Page 98 - Carbonate Facies in Geologic History

Supratidal (Sabkha) Evaporites and Related Carbonates of the Intertidal Zone 85

68. Caliche breccia and wavy laminites
Calcareous micritic crusts up to 1 or 2 m thick formed on surfaces of subaerial
exposure by continuous solution and reprecipitation of CaC0 3 over long periods
of time, usually in semi-arid climates where there are repeated long dry periods
alternating with a wet season. The wavy laminae are convex downward, develop-
ing progressively at the base of the crust. The continuous wetting, drying-out, and
calcite precipitation results in brecciation (Dunham, 1972, Fig. II-39. See also
under Diagenesis, this Chapter).
69. Vadose pisolite (Plate XVB)
Large (up to several cm in diameter) concentrically laminated concretionary
growths interpreted to form in limestones in the vadose zone. The balls are
commonly bound together by sparry cement or encompassing laminae. Other
evidence for in situ growth includes a tendency for the pisoids to form in down-
ward elongation and to fit together. They commonly show reverse grading. Piso-
lites may cut across normal bedding but may also follow it. They apparently
originate as fresh-water vadose caliche, or as "cave pearls", or perhaps also in the
littoral splash zone (Dunham, 1969a; Dunham, 1972, II-27, II-40).

70. Evaporite solution breccia
Breccia with abundant vein calcite usually in widely distributed and thin beds
with a sharp base and irregular top. Caused by collapse of carbonate layers due to
solution of interlayered anhydrite (Blount and Moore, 1969; Dunham, 1972, p.II-
51,52).
71. Salt hoppers and selenite bladed crystals (Plate XV C)
Isolated molds and casts of the typical crystal forms of halite and gypsum may
appear scattered within carbonate beds. The scattered slash-mark of selenite
blades is most common. These mark solution voids or carbonate replacements of
the soluble original minerals which earlier replaced carbonate sediment and grew
in its void spaces on sabkhas (salt flats).
72. Ptygmatic or enterolithic structure in gypsum-anhydrite
Contorted or thin beds of calcium sulfate probably caused by in situ crystal
growth and consequent volume increase under vertical confining pressure. The
structure has also been considered to form by coalescence of nodules perhaps
during compaction. That it is commonly an early diagenetic structure is evi-
denced by its presence in the upper meter of sediment in modern sabkhas of the
Persian Gulf. Tiny nodules and fine enterolithic structure are also described in
laminated evaporites considered basinal by Davies and Ludlam (1973, p.3533).
(For illustrations of evaporites considered intertidal, see Maiklem et al., 1969,
Shearman and Fuller, 1969, Fig.4).

73. Nodular, pearl, flaser-chicken wire anhydrite-gypsum
Nodular structures formed generally by compaction of various sizes and
amounts of calcium sulfate nodules in sediment of varying permeability. Such
structures are seen today forming on modern sabkhas but also result from com-
paction following burial (Shearman and Fuller, 1969, Fig.6, pI.2; Maiklem et aI.,
1969; Bebout and Maiklem, 1973).