Page 332 - Carbonate Facies in Geologic History

Chapter XI

The Rise of Rudists; Middle Cretaceous Facies
in Mexico and the Middle East

Geologists have long recognized the Cretaceous as a time of outstanding develop-
ment of carbonate platforms and offshore banks. These are known in an equa-
torial belt from 40° N to 20° S along the Tethyan seaway through southern Eu-
rope, in the Middle East, across southern Asia and the sunken Mid-Pacific and
around the Gulf of Mexico-Caribbean region. This Chapter focuses particularly
on Middle Cretaceous strata in these areas and on the role played in carbonate
accumulation by the rudist bivalves within them. Rudists added immensely to the
volume of material in Cretaceous buildups, forming a framework both within
shelf mounds and along shelf margins. They are significant constituents of reser-
voir rock in some of the world's greatest oil fields. In this Chapter a discussion of
rudist morphology and paleoecology is followed by a description of some typical
Middle Cretaceous facies patterns.

The Rudist Bivalves

In the Mesozoic, Pelecypods of the Pachydont group became well adapted to life
in more or less restricted marine environments and to both sheltered and rough
water. More than one hundred rudist genera evolved from the early rudist genus
Diceras, a thick-shelled, coiled-beak clam often found capping Jurassic coral
patch reefs and forming flank debris around such rises on the shallow sea bot-
toms.
In the Cretaceous rudists evolved into bizarre forms in which one valve rested
on the substrate or became attached while the other served as a lid or cap; the
valves usually articulated with massive teeth and sockets. When the rudists grew
in crowded conditions the prevalent form was an erect, tall, twisted, slender shell,
oriented during growth toward favorable food-bringing currents. Rougher water
caused stubby, trunk-like forms to develop. In growth form, rudists resemble the
smaller Paleozoic richtofenid brachiopods or even horn corals. The shoal water,
tropical reefy environment is highly competitive and a premium exists for rapid
growth. For rudists this resulted in many large species, as long as 11/2 m, having
porous, almost vesicular shells. The shell structure varies from simple laminae, as
in monopleurid and requienids, to thick, compact walls in some forms. The
important rock-building caprinid group has a very thick wall containing open
canals which occupied from 30 to 75% of shell volume. These porous walls,
together with a large central cavity, gave the caprinid rudists great original pore
space. The same high porosity was developed by radiolitid rudists whose thick
wall was regularly cellular. Figure XI-1 illustrates some typical rudists; Fig. XI-2
shows their distribution in time.