Page 348 - Carbonate Facies in Geologic History

The Great Middle Cretaceous Carbonate Banks of Central Mexico 335

2. Decrease in thickness of the Tamabra is progressive from east to west away
from the supposed source.
3. General bad preservation of the rudists and their lack of orientation indi-
cates that they are detrital particles and not oflocal derivation. Normal biological
attrition of carbonate particles weaken this argument as well as that of item l.
4. The amount of clay in the sediments shows that the rudists did not grow
locally.
5. Interstratification of the Poza Rica Tamabra facies with the pelagic faunas
of the typical Tamaulipas facies, the pelagic facies being in stratigraphic sequence
from Albian through at least Cenomanian, shows that the shallow water carbon-
ate facies could have been introduced gradually into the basinal environment. The
presence of lime mudstone bearing Tamaulipas pelagic microfauna inside of ru-
dist tests indicates a mixing of the two environments.
Evidence for the exposure and erosion of the top of the Golden Lane bank has
been pointed out, whereas a complete section of Late Cretaceous-early Tertiary
strata over the Tamabra belt is known. It has been considered that the Tamabra
represents forereef shoal sediment deposited in situ on the outer rim of the Golden
Lane bank. They are now 1000 m lower than the Golden Lane (Fig. XI-8). Cer-
tainly the rudists, corals, and hydrozoans found in the Tamabra did not exist in
water of such depth. If the bank was downwarped structurally it must have been
buried before Late Cretaceous time. Becerra's evidence for stratigraphic continu-
ity within the Tamabra and a gradual buildup of the facies through Middle
Cretaceous time is significant. A continuously penecontemporaneous origin for the
Tamabra material as detritus from the lip of the Golden Lane bank seems to be a
logical explanation.
Could the Middle Cretaceous relief have been the full 1000 m now structurally
indicated? Surface work by B. W. Wilson, B. Carrasco, and P. Enos on the out-
cropping banks in the Sierra Madre have clearly indicated that great relief existed
on the flanks of some of them. It is not clear just how much, despite the consider-
able difference in thickness between bank (1500-2000 m) and basin (300-400 m)
facies. Field studies have indicated faulting at bank margins as a response to the
Tertiary orogenic forces. This complicates discernment of stratigraphic onlap
relations. From analogy with Cretaceous to Recent banks in the eastern Gulf of
Mexico, it is tempting to equate the 5- to 6-fold thickness difference to an original
relief constructed by differential rate of sedimentation from bank to basin. Facies
show that the bank tops were barely awash, even intermittently exposed, and the
fauna of the basin sediments is consistently pelagic, but we cannot ascertain the
absolute depth difference. Perhaps there was a differential substrate subsidence
between bank and basin but it is tectonically improbable that circular patches on
the sea floor would subside five times more rapidly than surrounding areas. If the
banks built up without a considerably greater subsidence than the intervening
basins, they must have stood more than 1000 m in Cenomanian times. This is not
an incredible figure when one observes the amount of original relief preserved in
the Triassic Dolomites of the Tyrol.
Sedimentologic evidence of the peripheral facies indicates a considerable relief
but is not definitive as to how much. Slump features exist in thin to medium-
bedded limestones, coarse conglomerates contain clasts of previously lithified