EARLY TOURNAISIAN MUD MOUNDS FROM MISSOURI--PRECURSORS OF LATE TOURNAISIAN WAULSORTIAN REEFS

Earliest Mississippian (Tournaisian, Tn1) mud mounds in the Compton Limestone of southwestern Missouri represent the oldest Carboniferous buildups known in North America. Small mounds, rarely exceeding 25m in width and 4m in height, appear to represent the earliest organic buildups to follow the Frasnian-Famennian collapse of reefs. These small mounds occupy the identical paleogeographic position, not only as later Tournaisian Waulsortian reefs, but also late Mississippian and early Pennsylvanian buildups. Detailed study of two of these mounds indicates these oldest Carboniferous buildups share the internal texture and faunal composition, but not the size, of later buildups. The Compton mounds display recognizable core and flanking facies. The core consists of light-colored lime mudstone to wackestone with rare Stromatactis void fillings, sparse brachiopods, and fenestrate bryozoans. Flanking beds, composed of crinoidal packstone to grainstone, pinch out a short distance from the core facies.

Faunal collections from both the core and flank facies indicate relatively distinct faunas in each ecologic niche. The depauperate core fauna consists almost solely of scattered brachiopods. The core fauna of the Compton mounds is compositionally similar to the core faunas of buildups from the Fort Payne Formation of Kentucky, the Lake Valley Formation of New Mexico, and the Lodgepole Formation of Montana. However, with 12 identified genera, the Compton buildups are considerably less diverse than the later Tournaisian reefs of the region, which possess more than 25 recognized genera. The lower diversity of the early Tournaisian buildups is interpreted as representing a lag period following the collapse of reef communities during the Late Devonian.