Chaotic deposits are frequently reported in the geological literature
and are commonly interpreted as olistostromes or tectonic melanges. A
chaotic complex in the Cenozoic succession of Monferrato (NW Italy)
consists of interbedded mud breccia and burrowed silty clays that are
pierced by sheared mud breccias and embed carbonate-cemented blocks.
These may be represented by microcrystalline limestones or strongly
cemented matrix-supported breccias locally containing remains of
chemosymbiotic organisms (lucinid bivalves). Moreover, cylindrical
concretions, up to 15 cm in diameter and 1 m long, occur in the chaotic
complex and crosscut bedding planes at high angles. The cement of all
these lithified portions is mainly dolomite characterized by low
delta(13)C values (from -10.3 to -23parts per thousand PDB) and
delta(18)O values up to + 7parts per thousand PDB. The delta(13)C values
testify to precipitation of carbonates induced by microbial oxidation of
methane, whereas the markedly positive delta(18)C signature, ubiquitous
in the cylindrical concretions, is the evidence for the presence and
destabilization of gas hydrates.
The studied section provides a well-exposed example of the geological
record of the birth, life, and death of a mud volcano. Unsheared, soft
mud breccias represent mud flows along the flanks of the volcano,
whereas sheared mud breccias are the result of the injection of
unconsolidated overpressured fine-grained sediments, both taking place
during ``eruptive'' phases. They were followed by more quiet stages of
hemipelagic sedimentation, burrowing, and CH4 seeping. The cylindrical
concretions represent the first described ancient example of the
chimneys observed in present-day mud-volcano settings. They are the
remnants of a cold-seep plumbing network that crosscut the mud volcano
edifice. The chimneys were the pathway for the expulsion toward the sea
floor of gas- and sediment-charged fluids likely originated from
destabilization of methane gas hydrates. The association of mud breccias
and methane-derived carbonates may not be due to mass gravity flows but
can be primary and, therefore, is a diagnostic criterion for recognizing
chaotic deposits due to mud volcano activity in the geological record.