Trace sediment from weathering to lithification, classify clastic, chemical and organic sedimentary rocks, and read depositional environments, fluid flow and sea-level change from Ohio's sedimentary rocks

Learning goal: Describe how sediment becomes rock, from weathering through lithification; classify sedimentary rocks and minerals into the clastic (physical), chemical and organic divisions; identify the depositional environment of a sample from its grain size, sorting, structures and fossils; use fossils in sedimentary rock to track changes in sea level; evaluate how well different sedimentary rocks transmit groundwater, oil and natural gas; explain how coal, oil and natural gas form, including those in Ohio; and design a mining method that removes material without collapse.

From weathering to rock. Sedimentary rocks begin with weathering, the breakdown of rock at the surface. Mechanical weathering breaks rock into pieces without changing the minerals: water freezing in cracks expands by about 9% and wedges them apart, roots pry, and grains abrade one another. Chemical weathering changes minerals: feldspar reacts with slightly acidic water to form clay, iron minerals oxidize to rust-colored oxides, and calcite dissolves. Quartz resists both, which is why it survives as sand while the other minerals of granite become clay. Erosion removes the products, and water, wind or ice transport them. Transport sorts and rounds grains. Abrasion wears the corners off, so rounding generally increases with distance and time, but it is not a distance gauge: soft fragments round within a few kilometers, hard quartz takes far longer, and grains recycled from an older sandstone start out already rounded. A current of a given speed carries grains up to a certain size and drops larger ones. Deposition happens when the current slows. As layers pile up, burial follows, then compaction, which squeezes out water and presses grains together, and cementation, in which minerals such as calcite, silica or iron oxide precipitate from groundwater in the pores and glue grains together. Compaction and cementation together are lithification.

The three divisions. Clastic (physical) sedimentary rocks are made of fragments of older rock and are named by grain size. Grains larger than 2 mm make conglomerate if rounded (abraded, usually during transport by water) or breccia if angular (little abrasion, usually deposited close to the source). Sand, 1/16 to 2 mm, makes sandstone; quartz sandstone is nearly all quartz, while arkose keeps much feldspar because it was deposited close to its granite source before the feldspar weathered. Silt and clay, finer than 1/16 mm, make siltstone and mudstone; a mudstone that splits into thin layers is shale. Chemical sedimentary rocks precipitate from solution. When seawater evaporates in a shallow, restricted basin, gypsum and then halite crystallize, forming evaporites: rock gypsum and rock salt. Chert is microcrystalline quartz, and dolostone forms where magnesium-rich water converts calcite to dolomite. Organic (biochemical) sedimentary rocks are made from the remains of living things. Most limestone is biochemical: shells, coral and the tiny plates of plankton, all calcite or aragonite. Fossiliferous limestone shows the shells; coquina is loosely cemented shell debris; chalk is made of microscopic plankton plates. Coal is compacted plant material. Limestone can also precipitate chemically (travertine in caves and springs), so the division depends on how a sample formed, which is why evidence such as visible fossils matters.

Depositional environments. Every sedimentary rock carries clues to the place where its sediment was laid down. Grain size tells the energy of the water or wind; sorting and rounding give clues to how, and often how far, the grains were carried; structures and fossils tell the setting. River channels deposit sand and gravel in cross-beds, inclined layers built as ripples and dunes migrate, which dip downstream and record the current direction. Floodplains deposit mud with mud cracks (from drying) and root traces. Swamps accumulate plant material that becomes coal. Deltas stack sandstone, shale and coal. Beaches leave well-sorted, well-rounded quartz sand. Shallow, warm, clear seas grow limestone rich in shell fossils and reefs. Deep, quiet water lets clay settle into dark shale. Deserts build very large cross-beds of frosted, well-sorted sand. Glaciers leave unsorted till, clay to boulders mixed together. Evaporating lagoons leave salt and gypsum. A misconception to avoid is that grain size alone identifies an environment: fine mud settles in a quiet lake, a lagoon, a floodplain and the deep sea, and only the fossils and structures separate them.

Fossils and changing sea level. Fossils record not only life but depth and distance from shore. Land plants and roots mean land or swamp; shallow-water brachiopods, corals and crinoids mean a shallow, sunlit sea; fine dark shale with few bottom-dwellers means deeper or oxygen-poor water. When sea level rises and the shoreline moves inland (a transgression), deeper-water deposits come to lie on top of shallower ones; when it falls (a regression), shallower deposits lie on top of deeper ones. Here is a model worked through, a hypothetical Ohio outcrop read from the bottom up: coal with fern fossils (swamp, at sea level); gray shale with plant fragments (a muddy coastal bay); dark shale with brachiopods (a shallow sea has flooded in); limestone with crinoids (the shore is now farther away); gray shale again (the sea is shallowing); sandstone with cross-beds (a river or delta has built back out); coal (swamp again). Provided no erosion surface separates the layers, the sea advanced then retreated in one cycle. Repeated cycles like this in Ohio's Pennsylvanian rocks are widely interpreted as many rises and falls of relative sea level, although a delta switching its position can build a similar cycle, so geologists look for the same cycle across a wide area before calling it a sea-level change.

