Stratigraphic unit

Abstract: The Columbia River basalt has been sampled by 28 stratigraphic sections and by the mapping of selected areas. Petrographically, and also stratigraphically, the rocks assigned to this formation by Merriam, Lindgren, and Smith are divisible into 2 distinct varieties. The older, widespread in the Imnaha and John Day regions, is characterized by about 5% olivine, a silica content of 47 to 50%, and by notably higher Al 2 O 3 , MgO, and CaO than the younger. Many outcrops have a characteristic "greasy" appearance because of the presence of saponite after olivine and of nontronite and other clay minerals after chlorophaeite. The basalts at Picture Gorge and Turtle Cove, described by Merriam in 1901, are typical. The younger basalts are characterized by more than 20% tachylyte, little or no olivine, a silica content of 53 to 54%, and by notably greater amounts of K 2 O and TiO 2 . Nearly all the basalts that occupy the central and eastern part of the lava field N. of the John Day and Imnaha areas are of this kind. They constitute the Yakima basalt as defined by Smith in 1901. In the lower part of the Imnaha River canyon, and also in the John Day basin, the Yakima basalt rests with distinct structural unconformity on flows of the older Picture Gorge type. A late variant of the Yakima basalt emerged after warping and faulting had started to deform the Yakima flows in early Pliocene time. It contains more olivine and plagioclase and a distinctly higher percentage of Fe and titania than normal Yakima flows. Because of their separation in time, differences in chemical and mineral composition, and particularly because of the absence of transitional varieties between them, these lithologic and stratigraphic variations of the Columbia River basalt are considered to be products from separate magmatic hearths, and not differentiates of a hypothetical uniform magma.

Abstract: The interplay of local and global controls on accommodation and sedimentation generates basin-specific sequence stratigraphic frameworks that record cyclicity at multiple scales. There are no temporal or physical standards for the scale of any type of sequence stratigraphic unit. Sequences, systems tracts, and depositional systems can be defined at different scales, depending on the scope of the study, the resolution of the data available, and the local conditions of accommodation and sedimentation. A scale-independent methodology and nomenclature is key to the standard application of sequence stratigraphy. Stratal stacking patterns provide the basis for the definition of all units and surfaces of sequence stratigraphy. The same types of stacking patterns may be observed at different scales, in relation to stratigraphic cycles of different magnitudes. At any scale of observation (i.e., hierarchical rank), a specific type of stacking pattern defines a systems tract, and changes in stacking pattern mark the position of sequence stratigraphic surfaces. Beyond this model-independent framework, model-dependent choices with respect to the selection of the “sequence boundary” may be made as a function of the mappability of the different types of sequence stratigraphic surface that are present within the study area. The model-independent methodology, inherently simple and consistent, provides the flexible platform for a standard application of sequence stratigraphy across the entire range of geological settings, stratigraphic scales, and types of data available.