Page 189 - Carbonate Facies in Geologic History

176 Pennsylvanian-Lower Permian Shelf Margin Facies

SW Nf
SHELF "ARAOOX IASIN
HERMOSA FM 'ARADOX FM

Dolomi'ic ••• Ioid
________ - ' ''- - - - - -

Blod .hal. - .

~ Cha.'.' ..
'" Tubulo. Fa.om
" Shelly biacla ...
8 001i'.

Fig. VI-J Early mound development on southwest shelf of Paradox basin with cyclic and
reciprocal sedimentation. Mounds may be capped by oolite and quartz sand or tops may be
marked by an unconformity

Lithologically, the oldest buildups consist of cores of tubular foraminifera
with plumose codiacean? algae forming a boundstone (Fig. VI-5). No algal plates
are known. Scattered Chaetetes and Caninia are common in some of the masses.
Red algae occur as accessory elements but Komia is not reported, although the
buildups are within its stratigraphic range. Flank debris is developed impressively
but disappears a short distance away; the biocIastic-lithoclastic beds grade rap-
idly to spiculite. The bioherms have a vertical relief of a few meters and steep
slopes of several degrees (Fig. VI-5). Not enough sampling has been done to
ascertain whether the basinward northeastern sides of the features have a biota
that differs from the other parts.
Some interpretations can be made. The boundstone of tubular foraminifera
and algae is indicative of steeper shelf slopes because this assemblage is similar to
that capping some platy algal mounds seen in later Pennsylvanian strata. The
rapid gradation to spiculite indicates the same steep slopes. Porosity must have
been good originally; surface weathering shows much vuggy porosity despite
chertification (particularly of Chaetetes heads).

Algal Plate Mounds at Shelf Margins

The most common Late Paleozoic shelf margin buildups are lime wackestone
with abundant algal plates. These mounds are known from beds of middle Penn-
sylvanian to Wolfcampian age and occur throughout the southwestern U.S.A.
They rim many of the subsurface oil basins in Texas, Oklahoma, and New Mexico
and are known in outcrops at a basin margin in central Colorado (Minturn
Formation), and in New Mexico (Sacramento, Hueco, and San Andres Moun-
tains) rimming the Oro Grande basin. The facies provides excellent reservoir rock