Page 437 - Carbonate Facies in Geologic History

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Plate VIII. Bivalve Coquinas, Bioclastic Packestones,
Standard Microfacies 12

(A) Oyster shell hash, packstone with pelleted calcisilt matrix is represented by
sample No. 64, from the upper 100 m of the Cupido Formation (Lower Creta-
ceous), Arteaga Canyon, near Saltillo, Coahuila, Mexico. Sediment is of shallow
water, bank-interior facies. Note the umbrella effects, patches of white coarse
crystalline calcite under rounded shell fragments. Such biomodal sorting occurs
in the sediment where the larger shell fragments were abraded and worn by wave
action on the shoals and ultimately deposited with pelleted mud in quieter areas.
Note borings in the largest piece of oyster shell. Thin section, x 12

(B) Shell hash, packstone with abundant micritic matrix. Shell debris is exclu-
sively of bivalves, somewhat worn and with micrite rinds. Umbrella effects are
seen beneath shell where finer sediment has settled down leaving calcite-filled
original void space. Top is to upper left. The sediment represented by sample SP-2
was deposited in a shallow basin and forms a limestone bed in a dark shale
sequence in the Kossen basin below the Steinplatte reef in Austria (Fig. VIII-22),
Upper Triassic in age. Thin section, x 9