• Journal of the Geological Society of Korea
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• v.54 no.5
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• pp.477-488
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• 2018

Fluid muds commonly occur in estuarine environments, but their ancient examples have rarely been studied in terms of depositional characteristics and processes. Cores of estuarine channel deposits of the Early Cretaceous McMurray Formation, Alberta, Canada show various mudstone layers that possess depositional characteristics of high clay-concentration flows. These mudstone layers are examined in detail through microscopic observation of thin sections and classified into three microfacies (<1 to 25 mm thick) on the basis of sedimentary texture and structures. Structureless mudstone (Microfacies 1) consists mainly of clay particles and contains randomly dispersed coarser grains (coarse silt to fine sand). This microfacies is interpreted as being deposited by cohesive mud flows, i.e., fluid muds, which possessed sufficient strength to support suspended coarser grains (quasi-laminar plug flow). Silt-streaked mudstone (Microfacies 2) mainly comprises mudstone with dispersed coarse grains and includes very thin, discontinuous silt streaks of coarse-silt to very-fine-sand grains. The texture similar to Microfacies 1 indicates that Microfacies 2 was also deposited by cohesive fluid muds. The silt streaks are, however, suggestive of the presence of intermittent weak turbulence under the plug (upper transitional plug flow). Heterolithic laminated mudstone (Microfacies 3) is characterized by alternation of relatively thick silt laminae and much thinner clay laminae. It is either parallel-laminated or low-angle cross-laminated, occasionally showing low-amplitude ripple forms. The heterolithic laminae are interpreted as the results of shear sorting in the basal turbulent zone under a cohesive plug. They may represent low-amplitude bed-waves formed under lower transitional plug flows. These three microfacies reflect a range of flow phases of fluid muds, which change with flow velocities and suspended mud concentrations. The results of this study provide important knowledge to recognize fluid-mud deposits in ancient sequences and to better understand depositional processes of mudstones.

• Journal of the Korean earth science society
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• v.21 no.3
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• pp.337-348
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• 2000

The Middle Carboniferous Manhang and Geumcheon formations in Taebaek area consist of sandstones, shales, and limestones. The limestones of the formations contain abundant conodonts, fusulinids, crinoids, brachiopods, bryozoans, corals, etc. This study was carried out to investigate the microfacies of limestones and conodont faunas of the formations and to determine their paleoecology in detail. The limestones of the Manhang and Geumcheon formations of the study area consist of wackestone and packstone which are composed of crinoid fragments and other various fossil fragments. Some limestone beds of the Geumcheon Formation consist of only Chaetetes corals which indicate that the limestones deposited in a warm shallow sea. Conodonts found from limestones of the Manhang Formation are Neognathodus bothrops, N. medexultimus, Hindeodus minutus, Diplognathodus coloradoensis, D. edentulus, Idiognathodus delicatus, Streptognathodus elegantulus, and S. sp. And conodonts found from the limestones of the Geumcheon Formation are Neognathodus medexultimus, N. roundyi, N. dilatus, Gondolella bella, Diplognathodus coloradoensis, D. edentulus, Hindeodus minutus, Idiognathodus delicatus, and Streptognathodus elegantulus. Among these conodonts, Diplognathodus coloradoensis, D. edentulus, and Hindeodus minutu, are found generally from limestones which deposited in the shallow seas. According to the limestone facies and conodont faunas of the Manhang and Geumcheon formations of the study area, it can be concluded that the limestones of the formations deposited in the shallow sea.