Page 338 - Carbonate Facies in Geologic History

Regional Facies around the Gulf of Mexico 325

3. Some groups of rudists spanned the entire range from outer shelf oxygen-
ated and agitated water to interior backreef protected environments (e.g., re-
quienids and radiolitids).
4. In geneml, rudists could thrive better in lime. mud environments than filter
feeders such as Coelenterates and sponges. Requienid biostromes are always in
micritic sediments and caprinid mounds also contain much lime mudstone.
5. Many rudist fragments as well as the living shells were too large to be easily
coated or encrusted and accumulations of them are not bound together organi-
cally as in coral reefs.
6. Although not truly colonial organisms as are commonly found in reefs,
these peculiar mollusks, like corals, were capable of rapid growth into wave base
and of large bulk. Their wave resistance is indicated by flanking halos of win-
nowed shell debris derived from their shells. The fauna is often monotonous
where caprinids flourished as if all other organisms were choked out by thriving
rudists.
7. In general, individual rudist mounds on more or less stable shelves and in
many shelf margins are no higher than 10--15 m and many are much less, perhaps
giving a maximum depth for inception of rudist growth.
8. Because of the above facts, rudist buildups in shelf margin positions may be
expected to have more continuity than those formed chiefly by corals. They could
more easily withstand seaward moving off-shelf water and may be expected to rim
both lee and windward sides of offshore banks. These rims may be only a few
100 m wide and have variably gentle and steep slopes. Open sea sides of the banks
can be ascertained from the diverse faunal content of accessory organisms such as
corals, red algae, and hydrozoans.
9. The structure of rudists, and the fact that part of their shell was aragonite,
resulted in both high initial porosity and later partial solution under conditions of
meteoric water flow.
10. Rudists evolved rapidly from Late Jurassic to Late Cretaceous when they
abruptly became extinct, leaving reef-building corals and calcareous algae as
major framebuilders of the Tertiary. Sizeable Cretaceous rudist buildups are
known as far back as Aptian or Barremian time.

Shelf Margin and Platform-Bank Interior Facies of
Middle Cretaceous of Mexico and the Gulf Coast

Regional Facies around the Gulf of Mexico

In Middle Cretaceous time the Gulf of Mexico was almost completely bordered
by large carbonate platforms and offshore banks. These formed as a result of
extensive Albian and Cenomanian marine transgression which prevented influx of
terrigenous clastics from western North America. The wide areas of clear, tropical
water at the edges of the Gulf induced limestone accretion which built out ramps
from several positive areas around its subsiding center. From these ramps, plat-
forms soon developed (Fig. XI-5). Shallow shelf basins and intervening positive