Ohio's sedimentary bedrock. Ohio's bedrock is almost entirely sedimentary, laid down in or beside shallow seas during the Paleozoic Era. In southwestern Ohio around Cincinnati, Ordovician limestone and shale are packed with brachiopods, bryozoans, crinoids and trilobites; the trilobite Isotelus is Ohio's official state invertebrate fossil. Silurian dolostone underlies much of western Ohio and is quarried for crushed stone and tapped as an aquifer; Silurian rock salt of the Salina Group lies deep under northeastern Ohio and is mined beneath Lake Erie from Cleveland and Fairport Harbor. Devonian Columbus Limestone crops out in central Ohio, and above it lies organic-rich black Ohio Shale. Sandstones near the Devonian–Mississippian boundary and in the Mississippian include the Berea Sandstone, a building stone and oil and gas reservoir, and the Black Hand Sandstone of the Hocking Hills cliffs. Pennsylvanian rocks of eastern and southeastern Ohio contain the state's coal seams.

Fossil fuels. Coal forms where swamp plants accumulate in waterlogged, oxygen-poor water faster than they decay, making peat; burial and heat drive off water and gases, turning peat into lignite, then bituminous coal and, where heated further, anthracite, with carbon content rising at each step. Oil and natural gas form from the remains of marine plankton buried in mud that becomes organic-rich shale, the source rock. At temperatures of roughly 60 to 120 °C, reached at a few kilometers depth, the organic matter breaks down to oil; hotter, to gas. Oil and gas are less dense than water, so they migrate upward through pores and fractures until trapped in a porous reservoir rock beneath an impermeable cap rock.

Fluid flow: porosity and permeability. Porosity is the fraction of a rock's volume that is open space; permeability is how easily fluid flows through it, which depends on whether the pores are connected and how wide the connections are. A well-sorted sandstone that is only lightly cemented keeps much of the porosity and permeability of loose sand, making a good aquifer or reservoir. Sorting alone does not guarantee it: the same sandstone with its pores filled by cement, or squeezed by deep burial, can be tight, with little of either. Shale can have substantial porosity, but its pores are so tiny that fluid barely moves: low permeability makes it a cap rock or a barrier to groundwater. Fractured limestone and dolostone transmit fluids along cracks and dissolved channels. Ohio's deep Utica Shale holds gas and oil that moves only after the rock is fractured artificially from horizontal wells.

Worked example, a dichotomous key: Dichotomous means every step offers exactly two choices. This key is for a classroom set of sedimentary rocks. Step 1: Does dilute acid (applied by the teacher) make the rock fizz, on the solid surface or on powder scratched from it? Yes: go to 2. No: go to 6. Step 2: Does the solid, unpowdered surface fizz? Yes: go to 3. No, only the powder fizzes: dolostone. Step 3: Are shells or shell fragments visible to the eye? Yes: go to 4. No: go to 5. Step 4: Is the rock made almost entirely of loosely cemented shells with open gaps between them? Yes: coquina. No, the shells sit in firm rock: fossiliferous limestone. Step 5: Is it soft, white and powdery? Yes: chalk. No: fine-grained limestone, not resolved further by this key. Step 6: Are grains visible to the eye? Yes: go to 7. No: go to 10. Step 7: Are most grains larger than 2 mm? Yes: go to 8. No, sand-sized and gritty: go to 9. Step 8: Are the large fragments rounded? Yes: conglomerate. No, angular: breccia. Step 9: Are the grains nearly all clear or gray quartz? Yes: quartz sandstone. No, with many pink or white feldspar grains: arkose. Step 10: Is it black and lightweight? Yes: coal. No: go to 11. Step 11: Does it break into cubes (cubic cleavage)? Yes: rock salt (it also dissolves in water; never taste a specimen). No: go to 12. Step 12: Can a fingernail scratch it? Yes: rock gypsum. No: go to 13. Step 13: Does it scratch glass? Yes: chert (very hard, with conchoidal fracture). No: go to 14. Step 14: Does it split into thin layers? Yes: shale. No: mudstone or siltstone, not resolved further by this key. Test: a gritty rock that does not fizz, with sand-sized grains, many of them pink feldspar: step 1 no, step 6 yes, step 7 no, step 9 no, so arkose. A specimen that fits no ending is reported as not resolved, never forced to a name.

