AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Implications of Flume Studies for Shallow Marine Mud Deposition and the Stratigraphic Record
(1) Geological Sciences, Indiana University, Bloomington, IN.
Recent flume studies have shown that at flow velocities that transport and deposit sand (20-35 cm/s, 5 cm flow depth), muddy suspensions produce deposition-prone floccules that form migrating floccule ripples. The resulting mud beds appear parallel laminated at first glance, but reveal internal low angle cross-lamination and rib and furrow structure upon closer inspection.
In experiments at marine salinity, conducted with uniform as well as decelerating currents, suspensions of clay and quartz silt mixtures (sediment concentration <10 g/l). produced deposits with low relief coarse silt ripples at the base, followed by low angle inclined silty laminae and an increasing clay component upwards. The clay-rich upper portion of the deposits shows internal laminae and cross-laminae as well.
Storms are an important process for re-mobilization, transport, and re-deposition of muddy sediments in ancient and modern shelf seas. Muddy tempestites have been recognized and studied in modern shelf deposits, and ancient analogs have been identified throughout the rock record. Both ancient and modern muddy tempestites share sedimentary features such as normal grading and basal silt laminae, attributes that have variably been attributed to the action of sediment gravity flows, offshore flowing gradient currents, and a combination of wave induced mud fluidization and density flow.
Whereas fluid mud scenarios imply suspended sediment concentrations in excess of 10 g/l, the low concentration sediment suspensions (<10 g/l) of our experiments produce deposits with equivalent sedimentary features when given sufficient time for advective sediment transport. Graded muddy deposits on modern shelves and in ancient shelf successions thus may be of multiple origins. In addition to being the result of wave-triggered gravity flows, they might for example also reflect seasonal reworking of nearshore muds that are carried offshore by wind-driven circulation, storm induced downwelling of muddy suspensions, or even remobilization of bottom muds by tidal currents. Establishing the boundary conditions for these potential scenarios will require further experimentation.