Introduction to Stratigraphy

• Stratigraphy studies rock layers (strata) formed from sediments deposited in basins.
• Sedimentary rocks occur in layers called strata and these rocks are formed from sediments originally deposited in a basin.
• A basin is a depressed area surrounded by higher elevation land, which can be dry or filled with water.

Sedimentation

• Sediments record the memory of their origin as rocks, with the transport journey, and basin processes.
• Sediments include solid particles, salts in aqueous solution, and the remains of organisms.
• Particle size varies from boulders to clay flakes, depending on distance traveled, weathering, and energy changes.
• Sorting describes the range of grain sizes due to weathering and energy levels during transport.
• Sediment composition varies, with some being mostly quartz and others containing unweathered minerals.
• After deposition, sediments undergo physical and chemical processes, including soil formation on land and biogenic processes in marine environments.

Sedimentary Facies and Beds

• Sediments temporarily reside in locations along pathways before deposition.
• Varying depositional environments arise from collective sedimentation experiences.
• Facies are sedimentary characteristics indicative of depositional environment.
• Sedimentary facies represents sediment or rock characteristics, indicating the deposition environment.
• Facies change as real-time conditions evolve.
• Uniformitarianism suggests similar past environments produced rocks with familiar characteristics.
• Strata, known as beds, are generated with sediment deposition and facies migration.
• Geologists can map units called formations once beds from similar environments accumulate and become stone.

Basins as Historical Records

• Basins act as libraries, and layers of rock are the books that record global and regional geologic changes, plus changes within the basin.
• Sediments within the layers vary, providing the equivalent of text on pages.
• Strata or beds are individual pages, and their stories are facies with the pages laid down horizontally, building up vertically, reflecting the order in which they occurred which is superposition.
• Accumulation of pages/beds creates books/formations.
• Subsequent processes alter the record.
• Modifications like faulting, folding, and intrusions can potentially add confusion or clarification to the overall story, depending on the context.
• Erosion, metamorphism, and chemical alteration can cause strata material to be lost or altered.
• Unconformities occur when entire beds or formations are truncated, causing gaps in the storyline.
• Stratigraphers study the incomplete geological rock record.

Stratigraphic Metaphor

• Facies = words or notations on a book page
• Beds = pages within periodicals or books
• Formations = books with pages, complete or incomplete
• Groups of Formations = Shelves of books with similar topics
• Bookcase = stratigraphic record of the entire basin

Layers as Participants

• Layers of sediment record history and are influenced by tectonics, sea level, and lithification.
• Layers of sediment have impact on tectonics and sea level.
• Stratigraphy studies the history recorded in basin layers.

Basins and Sea Level

• Sedimentary basins are depressions where sediment accumulates; these can be epicontinental (on continents) or tectonic (at continental margins).
• Stratigraphy helps understand basin histories.
• Basins may or may not contain water.
• Stratigraphic sequences in dry basins are controlled by tectonics.
• Water-filled basins record water level changes due to climatic and astronomical variables.
• The preserved pattern of sedimentary deposits reveals history.
• Cratons, which are stable cores of continents made of igneous and metamorphic rocks, have exposed (shields) and buried portions (platforms).
• Basins exist and sedimentary strata accumulate on platforms

Accommodation Space

• Limited space in basins is affected by tectonics, global sea level, local water level, and sediment accumulation.
• Accommodation space is the available volume within a basin to accommodate sediment.
• Accommodation space changes with tectonics, sedimentation, water level, and eustatic sea level.
• Basin sediment accumulation fluctuates with Earth's system interactions.

Types of Basins: Tectonic Settings

• Basins are categorized according to their formation in various tectonic settings, each leaving unique geological signatures.

Rift Basins

• Rift basins form at divergent plate boundaries.
• Rifting in backarc basins at convergent subduction zones.
• Basin character evolves as rift widens.
• Intracratonic basins are landlocked while epicontinental basins develop bodies of water.
• Subsidence occurs due to lithosphere thinning.
• Graben subsides during crustal separation.
• Basins can stay dry unless intersecting body of water or in a wet climate.
• East African Rift includes terrestrial rift basin, Red Sea, and Gulf of Aden, which contains lakes fed by rivers.
• Lithosphere thinning can lead to volcanism, examples include Mt. Kilimanjaro, Mt. Kenya, Oldoinyo Lengai.

Passive Margins

• Passive margins are created from ancient rifting which are ocean basins that result from ancient rifting with absent tectonic activity.
• Myrtle Beach, SC, sandy beach versus abyssal plain with siliceous or calcareous ooze facies.
• Epicontinental marine basins are located around the globe today.
• Thickly sedimented basins are where major rivers flow into the ocean.

