Water escape fissures resembling ice-wedge casts in Late Quaternay subaquesous outwash near St. Lazare Québec, Canada

Post-depositional structures in late Quaternary subaqueous outwash near St. Lazare, Québec resemble icewedge casts but are interpreted as water escape fissures. Cryogenic origin is discounted because, in contrast with ice-wedge casts, the fissures have a higher depth to width ratio, do not form an intersecting network, and do not exhibit adjacent upward turning of strata. In addition, their truncation by the sub-littoral unconformitydemonstrates formation before regression of the post-glacial Champlain Sea, under conditions in which ground ice development was highly unlikely. The fissures probably formed in response to elevated pore pressures caused by melting of remnant glacier ice or by liquefaction of deeper units. Excess pore water pressure initiated upward flow of dilute sediment-water mixtures that became concentrated in planar zones (fissures) along which they entrained and removed sediment. Slumping of the fissure walls followed, either during or after water escape. Minor faulting over cavities created by melting ice or water escape along fractures in underlying strata may have controlled the morphology of the fissures.