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Wave and Gravity-Controlled Dispersal of Mud in the Lower Ordovician Beach Formation, Newfoundland, Canada: Multi-Scale Facies Heterogeneity and Diagenetic Framework


The suitability of mudstone to function as either source, reservoir or seal rocks depends on the sum total of all physical, biological and chemical rock attributes across an entire basin. This study presents sedimentological, ichnological and geochemical data of exceptionally preserved mudstones originating from hyperpycnal and wave-advected sediment gravity flows in the Lower Ordovician Beach Formation (Bell Island Group), Newfoundland. Seven mudstone facies are described, based on textural, compositional and ichnological characteristics. Mudstone deposits interpreted to originate from hyperpycnal flows are well-cemented, exhibit high chlorite-illite ratios, and contain well-developed grain size breaks with a tripartite-subdivision. Conversely, deposits of wave-enhanced sediment gravity flows are poorly cemented and composed of predominantly illite. The latter mudstones exhibit decimeter-thick combined-flow structures, as well as laterally discontinuous, unbioturbated mudstone layers with abundant mudstone-on-mudstone and mudstone-on-sandstone erosional contacts. Deposits originating from hyperpycnal flows are sparsely (0-10%) bioturbated by a shallow-tier assemblage of millimeter- to centimeter-sized Planolites, Trichophycus and rare Arenicolites, which exhibit simple to complex cross-cutting relationships. The compositional diversity of mudstone within this heterolithic shoreface succession is interpreted to be controlled by the starting composition and residence time of mud in the oxic and suboxic diagenetic zone. Low organic carbon loading from a non-vegetated early Paleozoic hinterland, combined with a high reworking frequency potentially result in a high remineralization efficiency and coevally low preservation potential of reactive organic carbon in this muddy shoreface environment. Burial efficiency and bioavailability are proposed to be critical variables which exert a significant control on macrofaunal colonization patterns and bioturbation intensities within mud-dominated shoreface paleoenvironments. A thorough facies description of shales at a range of scales in combination with compositional analyses can help to refine sequence stratigraphic models in nearshore-marine mud-dominated successions where stratigraphic architectural pattern are controlled by high supply rates of fine-grained sediment rather than long-term changes of accommodation imparted by eustatic sea-level change.