates. Similar relationships are exposed in the Doonersk anticlinorium in the central Brooks Range. These features are interpreted to indicate that the lower Paleozoic rocks of the eastern Brooks Range were tectonically assembled by accretionary processes along an active continental margin prior to Middle Devonian time. Subsequent uplift and erosion occurred prior to deposition of the Mississippian to Neocomian (Ellesmerian) passive margin sequence. End_of_Article - Last_Page 670------------

Post-Ellesmerian Depositional Sequences of Central North Slope Subsurface: ABSTRACT

Detailed electrical-log correlations of bedding in the Mesozoic to recent intervals define nearly time-equivalent stratigraphic units. Basinal depositional minima separate them into depositional cycles of 15 to 40-m.y. duration, and sequences of similar cycles correspond to the major episodes of Arctic tectonism. The close of the Sag River cycle in Pleinsbachian time ended the Ellesmerian sequence of accretionary tectonics and northerly continental provenance. Long, oscillating uplift to the northwest during the Jurassic Kingak cycle, and five or more subcycles of emergence along an ancestral Barrow arch rift shoulder during the Lower Cretaceous Kup River cycle show that the Barrovian sequence accompanied Arctic rifting. The Brookian sequence records a time of Arctic seaf oor spreading coincident with underthrusting of the North Slope block toward a convergent Pacific margin. A series of major overthrusts onto the block from this margin were sources for Lower Torok, Nanushuk, Schrader Bluff, Prince Creek, and Franklin Bluffs cycles. The lower Torok source was in a distant westerly direction, and those of the following cycles became progressively closer and more southerly, ending near the present position of the central and western Brooks Range. A collision between Alaska and Siberia in mid-Tertiary time initiated the Eurekan sequence of circum-Arctic compressional tectonics. The North Slope block was tilted northeast, and the Nuwok cycle was derived from the resulting regional erosion. Similar tilting and erosion beginning in the Pleistocene started the G bik cycle that is still being deposited. End_of_Article - Last_Page 674------------

Facies Comparison of Autochthonous and Allochthonous Permian and Triassic Units, North-Central Brooks Range, Alaska: ABSTRACT

Eight stratigraphic sections of Permian and Triassic rocks have been studied over a 30 km by 150 km area in the Endicott and Philip Smith Mountains of the central Brooks Range. Six of the sections are located on the Endicott Mountains allochthon, and the remaining two are parautochthonous columns in the Mount Doonerak area. The sections record a facies transition between the autochthonous Sadlerochit Group and Shublik Formation of the northeastern Brooks Range and the characteristically siliceous rocks of the allochthonous Siksikpuk and Otuk formations of the western Brooks Range. Laterally continuous and bioturbated beds of fine-grained sandstone, siltstone, and shale dominantly compose the Permian sequence, whereas the Triassic rocks consist of black shales, thin rhythmically bedded siliceous mudstones, and fossiliferous limestones. When the allochthonous sections are restored to a position south of the Mount Doonerak area, a general shallowing trend from southwest to northeast becomes evident within the reconstructed marine basin. To the south and west, the Permian sediments show a marked increase in silica content, with the occurrence of barite and a corresponding decrease in the thickness of the basal, coarser grained clastics. The Triassic formations also document an increase in silica and the presence of barite to the south and west, while becoming signi icantly sooty and phosphatic to the north and east. Ongoing petrographic and micropaleontologic studies of the field data will clarify these general paleogeographic relationships. End_of_Article - Last_Page 655------------

Stratigraphy of Endicott Mountains and Picnic Creek Allochthons, Killik River and Chandler Lake Quadrangles, North-Central Brooks Range, Alaska: ABSTRACT

