ABSTRACT Lithologies, depositional environments, stratigraphic architecture, and conodont biostratigraphy of Lower to Middle Mississippian rocks in the western Ozarks comprise five depositional sequences in ramps on the southern Burlington shelf. Aggradational ramps in the Kinderhookian to early Osagean St. Joe group were relatively strongly overprinted by Ouachita-related tectonism involving inferred recurrent passage of fore-bulge highs and associated basins across central and southern parts of the outcrop area. Significant effects of tectonism are southward facies shallowing onto the broad Kanoka ridge paleotopographic high associated with locally extensive marine and lesser subaerial erosion, sediment thickening and deposition of generally northward down-lapping, resedimented wedges with dislodged reef blocks and conglomerates into relatively rapidly subsiding basins, and formation of a regionally extensive paleosol at the top of the group. Back-stepping subsidence due to middle Osagean foundering of the Kanoka ridge was followed by rapid, long-distance progradation of middle- and outer-ramp facies in the Bentonville and Reeds Spring limestones. Tectonism at this time resulted variously in local folding, uplift, marine and subaerial erosion, and reversal of shelf bathymetry. Southward erosion of the Reeds Spring and Bentonville occurred at least in Oklahoma on rejuvenated segments of the Kanoka ridge. Overlying lower Meramecian limestones are shallow-water deposits truncated by a major unconformity.

Abstract This field trip examines the geology and geohydrology of a dissected part of the Salem Plateau in the Ozark Plateaus province of south-central Missouri. Rocks exposed in this area include karstified, flat-lying, lower Paleozoic carbonate platform rocks deposited on Mesoproterozoic basement. The latter is exposed as an uplift located about 40 mi southwest of the St. Francois Mountains and form the core of the Ozark dome. On day 1, participants will examine and explore major karst features developed in Paleozoic carbonate strata on the Current River; this will include Devil’s Well and Round Spring Cavern as well as Montauk, Round, Alley, and Big Springs. The average discharge of the latter is 276 × 10 6 gpd and is rated in the top 20 springs in the world. Another, Alley Spring, is equally spectacular with an average discharge of 81 × 10 6 gpd. Both are major contributors to the Current and Eleven Point River drainage system which includes about 50 Mesoproterozoic volcanic knobs and two granite outcrops. These knobs are mainly caldera-erupted ignimbrites with a total thickness of 7–8 km. They are overlain by post-collapse lavas and intruded by domes dated at 1470 Ma. Volcaniclastic sediment and air-fall lapilli tuff are widely distributed along this synvolcanic unconformity. On day 2, the group will examine the most important volcanic features and the southernmost granite exposure in Missouri. The trip concludes with a discussion of the Missouri Gravity Low, the Eminence caldera, and the volcanic history of southern Missouri as well as a discussion of geologic controls on regional groundwater flow through this part of the Ozark aquifer.

Abstract The Osage Plains are that part of the Central Interior Lowlands extending from the glacial limit southwestward through eastern Kansas, central Oklahoma, and north-central Texas (Fig. 1). The Interior Highlands adjoin the Osage Plains on the east and comprise two distinctly different highland areas, the domed Ozark Plateaus and the tightly folded mountains of the Ouachita Province (Fig. 2). The Osage Plains and Interior Highlands have little in common, physiographically or stratigraphically, and are included in the same chapter for convenience because of their proximity to one another. Situated near the center of the conterminous United States, the Osage Plains are a place of transition between regions of contrasting character. They lie between the humid east and the semiarid west and the glaciated Interior Lowlands on the north and the Coastal Plain of the Gulf of Mexico on the south. The ecotonal boundaries between forest and grassland lie within the Osage Plains, as does the boundary between calcic and noncalcic soils. The Osage Plains differ markedly from the Interior Highlands in many respects, including topography, geology, vegetation, and soils. The Interior Highlands can be thought of as western outliers of the Appalachian Highlands. The plateaus overlying the Ozark dome are similar in topography, rock types, and structure to the Interior Low Plateaus overlying the Nashville dome and to the Appalachian Plateaus farther east and north. Likewise, the tightly folded and faulted Ouachita Mountains are comparable to the Appalachian Mountains. The Quaternary stratigraphy of the Interior Highlands

Abstract U.S. 65 is a major north-south artery across Arkansas. In north-central Arkansas it traverses the Ozark Plateau Province, which represents a stable Paleozoic platform located on the southwest flank of the Ozark Dome and north of the Ouachita trough. In Searcy County the highway crosses the deeply incised valley of the Buffalo River (Fig. 1) about two-thirds of the distance from Harrison to Marshall.

