Page 343 - Carbonate Facies in Geologic History

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

such as Solenopora and Jania form the nuclei. These coated, grainstone beds are known to
overlie and flank caprinid mounds on the Cuesta EI Abra (SMF-11).
Bank interior facies of EI Abra (Standard Facies belt 8).
These strata are light-colored and well-bedded, varying from 30 cm to 3 m thick. They are
commonly cyclic; repetitions of strata progress upward in a shoaling sequence. Lowest beds
are burrowed micrite with Toucasia or miliolid grainstones. These pass upward through
laminated micrite to a cap of dolomite crust or intraformational pebble conglomerate. Eight
common bank interior-restricted marine microfacies are recognized in the various studies
noted above.
1. Miliolid grainstone: Thick-bedded (1 m) to massive strata, burrows in upper parts of bed,
some peloid grains but mostly tests of miliolid foraminifera (SMF-18) (Plate XXX C).
2. Peloidal grainstone to wackestone with some grapestone: Sparry cement, generally 75%
grains, less than 10% miliolids, 5% bioclasts. This sediment is derived from organic
pelleting of lime muds in an environment where lime precipitation continuously hardens
the grains and where gentle current partly winnows remaining lime mud (SMF-16).
3. Miliolid wackestone-packstone: Burrowed sediment which includes a few rudists (SMF-
19).
4. Molluscan wackestone: Toucasia and monopleurid rudists, oysters, and scattered milio-
lids. In places the oysters or rudists form biostromes (Plate VB).
5. Dolomite-lime mudstone laminated crusts: Formed by evaporation causing mineral pre-
cipitation and replacement. By analogy with Holocene sediments, lamination is probably
algal in origin. Disruption of the crust by drying and curling-up of the fragments results in
the construction of intraformational breccia which ftlls channels and is a common lateral
facies to the crusts (SMF-24).
6. Bulbous algal laminites with fenestral fabric: Mostly peloidal lime mudstone or homoge-
neous mudstone. Represent desiccated algal mats with scattered miliolids and ostracods
(SMF-20).
7. Planar fine mm laminites, nonfenestral, often pure dolomite, containing scattered miliol-
ids. Probably also induced by algal trapping of seasonal increments of lime mud.
8. Homogeneous lime mudstone or dolomite (SMF-23).
The outcrop areas of the platforms studied in Mexico all show strong facies
differentiation displaying the above standard rock types and illustrating clearly
their interpreted environments. The interiors of the platforms and large offshore
banks are wide expanses of the well-bedded EI Abra Formation. The margins
contain knolls of caprinid-radiolitid rudist buildups and much biogenic lime
sand. These facies are reasonably well understood. The foreslopes of the banks, on
the other hand, provide problems in interpretation in almost all places where they
have been studied. Is the common faulting at the edge of the massive carbonate of
the banks contemporaneous with deposition, or later? Just how steep was the
original slope? How much original relief was there on anyone bank-tens of
meters or hundreds of meters? To what extent can sedimentologic study of the
basinal sediments provide some estimate of depth? Clear answers to these ques-
tions are important in developing exploration models and in comparing Creta-
ceous shelf margins with others in the geologic record.
One of the best areas for display of shelf margin facies lies along the Tampico
highway about 10-12 km east of the city of Valles, San Luis Potosi (Figs. XI-3, XI-
6). Here extensive quarries have been cut into the eastern edge of the Valles
platform which has been uplifted along a faulted anticline in front of the Sierra
Madre for a distance of more than 100 km. The frontal, east-facing scarp, the
Cuesta EI Abra, exposes knoll reefs close to the shelf margin at the railway
loading station, Taninu!. Large springs and caverns along the escarpment are
present, making additional exposures possible. Detailed studies have been made