Geology tricks that make stratigraphy click

Q: What is the Law of Superposition and what are the exceptions?

A: Steno's Law of Superposition (1669): in an undisturbed sequence of sedimentary rocks, lower layers are older than upper layers. Fundamental to all relative dating. Exceptions: (1) Overturned folds — tectonic forces can flip entire sequences upside-down. Identify using way-up indicators: graded bedding (coarse at base), cross-bedding orientation, geopetal structures. (2) Thrust faults — older rocks can be pushed over younger rocks. (3) Intrusions — younger igneous rock cutting through older sedimentary rock (cross-cutting relationships override superposition). (4) Impact ejecta — can land far from source, deposited out of local order. Confirmation: use index fossils to verify relative age when structural complications exist.

Q: Describe the three types of unconformities and what each records.

A: Unconformity: a surface representing missing time — either erosion occurred or deposition stopped. (1) Angular unconformity: tilted or folded rocks below, horizontal strata above. Records: deposition, deformation (folding/tilting), uplift and erosion, then subsidence and renewed deposition. Classic example: Hutton's Unconformity at Siccar Point, Scotland (1788) — convinced Hutton of deep time. (2) Disconformity: both rock sequences are horizontal but an erosional surface separates them. Harder to identify — use fossils to detect missing time. (3) Nonconformity: sedimentary rocks deposited directly on igneous or metamorphic basement. Records an entire mountain-building cycle compressed to one surface. Grand Canyon example: the Great Unconformity at the base of the Cambrian Tapeats Sandstone represents ~800 million years of missing time.

Q: What is sequence stratigraphy and what are the three systems tracts?

A: Sequence stratigraphy: the analysis of sedimentary rock packages bounded by unconformities (sequence boundaries) driven by sea level changes. Developed by Exxon geologists (Vail, Mitchum, 1977) for oil exploration. Three systems tracts: (1) Lowstand Systems Tract (LST): forms during sea level fall and early rise. Shoreline moves seaward (regression). Deep-water submarine fans deposited at base of slope — important oil reservoirs. (2) Transgressive Systems Tract (TST): sea level rising. Shoreline migrates landward (retrogradation). Organic-rich shales deposited — source rocks for oil. Maximum flooding surface at top. (3) Highstand Systems Tract (HST): sea level high and stable, then falling. Shoreline progrades seaward. Deltaic and shallow marine sands deposited. Bounded above by sequence boundary (unconformity). Petroleum significance: LST sands = reservoirs, TST shales = source rocks, HST shales = seals.

Q: What is biostratigraphy and what makes a good index fossil?

A: Biostratigraphy: the use of fossils to date and correlate rock layers. Based on William Smith's principle of faunal succession (1815): different rock layers contain different fossils in consistent order everywhere. A biozone is a rock unit defined by its fossil content. Types of biozones: range zone (total stratigraphic range of a species), concurrent range zone (overlap of two species), interval zone (between two fossil events). Good index fossil characteristics: (1) Geographically widespread — found globally for correlation. (2) Short stratigraphic range — existed for a brief time, so precisely constrains age. (3) Abundant and easily preserved. (4) Easily identified. Best index fossils by period: Planktonic foraminifera and calcareous nannofossils (Cenozoic marine), conodonts (Paleozoic-Triassic), ammonites (Mesozoic), graptolites (Ordovician-Silurian), trilobites (Cambrian). Biostratigraphy + radiometric dating = integrated time scale.

Q: What is Walther's Law and how does it apply to interpreting vertical rock sequences?

A: Walther's Law (Johannes Walther, 1894): facies (rock types representing specific environments) that are found adjacent to each other laterally will also be found stacked vertically in a conformable sequence. In other words: as environments migrate laterally due to sea level change, they leave a vertical record at any one location. Application: during transgression (sea level rise), coastline migrates inland. A location that was once beach becomes offshore. Vertical sequence at that location from base up: beach sand (coarse) then nearshore sand then offshore mud (fining upward = transgressive). During regression: coarsening-upward sequence (offshore mud then nearshore sand then beach). Prerequisite: no unconformity between beds — the sequence must be conformable. Walther's Law allows ancient paleogeography to be reconstructed from vertical sections, and underpins all sequence stratigraphic interpretation.