Figure 15.1. Divisions of type-Cincinnatian strata. Geologists have traditionally defined sedimentary rocks on the basis of time, usually inferred from fossil assemblages, and on the basis of rock type. The Cincinnatian has been traditionally divided into three stages, shown at the far left, and there is currently disagreement over the relative durations of these three stages. A fourth stage, indicated by “G” and called the Gamachian Stage, is not present in the Cincinnati area. The divisions based on rock type are shown at the right. Most of those in modern usage are shown, but dozens of different named divisions have been proposed over the years and are not shown. The names currently used by the Ohio and Kentucky geological surveys differ, and dashed vertical lines indicate these “stateline stratigraphic divisions.” The Indiana Geological Survey recognizes the Kope, Whitewater, and Saluda Formations, and assigns all strata between the Kope and Whitewater to the Bull Fork Formation (not shown). Type-Cincinnatian strata have also been divided into six units that reflect cycles of sedimentation produced by the rise and fall of sea level. These depositional sequences, numbered C1–C6, are bounded by unconformities shown in gray that reflect falls in sea level that drained the seas from the Cincinnati area, resulting in no deposition of sediments. The actual duration of these unconformities is poorly known.
Earth scientists reconstruct conditions during the ancient past from a wide variety of clues from minerals, rocks, and fossils. Although no place on Earth today is exactly like the Cincinnati area during the Late Ordovician, comparisons with modern environments offer valuable insights into the interpretation of these clues. Of modern environments, the Persian Gulf is perhaps the most similar to the Late Ordovician of the eastern United States in terms of its climate, the size of the sedimentary basin, the gently dipping sea floor, the mix of carbonate sediment and clay, and the occurrence of storms that rework and deposit sediment.
Geography
“When Cincinnati Was in the Southern Hemisphere”
(Title of a presentation by Kenneth E. Caster for the Cincinnati Historical Society, 1974)
Global geography during the Ordovician has been reconstructed primarily through studies of the magnetic properties of Ordovician rocks. When sediments are deposited, iron-bearing minerals tend to align with the Earth’s magnetic field. As the sediments become cemented together and undergo lithification to become rocks, these miniature magnets are locked in place. By careful measurement of the orientations of these iron-bearing minerals, geophysicists reconstruct the latitude at which the sediments were originally deposited.
Four large continents dominated the globe during the Ordovician (Plate 1). Stretching from the South Pole into the northern hemisphere, Gondwana was the largest of these continents and consisted of modern day South America, Africa, Arabia, Antarctica, Australia, India, southeast Asia, and parts of China. Laurentia included most of present-day North America, except for New England and Maritime Canada, parts of the southeastern United States, and the west coast of the United States and Canada, all of which were added to Laurentia during subsequent tectonic collisions. Laurentia straddled the equator during the Ordovician and was rotated 45° clockwise from its present-day orientation. As a result, Cincinnati was situated at 20–25° south of the equator for the entire Late Ordovician. To the east of Laurentia along the equator sat the continent of Siberia-Kazakhstan, which consisted of present-day portions of central Asia. The fourth continent, Baltica, lay to the south at roughly 60° and was composed of northern Europe and Scandinavia.
Three major oceans separated these continents. The Iapetus Ocean separated Laurentia and Siberia-Kazakhstan from Baltica to the south. The Paleotethys Ocean lay between Gondwana and the continents of Baltica and Siberia-Kazakhstan to the west. The massive Panthalassic Ocean covered almost all of the Northern Hemisphere and would have dwarfed today’s Pacific Ocean.
Global sea level was high during the Ordovician, and although its position is difficult to constrain, the current consensus is that it was 100 to 200 meters higher than present-day sea level (see Figure 1.3). Several factors contributed to such a high position of sea level. Rates of sea floor spreading were high following the breakup of an older, Late Proterozoic supercontinent called Rodinia, causing the average elevation of the sea floor to be higher than normal. This raising of the “bottom of the bucket” forced ocean waters to spill onto the continents. In addition, the lack of polar ice caps in most of the Ordovician also would have raised sea level relative to today because water in modern glacial ice caps such as Antarctica and Greenland is produced from snow generated by evaporation from the ocean. Because of this high sea level, low-lying areas on the continents were flooded with ocean waters, much like the flooding of the present-day continental shelves, but to a much greater extent. During much of the Ordovician, most of Laurentia was submerged, with the exception of parts of the Canadian Shield, a low mountain range running from Minnesota toward Colorado, the Ozark region of Missouri, and the uplifting Taconic Mountains along the southeastern edge of Laurentia (Plate 1).