Geologic mapping in the Killik River and Chandler Lake quadrangles has delineated rocks of at least three of the major allochthons found in the De Long Mountains of the western Brooks Range: the Brooks Range (Endicott Mountains), Picnic Creek, and Copter Peak allochthons. Rocks characteristic of parts of the Tpnavik River and Nuka Ridge allochthons are also present. The Endicott Mountains allochthon forms the main crest and mountain front of the central Brooks Range. It is the structurally lowest of the allochthons in the region. Major rock units on the allochthon are: Upper Devonian Hunt Fork Shale and Noatak Sandstone, Upper Devonian-Lower Mississippian Kanayut Conglomerate, and Mississippian Kayak Shale of the Endicott Group; Alapah Limestone and Kuna formation of the Lisburne Group; and Permian Siksikpuk Formation and Triassic Otuk formation of the Etivluk group. Lower Cretaceous coquinoid limestone and, in some places, orogenic sediments of the Okpikruak Formation cap the allochthon. Total stratigraphic thickness of the Endicott Mountains allochthon is over 2,000 m (6,500 ft). Rocks of the Picnic Creek allochthon are widespread in the disturbed belt of the Brooks Range foothills, and structurally overlie the Endicott Mountains (Brooks Range) allochthon in the Killik River quadrangle and quadrangles to the west. The allochthon is not recognized with certainty east of the Killik River quadrangle. Major rock units on the Picnic Creek allochthon are: Upper Devonian(?) Hunt Fork Shale, Lower Mississippian Kurupa sandstone tongue (new name) of the Noatak Sandstone, and Mississippian Kayak Shale of the Endicott Group; Carboniferous Akmalik chert (new name) of the Lisburne Group; and Pennsylvanian Imnaitchiak chert (new name) and Otuk formation of the Etivluk group. Orogenic sediments of the Lower Cretaceous Okpikruak Formation form the top of the Picnic Creek al ochthon. Total stratigraphic thickness of these rock units is not over 1,000 m (3,200 ft). Plant fossils from a number of localities in the Kurupa sandstone are dated as Early Mississippian (Tournaisian-Visean) by B. A. Thomas and R. A. Spicer and have close affinities to material from eastern USSR. Palinspastic restoration of the Picnic Creek allochthon southward to a position south of the Endicott Mountains allochthon restores the Kurupa sandstone, the Akmalik chert, and the Imnaitchiak chert to their proper basin position. In the reconstructed basin, the Kurupa sandstone appears to represent the distal, southern end of the Kanayut-Noatak coarse-clastic wedge. The Akmalik chert and Imnaitchiak chert represent basinal equivalents of the Alapah Limestone-Kuna formation of the Lisburne Group and Siksikpuk Formation of the Etivluk group. Detailed stratigraphic and paleontologic studies of the Mississippian through Triassic rocks on both allochthons are in progress. End_of_Article - Last_Page 671------------

Comparison of Two Suites of Okpikruak Formation: A Point-Count Analysis: ABSTRACT

The Lower Cretaceous Okpikruak Formation lies unconformably on the Etivluk group in the Foothills thrust belt of the central Brooks Range. These deep marine sediments were shed northward during the first phases of Brookian deformation. The formation, as sampled, contains two petrographically distinct populations: the upper Kurupa-Oolamnagavik River drainage samples (KOR) of the Picnic Creek allochthon, and the Cobblestone Creek samples (CC) of the Endicott Mountains allochthon. The KOR samples have an average Q:F:L of 49:21:30 and Qp:Lv:Ls of 46:37:17. Variations in total detrital grain populations are greater for End_Page 682------------------------------ quartz (40-50%) and rock fragments (22-39%) and less for feldspar (17-27%). The KOR population is characterized by high proportions of polycrystalline quartz and low proportions of sedimentary rock fragments. Interpretation of detrital modal analyses suggests a complex source terrane involving orogenic recycling and mixing of sediments from a magmatic arc and a subduction system. In contrast to KOR samples, CC samples have an average Q:F:L of 16:11:73 and Qp:Lv:Ls of 11:34:55. Total grain populations vary most for rock fragments (56-85%) and quartz (6-27%) and to a lesser degree for feldspar (8-17%). The high variation in rock fragments suggests variable volcanic and sedimentary sources. Interpretation of detrital modal analyses also suggests complex sources but with a significant arc-orogen component. Both populations are characterized by varying proportions of diagenetic products, which include albite, calcite, chlorite, sericite, and epidote. End_of_Article - Last_Page 683------------

Fold-Nappes and Polyphase Thrusting in North-Central Brooks Range, Alaska: ABSTRACT

Ongoing study involving inch-to-the-mile mapping of a geologic transect along the Itkillik-Koyukuk Rivers is providing new information on the structural style and kinematic development of the central Brooks Range. The principal structures controlling north-directed structural telescoping are three detachments. These detachments are stratigraphically controlled and occur in: (1) Devonian Hunt Fork Shale, (2) Mississippian Kayak Shale, and (3) Permian and Triassic shale. The detachments and the subsidiary thrusts that branch from them form a thrust complex that developed in two stages. During the first stage, duplexes consisting of horses of Mississippian Lisburne Group platform carbonates formed where the Kayak Shale detachment ramped up to Permian and Triassic beds, imbri ating the intervening Lisburne Group. In the second stage, alpine-style fold nappes involving Devonian Kanayut Conglomerate and elements of the underlying Hunt Fork Shale were emplaced sequentially from south to north on thrust faults solely in the Hunt Fork Shale. Thrust faults that emplace the fold nappes merge with the previously formed detachment in the Kayak Shale. Complex deformation associated with both stages include: (1) ramping of duplexes in Lisburne Group rocks over previously imbricated Lisburne Group rocks; (2) regional folding of imbricated Lisburne Group rocks; (3) folded individual subsidiary faults within the duplexes of Lisburne Group rocks; (4) warping, folding, and thrusting of the Kayak Shale detachment associated with generation and emplacement of the underlying fo d nappes; and (5) faults that cut through the hinge area of fold nappes and thrust the trailing limbs of the fold nappes over the overturned leading limbs of the fold nappes. End_of_Article - Last_Page 667------------

