Eolian Sequence Stratigraphy—A Conceptual Framework
Gary Kocurek, Karen G. Havholm, 1993. "Eolian Sequence Stratigraphy—A Conceptual Framework", Siliciclastic Sequence Stratigraphy: Recent Developments and Applications, Paul Weimer, Henry Posamentier
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An approach to eolian sequence stratigraphy is to distill geological configurations into fundamental principles that can be quantified. Viewing an eolian system as a control volume, the sediment budget is defined by the flux, in which the influx is a function of the external sediment supply and the transport capacity of the flow, and the outflux is a function of spatial and temporal changes within the control volume. The depositional surface separates sediment in transport from the accumulation. The behavior of the depositional surface over time is described by the sediment conservation equation, and is a function of the downwind sediment transport rate and the concentration of sediment in transport over time, the latter measured largely by average dune height. The depositional surface rises over time, where the sediment budget is positive or influx exceeds outflux, and the balance is stored as the accumulation. A change from a positive sediment budget where an accumulation forms to one that is neutral (influx = outflux) or negative (influx < outflux) results in a bypass or erosional super-bounding surface, respectively. The super surface, therefore, bounds the accumulation, and accumulations and their capping super surfaces are the basic building blocks of eolian sequence stratigraphy. Geological processes that cause accumulation and the formation of super surfaces indicate a spectrum of eolian system types in which the end members are dry, wet, and stabilized systems. In dry systems, the water table has no effect on the substrate and no stabilizing factors occur so that deposition, bypass, and erosion along the substrate are controlled by the aerodynamic configuration alone. Airflow over dunes and interdune flats shows that the flats are at least potentially erosional, so that accumulation in dry systems is unlikely to begin until interdune flats have been eliminated by dune growth. In the rock record, therefore, dry systems are characterized by the absence of interdune flat accumulations. A typical cause of accumulation in dry systems is a downwind deceleration of flow, so that the depositional surface rises because of a spatial decrease in the transport rate. Super surfaces form where flow deceleration ceases for any reason, or the sediment saturation level of the influx decreases to the point where the downstream deceleration is no longer sufficient to maintain the positive sediment budget. The lower boundary of accumulation space is generated by subsidence and the upper boundary is aerodynamically controlled. Wet eolian systems are those where the water table or its capillary fringe intersects the depositional surface, so that deposition, bypass, and erosion along the substrate are controlled both by the aerodynamic configuration and the moisture content of the substrate. Accumulation occurs by a relative rise in the water table so that both dune and interdune-flat strata accumulate, and accumulation can be viewed as the result of a decrease in concentration over time. Super surfaces form when the water table becomes static (bypass surface) or falls (erosional surface). Accumulation space is defined by the position of the water table and the extent of subsidence. Stabilized systems are those in which stabilizing factors, such as vegetation or surface cementation, promote accumulation, either by causing a decrease in the transport rate downwind or reducing the concentration by stabilizing and raising the depositional surface. Super surfaces form whenever the factors causing accumulation wane to the point where a positive sediment budget cannot be maintained, or the stabilizing effects are enhanced to the point the system becomes inactive. Super surfaces can be further classified as to the nature of the substrate as stabilized or unstabilized, and dry or damp. The surfaces evolve so that the final configuration seen in the rock record commonly does not characterize initial or intermediate conditions. Preservation of accumulations and super surfaces occurs with subsidence below the regional baseline of erosion, or incorporation into the saturated zone; a third, less-permanent mechanism of preservation is stabilization of the accumulation by vegetation or other factors. In marine systems, the space for accumulation and the space for preservation coincide as accommodation space, but this need not occur in eolian systems where accumulations can build above preservation space and have little potential for being incorporated into the rock record.
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Applying depositional sequence stratigraphic concepts to the interpretation of siliciclastic depositional systems is becoming an increasingly important tool in petroleum geology. After a succession of breakthroughs during the 1970s and 1980s, sequence stratigraphic concepts now have entered a phase of intense application and documentation, especially with regard to successful implementation in the field of petroleum geology. Workers have applied these concepts to a variety of databases, ranging from outcrop to cores to electric logs and to multifold seismic data. Clearly, sequence stratigraphic concepts embody–not a rigid model or template–but rather a way of looking at geology. This volume has two purposes: to compile some recent applications of siliciclastic sequence stratigraphic concepts, and to present new studies focused on refining conceptual models. This memoir grew out of a 1991 symposium, "Variations in Depositional Systems Within a Sequence Stratigraphic Framework: Applications to Exploration," organized by the authors at the AAPG annual meeting at Dallas. Robert Loucks and Rick Sarg have edited a companion volume, also published by AAPG,“Entitled Carbonate Sequence Stratigraphy: New Developments and Applications.”