Grading in mud volcanic breccia from the Mediterranean Ridge

Abstract

An analysis was made of mud breccia sampled from mud diapir fields on the crest of the Mediterranean Ridge to investigate whether they are emplaced by extrusion resulting in debris flows or by intrusion into the hemipelagic sediments. Descriptive sedimentological analyses was combined with the measurements of clast sizes and grading, and the determination of the ratio of clasts to the mud matrix in the upper and lower parts of the mud breccia units in 15 cores taken from the tops and flanks of mud volcanoes. Observed grading in clast size in the upper and/or lower boundary layers of most mud breccias is inferred to be the result of extrusion and flow on the seafloor. Such mud flows may be divided into three sections consisting of graded beds at the top and bottom separated by a massive intermediate part. The best developed grading may result from further reworking of extruded mud breccia through debris flows down the flanks of the mud structures. The composition of the clasts show five lithologies for which the dominant one is pebbles of silty mudstone.