Page 53 - Carbonate Facies in Geologic History

40 The Stratigraphy of Carbonate Deposits

A
Fig.I1-18. Two common isopach patterns for carbonate time-stratigraphic units. Pattern B
forms with development of a carbonate platform around the basin and rapid rate of subsi-
dence of the basin center. Gradual and slow subsidence may result in detrital infilling of the
basin and the development of the pattern A .

The Interpretation of Thickness Maps in Carbonate Strata

Isopach maps are easily and rapidly constructed and may permit some tectonic
interpretations of rate of subsidence and location of structural trends and prov-
inces. This is a reliable procedure only if boundaries at the base and top of the
isopached unit appear logically to be time-stratigraphic, or if their onlap and
offiap relations are clear. Careful correlations within the isopached unit offer a
check on this. In carbonate strata, however, even if "time planes" are known,
accurate interpretations of an isopach map alone may be difficult. This is because,
as pointed out above, there exist dual mechanisms for carbonate deposition:
(1) Detrital accumulation washed into or created in situ within a shallow subsid-
ing basin; in this case greatest thickness lies at the center of the downwarping
basin. (2) Carbonate may be anomalously thick over an intrabasinal topographic
high which subsided regionally.
Furthermore, as Fig. 11-18 illustrates, different isopach patterns in carbonate
strata may result from simple basin filling or from gradual basinward accretion
of the shelf margin. Accurate distinction between them at an early stage of strati-
graphic information depends on lithofacies analysis as well as thickness informa-
tion.

Trends of Carbonate Buildups May Be Controlled
by Tectonic Movements in the Basement
It is important to relate regional structure to reef configuration and facies, for
purposes of predicting distribution of such carbonate bodies. For example, fault
movements on basement blocks may offer the primary control for linear carbon-