Findlay Arch Research Articles

Fluorite and Other Minerals from the Suever Quarry, Delphos, Van Wert County, Ohio

The Suever quarry is located near the western edge of the Findlay Arch mineral district, at the eastern edge of Van Wert County, Ohio, and at the southwest corner of the town of Delphos, Washington...

Fluorite of the Findlay Arch Mineral District, Ohio, Michigan, and Indiana

“HOW MANY TIMES HAVE WE BEEN DOWN THIS ROAD?”—a common question deliberated between my father and me in the wee hours of the morning, at least in part to remain awake, as we pass familiar landmarks...

Correlation of Middle Devonian Hamilton Group-equivalent strata in east-central North America: implications for eustasy, tectonics and faunal provinciality

Abstract An integrated approach, involving nearly all available biostratigraphic data, event and sequence stratigraphy, has been utilized in correlation of the Middle Devonian (latest Eifelian–Givetian) Hamilton Group and equivalent strata in north-central North America. This approach permits high-resolution correlation of strata equivalent to the Oatka Creek (upper Marcellus), Skaneateles, Ludlowville and Moscow Formations from New York into sections bordering the Michigan Basin in Ontario, Canada, as well as southern Michigan, northern Ohio and Indiana, USA. Most member and submember-scale units, herein slightly redefined and interpreted as 3rd and 4th order sequences, respectively, and their bounding condensed beds can be correlated regionally. Moreover, many faunal patterns also persist across this region, which, together with sequence stratigraphy, provides a bridge for correlation into the Michigan Basin. The detailed stratigraphy presented herein permits a more-resolved understanding of far-field tectonics, eustasy and biotic responses during the Middle Devonian. Allocyclic processes, primarily eustasy, played a key role in generating persistent sedimentary cycles. Episodes of rapid mud sedimentation occurred over large areas of the cratonic interior, distal to Acadian source terrains. The major Algonquin–Findlay Arch, which presently separates the Michigan Basin from the Appalachian foreland basin, was not present during deposition of these strata. Conversely, a roughly north–south trending region, running approximately through present-day Cleveland, Ohio, was first a local subsiding area during late Eifelian–early Givetian time and then underwent topographic inversion to form a local arch at which upper Hamilton units were condensed and then bevelled during the later Givetian; we infer that this feature may represent a migrating forebulge. Finally, fossil biotas do not show strong partitioning into Appalachian and Michigan basin faunal subprovinces during the early Givetian, as there appears to have been no physical barrier to migration at least in the study area. However, Hamilton-equivalent strata in the most proximal portion of the Appalachian Basin do show a relatively minor admixture of typical Michigan Basin taxa with normal Hamilton forms.

Geologic, fluid inclusion, and isotopic studies of the Findlay Arch District, northwestern Ohio

The Findlay Arch district is a cluster of noneconomic, carbonate-hosted mineral occurrences centered in northwestern Ohio. The principal host rocks are vuggy dolostones that range in age from Middle Silurian to Middle Devonian. Reef complexes, beds of nodular gypsum, collapse breccias, and fracture zones provided important stratigraphic and structural ore controls. The main-stage phases are brown, yellow, and purple fluorite, sphalerite, and galena. The sphalerite/galena ratios are 100 or higher. Early dolomite, pyrite-marcasite, and celestite I are premain-stage phases, whereas saddle dolomite, vug-filling calcite, celestite II, barite, and strontianite are postmain-stage phases.T h and T m measurements of fluid inclusions in fluorite and sphalerite indicate that these minerals were precipitated from a warm Na-Ca-Cl brine, likely derived from the northern Appalachian basin. The flow path is inferred from westward-diminishing salinity values that are documented in successive generations of inclusions. The dominant thermal trend was aregional temperature plateau that peaked at about 110 degrees C. Thermal and salinity data imply that the flow system developed during the Alleghenian orogeny when the burial depth of host rocks was at a maximum and when the brine was diluted isothermally as it migrated westward across the arch. Coexisting inclusions of oil and brine indicate that these fluids moved through the district together.Similar sulfur isotope compositions (-0.9 to +4.8 and -3.4 to +0.6ppm) in sphalerite and galena show that these sulfides are genetically related. These data generally agree with sulfur compositions of late Paleozoic seawater sulfate, which is consistent with an Alleghenian age for the Findlay arch sulfur source. Lead isotope compositions of galena (Haden, 1977) fall in the field of the Appalachian ores and diagenetic galena, indicating further that nonradiogenic Paleozoic sedimentary rocks to the east were the likely source of lead.Minerals in vugs lack a radiogenic component of strontium. Nevertheless, a basinal brine is required for the fluorite-sphalerite mineralization, because a rock capable of providing the salinity of the fluid inclusions was absent in the district. The strontium isotope composition of this brine apparently was reset, acquiring the signature of Salina gypsum of Upper Silurian age. A local source of strontium and sulfur is indicated for celestite I, because its S and Sr isotope compositions are nearly identical to those of the Salina anhydrite.Brown calcite in veins associated with the Bowling Green fault zone contains strontium that is slightly radiogenic relative to vug-filling calcite. This, along with the prehydrocarbon age of the brown calcite, indicates that these early veins and the fiuorite-sphalerite mineralization are not coeval.

