Abstract

A stratigraphic interval of 1846 feet belonging to the Lower and Middle Bird Spring Group and ranging from Lower Morrowan to Lower Missourian has been investigated petrographically in order to unravel the cyclic sedimentation of this miogeosynclinal limestone sequence. More than 2095 samples collected at an average interval of 10 inches were divided into eleven microfacies, given here in order of decreasing relative depth of deposition or increasing energy level: an almost pure calcisiltite (microfacies 0), a calcisiltite with scattered sand-size organic debris (microfacies 1), a calcisiltite with abundant scattered sand-size organic debris (microfacies 2), a grain-supported biocalcarenite with calcisiltite matrix (microfacies 3), a grain-supported biocalcarenite with clear calcite cement (microfacies 4), and a grain-supported oolitic biocalcarenite with clear calcite cement (microfacies 5). Microfacies designated as 0a, 1a, 2a, 3a and 4a are the detrital quartz-rich equivalents of microfacies 0 to 4 and include rocks with more than 10 percent of detrital quartz. The following microscopic parameters were measured: frequency and clasticity of crinoid fragments, oolites, pellets (fecal, lithic and algal coated) and detrital quartz; frequency of debris of brachiopods and bryozoans, ostracodes, calcispheres, arenaceous benthonic foraminifers and pyrite. These data, together with symbols representing many other minor components were plotted alongside the columnar section as variation curves showing the stratigraphic behavior of the parameters. A combination of these measurements with the above textural classification of the microfacies led to the drafting of a relative bathymetric curve (expressing in this environment also the energy level) which represents the final interpretation of the oscillations in energy level and related water depth during deposition. A total of 78 complete cycles were recognized. They are variants of 3 fundamental ideal cycles (or statistically most frequent successions of microfacies) established on the basis of the average values of all the measured parameters. Two of these ideal cycles characterize the Morrowan which consists of 46 cycles with features expressing a gradual transgression. A massive inflow of fine detrital quartz coincides with the deepest phase of the transgression. The third type of ideal cycle is typical of the Atokan-DesMoinesian-Missourian interval which consists of 32 cycles with features indicating a gradual regression. Therefore the investigated sequence is a complete major geological cycle of transgression and regression.

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