Geology tricks that make stratigraphy click

Superposition: lower layers are older than upper layers — unless overturned by tectonics. Steno, 1669.

Law of Superposition — Nicolas Steno (1669): 'At the time when any stratum was being formed, all the matter resting on top of it was fluid.' Therefore lower layers must be older. Exceptions: overturned folds (can flip sequence), thrust faults (older rock pushed over younger), impact ejecta. How to identify overturning: graded bedding (coarse at base), cross-bedding orientation, fossil orientation, geopetal structures (partial fill of void — shows which way was up). Combined with cross-cutting relationships and inclusions → full relative chronology of any outcrop. Foundation of all biostratigraphy, lithostratigraphy, and sequence stratigraphy.

Three unconformity types: angular (tilted below, horizontal above), disconformity (both horizontal, erosion gap), nonconformity (sedimentary on igneous/metamorphic).

AngularTilted below, horizontal above

DisconformityBoth horizontal, erosion surface

NonconformitySedimentary on igneous/metamorphic

Correlation: matching rock layers across locations using index fossils, key beds (tephras), and lithology.

Stratigraphic Correlation — Lithostratigraphic correlation: match by rock type — unreliable over large distances (same rock type can form at different times). Biostratigraphic correlation: index fossils — worldwide, precise. Key beds (marker horizons): distinctive ash layers (tephras), irridium-rich K-Pg boundary, iridium anomaly. Chemostratigraphy: stable isotope ratios (C-13, O-18) — global signals. Magnetostratigraphy: magnetic polarity reversals — global correlation. Sequence stratigraphy: correlate based on systems tracts (transgressive, highstand, lowstand) driven by sea level. Well-log correlation: gamma ray, resistivity logs from oil wells — subsurface stratigraphy. Type section: reference outcrop where a stratigraphic unit is defined.

Sequence stratigraphy: sea level cycles create predictable stacking of rock bodies. LST → TST → HST.

Sequence Stratigraphy — Sea level fall: erosion on shelf, sediment bypass to deep water → lowstand systems tract (LST) — submarine fans. Sea level rise (transgression): shoreline migrates landward → transgressive systems tract (TST) — backstepping. Sea level highstand: sediment progrades (builds seaward) → highstand systems tract (HST). Sequence boundary: unconformity at top of sequence (sea level fall). Maximum flooding surface: deepest water facies. Systems tracts linked by key surfaces. Oil exploration: reservoir rocks in LST (deep-water sands), source in TST (organic-rich shales), seal in HST (shales). Vail curves: global sea level change charts from seismic. Walther's Law underpins sequence stratigraphy.

Walther's Law: facies adjacent laterally are also found stacked vertically — as sea level changes, environments migrate.

Walther's Law — Johannes Walther (1894): 'Only those facies and facies-areas can be superimposed primarily which can be observed beside each other at the present time.' In other words: as sea level rises (transgression) → beach migrates inland. The sequence in any one location records environments that were once adjacent horizontally. Example: transgressive sequence from base up: beach sand → nearshore sand → offshore mud (as ocean deepened over the site). Conversely, regression → coarsening-upward sequence. Walther's Law allows reconstruction of ancient geographic environments from vertical sequences. Prerequisite: no unconformity between facies.

Radiometric dating: gives absolute ages for igneous beds and tuffs within sedimentary sequences. U-Pb most precise.

Radiometric Dating in Stratigraphy — Cannot directly date most sedimentary rocks (detrital grains have ages of their source, not deposition). Solutions: volcanic ash layers (tephras) within sedimentary sequence — date the ash (zircon U-Pb). Interbedded lava flows. Diagenetic minerals (glauconite — K-Ar, imprecise). Re-Os dating: organic-rich shales directly. Technique: find index fossil in rock layer → find nearest dateable volcanic layer → constrain fossil zone age. Example: Cambrian base ~541 Ma constrained by tuffs above and below first Cambrian fossils. Numerical time scale: biostratigraphy (relative) + radiometric (absolute) = Global Chronostratigraphic Chart.

Biostratigraphy: use fossils to date and correlate rocks. Biozones defined by first/last appearance of species.

Biostratigraphy — William Smith (1815, 'Strata' Smith): different rock layers contain different fossils in consistent order — 'principle of faunal succession.' Biozone: rock defined by fossil content. Types: range zone (total extent of species), concurrent range zone (overlap of two species), interval zone (between two events). First appearance datum (FAD) and last appearance datum (LAD): define zone boundaries globally. Biostratigraphic index fossils: planktonic foraminifera and nannofossils (Cenozoic marine), conodonts (Paleozoic-Triassic), ammonites (Mesozoic), graptolites (Ordovician-Silurian), trilobites (Cambrian). Combines with magnetostratigraphy and radiometric dates → integrated stratigraphy.

