Page 337 - Carbonate Facies in Geologic History

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

Ool;t. ~ (ool.d Dra in
land bar
.",dil' debris ~=~:;:g~~7~~~~~~~~~:;;
bioclastic

TOP Of T ... NINUL au ..... y. CLOSE TO E"'ST F.ONT Of V ... LLES P~TfORM
Mod.,ot.l, kigh ••• ,gy HE ... R V"'LLES, SLP, MEX.

10UCO'io.hn.It,;:,0~:~~§~~~~~~~~~~~~§~~~=
1 ~
lobric;
__
RUD IST BUILDUPS Of ... CTOP ... N .... NK WITH IN EL "'U'" SHELF, CEN. MEXICO
... tlle,.d ci rculation ond en.rGY Itrollll a. Cono.(.o IMP, Me",)
Open lea . id. IOkm
-----
Coprinid. -----
8·IOm

_-- &'I ../' t1 ......,.",. (l\
--,.--~ ./" rr Touc:cu lo and .....".--!M:::!o~no=I."".,"'"o ____ _
v ., \1 Clodophyllio V 'to V
IDE ... LlZED IUILDUP (T .... UlAR REEf) SHOWING I I0f ... CIES PROGUSSION TO SHO ... LlNG W"'TER-
INNER SHELf OF TEX ... S CIl ... 10N F,om K. To •• g , 1959 modili.d by .... Coogon
Fig.XI-4. Facies in three rudist mounds on shelves or along gently dipping shelf margins in
Texas and Mexico

ids together in extreme backreef positions, e.g., in the EI Abra of Mexico. Figure
XI-3 shows a facies profile with the ecological zonation of the major rudist
groups and associated biota. Figure XI-4 is an illustration of the composition of
typical rudist mounds,
Rudists differ somewhat from other common reefoid organisms in the geo-
logic record. This is true both in depositional environment and in habit of con-
struction.
1. Generally in the Middle Cretaceous (except for monopleurids) they inhab-
ited clear-water, pure carbonate environments without clay contaminants. In the
Late Cretaceous some of the radiolitids and hippuritids were also tolerant of the
clayey bottom.
2. They were somewhat tolerant of variable salinity and restricted circulation.
The absence of brachiopods, ammonites, and echinoids from typical rudist facies
indicates that the organism could withstand fluctuations in temperature, salinity,
and oxygen beyond the limits of normal marine water. Discovery of numerous
diastems and hard grounds within caprinid mounds in Texas, and the presence of
coal directly above caprinid (?) biostromes in the Middle East, indicates that
some rudists could survive intermittent exposure.