The Lima-Indiana Trend--Reservoir Characterization

The Lima-Indiana trend (informal designation) is an arcuate oil- and gas-producing trend that extends from north-central Ohio to east-central Indiana. It was the first giant field in the United States. Since its discovery in 1884 in Findlay, Ohio, nearly 500 MMBO and an estimated 2 tcf of natural gas have been produced from the trend. Production has been documented from the Ordovician Trenton and Black River reservoirs. The trend is located on a broad structural platform overlying the Findlay arch and the northern end of the Cincinnati arch. Reservoir quality is locally highly variable within the trend as illustrated by a 24-fold difference between the most productive areas in the trend (12,000 barrels/acre along the Bowling Green fault zone in Ohio) and the least productive areas (500 barrels/acre over much of the Trenton field in Indiana). Study of cores and logs has shown that this difference in productivity can bibe rectly related to two end members of reservoir character. Reservoirs with the best recovery are in fractured Trenton Limestone associated with tectonic features. Solution porosity in the reservoir rock and mineralization by white sparry dolomite and other minerals is also common along the fracture zones. Porosity and permeability are greatlymore » enhanced in these zones and account for the high productivity. Reservoirs with the poorest recovery are found where the Trenton Limestone has been regionally dolomid to produced a vuggy to intercrystalline pore system with highly variable porosity and permeability. Local reservoir enhancement by regional fractures is suspected, but cannot be documented.« less

Six Types of Trap or Reservoir-Producing Configurations in Trenton Limestone Reservoir, Northwestern Ohio: ABSTRACT

Six types of petroleum-producing configurations are recognizable in the oil and gas fields of the Lima-Indiana trend whose reservoirs are the Trenton Limestone of Ordovician age. The data that support recognition of these six structural, structural-stratigraphic, and possibly stratigraphic-permeability trapping configurations are mixed, but involve consideration of the pattern of 34 fields or pools on the main anticlinal trend of the Findlay arch as well as 12 smaller fields or pools to the northwest in the Michigan basin and 20 fields to the southeast at the updip edge of the Appalachian basin. The reservoir is mainly dolomite and the producing portion of the reservoir generally occurs near the top of the Trenton Limestone. The more porous dolomite has been analyzed chemically for Ca/Mg ratios, Na, Sr, Fe, and other elements in cores to supplement petrographic studies off the main oil field trend in Wyandot County. The six play configurations are the following. (1) An anticlinal trap along the crest of the Findlay arch. Here, as elsewhere, the seal and presumably the source are the overlying Utica Shale. (2) A faulted anticlinal trap on the western side of the Findlay arch. The fault, the Bowling Green fault, generally limits production to the upthrown eastern side. (3) An updip facies change from the Trenton Limestone into the overlying Utica End_Page 1917------------------------------ Shale to the south. This change, together with draping of the Utica over the underlying competent Trenton, traps petroleum along the southwestern extension of the Lima-Indiana trend. (4) Fracture systems related to regional fracturing of the early Paleozoic rocks. Apparently these systems have provided fracture-enhanced reservoirs. Similar configurations are well known in the Scipio-Albion trend in southern Michigan. Secondary dolomitization and sulfide mineralization are common in association with these fractured features. (5) Possible porosity-permeability traps, probably on structure, where dolomite is replaced laterally by dense limestone. (6) Small anticlinal terraces off the main arch system, related to minor production. These terraces appear as down-to-the-basin noses on struct ral maps. Considered together, the Lima-Indiana fields are a giant field extending 120 mi (193 km) in Ohio and another 50 mi (80 km) in Indiana. Before 1900, thousands of wells were drilled, as shallow as 1,100 ft (334 m), producing over 220 million bbl of oil at initial rock pressures between 100 and 450 psi (690 and 3,100 kPa). Exploration continues at a modest pace. End_of_Article - Last_Page 1918------------

