Allochthonous Origin of Small Carbonate Mud Mounds in the Compton Limestone (Mississippian, Kinderhookian), Southwestern Missouri
Jeremiah S. Jackson and Kevin R. Evans
Department of Geography, Geology, and Planning, Missouri State University, Springfield, MO 65897, [email protected]
Models concerning the origin of Waulsortian-type mud mounds have been controversial: allochthonous emplacement, in situ framework, and accretionary buildup processes have been proposed. Newly exposed mud mounds in the Compton Limestone in McDonald County near Jane, Missouri, may provide insight into the controlling mechanisms in the early development of these features.
The Compton Limestone has remarkably uniform bedding across much of its outcrop area. Near Jane, it is 3.0-8.5 m thick along a 250-m road cut, where it conformably overlies the Bachelor Formation, a 10-20-cm-thick sandy green-gray shale that rests with angular unconformity on the Upper Devonian Chattanooga Shale. Mounds are found in the upper part of the Compton Limestone, and the uppermost are draped by the Northview Formation, 0.2-1.0 m thick.
Mounds are asymmetrical, ranging from 3 m high and 9 m long to approximately 1 m in height and length. The mound cores are mostly composed of lime mudstone with irregular wispy silt seams; they are homogenous with no obvious framework, and fauna include brachiopods and disarticulated crinoid debris. The uppermost mounds have digitate pores approximately 4 cm wide, which are occluded by sparry calcite cement. The mounds have no obvious flank beds in the inferred upslope direction, and one mound can be traced downslope laterally to a feature that resembles a duplex structure.
We interpret mounds as cohesive masses that were derived from upslope slumping due to compaction of the Chattanooga Shale, flexure, and oversteepening of the depositional surface in this middle ramp setting.
AAPG Search and Discovery Article #90095©2009 AAPG Eastern Section Meeting, Evansville, Indiana, September 20-22, 2009