Page 349 - Carbonate Facies in Geologic History

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

Fig. XI-9. Regional Middle
Cretaceous (Albian) paleoge-
ography of Texas during for-
mation of Fredericksburg-
Washita groups. Devils River
and Stuart City trends consist
of rudist buildups at shelf
margins. For detailed facies on
Comanche shelf see Fig. II-9.
Outcrops are stippled. Names
of cities are abbreviated:
Houston, Laredo, San Antonio,
Austin, Ft. Worth, San Angelo

limestones and show locally strong graded bedding. Some basinal facies consist
solely of pelagic fauna in micrite with abundant black chert nodules, indicating
sedimentation well below wave base.

Deep Edwards or Stuart City Reef Trend

A major carbonate platform, termed in Texas the Comanche Shelf and a narrow
shelf margin (Stuart City reef-see Fig. II-9), closely related in facies to the Mexi-
can offshore banks, exist completely along the northern border of the Gulf of
Mexico. The platform has produced minor amounts of oil and considerable gas
along its south Texas edge (Fig. XI-9) and carbonate facies have been closely
studied here since the 1950s. The same microfacies types in the same lateral
sequence existing in the Mexican banks, are recognized here by Keith (1963), Rose
(1963) and Griffith et al. (1969). The latter study indicates the value of a compara-
tive analysis of combined surface and subsurface strata. Very detailed study of
microfacies of the carbonate shelf margin rocks of the Cuesta El Abra in Mexico
was used, with the help of computer analysis, to determine the minimum signifi-
cant parameters necessary to describe related facies in the Deep Edwards trend.
Use was made of a modern analog offered by the Florida Reef tract to confirm
conclusions about the environmental meaning of the facies sequence. Application
to the Deep Edwards subsurface study, where data and time were limited, was
made more accurate by such comparisons.