Sedimentary and Diagenetic Features in the Sulfide-Bearing Sedimentary Dikes and Strata of Lower Ordovician Dolomites, Decaturville, Missouri, U.S.A.
Additional studies of the Decaturville cryptovolcanic structure in western Missouri, USA, in a new exposure, has revealed additional information on distribution of sulfide minerals and sedimentary features associated with this structure.
The new roadcut exposes a nearly complete section of the Lower Ordovician Jefferson City and Cotter dolomites, permitting detailed examination of features about two km from the center of the structure along the inner margin of the ring fault which surrounds the structure.
The formations consist almost entirely of dolomite with some sandstone, shale, and chert. Sandstones, in beds from 3 to 300 cm thick in this area, and breccias are interpreted to mean a closer proximity to the source area than other exposures in the Ozarks. In addition, the beds have a cyclic arrangement, whose pattern suggests oscillation from very shallow marine conditions to subaerial exposure. This feature is also not apparent in exposure in the Ozarks although poorer exposures may have obscured necessary details in other places.
Sulfides occur both as stratiform blebs in a few layers and more particularly within the matrix and some breccia clasts in sedimentary dikes which occur in the upper part of the section. They make about 1% of the volume of the dike, sufficient to give the dikes a gray to dark gray color.
The sulfides of the dikes consist of pyrite, marcasite, some galena and sphalerite. The sulfides occur in the following forms: (1) pyrite and marcasite spheres, containing framboids, (2) fragments of pyrite-marcasite with porous pyrite framboids, (3) isolated pyrite framboids, (4) minute fragments of fine-grained marcasite, pyrite or mixed pyrite-marcasite fragments, (5) fragments of colloform pyrite, (6) limonite-coated colloform pyrite fragments, and (7) fragments of sphalerite(?). Each of these occurrences is illustrated and a paragenetic sequence for these is given.
It is postulated that the sulfides were transported into the dikes as detritals along with detrital sand and clay from source beds which can be defined for most dikes.
KeywordsIron Sulfide Pyrite Framboids Diagenetic Feature Breccia Clast Argillaceous Sandstone
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