principles of stratigraphy: deposition, succession, continuity and correlation

Transcription

1 Relative Age Dating Comparative Records of Time Nature of the rock record principles of stratigraphy: deposition, succession, continuity and correlation Stratigraphic tools biological succession of life: biostratigraphy magnetic reversals: magnetostratigraphy progression of sedimentary rocks from changes in sea level: sequence stratigraphy temporal excursions and sequential changes in chemical characteristics: chemostratigraphy Principles of Stratigraphy Depositional Succession Sedimentary rocks deposited as beds or horizons in rock units record and preserve depositional events beds often discontinuous beds can be eroded or lost result: a gap in the temporal record known as an unconformity or hiatus continuous deposition 100 years of sediment erosion for 50 years unconformity 150 year gap 150 years = 100 years lost + 50 years of erosion renewed deposition resultant rock record

2 Unconformities: Grand Canyon Characteristic Features Distinct change in rock type, age, orientation great unconformity or structure Principles of Stratigraphy Laws Governing Stratigraphic Relationships Superposition relative ages from sequence of rock deposition Initial Horizonality orientation of beds when deposited Lateral Continuity spatial correlation of individual horizons and rock units Cross-Cutting Relationships sequence of events record in rock relationships Nicolaus Steno

3 Law of Superposition Order of Layered Units Definition: A rock unit is younger than the one below and older than the one above Stratigraphic Column temporal succession of rock units deposition not necessarily continuous, but sequential youngest layer oldest layer Law of Initial Horizontality Contraints on Original Orientation Sediments are deposited as horizontal beds Principle applies to sedimentary rocks formed in an aqueous environment Grand Canyon: horizontal strata

4 Law of Initial Horizontality Evidence of Deformation Non-horizontal sedimentary rocks Modified by post-depositional events, e.g. folding folded rock units Law of Lateral Continuity Spatial Relationships Sediments form as continuous layers Individual horizons or layers thin or end only when the environment of deposition changes Enables correlation of beds with specific characteristics Correlation of rock units

5 Cross-Cutting Relationships Event Sequence Younger units cross-cut older units erosion surfaces intrusions unconformities Recorded Sequence of Events: fossiliferous sedimentary rocks (D; horizontal) 1. Deposition of fossiliferous sediments (A) 2. Folding (deformation) of A 3. Intrusion of igneous rock (B) 4. Erosion to create surface (C) 5. Deposition of fossiliferous sediments (D) igneous intrusion (B) younger intrusive rocks eroded surface (C; unconformity) deformed fossiliferous sedimentary rocks (A) Stratigraphic Correlation Columns Sequential order of deposition determined by correlation of separate, related stratigraphic records Unconformities may be recognized, uncertainties may persist H? G E C hiatus F D C G /H? E C B A F

6 Stratigraphic Methods Biostratigraphy Relative ages determined from fossil assemblages Biotic changes are a function of extinctions and evolutionary processes Datums record timing of biotic changes first appearances and last appearances Site A Site B age-dependent characteristics boundaries calibrated by absolute ages Biostratigraphy Temporal Records of Life Recognition of species unique to particular time intervals Index fossils youngest layer oldest layer

7 Stratigraphic Methods Magnetostratigraphy Approach based on intermittent, irregular reversal of the polarity of Earth s magnetic field Rocks record field at time of formation (cooling) magnetic axis N magnetic equator N S S axis of rotation Normal Polarity Reversed Polarity Stratigraphic Methods Magnetostratigraphy Magnetic signals preserved in stratigraphic sections show alternating sequence of polarity Series of polarity shifts: normal normal (modern) reversed (opposite) Polarity intervals are: reversed independent of lithology of varying duration

8 Stratigraphic Methods Magnetostratigraphy Sequence of polarity reversals recognized major intervals minor intervals require excellent stratigraphic resolution Globally uniform series of time-dependent reversals Ages determined by absolute dating normal reversed minor Stratigraphic Methods Magnetostratigraphy Record compiled from multiple, overlapping sequences Correlations to stages often based on biostratigraphy palynology - plant remains Cretaceous Magnetic Records

9 Sequence Stratigraphy: Principles Controls on the Sedimentation Process Sediment production and accumulation is controlled by: sea level water depth, accommodation space tectonic subsidence - accommodation space climate - weathering rates, grain production Cycles in these variables operate over different time scales Result from multiple studies of these cycles: a temporal record of changes in sea level Sequence Stratigraphy: Principles Parasequences Shallowing upward sequences produce defined patterns of sediments Rock record indicates shallowing

10 Sequence Stratigraphy: Principles Spatial Arrangement of Stratigraphic Units Lateral and vertical relationships in parasequences Predicable, recognizable sequences develop landward oceanward Sequence Stratigraphy: Principles Stacking of Parasequences Vertical sequences record sea level change: Progradational: shallowing-upward Aggradational: constant, static Retrogradational: deepening

11 Sequence Stratigraphy: Principles Depositional Sequence Sequential order of characteristic elements: sequence boundary, lowstand systems tract, transgressive surface, transgressive systems tract, maximum flooding surface, highstand systems tract, sequence boundary Relevance: defines sea level changes Stratigraphic Comparisons Correlation of Stratigraphic Records Magnetostratigraphy Biostratigraphy Sequence stratigraphy Cenozoic 2-65Ma

12 Stratigraphic Methods Chemostratigraphy or Isotope Stratigraphy Stratigraphic variations in specific chemical or isotopic characteristics stable isotopes: C (! 13 C), O (! 18 O), S (! 34 S) isotopes: ( 87 Sr/ 86 Sr) molecules organic matter Excursion in! 13 C in black shales across Cenomanian/Turonian boundary Eastbourne, Sussex Pueblo, CO Tarfaya, Morocco Chemostratigraphy: Examples Strontium Seawater Evolution Changes in 87 Sr/ 86 Sr related to sources of Sr increasing trend for past 150Ma 87 Sr/ 86 Sr ratios of ocean change with Sr sources: new crust (low) vs. weathering (high). Ratios can be age diagnostic for much of Cretaceous (K) and Tertiary (T)