The Columbia River Basalt Group comprises a tholeiitic flood-basalt province of moderate size, covering an area of about 2x10^5 square kilometers, with an estimated volume of about 2x10^5 cubic kilometers (Waters, 1962). The group is the youngest assemblage of flood basalt known, with an age range from about 17 to 6 million years ago; most eruptions took place between about 17 and 14 million years ago (Watkins and Baksi, 1974; McKee and others, 1977). It is the only flood basalt province of Phanerozoic age in North America.
Wide-ranging regional studies of the basalt, underway for the last 10 years, have been devoted primarily to defining stratigraphic and chemical relations for use in unraveling the history of the province and comparing the Columbia River Basalt Group with flood basalt elsewhere. Recently, these studies have been accelerated because of the need to know more about the stratigraphy and structure of the basalt as related to potential storage of nuclear waste within the basalt pile. A geologic map of the entire province is under preparation. Reconnaissance geologic maps of the basalt in most of Washington and northern Idaho have been completed (Swanson and others, 1979a) and eventually will be published in color (for example, Swanson and others, 1980). Studies in 1978 and 1979 were conducted by full-time and temporary personnel of the U.S. Geological survey under Interagency Agreement No. EW-78-I-06-1978 with the U.S. Department of Energy in support of the Basalt Waste Isolation Program, administered by Rockwell Hanford Operations, Richland, Washington. Personnel involved in this mapping project have been: J.L. Anderson, R.D. Bentley, G.R. Byerly, V.E. Camp, J.N. Gardner, P.R. Hooper, D.A. Swanson, W.H. Taubeneck, and T.L. Wright.
The flows cover a diverse assemblage of rocks ranging in age from Precambrian to Miocene. The pre-basalt topography had considerable local relief, 1000 meters or more in places, near the margin of the Columbia Plateau. some of the pre-basalt hills today stand high above the plateau surface, especially in the Spokane area. Little is known about the nature of the rocks or pre-basalt surface beneath the central part of the plateau. sparse evidence from a drill hole 3.2 kilometers deep just west of the Pasco Basin suggests that a thick weathered or altered zone caps a sequence of lower Tertiary volcanic rocks of mafic and intermediate composition at least 1,970 meters thick (Raymond and Tillson, 1968; Newman, 1970; Jackson, 1975; Swanson and others, 1979b).
The criteria used to recognize specific stratigraphic units include megascopic and less commonly microscopic petrography, magnetic polarity, and chemical composition. Megascopic petrography and magnetic polarity can be determined in the field and, taken together in over-all stratigraphic context, are generally sufficient to identify a particular unit. Ambiguities commonly arise, however, and chemical analyses provide an invaluable and independent guide for checking and correcting field identifications. In fact, use of chemical analyses for correlation purposes is so rewarding that no study of the basalt requiring identification of flows should be undertaken without provision for chemistry. Physical characteristics such as weathering color, size and shape of vesicles, thickness, and type of columnar jointing have been used by some past workers as correlation criteria, but we have found them unreliable because of lateral variability except in some local areas.
[Map,30K,InlineGIF]
The formal stratigraphic units of the Columbia River Basalt Group (figure 2) have been described in detail by Mackin (1961), Bingham and Grolier (1966), Schmincke (1967b), Swanson and others (1979b), Swanson and Wright (1978), and Camp and others (1979); only general statements are made here.
[Table]
Outcrops of the Imnaha Basalt, the oldest formation in the group, are confined to extreme southeast Washington, northeast Oregon, and adjacent parts of Idaho, where feeder dikes are known. Whether the Imnaha occurs farther west beneath younger rocks is conjectural. It covers a surface of rugged local relief and has an aggregate thickness of more than 500 m. Most flows in the formation are coarse grained and plagioclase phyric (Hooper, 1974). Five chemical types have been distinguished (Holden and Hooper, 1976; Kleck, 1976; Vallier and Hooper, 1976; Reidel, 1978). ... Most of the Imnaha has normal magnetic polarity, but the oldest and youngest flows known have reversed polarity, based on measurements with a portable fluxgate magnetometer. ...
