Stratigraphy and Historical Geology – Comprehensive Review

150 question-and-answer flashcards covering key concepts in stratigraphy, sedimentology, historical geology, geologic time, sedimentary structures, diagenesis, depositional environments, and associated geologic principles.

What is the literal meaning of the word "geology"?

It comes from the Greek words geo (earth) and logos (study), meaning the study of the Earth.

What are the three main components of the scientific method used in geology?

Gathering and analyzing facts, formulating hypotheses, and testing hypotheses to develop theories.

In geology, what is a paradigm?

A comprehensive theory that is extensively documented and held with a high degree of confidence.

What is the focus of physical geology?

The study of Earth’s materials, the processes acting on them, and the forces that cause changes.

What does historical geology investigate?

The origin and evolution of Earth’s continents, oceans, atmosphere, and life through time.

Define stratigraphy.

The study and interpretation of layered rock sequences in space and time to reconstruct Earth history.

Name two major economic uses of stratigraphy.

Locating mineral deposits and exploring for oil, gas, coal, and groundwater aquifers.

State Steno’s Law of Original Horizontality.

Sediments are originally deposited in horizontal layers; tilted layers were tilted after deposition.

State Steno’s Law of Original Continuity.

Sedimentary layers extend laterally in all directions until they thin out or meet a barrier.

State Steno’s Law of Superposition.

In an undisturbed sequence, the oldest layers are at the bottom and the youngest at the top.

Who is considered the "Father of English Geology" and why?

William Smith, for discovering the principle of faunal succession and publishing the first geologic map of England and Wales in 1815.

What does the principle of faunal succession state?

Fossil organisms succeed one another in a definite and recognizable order through geologic time.

Summarize Hutton’s principle of uniformitarianism.

"The present is the key to the past"; processes operating today also operated in the geologic past.

What are the three eras of the Phanerozoic Eon?

Cenozoic, Mesozoic, and Paleozoic.

Which geologic period is known for abundant dinosaurs and the first birds?

The Jurassic Period.

Which geologic period ended with a major dinosaur extinction?

The Cretaceous Period.

What major life event marks the Cambrian Period?

The appearance of abundant shelly marine organisms.

Which two Greek words form the basis of "stratigraphy"?

Stratum (layer) and graphia (writing or description).

Define lithostratigraphy.

The branch of stratigraphy dealing with the correlation and characterization of rock units based on lithology.

What is a stratotype?

The designated type section that serves as the reference for defining a stratigraphic unit or boundary.

List three common careers that use geology.

Mining and mineral industry, petroleum exploration, and environmental consulting.

What grain-size term describes particles between 64 mm and 256 mm?

Cobble.

On the Udden-Wentworth scale, what is the phi (φ) value of 1 mm sand?

φ = –1.

Define "sorting" in sedimentology.

A measure of the range of grain sizes and the scatter of sizes around the mean in a sediment.

What sorting descriptor corresponds to a standard deviation (σφ) of 0.4?

Well sorted.

What does positive skewness indicate in a grain-size distribution?

An excess of fine particles (fine-skewed).

Name three aspects of particle shape.

Form (sphericity), roundness, and surface texture.

Why do spherical grains settle faster than platy grains?

Because their shape reduces drag, increasing settling velocity.

What is imbrication in sedimentary structures?

An arrangement where elongated clasts overlap like shingles, typically dipping upstream.

Describe planar bedding.

Beds bounded by essentially parallel surfaces with no internal dipping laminae.

What is graded bedding?

A single bed that shows a vertical change in grain size from coarser at the base to finer at the top.

Which process typically produces normal graded beds in deep-marine settings?

Turbidity currents.

Define ripples in bedforms.

Small-scale ridges 5–20 cm long and 0.5–3 cm high formed by water or wind currents.

How do current ripples differ from oscillation ripples?

Current ripples are asymmetrical with a lee side facing downstream; oscillation ripples are symmetrical from wave action.

What type of cross-bedding results from straight-crested bedforms?

Tabular cross-bedding.

What bedding structure results from rapidly deposited climbing ripples?

Ripple cross-lamination.

Define flaser bedding.

Ripple-laminated sand or silt with thin mud streaks concentrated in ripple troughs.

In hummocky cross-stratification, what are "hummocks" and "swales"?

Convex-up laminae are hummocks and concave-up laminae are swales, formed by storm combined flows.

Name two load structures caused by soft-sediment deformation.

Flame structures and ball-and-pillow structures.

What is a flute cast used for in field studies?

Determining paleocurrent direction (the bulbous nose points upstream).

Define mudcracks and the environment that forms them.

Polygonal V-shaped fractures formed by desiccation in drying mud on floodplains, tidal flats, etc.

What are the three main textural components in Folk’s limestone classification?

Carbonate grains (allochems), micrite (carbonate mud), and sparry calcite cement.

Distinguish between micrite and sparite.

Micrite is microcrystalline carbonate mud; sparite is clear, coarse sparry calcite cement.

What carbonate rock name does Dunham give to a mud-supported limestone with less than 10 % grains?

Mudstone.

What is an "ooid"?

A coated carbonate grain with concentric layers formed in agitated, shallow marine water.

Which sedimentary rock is composed chiefly of quartz grains 1/16 mm to 2 mm in size?

Sandstone.

Define "arkose".

A feldspar-rich sandstone containing ≥25 % feldspar.

Name the three principal components used in QFL ternary plots.

Quartz (Q), feldspar (F), and lithic fragments (L).

According to Dott, when does a sandstone become a wacke?

When matrix exceeds 15 % of the rock.

What is a diamictite?

A matrix-supported conglomerate or breccia containing poorly sorted clasts.

Define "graywacke".

A dark, hard, matrix-rich sandstone of any composition, commonly chloritic.

