Page 342 - Carbonate Facies in Geologic History

The Great Middle Cretaceous Carbonate Banks of Central Mexico 329

30 cm) to medium-bedded, rhythmic bedded with slump structures (boudins) and surfaces of
discontinuity in bedding caused by gliding or channeling. Micritic texture with scattered
calcisilt particles such as micropeloids, tintinnids, calpionellids, and globigerinids. Laminae
and micrograded beds, cut and fIll structures, and occasional burrows. Micropeloid calcisilt
particles are detrital and mainly of shallow water origin (SMF-3).
Marginal breccia and conglomerates (Standard Facies belt 4).
Tamabra Limestone is the stratigraphic term used in Poza Rica. This is the detrital facies
of Coogan et al. (1972). Wilson et al. (1955) described it as coarse conglomerates marginal to
the EI Doctor bank. Carrasco (1971) described a massive body of this facies (30 m thick) at the
base of the EI Doctor or EI Abra limestone.
Such conglomerates are both rudstones and floatstones. Clasts are from 25 cm to 1 m in
diameter; they include fragments of previously lithified shallow-water limestone as well as
micritic clasts with planktonic fossils. Clasts are rounded to subangular, and may be well
graded. The boundaries with the micrite matrix are usually, not always, well-dermed. Edges of
the clasts commonly show truncation and erosion and may be somewhat weathered and
oxidized. In some areas, strata downslope at basin margins are principally biogenic micrite
with scattered large rounded blocks. Slumped beds and convolute strata occur, over- and
underlain by horizontal planar strata and indicate penecontemporaneous deformation on
slopes.
Shelf margin bioclastic facies (Standard Facies belts 4 and 6).
This term, used by Coogan et al. (1972) and Carrasco (1970) equates with the skeletal sand
and silt of Griffith et al. (1969). The facies occurs in the Tamabra (foreslope debris) and in halos
marginal to caprinid and radiolitid mounds on shelf margins where it is usually included in
the EI Abra Formation.
The strata may lack well-defined bedding. In other places thick, wedge-shaped foreset
beds occur and cross-lamination is common. Interbedded are ill-sorted, coarse debris and
much better sorted, finer, biogenic fragments. These sands may be mixed with biogenic
calcisilt. The fauna in outer slope beds (e.g. Tamabra of Poza Rica and the narrow fringe of
the Deep Edwards trend) contains fragments of caprinids and radiolitids (60% according to
Griffith et al. (1969), red algae, spongiomorph hydrozoans, sponges, and colonies of dendroid
corals, each group averaging around 5% each. Up to 5% lithoclasts may be present. The
bioclasts are exclusively of rudists and gastropods when higher on the shelf, forming halos
around caprinid mounds (SMF 4 and SMF 5).

Shelf margin rudist mounds (Standard facies belt 5) (Plate XXX E).
Caprinid micrite facies of Coogan et al.; organic reef of Griffith et al. and Carrasco. These
are accumulations formed by individual mounds of caprinids with some radiolitids. Alan
Coogan (personal communication) would distinguish the following biofacies subdivisions:
coral-algal, coral-spongiomorph, Monopleura, and caprinid. The buildups are on the order of
3-15 m thick and a few 100 m across. They are superimposed on each other on open sea edges
ofthe major banks. Examples are: northeast border of Actopan platform, the EI Doctor Bank
and the Cuesta EI Abra on the east side of the Valles platform. Some of the rudists are in
growth position but most are not. They are commonly disoriented in a micrite matrix.
Accessory organisms include corals, boring pholad bivalves, snails, some requienid rudists,
and various large foraminifera including Orbitolina, Dictyoconus, Chofatella, Cuneolina, and
Coskinolinoides. At the end of the Cuesta E1 Abra east of Valles, San Luis Potosi, Griffith et al.
(1969) estimated the following faunal distribution which indicates open circulation: caprinids
23%, radiolitids 15%, coralline algae, hydrozoans, and oolite 10% each, and dendroid coral
5% (SMF-7 c baftlestone).
Shelf-margin and bank-interior facies of EI Abra (Standard Facies belt 7).
This is a coated particle grainstone facies, following terminology of both Coogan et al.
and Griffith et al. It is termed the oolitic facies by Carrasco. Beds are from a few em to a meter
thick and cross-bedded. The sediment is grainstone of sorted, rounded, superficially coated
particles of lithoclasts, miliolids, and dasycladaceans. Alan Coogan (personal communica-
tion) reported the radiolitid Sauvegesia in this near backreef facies. Well-formed ooids are
rare; grapestone lumps and peloids are present. In some places on the shelf margin, red algae