Much of the Picture Gorge Basalt is apparently coeval with the middle part of the Grande Ronde Basalt, as judged by the interfingering of the two formations (Cockerham and Bentley, 1973; Nathan and Fruchter, 1974) and reconnaissance magnetostratigraphic work (Bentley and Swanson, 1977) ... The Picture Gorge crops out only in and surrounding the John Day Basin in north-central Oregon, ... Possibly the ancestral Blue Mountains uplift kept most flows from spreading northward out of the basin. The formation is at least 800 m thick; ... The Picture Gorge contains aphyric to highly plagioclase-phyric flows with compositions falling into a broad field known as Picture Gorge chemical type (Wright and others, 1973). ... The formation can be subdivided into three informal units based on field characteristics and magnetic polarity, according to R.D.Bentley (Swanson and others, 1979a). ...
The Grande Ronde Basalt is the oldest formation of the Yakima Basalt Subgroup and the most voluminous and areally extensive formation in the entire Columbia River Basalt Group, underlying most of the Columbia Plateau with an estimated volume of more than 150,000 cubic kilometers. Its thickness varies widely depending on underlying topography; the thickest preserved section exceeds 1000 meters in drill holes in the Pasco Basin, and sections 500-700 m thick occur in the Blue Mountains and other uplifted or deeply incised areas. Most flows in the formation are very sparsely plagioclase-phyric to essentially aphyric, although a few of the oldest flows in and near the Lewiston Basin contain abundant large plagioclase phenocrysts. Major element compositions fall in a broad range termed Grande Ronde chemical type. Flows having different compositions within this range are interleaved throughout the section, although flows of high-Mg type rather consistently overlie flows of low-Mg type in the western part of the plateau. The Grande Ronde Basalt is subdivided into four magnetostratigraphic units on the basis of magnetic polarity (Swanson and Wright, 1976b; Swanson and others, 1979b); these units provide the only useful subdivisions of the formation on a plateau-wide basis. Feeder dikes occur throughout the eastern half of the plateau and are apparently not confined to distinct swarms as formerly thought (Waters, 1961). The Grande Ronde conformably overlies the Imnaha Basalt, intertongues with the Picture Gorge Basalt, and conformably underlies and locally interfingers with the Wanapum Basalt. Commonly a thick soil and, locally, weakly lithified clastic sediments occur on top of the Grande Ronde; they indicate a significant time break, although probably no longer than a few tens of thousands of years judging from the local interbedded relations between the Grande Ronde and Wanapum Basalts in southeast Washington. ...
The Wanapum Basalt is the most extensive formation exposed at the surface of the Columbia Plateau but is much less voluminous than the Grande Ronde, probably having a volume of less than 10,000 cubic kilometers. On a local scale, the Wanapum conformably overlies the Grande Ronde, except for minor erosional unconformities or interbedded relations. On a regional scale, however, the Wanapum overlies progressively older basalt from the center toward the eastern margin of the plateau. Such onlap is not apparent along the northern and western margins, however. These relations suggest that the plateau had tilted westward before Wanapum time.
This formation, the youngest in the Columbia River Basalt Group, is about 13.5 to 6 million years old and contains flows erupted sporadically during a period of waning volcanism, deformation, canyon cutting, and development of thick but local sedimentary deposits between flows. The Saddle Mountains Basalt has a volume of only about 3000 cubic kilometers, less than one percent of the total volume of basalt, yet contains by far the greatest chemical and isotopic diversity of any formation in the group.
Flows within the Grande Ronde, Wanapum, and Saddle Mountains Basalts range from a few tens of centimeters to more than 100 m thick, averaging 30-40 m. The thick flows generally record ponding in pre-basalt valleys, in structurally controlled basins that developed during volcanism, or in narrow canyons eroded into older flows; such intracanyon flows are common only in the Saddle Mountains Basalt. Even the thinner flows generally show evidence of having ponded. This evidence consists of the columnar-jointed nature of the basalt. Such columns can apparently form only under static cooling conditions; their development therefore implies that the lava had ponded. What impounded the lava can rarely be determined. Natural levees several meters high have been observed in places and probably account for most of the ponding. Elsewhere, flows could have pinched out agains opposed topographic slopes. ...
