Rocks, Fossils and TimeMaking Sense of the Geologic RecordChapter 5 Stratigraphy deals with the study of any layered (stratified) rock, but primarily with sedimentary rocks and their compositionoriginage relationshipsgeographic extentMany igneous rocks such as a succession of lava flows or ash beds are stratified and obey the principles of stratigraphyMany metamorphic rocks are stratifiedStratigraphy Stratification in a succession of lava flows in regon!Stratified "gneous #ocks Stratification in sedimentary rocks consisting of alternating layers of sandstone and shale, in $alifornia!Stratified Sedimentary #ocks Stratification in Siamo Slate, in MichiganStratified Metamorphic #ocks Surfaces known as bedding planes separate individual strata from one another or the strata grade vertically from one rock type to another#ocks above and below a bedding plane differ in composition, texture, color or a combination of these features%he bedding plane signifies a rapid change in sedimentation or perhaps a period of nondeposition&ertical Stratigraphic #elationships 'etermining the relative ages of lava flows, sills and associated sedimentary rocks uses alteration by heat and inclusions(ge of )ava *lows, Sills +ow can you determine whether a layer of basalt within a se,uence of sedimentary rocks is a buried lava flow or a sill- ( lava flow forms in se,uence with the sedimentary layers! #ocks below the lava will have signs of heating but not the rocks above! %he rocks above may have lava inclusions! ( sill will heat the rocks above and below!Sill %he sill might also have inclusions of the rocks above and below, but neither of these rocks will have inclusions of the sill! So far we have discussed vertical relationships among conformable strata, which are se,uences of rocks in which deposition was more or less continuousUnconformities in se,uences of strata represent times of nondeposition and.or erosion that encompass long periods of geologic time, perhaps millions or tens of millions of years%he rock record is incomplete! %he interval of time not represented by strata is a hiatus!/nconformities *or 0 million years erosion occurredand removed 1 M2 of rocks and giving rise to a 3 million year hiatus%he origin of an unconformity %he process of forming an unconformity deposition began 01 million years ago (M2(), continues until 4 M2( %he last column is the actual stratigraphic record with an unconformity %hree types of surfaces can be unconformities5( disconformity is a surface separating younger from older rocks, both of which are parallel to one another( nonconformity is an erosional surface cut into metamorphic or intrusive rocks and covered by sedimentary rocks(n anglar nconformity is an erosional surface on tilted or folded strata over which younger rocks were deposited%ypes of /nconformities /nconformities of regional extent may change from one type to another%hey may not represent the same amount of geologic time everywhere%ypes of /nconformities ( disconformity between sedimentary rocks in $alifornia, with conglomerate deposited upon an erosion surface in the underlying rocks( 'isconformity (n (ngular /nconformity (n angular unconformity, Santa #osa ( nonconformity in South 'akota separating 6recambrian metamorphic rocks from the overlying $ambrian7aged 'eadwood *ormation( 8onconformity "n 099:, 8icolas Steno proposed his principle of lateral continity, meaning that layers of sediment extend outward in all directions until they terminate %erminations may be (brupt at the edge of a depositional basin where eroded where truncated by faults)ateral #elationships or they may be gradual where a rock unit becomes progressively thinner until it pinches out or where it splits into thinner units each of which pinches out, called intertonging where a rock unit changes by lateral gradation as its composition and.or texture becomes increasingly different ;oth intertonging and lateral gradation indicate simultaneous deposition in adrand $anyon, (ri?ona( Marine %ransgression in the >rand $anyon 'uring a marine regression, sea level falls with respect to the continentMarine #egression the environments paralleling the shoreline migrate seaward Marine #egression ( marine regression is the opposite of a marine transgression "t yields a vertical se,uence with nearshore facies overlying offshore facie sand rock units become younger in the seaward direction @ohannes Aalther (0B9C70:3D) noticed that the same facies he found laterally were also present in a vertical se,uence, now called !alther"s #a$AaltherEs )aw holds that the facies seen in a conformable %ertical se&ence $ill also replace one another laterally !alther"s la$ applies to marine transgressions and regressions Since the )ate 6recambrian, 9 maeologic %ime Scale Mostly, absolute ages for sedimentary rocks must be determined indirectly by dating associated igneous and metamorphic rocks(ccording to the principle of cross7cutting relationships, a dike must be younger than the rock it cuts, so an absolute age for a dike gives a minimum age for the host rock and a maximum age for any rocks deposited across the dike after it was eroded(bsolute 'ates for Sedimentary #ocks (re "ndirect (bsolute ages of sedimentary rocks are most often found by determining radiometric ages of associated igneous or metamorphic rocks"ndirect 'ating %he absolute dates obtained from regionally metamorphosed rocks give a maximum age for overlying sedimentary rocks )ava flows and ash falls interbedded with sedimentary rocks are the most useful for determining absolute ages ;oth provide time7e,uivalent surfaces giving a maximum age for any rocks above and a minimum age for any rocks below"ndirect 'ating "ndirect 'ating $ombining thousands of absolute ages associated with sedimentary rocks of known relative age gives the numbers on the geologic time scale