Grand Canyon: 1.8 Ga at bottom to 270 Ma at rim — 1.5 billion years of record (with Great Unconformity gap).

The Grand Canyon as Stratigraphic Record — Colorado River cut through ~1,900 m of rock. From bottom to top: Vishnu Schist (1.75 Ga) — metamorphic basement. Grand Canyon Supergroup (1.25–0.74 Ga) — tilted Proterozoic sediments. Great Unconformity: ~800 million years missing. Tapeats Sandstone (525 Ma) — transgressive Cambrian beach sand. Bright Angel Shale (515 Ma). Redwall Limestone (340 Ma, Mississippian). Supai Group, Hermit Shale. Coconino Sandstone (275 Ma) — desert dunes. Kaibab Limestone (270 Ma, Permian) — rim. Layers contain fossils recording Cambrian explosion, ancient oceans, reefs, deserts. Grand Canyon = 6 of the 7 geologic periods of Paleozoic visible in one cliff.

Lithostratigraphy: formal units — Formation (mappable), Member, Group. Type section defines each unit.

Lithostratigraphy — Formal units (International Stratigraphic Guide): Formation: basic mappable unit — distinctive rock type, named for geographic locality (Morrison Formation, Hell Creek Formation). Member: subdivision of formation. Group: two or more formations. Supergroup: two or more groups. Type section: reference outcrop where formation is defined — all other occurrences correlated back. Rules: oldest name takes priority. Lithostratigraphic units ≠ time units — same formation can be different ages in different places (diachronous). Time-stratigraphic units: System (= geologic period in rock). Chronostratigraphic vs geochronologic: Cretaceous System (rock) vs Cretaceous Period (time).

Petroleum system: source rock (organic-rich) + reservoir (porous) + trap (anticline/fault) + seal (impermeable) + timing.

Petroleum Systems Stratigraphy — Source rock: organic-rich shale (TOC >0.5%), buried to oil window (60–120°C, ~2–4 km) or gas window (120–220°C). Kerogen → oil → gas with increasing maturity. Reservoir rock: porous and permeable — sandstone (clastic), carbonate (limestone/dolostone), fractured. Porosity: space for fluids. Permeability: ability to flow. Trap: structural (anticline, fault) or stratigraphic (pinchout, reef, unconformity). Seal: impermeable cap rock (shale, evaporite). Timing: trap must exist before or when migration occurs. Migration: oil/gas expelled from source → moves through carrier beds → accumulates in trap. Sequence stratigraphy guides exploration: LST sands, organic-rich TST shales.

Chemostratigraphy: stable isotope ratios (δ¹³C, δ¹⁸O, Sr) vary globally with ocean chemistry — global correlation.

Chemostratigraphy — Carbon isotopes (δ¹³C): negative excursions = carbon input (volcanism, methane release); positive = carbon burial. Carbon isotope excursion (CIE) at PETM (56 Ma) correlates globally. Oxygen isotopes (δ¹⁸O): temperature signal — heavier O-18 in cold periods (ice locks light O-16). Benthic foram δ¹⁸O = Cenozoic temperature record. Strontium isotopes (⁸⁷Sr/⁸⁶Sr): changes with continental weathering and hydrothermal activity — globally uniform in ocean at any time → precise correlation. Iridium anomaly: K-Pg boundary worldwide → Chicxulub impact. Platinum group elements (PGEs): extraterrestrial signal. Carbon isotope stratigraphy: useful in Precambrian where fossils rare.

William Smith (1815): 'the map that changed the world' — first geological map of England. 'Strata' Smith proved faunal succession.

William Smith and Faunal Succession — William Smith (1769–1839): self-taught surveyor, dug canals across England. Observed: each rock layer contains distinctive fossils in the same order everywhere — 'principle of faunal succession.' Produced first geological map of England, Wales, and part of Scotland (1815) — 'the map that changed the world' (Simon Winchester). Colored by hand, 2.5 × 1.8 m. Used immediately by mining, engineering industries. Bankrupted himself publishing it; plagiarized by Geological Society (wealthy amateurs). Finally honored: Wollaston Medal (1831) — first recipient. His map is the foundation of all biostratigraphy and demonstrated the utility of geology for industry — birth of applied geology.