Page 340 - Carbonate Facies in Geologic History

The Great Middle Cretaceous Carbonate Banks of Central Mexico 327

fossils have been dredged from submarine outcrops far down the slopes below
some of these rims. This would ingi~~te ~QntimH7u~ buildups Qf th~ pl~tforms
since Jurassic time or possibly widespread faulting along the escarpment bases.
Arcer profiles across the platforms show that a pronounced ridge in Cretaceous
strata generally trends parallel to and just within the platform edges, presumably
evidence of marginal buildups (Bryant et aI., 1969). The same geophysical tech-
nique outlines the outbuilding of Early and Middle Cretaceous strata along slop-
ing ramps in the seaward direction and shows the final construction of the great
platforms.
These Early-Middle Cretaceous bank and platform areas have remained as
such until the present, and today's bathymetric configuration of the Gulf of
Mexico clearly outlines the apparent Cretaceous shelf and basin areas. A narrow
inlet must have entered the Gulf at the south, at about the position of the present
island of Cuba. The Antilles orogenic belt was at that time a geosynclinal trough.
The western side of the Gulf was bordered by another area of great subsi-
dence, the Mexican geosyncline which was filled by sediments. Thick carbonates,
measured in thousands of meters, now observed in the eastern Sierra Madre,
constitute evidence for a miogeosyncline in which basinal limestones and im-
mensely thick carbonate banks formed. The western boundary of this area is
buried beneath Tertiary volcanics along central and western Mexico. Presumably
these strata mask a series of Mesozoic orogenic islands and a source area for both
Early and Late Cretaceous sandstones.
Middle Cretaceous climate is comparatively well known. Cycad stumps and
dinosaur tracks, occurring in littoral sediments of Albian age cropping out in the
Edwards plateau country of central Texas, indicate a tropical climate north of 30
degrees latitude. Albian evaporites from the North Texas-Tyler basin and the
Maverick basin in south Texas, as well as from Guatemala and Cay Sal off Cuba,
indicate periodic dry periods as well as a hot climate. The great thicknesses of
lime mud sediment also indicate tropical seas. Vast quantities of this sediment not
only make up the shelf platforms but in addition accumulated in foreslope posi-
tions and in marginal offshore basins. Only in the northern coastal areas does
much shale and sand occur. Sands appear to have been introduced from the
Appalachian area in the east and from a western source in northern Mexico,
north of Monterrey and Saltillo. The coastal stretch between these two areas is a
narrow belt of sand, which crops out in southern Oklahoma and north central
Texas and shows that minor river systems drained the midcontinent area much
as today. Occasional influx of clay resulted in widespread dark shales which
punctuate the dominantly carbonate stratigraphy in bands around the whole
perimeter of the northern Gulf, e.g., Pine Island-La Pena-Otates of Aptian age, the
Albian Kiamichi shale and the Cenomanian Grayson-Del Rio formation.

The Great Middle Cretaceous Carbonate Banks of Central Mexico

Figure XI-5 indicates the distribution of known banks of this age in central Mex-
ico. They include the Toliman, El Doctor, Actopan, and Valles platforms, and the
Faja de Oro (Golden Lane). The bank or platform facies is generally termed the