The Carboniferous MTD Complex at La Peña Canyon, Paganzo Basin (San Juan, Argentina)

The La Peña Canyon section (San Juan Province, western Argentina) provides outstanding examples of different varieties of mass-transport deposits (MTDs) and related sediments, showing a wide range of lithologies (from mud to sand dominated), scales and styles of deformation (from imbricate thrusts comprising the entire thickness of the deposit down to meso- and microscale folding), and mechanical responses (ductile, brittle) according to the rheology of the sediment. Specific observations include (i) the amalgamation of MTDs and the recognition of amalgamation surfaces; (ii) the tendency for MTDs to make up a progressively smaller proportion of the succession as the basin fills; (iii) the progressive disaggregation and homogenization of the protolith, from clear boundaries between a muddy matrix and sandy blocks and slabs up to almost complete mixing generating a “holomictite” (nom, nov.), via intermediate stages of peperite-like lithologies; (iv) slow deformation contemporaneous with deposition of overlying sediments (progressive slumping), suggesting emplacement or post-emplacement modification by creep; and (v) the tendency to find clean, structureless gravel at the sole of some larger MTDs, suggesting a mechanism for MTD sliding involving a layer of gravel with overpressured pore fluid, combining models of hydroplaning and linked cavity systems.