Rib-and-trough structures in a tundra lake

I have modified the original slightly and have removed figure captions and references —William W. Shilts

Permafrost features under arctic lakes
"Rib-and-trough patterns are by far the most common shallow-water features in lakes in southeastern Keewatin. This pattern forms a fringe of varying width along much of the shoreline and around many of the islands of most lakes. The rib-and-trough pattern consists of a series of 2- to 3-m wide, cobble and boulder-covered ridges that stand 0.5 to 1 m above intervening 2- to 3-m wide troughs. Troughs are commonly filled with boulders larger than 50 cm in diameter. Nearshore, the ribs usually trend at right angles to the shoreline or directly down the steepest slope of the lake bottom. They occur at regular intervals with spacings from ridge crest to ridge crest of 4 to 6 m. Offshore, the ribs may bifurcate, turn obliquely or parallel to the slope, or pass into individual, cobble-covered lumps surrounded by boulder-filled depressions. Where ribs have been subaerially exposed by a permanent drop in lake level, they appear to have evolved into mud boils or sorted circles.

Terrain adjacent to rib-and-trough patterns is usually a till plain. The patterns are rarely observed adjacent to the sandy, gravelly sediment of eskers or raised nearshore marine sediments. They are also uncommon offshore from terrain containing ice wedge polygons. Where excavated, both ribs and troughs were underlai largely by till, or by till modified by solifluction or wave action. The ribs are overlain by a one-clast-thick mantle of cobbles, and troughs are filled with a similar mantle of boulders. The cobbles on the ribs appear to be pressed into the underlying sediment. Nearshore, where ribs are submerged to depths of less than 1 m, or where they are exposed during low water stages, the till is covered by 40 to 50 cm of well sorted, gray, medium- to fine-grained sand, which is in turn overlain by cobbles or small boulders. At two sites, this sand covering was observed to be pierced by diapiric structures extending upward from the underlying red till. The sand appears to pinch out offshore so that portions of ribs lying more than about 1 m below the water surface consist of till directly covered by cobbles. Small, fresh-appearing mud boils were noted on the offshore portion of ribs. In some parts of Kaminak Lake, boulders in the troughs are partially or completely submerged by lacustrine silt or sand.

Excavations and probing to permafrost at several nearshore locations revealed that the frost table was undulating. Where probed, the frost table had a relief of 10 to 20 cm, rising slightly beneath troughs and being slightly depressed beneath ribs. Permafrost depths at the water's edge ranged from 60 to 140 cm below the rib-and-trough surface in early August 1973.

Rib-and-trough patterns have not been observed along portions of shoreline where the lake bottom has a steep slope. Although there seems to be a critical bottom slope beyond which these features do not form, no slope measurements have been made. The patterns have not been observed in the marine or estuarine environments of the present coast of Hudson Bay even though several tens of miles of marine shoreline were examined on low-altitude aerial photographs."

Rib-and-trough patterns are similar in many respects to mud boils or sorted circles, which usually occur on adjacent terrain. Both types of features are composed of till or other poorly sorted silty sediment with low liquid limit and low plasticity index; both are underlain by an undulating frost table; diapiric structures have been observed in both. The rib-and-trough structure sometimes assumes the form of a sorted circle, a feature thought by the authors to be a form of mud boil. Because of these similarities and the common occurrence of the features side by side, we have inferred that the genesis of the rib-and-trough pattern must be similar in at least some respects to genesis of mud boils.

Build-up of cryostatic pressures during freeze-up is thought to cause sub-lacustrine diapiric extrusion similar to that described for mud boils. When lake ice forms and freezes to the bottom, the frost table descends through the rib-and-trough structure, contacting the highest parts of the permafrost table (under the trough) first and creating an unfrozen cell in the core of the rib. As the freezing front advances from the top, sides, and base of the ribs, water is driven in front of it into the unfrozen core. This increase in porewater pressure liquefies sediment in the core and builds up cryostatic pressures that cause the sediment to move toward areas where pressure can be released. The most likely place for pressure release, presumably accompanied by extrusion of the liquefied sediment, would be just lakeward or downslope from the contact of the lake ice with the bottom. As lake ice thickens, this contact moves downslope, and points of extrusion move with it until ice attains its maximum thickness of about 2 m. Thus, the rib is a favorable site for cyclic sediment extrusion, which gives it its positive relief. The sediment that is extruded is replaced by similar sediment that enters the lake by solifluction from the active layer on shore.

The original irregularities in the sublacustrine permafrost table may have been caused by tongues of sediment entering the lake from mud boils, by varying thicknesses of lake ice caused by varying thicknesses of snow cover or drifting, or by flooding of a surface already ornamented by mud boils with their underlying, irregular permafrost table already developed. Thus, the regular rib spacing may be a function of the average spacing between mud boil centers, 4 to 6 m. Any such irregularities that occur at the shoreline at its freeze-up position would be perpetuated lakeward as rib formation progressed. An irregular permafrost table at the shore should lead to the development of ribs, which extend themselves lakeward to water depths equivalent to the average annual ice thickness. Since ribs require a shallow frozen (or impermeable) substrate for their formation, sublacustrine areas covered by the rib-and-trough patterns are assumed to be underlain by permafrost at shallow depth.

Updated 08/28/2009 CAB