The fossiliferous and bioclastic Lexington Limestone underlies the inner Blue Grass region of central Kentucky. The formation is mostly of late Middle Ordovician age, but the upper part is equivalent to the basal beds of the type Cincinnatian Series (Late Ordovician) of southwestern Ohio. The Lexington rests disconformably on the Tyrone Limestone of Wilderness Age and is overlain conformably by interbedded limestone and shale of the Clays Ferry Formation. In the east-central part of the outcrop area, the Lexington is more than 320 feet thick, but it thins to the north, west, and southwest by the intertonguing of the upp•er part with the Clays Ferry Formation. The Lexington Limestone is abundantly fossiliferous, and both the calcarenites and calcisiltites are composed largely of fossil fragments. Bryozoans crinoid debris, brachiopods, mollusks, and stromatoporoid~ are all common. The Lexington is unusually phosphatic. Calcarenites average 2.4 percent P~Or.. The phosphate, present as cryptocrystalline carbonate-fluorapatite, occurs as fillings and replacements of small fossils. The Lexington Limestone has been divided into 11 members. These are the Curdsville Limestone Member (mostly calcarenite), the Logana Member (calcisiltite and shale) the Grier Limestone Member (fossiliferous limestone), the P~rry ville Limestone Member (calcilutite), the Brannon Member (calcisiltite and shale), the Sulphur Well Member (bryozoan limestone), the Tanglewood Limestone Member (calcarenite), the Devils Hollow Member (gastropodal calcirudite and calcilutite), and the Greendale Lentil, the Stamping Ground Member, and the Millersburg Member (nodular fossiliferous limestone and shale). The members are complexly intertongued. The foss.iliferous limestone and nodular fossiliferous limestone and shale units formed in marine water of the infralittoral zone where conditions were most favorable for growth of the fauna. Calcareous skeletal material that formed in the infralittoral zone was broken and sorted and deposited in shallow wateT by tidal currents to produce crossbedded calcarenite; calcareous fines were transported from shallow water and deposited in deeper water of the circalittoral zone along with terrigenous silt and clay from a distant source to produce interbedded calcisiltite and shale. Shallow subtidal and inte•rtidal calcilutite was deposited as pelleted lime mud in lagoons behind calcarenite bars. Terrigenous clay and silt derived from distant tectonic lands in and bordering the Appalachian geosyncline were deposited below wave base and in shallower water where they were trapped by the baffling effect of the epifauna and flora. Transgression and regression of the rock types resulted fr?m varying rates of subsidence. During the time of deposition of the lower half of the Lexington Limestone, the sea floor sloped northward; calcarenite, formed above surf base in the south, grades successively northward to fossiliferous limestone, nodular fossiliferous limestone and shale, and interbedded limestone and shale. During deposition ...
New geologic mapping and application of stratigraphic details previously unknown in Belt rocks of Middle Proterozoic age in the northwestern corner of Montana has led to the definition of a previously unrecognized thrust belt. The zone, herein named the Libby thrust belt, is a complex of thrust and high-angle normal and reverse faults about 15 mi wide and 100 mi long that extends southerly from the Canadian border through Libby, Montana, to the Hope fault zone. Dominating the bedrock of the area is the Belt Supergroup, a sequence of low-grade metamorphic strata at least 50,000 ft thick that accumulated as siliciclastic and carbonate sediments along the continental margin about 1,400-900 m.y. ago. The lowest exposed strata (Prichard Formation) contain turbidites and black pyritic argillites representing early deposits in a rift zone. The base of the Prichard is not exposed. Above the Prichard is the Ravalli Group, a sequence of quartzites, siltites, and argillites showing shallow-water features and that suggest the rate of basin filling approximated the rate of basin subsidence. The Ravalli Group includes in ascending order the Burke, Revett, St. Regis, Spokane, and Empire Formations. Overlying the Ravalli Group is the middle Belt carbonate, an informal grouping which includes the stratigraphically equivalent and laterally interfingering Wallace and Helena Formations. The Wallace is characterized by carbonate-bearing siliciclastics; whereas, the Helena is characterized by cyclic deposits of calcite-and dolomite-cemented silt beds that alternate with dolomite beds. Overlying the middle Belt carbonate is the Missoula Group, a sequence of red and green siliciclastic beds, some carbonate, and the Purcell Lava. Formations of the Missoula Group in ascending order include the Snowslip, Shepard, and Mount Shields Formations, the Bonner Quartzite, and the McNamara and Libby Formations. Most formations reflect shallow-water deposits of braided stream, mud flat, and shallow-shelf environments. The top of the Missoula Group is eroded and is overlain disconformably at a few places by the Flathead Quartzite of early Middle Cambrian age. Paleozoic sedimentary rocks are sparse in the area and are confined mostly to the Libby thrust belt. They include, in ascending order, the Middle Cambrian Flathead Quartzite, the Wolsey Shale, the Middle and Upper Cambrian dolomite of Fishtrap Creek, and one small outcrop area of unnamed Ordovician quartz arenite and dolomite beds (mapped with Cambrian dolomite).
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