Eruption and emplacement of flood : An example from the large-volume Teepee Butte Member, Basalt Group

STEPHEN P. REIDEL Department of Geology, State University, Pullman, Washington 99164 TERRY L. TOLAN Geology Department, Portland State University, P.O. 751, Portland, 97207

ABSTRACT 117° Flows of the Teepee Butte Member, Grande Ronde Basalt, issued from a vent system in southeastern Washington, northeastern Oregon, and western Idaho. Three distinct basalt flows were erupted: the Limekiln Rapids flow, the Joseph Creek flow, and the Pruitt Draw flow. Together these mappable flows cover more than 52,000 km2 and have a volume exceeding 5,000 km3. A portion of the vent system for the Joseph Creek flow is ex- posed in cross section in Joseph Canyon, Washington; it is one of the best preserved Columbia River Basalt Group vent complexes known. The vent complex is about 1 km in cross section, 30 m high, and composed of deposits characteristic of Hawaiian-type volcanism. The vent is asymmetrical; the eastern rampart consists of intercalated py- roclastic deposits and thin pahoehoe flows; the western rampart is composed wholly of pahoehoe flows. Vent deposits indicate a complex sequence of events accompanied by lava fountaining and deposition of tephra and spatter-fed flows. Deformation of the vent ramparts and breccia deposits in the vent complex suggest several lava rise/fall cycles that led to fissure widening by wall collapse. A lava pond formed in the fissure at the end of the eruptive activity. Flows of the Teepee Butte Member are compositionally homo- geneous and were emplaced as sheet flows, each having several local flow units. Our study supports the importance of linear vent systems and the westward Palouse Slope, along with the large-volume lava flows, in controlling the distribution of Columbia River Basalt Group flows. Other factors, including the number of active fissure segments Figure 1 A. Location of the known portions of feeder dikes for the and topography, modified the shape of the flows and the number of three Teepee Butte Member flows. flow units.

INTRODUCTION time issued from linear vent systems tens to hundreds of kilometers in Flood-basalt volcanism on the occurred between length and produced huge sheet flows that ultimately buried tens of thou- 17.5 and 6 Ma when the Columbia River Basalt Group was erupted from sands of square kilometers of the Columbia Plateau (Reidel and others, fissures in eastern Washington, northeastern Oregon, and western Idaho 1989b). Previous studies suggest that many of these basalt flows were (Figs. 1 and 2). The largest eruptions occurred during Grande Ronde erupted and emplaced during a relatively short period, ranging from sev- Basalt time (about 17.0 to 15.6 Ma) when 85 vol% of the basalt was eral days to weeks or months (Shaw and Swanson, 1970; Mangan and erupted (Reidel and others, 1989b; Tolan and others, 1989). The Grande others, 1986; Wright and others, 1989; Reidel and Fecht, 1987). Ronde Basalt covers more than 149,000 km2, with an estimated total Little is known about the processes that produced these flood-basalt volume exceeding 148,000 km3. At least 120 flows are in the Grande flows. This is mostly due to the lack of adequate exposures; vents of the Ronde Basalt, many of which exceed 1,000 km3 in volume and traveled Grande Ronde Basalt are typically buried by younger flows and require a more than 300 km from their fissure systems (Reidel and others, 1989b); fortuitous combination of uplift and erosion to expose any portion of these flows are currently the largest known terrestrial basalt flows. them. Few observable connections between dikes and vents are known Enormous outpourings of tholeiitic lava during Grande Ronde Basalt (Swanson and others, 1975, 1979b; Swanson and Wright, 1981; Reidel

Geological Society of America Bulletin, v. 104, p. 1650-1671, 19 figs., 2 tables, December 1992.

