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Stratigraphy Concepts in Stratigraphy Basic Concepts Lithostratigraphy Sequence Stratigraphy Sea level and sediment supply Consequences of changes in sea level Types of sequences Biostratigraphy Other Types of Stratigraphy Younger Basic Principles Steno (1669) Principal of original horizontality Older Sediments deposited as essentially Younger horizontal beds Principal of superposition Each layer of sedimentary rock (sediment) in a tectonically undisturbed Older sequence is younger than the one beneath it and older than the one above it Basic Principles Basic Principles Hutton (1700s) Walther (1884) Principle of Uniformitarianism: The Walther’s Law: Only those facies and processes that shaped Earth throughout facies-areas can be superimposed geologic time were the same as those primarily which can be observed beside observable today each other at the present time “The present is the key to the past” Only applies to conformable successions – Sometimes there are i.e., no major breaks in sedimentation environments/conditions that do not Vertical successions do not always reproduce have good modern analogues horizontal sequence of environments 1 SSeevveerraall km km,, 10s of 10s of kmkm FFFooorrreshoreshoreshore sandse sandse sandstttoneoneonesss SSShhhooorrreeefffaaace sce sce saaandstndstndstonesonesones 0s of m0s of mIIInnnttteeerrrbbbeeedded sandstdded sandstdded sandstonesonesones///shashashallleseses 11 “““DDDiiissstttaaalll””” shal shal shaleseses “Shalier (fining) upward” succession Lithostratigraphy Lithostratigraphy Formation Formation Fundamental unit of lithostratigraphic Generally considered to be tabular in classification geometry A body of rock identified by lithic Large enough to be mappable at the Earth’s surface or traceable in the characteristics (composition, colour, subsurface sedimentary structures, fossils, etc) and Existing formations range from a few m stratigraphic position to several 1000s of m thick Traceable for a few km or several 1000 km Lithostratigraphy Formation Todilto Names (unfortunately...) may change at Entrada political boundaries or from one region to another Names generally based on geographic Chinle locations Contacts between formations established at obvious lithologic changes (sharp or gradational; lateral or vertical) 2 rd Lithostratigraphy a w ardard Members upw Subdivisions of formations ning-png-upw Possess characteristics that distinguish it e er-u from other parts of the formation s CoarSandiShoali Not all formations are subdivided into members Beds Smallest formal lithostratigraphic unit Used only if official designation is useful Lithostratigraphy Lithostratigraphy Groups Units described at “type sections” Two or more formations related Outcrops, well logs lithologically Independent from inferred geologic history Component formations may change Based on objective, identifiable characteristics laterally (e.g., due to facies changes) Interpretations of geologic history may change Supergroups with time Assemblage of related or superimposed Diachronous to some extent groups Produced by shifting depositional environments Sometimes useful for regional syntheses SSeevveerraall km km,, 10s of 10s of kmkm FFFooorrreshoreshoreshore sandse sandse sandstttoneoneonesss SSShhhooorrreeefffaaace sce sce saaandstndstndstonesonesones 0s of m0s of mIIInnnttteeerrrbbbeeedded sandstdded sandstdded sandstonesonesones///shashashallleseses 11 Sand “““DDDiiissstttaaalll””” shal shal shaleseses T1 Shale 3 Sand T2 Sand T3 Shale Shale Problems with Lithostratigraphy Different facies represent different depositional environments As laterally contiguous environments Formation “A” T4 shift with time, facies boundaries shift Formation “B” so that the facies of one environment lie above those of another environment Walther’s Law Problems with Problems with Lithostratigraphy Lithostratigraphy Timelines cross lithologic boundaries Quest: Find/define “timelines” that will Obscures genetic relationships when permit depositional histories to be defined we are reconstructing geologic history with precision Timelines: stratigraphic surfaces generated by the interplay of tectonics, eustacy (global sea level) and sediment supply Use links between sea level, sediment supply and other factors to develop predictive models 4
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