Page 65 - Carbonate Facies in Geologic History

52 The Stratigraphy of Carbonate Deposits

in areas of variable tectonic activity. They are found over interiors of large car-
bonate banks situated in active geosynclines (e.g., Lofer facies of Dachsteinkalk)
and across shelves and shallow basins a thousand miles from the nearest orogenic
belts and well within the stable interior of cratonic blocks (e.g., cyclical Ordovi-
cian to Mississippian in the Williston basin). Cycles may pass from one tectonic
unit to another, changing in facies but still maintaining a cyclic character. The
Mississippian Madison Group shows this in passing from the Williston basin on
to the Central Montana high, as do the Pennsylvanian cyclothems traced from the
Pedernal uplift to the Oro Grande basin of New Mexico. Very possibly some
world-wide mechanisms operate to cause sea level fluctuations and in addition
more local causes, such as tectonic uplift and weather changes, add to and compli-
cate the pattern (Wanless, 1972, p. 41).
The favored eustatic mechanism for cyclicity, particularly in the Late Paleo-
zoic, has long been continental glaciation and many papers discuss this (Wanless
and Shepherd, 1936; Wanless, 1972). This is a very attractive mechanism for Late
Paleozoic strata, which are strikingly cyclic in the northern Hemisphere and
which in general correlate in time with a long period of southern Hemisphere
glaciation. Cyclicity of Late Paleozoic strata, however, is so clearly manifest and
so easy to recognize mainly because they commonly consist of interbedded sand-
stones, shales, and limestones. Even at a distance, most Pennsylvanian beds may
be recognized by their "ledgy" character. The terrigenous influx is undoubtedly a
response to the widespread Late Paleozoic orogeny.
It is important to consider that pure carbonate shelf cycles are prevalent in all
parts of the geologic column. They are, for example, particularly well-developed
in Cretaceous and Jurassic beds in both Europe and North America, as well as in
Devonian strata. If glacially induced eustatic sea-level movements are generally
responsible for sedimentary cycles, a more or less continuous waxing and waning
of polar ice caps is called for, at least throughout Phanerozoic time.
Perhaps other causes for eustatic sea-level movements exist, e.g., megatectonic
(Wells, 1960), but evidence shows that sinking of oceanic plates or uplift of the
mid-oceanic rises accounts for only a small amount of sea level displacement.
Duff et aI., (1967, p.246) quoted figures from Menard of a sea level rise of 0.2-
0.3 cm per 1000 years based on megatectonic adjustments in the Pacific since the
beginning of the Tertiary. Mathews (1974) concluded that, except for the most
tectonically active island arcs in the northwestern Pacific, a tectonic displacement
rate of 1 m per 1000 years is average. See Mathews' review of Christensen's study
of the San Joaquin Valley of California (1974, p. 68). Hallam studied upper Liassic
geologic history in western Europe and estimated that sea level may have risen
about 15 m in 3 million years (during 3 Toaracian ammonite zones formed during
a presumably nonglacial period in earth history). This is a rate of only 0.5 cm per
1000 years. These data are not as accurate as are needed but they indicate clearly
that (1) megatectonic forces are presumably continuously operating to cause sea
level variations and (2) that regionally none of these rates approach the most
recent glacially induced eustatic fluctuations of the Late Pleistocene. During the
last 120000 years, sea level dropped in spurts of several meters per 1000 years at
an over all rate of 0.8 m per 1000 years. More recently, during the last 15000
years, sea level rose at almost 8 m per 1000 years!