Litho-, bio-, and sequence stratigraphy of the Boyle-Portwood Succession (Middle Devonian, Central Kentucky, USA)

Abstract

The Middle Devonian (Givetian) of central Kentucky is represented by a complex and somewhat enigmatic succession of sandy dolomitic limestones and chert-rich wackestones, silty dolostones, and thin dark shales belonging to the Boyle Formation and Portwood Member of the New Albany Formation. The lower contact of the Boyle Formation is a distinct, regionally angular unconformity on rocks of Late Ordovician to mid-Silurian age. New exposures of Middle Devonian strata in the vicinity of Irvine, Estill County, Kentucky, reveal a series of distinctive depositional sequences, each with a sharply erosive base, overlain by phosphatic grains, fish teeth, and bone-rich quartz sandstone. The lower sequences, forming the Kiddville Member, are siliciclastic-rich, Zoophycos-bioturbated, silty, dolomitic carbonates that contain typical Hamilton brachiopod associations. The Kiddville is missing in some successions apparently owing to erosional overstep by upper Boyle units (Beechwood Member). The upper Boyle, Beechwood Member, is massive and cherty carbonate, which is highly variable in thickness, owing to major erosional truncation/karstification that locally removes the Boyle completely. Conodont studies indicate that much of the upper Boyle is of Po. ansatus Zone and thus middle Givetian in age. It is probably equivalent to the Moscow Formation of New York State. The overlying Portwood Member of the New Albany Shale belongs to the later Po. ansatus and “Oz.” semialternans zones and consists of dark-brownish-grey shales and muddy dolosiltites. In many localities to the west, the basal unit is a lenticular dolomitic breccia, the Duffin Bed, which may represent a lowstand karst breccia that mantles the irregular upper contact of the Boyle Formation at a regional unconformity. A lower dark shale contains rare small brachiopods Emmanuella and Leiorhynchus, suggestive of the lower Tully fauna of the Appalachian Basin; locally, this shale appears to fill paleokarst in the Boyle carbonates. It is overlain by a dolosiltite that has a highly distinctive “megaburrow” trace fossil assemblage on the base and yields rare Tullypothyridina and the conodonts Po. ansatus and “Po.” alveoliposticus, diagnostic of the Lower Tully Limestone in the Appalachian Basin. It is overlain by dark grey to black, typically deformed (seismites) calcareous mudstone of the middle Portwood Member. An upper pale grey, lenticular massive dolomitic bed rests sharply on the middle Portwood in channel-like scours with relief of about 0.5 m and a width of up to tens of meters. This bed yields poorly preserved corals, atrypid brachiopods (Spinatrypa sp.), and phacopid trilobites as is typical of the Upper Tully member. Overlying thinner bedded rhythmically bedded calcareous mudstones and dark shales of the uppermost Portwood yield conodonts of the “Oz.” semialternans Zone; this lithology resembles the Fillmore Glen interval of the Upper Tully Limestone. The sharply overlying black shale of the Trousdale Member yields hermanni Zone elements, indicating correlation with the basal upper Givetian lower Geneseo black shales of the Appalachian Basin. The highly variable thicknesses, complex internal discordances, and abundant seismites of the Portwood and Boyle Formations suggest a dynamic interval with regional far-field tectonics associated with the onset of the third tectophase of the Acadian Orogeny. Despite these tectonic effects, the presence of a consistent internal sequence stratigraphy that correlates with that of the Appalachian Basin and elsewhere reflects strong eustatic effects.