Page 60 - Carbonate Facies in Geologic History

Widespread Uniform Single Lithosomes 47

lower Gulf of Mexico coastal plain !). This is true for surfaces of the interior of
present day carbonate banks and for modern shelves built from the continental
blocks.
By analogy with present shelf depths, whose Holocene sediments much resem-
ble ancient limestones, we can safely assume that in the geologic past, shelves and
platforms hundreds of miles wide were covered with water only a few tens of
meters deep. Again, by analogy with actualistic models, it is hardly conceivable
that tidal currents and wave action in such widespread and shallow seas could
have been very effective. Yet, many deposits across the North American craton
contain uniform sequences of rock types for hundreds of miles and such deposits
commonly include beds of clean quartz sandstone and pelletoid or oolitic lime
grainstones! These are clearly the result of wave and/or current activity and yet
are widely distributed over thousands of square miles. On such flat-bottomed,
shallow seas not much wave energy can be generated and tidal effects are severely
restricted (Keulegan and Krumbein, 1950). It is thus probable that such deposits
were formed by seaward progradation of shorelines and offshore oolite bars.
Studies of the mechanism of accretion of lime muds in Florida Bay, on the
tidal flats in the Bahamas, and along the Persian Gulf, reinforce this conclusion.
Shelf areas of the earth are now covered by wide expanses of shallow marine,
tropical water as a result of the post-Wisconsin sea level rise. Fine lime sediment
is produced abundantly in some of these waters, perhaps mostly by breakdown of
algae and attrition of calcareous tests of a wide variety of organisms. This fine
sediment is carried continuously landward on flood and wind tides and trapped
effectively against the shore on tidal flats which build progressively seaward. A
regressive sequence, about 4000 years old is well documented by coring on parts
of Andros Island and in the Persian Gulf. As pointed out in Chapter I, this
sedimentation has been extraordinarily rapid since the beginning of the Holocene.
Thus progradation over a very flat surface would result in quiet water, open
marine muds overlain by sheets of grainstone, which in turn are overlain by wide
belts of shallow water carbonate muds and evaporites bearing sedimentary struc-
tures which form today in intertidal areas. Unless such deposits were formed in
tides which daily transgressed inland hundreds of miles, we must conclude that
these facies result from environments migrating over thousands of years. The
reader is referred to an extensive explanation of the origin of carbonate facies over
these surfaces and their diachronic mode of deposition by Shaw (1964) and Irwin
(1965). Stratigraphers recognize two types of such strata: uniform single litho-
somes and sedimentary cycles.

Widespread Uniform Single Lithosomes

The midcontinent area of North America, including the Williston basin, typically
contains "layer cake" strata as does also the Arabian shield and Russian platform.
Some of these lithic units cover remarkably wide areas. Krumbein and Sloss
(1963, p. 374-378) considered some of them "time-parallel rock units". Considera-
tion of facies within these strata shows that it is unlikely that some of them were
deposited in contemporaneous layers. Certain of the beds, such as cross-bedded