Regional hydrogeochemical patterns in ground water of Northwestern Ohio and their relation to Mississippi valley—type mineral occurrences

A cluster of minor Mississippi Valley-type deposits occurs in northwestern Ohio. The district, which forms a northeasterly trending belt that cuts across the Findlay Arch, extends from the Indiana border to the Lake Erie Islands. The minerals of the deposits — chiefly celestite, fluorite and sphalerite with lesser amounts of barite and galena — show variation in both geographic and stratigraphic distribution. Dolomites of Middle Silurian to Middle Devonian age, which are the host rocks, also form an important aquifer system. The deposits are of interest because they might be indicators of economic mineralization at depth. Through a reconnaissance study, one-hundred ground-water samples from shallow wells (less than 50 m deep) were collected across an area of approximately 19 000 km 2. Recharge takes place in the southern part of the area while ground-water flow is northward towards Lake Erie. The majority of the samples are high in sulfate with the source being evaporites within the carbonate sequence. The remainder of the water samples are rich in bicarbonate. Trend surface maps for the major constituents indicate that the ground-water chemistry for the region is established chiefly by the lithology and the flow system. Trend surface maps for F −, Sr and Ba reflect the geographic distribution of the minerals in the deposits. Correlations are weakened, however, due to the influence of geochemical barriers such as SO 4 2− on Ba and Sr, and Ca on F −. The map for Pb follows the trends of the major constituents instead of the mineralization. In a detailed study across northwestern Sandusky County, which lies near the center of the district, 46 samples were collected in an area of 78 km 2. Trend surface maps for Ca, Mg, SO 4 2− and total dissolved solids reflect the chemistry of the bedrock and display concentrations that increase along the local flow path. Maps for F −, Sr and Ba correlate with mineralization in the vicinity, the first of these displaying a local trend and the last two correlating with regional trends.

Stratigraphic and Sedimentologic Controls of Gas in Shale--Example from Upper Devonian of Northern Ohio

The gas-bearing Devonian Ohio and Chagrin Shales along Lake Erie have produced marginally commercial volumes of gas for over 100 years. These shales thin depositionally westward from a maximum of 2,000 ft (600 m) at the Pennsylvania-Ohio border to less than 500 ft (180 m) near Sandusky, Ohio. West of Sandusky, they have been truncated by Holocene and preglacial erosion on the Findlay arch. Westward thinning is accompanied by a facies change from gray shale and siltstone in the east to black shale in the west. The Ohio and Chagrin Shales were deposited on a turbidite slope and basin plain and all were derived from the Catskill delta on the east. The Late Devonian epeiric sea was at least 700 ft (215 m) deep and was poorly oxygenated. There are three major lithologies--black bituminous shale, greenish-gray shale, and siltstone. Black bituminous shale is most abundant in the west, whereas gray to greenish-gray shale and siltstone are most abundant in the east, where they constitute 75% of the section. Zones productive of natural gas occur most commonly in the greenish-gray shale and siltstone. These rocks contain abundant thin silt laminae which probably act as permeable conduits and reservoirs for gas. This gas appears to have had its origin in underlying black shales. Sedimentologic study helps explain the origin of both small- and large-scale interbedding of the greenish-gray and black bituminous shale and provides guides for improved gas exploration in the Appalachian basin.

