SEQUENCE STRATIGRAPHY 159 SEQUENCE STRATIGRAPHY P P McLaughlin Jr, Delaware Geological Society, Newark, DE, USA ß 2005, Elsevier Ltd All Rights Reserved controls, the sequence concept provides a framework for understanding the evolution of depositional systems through time, making it a powerful predictive tool for stratigraphic analysis Introduction: What is Sequence Stratigraphy? Development of the Concept Sequence stratigraphy is one of the major unifying concepts of the geosciences to arise in the twentieth century Rooted in the cross-fertilization of regional facies mapping and geophysics, sequence stratigraphy provides an invaluable approach to practical problems in applied geology and fundamental scientific questions in Earth history It incorporates a variety of disciplines of stratigraphic geology (i.e lithofacies analysis, biostratigraphy, and chronostratigraphy) and is intrinsically related to a number of other areas of Earth history, notably sea-level change, tectonics, and palaeoclimate Sequence stratigraphy is defined as the study of rock relationships within a chronostratigraphic framework of repetitive genetically related strata bounded by surfaces of erosion or deposition or their correlative conformities The fundamental starting point for sequence stratigraphy is the sedimentary facies, which is a lithostratigraphic body characterized by distinct lithological or fossil characteristics, generally reflecting a certain origin A group of sedimentary facies genetically linked by common processes and environments comprises a depositional system These depositional systems can be grouped together within a framework of unconformity-bound relatively conformable stratigraphic packages called sequences Early publications on sequence stratigraphy emphasized the relationships between global sea-level change, or eustasy, and large-scale stratigraphic patterns This work provoked serious debate about the importance of global sea-level change as a genetic control on stratigraphy Recent work integrating sequence-stratigraphic analysis with isotope data is providing new insights into the relationships between ice-sheets, climate, and sea-level, and is helping to clarify the role of eustasy in the evolution of stratigraphic successions However, sequence stratigraphy is more than a record of global sea-level: it is a practical stratigraphic tool Sequences are a product of the interplay of eustasy, tectonics, and sediment supply As a result, they can be recognized and correlated regionally, regardless of whether global sea-level change was the dominant control With an understanding of these Sequence stratigraphy has seen major growth and development since the 1970s However, the roots of the field extend back to the 1940s, when LL Sloss coined the term ‘stratigraphic sequence’ in his regional facies mapping of the Palaeozoic of North America Sloss defined stratigraphic sequences as ‘‘rock stratigraphic units of higher rank than group, megagroup, or supergroup, traceable over major areas of a continent and bounded by unconformities of interregional scope’’ He recognized six sequences and gave them Native American names derived from localities where they are well developed: Sauk, Tippecanoe, Kaskaskia, Absaroka, Zuni, and Tejas In the 1960s and 1970s, the concept of the stratigraphic sequence was applied to the geophysical data collected by oil companies that were using new seismic-imaging tools to obtain a picture of basin and stratigraphic architecture Under the leadership of former Sloss student Peter Vail, researchers at Exxon and its predecessors recognized stratigraphic patterns on seismic lines that they believed corresponded to the same types of sequences and unconformities mapped by Sloss In addition, they identified ‘onlap unconformities’ within marine successions in basins on different continents and inferred global sea-level control and a worldwide extent for ‘onlap cycles’ These seismic-derived sequence-stratigraphic concepts were brought into the public domain with the 1977 publication of seminal papers by Vail and collaborators in AAPG Memoir 26 The ‘depositional sequence’ was defined as a stratigraphic unit composed of a relatively conformable succession of genetically related strata bounded at its top and base by unconformities or their correlative conformities Conceptually, depositional sequences resemble Sloss cratonic sequences, but represent much shorter time intervals The AAPG memoir also provided the first published documentation of the Exxon group’s view of the relationship between inter-regional unconformities and global cycles of sea-level, including documentation of coastal onlap curves established from seismic stratigraphic records on different continental margins Other papers detailed methods for determining sea-level change from coastal onlap and for