Limestones deposited in downslope or in basinal positions possess several distinctive criteria based on microfacies, bedding, and fauna. The microfacies include (1) homogeneous lime mudstone, (2) millimeter laminated lime mudstone, and (3) common micropelletoid lime grainstone (calcisiltite). The latter may show fine ripple cross-lamination. These rock types may occur in graded sequences but non-graded alternations are also common. Several kinds of minor sedimentary rhythms are recognized. The limestone types are generally dark but light-colored and even pink or red varieties are known. Distinctive monotonous bedding, consisting of planar, relatively thin (½ to 2 foot) limestones interlayered regularly with thinner beds of dark shale, is common. Chert beds may be intercalated in such sequences. Spectacularly large cut and fill structures (stretching more than 100 yards) and presumably caused by submarine penecontemporaneous sliding are seen in places. Convolute bedding and flame structures indicative of soft sediment slumping are rare. In several locations lime mud mounds are seen to occur slightly upslope from basinal lime mudstones.
Distinctive faunal assemblages exist in these beds. Siliceous and calcareous microplankton (radiolarians, diatoms, calcispheres, and tintinnids), sponge spicules, graptolites, pelagic or nektonic pelecypods, pelagic foraminifera (globigerinids), pteropods, and certain ammonoids are especially characteristic.
Field observations of this limestone type show that it commonly forms a sort of apron down very gentle slopes from typical carbonate shelves, and in many caWs regionally is peripheral to a depositional basin, the center of which contains a thin section of dark shale. Recognition of this facies permits one to predict proximity to a carbonate shelf margin. Stratigraphic relations at the foot of several such shelf margins indicate that basins in which this type of deposition occurs may be from 300 to 2,000 ft deep. The even and regularly bedded lime mudstone and calcisiltite, even though found also in geosynclines, contrast with characteristic limestone turbidites (allodapic) limestones which are more common accumulations down from relatively steeper slopes in more tectonically active environments, and perhaps downslope from shelves with higher water agitation. Numerous examples of “deeper water” but nonturbiditic limestones are discussed from basins and geosynclinal troughs in the Late Paleozoic of the western U.S.A., the Cretaceous and Jurassic of Mexico, the Middle East, and Europe.
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Depositional Environments in Carbonate Rocks
One of the principal tasks of the geologist is to determine the depositional environments in which rocks are deposited. Although regional environmental interpretations of transgressions and regressions, movements of shoreline, and gross aspects of continental and marine sedimentation have been understood since stratigraphy became an established branch of geology, only recently has the science of sedimentology come up with criteria for environmental recognition of specific outcrops, wells, or even hand samples. This observation is especially true of carbonate rocks. The papers in this volume will provide a key to the subject of recognition of depositional environments in carbonate rocks. Based on a symposium held in Los Angeles, California, on April 1967, at the joint meeting of AAPG and SEPM.