Page 335 - Carbonate Facies in Geologic History

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

rinids were environmentally more tolerant than some other rudists and probably
could withstand the quieter backreef realm of restricted circulation and variable
salinity as well as inhabit waters of more open marine circulation at the shelf
margin.
The caprotinids are a reef-forming group related to monopleurids. They are
common in Europe as well as in the Gulf of Mexico province. They had a thick,
two-layered wall and often a bulky overhanging free valve. The caprotinids in the
Texas shelf built substantial portions of rudist mounds in the Edwards Forma-
tion, occupying the upper zone with the radiolitids. They look like caprinids in
the outcrops. These forms have not been widely recognized in shelf margin posi-
tions in Mexico or in "Deep Edwards" on the Stuart City reef trend.
4. Another important group of rudists of Aptian and younger age is the
radiolitids. They are the important reef-formers of the Late Cretaceous but also
are common in Middle Cretaceous strata. These forms possess thick walls com-
posed of closely spaced radial and concentric plates. Adaptation proceeded to the
extent that the attached valve was more or less a heavy keg-like or conical form.
The upper valve was a mere cap which could easily be clamped shut perhaps
during subaerial exposure and permitted an intertidal existence for some genera.
Ecologically the group was widely adapted. They are known in backreef bio-
stromes, at tops of shelf mounds and, in the Middle Cretaceous, commonly at shelf
margins and on foreslopes where they were associated with and gradually re-
placed by corals, spongiomorphic hydrozoans, red algae, and sponges, a biota
inhabiting water of normal marine salinity. The robust construction of many
members of the group as well as their abundance in the outer shelf marins would
indicate a surf-resistant habit but other forms (Durania and Sauvegesia) existed in
quieter water.
In a detailed study of a Cenomanian platform east of Rome (Polsak et ai.,
1970; Carbone et a!., 1971) radiolitid dominance of the rudist faunas is reported to
be internal to the shelf marginal caprinids although considerable faunal overlap
occurs. It is possible that increased adaptation of the radiolitids during Ceno-
manian and later times permitted their existence in more restricted environments.
The facies analysis by Carbone et al. (1971) indicated a subtidal environment for
the radiolitids and caprinids existing on shoals at the platform margin studied at
Roca di Cava. In this area, both seaward and bankward, more micritic sediments
were deposited than at the shelf margin.
Accessory organisms in Cretaceous buildups include oysters and the oyster-
like genus Chondrodonta. These occur in heaps of shell debris and are known in
the upper parts of shelf mounds. Corals, mainly dendroid branching forms such as
Cladophyllia, may form biostromes beneath the shelf mounds and occur on the
foreslopes of rudist shelf margins. Knobby growth forms of coral also occur, like
those found in Jurassic reefs, but the shallow-water coral forms seem to have been
replaced at some quieter water shelf edges by the rudists. Hydrozoan spongio-
morphs are known in foreslopes and outer shelf-margin positions as in the Juras-
sic. Cretaceous algae are abundant in the carbonate realm and the major algal
groups generally occur with certain of the major rudist groups: radiolitids with
the red algae in an outer shelf margin position, codiaceans with the tolerant
caprinids over a wide environmental range, and algal stromatolites and requien-