Page 350 - Carbonate Facies in Geologic History

Deep Edwards or Stuart City Reef Trend 337

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$t"ort City R •• f
b:.:::J Poro.it1 ~b
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Fig. XI -10. Section across Deep Edwards or Stuart City shelf margin trend through southern
Atascosa and northern Bee counties, Texas. After Griffith et al. (1969), courtesy of Society
of Economic Paleontologists and Mineralogists

Figure XI-lO is a section across the shelf edge based on the above work (cf.
Fig. XI-6). Essentially the facies progression is from peloidal, miliolid, and coated-
grain micritic sediment, extensively dolomitized far behind the bank edge, to a
marginal peloidal grainstone-wackestone facies in which grains with micritic
rinds are common. This peloidal sand grades within a mile or so to shelf-margin
facies, winnowed bioclastic sand banks interstratified with caprinid wackestones.
In the outermost part of this 8 km wide marginal belt, the sandy debris contains a
diverse fauna with button and clump-like spongiomorph hydrozoans (e.g., Actin-
stromonia), numerous species of corals, including the dendroid Cladophyllia and
the compact Diploastrea. Some of the reefy organisms are also imbedded in lime
mud. The marginal belt abruptly gives way to dark shaly pelagic lime mudstone
younger than the shelf margin carbonates.
The following are pertinent facts of facies distribution and structural configu-
ration:
1. The marginal belt is only 5- 10 km wide and is very long and unusually
regular. It stretches more than 1000 km along the coast from north of Laredo,
Texas in Webb County, to north of New Orleans, Louisiana, whence it passes out
into the Gulf.
2. It is indicated that the rim was a topographic high, rising above the shelf
area, by the absence of Georgetown (Uppper Albian) strata over its top in some
places and thinning and onlap of the Georgetown by the overlying Del Rio Clay.
The "Georgetown" or Atacosa basinal mudstones are considered to have filled in
on the gulfward side of the shelf margin and overlapped it. This interpretation
would require considerable bathymetric relief to have existed, measured in
hundreds of meters. Cross section reconstructions (Van Siclen, 1958) show the
slope on the seaward side to have been about 2 degrees, somewhat less than on
the great offshore banks of central Mexico.
3. The belt, where best delineated, shows faulting on its gulfward side just as is
present at Cuesta EI Abra on the east margin of the Valles platform and suspected