Page 227 - Carbonate Facies in Geologic History

214 Late Paleozoic Terrigenous-Carbonate Shelf Cycles

_---- -------_'.$111

SII ll"l'" (AtCJltU$lON ~~~,.,~ _______________
_

=:i .-cw ... L IlAJt,. ateIUS'
~ .LD$ItA,C

Fig. VII-S. Stratigraphic facies model showing development of sedimentary cycles through
reciprocal sedimentation on the Sacramento shelf and eastern Oro Grande basin, southern
New Mexico. The shelf is only a few km wide and each cycle only a few tens of meters thick.
Sedimentation occurred at surfaces A- B-C at times when sea levels A- B- C existed. Basinal
clastics, shelf carbonates, and basinal carbonates were deposited in turn. From Wilson (1967
and 1972, Fig.4)

Galloway and Brown (1973) pointed out several facts indicating local (intraba-
sinal) controls on cyclicity during continuous subsidence of the north Texas area,
rather than eustatic sea-level changes of great magnitude.
1. No old soil zones or widely traceable disconformities are observed on the
shelf.
2. The general stratigraphy indicates almost continuous fluctuations of envi-
ronments instead of widespread traceable individual cyclic units. There is much
variability. Even the limestones on this shelf are not continuous beds but can only
be correlated as general zones. Much marine reworking of terrigenous clastics
occurred and a continuous interplay of terrestrial and marine sediments existed.
3. Channels do not get deeper toward the shelf edge, as would be expected in
cases of drastic sea-level drops. There is no general dissection of the shelf margins.
4. The sandstone patterns are clearly of distributaries crossing the shelf giving
the possibility of prevalent lateral shifting of clastic sources.
5. Despite the tendency toward basin ward migration, the shelf margins are
localized and vary somewhat in stratigraphic position. Clastic wedges on basin-
ward slopes of the shelf margins are also localized and correlate with five deltaic
lobes within the Virgil ian strata alone.
Galloway and Brown (1973, p.1212) conclude that "depositional or erosion
events occurring simultaneously over the entire eastern shelf are not indi-
cated ---. Although extrabasinal controls probably were operative to some ex-
tent, the depositional fabric was determined by interplay of locally active deltaic
lobes".
This explanation for cyclicity is essentially that of Moore (1959) for the Yore-
dales whose sedimentary pattern is very similar to that of the North Texas Penn-