Gaps in the recordGEOL 342: Sedimentation and StratigraphyLecture 15: lithostratigraphy7 April 2005Assoc. Prof. A. Jay KaufmanLithostratigraphyAs we will learn, the sedimentary layers that geologists use to read Earth history (like thepages of a book) are variably preserved and exposed in only certain places. With no singlesection recording continuous sediment accumulation, it is possible to reconstruct geologicalhistory only through our ability to correlate widely separated strata in space and time. Thematching of sedimentary properties (e.g., siltstone, limestone, clay) of rock units is known aslithostratigraphy. It is useful in local areas and essential in geological mapping, but there isalways the danger that even in small areas rock units cut across time planes. A French chemist, Antoine Lavoisier, first applied uniformitarian concepts of geology ina comparison of modern depositional systems with their ancient rock equivalents. By 1789Lavoisier recognized that gravel can only be moved by waves near the shore whereas finersediment can be carried into deeper water, and that each environment had distinctive organisms.He further reasoned that these environments would shift towards the land or out to sea if sea levelrose or fell, respectively.As we have discussed previously, a facies is the sum of the lithological andpaleontological characteristics of a deposit in a given place. Specifically, a lithofacies thendenotes a consistent lithologic character within a formation excluding its fossil content. Atpresent the term facies usually implies some genetic or environmental information (rather thanstrictly descriptive), as the lithologic response to a depositional system.The principle that facies that occur in conformable vertical succession of strata also occurin laterally adjacent environmentsis known as Walther’s law of correlation of facies.Facies in marginal marine environments most often change due to fluctuations in sealevel. Facies are transgressive (move shoreward) when the sediment supply is overwhelmed by arelative rise in the ocean, or when the margin subsides tectonically. On the other hand, when theshoreline moves seaward due to enhanced sediment supply (progradation) the facies areregressive. Transgressive-regressive facies patterns form most of the marine stratigraphic record.1When time lines (representing depositional surfaces of equal age) cut across faciesboundaries the deposit is considered to be time-transgressive.Rock layers that laterally thin to a point where they vanish are known as pinch outs. Theresult of a series of pinch outs due to facies migration during transgression or regression areinterfingering relationships. The traditional model of symmetrical transgression and regressionoften taught at the introductory level is an unlikely scenario due to net erosion, reworking andprogradation of sediments during sea level rise. The result is apparent rapid transgression(flooding) and abrupt facies changes. The transgressive phases apparently accumulate only onrapidly subsiding margins where these sediments sink and are preserved before they can bereworked.While we have discussed the base level of erosion previously as being sea level, but onecould apply this concept more broadly to mean the level above which sediments cannotaccumulate permanently, otherwise known as the base level of aggradation. It is an imaginarysurface of equilibrium between the forces of erosion and deposition.Preservation of the sedimentary record is the rare exception. Sea level rise maytemporarily preserve sediments, but ultimately preservation depends on net subsidence.Gaps in the recordWhile radiometric age constraints imply that on average ancient sediments accumulate on theorder of meters per thousands of years, modern observations indicate orders of magnitude greateraccumulation rates (meters per year to hundreds of years) implying that the stratigraphic record is veryincomplete. Indeed every bedding plane could represent some cryptic surface of missing time (anunconformity) which is called a diastem. Because sea level fluctuates up and down, most geological timeis not represented in the sedimentary record.2It appears that only during times of sea level rise without subsequent erosion that sediments can actually accumulate, which occurs most often when the basin floor is subsiding rapidly. Unconformities are defined as temporal breaks in a stratigraphic sequence. These include:1) angular unconformity2) nonconformity3) disconformity4) paraconformity – a disconformity for which there is not immediate evidence of missing sediment, but abrupt changes in fauna indicate adjacent beds are of significantly different ages.What does the Grand Canyon tell us about unconformities and the base level of erosion?In the field, unconformities are often recognized by 1) basal conglomerates or breccias aboveerosional surfaces which shows relief, 2) hardgrounds, 3) ferruginization or chertification in carbonates, 4)paleontological or structural anomalies.The time missing in an unconformity is known as a hiatus or lacuna. The origin and history of anunconformity may be revealed by the study of temporally calibrated cross sections from the edge ofsedimentary basins.3Since no single section likely records continuous sedimentation it is important to demonstrateequivalence of widely separated sections through the process of correlation to piece together a completehistory of the planet.Oftentimes key lithologies or sedimentary structures (marker beds) are used in a lithostratigraphiccorrelation between outcrops, assuming these represent unique events (i.e., glacial diamictites). This effortis complicated by facies changes as well as unconformities in the succession that remove rock and time (seePerspective article by AJK in Science, provided as a link to the course webpage).The divisions erected in lithostratigraphy are arranged in a hierarchial system, including: group,formation, member and bed. A bed is a distinct layer in a rock sequence. A member is a group of bedsunited by certain common characters. A formation is a group of members, again united by characters withfeatures in