lecture 8a: stratigraphy, paleomagnetism

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Lecture 8a: Stratigraphy, Paleomagnetism. Questions How is stratigraphy related to analysis of sedimentary environments? What happens when sea-level varies? How do variations in the terrestrial magnetic field get recorded in rocks and used by geologists to reconstruct history? Reading - PowerPoint PPT Presentation

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  • *Lecture 8a: Stratigraphy, PaleomagnetismQuestionsHow is stratigraphy related to analysis of sedimentary environments?What happens when sea-level varies?How do variations in the terrestrial magnetic field get recorded in rocks and used by geologists to reconstruct history?ReadingGrotzinger & Jordan, chapters 8 (again) and 14

  • *Principles of Stratigraphy (revisited)Recall the fundamental principles of stratigraphy: original horizontality, superposition, cross-cutttingA more detailed study brings up three major themes:Uniformitarianism: the interpretation of ancient deposits by analogy to modern, observable environmentsCyclicity: climate, sea-level, annual, tidal variations, etc., all generate repeating cycles of sedimentationHierarchy: basic stratigraphic principles apply across a wide range of space and time scalesDefinitions of stratigraphic elements: Rock units are organized into a hierarchy of classifications

    There are also supergroups and subgroups, used when original group definitions later prove inadequate to describe important associations.

    Name

    Typical thickness

    Lateral Continuity

    Group

    > 1000 m

    Continent-wide

    Formation

    100-1000 m

    1000 km

    Member (Lens, Tongue)

    10-100 m

    100 km

    Bed or Flow

    1-10 m

    10 km

  • *The boundaries between rock units can be conformable or unconformable.Conformable describes continuous deposition with no major breaks in time or erosional episodes. This definition is scale-dependent just how long or large a gap is an unconformity depends on the size duration of the units being divided.A vertical succession of strata represents progressive passage of time, either continuously at the scale of observation (conformable) or discontinuously (unconformable).A lateral succession of strata represents changing environments of deposition at the time of sedimentation or diagenesis.Each recognizable environment in a lateral succession is called a facies.Stratigraphy:definitions

  • *Disconformity is used when beds above and below are parallel but a well-developed erosional surface can be recognized, by irregular incision, soil development, or basal gravel deposits on top.Paraconformity is used for obscure unconformities where correlation with time markers elsewhere indicates missing strata, even though no evidence of a gap is present locally.Nonconformity is used for deposition of bedded strata on unbedded (usually igneous or metamorphic) basement.Stratigraphy:definitionsUnconformities are usually divided into four types:Angular unconformity is used when layers below are clearly tilted or folded and then eroded before deposition continues on the eroded surface

  • *Stratigraphy:definitionsAny package of sedimentary strata bounded above and below by an unconformity (of any kind) is a sequence.Traditional stratigraphy uses formations as the fundamental units of the rock record, withinterpretation of sedimentary environments as the essential product of stratigraphic studies.Sequence stratigraphy makes sequences the fundamental units of the rock record and emphasizes periods of deposition and nondeposition (episodes of rising and falling sea level?) as essential information. Sequence stratigraphy grew out of seismic stratigraphy; unconformities are easily distinguished in seismic records, but lithology is often unknown.Sedimentary accumulation (hence the boundaries of sequences) is controlled by changes in base level, the elevation to which sediments will accumulate if the local land surface is too low, or erode is the local land surface is too high.

  • *Stratigraphy: Base LevelOn land, base level is set by the equilibrium profile of river systems.In marginal marine settings, base level is often the same as sea levelIn the deep sea there is no base level and sedimentation is controlled only by sediment supply.Changes in base level allow the sedimentary record to preserve evidence of geological events: Relative sea level change is the most important determinant of changes in base level. Local tectonic uplift or subsidence changes base level and leads to erosion or accumulation. Changes in water supply or sediment load affect the equilibrium profile of a river and therefore the base level downstream.

