Page 196 - Carbonate Facies in Geologic History

Algal Plate Mounds at Shelf Margins 183

Fig. VI -11. Detailed map of Yucca mound area west flank of northern Sacramento Mountains,
Otero County, New Mexico. Circled numbers are measured sections through lower Holder
F ormation of Late Pennsylvanian age. Wilson (1967 a, 1972)

small biohermal cores are seen downslope (Fig.VI-ll); these diminish in size
basinward and are enveloped by grey-green silt and sand; forestet flank beds are
missing. On the other hand, across the shelf, shoal-water conditions developed
when the total accumulations had reached wave base. Horizontal capping beds of
cross-bedded lime grainstones developed over the mound-flank bed complex.
They contain battered debris of foraminifera, dasycladaceans, and gastropods, in
places considerably altered by surface exposure. This final stage in development
of such a mound complex is seen on Figs. VI-10, VI-25.
Porosity within algal plate-mound facies is complex and irregular. The sedi-
mentary fabric is explained best by exposure to meteoric water before much
lithification had occurred. Collapse and flowage of lime mud within a framework
of delicate, but rigid, algal plates and skeletal fragments resulted in a jumbled
brecciated mass of sediment with many syngenetic fractures. Stromatactoid cavi-
ties were formed and partly filled by geopetal mud. Leaching and solution acted
on this heterogenous mass and attacked particularly the poorly calcified aragonit-
ic algal plates; probably, at the same time, partial infill by internal sediment and
drusy calcite was occurring. Multiple porosity levels in reservoirs show that this
happened many times. On outcrops many biohermal cores are composite show-
ing erosion before deposition of higher parts. This is probably due to sea level
drop and exposure because zones of reddish conglomerate occur at such horizons.
Thickness of such reservoirs in the subsurface is not great but locally good inter-
connection of pore space is present through the non tectonic and secondarily
leached fractures.