TEXTURAL FEATURES AND FIELD CONTEXT TO EVALUATE SEDIMENTARY VERSUS TECTONIC MIXING OF CLASTS AND BLOCKS IN DEFORMED MÉLANGES

Sedimentary mixing of clasts and blocks into block-in-matrix mélanges is easily recognized in units that show little post-mixing deformation, but primary sedimentary textures become increasingly difficult to discern with increasing strain and recrystallization. Undisputed clastic sedimentary rocks, such as sandstones (for petrographic scales) and conglomerates (for macroscopic scales), suggest some textural relationships that are unique to sedimentary mixing. Pressure solution geometry (fitted convex and concave corresponding surfaces) along surfaces at high angles to the matrix foliation/cleavage is one such relationship, but this requires a clast/block-to-clast/block contact, so it is most commonly seen at petrographic scales in sandy matrix. A more general relationships include abrupt (no drag) truncation of internal bedding or foliation of a block/clast against boundaries at high angles to matrix foliation and somewhat similar of rounded grain/clast/block boundaries at high angles to the matrix foliation adjacent to little-deformed material (no evidence of tectonic rounding). Similar relationships characterize the contacts of internal mineral cleavage of detrital mineral grains with their detrital grain boundaries. In strongly deformed sedimentary mélanges, the above textural relationships may be obscured by tectonic strain over most of the rock volume, but the sedimentary relationships are preserved in strain shadows of clasts/blocks or in lensoidal domains). Because large-scale (multimeter and greater) high-quality exposures of block-matrix boundaries are rare, the sedimentary textures are commonly best observed at petrographic scale. The clast population of matrix mimics the block population of the host mélange. Basal sedimentary contacts of deformed sedimentary mélanges are rarely observed, especially since large-scale reaches of such contacts are rarely exposed; this is similar to the block-matrix problem. Because of rheologic contrasts between blocks and matrix as well as mélange horizons and flanking non-block-in-matrix units, much of the length of original sedimentary mélange contacts and block perimeters are faulted, but there is local preservation of the original sedimentary contacts where there are irregularities (reentrants) along the contacts.