Chronology of Holocene glaciation, central Brooks Range, Alaska

The central Brooks Range was glacierized in the highest, north-facing cirques during late-middle to late Holocene (Neoglacial) time. This Neoglaciation involved at least 5 major cirque-glacier expansions of similar magnitude, as based on lichenometric mapping of more than 50 glaciers and radiocarbon dates directly associated with 5 moraines. Initial stabilization of debris-covered glaciers took place by early Holocene time, but evidently no moraines formed during this interval. Few morainal ridges are preserved that date from the older expansions, but they have been lichenometrically dated (± 20% age reliability) to 3 separate intervals: 4400, 3500, and 2900 yr B.P. Twelve morainal complexes have ridges indicative of glacial advance that lichenometrically date at 1800 ± 500 yr B.P. Relict lichens that are now emerging undisturbed from beneath a receding glacier toe imply that this time was one of prolonged recession in at least some parts of the Brooks Range, however. Radiocarbon analysis of dead moss at this same site dates a subsequent Neoglacial advance across the cirque floor at 1120 ± 180 yr B.P. Our data suggest that for the past ∼1,100 yr, cirque glaciers have been continuously in more extended positions than they are today. During this glaciologically favorable interval, the last two major advances occurred at 800 ± 150 and 390 ± 90 yr B.P. (A.D. 1410–1600). Glaciers across the central Brooks Range stayed close to their maxima until A.D. 1640–1750. Historical photographs and lichenometry show that retreat was most rapid after A.D. 1870 and decelerated after the mid-1900s. Recession from this most recent Neoglacial maximum has amounted to 150–700 m and continues at present. The cirque glacier advances were accompanied by equilibrium-line altitude (ELA) depressions of 100 to 200 m below levels maintained in the late 1970s. Environmental lapse rate estimates and 1977–1981 glaciologic-meteorologic measurements suggest that summer air temperatures accompanying Neoglacial maxima were, respectively, 1 °C or 3 to 4 °C cooler than those of the late 1970s. Across the central Brooks Range, the Neoglacial maxima ELAs rise northeastward and northward from 2 to 5 m km −1 .

A late Wisconsin and Holocene vegetation history from the central brooks range: Implications for Alaskan palaeoecology

Five pollen diagrams reveal late Wisconsin and Holocene vegetation changes in the Walker Lake/Alatna Valley region of the central Brooks Range, approximately 100 km west of the area studied by D. A. Livingstone (1955, Ecology 36, 587–600). New insights into the vegetation history of this region are provided by calculations of pollen influx and by the use of linear discriminant analysis to separate Picea glauca and P. mariana pollen. Three major pollen zones are identified: (1) a basal herb zone, characterized by high percentages of Cyperaceae, Gramineae, Salix, and Artemisia, and low total pollen influx; (2) a shrub Betula zone with increased total pollen influx and very high percentages of Betula pollen, predominantly in the size range of B. nana and B. glandulosa; and (3) and Alnus zone dominated by Alnus pollen. Lakes currently within the boreal forest or near tree line show relatively high percentages of Picea pollen in the Alnus zone. Several striking vegetation changes occurred between ca. 10,000 and 7000 yr B.P. Between ca. 11,000 and 10,000 yr B.P., Populus balsamifera pollen percentages as great as 30% indicate that this species was present at low-elevation sites near Walker Lake. These populations declined abruptly ca. 10,000 yr ago and have never regained prominence. About 8500 yr B.P., Picea glauca pollen reached 10–15%, indicating the arrival of P. glauca in or near the study area. P. glauca populations evidently decreased ca. 8000 yr ago, when Picea pollen percentages and influx fell to low values. About 7000 yr B.P., Alnus pollen percentages and influx rose sharply as alder shrubs became established widely. Picea once more expanded ca. 5000 yr ago, but these populations were dominated by P. mariana rather than P. glauca, which increased slowly at this time and may still be advancing northward. Some vegetation changes have been remarkably synchronous over wide areas of interior Alaska, and probably reflect responses of in situ vegetation to environmental changes, but others may reflect the lagged responses of species migrating into new areas.

