GEOL 342: Sedimentation and StratigraphyLecture 16: lithostratigraphy19 April 2005Assoc. Prof. A. Jay KaufmanBiostratigraphyExcept locally, lithostratigraphic units are not generally reliable as time markers.This is due to variations in facies as well as the progradation of units. Good lithologictime markers might include event beds, like tempestites, glacial diamictites or ash falldeposits formed during a single episode. However, the potential cyclicity of these eventsmakes their worldwide use problematic. Today our primary sources of temporalcorrelation come from 1) radiometric constraints, 2) remnant magnetism, and 3) fossils.Biostratigraphy is the study of the temporal and spatial distribution of fossilorganisms. The evolution of life, like radiometric decay, provides a unidirectionalhistory, although this is often ambiguous and unpredictable. Fossil sequences must bedetermined by superposition focusing on the origin or extinction of a species as uniqueevents in Earth history. These two events divide relative time into three segments:before, during, and after. The history of geological thinking can similarly be subdivided.A short history of biostratigraphyBefore Nicholas Steno (1638-1686) people believed that fossils either fell fromthe sky or grew in the rocks. Steno, however, recognized the similarity between modernshark teeth and certain “tongue stones” that were found in particular geological units.Steno's work on shark teeth led him to the more general question of how any solid objectcould come to be found inside another solid object, such as a rock or a layer of rock. He1followed up these observations with those on the formation of rock layers and the fossilsthey contain. These studies lead to the development of the key principles used today byall geologists, including 1) the principle of original horizontality, 2) the principle ofsuperposition, and the principle of cross cutting relationships.Even with the advances that Steno made, by the end of the eighteenth century,geologists still had a confused perception of the rocks they studied. In particular, theyhad great difficulty reconstructing the original order of widely separated sedimentarylayers. That began to change around 1800, thanks in large part to an obscure British canalsurveyor named William Smith (1769 - 1839).Smith, who had little formal education, traveled throughout England and spent sixyears supervising the digging of the Somerset Canal in southwestern England. Along theway he became well acquainted with the rocks through which he cut the canals. He wassurprised to find that the fossils in the layers often were arranged in the same distinctiveorder from the bottom to the top of the rocks. And as he traveled across England, hediscovered the same sequences of fossils in rock layers. Each type of animal, he realized,had a widespread existence for a particular span of time, a span that partially overlappedwith that of other animals. This became known as the Law of Faunal Succession – fossilorganisms succeed each other in the stratigraphic record in an orderly and recognizablefashion. These observations made it possible for Smith to recognize the order in whichrocks had been formed throughout much of England, and then to draw the firstcomprehensive geological map.In addition to initiating the science of biostratigraphy, Smith preceded Darwin bydeveloping the Principle of Organic Evolution – life forms on Earth have developedgradually from one or a few simple organisms to more complex organisms2In 1842 Alcide d'Orbigny developed the concept of the biological stage. Stagesare major subdivisions of strata, each systematically following the other and containing acharacteristic assemblage of fossils. Stages are defined from the first occurrence to theextinction of major groups of organisms, and should be considered as low resolutionstratigraphy. These stages lasted tens to hundreds of millions of years. For example, theTriassic origin and end Cretaceous mass extinction of the dinosaurs represents stageduration of ca. 175 Ma.Albert Oppel, a student of d’Orbigny further refined his predecessor’sobservations and developed the Concept of Biologic Zone in 1856. Biozones are smaller-scale stratigraphic units that include all strata deposited during the existence of specificfossil organisms. Biozones are based on overlapping stratigraphic ranges of organisms,and are used widely for correlation. When interpolated with numerical age informationderived from radiometric dating, biozones and the divisions of the geological time scalecontaining biozones (periods, epochs, stages, etc.) can be given chronological values, aprocess sometimes referred to as biochronology. BiozonationThe preserved earliest (lowest) occurrence in a section for a particular species isits first appearance datum or FAD, and the last (highest) occurrence is the LAD. Theoccurrence of fossils in local sections is also controlled by environmental factors at thetime of deposition, as well as by diagenesis after deposition. Thus the local FAD (orLAD) may or may not coincide with the actual evolutionary events.The absence of fossils in any section, but particularly at the FAD or LAD may be theresult of: 1. immigration (biogeographic distribution and refugia) or emigration2. facies control3. erosion4. deformation by diagenesis or metamorphism35. poor or no sampling6. mis-description of fossilThe key to good zonation is the collection of good index fossils. It is important torely on fossils that are 1) common, easy to identify, and most-likely to be preserved, 2)relatively independent of benthic facies, 3) widespread geographically (endemic vs.cosmopolitan), and short ranging.Biostratigraphically important groups for different time intervals include:PaleozoicTrilobites, Conodonts, Graptolites, FusilinidsMesozoicAmmonites, Foraminiferans, Pollen, Ostracodes, other marine microfossils CenozoicPollen, Foraminifera, Diatoms, Radiolarians, Small Mammals, other marine microfossils Zone classificationsinterval zone – bounded by two specific first or last occurrences of taxaconcurrent range zone – an interval zone defined by the overlap of two or more taxataxon range zone – based on the first and last appearance of a single taxalineage zone – based on