LINEAR AND TEAR-DROP ORNAMENTATION DURING LOW TEMPERATURE RHEOLOGICAL DEFORMATION AT BOUNDARY LAYERS IN ROCK AND SEDIMENT: A SMALL ANALOGY WITH THE FORMATION OF SOME GLACIAL LANDFORMS

Linear grooves, striae, or ridges, on bedding planes or striated and polished fault surfaces are common in metamorphic terrains. The linear forms are generally considered to be stretching lineaments, attributed to flexural slip of semi-competent layers relative to each other, and produced by surface asperities. Lineament orientation occurs parallel to the direction of layer stretching with orientations oblique to the axial planes of non-cylindrical folds.

Slickensides, grooves, ridges and small blunt-nosed, tear-drop ridges, have been found on fault surfaces of weak metamorphic bedrock at Heath Steel Mines, New Brunswick. The structures were a result of friction on fault surfaces during glacier overridding and unloading.

Grooves, ridges and tear-drop-shaped structures found on bedding planes of lithified sediments are commonly attributed to current scouring, although post-depositional rheoplasis has been cited as a possible origin for some examples of soft-sediment deformation of units. However, large isoclinal folds and complex curvilinear folds are ubiquitous diapiric structures in glaciolacustrine sediments along river valleys south of Prince George, British Columbia. These structures are tight isoclinal folds that extend vertically through up to 25m of section and are associated with smaller sheath-type and curvilinear-hinged folds. Following deposition, locally elevated pore water pressures resulting from rapid sedimentation and texturally heterogeneous layering facilitated plastic deformation and intrastratal flowage. Flexural slip, flowage and fluid expulsion along coarse-grained layers, resulted in boundary scour and the formation of grooves, ridges, and tear-drop ridges on surfaces of confining layers comprising the diapiric structures. Analysis of facies relationships and the geometry of the structures indicate that diapirism resulted from subaqueous slumping along the margin of a glacier-dammed lake during rapid drainage. The wide-scale slumping produced an identifiable, stratigraphically delimited, deformation unit.

These low-temperature rheoplastic and friction-generated micro structures are analogous with much larger streamline forms (e.g. ridge, groove, drumlin, crag and tail) and probably share similar mechanics of formation.