Abstract The Pennsylvanian System in the Mid-Continent area consists of five series, from the lowest upward —the Morrowan, Atokan, Desmoinesian, Missourian, and Virgilian. In the Arkoma basin the Pennsylvanian rocks were deposited upon Chesterian rocks without apparent unconformity. On the platform area they were deposited unconformably upon an eroded surface of earlier Mississippian rocks; along the Nemaha uplift, upon all earlier systems including the Precambrian, and in the center of the Ozark dome upon Ordovician rocks. The Arkoma basin is a narrow wedge of thick Morrowan, Atokan and Desmoinesian rocks bordering the Mid-Continent platform. Morrowan strata are thickest in the western part of the basin. They occupy a belt about 100 miles wide in southeastern and eastern Oklahoma, and northern Arkansas. Rocks of Atokan age overlap the Morrowan strata on the southern part of the platform. The Atokan is more than 9,000 feet thick at places in the basin and thins rapidly northward as it grades into a platform type of sediment marked by clastic limestones. The Desmoinesian Series is ten times as thick in the basin as it is on the platform. The basinal sediments are predominantly shale with massive sandstone tongues, and some coal beds. The platform facies is markedly cyclical, consisting of repeated sandstone, coal, limestone, shale sequences. The Arkoma geosyncline ceased to exist at the close of Desmoinesian time and upper Pennsylvanian rocks are no thicker in the Arkoma basin area than they are on the platform. They are clastic and are in a pattern of intricate overlaps, pinchouts, and conglomerate tongues in the southwestern part of the area where sedimentation was influenced by movements in the Arbuckle uplift. On the platform upper Pennsylvanian rocks are predominantly shale with a succession of limestone units, many of which are rich in fusulinids. The Pennsylvanian-Permian contact is customarily drawn below the first occurrence of Sckwagerina, herein, the basal unit of the Council Grove group. The Admire group (below) has been placed in the Permian because the first significant physical break below the Council Grove is at the base of the Admire. The fusulinid boundary is here considered an illogical one, and it is suggested that the Herington limestone-Wellington shale contact is the most desirable boundary. At the close of Krebs time (early Desmoinesian) a system of northeast-southwest trending normal faults displaced the rocks in and adjacent to the southwestern flank of the Ozark uplift. Uplift and erosion at the end of the Desmoinesian resulted in unconformity and overlap. Uplift in the Arbuckle Mountains became strong in Virgilian time and the Virgilian sediments of the adjacent area reflect the movements in coarse clastics, unconformities and overlaps. In late Permian or post-Permian time the rocks were folded into the present structural features—Prairie Plains homocline, Forest City basin, and local anticlines and synclines. Pennsylvanian fossils of the area are in general of long-ranging species and are of little stratigraphic use. Fusulinids, some ammonoids, some brachiopods, and spores are useful guide fossils. Rocks of the system yield almost all of the coal of the area, and important amounts of ceramic products, cement, petroleum and natural gas.

Abstract Rocks of the Mississippian System, which originally covered all the Ozark region in Missouri and adjacent states, consist mainly of carbonate deposits. Study of the nearly continuous outcrops of Mississippian formations now found on the flanks of the Ozark dome leads to recognition of several distinct types of carbonate rocks and permits determination of their regional stratigraphic relations. The widespread occurrence of siliceous sediment in the form of chert nodules and beds is an outstanding peculiarity of some formations, whereas chert is virtually unknown in others. This dissimilarity, and the observation that chert associated with various stratigraphically distinguishable parts of the Mississippian deposits in this region, are features which cannot be ignored in research on the nature of sedimentation that produced the carbonate rocks. Most of the differences are primary. Data and conclusions given in the present paper are based largely on field work undertaken in 1953 for the purpose of lithologic sampling, but they embrace also extensive previous acquaintance with the rocks discussed.

