Fossils are found in sedimentary rocks, which come in two basic types: siliciclastic and chemical. Siliciclastic rocks are made up of broken pieces of pre-existing rock. These pieces, or sediments, form by the process of weathering (the mechanical or chemical breakdown of rocks into smaller pieces by water, ice, wind, plant roots, etc.). Agents of erosion, usually streams, rivers, glaciers, or wind, then transport these fragments away from the source rock. Eventually, the sediments settle out of the transporting medium and are deposited in layers or beds. These sediments may then be buried, compacted, and cemented together by minerals, to form a siliciclastic sedimentary rock.
Gravel, sand, silt and clay are typical sediments – they form conglomerate, sandstone, siltstone, and claystone. The term “mud” refers to silts and clays, which make up mudstones. A thinly layered mudstone is called a shale. Finer-grained rocks like shale tend to form in quieter, deeper water settings, while coarser-grained rocks like conglomerates and sandstones tend to form in shallower settings where current and wave energies are higher.
Chemical sedimentary rocks are made up of minerals that precipitated directly out of seawater and settled to the bottom. These minerals may precipitate inorganically as a result of evaporation (like rock salt and gypsum deposits). Such evaporite deposits indicate arid or semi-arid conditions. More commonly, though, the minerals are biochemically precipitated when organisms make skeletons for themselves. When the organisms die, their skeletons become sedimentary particles that may accumulate on the seafloor intact, or be smashed into pieces or even into a fine mud. Note that chemical sediments usually form in place, rather than being transported in from elsewhere. Limestones are chemical sedimentary rocks made from calcium carbonate (CaCO3); such carbonates are extraordinarily common rocks on Earth’s surface. Limestones form particularly well in shallow, tropical marine environments.
Sedimentary rocks typically form as originally horizontal layers, or strata. These may later be tipped, folded, or otherwise deformed. Layers of rock can be subdivided into units that have a distinctive and clearly definable composition or character; these units are called formations. One formation may insensibly grade into an overlying one, or the contact may be sharp or discontinuous. Often the boundary between two formations is an unconformity, or ancient erosion surface. An unconformity represents a gap in the rock record, where rocks are missing because they have been eroded away. For example, the actual Ordovician-Silurian boundary is missing in western Ohio’s rock record. Upper Ordovician rocks are separated from Lower Silurian rocks by an unconformity. This gap was probably formed when sea levels fell in the Latest Ordovician, exposing western Ohio’s rocks to the air and allowing them to erode. Sea level then rose again and deposition of sediments resumed in the Early Silurian.
Let’s first consider Ohio as a whole. Figure 1 is a map of Ohio’s bedrock geology. Note where rocks of different ages outcrop. Early Paleozoic rocks make up the bedrock in western Ohio, with the oldest (Ordovician) rocks primarily in the southwest corner of the state. These older rocks are brought up to the surface by the Cincinnati and Findlay Arches, broad upwarpings of Earth’s crust (see Figure 2). In eastern Ohio, Early Paleozoic rocks are buried deeply under younger rocks.
P Can you see how the geological structures in Ohio are reflected by the bedrock geology?
During this trip we will be looking at Ordovician and Silurian rocks that were deposited between about 435 and 445 million years ago. World geography was quite different at this time than it is today, as shown in the paleogeographic reconstruction in Figure 3. The southern continents—Africa, India, South America, Australia, and Antarctica—were already sutured together into the supercontinent Gondwana, which sat around the South Pole. Continental ice sheets, or glaciers, grew on Gondwana during the Late Ordovician; this Ice Age may have had global effects on sea level and climate, and may be implicated in the end-Ordovician mass extinction event. Meanwhile, North America at this time straddled the equator, and Ohio was located south of the equator at a tropical to subtropical latitude.
P Is it surprising to you to think that 430 million years ago, Africa was covered by ice while Ohio basked in a tropical paradise?
Sea levels were high during much of the Early Paleozoic, and large areas of the continents were flooded, forming shallow marine environments called epicontinental or epeiric seas. Paleontologists love epeiric seas because lots of sediments and fossils accumulated in them, and, since sea level is now quite low, these rocks are exposed and easily accessible for study. In fact, some have estimated that up to 90% of the fossils we’ve discovered are found only in these shallow continental sea environments.
P How might our view of marine diversity be skewed by the disproportionate representation of shallow sea dwellers?
P Imagine an Ohio sitting at a tropical latitude, and covered by a very shallow sea. What would it look like? What sorts of organisms would you expect to see?
During the Ordovician Period (443-490 Ma), most of the North American mid-continent was submerged under an epeiric sea. The eastern seaboard was undergoing the Taconic Orogeny, a mountain-building event involving the collision of North America and a volcanic island arc. Tectonic activity included huge volcanic eruptions, as evidenced by massive Ordovician ash beds found throughout eastern North America, and the uplift of a mountain chain along the eastern margin of the continent. As these mountains eroded, they shed siliciclastic sediments (clay, silt, sand, etc.) westward into the mid-continent sea (see Figure 4). Ohio was affected by these new sediments – the Lower Ordovician Trenton and Black River Limestones grade into the more siliciclastic Middle and Upper Ordovician formations.
P How can you tell limestone and siliciclastic rocks apart?
Upper Ordovician rocks well-exposed in southwest Ohio define the Cincinnatian Series. This package of rocks is famous worldwide for its well-preserved and abundant fossils. The fossiliferous nature of these rocks was first described in 1825, and Cincinnatian fossils have since become a standard basis for museum exhibits on Paleozoic marine life.