Oil- and Gas-Producing Configurations in Trenton Limestone, Northwestern Ohio: ABSTRACT

Five types of petroleum-producing configurations are present in the Middle Ordovician Trenton Limestone around the Lima-Indiana field in northwestern Ohio. The first, an anticlinal trap, is present along the Findlay arch. Closure is provided on top of the Trenton by regional carbonate bank buildup, folded with and capped by the Utica Shale. The second, a faulted anticline, is present along the Bowling Green fault in Wood, Hancock, and Lucas Counties. A high-angle reverse fault along the crest of the Findlay arch juxtaposes dolomitized Trenton rock with the overlying Utica Shale. This configuration has accounted for significant oil and gas production. The third trap type consists of an updip facies change from Trenton Limestone to Utica Shale, with draping of the thickened shale over the Trenton Limestone. The fourth type, in the Michigan basin, is the fracture systems and dolomitization in the Albion-Scipio trend. The fifth, and less well documented, is a porosity trap in dolomitized upper parts of the Trenton. Dolomitization may function in two ways, both as a prerequisite to formation of sufficiently porous reservoir rock with other trapping mechanisms. Other Trenton fields are not accounted for by these five configurations. Stratigraphic and structural cross sections from the top of the Trenton to the Knox unconformity, as well as structural and isopach maps of the area, show a major carbonate buildup of the Trenton in a northeast-southwest trending arc in northwestern Ohio. A broad carbonate platform with wedge-top dolomitization in an island environment is postulated as an alternative to a regional erosional unconformity between the Trenton Limestone and Utica Shale. End_of_Article - Last_Page 1583------------

Gas-Bearing Ohio Shale along Lake Erie--Stratigraphy, Petrology, and Sedimentology: ABSTRACT

Increased interest in low-yield gas-bearing strata has led to reexamination of the Ohio Shale in the vicinity of Lake Erie where it has had a history of marginal commercial production for over a century. These strata thin depositionally westward from a maximum of 2,000 ft (600 m) at the Pennsylvania border to less than 600 ft (180 m) near Sandusky where they are truncated by erosion on the Findlay arch. Common lithotopes are black and gray shale, siltstones, and rare carbonate rocks. Black shales are most abundant in the west where gray shales and siltstones are rare. In the east, gray shales and siltstones constitute 75% of the section and the remainder is mainly black shale and minor carbonate rocks. Thin-section petrology revealed that the black shale commonly contains, by volume, less than 20% silt-size quartz and feldspar, 30 to 60% clay and mica, and 15 to 35% organic material. Gray shale commonly contains less than 30% silt-size quartz and feldspar, 50 to 90% clay and mica, and less than 10% organic material. Much of the silt is concentrated in discrete laminae a few grains thick. Production records are related to facies distribution to provide an exploration tool. Cross sections and paleocurrents indicate that the Upper Devonian prodeltaic muds and turbidite siltstones were deposited episodically in a euxinic basin. End_of_Article - Last_Page 1576------------

Modes of evolution and their chronostratigraphic significance: evidence from Devonian invertebrates in the Michigan Basin

The origin and evolutionary histories of the two most abundant and diverse genera of invertebrates (Prosserella: Brachiopoda and Syringostroma: Stromatoporoidea) in the Detroit River Group and associated rocks in the vicinity of the Michigan Basin appear to support the models of either allopatric speciation and punctuated equilibria (Eldredge and Gould, 1972) or quantum evolution (Simpson, 1944; 1953). Four morphotypes of Prosserella arose almost simultaneously (a “burst”) just below the base of the Detroit River in sandstone deposited near the axis of the Findlay Arch and persisted without evidence of significant progressive evolutionary change (a “trend”) until their almost simultaneous extinction by lineage termination near the top of the Detroit River. Neither ancestors nor descendants of Prosserella have been recognized and even the familial placement of the genus is uncertain. The genus, its species and morphotypes probably arose by means of very profound genetic or chromosomal “revolutions” that probably took place in small allopatric populations; each population quickly increased in abundance and geographic range and persisted without further significant morphologic modification until its extinction.S. ristigouchense from the Lower Devonian of New Brunswick is the probable ancestor to six (or seven) species of Syringostroma that appear almost simultaneously (another “burst”) in lower Detroit River carbonate rocks deposited near the eastern margin of the Michigan Basin. Two of these new species are known only from reefs of early Detroit River age (a “crash”), four species persist without significant morphological change to at least the end of Detroit River deposition, and one of these was the probable ancestor to yet another newly evolved species that is abundantly represented in the conformably overlying Columbus Limestone.The “founding fathers” of chronostratigraphy were pre-Darwinian and based their concepts and methods on assemblages of co-occurring taxa of unknown phylogenetic relations (assemblage-zones, concurrent range-zones and Oppel-zones) rather than on the range-zones of successional species in the same phylogenetic lineage (lineage-zones). Assumptions of these early biostratigraphers concerning the temporal relations of taxa are in close accord with the premises of punctuated equilibria and quantum evolution. The development of the chronostratigraphic system during the 19th century attests to the success of these early methods which depend for their precision on the number of taxa used and determination of the significance of morphologic differences among these taxa and their geographic and stratigraphic distributions.

