NOT SO RANDOM: SYSTEMATICS OF MAPPING MÉLANGE (Invited Presentation)

When mapping mélange one must “saturation map” recording every possible outcrop instead of locating points along an extended contact. Whereas one “dot” may be a single outcrop too small to outline at map scale, multiple separate outcrops belonging to the same geomorphic lump may constitute a single block that may be outlined by a contact if large enough relative to map scale. Similarly, the continuity of a geomorphic form and internal coherence can distinguish intact mélange bedrock from landslide deposits. Exposures of landslide deposits reveal material much more disaggregated than mélange bedrock. Geomorphology may be misleading at times for indurated sedimentary mélange matrix with little late deformation: such matrix can form resistant outcrops and hold steep slopes. Bounding contacts of a mélange zone are mapped as ordinary geologic contacts, but in some cases, there is a gradation between the mélange and bounding non-mélange units, making the assignment of a contact difficult. For deformed sedimentary mélanges (possibly the most common variety of mélange) bounding contacts as well as block-matrix contacts may be depositional or reactivated as faults, whereas for tectonic mélanges the bounding contacts and internal contacts are faulted. In any sort of mélange, orientation of bedding or foliation within blocks of mélange is commonly sub-parallel to the foliation in the matrix, owing to the lens to sheetlike shape of many blocks. Matrix types can vary from shale/mudstone to sandstone/pebbly sandstone/conglomerate with sandy matrix being at least as common as shale. One cannot assume that sandstone outcrops are blocks in unseen shale matrix. The clastic sedimentary matrix may be siliciclastic or clastic serpentinite with local gradation between the two. Care must be taken to distinguish mélange matrix from hard, cemented products formed by surficial processes. Examples include carbonate cementation in streambeds (common in areas with serpentinite) and intense soil development (especially with silica cementation).