Page 362 - Carbonate Facies in Geologic History

Stratigraphic Principles 349

7. Carbonate basins may be filled by gradual deposition and regular sedimentary
thickening from margin to center as continued subsidence takes place or by the
progradation of shelf margins with either step-like or steady outbuilding of plat-
forms.
8. A process of cyclic and reciprocal sedimentation fills basins which develop
well-defined depositional topography at margins and which are affected by per-
sistent sea-level fluctuations. The shelf grows up and out during high sea-level
stands while the basin is sediment-starved; at lower sea-level stands the shelf
remains above the equilibrium profile; sediment by-passes the shelf and fills in the
basin.
9. On a simple isopach map of a carbonate time-stratigraphic unit, a positive area
may be represented either by an abnormally thin or abnormally thick area, de-
pending on the amount of carbonate buildup over buried structural highs, and on
the depth of water over the "high" when carbonate sedimentation commenced.
10. In basins it is common for groups of "pinnacle reefs" to develop on the edges
of submerged platforms parallel to major surrounding platforms. These are re-
sponses to later carbonate accretion either on structural or paleogeomorphic
"highs."
11. In basins, positive linear trends (e.g., edges of basement fault blocks) may
develop carbonate buildups. These linear buildups can be multiple and cut across
basins or form their shelf margins.
12. Correlation along shelf margins may be difficult but electrical log tracing of
clinoforms downslope, correlation of thin clastic zones of basin and shelf, careful
use of detailed paleontology and seismic profiles aid in this procedure.
13. The principle of rapid prograding sedimentation is applicable also to the
construction of the common shelf cycles characteristically seen on carbonate
platforms. The calculated rates of shallow water carbonate accumulation derived
from Holocene studies indicate that progradational accumulation is more rapid
than paleontological zonation can resolve. Assuming that inundation is more
rapid than gradual progradation, the best correlation lines are boundaries be-
tween cycles where a rapid change occurs upward to more marine conditions.
14. Shelf cyclicity is prevalent in many varied tectonic settings, in rock of many
different types, and so widespread in the geologic record that one may admit it to
be a normal geologic process.
15. Shelf correlations using sedimentary cycles may be checked by attention to
key beds originating from presumed rapid inundation of planar surfaces (e.g.,
coals) or representing still-stands in sedimentation (concretion zones, glauconite
beds, burrowed horizons), or representing geologically instantaneous events
(zones of quartz silt formed by dust storms or bentonites formed from falls of
volcanic ash).
16. Shelves also contain widespread, somewhat thin, carbonate rock units of
fairly uniform facies and paleontology. These represent slow, extensive sedimenta-
tion beneath deep enough water so that open circulation prevailed and not much
progradation of shallow water carbonate facies took place.