Page 97 - Carbonate Facies in Geologic History

84 Outline of Carbonate Petrography

zation in tropical tidal flats (Shinn et aI., 1965, p.1l8, 119; Shinn et al., 1969,
p.1217).
61. Minor beach deposits
Thin beds of fine sand and shell hash with low angle cross bedding formed on
beaches separating the edges of flats from large lagoons. In many places these
beaches are exclusively of cerithid snail shells (Shinn et aI., 1969, p.1212, Fig. 15).
62. Burrows
Filled traces of organic burrowers, dominantly vertical, rather long, distinct,
and isolated in sediment that is intertidal and partly lithified as contrasted with
subtidal sediment that is thoroughly homogenized, horizontally burrowed with
shallower penetration (Frey in Perkins, 1971).
63. Tracks and trails on bedding planes
Organic markings of land or amphibious animals on bedding planes usually
indicate intermittent exposure in lime muds which are readily hardened only by
subaerial exposure. Arthropods, snails, mollusks, and vertebrates all commonly
leave trails and tracks on such surfaces (Perkins, 1971).

Supratidal (Sabkha) Evaporites and Related Carbonates of the Intertidal Zone
(Standard Facies Belts 8 and 9 under a Rigid Evaporitic Climate)
64. Sheet cracks in limestone or dolomite
Planar cracks concordant to bedding attributed to shrinkage of sediment,
perhaps due to drying out or to de-watering in an aqueous environment with
varying brine concentration (Fischer, 1964, p.148).
65. Zebra limestone or dolomite
Rock banded by parallel sheet cracks filled with calcite or dolomite druse.
Bands are at an angle to normal bedding. It is not known whether the steep angle
represents traces of bulbous algal mats coating lime mud mounds or failure by
slumping of piles of lime mud along lines normal to stress. Perhaps a shrinkage
process causes the structure. Such oblique bands are known in sediment within
fissures (Fischer, 1964, p.135).
66. Druse veins
Concordant and discordant veins of sparry calcite in the form of coarse druse
or palisade crystals filling an irregular cavity or opening. The walls of the fissure
are lined with projecting crystals usually of the same minerals as those of the host
rock (Fischer, 1964; Dunham, 1972, p.II-3, 11-47).
67. Tepee structures
Cross sections of very large desiccation polygons with prism cracks. In a cross
section they appear as disharmonic folds, resembling chevrons or inverted, de-
pressed V s. The structures are obviously caused by expansion of the rock, pre-
sumably during hydration of anhydrite interbeds or failure of the limestone dur-
ing expansion caused by crystal growth or extreme variations in temperature.
Parallel beds below and above such features clearly show them to be syngenetic
(Newell et aI., 1953, p.126; Kendall, 1969, Fig.9; Assereto and Kendall, 1971;
Smith,1974c).