Sedimentation in Rift Basins

• Sediments come from weathering of surrounding horsts, volcanic eruptions, or marine/freshwater sources.
• Rift valley basin sediments consist of a mix of sources, with domination of quartz and feldspar from surrounding mountains.
• Arkosic sandstones are often the sediment is dominated by quartz and feldspar, often producing arkosic sandstones.
• The Gulf of Aden and the Red Sea are flooded rift valleys, sediments record weathering and substantial accumulations of materials deposited in these seas.
• Passive-margin deposition occurs along shorelines of rift zones.
• East coast of the United States is an ancient passive margin from Pangaea rifting.
• Key evidence is the Mid-Atlantic Ridge and Triassic/Jurassic rift basins running parallel along the east coast.
• Coastal wave action winnows out nearly everything but quartz sand leading to quartz arenites.
• Not all rift zone basins flood with water, and none rift forever.
• Failed rift basins can stop rifting or can continue for millions/hundreds of years forming ocean basins.
• Reduced space within basin ceases stretching, and eventually the eroding sediments continue to fill the basin.

Subduction-Related Basins

• Trench basins, forearc basins, backarc basins are locations where sediments can accumulate at subduction zones.
• Deepwater trench basins form where subducting and overriding plates meet.
• Forearc basins form between volcanic arc and trench.
• Backarc basins form opposite the subduction zone, with extensional nature.

Sediments in Subduction Zone Basins

• Sediments are produced in the water column, weathered from the overriding plate, and scraped off the subducting plate.
• Organisms generate sediments from the water column, generally siliceous or carbonate.
• Land-sourced sediments arrive via submarine mass movements like turbidity currents.
• Sediment is conveyed atop subducting plate.

Forearc Basins

• Form between accretionary wedge and volcanic arc areas and on land and underwater.
• Location is dependent upon oceanic and continental lithosphere, or oceanic and oceanic lithosphere subduction.
• Result from downwarping, or flexure, of lithosphere created by the subducting plate and consists of siliciclastic matter.
• Sediment originates from volcanic mountains and deposits formed during eruptions, along with biochemical life forms.
• Lithic sandstones are common because sediments do not travel long distances before deposition.
• Central Andean Forearc contains two forearc basins, the Atacama Bench and the Iquique Terrace.
• Sediments undergo intense metamorphism.
• Metamorphic signatures, such as those in Franciscan Blueschist, have historical context.
• Ocean floor sediments alter to present form during subduction and metamorphism.

Foreland Basins

• The inland portion of the backarc region at subduction zones are called foreland basins.
• Foreland basins form parallel to mountain belts, resulting from crustal thickening and downwarping.
• Lithospheric flexure is the process of the crust thickening and the downwarping.
• Sediment source is material eroded from nearby mountain belt.
• Proximal sedimentary strata are much thicker than distal strata.
• Characteristics can provide information about depth/lateral extent of ancient basin.
• Persian Gulf serves as foreland basin for the Zagros Mountains of Iran.
• Appalachian formation left behind a record of successive foreland basins during the Taconian, Acadian, and Alleghenian Orogenies.
• Valley & Ridge province contain foreland basin deposits of Appalachian mountain belt.
• Appalachian foreland basin strata contain deposits of coal and natural gas.
• Western Interior Seaway developed during the Cretaceous North American west, which also preserves marine invertebrate and reptilian fossils.

Strike-Slip Basins

• Strike-slip or pull-apart basins form due to land subsidence along strike-slip faults of shearing forces rip the lithosphere.
• San Andreas Fault Zone in California and North Anatolian Fault Zone in northern Turkey are modern examples.
• The Gulf of California is the result of numerous pull-apart strike-slip basins.
• Smaller examples on Hayward Fault in Fremont, California.

Sediments in Strike-Slip Basins

• Typically consist of materials weathered from the uplifted sides of the basin.
• Sediments include marine biogenically-derived sediments and sediments derived from shoreline processes in the Gulf of California.

Basic Principles of Stratigraphy

• Interactions between Earth's spheres have evolved throughout history.
• Biosphere introduction in Archean Eon significantly affected system interactions.
• The interplay between the spheres has created a long rock record, which is contained in basins as sedimentary rocks.
• Sediments accumulate in layers, forming the strata of the stratigraphic record.

Strata and Stratigraphy

• Sedimentary layers are called strata.
• Strata form the basin stratigraphic record.
• Stratigraphy is layer writing in Latin.
• Weathered sediments erode into basins and deposit in laterally extensive, horizontal layers.
• Strata forms bedding and beds.
• Beds of strata accumulate into formations, the functional unit of stratigraphy.
• Formations are collections of strata deposited in related environments; represent a collective facies.
• Facies show rock with specific characteristics that make up a sandstone that was once a beach.
• Principle of original horizontality is where strata form in a flat orientation.
• Sediments lithify into rock formations originally laid down horizontally.
• Younger strata lie on top of older strata which is the principle of superposition.