Abstract Pennsylvanian rocks generally underlie the Cumberland Plateau and Warrior plateau. To the south and west they continue beneath Mesozoic sediments of the Coastal Plain. To the north they form the Allegheny plateau. Lithologically, the strata consist of sandstone, shale, conglomerate, coal, underclay, and minor ironstone and limestone. Black shale is prominent in western Mississippi. Geology of the Pennsylvanian is illustrated by five correlation sections, four isopach and lithofacies maps, and three structure maps. Deposition began during the early Pottsville in two separated areas and then spread by basal onlap beyond the present day erosional limits. By middle Pottsville time, Pennsylvanian deposits were continuous with those of the Eastern Interior basin. Thickest connection was probably across the Clifton saddle in western Tennessee, and even the Nashville dome was covered. Trends of deposition were generally parallel to the Cincinnati arch, swinging from northeast in Kentucky and Tennessee to northwest in Mississippi, as the arch similarly swings to join the Ozark dome. Pennsylvanian rocks were later folded and thrust in southwest-trending “no-basement” structure of the Appalachian revolution. This structure system may intersect or join with northwest-trending Ouachita structure deep in the subsurface in southeast Mississippi. More recent epeirogenic movements have formed a broad southeast-plunging nose in southern Tennessee and northern Alabama where Pennsylvanian beds have been most deeply eroded.

The Mississippian formations and their varied faunas in the type area in the upper Mississippi Valley suggest a wide variety of paleoecological environments. The rock types include black paper-thin shale, greenish clay shale, massive mudstone and siltstone, sandstone, conglomerate, breccia, lithographic limestone, oölitic limestone, fine-grained earthy and dolomitic limestone, dolomite, coarse-grained crinoidal and other types of limestone. The sediments were laid down in relatively shallow seas. The area is structurally a part of the Central Interior Lowlands. The structure is essentially that of a platform of crystalline igneous rocks overlain by a relatively thin cover of sediments. Larger positive areas that surrounded the area include the Wisconsin lobe of the Canadian Shield, the Cincinnati anticlinal area, the Nashville dome, the Ozark dome area, and a landmass along an anticlinal fold that extended northeasterly across Nebraska and adjacent States. Local folds may also have been landmasses at times. Important among these are the Lincoln and Pittsfield-Hadley anticlines and an unnamed feature that extended northeasterly as a slightly submerged area across Fayette, Shelby, Douglas, and Champaign counties, Illinois. Sediments were probably received from all these postulated landmasses. Some of these landmasses served as barriers at times and thus made local seas. At other times the entire area was submerged. The oldest fauna considered is that of the Grassy Creek shale of Devonian or Mississippian age. This unit consists largely of paper-thin black shale beds. It was probably formed in stagnant water in an area partly enclosed by land barriers. The fauna consists of conodonts, other fish remains, spores, and linguloid brachiopods. The muds that formed the Grassy Creek shales were derived from low-lying lands. The Saverton fauna lived in an environment in which greenish-gray mudstones were being deposited. It had a large benthonic invertebrate fauna but included conodonts, other fish remains, and spores. The Louisiana limestone is typically a dense lithographic rock with dolomitic clay partings. The fauna includes a large and varied benthonic assemblage. A relatively thin series of shales, oölites, and limestones lying on the Louisiana limestone has been referred to the Glen Park formation by some authors but is called the “Hamburg” oölite by others. This series is overlain by the greenish-gray siltstone, dark clay shales, and fine-grained olive sandstones of the Hannibal shale. The “Hamburg” strata contain an assemblage of small brachiopods, pelecypods, pieces of Bryozoa, and some small gastropods. The assemblage has been called a dwarf fauna, but the smallness of the individuals that make up most of the assemblage may be a result of sedimentary sorting. The “Hamburg” oölite is thought to have been deposited in very shallow water. The Hannibal fauna consists mainly of brachiopods and pelecypods and suggests a shallow marine benthonic environment with several burrowing types of life being prominent. The Chouteau limestone is principally an argillaceous fine-grained limestone, but it also contains beds of medium crystalline limestone and some dolomitic limestone. It contains a large and varied fauna mainly of benthonic invertebrates. The Sedalia-Burlington-Keokuk limestone series is predominantly a coarsely crystalline cherty crinoidal limestone, with minor amounts of shale and dolomite. The shallow benthonic fauna is dominated by crinoids, although many other forms are quite numerous. Crinoid columnals probably were moved about before consolidation. The Warsaw and Spergen rocks contain more shale and argillaceous limestone than the immediately underlying rocks. Sedimentary structures suggest shallow water as do the faunules. Oölites may be rare or absent; fenestellate bryozoans are important components of the diverse shallow benthonic faunas. The St. Louis and Ste. Genevieve formations are typically fine-grained to dense limestone, and considerable thicknesses of oolitic limestone occur in each formation. Gypsum and anhydrite are known from subsurface sections of the St. Louis. Breccias and conglomerates are conspicuous locally, and dolomites and fine sandstones may be present. Sedimentary structures suggest a shallow-water origin. Shallow benthonic faunas are more abundant in some beds than in others but are less varied as a rule than in the Burlington limestone and adjacent formations. Nektonic life is represented by fish remains. Conspicuous forms in the benthonic fauna are the coral Lithostrotion proliferum and the echinoid Melonechinus. The area was probably land during most of Chester time.

