Abstract The Cincinnati arch is a continual anticlinal trend approximately parallel with the Appalachian Mountains. There are two independent domes along the crest—Nashville dome in Tennessee and Jessamine dome in central Kentucky. Sedimentary rocks, in an essentially normal sequence, range from Upper Cambrian or possibly older, to the Dunkard Series (Permian), and these rocks contain many units productive of gas and oil. Cambrian .—Trempealeau dolomite, with vuggy porosity locally, produces gas in northeastern Ohio, and oil was discovered in the unit in Morrow County in 1959; the fields are small and disconnected, and accumulation is controlled by post-Cambrian topography. Oil and gas were found recently in wells in the northwest corner of Pennsylvania; although these wells reached Cambrian rocks, they do not produce from a Trempealeau equivalent. In Ohio there have been good shows of oil and gas in crystalline dolomitic zones in Upper Cambrian rocks beneath the Middle Ordovician unconformity. Ordovician .—Gas from seeps in Hancock County, Ohio, was used by the first settlers. The first well, drilled near Findlay to a depth of 1,092 ft in the Trenton Limestone, is considered one of the most important discoveries. The field became known as the Lima-Indiana oil and gas field. The entire Trenton-Black River-Chazy section, 600 -ft thick, is ordinarily termed “Trenton.” Edward Orton determined that the exceptionally great porosity in the upper part, caused by the granular or crystalline character of secondary dolomite, controlled the accumulations. Noncommercial Trenton gas production has been developed in many directions from the Lima-Indiana field. Silurian .—In 1887, near Lancaster, Ohio, gas was discovered at 2,000 ft in a Lower Silurian zone arbitrarily named “Clinton.” Since that time, one of the most valuable gas fields in the world has been developed in Ohio along the western extent of this sandstone body, on the east flank of the Cincinnati arch. On the basis of sandstone thickness and continuity, virgin pressure, and delivery, this large gas field can be divided into three productive areas. In the northern part, production is from elongate lenses which overlap to such an extent that several have been found in most producing wells; in the southern extension of the northern area, the productive unit is a relatively continuous sandstone body averaging 22 ft thick; and in the unconnected eastern productive area, reservoir characteristics are more comparable to the southern than the northern fields. The Upper Silurian Salina Group contains a dolomitic unit, called “Newburg,” which yields gas and some oil in more than 20 small pools in the area of the “Clinton” fields. Accumulations are shown to be controlled by closed structure. Devonian .—The Oriskany Sandstone, which is 100-300 ft below the top of the “Big Lime,” is thought to have been deposited over a post-Silurian erosion surface. It is present as a fairly regular sandstone body in an area comprising 14 counties in eastern Ohio, and is productive of gas and oil in several of them. In Kentucky, Middle Devonian Onondaga Limestone is called “Corniferous.” It is a siliceous, magnesian limestone which locally in central Kentucky yields moderate quantities of gas in several fields, from a very porous zone in the upper part. “Shale gas” has been developed in several commercially important fields in southern Indiana, eastern Kentucky, southeast Ohio, and western West Virginia by fracturing the Upper Devonian brown or black bituminous shale with high explosives. Estimates of 400 million cu ft per well, ultimate yield, have been made. Sandstone, productive of gas and oil, is developed locally in the Devonian shale in western Pennsylvania, certain areas of eastern and southeastern Ohio, and West Virginia. At Macksburg, Ohio, a sandstone 350 ft below the top of the Devonian shale has yielded gas and some oil; this unit is correlated with the Gordon Sand of West Virginia. Mississippian and Pennsylvanian .—Shallow fields which produce from Mississippian and Pennsylvanian rocks were discovered in southeastern Ohio soon after completion of the Drake well. In eastern Ohio, eastern Kentucky, and western West Virginia, at least 15 units in the Mississippian and Pennsylvanian Systems have yielded gas in commercial quantities. Probably an equal number is present in the western part of the province. Of these, the most important are the First and Second Berea Sands in the basal part of the Mississippian. Gas production from none of these has had the commercial importance of reservoirs in older rocks.

