Page 204 - Carbonate Facies in Geologic History

Platy Algal Mounds in Basins and on Shelves 191

'IEL.O 0_ WU ...
WIT" ALGAL,.
"LATE ROCK Of
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0111 TCWNS[ NO "01[ .. ..
1------1 " ..
10 KM

LtA COUNTY

TECTONIC GRAIN ANO FACIES MAP OF LOWER WOLFCAMP-
ALPHABET ZONES LEA COUNTY, NEW MEXICO

Fig.VI-19. Paleotectonics and facies of Lower Wolfcamp Alphabet zones, Lea County, New
Mexico on shelf north of Delaware basin

Understanding of shelf mounds both from Kansas and northern Texas (e.g.,
Possum Kingdom Dam area) suffers from a lack of published information about
the western downdip (subsurface) relations. On the other hand, extensive three-
dimensional subsurface study of Wolfcampian algal plate lenses has been made
across the shelf north of the Delaware basin. In New Mexico the Baugh field
"Alphabet" zones A through D (Fig. VI-19) are developed in thin cyclic lime-
stones, the middle and upper portion of each being brecciated algal plate bio-
stromes. Reservoir development is best where this facies is developed; it preferen-
tially occurs on downthrown sides of growth faults and slightly downslope from
structural crests. The upper portion of each cycle is a reddish, brecciated "soil zone"
developed at marine regressive stages.
Thus, while platy algal mounds are best developed on shelf margins, the plants
actually inhabited a variety of paleogeographic settings and their distribution and
size of accumulation varies in part according to structural movement and in part
according to vagaries of climate and terrigenous influx.