Page 183 - Carbonate Facies in Geologic History

170 Pennsylvanian-Lower Permian Shelf Margin Facies

~ lalins

r;;l Pun
L.:.J evopori •• t.
Q Wolfco",p
t..:.:J u'oporite,
rv CatbOnal.
~ buadupi
a rog_nic fronh

..-- Sediment tron.port

Fig. VI-l. Pennsylvanian-Wolfcampian paleotectonic map of United States with evaporite
basins and carbonate shelf margin buildups indicated

of these long, narrow uplifts is crudely rectilinear. All have extremely steep sides
and are clearly faulted blocks. They shed extensive feldspathic debris from the
eroded Precambrian granitic and gneissic basement. Figure VI-l shows them,
their intervening basins, and areas of rimming carbonate margins.
The southwestern extension of the shield was the most affected by this frag-
mentation. The northern areas of the Midcontinent remained stable. Much sandy
and muddy terrigenous sediment was shed to the southwest from the distant
eastern Canadian area through the Ohio-Illinois basins, east of the Ozark dome,
but the stable western side of the shield supported shelf deposits of clean ortho-
quartzite (Tensleep-Wells-Quadrant). In addition to the narrow, linear horsts like
the Nemaha Ridge and Ancestral Rocky trends, three moderately uplifted equidi-
mensional domes formed near the southern and eastern geosynclines: the Llano,
Ozark, and Cincinnati-Nashville uplifts. These did not shed much sediment into
adjacent basins but remained mildly positive from Pennsylvanian to the present
time. They subtly affected cyclic sedimentation on the shelves separating them.
The unstable tectonic setting described above is clearly and extensively re-
corded by Late Paleozoic facies because the activity proceeded during major
marine incursions beginning as early as Late Mississippian. These incursions
reached a maximum in Des Moinesian-Missourian time and a gradual, though
intermittent, tendency toward regression occurred thereafter. Clastics shed from
uplifts in geosynclinal belts and from horst blocks within the craton, become
increasingly prevalent in Virgilian and Wolfcampian strata. Halos of carbonate