Page 439 - Carbonate Facies in Geologic History

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Plate IX. Grainstones

(A) Grainstone (pelsparite), standard microfacies 16. Peloids ~omposed partly of
micritized and rounded, abraded, tubular foraminifera. Top of a cycle in the
Pennsylvanian Virgil ian Holder Formation in the Sacramento Mountains (see
Chapter VI). Sample 23-39 from the west end of a ridge north of and above
Alamogordo, Indian Wells Canyon, New Mexico. Thin section, x 15
(B) Onkoidal grainstone, standard microfacies 13; MF-9, Chapter VIII. Sam-
ple HG-6 is Late Triassic (Norian) near backreef sediment behind the Hohe Gall
coral ramp (Chapter VIII). Particles are dasycladaceans and coral debris coated
with micrite and nubicularid foraminifera and blue-green algae. The coarse sedi-
ment is cross-bedded and represents very shallow shoal sands probably at or close
to the reef crest in the splash zone. Note the coarse, centripetally oriented crystals
of cement between the grains. Thin section, x 10

(C) Lag grainstone, standard microfacies 14. A composite of mixed resistant
particles of blackened, phosphatized and iron-stained lithoclasts and peloids and
badly worn, resistant echinoderm and foraminiferal pieces. Other "lags" com-
monly possess bones and teeth. This sample 23-38 is from a thin persistent bed at
the contact of two sedimentary cycles in the Upper Pennsylvanian (Virgilian)
Holder Formation on a ridge of Indian Wells canyon above Alamogordo, Sacra-
mento Mountains, New Mexico. The bed persists across the shelf area, trans-
gresses older cycles and represents a long period of non-deposition in shallow
marine reducing environment. Its grains have been compacted by solution possi-
bly owing to occasional saturation by meteoric water. Thin section, x 15