The Imo Formation of northeastern Arkansas and the Rhoda Creek Formation of south-central Oklahoma contain rich ammonoid assemblages of approximately equivalent age. The Imo ammonoids include Anthracoceras discus Freeh, 1899; Cravenoceras friscoense (Miller and Owen, 1944); Cravenoceras mapesi n. sp.; Cravenoceras bransoni n. sp.; Rhadinites n. gen. (type species: Cravenoceras miseri Gordon, 1965); Eumorphoceras richardsoni McCaleb, Quinn and Furnish, 1964; Eumorphoceras imoense n. sp.; Peytonoceras ornatum Saunders, 1966; Syngastrioceras imprimis n. sp.; Somoholites cadiconiformis (Wagner-Gentis, 1963); Delepinoceras bressoni Ruzhencev, 1958, and Paradimorphoceras sp. Of these, the Rhoda Creek assemblage contains Cravenoceras friscoense; Cravenoceras bransoni n. sp; Rhadinites miseri; Eumorphoceras imoense n. sp. (?); Somoholites cadiconiformis and Delepinoceras bressoni. Ammonoid faunas which include common or similar elements are known from the British Isles, France, Spain, Belgium, Holland, Germany, Poland, Yugoslavia; from the Soviet Union, in the Donetz Basin, the Southern Urals, Novaya Zemlaya and Central Asia, and from the Algeria-Morocco border area. The interval represented by the Imo-Rhoda Creek ammonoid fauna should be known as the Eumorphoceras richardsoni-Cravenoceras friscoense Assemblage Zone. This overlies the well-known Eumorphoceras bisulcatum-Cravenoceras richardsonianum Zone, represented in the Sand Branch Member of the Caney Formation in Oklahoma, and in the upper portion of the Pitkin Limestone, north-eastern Arkansas. The E. richardsoni-C. friscoense Zone correlates to the Arnsbergian Stage (upper Eumorphoceras , E 2 ) of the lower Namurian, between the E. bisulcatum s.s. (E 2 a) and Nuculoceras nuculum (E 2 c) Zones of the standard European succession.

ABSTRACT Geologic mapping on the Chortís block has defined two major Mesozoic units which are older than the well-known, lower Cretaceous Yojoa Group. The two units have been considered part of the same stratigraphic element, the Honduras Group. The Agua Fría Formation, a dominantly clastic mixed continental-marine unit, has been dated as Middle Jurassic based on recent discoveries of ammonites and plant fossils. Dates as old as Rhaetian for similar rocks have been reported in the literature, but have not been confirmed. No unfaulted contact between the Agua Fría Formation and the Yojoa Group has been found. The Yojoa Group is commonly underlain by a conglomeratic unit, the unnamed siliciclastic member of the Honduras Group, that is lithologically distinct from the Agua Fría Formation. Available palynological data indicate that the unnamed siliciclastic member is restricted to the lower Cretaceous. The Honduras Group as currently defined is an inappropriate stratigraphic term. The two units of the group are not contiguous and have significant differences in lithology, and, especially, in age. Inasmuch as its usage is causing stratigraphic confusion. I recommend abandoning the term Honduras Group. The Agua Fría Formation is left as an independent stratigraphic element. The unnamed siliciclastic member is herein named “the Tepemechín Formation” after a river south of Lake Yojoa. Although thicknesses as great as 1200m have been reported for the Agua Fría Formation, it is limited to rather narrow basins probably of strike-slip origin. By contrast, the Tepemechín Formation is widespread yet thinner and was deposited immediately prior to the deposition of the Yojoa Group limestones. Sedimentologically, the formation is similar to known rift deposits. Rifting probably occurred in the earliest Cretaceous and was followed by subsidence represented by the limestone. Thus, the Chortís block experienced strike-slip faulting in the Middle Jurassic followed by rifting throughout the block in the Early Cretaceous whereas neighboring regions experienced “pure” rifting from the Middle Jurassic to the Cretaceous as evidenced by thick deposits of continental redbeds.