EARLY POST-DEPOSITIONAL BEDDING-PLANE-PARALLEL MELANGES CREATED BY SHEAR AND LIQUEFACTION: A COMMON BUT LARGELY MISINTERPRETED ORGANIC-RICH MUDROCK FACIES

Many organic-rich mudstone sequences contain cm- to m-scale beds of cryptic mélange consisting of small (usually <20 cm) quadrangular to rounded or folded clasts with truncated oblique-to-bedding laminae floating in a poorly-bedded matrix. Clasts at high angles to bedding have thickened and crinkled laminae indicating deformation preceded full compaction. There is often an upward progression from partly organized masses of clasts with little matrix to isolated clasts in mostly matrix, with less common similar lateral transitions. Parent material can be below and/or above the mélange with downwardly hanging pendants or upwardly projecting “flames”. The massive mudstone matrix suggests partial liquefaction accompanied the bedding-parallel shear producing clasts with both brittle and ductile features. Some beds even resemble augen gneiss with winged porphyroclasts! In 1989, PEO half-jokingly termed the clasts “dead horses” (derived from "horse" for a fault-bounded sliver of rock and its flattened or prone position), hoping in vain to provoke interest.

When noted at all, "dead horse" mélanges have been interpreted as depositional units such as turbidites, rip-up clasts indicating subaerial exposure, slumps, or seismites, but we interpret them as “early” shear and dewatering-related units that did not have a free surface at the time of deformation. These are very common; in lacustrine sequences of the Triassic-Jurassic of the eastern US, nearly every sedimentary cycle with a dark gray to black mudstone has such a layer, amounting to hundreds of beds. Similar beds are abundant in the Eocene Green River Fm and are found in many other organic-rich, laminated mudstone sequences.

Assuming these mélanges are depositional leads to very serious mistakes in environmental interpretation. Moreover, because they formed post-depositionally, between pre-existing beds, and their formation was controlled by a combination of specific rheology at unknown depths and pressures (with or without specific triggers) each bed cannot be treated as resulting from a specific event, and a stratigraphy of sequential beds cannot be interpreted as a history of events. Given their virtual lack of recognition, the effects of these mélanges on diagenesis or fluid and gas generation and migration are wholly unknown.