Part II Lithostratigraphy and Chronostratigraphy: Introduction EB. VAN HOUTEN Lithostratigraphy and chronostratigraphy have long Among the many advances in stratigraphic analy­ been central concerns in the study of sedimentary sis several have been especially effective. Recogni­ basins. In the past, however, the focus was com­ tion of regionally extensive unconformity-bound monly on local problems or on conventional codes sequences (Sloss 1963), corroborated by seismic for cataloging global geology. During the past stratigraphy (Vail et al. 1977), emphasized the need several decades a rapid rise of interest in basin analy­ for reliable means of regional correlation, and it sis, generated in part by petroleum exploration and increased the significance of unconformities in basin stimulated by the plate-tectonics paradigm, has led analysis. This has led to a more general interest, the to "the greening of stratigraphy" (Sloss 1984). With meaning of hiatuses in basin history and clues to it has come an expansion of pertinent techniques and lapses in normal sedimentation. Appreciation that methods, recently reviewed by Miall (1984), who sedimentation may be essentially discontinuous, points out that stratigraphy now requires a synthesis consisting of increments and gaps (Sadler 1981), of many sorts of geologic data. Concurrently, the has improved our understanding of "rates of accumu­ principal kinds of basins have been delineated in a lation." Development of the graphic correlation plate-tectonics framework (Bally and Snelson 1980; technique (Shaw 1964) provided a reliable way to Mitchell and Reading 1986). This has increased the discover changes and differences in subsidence his­ role of detailed description, accurate correlation, tory. In the past this method was applied mostly to and precise interpretation of their basin fill because successions with rigorous biostratigraphic control; that is the main source of information about the sub­ its broader value has not yet been fully appreciated. sidence history. A backs tripping method of analyzing stratigraphic Early in the "greening" Francis Pettijohn recog­ sections can produce tectonic subsidence curves of nized the importance of careful study of the sedi­ passive margins which reflect thermal contraction of mentary rock record - a focus that was a persisting the heated lithosphere (Sleep 1971). A continuing theme throughout his constructive career. In partic­ complementary concern has been the identification ular, Pettijohn's extensive work on sedimentary and interpretation of transgressive and regressive structures, as well as that of his many students, is alternations and the role of global rise and fall of sea reflected in the Atlas and Glossary of Primary Sedi­ level. Construction of oxygen, carbon, and sulfur mentary Structures (Pettijohn and Potter 1964) and isotope curves has contributed to the correlation of in Paleocurrents and Basin Analysis (Potter and Pet­ sea-level changes as well as to the interpretation of tijohn 1977). This, in tum, contributed immensely the marine environment. Identification of vertical to the development of facies models. Use of these patterns of magnetic reversals in sedimentary rocks two-dimensional vertical profiles (Visher 1965; affords still another new source of chronostrati­ Walker 1979) in reporting and analyzing sedimen­ graphic markers, especially when they have been tary sequences has helped alert students to the vari­ calibrated with the detailed radiometric time scale ety of successions of facies, and encouraged them to (Berggren et al. 1985). These methods of correlation try to find some order in the lithostratigraphic are especially useful today when there is an enthus­ record. iastic interest in finding allocyclic control expressed 64 EB. Van Houten extensively in the stratigraphic record. With this rate of 1 metre per 1,000 years, and that a significant trend has come an increased awareness of the effect change in the rate of sedimentation recorded within of Milankovitch orbital cycles in producing some of the deposit probably resulted from a major change in the patterned sedimentary sequences. provenance. Each of the papers assembled in this section Bond, Kominz, and Grotzinger have analyzed the elaborates a particular method or technique of lower Paleozoic stratigraphic record of the Cordille­ lithostratigraphic or chronostratigraphic analysis ran and Appalachian orogens to test whether multi­ that contributes to the more general study of ple orders of sea-level change prevailed then. They sedimentary basins and will have an increasingly have constructed subsidence curves by the back­ important role in the future. Miall has reviewed stripping method which show that in general the changing ideas about detrital