Concepts Related to Subdivision of the Rock Record Stratigraphy Concepts Related to Subdivision of the Rock Record

Inextricably Related Issues in Geology (that must be distinguished!) Material Units: The Rock Record Have physical extent and consist of tangible material The material record of events that have occurred during the passage of geological time Geological Time: The Abstraction of Time Since the Advent of the Earth Measured in years before present Divided into discrete intervals Ma; millions of years before present Ga; billions of years before present

Subdivision of the Rock Record Lithostratigraphy – study of the physical relationship among rock units; no time connotation other than superposition Otbo physical properties and stratigraphic position relative to other lithostrat units Chronostratigraphy – integrated approach to establishing the time relationships among geologic units

Chronostratigraphy: Integrated Approach to Establishing the Time Relationships Among Geologic Units Biostratigraphy Study of the fossil record with emphasis on faunal succession to establish relative time relationships The correlation web Magnetostratigraphy Study of the magnetic properties of rock units for the purpose of correlation otbo magnetic polarity reversals Allostratigraphy Study of rock units defined by unconformities and other features generated by base level change Geochronology various techniques, especially isotope geochemistry, to establish the absolute age of rock units

M o r e L a t e r

Stratigraphic Correlation

Lithostratigraphy Formations, Members, Groups, etc. Material Units and "Classical Layer Cake Stratigraphy“ catastrophism: continuous layering = time equivalence NO! Further organization and subdivision of the rock record otbo Relative age Superposition, unconformities, cross cutting relationships, included fragments Original Horizontality Lateral Continuity

Objective Subdivision of the Stratigraphic Record Into Distinct Lithostratigraphic Units Formations with a type section, geographic or lithologic name, and definition based on limited and distinctive lithologic variability consistent stratigraphic context “extensive” map distribution in the surface or subsurface Groups and Supergroups Are formations lumped otbo stratigraphic association Members and beds Subdivisions of Formations lithostrat units with less areal extent defined as it is useful

Actualism and "Genetic Stratigraphy" Recognition of Uniformitarianism the relationship between modern processes of sedimentation and the rock record

Actualism and "Genetic Stratigraphy" Sediments with distinct lithologic aspect are deposited in only limited areas in a given time period. This results from the limited lateral extent of contiguous depositional environments. Distinct depositional environments migrate in space, through time so that lithofacies units are inherently diachronous (variable in age throughout)

Diachronous Stratigraphic Units

Diachronous Stratigraphic Units Basement

Actualism and "Genetic Stratigraphy" Sedimentary (litho)Facies: lithologic aspect; a result of depositional process not age. Lithologically distinct sediment bodies deposited synchronously in adjacent sites (distinct lithofacies) must be separated across some kind of boundary. Such contemporaneous and physically associated strata are said to be in Facies Relationship. Lithologic aspect in space is not directly equitable to time but rather depositional process at that place at some time.

Geometry of Facies/Lithostratigraphic Relationships

Actualism and "Genetic Stratigraphy" Depositional Processes -----> Geological Product Sedimentary Depositional Environments Sedimentary Lithofacies Defined by Lithology Sedimentary structures (physical, chemical, deformational, etc) Geometry: bedding scale and outcrop scale, vertical trends Paleocurrent indicators Fossils

Walther's Law of Correlation of Facies “Only those lithofacies which are a product of sedimentary environments found adjacent to one another in the modern can be occur superimposed in continuous, uninterrupted stratigraphic succession.” Also, lith units in conformable vertical relationship were in lateral relationship during deposition

Autostratigraphic Variation Autocyclicity Lithologic variability, within distinct geomorphic settings, in space and time may be the result of variations in depositional conditions inherent to that depositional environment----->

External Controls on Stratigraphic Architecture: Allocyclicity Variation in lithologic character due to regional scale variations in Base Level level at the earth's surface above which permanent sediment accumulation does not occur Closely related to local sea level Basin Subsidence (tectonic) Eustacy (global sea level) Sediment input (climate and topography/tectonics)

Unconformity

Transgression and Regression Geometric relationship of idealized "graded, shore parallel facies belts“ deepening upwards, shoreline moves landward through time: Transgression Basin Subsidence, +/- Sea Level Rise, +/- Sediment Starvation (With Subsidence) May (rarely) leave a distinct material record---> Marine flooding surfaces Ravinement surfaces Landward shift in sedimentary facies up section

Transgression Geometric relationship of idealized "graded, shore parallel facies belts“ Fining Upwards Sequence: FUS More basin-ward facies overlie more landward facies Compared to depositional systems models

Transgression and Regression Shallowing upwards, shoreline moves basinward through time Regression sea level drop, +/- uplift, +/- sediment supply Progradation; Normal Regression excess sediment supply relative to accommodation space Forced Regression: Relative sea level drop and formation of erosion surfaces: Unconformity (surface of subaerial exposure) Soils; kaolinitized, clay-rich layers Angular discordance with underlying units (disconformity) Plant remains, rooted zones Non-genetic stratal relationships: Abrupt Basinward Shift In Sedimentary Facies Upsection Strata across lithologic boundaries NOT in accordance with Walther’s law

Regression Geometric relationship of "graded, shore parallel facies belts“ Coarsening Upwards Sequence: CUS More landward facies overlie more basin-ward facies Compared to depositional systems models

Causes of Base Level Change Relative Change Eustatic Change

Sea Level Cycles 1st Order Cycles 2nd Order Cycles 3rd Order Cycles 100’s my 100’s of meters 2nd Order Cycles 10’s my 3rd Order Cycles 1-10 my 10’s of meters Falling Rising

Transgression/Regression vs. Relative Sea Level Relative Sea Level Curves

Transgression/Regression and Relative Sea Level