Page 50 - Carbonate Facies in Geologic History

The Effect of Differential Subsidence during Deposition 37

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No subsidence-moderate sedimentation ffll~ ______ ~======~
Subsidence < sedimentatm
off inundated basement block

Subsidence> sedimentation
in basin center
Fig. 11-15. Stratigraphic profiles resulting from variations in rates of subsidence versus rates of
sedimentation

Facies Belts Vary in Width and Uniformity along Such Ramps

A typical facies complex described above usually consists of more or less wide-
spread and uniform sedimentation across the shelves and within basins, and much
narrower belts showing rapid change of contrasting sedimentary facies across the
intervening shelf margin. Shelf and basin sediments commonly extend across tens
or even hundreds of miles, whereas individual facies bands of shelf margins are
only a mile or two wide. Generally, the steeper the slope, the narrower the facies
belts (Figs. II-13 and II-14).

The Effect of Differential Subsidence during Deposition

Modification of the above simple model based on linear subsidence may be
expected if subsidence rate increases toward the basin during deposition. This
may be due either to differential downwarping of the basin or to contempora-
neous faulting. The effect of varying subsidence with sedimentation rate is out-
lined by Meissner (1972, p. 212, see Fig. II-1S).
The most common cases seen in the geologic record are those displaying
progressive outbuilding as carbonate sediments are produced and accumulated
faster than basin subsidence can accomodate them. For example, the well-known
Permian Reef Complex can be seen to have prograded basinward more or less
continuously over several miles during Late Permian time alone. Shelf margins
around the Delaware and Midland basins show a general pattern of several
abrupt basinward marine regressions in step-outs, diagrammed nicely by Van
Siclen (1972, see Fig. 11-16).