- Copyright © 2003 Society of Exploration Geophysicists
Facies maps developed by William Krumbein, Lawrence Sloss, and Edward Dapples in the 1950s reflected areal lithologic change within stratigraphic units. From these maps it became obvious that stratigraphic intervals that contained major unconformities could not yield interpretable paleogeographic and tectonic patterns.
To understand regional patterns a new stratigraphic subdivision was required, and the term “sequence” was chosen. If regional patterns were to be understood, it became necessary to define stratigraphic intervals that did not include these unconformities. Sequences differed from the classic dual classification of rock and time-rock intervals, which included lithologic units of Group, Formation, and Member, and time-rock units of Systems, Series, and Stages. The latter were placed in a time framework, including Periods, Epochs, and Ages. These subdivisions were based on their fossil content, and did not reflect depositional history. Exploration and production of hydrocarbons require understanding the depositional origin of stratigraphic units, not simply their subdivision, which was only useful for determining the time and place for deposition of stratigraphic units of a similar age. Unconformity surfaces often reflect long periods of nondeposition, erosion, and truncation that separate units deposited in similar tectonic and geographic settings.
The determination of stratigraphic intervals that had a common tectonic and paleogeographic framework was found to be essential for interpreting their depositional history. Facies maps that included stratigraphic units of disparate origin did not reflect coherent structural or paleogeographic patterns. It was learned that to profitably explore and develop hydrocarbon reserves, it was necessary to recognize both major and minor unconformity surfaces that separated intervals. This allowed the recognition of depositional systems, tectonic history, and stratigraphic patterns which indicated source rocks, paths of hydrocarbon migration, and reservoir seals.
History of the sequence concept
Sloss (1963) suggested a sequence subdivision of the geologic history of North America. It included seven major subdivisions commencing with Sauk, …