Page 99 - Carbonate Facies in Geologic History

86 Outline of Carbonate Petrography

Eolianites-Carbonate Dunes

General references are Ball, 1967b, Ward, 1970
74. Steep cross beds, dipping shoreward in large sets
Dips from 25 degrees to 45 degrees common, progressive migration of dunes
downwind resulting in pronounced crossbedding directions shoreward but within
an arc of 180 degrees. The preservation oflarge "sets" of cross beds, up to 15-20 m
is possible.
75. Preserved dune forms
Upward convex forms formed by spillover lobes preserved in eolianites be-
cause of rapid cementation in the vadose zone in some climates and because of
protection by an impermeable caliche crust.
76. "Ribbing" caused by differential cementation oflayers
Alternating fine and coarse layers showing on weathering a characteristic [me
ribbing. This is due to differential cementation because generally the finer sand
sizes hold water by capillary attraction long enough for CaC0 3 cementation,
whereas water drains from the coarser layers which remain loose and therefore
weather deeper.
77. Red zones
Old soil zones represented by limonitic red or orange-weathering, brecciated,
powdery and chalky beds containing chunks of cemented eolianite.
78. Snails and calcified insect cocoons
These occur in old soil zones. The snails are the only body fossils commonly
preserved. Tiny egg-shaped cocoons Occur rarely in these zones.
79. Root casts and root hairs
Rhizocretions are calcareous lined tubes or solid cylindrical forms resulting
from calichification and cementation along roots. Even root hairs may be so
preserved as tiny calcareous sheaths. The characteristic dense network and down-
ward decrease in size of branches distinguish these from burrows which they
superficially resemble. These masses occur commonly below old soil zones.

Environmental Analysis of a Carbonate Thin Section

A great amount of environmental information is available from just a single thin
section, as illustrated by study of Plate I. The field is about 1.0 by 1.4 cm and
magnification is X 16, plane light. The sample is from the top of a 3 1 / 2 m thick bed
of gently foreset strata forming a capping bed above strongly foreset dipping
strata constituting biohermal flanking beds (Fig.XII-6). It is a variety of SMF -5. It
is of Virgilian (Late Pennsylvanian) age and is from the Sacramento Mountains,
north wall of Dry Canyon above New Mexico State highway 52, 3 miles northeast
of Alamogordo, New Mexico. This single thin section indicates the water salini-
ty, wave energy, coherence and oxidation of substrate sediment, how the sediment
accumulated, and to some extent its later diagenetic history. Proper vertical orien-
tation of the sample is also seen from its fabric.