What minerals dominate carbonate rocks?

Calcite (CaCO₃), aragonite (CaCO₃ polymorph), and dolomite (CaMg(CO₃)₂).

What is the typical mineralogy of modern reef-building corals?

High-magnesian calcite and aragonite in their skeletons.

Name the primary atmospheric gases erupted during volcanic outgassing on early Earth.

H₂O (steam), H₂, HCl, CO, CO₂, and N₂.

What is the Carbonate Compensation Depth (CCD)?

The depth in the ocean below which calcite dissolves faster than it accumulates, leaving little carbonate sediment.

Define pelagic sediment.

Deep-sea sediment that settles from suspension, often biogenic ooze or red clay.

What rock forms from siliceous ooze after diagenesis?

Chert.

What does an angular unconformity represent?

An erosional surface where tilted or folded older strata are overlain by younger, flat‐lying layers.

What is Walther’s Law of Facies?

Vertical successions of facies reflect lateral changes in depositional environments that existed side by side.

Which radiometric isotope pair has a half-life of 4.5 billion years?

Uranium-238 to Lead-206.

How old is a rock with a parent/daughter ratio of 1:3 and a half-life of 57 Ma?

Approximately 114 million years (two half-lives).

What dating method uses trapped light energy in quartz and feldspar?

Luminescence dating (optically stimulated luminescence).

What does a high gamma-ray log response typically indicate in well logging?

Shale or clay-rich strata with abundant radioactive potassium and thorium.

Which marine microfossils are most useful for biostratigraphic zonation in the Cenozoic?

Planktonic foraminifera and coccolithophores.

Define "biozone".

The fundamental biostratigraphic unit representing a body of rock characterized by a particular fossil taxon or assemblage.

What is magnetostratigraphy based on?

Reversals of Earth’s magnetic polarity recorded in rocks.

Name the three basic rock types.

Igneous, sedimentary, and metamorphic.

What two processes create sedimentary rocks?

Deposition of sediments and their lithification (compaction and cementation).

Which sedimentary rock forms by precipitation from evaporating seawater?

Rock salt (halite) or rock gypsum (gypsum).

Define "peat".

A compacted deposit of partially decayed plant material that can lithify into coal.

What climate indicator is provided by tillites and striated pavements?

Evidence of past glacial environments.

What depositional environment is characterized by well-sorted, rounded quartz sand and large-scale cross-beds?

Aeolian (desert dune) environment.

Which river system type has low sinuosity and multiple shallow channels?

Braided fluvial system.

What sedimentary sequence typifies a meandering river point-bar deposit?

Fining-upward from channel lag gravels through trough cross-bedded sands to ripple-laminated silts and muds.

Define "delta front".

The steeper, seaward-facing part of a delta where sediments prograde into the basin.

Which delta type is shaped mainly by wave reworking?

Wave-dominated delta (often arcuate).

What facies are typical of a carbonate peritidal cycle?

Subtidal lagoon muds grading upward to intertidal ripple-marked flats and capped by supratidal algal laminations or mudcracks.

What primary reef frame-builders dominate modern tropical reefs?

Scleractinian corals and calcareous algae.

Which geologic period experienced the "Cambrian Explosion" of animal life?

The Cambrian Period, ~541 Ma.

Name the principle that sedimentary facies are deposited beside each other in space and seen vertically in time.

Walther’s Law of Succession of Facies.

Who calculated Earth’s age at 4.55 billion years using lead isotopes?

Clair Patterson in 1956.

What forms the stable interior portion of a continent that has not deformed since the Precambrian?

A craton (where exposed, called a shield).

What are banded iron formations (BIFs) and why are they important?

Precambrian chemical sedimentary rocks of alternating iron oxides and chert, recording early oxygenation events.

Which fossil group is key for correlating Paleozoic marine rocks?

Trilobites.

What does the term "Proterozoic" mean?

"Earlier life"; the eon following the Archean, spanning 2.5 Ga to 541 Ma.

What is the primary difference between prokaryotic and eukaryotic cells?

Eukaryotic cells have a nucleus and organelles; prokaryotic cells do not.

Define "stromatolite".

Layered microbial (cyanobacterial) mats that trap and bind sediment, common in Precambrian carbonates.

What is the oldest widely accepted age of Earth from radiometric dating?

Approximately 4.56 billion years.

Explain the nebular hypothesis.

The solar system formed from a rotating cloud of gas and dust that collapsed under gravity, forming the Sun and planets.

What is an ooid’s typical environment of formation?

Warm, shallow, agitated marine waters such as shoals and tidal channels.

Describe a turbidite deposit using Bouma sequence terminology.

A graded bed with divisions A-E: massive sand (A), planar-laminated sand (B), ripple-laminated sand (C), laminated silt (D), and pelagic mud (E).

Name two major types of glacial depositional features found in continental environments.

Tillites (diamictons) and outwash plain sediments.

What key sedimentary structure indicates tidal influence in sandstones?

Herringbone cross-bedding.

Which isotope pair is most useful for dating Quaternary organic material?

Carbon-14 to Nitrogen-14.

What half-life does carbon-14 possess?

Approximately 5,730 years.

Define "fission-track dating".

A radiometric method that counts microscopic tracks left by the spontaneous fission of uranium-238 within minerals.

What is a chronostratigraphic unit?

A body of rock defined by its age and bounded by isochronous surfaces, e.g., a system or series.

How does an index fossil aid correlation?

It is geographically widespread, abundant, and limited to a short geologic time span, allowing precise age correlation of strata.

What is the main difference between a disconformity and a paraconformity?

Both have parallel strata, but a disconformity shows an erosional surface whereas a paraconformity shows no obvious erosion.

Which sedimentary rock forms from compacted plant debris?

Coal (bituminous or lignite depending on rank).