LITHOLOGY AND ORIGINS OF MUD MOUNDS IN THE COMPTON LIMESTONE (KINDERHOOKIAN SERIES), MCDONALD COUNTY, MISSOURI

Waulsortian mounds are common in lowermost Carboniferous (lower Mississippian subsystem) successions around the world. Classic buildups, up to 300 m thick, near Dinant, Belgium are type examples; they commonly show several stages of development with downlapping clinoforms of grainstone-rich flank beds. In contrast, the cores of build-ups commonly are lime mudstone to wackestone with skeletal material, stromatactis, and other marine cements, but the proposed mound-forming framework organisms responsible for the binding or trapping of carbonate muds has been somewhat controversial. Other proposed mechanisms for the growth of build-ups include in situ hydrocarbon seeps, hydrothermal vents, or upwelling. Factors such as paleobathymetry, paleogeography, and position in slope or ramp settings may have influenced their development.

Recent road construction along US Hwy 71 has exposed a series of lower Mississippian (Kinderhookian Series) mounds in the Compton Limestone near Jane, Missouri that vary from 2 – 12 m wide and are up to 4 m thick. Comparable mounds are exposed in bluffs along the Elk River in Noel, Missouri and a few other locations in southern Missouri and northern Arkansas. The mounds are diminutive and differ significantly from Waulsortian-type mounds; key features of their distribution and morphology suggest a complex origin.

The Compton Limestone is a fine-grained peloidal grainstone and neomorphic wackestone, containing minor fossil debris. Mound facies also are composed of fine-grained peloidal grainstone and wackestone but tend to contain more bryozoans and pockets of crinoidal hash with fine fracture- and vug-filling carbonate cements. Most geopetals indicate that these mounds have had little or no rotational movement, although one isolated brachiopod shows at least 40° of rotation and was collected near a small syndepositional microfault. Mounds lack flank beds and a have a shallow slopes that taper out laterally or form contiguous beds. The largest mound cuts the bed that it overlaps but constitutes that same bed on the opposite side. That bed can be traced laterally to two syndepositional duplex structures. Provisionally, we interpret these build-ups as allochthonous slide blocks or mud mounds that accumulated in shallower water and were transported to a deeper-water setting.