Stratigraphic and Environmental Significance of Sedimentary Structures in Cayugan (Silurian) Tidal Flat Carbonates, Northwestern Ohio

Detailed studies of outcropping rocks of Cayugan age in the Findlay arch area disclose numerous and diverse types of sedimentary structures, the majority of which have not been recognized or described previously in these strata. The recognition of these structures provides a new approach to the lithostratigraphic and environmental analysis of Cayugan rocks in the Findlay arch region. The 170-ft (52-m) sequence of Cayugan rocks which crops out in the Waterville, Ohio, area is the thickest sequence of Cayugan rocks exposed in Ohio along the southern margin of the Michigan basin. These rocks include the uppermost part of the Greenfield Formation, nearly all the Tymochtee Formation, and the lower part of the Put-in-Bay Formation. The 130-ft (40-m) section of the Tymochtee Formation is the thickest surface section of this unit and the only section in which both the upper and lower contacts are exposed in the Findlay arch region. The section of the Tymochtee Formation is designated as a principal reference section for the Findlay arch region; portions of this section can be correlated physically with the type section of the Tymochtee Formation. Sedimentary structures provide criteria for the recognition of four new members in the Tymochtee Formation, which are, in ascending Stratigraphic order, the Granger Island, Ovitt Road, Maumee River, and Roche de Boeuf Members. The principal types of rocks developed from Cayugan sedimentation in northwestern Ohio are fine-grained, mostly unfossiliferous dolomites and shaly dolomites, which contain algal stromatolites, shrinkage cracks, ripple marks, tidal channels, gypsum, pseudomorphs after gypsum, celestite, and molds and casts of halite. Collapse has occurred in some rocks due to the solution of evaporite minerals. By analogy with Holocene sediments, it is concluded that the Cayugan rocks were formed on an evaporite-carbonate tidal flat wherein the conditions of sedimentation were not unlike those found today in the Persian Gulf. Subenvironments in the Cayugan tidal flat were mainly intertidal, but ranged from subtidal to supratidal. During Cayugan time northwestern Ohio was an evaporite-carbonate tidal flat rather than a reef bank complex as has been suggested previously. Strandlines within the tidal flat were probably oriented mainly northwestward, in opposition to the present northeastward trend of the Findlay arch through northwestern Ohio. It is probable that the Findlay arch did not affect sedimentation in northwestern Ohio during Cayugan time.

Minor epigenetic, diagenetic, and syngenetic sulfide, fluorite, and barite occurrences in the central United States

Metallic sulfides, fluorite, barite, and celestite are widespread in the sedimentary rocks of the central United States. Many occurrences are epigenetic concentrations either in known major mineral districts or in clusters of deposits that warrant further exploration for potential ore. Evaluation of trace-element composition, of fluid inclusions, of depositional temperature, and of isotopic composition of sulfur may help discriminate potentially economic deposits from even more widespread, probably noneconomic, syngenetic occurrences. Known major mineral districts and many promising sulfide occurrences are located along complex major fault systems and particularly over structural domes at intersections of fault systems. Economically promising sulfide deposits occur along a west-trending fault system crossing Kentucky and Missouri near the 38th parallel, particularly at associated cryptoexplosion structures along it, at the intersection of the fault system with the Cincinnati arch. Elsewhere in the central United States, promising sulfide occurrences are located along the crest of the Findlay arch, near Serpent Mound, Ohio, along the northwest-trending Mount Carmel fault in southwestern Indiana, and buried at depth in oil fields of west-central Kansas.Other types of deposits that are productive or may have some future economic interest are locally economic and possibly syngenetic red-bed copper occurrences in Oklahoma, Texas, Kansas, and Pennsylvania. Most widespread are the black shale and pyritic sandstone beds, some of which have been a source of pyrite for sulfur and sulfuric acid in conjunction with coal mining. Bedded barite deposits of possibly diagenetic origin have been mined in Arkansas. Diagenetic siderite concretions containing sulfides and barite were widespread sources of iron in the 19th century.