ABSTRACT The Saint Francois Mountains are the physiographic expression of the central part of the Ozark Dome of southeastern Missouri. The mountains are made up of a quaquaversal-dipping series of Paleozoic units cored by the Mesoproterozoic-aged rocks of the broader Saint Francois Mountains terrane. The Saint Francois Mountains terrane lies within the Eastern Granite-Rhyolite province along the eastern margin of Laurentia and contains at least four mapped caldera complexes (Eminence, Lake Killarney, Butler Hill, and Taum Sauk), associated volcanic and volcaniclastic rocks, and four distinct types of intrusive units. The Mesoproterozoic rocks represent two major pulses of magmatic activity: (1) an older 1.48–1.45 Ga episode of caldera-forming volcanism and associated subvolcanic to massif-type granitic intrusions; and (2) a younger 1.33–1.28 Ga episode of bimodal intrusions. Volcanism included primarily high-silica rhyolite and volcaniclastic sediments associated with caldera-forming volcanism with lesser amounts of basalt and basaltic andesite that formed as flows and subvolcanic intrusions. The older (ca. 1.4 Ga) intrusive rocks can be divided into three broad categories: (1) granite massifs including the Butler Hill/Breadtray massif-type granites, (2) caldera ring–type granites such as the Silvermine Granite, and (4) mafic- to intermediate-composition intrusive rocks such as the Silver Mines Mafic Series. The younger (ca. 1.3 Ga) bimodal intrusions are represented by the highly evolved felsic Graniteville-types granites and the gabbros of the Skrainka Mafic Group. This field guide provides an overview of the magmatic history of the Mesoproterozoic rocks exposed in the eastern Saint Francois Mountains. Field-trip stops are divided into two days, highlighting well-known stops and lesser-known localities that illustrate the magmatic activity of one the premier igneous locations in the midcontinent region. The field trip is focused on two main areas. Day 1 focuses on the rhyolite sequence and associated caldera-forming eruption of the Taum Sauk caldera. Day 2 focuses on the volcanic rocks and granitic intrusions related to the Butler Hill caldera and ends with a visit to one of the youngest granitoids in the terrane, the Graniteville Granite. The field guide presents a summary of the volcanic history and petrogenesis of the Saint Francois Mountains rhyolites and granites.