Alastair H.F. Robertson2 and Achim Kopf3


Petrographic and mineralogical studies of clasts and matrix in the Milano and Napoli mud volcanoes drilled during Ocean Drilling Program Leg 160 provide important clues about depositional processes, provenance, and the tectonic setting of deep-sea sediments that accumulated before genesis of the Mediterranean Ridge mud volcanoes.

The clasts recovered from both mud volcanoes are mainly mudstone and claystone, calcareous siltstone, quartzose sandstone and siltstone, shallow-water–derived limestone, and pelagic carbonate. Biostratigraphic evidence indicates mainly early and middle Miocene ages for fossiliferous clasts, with the older microfossils being reworked. Textural evidence (grading) suggests that the clastic lithologies are turbidites. Sources of the sandstones were mainly plutonic igneous and subordinate metamorphic rocks. Sporadic, exceptionally well-rounded, quartz grains may be of eolian origin and were probably derived from the Precambrian basement of North Africa. Shallow-water–derived carbonates were also redeposited as calciturbidites, with variable admixing with siliciclastic and pelagic carbonate, which indicates shared depositional pathways. Pelagic carbonates (packstones) were redeposited basinward as calciturbidites from a deep-sea carbonate slope that formed part of the North African continental margin. During the middle Miocene, the background sedimentation was in situ pelagic carbonate. In addition, relatively rare, texturally immature, lithic sandstones include serpentinite, basalt, and radiolarian chert, of ophiolite-related origin. This material was derived from the orogenic areas to the north, possibly originally from the higher thrust sheets of Crete, before late Miocene and subsequent erosion and extensional downfaulting.

The texture and composition of the mud and clay matrix provide clues about the mode of the mud volcanism. The matrix of the mud debris flows includes numerous, small, angular clasts of unfossiliferous claystone and shardlike fragments, which show pseudolamination, microshearing, and crosscutting veinlets, features that are taken as evidence for the involvement of high fluid pressure in clast and matrix formation (hyrofracturing).

1Robertson, A.H.F., Emeis, K.-C., Richter, C., and Camerlenghi, A. (Eds.), 1998. Proc. ODP, Sci. Results, 160: College Station, TX (Ocean Drilling Program).
2University of Edinburgh, Department of Geology and Geophysics, West Mains Road, Edinburgh, EH9 3JW, United Kingdom. Alastair.Robertson@glg.ed.ac.uk
3Geologisches Institut, Albert-Ludwigs-Universitšt Freiburg, Albertstrasse 23B 79104 Freiburg, Federal Republic of Germany.