Page 351 - Carbonate Facies in Geologic History

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

on the west side of the Golden Lane bank. As usual, it is difficult to distinguish
faulting from original bathymetric relief on the front.
4. The facies are slightly regressive within the marginal belt. Wells drilled near
its abrupt edge penetrate restricted marine carbonate of the shallow platform at
the very top, followed downward by rudist wackestone and bioclastic debris,
rudist and coral debris, and basinal lime mudstone. As much as 650 m of such
sequences are known; the maximum of reefy rock known may be as much as
500m.
5. In contrast, the backreef platform facies is extensive and well known. It
includes both shallow basinal strata in the North Texas and Maverick basins and
shelf sands and dolomitized intertidal muds on the central Texas, San Marcos
platform. Serveral zones of faulting, marking grabens, trend northeast across the
San Marcos platform. These began to accumulate sediment in Cretaceous time.
Isopachs of the Comanchean strata show a persistent axis of thickening 10-
20 miles shelfward of the narrow reef trend, marking the Karnes trough, which is
one of these grabens. Numerous oil fields occur in backreef dolomites associated
with the upthrown sides of these faults.
The geologic history of the platform begins with an Aptian transgression
followed by major construction of the platform during most of Albian time.
Upper Glen Rose and Fredericksburg strata show progressive restriction in the
basins as well as over the intervening San Marcos platform. Minor sea-level
fluctuations laid bare the San Marcos platform periodically. The Comanche shelf
built up to sea level toward the end of Albian time but during the Albian-
Cenomanian transgression was completely submerged by deeper water sedi-
ments, the dark pelagic "Georgetown" limestone. During the marine advance and
shale influx which ended reef growth, the San Marcos platform shrank to half its
original size and the pelagic and open marine benthonic facies of late Albian time
shifted northward across the North Texas embayment and on to the Comanche
shelf northeast of the Llano uplift. Presumably during this time the outer basin
beyond the reef trend was starved of sediment until latest Albian-Cenomanian
time (Van Siclen, 1958).
The large Cretaceous shelf of the northern Gulf of Mexico with its platforms
and basins (Fig. XI-5) offers an instructive example of how structurally positive
areas, such as the Ocala Uplift, San Marcos Arch, and Llano Uplift, served as the
nuclei for the development of carbonate ramps and platforms. The presence of
Jurassic and perhaps early Cretaceous salt in the intervening shelf basins shows
that the tectonic framework was established early in the Mesozoic.
There exist some unanswered structural questions about the Deep Edwards-
Stuart City trend. Why does it appear so regular? Was its formation induced by
carbonate shoal facies developed over an old ridge in the Ouachita orogenic belt
or over the fractured southern edge of a Gulf of Mexico plate which has migrated
northward under the Texas-Louisiana coasts? Either explanation could account
for the regular form and narrowness of the belt. Does the Albian-Cenomanian
shelf margin trend coincide with that of the Lower Cretaceous Sligo-Cupido
Formation? The prevalent regressive patterns of other shelf margins would indi-
cate that the Middle Cretaceous margin should be somewhat gulf ward of the
Cupido-Sligo.