During the Ordovician, the Cincinnati-Dayton area was at 15-20o south latitude, and covered by a shallow sea that varied up to about 50 meters depth (tending to be deeper towards the present-day north). Sediments accumulating in the sea were a mix of shales and limestones, depending on water depth. Hurricanes were common, producing distinctive rock layers called tempestites. This type of setting is usually termed a “storm-dominated shallow tropical marine ramp”.
P Think about what a tempestite might look like. What do you expect fossil skeletons in tempestites to be like?
The Cincinnatian Series is subdivided into several stages (see Figure 5). We will be focusing on the Richmondian Stage, which is the most fossiliferous. The increased abundance and diversity of marine life during this stage have led some to refer to the “Richmondian Invasion”. The majority of Richmondian (and indeed all Paleozoic) marine animals were sessile suspension feeders, which means they stayed fixed in place on the sea floor and filtered food particles out of the water. Brachiopods, crinoids, bryozoans, tabulate and rugose corals, and clams all are sessile suspension feeders, and are common Richmondian fossils. Common mobile animals include trilobites (including Isotelus, Ohio’s state fossil and one of the largest trilobites of all time), snails, nautiloid cephalopods, and starfish. Figure 6 shows reconstructions of some of these animals.
P How many of these animals do you think you could recognize in a rock?
Sea level fell briefly at the end of the Ordovician, producing an erosional unconformity between Ordovician and Silurian formations in western Ohio (see Figures 5 and 7). Shallow seas returned to Ohio quickly, though, and marine rocks of Early Silurian age are present in southwest Ohio. As the Silurian got underway, the mountains formed by the Taconic orogeny were eroding down, and not shedding as much siliciclastic sediment to the west. This drop in siliciclastic input allowed limestones to again predominate in Ohio, as they had before the orogeny began (see Figure 4).
During the Middle Silurian, two depressions or basins in the Earth’s crust developed, one in Michigan and one in north-central Ohio. Water depths in these basins may have reached 180 meters. Shallow water barrier reefs made of tabulate corals and stromatoporoid sponges surrounded these basins. Reefs are structures that rise up from the sea floor and are made from the skeletons of carbonate-secreting organisms. These Silurian reefs are notable for the diversity of organisms that helped construct them – prior to this, reefs were usually built by a single type of organism. Bowling Green sits on top of some of these tabulate-stromatoporoid reefal rocks.
P Picture a modern coral reef in your head. How different do you think a reef made by other organisms, like sponges and algae, would be?
In Late Silurian times, the basins began to dry up, as less seawater was supplied to them. Probably, the barrier reefs grew extensive enough to nearly completely block water flow into the basins. As the seawater in the basins evaporated, it produced thick deposits of rock salt and gypsum. These deposits are extensively mined today.
Large Silurian reefs are not as common in southwest Ohio as they are to the north. Rather, fine-grained limestones predominate. However, occasionally one can find fossilized mud mounds, composed of bryozoans, crinoids, and algae. These mounds are not considered by most to be true reefs, but do represent a buildup of organisms in a local area.
All the Silurian rocks in Ohio were probably once as fossiliferous as the Ordovician rocks already discussed. Unfortunately, most of Ohio’s Silurian rocks have been dolomitized. This means that the original calcite (CaCO3) forming the sediment and the skeletons has been replaced by the mineral dolomite (MgCO3). This replacement was not gentle – few fossils are well-preserved and most were obliterated all together! (This is why we’re driving so far from Bowling Green to see fossiliferous rocks.)
One package of rocks that escaped dolomitization is the Brassfield Formation, which is part of the Llandoverian Series of the Lower Silurian (see Figure 7). (Llandovery is a Welsh placename, spelled Llanymddyfri in Welsh; ask Dr. Yacobucci to try to pronounce it for you.) We will see the Brassfield Formation on our last stop of the field trip. Fossils are common in this unit, especially crinoid fragments.
P Do you think the Brassfield Formation will look different from the Ordovician rocks? How different do you expect the fossils to be?
The area around Caesar Creek, southeast of Dayton in Warren County, was noted by collectors as being particularly fossiliferous back in 1878. Rocks exposed here are of Richmondian Age, about 445 Ma. Concerns over flooding led to the construction of a dam across Caesar Creek between 1971 and 1978. The Caesar Creek Dam is 165 feet high and 2750 feet long, in case you were curious. The water pooling behind it forms Caesar Creek Lake, the deepest lake in Ohio (115 feet). Unfortunately, the lake now covers some previously exposed rock sections, but the emergency spillway excavated beside the dam exposes the Waynesville, Liberty, and Whitewater Formations (see Figure 5). A stratigraphically lower section is exposed near the dam face, where the Arnheim and lower Waynesville Formations are viewable. We will stop at both exposures. Fossil collecting is allowed here, but you may not use tools or take specimens larger than your palm.
Goes Station is east of Dayton and north of Xenia, in Greene County. A roadcut here along U.S. Route 68 exposes the Drakes Formation, which sits on top of the formations at Caesar Creek (see Figure 5). The Drakes Formation was deposited in an environment quite different from that of the formations at Caesar Creek—we’ll see if you can figure out the depositional setting and find any fossils. Also exposed at the top of this roadcut is the contact between the Ordovician Drakes Formation and the Lower Silurian Brassfield Formation.
This state preserve is in Miami County, north of Dayton and just southeast of Tipp City, between Ohio Route 202 and the Miami River. At this site, a stream fed by underground springs falls 11 meters over a cliff. The cliff top is made of the Lower Silurian Brassfield Formation, while the lowest exposed rocks are Upper Ordovician. The Brassfield is fossiliferous, so we’ll try to find crinoids, brachiopods, corals, and bryozoans, and compare Brassfield fossils to the Cincinnatian fossils at Caesar Creek.