Page 55 - Carbonate Facies in Geologic History

42 The Stratigraphy of Carbonate Deposits

tive element; Central Basin Platform of the West Texas Permian basin (with
more than 100 milligals rise in gravity across it); and the major Alberta reef trends
with north-northeast alignment, the Leduc-Rimbey trend of this area having a
faint underlying positive gravity anomaly (Fig. 11-19). The Late Paleozoic algal
plate mounds also display multiple linear trends.
These linear trends, presumably resulting from basement faulting, exist be-
cause of the sensitivity of carbonate production to any kind of pre-existing topo-
graphic high, whether structural or geomorphic. Carbonate accretion may be
expected to occur along any sudden break in slope, perhaps a response to drape on
basement faults. Once initiated, growth is apt to be continuous and rapid. Topo-
graphic control for Holocene barrier reef development has been advocated by
Purdy (1974a and b) who relates it to a late Pleistocene karstic surface as well as
underlying Tertiary faulting. In addition, basinal areas commonly contain numer-
ous areas of small circular and elliptical buildups of carbonate, developed as a
result of preferential accretion atop slight topographic rises. Note that lines or
elongate areas of such pinnacle mounds or reefs are known in several basins
discussed in Chapters IV and V. These may be structurally induced along base-
ment faults or shifting blocks. Massive carbonate caps are known to grow on
rising salt domes both in the Gulf of Mexico and in the Persian Gulf. In the latter
sea only a very few meters of original relief at depths of 30 or more meters is
sufficient to cause pure CaC0 3 to accrete rapidly over the elevated humps.

Cyclic and Reciprocal Sedimentation

Alternation of basin and shelf sedimentation results when the buildups of shelf
margin slopes around a basin are formed during periodic sea level fluctuations,
particularly when the climate and lack of hinterland relief encourage the carbon-
ate producing process to operate efficiently.
Fig. 11-20 shows that during high stands of sea level the shelf margin is pro-
tected from clastic influx, which is caught many miles away near the shoreline. In
the clear shelf water, carbonate production and sediment stabilization by organic
and inorganic processes are particularly efficient, resulting in rapid accumulation
on the shelf and its margin and almost none in the basin. If sea level remains high,
the carbonate shelf margin builds out, followed in regressive sequence by its
several shelf facies belts (lime sand, lagoonal lime muds, tidal flats and saline
lagoons and supratidal sebkhas). At the same time any minor accumulation oc-
curring in the deeper basin consists only of pelagic and windblown material in an
euxinic environment. Additionally, perhaps at the foot of the constructed plat-
form, some fine carbonate detritus is deposited from the shelf. Obviously the
basin starves during this time, although in many cases a thin, dark limestone or
shale unit may be formed completely across its floor.
A second phase of sedimentation occurs when sea level drops. Clastics are
reworked by rivers, particularly if tilting of the hinterland has increased gradients.
Sands and clays are carried out in channels across the shelf, bypassing the margin
and accumulating in the basin, filling it, and reducing the marginal relief created
during the high stand carbonate phase. There is a growing awareness among