The eastern United States was divided into several geographic regions during the Late Ordovician (Plate 12). Extending from central Kentucky northward into Ohio and Indiana was a shallow marine carbonate area known as the Lexington Platform. Water depths on the Lexington Platform deepened to the north. The Lexington Platform was bounded on the west by a deeper water trough known as the Sebree Trough. Far to the east rose the Taconic Mountains, produced by the collision of Laurentia with a small plate or island arc, such as the modern-day islands of Japan and the Aleutians. This collision warped the southeastern margin of Laurentia to form a deep water trough called the Appalachian Basin that separated the Taconic Mountains from the Lexington Platform. Into this trough, sediments shed from the eroding Taconic Mountains built a series of northwestward-advancing deltas called the Queenston Delta. “Red-bed” tidal flat sediments typical of the Queenston can be seen in the latest Ordovician strata on the eastern side of the Cincinnati Arch in Adams County, Ohio.
Climate
The climate of the Late Ordovician was warm, with much more even temperatures from the pole to the equator than seen today. High concentrations of carbon dioxide in the atmosphere (referred to as the partial pressure of carbon dioxide, or pCO2) caused these warm conditions. Computer models and limited geochemical data from Ordovician soil deposits suggest that levels of atmospheric pCO2 were nearly sixteen times greater than today, compared to the 20 percent increase in pCO2 witnessed in the past half-century.
Warm temperatures at the poles inhibited the formation of ice caps, such as today’s continental glaciers on Antarctica and Greenland and the sea ice over the Arctic Ocean. In the last million years of the Ordovician, atmospheric pCO2 levels dropped precipitously, triggering the rapid growth of polar glaciers and a geologically brief 160 meter global sea level fall. This fall in sea level drained the seas from the Cincinnati area, producing an erosional division between the Ordovician and Silurian strata called an unconformity.
In addition to this end-Ordovician fall in sea level, evidence for six cycles of global sea level change is preserved in the Ordovician near Cincinnati (Figure 15.1). The evidence for these cycles comes from packages of rock known as depositional sequences, which are bounded by unconformities, or surfaces that record the erosion and weathering of sediments. Each depositional sequence begins with a relatively thin interval of rock that records local deepening of the oceans and ends with a much thicker interval of rock that records progressive shallowing of the oceans. These same sequences can be recognized across the United States and in Estonia. The fact that these sequences are not just local features is strong evidence that they reflect global sea level changes rather than local tectonic changes. In the Cincinnati area, these six depositional sequences also contain evidence of shorter-term variations in sea level, but it is currently unclear whether these represent global or regional changes in sea level.
From studies of today’s geography and climate, as well as direct evidence from Ordovician strata, geologists have reconstructed the Ordovician climate of the Cincinnati area. Today, regions near 30° north and south of the equator are characterized by deserts, which form as air rising near the equator descends and forms a series of high pressure cells that inhibit rainfall. In Ordovician rocks of the Cincinnati area, direct evidence of such a semi-arid climate can be seen in the finely laminated dolomitic tidal flat deposits of central Kentucky. Similar deposits occur in semi-arid regions today, where high tides deposit thin laminae of sediment, but high salinity and temperature exclude animals that might burrow and disrupt the sediment. Dolomite itself can have a variety of origins, but the texture and occurrence of dolomite in Cincinnatian rocks matches that found in modern semi-arid settings with high rates of evaporation. Trade winds characteristic of subtropical latitudes would have blown westward across the Cincinnati area during the Ordovician.
