Page 57 - Carbonate Facies in Geologic History

44 The Stratigraphy of Carbonate Deposits

Correlation Problems along Edges of Shelf Margins

Usually bed-by-bed correlation becomes difficult along a carbonate shelf margin.
For one thing, the even bedding planes of the shelf facies give way to widely-
spaced irregular "breaks" or to massive unbedded strata characteristic of some
particular organic accumulations at the shelf margins. In addition, considerable
changes of thickness coincide with the more closely-spaced changes in facies at
the shelf margin, making rock-unit correlation particularly difficult. The normal
condition is for limestone strata to thicken gradually to the shelf break and
suddenly thin at the same place where they dip over into the basin. Such steep
margins, where thickness and facies changes are rapid, are hard to evaluate in the
geologic record. This is true even on outcrops where major faults may commonly
occur at the line of the slope break and obscure the significant correlative hori-
zons. For example, despite many years of study, argument still prevails concern-
ing the cause of the great topographic relief on the margin of the Cuesta EI Abra
(outcrop) and the Faja del Oro (Golden Lane) of Mexico. The prevailing view is
that it is depositional, but an interpretation of downfaulting along this trend and
a disbelief in the fact of great depositional relief on such banks is still maintained
by some experienced geologists (Coogan et al., 1972).
In many respects, subsurface data are more convincing than outcrop studies
for correlations along normal depositional topography at shelf margins. Four
techniques are used:
1. Tracing of thin carbonate horizons bas inward : Units can be followed down
the flanks of thickened carbonate masses at the break in the slope. Such key beds
may be seen to "drop" basinward for hundreds of feet and may be traceable well
into the basin itself. They are best recognized on electrical or radioactivity logs as
a series of special patterns or signatures and generally considered to represent
"time lines" or depositional surfaces. Fig. 11-22 is an excellent demonstration of
the use of such markers in the subsurface correlation across a Late Paleozoic shelf
margin. Here middle Wolfcampian beds show a 3 degree depositional slope.

2. Sandstone-siltstone correlations: Where slopes are very steep, bed by bed
correlation may not be possible but another technique is usable if cyclic and
reciprocal sedimentation has occurred. Sandstone beds on the shelf may have
correlatives in basinal sandstones even though the two series are disconnected
across the shelf. At lower sea level stands, sand may migrate as regressive sheets
across the shelf and spill over into the basin, either bypassing the exposed carbon-
ate edge in channels or being windblown. Comparative sections with multiple
sandstone may be used to effect good correlations. For a good diagram of this see
Meissner (1972, p.218, Fig. 11 and Plate II) where the Bell Canyon sands are
correlated with 6 sands of the Yates Formation shelf across a depositional topo-
graphic relief of 400 m in the Permian Reef Complex. See also the Bolliger and
Burri (1970) correlation across the Swiss Jura reef front.
3. Paleontologic zonation: Whenever possible paleontology is critical to the
certification of such correlations. It is significant that large amounts of topo-
graphic relief at the edges of subsurface basins were first recognized in the Late