Page 43 - Carbonate Facies in Geologic History

30 The Stratigraphy of Carbonate Deposits

carbonates in meters

Fig. II-7. Thickness of Lower Ordovician around North American craton. The great "halo" of
shallow marine and intertidal carbonate portrayed is mainly depositional and its outward
thickening forms platforms on all sides of the central cratonic axis

develop on isolated highs in offshore basins, probably originating at lower sea level stands.
Once started, they manage to maintain themselves, and grow upward through rapid accumu-
lation in spite of considerable subsidence. Small examples are often termed pinnacle reefs.
These may appear in lineations and rise from platforms developing over irregularities or fault
scarps along its surface.
The slopes and flat tops of such banks resemble those of the major platforms. Unlike the
platforms adjacent to cratonic blocks, whose facies progression faces the seaward side, the
belts completely encircle the great offshore banks, having only minor facies differentiation on
the windward side. For example: the Great Bahama Bank (Cretaceous to Recent age; Fig. II-
5), the Cretaceous Valles and Golden Lane platforms of central Mexico; the Central Basin
platform of West Texas, Middle and Late Permian; the Middle Triassic Dolomites of South
Tyrol, northern Italy (Fig. II-6); Pennsylvanian banks (including Jameson Field and the
Horseshoe "Atoll") in the Midland Basin (Fig. VI-14); Silurian pinnacle reefs in the Michigan
Basin (Fig. IV-ll); and the Zama area buildups of northern Alberta, Middle Devonian.

2. Carbonates Developed off Major Cratonic Blocks
during Great Regional Subsidence

a) Major platforms and ramps built out from cratonic blocks: These occur at edges of miogeo-
synclines or pericratonic basins. Shallow carbonate sediments are built out to form large
sloping ramps which evolve rapidly into platforms whose outer (seaward) slopes may range
from 1 or 2 degrees to as much as 30 degrees. Thicknesses may be on the order of hundreds or
even a few thousand meters. Facies belts on the edges of such platforms generally may be only
a mile or two wide, whereas the interior facies may be tens of km across, with almost flat
surfaces (slopes of 30 cm per km are common). An example is the Lower Ordovician of North