Page 374 - Carbonate Facies in Geologic History

Type I. Downslope Mud Accumulations 361

Three Major Types of Shelf Margin Profiles

The numerous examples of shelf margin profiles presented in Chapter IV through
XI may be grouped into three classes (Wilson, 1974): I-Downslope carbonate
mud accumulations, II-Ramps of knoll reefs, and III-Framebuilt reef rims
(Fig. XII-3). The classification is based on the slope of the stratigraphic profile and
the composition, shape, and disposition of subunits making up the marginal
carbonate mass (e.g., mud mound cores, frontal knoll reefs, reef flats, flanking
beds, patch reefs, capping beds, lime sand bars, eolianite dunes, aprons of slope
sediments and talus conglomerates). The above grouping is empirical and its
rationale is incomplete but the recognition of certain types of shelf margins can be
a valuable tool for both outcrop mapping and subsurface petroleum exploration.
It may aid in the prediction of the steepness of the slopes off the shelves and in
understanding the character of the sediment expected along them.

Type I. Downslope Mud Accumulations

These are linear trends of bioclastic lime mud or belts of mounds located on the
foreslope of the shelf margin with upslope sand beaches and islands. The down-
slope sediment contains variable amounts of somewhat limited and specialized
sessile organisms which tend to trap and baffle sediment. Thus, the downslope
muds may occur as superimposed (stacked-up) mounds, usually loaf-shaped bod-
ies although such forms are not always observable. Locally, the slopes of such
accumulations may be steep-from 25 to 30 degrees, about the angle of repose of
constituent sediment, but the overall slope into the basin may vary from a degree
or two, to as much as 25 degrees. This variation may be tectonically controlled
because organic construction is not a factor in such buildups.
On steep slopes the carbonate mud may accumulate far enough downslope to
be below the photic zone, and perhaps in water a hundred or so meters deep,
although the sediment is derived from much shallower water. In such cases where
clay is transported into the basin, well-defined mounds are separated by dark
shales.
Where gentle foreslopes prevail, carbonate mud mounds will still develop but
in shallow, photic zone water. The tops of such individual mounds may reach into
the zone of wave action. When this occurs, organic growth on the tops may
develop a reef framework and grade into Type II or III. But, even where this is
observed, the uppermost part of the slope, which is generally well within the
turbulent zone, is composed essentially of lime sand shoals, beaches, dunes, and
islands, and normally contains few or no framebuilding and sediment trapping
organisms. Examples are:
1. Capitan Formation of the Permian Reef Complex of the Guadalupe Mountains of
New Mexico and West Texas (Achauer, 1969; Dunham, 1972).
2. Phylloid algal shelf-margin buildups of Pennsylvanian and Early Permian age in the
southwestern U.S.A., (Plumely and Graves, 1953; Wray, 1962; Wilson, 1967a; Heckel and
Cocke, 1969). Capped in places by tubular foraminiferal-Thbiphytes boundstone creating
Type II.