Directional Properties, Paleoslope, and Source of the Sylvania Sandstone (Middle Devonian) of Southeastern Michigan and Northwestern Ohio

ABSTRACT Measurement of maximum inclination azimuths of 202 cross-bed sets at 7 exposures of Sylvania Sandstone in southeastern Michigan and northwestern Ohio revealed preferred inclination of cross-bedding down the northwestward regional dip, in the direction of maximum thickness increase of the formation, and in the direction of decreasing stratigraphic hiatus of the unconformity at the base of the formation. Preferred orientation of long dimensions of Sylvania sand grains is parallel to the azimuth of inclination of the cross-bed set containing them. Although primary sedimentary structures, sand fabric, and stratigraphic and structural relationships superficially suggest a source in the direction of the Findlay Arch to the southeast, the sand probably came from Wisconsin. Because of the relation of permeability to sand fabric, perhaps the relation of sand fabric to cross-bed orientation, regional dip, and direction of maximum formation thickening, as reported here for the Sylvania Sandstone, could be of value in reservoir evaluation.

Evidence of Unconformity at Top of Trenton Limestone in Indiana and Adjacent States

Subsurface data indicate that in Indiana and parts of adjacent states the Trenton Limestone was exposed to subaerial erosion before the deposition of the overlying shale, which generally is considered to be Cincinnatian in age. The subsurface data include: possible truncation of the Trenton Limestone in southern Indiana; knife-sharp contact between the Trenton Limestone and the overlying shale; concentration of pyrite, angular chert, phosphatic grains, and brecciated limestone and dolomite; solution pits on the surface of the Trenton Limestone and solution cavities within the Trenton Limestone; and local relief on the surface of the Trenton Limestone. East of a line, here called the hinge line, which extends from the Findlay arch to the eastern edge of the Ozark dome, the writer has not observed evidence of unconformity.

Lower Silurian and Cambro-Ordovician Sedimentation of Northern Appalachian Basin: ABSTRACT

Subsurface studies in the Northern Appalachian basin reveal several different histories of sedimentation (Cambro-Ordovician and Lower Silurian). Too few wells have been drilled to the Precambrian to reveal anything but very regional data on the Cambrian sediments. The Cambrian thickness varies from 13,000 feet on the outcrop in south-central Pennsylvania to 900 feet in northeastern Ohio and 0 feet in southwestern Ontario. The section is predominantly dolomite and sandstone. The Upper Cambrian isopach map shows a probable regional high in northeastern Ohio. This high seems to trend north-south. Two regional unconformities have been detected within this section. Middle-Ordovician sedimentation marked a time of emergence with the Adirondack-Tazwell axis in the center of the basin forming a structural and facies barrier separating for the first time the Appalachian basin into two distinct basins of deposition. On the northwest flank of the Allegheny synclinorium, the Crawford arch came into existence along with the Olean embayment and Erie trough (Chatham Sag extension). Unlike the more pronounced Adirondack-Tazwell feature, the Crawford arch did not act as a facies barrier. The Middle Ordovician sediments encountered so far are argillaceous limestones. The exception to this being the dolomitized productive areas on the Findlay arch. Upper Ordovician sedimentation shows regional east to west thinning with occasional interruptions across local highs. These sediments are predominantly shales with occasional layers of siltstones or sands. The subsequent Lower Silurian deposits also show regional thinning eastward with local variations. Lower Silurian production appears to be confined to within the 200-400-foot isopach interval. Variations within this isopach interval are critical as to the quality of production to be found. Detailed study reveals the productive sand bodies are deltaic rather than offshore bars or shoe-string sands. End_of_Article - Last_Page 1252------------

