Depositional Topography--Sedimentation Model for Explorationists: ABSTRACT

Abstract

The term refers to large-scale topographic irregularities formed by the processes of deposition in and beside a body of standing water whose bottom extended below the depth of significant wind-driven wave motion. This specifically excludes compaction features and terrestrial landforms. Modern examples include river deltas, barrier reef-lagoon complexes, and broad continental embankments. These have three principal elements: (1) a relatively horizontal part close to sea level (shelf or undaform), (2) a more steeply sloping part (slope or clinoform), and (3) a flatter deep-water part (or fondoform). Ancient examples of depositional topography are extensively developed in the Pennsylvanian and Permian rocks of western Texas, where frequent fluctuations in relative sea level produced many cyclic intercalations of carbonate and clastic sediments End_Page 422------------------------------ with varied paleotopographic forms. The cyclicity was generated largely by eustatic changes in sea level superposed on slow, continuous elevation of the source areas and corresponding subsidence of the depositional areas. The distribution of lithologies was controlled primarily by the topography, of depositional origin, which determined the sites of higher energy expenditure and therefore coarser sediment accumulation, and vice versa. These sites were shifted significantly by marked changes in sea level, the secondary control of sediment distribution. Conventional direct control of was not significant. Under these circumstances the appropriate sedimentation model is one based on the inductive concept of depositional topography, not tectonic control. This model links the sedimentary processes in fluvial, marine-shelf, and deep-water environments through the effects of the energy of waves, currents, and organisms on the sediments. The model is particularly appropriate for stratigraphic exploration because, where conditions are favorable, the depositional topography can be mapped as a series of continuous paleotopographic surfaces to which the discontinuous pattern of reservoir lithologies can be related in ways that are useful for understanding and prediction. End_of_Article - Last_Page 423------------