GEOL 202: Earth Surface Processes and Deposits Lithostratigraphic correlation “demonstration of equivalence” JT, ’14 1 Back to lithocorrelation: tracing lithologic similarity lithosome boundaries are rarely (never?) isochrons ★ therefore - formation boundaries are rarely isochrons! ★ – defining them as isochrons would not be “code” correct anyway JT, ’14 2 example - Cambrian, Grand Canyon time lines the “Great Unconformity” JT, ’14 E.D. McKee, 1954 3 Time-transgressive units ★ Time-lines are not horizontal – modern example from the middle East progradation JT, ’14 Ager, 1981 4 PCQ) This cross-section of facies relationships demonstrates (all that are true) a) lithocorrelation ★ b) biozone correlation ★ c) Walther’s Law ★ d) a regressional sequence ★ JT, ’14 5 original stratigraphy - Devonian of eastern N. Am., the Catskill Fan-Delta correlation of lithosomes model JT, ’14 Time-equivalence line Dunbar and Rogers, 1957 6 reinterpretation - correlate by time JT, ’14 Time-equivalence line Dunbar and Rogers, 1957 7 correlation where facies change: lateral correlation must not violate Walther’s Law ★ but - more than one change is permitted! ★ Niobrara shale Niobrara chalk and ls. Krumbein and Sloss, 1963 JT, ’14 8 The result: complex lateral nomenclature: Cretaceous in the west Krumbein and Sloss, 1963 datum: Fox Hills Sandstone and equivalents JT, ’14 9 what’s a “datum”? ★ in any correlation construction we need a horizontal tie-line – our “datum” ★ can be a – lithologic marker – an unconformity – an isochron JT, ’14 10 correlation based on picking unconformities ★ nature and duration of the unconformity – long-lived, diachronous; or – short-lived, more synchronous – overlying strata may be isochronous even if the underlying units are not ★ deformation event associated? – angular unconformity? JT, ’14 11 PCQ) At this locality, What part of the outcrop is most likely nearly an isochron? a) the beds below the unconformity. ★ b) the deformation event that tilted the beds below the unconformity. ★ c) the unconformity. ★ d) the beds overlying the unconformity. ★ JT, ’14 12 improving lithologic correlation characteristics ★ match a succession of strata – as many in a succession as possible – with enough, some can be missing Example ... JT, ’14 13 Example - Colorado Plateau ★ Correlation based on – lithology – order of units Note missing units to the east is expected! Why? JT, ’14 14 Using lithologic characteristics ★ using geophysical attributes – down-hole logging ★ Future lectures using “marker” or “key” beds that are isochrons – sea-level maxima – an ash – any widespread “event” JT, ’14 15 correlation using sea-level maxima. (note these are interpreted facies) Horizontal time lines s.l. highstand JT, ’14 Ager, 1981 16 tephrostratigraphy - isotopically datable isochrons Bishop ash Prothero & Schwab,’96 ★ ★ JT, ’14 a Yellowstone ash at Boca Res. isotopically date the tephra somewhere it has crystals correlate everywhere else using glass bulk chemistry 17 local ash deposits undated crystal/lithic tuff (ca. 3.1MA) at Chalk Bluff the “Mazama" ash at Bordertown, ca. 6,800 JT, ’14 18 ash (tephra) beds mineral crystals may be datable isotopically ★ glass in ash may have a unique and characteristic eruption chemistry ★ – dated elsewhere? – glass is more widespread than mineral grains » why? JT, ’14 19 Lake Tahoe - debris fans possibly related to earthquakes Catastrophic event deposits also are isochrons JT, ’14 20 Example: two tsunami run-up deposits from historic subduction-zone earthquakes 1960 1700 JT, ’14 21 “event beds” tsunami deposits, earthquake-induced slides, slump, flows; impact beds ★ if we can date the event ★ – then use the event bed as a dated horizon everywhere it occurs JT, ’14 22 PCQ) How many types of correlation are likely here: a) lithologic ★ b) isotopic ★ c) biostratigraphic ★ d) sea-level ★ JT, ’14 23 correlation example / exercise: datum - top of Foxfire Sandstone ★ JT, ’14 Add lithostratigraphy tie lines 24 with lithostratigraphic tie lines Now add chronostratigraphic tie lines: ash JT, ’14 25 with time tie lines ★ now add maximum transgression tie-lines lith correlation ash correlation JT, ’14 26 with time tie lines ★ now reconstruct based on time ( “isochron datum”) lith correlation ash correlation maximum s.l. correlation JT, ’14 27 new datum - time lines shoreline excursion maximum water depth (sea-level highstand) JT, ’14 28
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