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The Westphalia Limestone of the Northern Midcontinent: A Possible Ancient Storm Deposit
Stanton M. Ball
Two very different rock types, a fusulinid lime packstone to grainstone and an ostracode coaly lime mudstone, form the vast majority of northern Midcontinent Westphalia (Stranger Formation, Douglas Group, Virgilian, Pennsylvanian) outcrops. A mixed-fossil lime wackestone is the principal Westphalia rock type in the subsurface.
Petrologic characteristics such as mixed faunas, abrasion and polish of grains, grading in the form of hands of larger invertebrates in the fusulinid sandstone, lime mud rip-clasts as much as 7.5 inches in long dimension, shale pebbles, local current lineation, and the concentration of trillions of fusulinids along with many other invertebrate grain fragments within the fusulinid lime packstone to grainstone are believed to record a current-transported sediment load derived from a shallow seafloor area estimated very conservatively at 1450 square miles.
The ostracode coaly lime mudstone displays well-preserved lamination, extremely limited fauna interpreted as indicative of a fresh-to-brackish water habitat, and contained plant debris. This rock type lacks physical connection with the fusulinid lime packstone to grainstone facies. Both outcropping rock types overlie deposits of different types in different areas.
The subsurface mixed-fossil lime wackestone facies extends over the largest area and is westward of the outcrop facies. Although widespread, the Westphalia is much less so than most Midcontinent late Missourian-early Virgilian limestones and is completely absent from large areas within its limits of extent.
Ball (1964) documented the regional depositional framework for the Douglas Group, which includes the Westphalia Limestone and showed that depositional basin margins were persistently located within the area of, and to the east-southeast of, present-day outcrops. Within this depositional framework, the mixed-fossil lime wackstone of the subsurface is believed to have been the source of most constituents of the centrally-located fusulinid lime packstone to grainstone facies as well as the lime mud of the eastward facies, the ostracode coaly lime mudstone. This fit within the regional depositional setting as well as the petrologic characteristics cited above, when compared to those of Recent storm deposits given below in the introduction, indicate that the fusulinid sand and the ostracod rock are shore zone tidal and supratidal storm deposits.
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