facies architecture, and subsidence decayed exponentially with time in he concludes that in many situations two­ response to slow cooling of the crust. Significantly, dimensional vertical profiles are inadequate for however, the curves also reveal systematic deflec­ accurate understanding of complex three­ tions. These suggest that the long-term subsidence of dimensional sedimentary deposits. In an effort to the Paleozoic passive margins was modified by generate some new unifying concepts, he empha­ 40-60 m.y. and 2-10 m.y. patterns of eustatic rise sizes the importance of recognizing a hierarchy of and fall of sea level that were similar to patterns that physical scales and three-dimensional architectural prevailed during Jurassic time. elements, or depositional units, characterized by Nemec demonstrates that sequences of coal meas­ their lithofacies composition, and external and ures provide important clues to short-term intrabasi­ internal geometry. In this approach his focus is on a nal differential subsidence not accounted for in six-fold hierarchy of bounding surfaces, or bedding general basin modelling. Noting that beds of coal contacts, as a source of detailed information about commonly accumulate during basin-wide hiatuses in the relation between duration of events and the scale normal sedimentation, he regards them as essen­ and geometry of the product. tially chronostratigraphic markers. Then he uses the Shanmugam returns to an old stratigraphic theme coal measures as "biological" events and analyzes - unconformities. He identifies erosional unconfor­ the data with the graphic correlation technique mities of local to global scale, discusses the origin of (Shaw 1964). By this means he identifies changing subaerial and submarine varieties, and reviews rates of tectonic basin subsidence and sediment criteria for recognizing them. In emphasizing their accumulation in a foreland basin, an intracratonic role in basin analysis Shanmugam points out some of basin, and two extensional basins. The results sug­ the effects of eustatic and tectonic controls of sea gest that other factors, such as compaction and level, as well as problems of distinguishing between sediment loading, have relatively little effect on sub­ them, especially in seismic stratigraphic records. In sidence. petroleum exploration, erosional unconformities are Johnson and his colleagues describe a method for important, especially where they mark the upper calculating stratigraphic variability based on the boundary of increased porosity, separate reservoir magnetic-reversal time scale. Use of these time lines and source rocks, or provide avenues for migration for lateral tracing and correlation of Miocene fluvial of hydrocarbons. deposits in Pakistan reveals both lateral stratigraphic Clifton, Hunter, and Gardner have analyzed a nonuniformity between sections and local unsteadi­ thick late Cenozoic shallow marine and shoreline ness, or vertical variation in sedimentation, within deposit, with the aim of isolating the major factors sections. The authors address the question of how controlling its transgressions and regressions. For much of the observed variability is in the strati­ this purpose ten depositional facies, identified on graphic record and how much may be an artifact of the basis of biota, sedimentary structures, and tex­ sampling. In spite of recognized complications, the tures, have been reconstructed in a succession of identified variation in sedimentation from place to recurring transgressions and regressions. The con­ place and from time to time is a measure of the sistent pattern of alternations compares closely with dynamic processes at work. With continued refine­ a revised Pleistocene eustatic sea-level curve gener­ ment and extension the magnetic-reversal time scale ated by these authors. The study also reveals that the will become an increasingly useful chronostrati­ basin subsided throughout its history at an average graphic tool. Part II. Lithostratigraphy and Chronostratigraphy: Introduction 65 References Geology 89:569-584. SHAW, A.B. (1964) Time in Stratigraphy. New York: BALLY, A.W. and SNELSON, S. (1980) Realms of subsi­ McGraw-Hill, 365 p. dence. In: Miall, A.D. (ed) Facts and Principles of SLEEP, N.H. (1971) Thermal effects of the formation of World Petroleum Occurrence. Canadian Society Atlantic continental margins by continental breakup. Petroleum Geologists Memoir 6, pp. 9-94. Geophysical Journal Royal Astronomical Society BERGGREN, W.A., KENT, DV., FLYNN, J.J., and 24:325-350. VAN COUVERlNG, lA. (1985) Cenozoic geochro­ SLOSS, L.L. (1963) Sequences in the cratonic interior of nology. Geological Society American Bulletin 96: 1407- North America. Geological Society America Bulletin 1418. 74:93-113. MIALL, A.D.