The upper surface of a flow is rarely exposed in plan view. Where seen, the surface is rather flat, smooth, filamented, and locally ropy -- surface features characteristic of lava ponds at Kilauea. The surface of a flow with a rubbly upper zone is rough, has a relief of as much as 6 m, and otherwise appears unlike typical surfaces of ponded flows.
Many flows entered water and formed pillows. Recent studies (Jones, 1968; Moore, 1975) have demonstrated conclusively that pillows are nothing more than the subaqueous equivalent of pahoehoe toes. Many of the pillowed flows occur near the margin of the plateau as it existed at the time of eruption, apparently because lakes were formed as flows ponded rivers draining from marginal highlands. Other pillowed flows are much more extensive, perhaps signifying entry into shallow lakes standing on the plateau surface. One such extensive flow, in the Priest Rapids Member, is pillowed throughout an area of tens of square kilometers in the Cheney-Palouse scabland southwest of Spokane.
In places, lava deltas (Fuller, 1931; Moore and others, 1973) formed as lava poured into shallow lakes and ponded streams. The direction of dip of foreset "bedding" defined by elongate pillows and thin sheet flows in the lava deltas indicates the local flow direction of the lava. Particularly good examples of lava deltas can be seen near Malden south of Spokane (Griggs, 1976), near the mouth of Moses Coulee (Fuller, 1931), and at the mouth of Sand Hollow south of Vantage. ...
One of the major results of recent mapping has been identification of sources for most stratigraphic units and even single flows. ... The feeder dikes average about 8 m wide but vary from a few centimeters to more than 60 m. They may tend to thin upward but this is far from certain. The dikes generally trend north to north-northwest. They cannot be traced far along strike, in part because of exposure problems. Obviously related dike segments, offset a few meters to form an en echelon pattern, form systems extending tens of kilometers. Compound or multiple dikes, consisting of two or more pulses of magma related to the same intrusive event, are common, but composite dikes, containing two or more phases of contrasting compositions, have not been reported. In other words, each fissure was used just once, not repeatedly.
The chance of finding a dike connecting with a flow it fed is small, owing to problems of exposure. Nonetheless, several dikes have been found displaying such a connection. The top of most such dikes is rubbly, apparently consisting of slabs of crust once floating on a flow before it poured back into the fissure. ...
Many other dikes can be inferred to correlate with particular flows, or at least sequences of flows, on the basis of chemical composition and magnetic polarity. In this way, feeders have been identified for most of the named stratigraphic units.
The shape and extent of vent systems for specific flows or related sequences of flows can be reconstructed from the locations of feeder dikes, thick piles of degassed flows (presumably near their vent), abundant collapsed pahoehoe (Swanson, 1973), and accumulations of basaltic tephra (in places occurring in still recognizable spatter cones and ramparts). Such reconstructions show that eruptions of single flows or related flows took place from fissures concentrated in long, narrow vent systems on the order of tens of kilometers long and several kilometers wide (Swanson and others, 1975, and later work).
Vent systems for the Grande Ronde Basalt are distributed across the eastern half to two-thirds of the Columbia Plateau, but those for other units are more restricted (Swanson and Wright, 1979). For example, feeder dikes for the Picture Gorge Basalt are confined within the John Day Basin and neighboring areas, those for the Frenchman Springs Member within a zone 60 kilometers wide and probably more than 200 kilometers long, and those for the Ice Harbor Member within a zone 15 kilometers wide and about 90 kilometers long. On a still finer scale, vent systems for specific flows are nearly linear, as they occur along single dikes or closely spaced related dikes. Examples are the vent system for the Roza Member (probably less than 5 kilometers wide and now known to be more than 165 kilometers long (Swanson and others, 1975; Hooper, 1978)), the basalt of Robinette Mountain in the Eckler Mountain Member (a single dike extending at least 25 kilometers across the Blue Mountains), the basalt of Basin City in the Ice Harbor Member (possibly a single dike at least 50 kilometers long), and several chemically distinct flows in the Grande Ronde Basalt (Wright and Swanson, 1978). ...
Return to:
[Report Menu] ...
[Circular 838 Menu] ...