The Cincinnati area was subjected to frequent hurricanes, which produced the distinctive alternations of limestone and mudstone, often called shale, although few true shales exist in the Cincinnati area (Figure 15.2; see Figure 4.6). Although there is considerable variation in individual storm deposits as a result of differences in sediment supply, water depth, proximity to the hurricane, and the strength of the hurricane, most of these deposits contain at least some of the characteristic features produced by storms. Storm-generated deposits are well known not only from sediment cores on modern continental shelves, but also throughout the geologic record. Storm beds typically have an erosional base, which reflects progressively intensifying currents and waves as the storm approaches. This erosional base is often overlain by a shell-rich limestone, in which the size of shell fragments decreases upwards, which may in turn be overlain by laminated siltstone and burrowed mudstone. Such an upward decrease in shell and sediment grain size is called normal grading by sedimentologists and reflects deposition during the waning phase of a storm when storm-generated waves and currents weakened and deposited the sediment they carried. Laminated siltstone may display horizontal planar laminations, hummocky cross-lamination, or wave-ripple lamination, all of which can be produced by strong storm-generated waves and currents.
Figure 15.2. Destruction of benthic communities by storm-generated waves and currents. During calm, pre-storm conditions, benthic communities of organisms develop on the sea floor. Under storm conditions, high winds generate large waves that stir up fine-grained bottom sediments into suspension. Stronger wave and current forces can displace benthic organisms, and suspended sediment can clog feeding and respiratory mechanisms of organisms, and even smother entire benthic communities. Mobile organisms can escape if burial is not too severe, but storms can be lethal for many immobile benthic organisms. Following a storm, barren sediment can cover the bottom until benthic communities again develop from larval settlement or immigration. Modified after Hinterlong (1981). Courtesy of Wayne D. Martin.
Cyclical changes in the character of storm beds are well developed in some Cincinnatian deposits, such as the Kope Formation. At a broad scale, these roughly meter-thick cycles consist of a mudstone-rich unit and a limestone-rich unit (see Figure 4.6). The mudstone-rich unit consists of 3–5 cm beds of normally graded mudstone, with uncommon thin laminated siltstone beds. Limestone-rich units consist of shelly limestone beds, with a lesser amount of thin mudstone and siltstone beds. The alternation between these two units was originally thought to reflect changes in sea level, but recent studies suggest that these cycles may instead reflect changes in the average frequency and intensity of hurricanes over tens of thousands of years.
Oceanography
Compared to many modern carbonate settings, Ordovician limestone of the Cincinnati area is unusual in several regards. Most modern and ancient warm water carbonate deposits contain a wide variety of grain types, including skeletal grains (the shells of organisms), ooids (small, spheroidal, concentrically laminated grains), peloids (ovoid grains produced primarily as fecal pellets), and intraclasts (pieces of semi-cemented carbonate sediment that have been eroded and redeposited). In the type-Cincinnatian, ooids are absent, peloids are uncommon, and intraclasts occur sparingly and only in particular horizons. In contrast, skeletal grains of brachiopods, bryozoans, echinoderms, molluscs, and trilobites dominate most limestone in the Cincinnatian (see Figure 4.2). Most modern warm water carbonate deposits contain abundant lime mud, called micrite, produced primarily by the photosynthetic activities of algae. In comparison, most limestone in the Cincinnati area contains only minor amounts of micrite. Sediments in modern warm water carbonate environments are prone to undergo cementation within a few centimeters below the sediment surface, and if currents or waves strip away the overlying uncemented sediment, this exposes a hard concrete-like surface on the sea floor, known as a hardground. Hardgrounds can be important substrata upon which encrusting organisms such as bryozoans and corals may attach. Although hardgrounds do occur in the type-Cincinnatian, they are relatively uncommon compared to warm water settings both today and in the past. The features that typify limestone of the Cincinnati area—abundant skeletal grains, a lack of ooids and peloids, minimal micrite, and uncommon hardgrounds—are typical of carbonates deposited today in cool temperate to polar waters.
The presence of cool water carbonates at tropical latitudes at first seems like a paradox, but such conditions occur today where coastal up-welling brings cool water up to the surface from depths of less than 200 meters. These cooler waters also contain abundant nutrients, which generate phosphate deposits. Indeed, the type-Cincinnatian is rich in phosphate, particularly in strata of Maysvillian age. Phosphate is also found in abundance in Upper Ordovician strata near Nashville, Tennessee, suggesting that the entire carbonate platform from Cincinnati to Nashville was a site of upwelling of cool, nutrient-rich water during much of the Late Ordovician. Upper Ordovician rocks of the Cincinnati area contain a greater amount of lime mud, more hardgrounds, and less phosphate, which collectively suggest a decrease in the intensity of upwelling in the latest Ordovician.
