Page 192 - Carbonate Facies in Geologic History

Algal Plate Mounds at Shelf Margins 179

E

II -9-

" 13
T II T
-9- 35
N
23 2'
"
Thickness of ArB Interval
[ower lsma~ zone
ConIOU'iIIlo,,,,,I.1 tool l;l3m l
29
D Carbonate build-up 23
IOfeet thick or more
Maximum permeability
of IOmd. or more
0 1 2miles
I ! I • ! 32 33
0 2 3km 26
~
Fig. VI-6. Thickness of algal plate mounds at Ismay field, Utah and Colorado. The pro-
nounced northeast trend of the buildups is at right angles to the overall productive trend.
Darker crosshatch areas mark optimum permeability. From Choquette and Traut (1963,
Fig.4), courtesy of Four Corners Geological Society

In the Sacramento shelf cycles (Wilson, 1967 a) the mounds have a preferred
position in the vertical sequence of rock types. The cycles formed due to alternat-
ing transgression and regression across the shelf and the mounds began growth
after the initial transgression, coincident with terrigenous influx, had concluded.
Thus, the carbonate buildups occurred in a clay-free environment, at times when
relative sea level was either stable or slowly dropping. They represent offshore
sedimentation at the inundative, nonclastic phase of the cycle at the "turn-a-
round" time between transgression and regression (Fig. VI-12). Generally, the algal
plate facies developed after deposition of normal marine bioclastic lime wacke-
stone or shale. What caused the accumulations to localize is not clear; perhaps
hydrographic factors are responsible. A moundlike core (bioherm) developed in
quiet water as an accumulation of lime mud with a variable content of algal plates
with bryozoans and a scattered normal marine fauna. Algal plates appear to
increase in abundance above the base constituting 20-40% of the volume of the
rock. Later the sequence of additional facies developed which are summarized at
the end of this chapter and diagrammed on Fig. VI-25.
In the Big Hatchet Mountain area, where steep depositional slopes prevailed
toward the Pedregosa basin, slumps off bioherms resulted in rubble accumula-
tions of coarse conglomerates and breccias on basinal sides of the mounds