Five Minutes of Terror: The Application of Geomorphic Threshold Theory to Deep-Marine Depositional Systems

Lesli J. Wood1

ABSTRACT

Deep-marine depositional systems are by nature chaotic, with their architecture and cyclicity a product of crises–high-volume sediment transport and deposition, followed by periods of little sedimentation. Although similarity in form between submarine systems and fluvial-deltaic systems has led to assumptions of similarity in process, in actuality deep-marine systems most closely resemble subaerial systems under conditions of crisis, pushed beyond a critical threshold of stability. Submarine gravity flows occur when the conditions of existing deposits in the source area pass a critical threshold, forcing a readjustment and sending an episode of gravity sedimentation through the system. The volume of gravity flows, their extent, and their frequency all influence stacking patterns and distribution of subsequent deposits. Sea-level fluctuation can directly affect gravity-flow volumes by basinward- or landward-forced dislocation of major sediment source zones; however, gravity-flow frequency is more likely a function of material stability and frequency of destabilizing tectonic events. Fine-grained systems have high critical thresholds of failure resulting in infrequent, widely dispersed flows. Conversely, coarser grained systems have low critical thresholds of failure resulting in frequent, but confined, flows. Critical thresholds vary among settings and depend on sediment type, slope, tectonic regimes, and conditions of relative sea level. Impulses of increased sedimentation or tectonic events that force one system past its critical threshold to elicit a response may have no effect on an adjacent system that exists in a different state of dynamic equilibrium. Such examples of system divergence do not fit basin-wide models of cause and effect and complicate models for timing of deep-marine sedimentation.

1Bureau of Economic Geology, The University of Texas at Austin, University Station, Box X, Austin, TX 78713-8924; phone: (512) 471-0328, fax: (512) 471-0140; lesli.wood@beg.utexas.edu.