Additional evidence from the rich fossil faunas supports the interpretation of cool waters, followed by a return to warm water in the latest Ordovician. During the Late Ordovician, the western United States and Canada straddled the equator. Their carbonate sediments are typical of modern warm water settings, so their faunas are interpreted to reflect warm water conditions. These areas contain abundant corals and stromatoporoids, with a diverse array of brachiopods and trilobites. In particular, colonial rugosan and tabulate corals (for example, Tetradium), solitary corals (Grewingkia, Streptelasma), several brachiopods (Glyptorthis, Plaesiomys, Rhynchotrema, Hiscobeccus, Lepidocyclus, Holtedahlina, and Leptaena, for example), trilobites (Ceraurinus), and diverse cephalopods are characteristic of this warm water fauna. These organisms are absent from Edenian and Maysvillian strata in the Cincinnati area, but appear in the Richmondian as the limestones begin to reflect a return to warm water, low-nutrient conditions.
Type-Cincinnatian rocks differ from typical carbonate platform deposits in another significant aspect: the abundance of terrigenous mud, that is, clay produced by the weathering of silica-rich minerals such as feldspar. The earliest influx of this mud closely coincides with the beginning of nutrient-rich, cool water deposits at the base of the Lexington Limestone, which underlies type-Cincinnatian strata. The arrival of cool water, nutrients, and siliciclastic mud appears to have been triggered by the uplift of the Taconic Mountains to the east. Fine-grained terrigenous clay and silt were supplied by the Queenston Delta, as such sediments could easily have stayed suspended in the water across the Appalachian Basin until deposition in the Cincinnati area. These muds were best able to accumulate in relatively calmer deep water environments that were less disturbed by storms.
The Persian Gulf is similar in many respects to eastern Laurentia during the Ordovician. Both were developed as foreland basins, that is, deep water troughs adjacent to uplifting mountains. Both possess a carbonate platform that dips gradually into deep water. The sedimentary basins are similar in size and climate, and both sit at subtropical latitudes prone to storms. Large deltas supplied by abundant terrigenous sediment advanced into both basins. The Persian Gulf is notably different in that it lacks upwelling of cool, nutrient-rich water, which underscores that no modern setting is exactly analogous to Cincinnati during the Ordovician. Some have suggested the modern Bahama platform was similar to the Cincinnati area during the Ordovician, but the Bahama platform is flat-topped rather than gently dipping, is not a foreland basin adjacent to uplifting mountains that feed abundant terrigenous sediment to advancing deltas, and lacks upwelling of cool, nutrient-rich waters.
Marine Environments of the Cincinnati Arch
Four major sedimentary environments were present during the Late Ordovician of the Cincinnati Arch (Figure 15.3). Today, distinctive features of the rocks and fossils characterize these environments. Each of these environments is interpreted based on distinctive rock types and sedimentary structures that are found in similar settings today.
Tidal flat environments today are flat, nearly featureless areas that form between the low tide line and the high tide line. These areas are covered daily by tides, but are subjected to extreme variations in salinity and temperature on a daily basis.
In Upper Ordovician rocks of the Cincinnati Arch, tidal flat environments are preserved as laminated to burrowed dolomite and dolomitic limestone containing small amounts of clay (Figure 15.4A). The presence of dolomite suggests strong levels of evaporation, which would have drawn magnesium-rich brines through fine-grained limestone and converted it to dolomite. Although such conditions form today in relatively arid settings, the lack of other evaporite minerals such as halite or gypsum in these tidal flat facies argues more for a semi-arid environment. Wave-formed ripple marks attest to the shallow water environment in which these rocks were deposited, and desiccation cracks (“mud cracks”) indicate the drying and shrinking of the mud when it was exposed during prolonged low tides. Some of these strata contain numerous closely spaced planar laminae that record the deposition of individual layers of carbonate mud during incoming tides and storms on the highest part of the tidal flat, which remained above average high tide. In places, these laminae are penetrated by short, vertical burrows, which suggest areas somewhat lower on the tidal flat where burrowing organisms would not have been killed by drying out and overheating during low tides. Elsewhere in the Cincinnati region, tidal flat deposits lack planar lamination and are thoroughly burrowed by soft-bodied organisms such as polychaete worms and arthropods. Such extensive burrowing would require more frequent and persistent submergence during a tidal cycle, as on the lowest portions of the tidal flat. Many of these burrows are filled with the distinctive green iron mineral glauconite. Tidal flat deposits are most common in central Kentucky, but some tidal flats advanced as far northward as southern Indiana, as can be seen in exposures of the Saluda Dolomite near Madison, Indiana.
