DIAGENESIS OF UPPER CARBONIFEROUS ROCKS IN THE OUACHITA FORELAND SHELF IN MIDCONTINENT USA - AN OVERVIEW OF WIDESPREAD EFFECTS OF A VARISCAN-EQUIVALENT OROGENY

Diagenesis of Upper Carboniferous foreland shelf rocks in southeastern Kansas took place at temperatures as high as 100-150 degrees C at a d epth of less than 2 km. High temperatures are the result of the long d istance (hundreds of kilometers) advection of groundwater related to c ollisional orogeny in the Ouachita tectonic belt to the south. Orogeni c activity in the Ouachita area was broadly Late Carboniferous, equiva lent to the Variscan activity of Europe. Mississippi Valley-type Pb-Zn deposits and oil and gas fields in the US midcontinent and elsewhere are commonly attributed to regional groundwater flow resulting from su ch collisional events. This paper describes the diagenesis and thermal effects in sandstone and limestone of Upper Carboniferous siliciclast ic and limestone-shale cyclothems, the purported confining layer of a supposed regional aquifer. Diagenesis took place in early, intermediat e, and late stages. Many intermediate and late stage events in the san dstones have equivalents in the limestones, suggesting that the causes were regional. The sandstone paragenesis includes siderite cement (ea rly stage), quartz overgrowths (intermediate stage), dissolution of fe ldspar and carbonates, followed by minor Fe calcite, pore-filling kaol inite and sub-poikilotopic Ca ankerite (late stage). The limestone par agenesis includes calcite cement (early stage); megaquartz, chalcedeny , and Fe calcite spar (intermediate stage); and dissolution, Ca-Fe dol omite and kaolinite (late stage). The R(m) value of vitrinite shows a regional average of 0.6-0.7%; Rock-Eval T-max suggests a comparable de gree of organic maturity. The T-h of aqueous fluid inclusions in late stage Ca-Fe-Mg carbonates ranges from 90 to 160 degrees and T-mice ind icates very saline water (> 200 000 ppm NaCl equivalent); delta(18)O s uggests that the water is of basinal origin. Local warm spots have hig her R(m), T-max, and T-h. The results constrain numerical models of re gional fluid migration, which is widely viewed as an artesian flow fro m recharge areas in the Ouachita belt across the foreland basin onto t he foreland shelf area. Such models must account for heating effects t hat extend at least 500 km from the orogenic front and affect both sup posed aquifer beds and the overlying supposed confining layer. Warm sp ots indicate either more rapid or more prolonged flow locally. T-h and T-mice data show the highest temperatures coincided with high salinit y fluids.