Depositional Topography--Examples and Theory

Abstract

Depositional topography which in profile resembles the classic delta is demonstrated with electric well-log cross sections and isopach maps in the late Pennsylvanian and early Permian of western Texas. Distinctive limestone beds can be traced readily over the undaform (top-set beds), and down the clinoform (fore-set slope) where they thin and commonly disappear, or in some places grade into resistive black shale which extends far out onto the fondoform (bottom-set beds). The limestones are thickened in many places along the outer edge of the undaform into organic reefs (and banks) which are termed undaform-edge reefs. These grew when relative sea-level rose, which caused most terrigenous detritus to become trapped in the stream valleys and estuaries. The clay and sand were deposited at sea mostly when its level fell, or after it had remained stable for some time. These two predominant lithologic types--limestone, and shale with sandstone--recur again and again, producing an hierarchy of sedimentary cycles that reflect the sea-level fluctuations. The cycles vary across strike from mere changes in the rate of deposition on the fondoform through more typical cyclothems to presumed changes in the rate of erosion in the source areas. Waves and currents on the undaform sorted the detritus supplied it into dissimilar fractions, a kind of sedimentary differentiation. The coarser fraction, potential reservoir rock, ordinarily remained on the undaform, but at times large amounts were carried onto the fondoform. The finer fraction ordinarily settled in deeper water beyond the edge of the undaform, especially on the clinoform. The younger prominent clino limestones occur successively farther west, thus recording the progressive filling-in of a relatively deep sea by undaform advance. This process, plus sedimentary differentiation, produced a distinctive vertical stratigraphic succession identical with what has been termed the geosynclinal sequence. The essential conditions for this sequence are merely a sea floor well below the depth of wave agitation and a good supply of terrigenous detritus. Similar depositional topography is evident in many other places, including the Mesozoic and Tertiary of Texas and Louisiana. The major undaforms in this region constituted the continental shelves of their time, and the clinoforms the continental slopes. Recognition of depositional topography improves stratigraphic correlations, and the understanding of depositional processes and history. Reservoir beds related to the topography can be mapped and traps sought along them, and the relations of depositional topography to structure may disclose structural features.