Figure 15.3. The four principal sedimentary environments of the type-Cincinnatian. Cincinnatian seas generally deepened northward from shallow water environments in central Kentucky to deeper water environments in Ohio and Indiana. The boundaries between these four environments correspond to sea level, fairweather wave base, and storm wave base. Wave base reflects the depth at which waves can move sediment on the sea floor and this depth increases with the height and period of waves, both of which increase during storms. The locations of these environments changed over thousands to millions of years, with environments shifting northward (as shown in figure) during times of slowly rising sea level and retreating southward during times of rapidly rising sea level. Falls in sea level resulted in the draining of the seas from the Cincinnati area.
Most tidal flat deposits in the Cincinnati region are unfossiliferous, but locally the burrowed deposits contain a sparse fauna of bryozoans, and more rarely ostracods, brachiopods, stromatoporoids, and rugosan and tabulate corals. This restricted group of organisms presumably would have been capable of tolerating fluctuations in salinity and temperature. Preservation of these fossils is typically poor, as a result of dolomitization, which tends to destroy fine details.
Figure 15.4. A. Out-crop photograph of finely laminated dolomite deposited in a tidal flat environment. B. Outcrop photograph of rubbly weathering, nodular limestone and mudstone deposited in a shallow subtidal environment. C. Outcrop photograph of interbedded limestone and mudstone deposited in a deep subtidal environment, with the limestone beds recording deposition during hurricanes. D. Out-crop photograph of mudstone with thin beds of limestone and silt-stone, all deposited in an offshore environment.
Behind the history of every sedimentary rock there lurks an ecosystem, but what one first sees is an environment of deposition.
Edward S. Deevey 1965, 592
Shallow subtidal environments today are shallow marine environments below the low tide line, but above fair weather wave base, the depth to which waves can stir the sediment during calm weather. Fair weather wave base is typically only a few meters on coasts protected from large oceanic waves. In modern carbonate settings, shallow subtidal environments are adjacent to tidal flats and are characterized by intense burrowing by soft-bodied organisms such as worms and arthropods.
Shallow subtidal deposits are found in the type-Cincinnatian and are likewise characterized by highly burrowed shallow marine deposits that grade upwards into tidal flat deposits, indicating that the two were deposited in laterally adjacent environments. In the type-Cincinnatian, shallow subtidal deposits consist of nodular to very thin wavy-bedded shelly limestone and fossiliferous mudstone (Figure 15.4B). Because of the thinness and waviness of the limestone beds, these rocks weather to a characteristic rubble of fist-sized limestone nodules. This distinctive bedding results from the pervasive burrowing of the sediment by soft-bodied organisms. Although storms certainly reworked the sediment and deposited the characteristic well-sorted layers of shells overlain by layers of mud that are preserved in some places, subsequent burrowing mixed these layers, producing pods of shell-rich and shell-poor material. Preferential cementation of these churned sediments produced pockets of well-cemented shelly material surrounded by non-cemented zones rich in clay.
Shallow subtidal limestone in the Cincinnati area is locally rich in phosphate, particularly in the Maysvillian. Much of this phosphate occurs as infillings of bryozoan zooecia, the porous skeletons of echinoderms, and the larval shells of pelecypods, gastropods (such as Cyclora), and monoplacophorans. The presence of this phosphate indicates large amounts of decaying organic matter within the sediment. By dissolving pieces of shallow subtidal limestone in vinegar or dilute hydrochloric acid, one can see the rich fauna preserved by this phosphatization. Shallow subtidal rocks are broadly distributed over the Cincinnati Arch and occur from the southern edge of the Ordovician outcrop belt in southern Kentucky to the northern limit of Ordovician rocks in central Ohio and Indiana. The Bellevue, Mt. Auburn, Oregonia, and Whitewater Formations all accumulated within shallow subtidal environments.
