Page 451 - Carbonate Facies in Geologic History

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Plate XV. Onkoids, Pisolite and Mudstone Backreef Facies

(A) Onkoidal wackestone-packstone, Mumienkalk, standard microfacies 10 and
22 and facies J3, Chapter IX. Large Girvanella-bored algal biscuits encrust shell,
coral, and snail fragments. The matrix consists of a wackestone with fine shell
debris. The white spots are tiny onkoids. The sediment represents a deposit of
shallow water with somewhat restricted circulation. Such sediment may not rep-
resent the original depositional environment of the algal nodules which require
moving water to turn them over at intervals. This condition is not consistent with
the presence of micritic matrix. The nodules may have grown on sides of a
channel and have been buried in deep or shallower water in a muddy facies. The
sample is from Les Sagnettes south of La Chaux-de-Fonds and from the main
algal bed in the MaIm (Upper Jurassic) sequence, Jura Mountains of northern
Switzerland (see Chapter IX, Fig. IX-3). Photograph is courtesy of KQninklijke
Shell Exploration and Production Laboratory and Martin Ziegler. Polished sur-
face, x 2

(B) The well-known pisolite facies of the Upper Permian Capitan Limestone
of Guadalupe Mountains, New Mexico. The exact origin of these particles, which
were originally considered to be onkoids is still debated. The sample probably
represents reworked cave pearls, or vadose pisoid concretions, formed either by
meteoric or marine splash-zone water in porous carbonate sediment subjected
alternately to wetting and extreme drying (see Chapter III and VIII). Note the in-
filtered crystal silt, of possible vadose origin, between the pisoids (Dunham,
1969 b). Sample PRC-6 from switchback curve on the road up Walnut Canyon to
Carlsbad Cavern. Thin section, x 15. (pisoid illustrated is 0.5 cm long)
(C) Homogeneous pure lime mudstone with blades of replacement selenite
crystals, standard microfacies 23. Sample DK-35 is from the evaporitic top of an
Arab D cycle and consists of chalky lime mudstone with early replacement gyp-
sum, probably a sabkha sediment like that on present littoral salt flats. Iraq
Petroleum Company, Dukhan 51 well, about 6900ft depth, southern Qatar. Peel,
x 18