Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
Invited Presentation, Rutgers University, Dept. of Geology, February 26, 2003
of Sediment Deposition and Salt Tectonics on
Bruno C. Vendeville
Rock salt's rheological properties make it a weak material that typically responds rapidly to loading by even small volumes of sediments. In deep-sea fans (e.g., the U.S. Gulf of Mexico, West Africa, eastern Canada, Brazil, and the Mediterranean), progradation of large sediment wedges has created a regional bathymetric slope causing differential loading of the underlying salt. The bathymetric slope represent a gravity instability that triggers seaward spreading the whole sediment wedge, this spread being accommodated by distal contraction, midslope translation, and proximal extension. Extension allows salt to rise reactively as diapiric ridges, then passively as circular diapirs. During passive growth, interaction at local scale between salt flow and sedimentation continuously molds the diapir's flanks and thus determines the diapir's final geometry: rapid aggradation leads to steeply flanked diapirs, whereas slow aggradation favors the formation of upward-widening, extrusive salt bodies (overhangs, tongues, and nappes).
At regional scale, the location, shape, and timing of regional depocenters control salt tectonics. Deposition of large, subcircular sediment lobes triggers radial gravity spreading, which is accommodated by the formation of a network of annular grabens underlain by salt ridges and polygonal fault blocks that eventually evolve into circular depocenters (minibasins). When the location of regional depocenters shifts, the locus of salt tectonics also shifts.
On the other hand, salt movement also affects depositional patterns at the local scale. Salt movement creates bathymetric lows that preferentially trap sediments, while the crests of rising diapirs form bathymetric highs block or that divert sediment pathways. In shallow waters, rising salt domes can be capped by carbonate lentils that develop only above the diapir's crest. Analysis of data from regions of currently active salt tectonics (e.g., the Mediterranean) illustrates how bathymetric scarps and local slopes generated by salt movement also control the pattern of clastic rocks. Steep, local slopes caused by normal faulting or buckle folding are characterized by deposition of intercalated turbidites (gentle bathymetric relief) and local debris flows (collapse of bathymetric scarps). Detached grabens caused by thin-skinned extension above salt also influence depositional patterns. Slope-parallel grabens act as depositional drains that channel turbiditic flows downslope, whereas slope-perpendicular grabens can intersect channels, interrupting such flows.