Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
BEG Seminar, January 24, 2003
Volcanoes, Shale Diapirs, and Fluid Systems:
Lesli J. Wood
Mud volcanoes are explosive "mud mounds" that sometimes reach several tens of kilometers in diameter and can be hundreds of meters high. They are found principally in convergent-margin settings around the world. These settings are most commonly areas of rapid sedimentation, lateral tectonics, or geologically recent magmatic activity. Although thousands of mud volcanoes have been documented in both subaerial and submarine settings, it is likely that hundreds of thousands exist. In the larger picture, mud volcanoes are only a single component; they are the pressure release valves, if you will, of the larger systems that lie at depth: mobile shale basins. Mobile shale basins occur throughout the world. Some examples are the North Sea, the Caspian Sea, offshore Morocco, Niger Delta, Mackenzie Delta, the Barbados Accretionary Prism, offshore Trinidad, New Zealand, Borneo, Venezuela, onshore Texas, and the Gulf of Mexico. Mobile shale basins may be characterized by buried diapirs, shale walls, and deep mobile shale bodies that exhibit no explosive nature.
Mobile shales have their own internal means of propulsion. When shale burial depths reach certain diagenetic transition zones, such as the montmorillonite to illite transition limit (approximately 100 degrees C) the result is expulsion of lattice-locked water and rise of fluid pressure in the shales. Fluid increase can also happen due to thermal expansion of pore fluids or hydrogen gas generation during subsidence. Active tectonic movement can not only destabilize fluidized shales but can also compress deep shales with an explosive result similar to squeezing a mud pie in both hands. These systems, unlike salt, can go through long periods of dormancy while their fluids "recharge" until another phase of migration occurs.
Mud volcanoes and shale diapirs are erratic phenomena that influence fluvial, coastal, and marine sedimentation in complex ways. Shale mobility rise or subsidence can cause changes in base level, and uplifts can alter river courses and affect upstream and downstream sediment types, avulsion histories, channel pattern, and channel locations. Deltaic systems prograde around uplifted diapirs and volcanoes and often dump significant sediment loads due to these local changes in accommodation space and base level. Volcanoes and diapir activities in the offshore environment likely have a similar effect on shelfedge, slope, and deep marine depositional systems, although the effect of active tectonics on these marine systems is poorly understood. Mud volcano fields are often associated with gas hydrate development in deep marine environments. The combination of eruptive seafloor volcanoes and easily destabilized frozen methane (gas hydrate) in the shallow seafloor create the potential for explosive submarine landslides, methane release into the oceans and atmosphere, tsunamis, and both offshore and onshore infrastructure destruction.