  • *Stratigraphy: Base LevelThe parameters of the curve for each river are different. Changes in these parameters will cause the river to aggrade or incise to reach a new base level. Parameters include the elevation of the headwaters, which may change by uplift or erosion; the elevation of the mouth, which may change up uplift or sea-level change; the sediment supply, the water discharge, the type of rock being cut.On land, base level is set by the equilibrium longitudinal profile of river systems, which evolve to a characteristic shape:

  • *Stratigraphy: Base LevelThe placing of an artificial knickpoint in a river by building a dam has curious consequences, both upstream and downstream.A waterfall must retreat because it is steeper than the equilibrium gradient for the reach of the river below the falls.A sudden drop in base-level leads to the formation of river terracesA knickpoint (resistant bed or lake) where the form of the river is interrupted leads to a nested set of river profiles.

  • *Stratigraphy: Relative Sea LevelRelative sea level is the depth of water relative to the local land surface. Relative sea level can change due to local vertical tectonic motions or due to eustatic sea level variations (i.e. global changes in the volume of ocean water or of the ocean basins).In both sequence and traditional stratigraphy, the critical events that determine the locations of environments and unconformities are transgressions and regressions.A transgression is a landward shift in the coastline, and hence a landward shift in all marginal marine environments. A regression is a seaward shift in the coastline.A drop in relative sea level always causes a regression. A transgression hence requires rising relative sea level. However, rising sea-level can result in transgression, stationary shorelines, or regression depending on sediment supply. This asymmetry results because sediment flux from land is always positive, and because transgression during sea-level fall would create unstable, over-steepened long-valley profiles.

  • *Stratigraphy: Relative Sea LevelWhether transgression or regression occurs controls the preservation potential and vertical succession of environments like barrier islandsrising sea-level can result in transgression, stationary shorelines, or regression depending on sediment supply.

  • *Stratigraphy: Walthers LawWe are now ready to state the third fundamental tenet of traditional stratigraphy, lateral continuity, which is expressed by Walthers Law:In a conformable vertical succession, only those facies that can be observed laterally adjacent to one another can be superimposed verticallyThat is, if the lateral shifting of sedimentary environments is controlled by continuous changes in base-level, each point accumulating sediments vertically passes through all intermediate environments continuously.Thus, e.g., deep-sea sediments directly overlying a terrestrial flood-plain facies demands an unconformity in between.Consider again the vertical succession of beach facies, which maps the lateral succession of beach facies onto a single point as the beach progrades outwards during a regressive relative sea-level rise.

  • *Stratigraphy: Transgression and RegressionIn vertical succession, transgression is recognized by progression from inland towards deep water sediments moving up section; regression, if preserved, is recognized by progressively shallower water facies moving towards continental settings as you go up section.

  • *Stratigraphy: Transgression and RegressionThe ideal sequence consists of a transgressive clastic formation, a carbonate formation deposited when essentially the whole continent was flooded, and a regressive clastic formation (less often preserved after erosion).On a regional-continental scale, transgression is recognized by lateral migration of environments with time, from the coast towards the interior, and regression by migration of environments towards the coast.

  • *Sequence StratigraphyOn a continental scale in North America, there are recognized six major transgressions and regressions, bounded by five major regional unconformities. These sequences were named in North America by Sloss (1963), but they correlate fairly well with patterns seen on other continents. They are therefore interpreted as major changes in eustatic sea level, not as continental-scale uplift and subsidence.

  • *Sequence StratigraphySuperimposed on the major Sloss sequences are second-order cycles of transgression and regression usually called Vail curves, and superimposed on these are third-order cycles that are correlated with individual reflectors in seismic sections of marine strata. Tracing and correlating these sequences is the main project of sequence stratigraphy.Repeated transgressions and regressions, presumably related to cyclic rises and falls of sea level, lead to cyclic sedimentation episodes in sedimentary basins. In particular, the Pennsylvanian strata of the eastern U.S. show at least 50 distinct cylcothems consisting of the triplet of deposits: marine-fluvial-coal. Each is a regression, probably caused by withdrawal of water from the oceans during a glacial advance.

  • *Causes of sea-level changeRelative sea level can change due to local or regional tectonics, which cause vertical motions (uplift and subsidence). Global sea level can only change by altering either the volume of sea water or the volume of the ocean basins themselves.On time scales of 103105 years, glaciation can quickly

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