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Icing Mound on Sadlerochit River, Alaska

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Icing Mound on Sadlerochit River, Alaska Short Papers and Notes ICING MOUND ON SADLERO- Sadlerochithas emerged from the CHIT RIVER, ALASKA* mountains, has arelatively low gra- dient and flows in a broad, braided bed Icing mounds- small to large domes, characterized by manyanastomosing mounds,and ridges resulting from an shallowchannels separated by bare, upwardarching of soiland ice asso- gravelly and bouldery bars (Fig. 2). AS ciated with fields of aufeis - have been is characteristic of all riversof the Arc- described in detailfor Siberia1. Icing tic Slope, the change from a single deep mounds have also been mentioned brief- channel to a braided pattern of many ly inconnection with aufeisfields in shallow channels allows the formation Alaskaby Leffigwell (ref. 2, p. 158) of an aufeis field every year near the and Taber (ref. 3, p. 1528; ref. 4, p. 249), mountain front. During the fall the but there is little descriptive literature shallow channels freeze early and the on this phenomenon in theNorth Amer- obstruction of the resulting ice causes ican Arctic. One such icing mound was the river to overflow its bars;this over- examined briefly by the author on June flow then freezes and by repeated freez- 25,1959 during the course of a trip down ing,overflow and freezingsuccessive the SadlerochitRiver in northeastern layers of ice are built up toform an Alaska (Fig. 1). aufeis field. Another factor contributing Fig. 1. Locationmap, Arctic Slope, Alaska. The SadlerochitRiver rises in the to the formation of large aufeis fields is Franklin Mountains of the eastern a source of water that persists for some Brooks Range and flows northwardin a time after freezingbegins. Because a relatively broad, shallow valleythrough major tributary of theriver has its foothills and coastal plain to the Arctic source in large, deep lakes (Lake Peters Ocean.At approximately 69"50'N. the and Schrader Lake) and other tribu- taries maybe spring fed, the Sadle- * Publication authorized by the Director, rochit River continues to flow longafter US. Geological Survey. freezing begins each fall. 145 146 PAPERSSHORT AND NOTES In late June 1959 patches of ice as ter;the low terrace is rarely flooded much as 8 feet thick and low ice-shoved and is generally free of aufeis. The wide ridgesremained from the extensive gravelly highterrace thatflanks the low aufeis fieldthat had filled the active bed terrace on the west has to the south a .".::::<: Flood-plain alluvium Escarpment rj..::::: - @ Low-terrace alluvium 0 Icing mound High-terrace deposits Area shown in Fig. 3 Glacial-outwash deposits Scale of Feet Fig. 2. Surficial geology of Sadlerochit River valley in vicinity of icing mound. of the river during the preceding win- separate aufeis field built by a spring- ter. A channel-scarred low terrace that fed tributary, which parallelsthe Sadle- lies several feet above the river bars rochit River. This high terrace stands consists of gravelly and bouldery allu- about 15 feet above the river level. The vium with a veneer of organic silt and Sadlerochitvalley is bordered on the hesand. The flood-plain alluvium is east by a 30-foot escarpment marking covered by water during normal high- the edge of a highbouldery terrace, water stages and by aufeis in the win- which is probablyformed of glacial SHORT PAPERS AND NOTES 147 Fig. 3. Field sketch of icingmound. outwash deposits.The entire area is well the mound were in places still covered within the zone of continuous perma- with patches of surface ice. frost; depth of seasonalthaw ranges The exposed ground ice was generally from about 18 to 24 inchesnear the a very clear, white, sharply folded tab- coast to about 3 feet in the foothills. ular massapproximately 4 feet thick The icing mound occurred at the in- (Fig. 5). It appeared to be of uniform ner edge of the low terrace adjacent to thickness althoughthere may have been the aufeis field alongthe east side of the a slightthickening near the apex of river (Fig. 3). It consisted of a sinuous the fold.The ice was characterized ridge about 250 feet long,which ter- by a well-developed vertical col~mnar minated in a lowdome about 20 feet structure, whichwas accentuated by high at the north end and in a lower melting. The ice was overlain by 2 feet pointed “tail” at the south end (Fig. 4). of dark-grey, highlyorganic silt and A partof the “tail”, which was immedi- fine sand. The contact between the ice ately adjacentto a then active river andsilt was minutely irregular; the channel, had collapsed and revealed an bottom surface of the ice was generally underlying layer of ice. The ridge was smooth. The exposed faceof the ice was broken by several shallow cracks along mostly clean and there had been no ap- the crest; elsewhere the slopes were preciable slumping of the silt. At this smooth. The vegetation on the mound