Worked example, measuring porosity (model values): Fill a 250 mL beaker to the 200 mL line with dry, well-sorted sand. Pour water from a graduated cylinder until the water just reaches the top of the sand; 70 mL was used. Porosity = 70 ÷ 200 = 0.35, or 35%. A mixed sand and clay sample in the same test takes 48 mL: 48 ÷ 200 = 24%. The small clay grains fill the spaces between sand grains, so poorly sorted sediment has lower porosity. Permeability needs a separate test: time how long 100 mL of water takes to drain through each column.

Worked example, an idealized pillar-load calculation: Room-and-pillar mining leaves square pillars of rock to hold up the roof. Suppose pillars 20 m × 20 m on a 30 m grid, 200 m deep, with overlying rock of density 2,500 kg/m³. Each pillar supports its own area plus half of each surrounding room, a 30 m × 30 m = 900 m² tributary area. Extraction ratio = (900 − 400) ÷ 900 = 55.6%. Vertical stress at 200 m = 2,500 × 9.8 × 200 = 4.9 MPa; pillar stress = 4.9 × 900 ÷ 400 ≈ 11.0 MPa. If laboratory tests of the rock gave a strength of 30 MPa and the design requires a safety factor of at least 2, then 30 ÷ 11.0 ≈ 2.7 passes. Mining the pillars down to 15 m × 15 m would raise extraction to 75% but pillar stress to 4.9 × 900 ÷ 225 = 19.6 MPa, and 30 ÷ 19.6 ≈ 1.5 fails. This tributary-area check is only a first step: a real pillar is weaker than a small laboratory sample because of joints and its shape, and roof spans and floor strength must also be checked, so engineers use a reduced design strength.

Worked example, reading a depositional environment: A hypothetical sample is a sandstone of well-rounded, well-sorted quartz grains, with cross-beds up to 5 m high and no fossils. Well-rounded and well-sorted grains mean much abrasion and steady sorting, by long transport or by reworking of older sand; very large cross-beds are built by migrating dunes; no marine fossils: a desert dune field. A second sample of the same grain size but with small cross-beds, plant fragments and interbedded shale points to a river channel.

Investigation, porosity and permeability of sediments: Materials: three clear plastic cups with small holes punched in the bottom and lined with a coffee filter, gravel, sand and a sand-clay mixture, a graduated cylinder, a stopwatch and a collection tray. Measure porosity for each sediment by the water-filling method in the worked example (in a cup without holes). Then pour 100 mL of water into each cup with holes and time how long it takes to drain and how much is collected. Record porosity (%), drainage time (s) and volume retained (mL) in a table. Result that supports the claim: gravel drains fastest, sand next, sand-clay slowest, even though porosities may be similar; permeability depends on pore size and connection, not on porosity alone. What the model cannot show: cementation, which reduces both properties in real rock, and fractures, which can make an otherwise tight rock permeable.

Design task, a mining method: Salt is to be mined by room and pillar at 400 m depth under rock of density 2,500 kg/m³; tests give a design pillar strength of 40 MPa, and a safety factor of at least 2 is required. Rooms must be 12 m wide for the machines. Choose a square pillar width, calculate the extraction ratio and the safety factor, compare it with one alternative width, and justify your choice in terms of both safety and the amount of salt recovered.

Key-building task: Write your own dichotomous key for the six specimens you will classify in the independent performance below: every step two choices about one observable property. Give it to a partner to test on the specimens with their labels hidden, and record any specimen the key misnames or cannot resolve and the step you would change.

Representation task: Draw a schematic vertical column, not to scale, for the hypothetical Ohio outcrop in this lesson: coal, gray shale, dark shale with brachiopods, crinoidal limestone, gray shale, cross-bedded sandstone, coal. Beside each layer write the environment and draw an arrow showing whether the shoreline moved inland (relative sea-level rise or less sediment) or seaward. Label the transgression and the regression, and name one other cause, such as a delta building out, that could produce the same upward change.

Independent performance: Collect or be given six sedimentary specimens. For each, record grain size, sorting, rounding, composition, structures and fossils; classify it as clastic, chemical or organic; name it; and state its depositional environment with the evidence. Success criteria: every classification uses at least two observations; one specimen is linked to an Ohio rock unit it resembles; one is evaluated as an aquifer or cap rock with reasons; and the report states which identification you are least certain of and what test would settle it.