New Upper Paleozoic and Lower Mesozoic Stratigraphic Units, Central and Western Brooks Range, Alaska

The sequence of rocks described lies primarily in the DeLong and Endicott Mountains in the western and central parts of the Brooks Range, northern Alaska. New names are given to dominantly siliceous and shaly sedimentary rocks of Mississippian through early Middle Jurassic age. The Kuna Formation (new name) consists dominantly of black bedded chert, shale, limestone, and dolomite within the Lisburne Group; some beds are phosphatic, and mafic extrusive igneous rocks occur in a few areas. Paleontologic data suggest an Early Mississippian to Early or Middle Pennsylvanian age for the Kuna Formation. Because the formation intertongues eastward with light-colored shallow-water carbonate rocks of the Wachsmuth and Alapah Limestones of the Lisburne Group, it is also assigned to t e Lisburne Group. Light-colored shallow-water carbonate rocks of other formations of the Lisburne Group were apparently also deposited on the south, but have been telescoped northward by thrust faulting over the darker Kuna Formation. The formation ranges up to 100 m in thickness. The Etivluk Group (new name) is composed of dominantly siliceous beds of the previously named Siksikpuk Formation and overlying Otuk Formation (new name). New paleontologic data suggest that the Siksikpuk is of Pennsylvanian, Permian, and Early Triassic age. The Otuk Formation consists of up to 100 m rhythmically interbedded chert, silicified limestone, and shale previously assigned in this area to the Shublik Formation. A thin but distinctive zone of early Middle Jurassic organic shale and dolomitic limestone ocally present within the top of the Otuk constitutes the Blankenship Member (new name), which is up to 7 m thick. Paleontologic data indicate that the Otuk Formation ranges in age from late Early Triassic to early Middle Jurassic. These dominantly siliceous stratigraphic units appear to be limited to the allochthons of the western and central Brooks Range; they are not known to be present on the autochthonous complex of the northeastern Brooks Range and subsurface Arctic Slope.

New Upper Paleozoic and Lower Mesozoic Stratigraphic Units, Central and Western Brooks Range, Alaska: ERRATUM

New Upper Paleozoic and Lower Mesozoic Stratigraphic Units, Central and Western Brooks Range, Alaska: ERRATUM

Geochemical evidence for a brooks range mineral belt, Alaska

Geochemical studies in the central Brooks Range, Alaska, delineate a regional, structurally controlled mineral belt in east—west-trending metamorphic rocks and adjacent metasedimentary rocks. The mineral belt extends eastward from the Ambler River quadrangle to the Chandalar and Philip Smith quadrangles, Alaska, from 147° to 156°W. longitude, a distance of more than 375 km, and spans a width from 67° to 69°N. latitude, a distance of more than 222 km. Within this belt are several occurrences of copper and molybdenum mineralization associated with meta-igneous, metasedimentary, and metavolcanic rocks; the geochemical study delineates target areas for additional occurrences. A total of 4677 stream-sediment and 2286 panned-concentrate samples were collected in the central Brooks Range, Alaska, from 1975 to 1979. The −80 mesh (< 177 μm) stream sediment and the heavy (specific gravity > 2.86) nonmagnetic fraction of the panned concentrates from stream sediment were analyzed by semiquantitative spectrographic methods. Two geochemical suites were recognized in this investigation; a base-metal suite of copper-lead-zinc and a molybdenum suite of molybdenum-tin-tungsten. These suites suggest several types of mineralization within the metamorphic belt. Anomalies in molybdenum with associated Cu and W suggest a potential porphyry molybdenum system associated with meta-igneous rocks. This regional study indicates that areas of metaigneous rocks in the central metamorphic belt are target areas for potential mineralized porphyry systems and that areas of metavolcanic rocks are target areas for potential massive sulfide mineralization.

A Cirque-Glacier Chronology Based on Emergent Lichens and Mosses

AbstractRecession of “Golden Eagle” glacier in the central Brooks Range is exposing undisturbed lichen-covered boulders. Radiocarbon analysis of dead moss surrounding these boulders dates a Neoglacial advance across this site at 1120±180 years b.p Measurements of the preserved lichens indicate that a minimum ice-free period of 1500–2500 years preceded this glacial expansion.

A Cirque-Glacier Chronology Based on Emergent Lichens and Mosses

AbstractRecession of “Golden Eagle” glacier in the central Brooks Range is exposing undisturbed lichen-covered boulders. Radiocarbon analysis of dead moss surrounding these boulders dates a Neoglacial advance across this site at 1120±180 years b.p Measurements of the preserved lichens indicate that a minimum ice-free period of 1500–2500 years preceded this glacial expansion.