Formations and Walther's Principle

• Sediments are affected and also affect the basin after being deposited into a basin.
• Sediment grain size is dependent upon the energy level of the system.
• Beaches, locations that are full of sand have energy levels that are well-suited to grain size.
• Flora and fauna adapted to living in them and the unique structures are how we determine local facie
• Facies are the characteristics of a rock, formed under its unique depositional environment.
• Sedimentary facies display a gradual decrease in grain size from areas well above the tidal zone (supratidal) to areas well below wave base (deep marine).
• Changes in sea level is one of the key variables that affects how strata develop through facies in seashore locations.
• Transgression is rising sea level while regression is dropping.
• Walther's Principle is named after German geologist Johannes Walther and describes changes in sea level lead to stratigraphic stacking, or sequences.
• Siliciclastic basins and carbonate basins are environments where can apply Walther's Principle.
• Progression from conglomerate to sandstone to siltstone to mudstone going shoreward to deep marine reflects a fining up sequence.
• Fining-upward is transgressive and Coarsening upward is regressive
• Changes in sea level determine the sequence of deposition.
• Boundary between a transgressive and regressive sequence is a Maximum Flooding Surface.
• A Sequence Boundary is at the top of regressive sequence and it is erosional.

Pattern Matching

• Pattern matching and stratigraphic correlation connects layers across distances.
• Patterns in measured stratigraphic sections don't always perfectly line up.
• Bedding thickness varies over distances, with beds pinching out between outcrops.
• New beds are sometimes introduced.
• Reasons include changes in shoreline shape, bays, water chemistry, barrier islands, climate, inflowing river channels, continental shelf slope, and variety of environmental factors.
• Post-lithification processes can erase physical and chemical characteristics of the rocks through compaction and diagenesis.
• Stratigraphers use complementary methods to aid in correlation between the sites.
• Patterns emerging from measured stratigraphic sections represent environmental changes of all scales.
• Patterns also record interactions.
• Stratigraphers developed various subfields to provide focused study areas.
• Fields include Chronostratigraphy, Lithostratigraphy, Biostratigraphy, and Magnetostratigraphy.

Lithostratigraphy

• Lithostratigraphy is the study of rock layers with interest in the sediment grains that make up the layers, composed of greek words lithos meaning rock, and strata meanings layers.
• Mineral composition, shape, and sorting are evidence describing "facies" of rock.
• Stacked strata in section vary from bottom to top, providing a history of sediment deposition, changes in sea level, climate fluctuations, and the time scales.
• A layer of rock must be mappable across an area with similar characteristics and distinct boundaries (contacts) in order to be designated as a formation.
• Basic lithostratigraphic unit is the formation, also used by field geologists when creating geologic maps.
• Type section represents the outcrop for which the standard description of the unit was created.
• Contacts, mappable formations lying above and below.
• Members are lithologically distinct beds within the larger formation.
• Type Cincinnatian is the ideal section for the study of beds, because they are often laterally extensive and traceable. Roadcuts, cliff faces, and other locations are ideal study areas where they will vary in thickness and content to help reconstruct the history, but helps not just at one location, but overall.

Lithostratigraphic Hierarchy

• Flow (if volcanic materials are included) → Bed → Member → Formation (Basic Lithostratigraphic Unit) → Group → Supergroup
• Packages known as groups can put formations together.
• Packages of groups are supergroups are assembled to use good sense in situations where rock formations are closely related.

Biostratigraphy

• Biostratigraphy is used to correlate patterns and beds using fossils.
• Fossil include body fossils or trace fossils, evidence of movement or activity
• Based upon the existence of a particular diagnostic feature within strata specific to fossil material.
• Fossil material can be traced over great distances, much like lithostratigraphic layers.
• Biostratigraphic units vary and not a basic goal of biostratigraphy.
• Communities within an ecosystem vary across an environment, depending upon physical and biological conditions.
• Can be correlated beds which assist when understanding the paleoecological history of a location.

Index Fossils

• Fossil assemblages vary a great deal over basins, and it is important to consider shorelines.
• Index fossil mark short lived fossil that can serve well as markers for important intervals.

Biozones

• Biozone is the basic element of of biostratigraphy with its own unique set of terminology.
• The types of biozones are range zones, interval zones, assemblage zones, abundance zones, and lineage zones.
• No hierarchical system of designations with overlapping biozones within a single rock unit.
• Consist of two varieties of range bones, taxon range zones which is defined as a body of strata, and concurrent range zones which is are only different from the taxon range zone
• Interval zones are also known as fossiliferous strata that exist between two defined stratigraphic horizons.
• Lineage zones are defined by their evolutionary importance
• Assemblage zones are defined by the lower and upper boundaries of strata
• Abundance zones represent a body of strata where the abundance of a particular taxa is significantly greater than usual.

