Paleoenvironmental and diagenetic implications of selected siderite zones and associated sediments in the Upper Atoka Formation, Arkoma Basin, Oklahoma-Arkansas
Campbell, Michael David
Baker, Donald R.
Master of Arts
Occurrences of "clayband" and "black-band" siderite and zones of siderite-replaced marine fossils and associated sediments are reported in selected sandstone and shale sequences of the upper Atoka Formation of Arkansas and Oklahoma. Outcrops near Pocola, Oklahoma and Hackett, Arkansas were examined to characterize the paleoenvironmental conditions of deposition that may have promoted the genesis of siderite. Eight types of bedded or redistributed siderite are defined, each showing specific responses to bioturbation, to physical erosion and transport, and to chemical replacement and redistribution. The clay minerals contained within the siderite zones and associated shale consist of kaolinite and illite in nearly equal proportions, with the former also possibly consisting of a kaolinite-type chamosite and the latter consisting of a significant 2M muscovite polytype. Interpretations based on sedimentary structures, texture and mineralogy of the sediments associated with the siderite zones indicate that specific sequences may have been deposited in a tidal flat or intra-deltaic environment in proximity to normal marine conditions. A primary or syngenetic origin is proposed for the bedded types of siderite occurrence. Other sequences indicate that marine conditions also prevailed. Physio-chemical requirements for siderite genesis suggest that siderite does not form syngenetically in a marine depositional environment but could syndiagenetically result under normal post-burial conditions involving organic-rich material (fecal matter?) or a subsurface environment with a high Fe+2:Ca+2 ratio and low SO3 in solution. These conditions could promote characteristic siderization of calcareous material within marine sediments in proximity to or down the geohydraulic gradient from areas of either syngenetically-produced siderite or ironrich subsurface water derived from coal-forming swamps. Paleogeographic control of a siderite facies is a distinct possibility, within certain limits, if syndiagenesis plays a subordinate role in siderite genesis or if the character of the siderization of marine sediments can be contrasted to the character of the siderite formed syngenetically, as indicated in this investigation.