Shallow subtidal rocks are exceedingly fossiliferous in most places, reflecting the abundance of life in this shallow marine habitat. Most commonly, shallow subtidal rocks are packed with large brachiopods, such as Platystrophia, Hebertella, and Rafinesquina. Many of these have thick or coarsely ribbed shells, presumably for protection against waves and currents. Platystrophia, in particular, has a greatly thickened pedicle valve near the hinge, which would have increased the stability of the shell on the sea floor. Disarticulation, breakage, and abrasion of these shells are widespread and attest to the damaging effects of waves and currents. Large bryozoans are often abundant and include branching, encrusting, sheet-like, and massive forms. As is true for the brachiopods, these robust bryozoan skeletons reflect the intensity of waves and currents in this shallow water environment. Molluscs are present in shallow subtidal rocks, particularly the byssally attached pelecypods Ambonychia and Caritodens, the carnivorous cephalopod Treptoceras, and the gastropods Lophospira and Cyclonema. Cyclonema is commonly associated with crinoids, on which it may have been a parasite. Lophospira has been interpreted as a scavenger. Crinoids and trilobites occur, but most specimens are disarticulated rather than whole, presumably owing not only to waves and currents, but also burrowing organisms.
Deep subtidal environments today are those that lie below fair weather wave base, but above the wave base of all but the most powerful storms or hurricanes, which would have extended to depths of a several tens of meters. In these settings, the sedimentary deposits are characterized by the alternation of sandy and shelly beds deposited during storms and muddy beds that reflect quiet water deposition during weak storms or during periods between storms. Deep subtidal environments are adjacent to and slightly deeper than shallow subtidal environments.
As on modern shelves, storm deposits are the most conspicuous feature of the deep subtidal environment in the type-Cincinnatian, with roughly equal proportions of thin to medium-bedded shelly limestone, laminated siltstones, and mudstone (Figure 15.4C). Burrowing is much less intense here than in deep subtidal environments, resulting in thicker and more laterally continuous limestone beds. Beds of siltstone are commonly rippled or display internal planar or hummocky lamination generated by strong storm currents and waves. At times, storms occurred with sufficient frequency that they commonly eroded through the mudstone layer capping the deposit from the previous storm, such that the shelly layer from one storm was deposited directly on the shelly bed of the previous storm. This phenomenon, known as amalgamation, produces thick layers of limestone with subtle internal erosion surfaces that separate individual storm beds, thereby producing what are known as multi-event beds. In some cases, a one-foot thick bed of limestone may record half a dozen storm events. Deep subtidal rocks are as broadly distributed over the Cincinnati Arch as shallow subtidal rocks. The Fairview, Corryville, Sunset, and Liberty Formations accumulated in deep subtidal environments.
Deep subtidal rocks of the type-Cincinnatian contain an abundant and diverse fauna. Preservation is commonly better than in shallow subtidal rocks, with less overall disarticulation, breakage, and abrasion, suggesting less exposure to the damaging effects of waves and currents. Many brachiopod genera may be present, including Rafinesquina, Strophomena, Leptaena, Hiscobeccus, Platystrophia, Plectorthis, Glyptorthis, and Plaesiomys. All have shells that are thinner and finer-ribbed than those in the shallow subtidal. Bryozoans can be abundant and also tend to be thinner and less massive than in the shallow subtidal. Molluscs are common, with similar forms as in the shallow subtidal, as well as the byssally attached pelecypod Modiolopsis. Crinoids (Glyptocrinus, Pycnocrinus, and Iocrinus) and edrioasteroids (Carneyella, Isorophus, and Streptaster) are locally abundant, and beds and pockets containing fully articulated specimens are not unusual. Trilobites (Isotelus and Flexicalymene) are common both as individual sclerites and as articulated specimens. The frequency of articulated crinoids and trilobites suggests early burial and less frequent disturbance by waves, currents, and burrowing organisms. In the uppermost Cincinnatian, solitary corals (Grewingkia and Streptelasma), and the encrusting tabulate coral, Protaraea, are conspicuous additions to the deep subtidal fauna. Trace fossils are common in the siltstone beds. Chondrites and Trichophycus were the burrows of deposit feeding worms or arthropods. The U-shaped burrows of Diplocraterion were the homes of a polychaete worm, but whether it was a suspension feeder, a stationary deposit feeder, or an ambush carnivore is uncertain, as modern examples of all such U-tube builders are known. Paleophycus records the horizontal burrowing of another scavenging or deposit feeding worm.
