Microfacies and Sedimentary Structures in Deeper-Water Lime Mudstones: ABSTRACT

Abstract

Limestone deposited in downslope or in basinal positions possesses several distinctive criteria based on microfacies, bedding, and fauna. The microfacies include (1) homogenous lime mudstone, (2) millimeter-laminated End_Page 485------------------------------ 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 generally are dark, but light colored and even pink or red varieties are known. Distinctive monotonous bedding, consisting of planar relatively thin (½- to 2-foot) limestone beds 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 structur s indicative of soft sediment slumping are scarce. 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 places 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. In several locations lime mudstone mounds are seen slightly upslope from basinal lime mudstone. Both the even and regularly bedded lime mudstone and calcisiltite and the mudstone mounds contrast with characteristic limestone turbidite (allodapic limestone) which probably represents accumulations down from relatively steeper slopes, in more tectonically active environments, and perhaps downslope from shelves with higher water agitatio . Numerous examples of deeper-water but non-turbiditic limestone are discussed from basins and geosynclinal troughs in the late Paleozoic of the western United States, the Cretaceous and Jurassic of Mexico, the Middle East, and Europe. End_of_Article - Last_Page 486------------