Page 59 - Carbonate Facies in Geologic History

46 The Stratigraphy of Carbonate Deposits

TYPICAL SEISMOLOGICAL SECTiON
MORALILLO MORALILLO-CERRO AZUL CERRO AZUL
(Old Golden Lane)

2.500 Met e r s
~oo

Fig.II-21 Seismic profile interpretation across west side of Golden Lane bank south of
Tampico, Mexico; slightly modified after Guzman (1967, Fig. 5)

Paleozoic beds of West Texas where close biostratigraphic control was made
possible through the use of large robust foraminifera, the fusulinids. Twenty or
more years ago, much difference of opinion and discussion among petroleum
geologists concerned the evaluation of fusulinid determinations in West Texas
and New Mexico. Fusulinids are predominantly shelf and upper slope organisms
but commonly form detrital particles, and readily drift downslope to be reworked
into toe-of-slope limestones. Such beds at the basin margin obviously cannot be
older than the fusulinids found in them, but can always be younger than their
faunas indicate if reworked forms are present. Normally, resolution of seemingly
anomalous fusulinid occurrences is made if biostratigraphic data are plotted on
carefully correlated electric log cross sections.

4. Seismology: Seismic profiles probably offer the clearest demonstration of
subsurface topographic relief and the ability to distinguish such from faulting and
structural downwarping, Guzman (1967), Fig. 11-23) offers an illustration of this
for the Golden Lane in Tamaulipas, Mexico.

Stratigraphic Sequences in Carbonates of Epeiric Seas
on Shelves and in Shallow Basins

Sequences of both uniform and cyclically alternating, geographically widespread
thin rock units are common behind shelf margins. Such sequences include open
and restricted marine and evaporite environments of the carbonate facies spec-
trum (belts 7- 9). They also occur in shallow cratonic basins where a wider range
of facies belts from the basin center to the shelf may be present but wide and
irregular. Petroleum geologists have long termed such sequences "onionskin" or
"layer-cake" stratigraphy. The surfaces over which such strata are deposited are
extremely flat, generally dipping only 20-30 cm per km (less than the slope of the