date the ground was still frozen about as well as on the flood plain was pre- 6 inches below the surface. Underneath dominantlycottongrass. The flanks of the ice was a flat-floored vaulted cavity 148 SHORT PAPERSNOTES AND 10 to 12 feet high; the floor of the cavity The occurrence of pingos and other was covered by a shallow pool below ice moundshas long been notedin arctic which there appeared to be silt and Alaska and their localization in areas gravel. The pool was being fed by seep- of relatively deep thaw, suchas recently age from the river as well as by melting drained lake basins and deltas, is well of the ice.The apparent freshness of established. The mechanics of the for- the mound and cracks, andthe fact that mation of such mounds, however, have last season’s(1958) cottongrass was been subject to divers views. Leffing- normal to the surface of the mound well (ref. 2, pp. 150-5) and later writers rather than vertical seemed to indicate have ascribed the growth of ice mounds that the featurewas formed during the in northern Alaska to hydrostatic pres- preceding fall or winter. Several other sure operating at frostdams; Porsild lowmounds, seen only at adistance, (ref. 5, p.55) in reviewingsimilar occurred in the vicinity of the aufeis mounds suggested that the formation of fieldand were assumed to besimilar manypingos is caused by expansion to the one examined. during freezing of localized ground wa- Fig. 4. View south, showing sinuous form of icing mound. Brooks Range in background. Origin ter belowdrained lake beds; Sharp The uplift of the icing mound is con- (ref. 6, p. 421) believed that the forma- sidered to be related to the formation tion of some small Alaskan ice mounds of groundice. That the groundice was due to the growth of groundice representsburied lake or riverice is fed by seepagein theactive zone; Sum- discountedbecause (1) the minutely gin (quoted in ref. 1, pp. 111-5) attrib- irregular upper surface of the ice indi- uted the heaving of icing mounds both cated a growing upward of ice crystals to forcesproduced by the growth of into the overlying silt and (2) the ground ice and to hydrostatic pressures; banding that is such a prominent fea- Taber (ref. 3, p.1528) held that most ture of aufeiswas lacking inthis ice-mounduplift was caused by the ground ice. force of crystallization in ground-ice SHORT PAPERSNOTES AND 149 layers.More recently, Müller (ref. 7, zoneproceeded downward ground ice pp. 56-70, 97-101) distinguishedtwo grew in the organic silt.That the ground major mechanisms that produce pingos: ice was localized was indicated by the oneinvolving primarily hydrostatic absence of any depression around the pressure in an open system, as in East uplifted area and by a general lack of Greenlandpingos; and asecond in- thermokarst features and patterned volving crystallization pressures accom- ground in the low terrace. Localization panying an advance of permafrost into may have been caused by several fac- intra-permafrostground water in a tors: greater permeabiIity of the gravel closedsystem below lake beds, as in in buried meander channels may have the Mackenzie Delta. Although Müller allowed a flow of ground water suffi- restricts the term “pingo” to mounds cient to keep the channelunfrozen thatare caused by forcesoperating longer than the surrounding area; fur- within permafrost in contrast to icing thermore, the merging of the frozen mounds that are formed entirely within activezone and permafrostmay have the active zone, similar forcesare prob- occured sooner in the area adjacent to ably involved in both. The author be- the moundbecause of adifference in lieves that the Sadlerochit icing mound degree of heat conductivitybetween Fig. 5. Folded ground-ice lens revealed by collapse of part of icing mound. Fracture at apex is being enlarged by melting. showedevidence that upliftinvolved these areas. Growth of the ground ice both cryostatic pressures accompanying continued as long as water was avail- growth of a ground-ice lens and later able or until the water-bearing gravels hydrostaticpressures accompanying a werecompletely frozen. restriction of ground-water flow. Growth of the ground ice proceeded During icing of the Sadlerochit River bycontinuous growth of icecrystals in the preceding fall, overflow from the fedby water frombelow. That the river seeped through permeable alluvial growth of the ice was not bythe forma- gravel of a meander channel buried by tion of successive ice layers or wedges silt. This seepage providedan abundant was shown by the lack of banding in local source of water for the growth of the ice. During the initial stage, forces groundice. As freezing inthe active accompanying the formation of ice- 150 NOTES AND PAPERS SHORT either the force of crystallization or the more, a slight rise in river level would force of expansion-caused doming of allow water to flow through the cavity the ice lens and overlying silt.
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