Page 346 - Carbonate Facies in Geologic History

The Great Middle Cretaceous Carbonate Banks of Central Mexico 333

lies over a structural culmination of the Mexican part of the Late Paleozoic
circum-Gulf orogenic belt.
4. The edge of the platform consists of a string of individual paleomorpholo-
gic or structural hummocks in which oil accumulated under a seal of Tertiary-
Late Cretaceous? argillaceous strata. These highs lie along the eroded top of the
thick mass of EI Abra Limestone. They are probably erosional and not purely
depositional but their resistance to erosion may have resulted from original
mound areas of thick-shelled rudists. Because of early drilling practices, the rock
of the Golden Lane EI Abra Formation is very poorly known.
5. The Middle Cretaceous limestone mass in central Mexico is regionally
overlain by the pre-Turonian unconformity of the Gulf area, and much karstic
collapse, cavern formation and porosity development in the EI Abra occurred
probably before Late Cretaceous deposition. Although it seems reasonable for the
bank to have been exposed during formation of the regional pre-Turonian uncon-
formity, this cannot be proved because of subsequent erosion of critical strata.
The top of the Golden Lane bank was quite possibly intermittently emergent
from Cenomanian to Oligocene time. A latest Cretaceous conglomerate derived
from EI Abra facies shows exposure and erosion in latest Cretaceous and before
the Paleocene. Late Cretaceous through Eocene beds overlapping the great bank
were removed by uplift and gulfward tilting of the mass in pre-Oligocene time.
6. The Poza Rica trend consists of a series of structural highs parallel to those
of the Golden Lane but they are structurally much lower. They are covered by a
complete section of Late Cretaceous and Tertiary beds. Any post-Cenomanian
sea-level drops which exposed the EI Abra banks could not have affected the Poza
Rica trend.
7. The Poza Rica strata (the Tamabra) are, however, more dolomitic and
fragmented than those in the Golden Lane. They have suffered extensive neo-
morphic alteration. The beds are thickest near the Golden Lane bank (500 m) but
thin considerably within 20 km northwestward away from the bank.
8. The Tambara contains a mixed and interbedded biota of pelagic lime
mudstone and shallow water benthos.
9. The thick Tamabra carbonate facies in Poza Rica is interrupted by numer-
ous thin shaly units (perhaps bentonites) which can be traced over part of the
field. Some of these have pelagic microfossils.
Barnetche and Illing (1956) and Coogan et al. (1972) considered the Tamabra
facies as forereef shoals, in situ organic growth, pointing out that generally the
fauna was more normal marine than that of the EI Abra. It has radiolitid rudists,
hydrozoan spongiomorphs, corals as well as caprinids, and almost no miliolids. It
thus contrasts greatly with the Golden Lane EI Abra which, although poorly
known, is seen to be mostly restricted marine. Coogan et al. (1972) noted a
sequence 90 m thick in one core which they interpreted as upward shoaling, thus
in situ, accumulation. They believed that the chain of Poza Rica fields produced
from rudist reef accumulations. These occupy a chain of positive uplifts now
represented by thin areas of post Jurassic to Mid-Cretaceous strata over what
appear to be fault blocks (Fig. XI-8). These are considered comparable to such
offshore British Honduras banks as Turneffe, Glovers reef, and Chinch ora.