The thick, tabular limestone beds of the deep subtidal are well suited as building stones. Many old quarries were established in deep subtidal rocks and many of those stones can now be found in old building foundations and rock walls. Two intervals of deep subtidal rocks frequented by quarrymen were the River Quarry Beds (now called the Point Pleasant Formation) and the Hill Quarry Beds (now called the Fairview Formation). Although the River Quarry Beds near Cincinnati are now largely under the Ohio River, whose level was raised during the construction of dams, they can still be seen near Point Pleasant, Ohio, along the crest of the Cincinnati Arch. Many of the Hill Quarries can still be seen in the bluffs south of the University of Cincinnati and flanking the Mill Creek Valley.
Offshore environments on modern coasts lie below the wave base of most storms, but are sometimes affected by the most severe storms and extend to depths of several tens of meters. In these modern settings, deposition is dominated by muds, which can accumulate when currents and waves are weak. Rare, exceptionally strong storms are capable of moving shells and sediments even at these depths and produce thin storm beds, although these make up a minority of the deposits. Offshore environments are adjacent to and somewhat deeper than deep subtidal settings.
In the type-Cincinnatian, offshore rocks contain a greater proportion of mudstone (commonly near two-thirds), but in other regards are quite similar to the deep subtidal (Figure 15.4D). The less frequent occurrence of storm beds in offshore deposits indicates less frequent disturbance by storm-generated waves and currents. As a result, amalgamation is much less common in the offshore, and most limestone beds are single-event beds and record the passage of a single hurricane. Many of the thick mudstone layers found in the offshore are also the result of storm deposition, as indicated by the presence of multiple 2–3 cm fining-upward mudstone beds, each with a slightly silty interval at its base. Each of these thin layers records the minor disturbance of the sea floor by a hurricane, with settling of silt and then clay following the storm. Frequently, such mud layers would blanket the bottom, smother the fauna living on the sea floor, and preserve articulated crinoids and trilobites. Offshore rocks occur as far south as north-central Kentucky but extend beyond the northern limit of Ordovician exposures in central Ohio and Indiana. The Kope and Waynesville Formations largely reflect offshore settings.
Offshore strata contain an abundant and diverse fauna, characterized by small, thin, and delicate fossils, suggesting generally quiet water conditions, except during rare, severe storms. Common brachiopods include the highly gregarious Dalmanella and Sowerbyella, and the burrowing inarticulates Pseudolingula and Leptobolus, which are sometimes found preserved inside their vertical burrows. Although sheet-like bryozoans do occur, thin branching forms and flat disc-shaped forms are more common. Molluscs are diverse and abundant in a few widely traceable horizons, which are conspicuously poor in brachiopods and bryozoans. Common molluscs include the byssally attached pelecypods Ambonychia and Modiolopsis, the burrowing pelecypods Deceptrix and Rhytimya, the scavenging gastropods Lophospira and Liospira, the monoplacophoran Sinuites, and the cephalopods Treptoceras and Cameroceras. Trilobites are numerous and frequently fully articulated. The burrowers Flexicalymene, Gravicalymene, Cryptolithus, and Isotelus are the most common, but the spiny swimming Acidaspis can be abundant in some crinoid-rich layers. Suggestive of deep water settings is the presence of the blind trilobites Cryptolithus and Triarthrus in offshore strata. Crinoids are also numerous and are frequently articulated. The most common genera are Cincinnaticrinus and Ectenocrinus, whose ossicles may comprise entire beds of limestone. Given the tens of kilometers over which such beds can be traced, the number of crinoid individuals must have been astronomical. As in the deep subtidal, trace fossils are numerous in beds of siltstone. Chondrites, Diplocraterion, Trichophycus and Paleophycus are all common. The trilobite burrow Rusophycus is also common, and examples of Rusophycus made by Isotelus and Cryptolithus have been reported, but ones produced by the calymenids Gravicalymene and Flexicalymene are much more common.