Fossil Lagerstätten, Epiboles, and Outages

• Lagerstätten (storage place) is an exceptional fossils with rich deposits.
• Epibole beds are suddenly very abundant and represent a sudden flourishing.
• Outage represent the disappearance of a fossil or community.
• Lagerstatten, Epiboles, and Outages are used to be important biostratigraphic marker beds.

Faunal Succession

• Way evolution has influenced life on Earth presents a distinct sequence of appearances of organisms through time. Example, humans and dinosaurs never lived together.
• Important patterns develop across the fossil record for paleontologists to identify rock units.

Chronostratigraphy

• Chronostratigraphy based upon time correlates with aspects of biostratigraphy and numerical dating to constrain the time of deposition in basins.
• Global chronostratigraphic names can be different than regional names for these periods of time, such as Cambrian and Devonian.
• Locality name.
• International Commission on Stratigraphy (ICS) has created a time chart
• Each boundary on this is marked by a GSSP (Global Stratotype Section and Point). A chronostratigraphic "type locality"

Comparison of Chronostratigraphic and Lithostratigraphic Designations

• "Pennsylvanian" is a regional example, and in Chronostratigraphy has similar names
• Globally agreed upon
• Marked with radiometric dates and fossils. Not all boundaries have agreed upon GSSPs.

Golden Spikes

• Metaphor used to mark moments of geological importance where significant changes, or shifts, occurred and also used to mark a boundary on the geologic time scale (note the “golden spikes” on the ICS timetable linked above).
• Common name for the more technical Global Boundary Stratotype Section and Point, or GSSPs, that are used to define most boundaries on the geologic time scale.
• Rules for marking these follow:*
• A GSSP has to define the lower boundary of a geologic stage (body of rock deposited during an Age).
  The lower boundary has to be defined using a primary marker (usually first appearance datum of a fossil species).
• There should also be secondary markers (other fossils, chemical isotope signatures, geomagnetic reversal).
• The horizon in which the marker appears should have minerals that can be radiometrically dated.
• The marker has to have regional and global correlation in outcrops of the same age
• The marker should be independent of facies.
• The outcrop has to have an adequate thickness
• Sedimentation has to be continuous without any changes in facies
• The outcrop should be unaffected by tectonic and sedimentary movements, and metamorphism
• The outcrop has to be accessible to research and free to access. This includes that the outcrop has to be located where it can be visited quickly (International airport and good roads), has to be kept in good condition (Ideally a national reserve), in accessible terrain, extensive enough to allow repeated sampling and open to researchers of all nationalities.

Magnetostratigraphy

• Correlates using Earth’s magnetic field inclination and declination, also known as normal and reverse polarity.
• Correlating times of magnetic reversal, or times when the Earth’s magnetic poles have reversed polarity, with “Geographic South” becoming “Magnetic North”, and vice versa.
• Relies on minerals’ magnetic properties at deposition time.
• Magnetozones are portions of the stratigraphic section that share polarity from the same time period while magnetic chrons within these zones allow numerical dating.
• Zones/chrons provide info about a time period and complement tools from other stratigraphy types.
• Many GSSPs are marked by magnetozone boundaries, which includes changes in the Earth’s magnetic field polarity
• Magnetic polarity also marks where boundaries of the Gelasian Stage of the Pleistocene lies.

Measuring Stratigraphic Sections

• Critical work is done primarily in the field. and measures sections on multiple scales.
• It allows direct exploration of brand new stratigraphic sections created by human or natural processes
• Planning field outing begins with background research collected from geologic maps of an area academic and state geologic survey publications.
• Detailed fieldwork consist of measuring sections, photography, sample collection, and sketching.
• Scales are meter-scale of centimeter-scale.
• Data recorded at intervals relative to known datum.
• Detailed stratigraphy with drawing of stratigraphic columns.
• Compaction leads to burial, and with burial, heating which creates secondary processes and give info from fossils and other sedimentological evidence.
• Microstratigraphy is environmental events and processes at sub-millimeter to a meter of scale.
• Fine layers of alternating gypsum crusts resulting from periodic evaporation represents microstratigraphic evidence.
• Larger scale descriptions in decimeters to thousands to study broad basin events.

Stratigraphy Not Just Earth Thing

• Stratigraphic principles apply outside of Earth such as on Mars.
• Shaler outcrop shows what appears to be layered strata and possibly an ancient lake bed.