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Time and the Stratigraphic record • Lithostratigraphy revisited • Asymetric cyclicity • The imperfect recorder • Facies and temporal correlation Lithostratigraphic concepts, 1 • Facies • Walther’s Law • Time-transgressive • Sharp contact • Pinch-out and intertounging • “to grade” or intergrade Lithostratigraphic concepts, 2 • Transgression and the fining upward sequence • Regression and the coarsening upward sequence • Prograde vs. retrograde • Aggrade vs. degrade Symetric or asymetric sea level change? • Fining upward sequences are stratigraphically rare • Coarsening upward sequences are more common • Why? • Reworking of fining upward sequences by high energy nearshore processes during SL rise • Sediment character results in differential preservation Example: Coal Cyclothem • Traditional model implied gradual transgression (Right curve) • Modern model assumes rapid transgression (Left curve) • Discontinuity is seen as a function of erosion and reworking during transgression, not regression Paradigm shift? • The asymetric model has gained widespread acceptance, but is it the final word? • Remaining questions- • Is the rapid transgression an apparent one due to preservational differences? • Strata from rapidly subsiding margins preserve thick transgressional sequences • Asymmetry may also arise from variable sediment supply (Example: Devonian Hamilton Group) Base level and sediment accumulation • Marine sediments compose the majority of the stratigraphic record due to their position relative to • Base level of erosion [Powell, 1875; Gilbert, 1914] • Base level of aggradation [Barrell 1917; Dunbar & Rogers 1957] • Profound influence on the nature of the record Stratigraphic model of Ager Gaps due to • Erosion • Non-deposition • More rock is missing than accumulating • Gradual biotic transitions may appear abrupt or continuous Examples: Facies models plotted versus time “More gaps than record” [Ager, 1980] • Grand Canyon strata are almost 1 mile thick, but remarkably incomplete • Represent less than 10% of Paleozoic time (590 to 250 Ma) The Inuit have 44 words for “snow”; Stratigraphers have almost as many for time and missing time! • Angular unconformity • Nonconformity • Disconformity • Paraconformity • Hiatus • Degradational vacuity • Lacuna • Diastems • Diacronous (time-transgressive) Time and place from a geologic Perspective Time in geochronologic units Time-rock in chronostratigraphic units Major unconformaties (Angular, non- and disconformities) are associated with lacuna Paraconformities and diastems • A minor unconformity (Paraconformity) is the expression of a diastem • Barrell diagram shows diastems in respose to changes in base level Example barrell diagram for Cedar Mesa SS, UT Estimating lost time •Lacuna - time gap in the sequence resulting from a hiatus and a degradational vacuity. •Potential response to the transgression/regression cycle •Marine realm we talk of Non-depositional hiatus Erosional hiatus Measuring the lacuna • The depth section (blue box) must be reploted as a time section (red box) • Requires identification of time lines by biostrat or some other method Regression and erosion • Unconformity develops in response to regression • A degradational vacuity or erosional hiatus (blue wedge) develops as base level moves seaward (basinward), removing sediment deposited from T0 to T7 Transgression and the hiatus • At the maximum extent of the regression, much of the section lies above base level and thus accumulates no sediment. • The sea then transgresses over the unconformity surface. • The time associated with the hiatus (non-deposition) is outlined in red.
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