Trenton Structure in Ohio, Indiana, and Northern Illinois

This paper presents concepts of Trenton structure and of geologic history in Ohio, Indiana, and Illinois that are not commonly found in the literature. The literature is reviewed in order to show the origin of structural thought patterns, some of which have been preserved by repetition without any re-examination. This review shows great need for better understanding and for more careful use of structural terminology. The Cincinnati Arch geologic province includes all the structure that separates the Appalachian, Michigan, and Illinois basins. The regional aspects of this structure have resulted from subsidence in the basins, rather than from uplift between the basins; consequently, there are no continuous structural axes. In the Appalachian basin subsidence began in pre-Trenton time. In the Michigan basin subsidence began in post-Niagaran Silurian time. In the Illinois basin subsidence began in Devonian time. There was no west dip in Indiana and, consequently, no Cincinnati before late Devonian time. The major structural features in the Cincinnati Arch geologic province are: Cincinnati arch, Findlay arch, Francesville arch, and Indiana-Ohio platform. The three lose their identity in the platform which covers an area of 10,000 square miles in which there is no arched structure. Northern Illinois is in the Wisconsin Arch geologic province which had a different geologic history. The Oglesby fault is shown where the LaSalle anticline was first described. The upthrown block between this fault and the Sandwich fault is here shown as the Ottawa horst. Data for the interpretation of the Tuscola fault were supplemented by carefully located core-drill holes in three counties. In Indiana the Logansport fault and the Francesville have been taken from a map contoured by use of 50-foot intervals. No arching extends from Indiana into Illinois, and none of the various Kankakee arches found in the literature will withstand the test of careful examination. The term Kankakee arch should be dropped. Thickness of sedimentary columns between the top of the Trenton limestone and granite are used to show present structure does not reflect the configuration of the granite surface. The original configuration of the granite surface can be used to explain northward thinning from Kentucky to the Ontario Peninsula. Arkosic materials should not be used for age determinations or for long-range correlations. The first sediments above the granite in the Ontario Peninsula may have been deposited in Ordovician time.

Cambrian and Ordovician Rocks in Michigan Basin and Adjoining Areas

Sandstone, limestone, and dolomite of Upper Cambrian and Ordovician age either are exposed at the surface or underlie glacial drift in the Northern Peninsula of Michigan. The outcrop of these rocks extends southward from northern Michigan into Wisconsin and Illinois. Eastward across the Northern Peninsula, Middle Ordovician rocks overlap progressively older rocks, and on Manitoulin and adjacent islands Black River rocks of Middle Ordovician age rest on pre-Cambrian. This same stratigraphic relationship is observed in other outcrop areas in Ontario and in deep wells that have penetrated the full section of sedimentary rocks. Pre-Black River erosion removed most of the older Paleozoic rocks from the underlying pre-Cambrian in southwestern Ontario and in some places only a t in remnant of Upper Cambrian sandstone and some dolomite occurs between Middle Ordovician and pre-Cambrian rocks. Cambrian and Lower Ordovician (sub-Trenton) sandstone and dolomite range in thickness from less than 500 feet to more than 3,500 feet in Michigan. The aggregate thickness of these rocks increases southwestward from Michigan toward Illinois and thins southeastward over the Findlay arch in southeastern Michigan and northwestern Ohio. The rocks increase in thickness eastward into the Appalachian basin from the Findlay arch. The widespread limestones and dolomites of Middle Ordovician age rest on eroded Lower Ordovician and Upper Cambrian rocks in Michigan, northern Indiana, and northwestern Ohio and range in thickness from less than 200 feet in the Northern Peninsula to more than 900 feet in southeastern Michigan. These rocks are thickest a short distance east of the center of the present Michigan basin and are thinner over major anticlinal folds. This indicates that the basin was depressed in an area somewhat similar to its present form as early as Middle Ordovician time. Oil and gas were commercially produced from the sub-Trenton rocks in Ohio on the east flank of the Findlay arch in several small areas. Showings of oil and gas in these rocks have been reported from wells in northeastern Indiana, southeastern Michigan, and southwestern Ontario. The Trenton limestone, which is the producing formation in the Lima-Indiana field, is productive in small areas in southeastern Michigan and southwestern Ontario.