Page 198 - Carbonate Facies in Geologic History

Platy Algal Mounds in Basins and on Shelves 185

Cyel ..

l00"'L

100 ..
I":::~ Copping grain,t'one t},g,ol Carbonat. breccia
Slope foci •• .
~ Algol pial. mov"d.
dark .hale, lill. Lu,
I:(·.~:.I flook bod. i:. -=1 Sondllon.

Fig. VI-13. Cyclic sedimentation ofVirgilian strata at shelf margin in Horquilla Formation at
Cement Tank Canyon, west side of Big Hatchet Mountains, southwestern New Mexico.
Terrigenous slope sediments alternate with algal plate mounds and carbonate slope breccias
and flank beds. Cycles are in brackets. Not much vertical exaggeration. From dissertation by
Martin Schiipbach, Rice University, Houston, Texas

Platy Algal Mounds in Basins and on Shelves

Platy algal mounds are also part of large biohermal masses developed as major
offshore banks in the subsurface Midland basin. These have steep sides (8-
10 degrees) and many form very thick masses. For example, the Scurry County,
Jameson, and Nena Lucia buildups and oil fields of northwest Texas may have
had original depositional relief of a hundred meters (Fig. VI-14). Such large banks
developed in basins owing to rapid carbonate sedimentation over the crest or on
flanks of previously existing topography~ither erosional or tectonic. In areas of
rapid subsidence, only the highest parts of any area managed to remain in the
photic zone long enough for algal and carbonate mud accumulation to be main-
tained. Large isolated banks thus developed in the Midland basin away from the
major carbonate shelf margin. In a faunal study of strata comprising Nena Lucia
bank, Toomey and Winland (1973) indicate that this Middle Pennsylvanian
buildup had 100 m of relief on its open sea (northwestern) side. It is about 20 km
long and 3-5 km across. The buildup consists principally of brecciated lime
wackestone with abundant algal plates. The northwestern side has a concentra-
tion of porous, partly winnowed algal plates and peloidal foraminiferal wacke-
stone to packstone. Paleontological changes across the bank were noted. Fusulin-
ids, smaller mobile foraminifera, calcispheres, encrusting foraminifera (Tetratax-
is), red algae, and encrusting bryozoans are concentrated with the phylloid algae
on the open water side of the bank. After formation the buildup was buried by
transgressive silty crinoidal grainstone and later by dark shales and limestone.