ABSTRACT A succession of Ordovician and Mississippian carbonates, separated unconformably, is exposed across the southern flank of the Ozark Dome in southwest Missouri. Deposits of both periods exemplify typical facies of the Midwestern United States: carbonate tidal-flat assemblages for the Early Ordovician and carbonate shelf environments for the Early–Middle Mississippian. The basic stratigraphic sequence of these deposits has been known for over a century, but interesting features remain to be addressed. Thin discontinuous sandstones are present within the Early Ordovician Cotter Dolomite, but the informal Swan Creek sandstone member seems anomalous. This sandstone can exceed 5 m in thickness and is fairly continuous across southwest Missouri. Most Ordovician sandstones in Missouri mark major transgressions above regional unconformities, but not the Swan Creek, and there is no obvious source of the sand. Therefore, we hypothesize that the Swan Creek represents reworked eolian dunes blown across the broad peritidal environment. Clastic sandstone dikes, apparently sourced from the Swan Creek, cut across beds of Cotter Dolomite near faults. We propose that these dikes are evidence of local faulting and seismicity during the Early Ordovician. Early and Middle Mississippian limestones comprise a sequence of shelf deposits, although mud mounds and other facies changes near the Missouri-Arkansas line mark the edge of the Mississippian shelf and the transition to a ramp setting. Early Mississippian carbonate deposition was interrupted by a short and localized influx of siliciclastic sediment comprising the Northview Formation. The Northview has additional characteristics consistent with a river-dominated deltaic deposit, which we suggest as its origin. If correct, this hypothesis implies that the history of tectonic features in the Midwest is more complicated than yet known. Finally, facies changes within and between the local Mississippian formations may record an early crustal response to the impending Ouachita orogeny farther to the south.

Abstract Middle Cambrian–Lower Ordovician (Sauk megasequence) rocks of southern Missouri have attracted interest because they host the large Mississippi Valley-type ore deposits of that region. A Proterozoic igneous and metamorphic complex nonconformably underlies this megasequence and Mississippian and Pennsylvanian strata unconformably overlay these units. Strata of the Sauk megasequence in southern Missouri include, in ascending order, the Cambrian Lamotte Sandstone, Bonneterre Dolomite, Davis Formation, Derby-Doerun Dolomite, Potosi Dolomite, and Eminence Dolomite, which are overlain by the Ordovician Gasconade Dolomite, Roubidoux Formation, Jefferson City Dolomite, and Cotter Dolomite. Cambrian depositional facies in southern Missouri are small- to large-scale, unconformity-bounded, transgressive-regressive sequences, and are characterized by distinct facies in linear belts that developed on and adjacent to basement highs. These include intrashelf-basin facies distal from basement highs, platform-edge facies in narrow belts adjacent to highs, and back-reef facies proximal to and within the St. Francois Mountains. A distinct rift-graben facies is characteristic of deposition of these units in the Reelfoot rift. An epeiric sea that extended over the region controlled Lower Ordovician deposition. Lower Ordovician strata are com-posed of five third-order sequences, punctuatedbyregional and sub regional unconformities. The strata are characterized by facies that have regional lateral continuity.

Abstract The Jerusalem field produces dry gas from two sands in the lower part of the Atoka Series. The upper sand is called the “Allen” and the lower sand the “Barton” or “Barton ‘B’.” A third sand, the McGuire, overlies the Allen Sand and may prove to be productive over a very small area on the structurally highest part of the field. The gas is trapped in a faulted anticline. The Allen reservoir is limited downdip on both sides of the fault by a gas-water interface. The Barton Sand reservoir is limited on the west and the north flanks by lack of permeability and porosity, on the south by a normal fault, and on the east by a gas-water interface. The oldest rock penetrated by drilling in the Jerusalem field was in the Carter Oil Co. No. 1 Morrilton Lumber Co. This well bottomed at 3,500 ft in the Fayetteville Shale of Mississippian age. The deeper formations have not been tested in this area. Three formations of Ordovician age can be considered to be potential reservoirs, not only here but also in a large part of the Arkansas valley, particularly along the north rim of the basin—the St. Peter Sandstone, the Cotter Dolomite, and the Powell Dolomite. Twelve wells have been drilled in the process of defining the field limits. Six were producing wells and six were dry holes. Two of the producing wells are completed dually. The writer believes Jerusalem field is one of many small but economical fields which will be discovered on the north rim of the basin. The estimated gas reserve attributed to this field ranges from 18 to 25 billion cu ft.