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125 122 12V 120°

Figure IB. Extent of the Columbia River Flood-Basalt province and the informal structural subprovinces. (Inset map is Fig. 1 A.)

and others, 1987; Hooper and Reidel, 1989), especially for large-volume Fig. 2). Each informal member consists of one or more flows that share eruptions (>1,000 km3). similar characteristics (for example, lithology, paleomagnetic polarity, and At Joseph Creek, Washington (Fig. 1), however, a dike that fed one geochemical composition) with respect to their stratigraphie position. The of the large-volume flows of the Teepee Butte Member of the Grande Teepee Butte Member is the second oldest member (Fig. 2) and, because Ronde Basalt (Reidel and others, 1989b; this study) can be traced upward of its distinctive chemical composition, is one of the most easily recognized into a well-exposed vent complex (Reidel and others, 1987). In describing members in the Grande Ronde Basalt. this vent complex and the general physical and chemical characteristics of its extrusive products, we will examine the eruptive history of this repre- Stratigraphie Nomenclature of the Teepee Butte Member sentative large-volume flood-basalt eruption. These new data, coupled with our present knowledge, allow us to evaluate and place constraints on Reidel (1983) and Reidel and others (1989b) used chemical compo- flood-basalt volcanism on the Columbia Plateau. sition and stratigraphie position to identify two distinct units in the Teepee Butte Member. In this study, we recognize a third unit stratigraphically TEEPEE BUTTE MEMBER below the other two. From oldest to youngest, we propose the following nomenclature (Fig. 2) for the Teepee Butte units: (1) basalt of Limekiln

The Grande Ronde Basalt has 4 magnetostratigraphic units (Swanson Rapids (new unit), (2) basalt of Joseph Creek (formerly the high Ti02/ and others, 1979b) and 17 informal members (Reidel and others, 1989b; high MgO unit), and (3) basalt of Pruitt Draw (formerly the low

Geological Society of America Bulletin, December 1992 1651

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£ / MEMBER /

6.0

(SEE SWANSON AND OTHERS (1979b) AND TOLAN AND OTHERS (19891 FOR SUBDIVISIONS OF THIS FORMATION) BASAL T SADDL E MOUNTAIN S n_ D PRIEST RAPIDS 14.5 O rr ROZA 0 FRENCHMAN SPRINGS 15.3 BASAL T > IANAPU M 1- ECKLER MOUNTAIN

SENTINEL BLUFFS < 15.6 CO SLACK CANYON Figure 2. Stratigraphie nomenclature of Columbia LU < FIELDS SPRING River Basalt Group (from Reidel and others, 1989b; and z m IU WINTER WATER CM Tolan and others, 1989) and the units within the Teepee 0 ir UMTANUM Butte Member. LU CO 0 ORTLEY o > ARMSTRONG CANYON a 5 CQ 15.6 LU li. Q MEYER RIDGE a. z GROUSE CREEK 0 CM X < CC d EE WAPSHILLA RIDGE m t- a MT. HORRIBLE < z CHINA CREEK -) < H —1 IT DOWNEY GULCH o O CENTER CREEK LU 0 ROGERSBURG a

BASALT OF PRUITT DRAW cc s TEEPEE BUTTE BASALT OF JOSEPH CREEK BASALT OF LIMEKILN RAPIDS

BUCKHORN SPRINGS 16.5

17.5 UNDIFFERENTIATED BASAL T IMNAH A

TiOj/high MgO unit). Furthermore, we propose that the Teepee Butte precipitation of sulfide minerals (Reidel, 1983). Ti02, Nb, Zr, Sr, and Y, Member be formalized. The type and reference localities, as well as de- in contrast, neither vary greatly nor show any systematic change with scriptions of each unit, are presented in Table 1 and the following sections. distance from the vent. Martin (1989) recognized six subtle, chemically defined and spatially Chemical Composition separated stratigraphic units in the Roza Member of the Wanapum Basalt (Fig. 2). He argued that these stratigraphic subdivisions represented sepa- Flows of the Teepee Butte Member have the highest MgO concentra- rate active segments of the Roza dike system. A similar distinction in

tions of any flows within the R( magnetostratigraphic unit (Fig. 3). This flow-unit compositions is not present in the Teepee Butte Member. Com- characteristic, coupled with the high Ti02 of the Limekiln Rapids and positional variation within a flow unit is as great as that between flow units Joseph Creek flows and low Ti02 of the Pruitt Draw flow (Fig. 3), make of the same flow. We thus conclude that the flows of the Teepee Butte the member easily recognizable. The three flows plot in separate fields Member represent distinct eruptive units, the nature of which we examine with some overlap (Fig. 4). The flows of the Teepee Butte Member have in this paper. lower P2O5 than does the underlying Buckhorn Springs member, allowing the two to be easily distinguished (Reidel and others, 1989b). Lower Ti02, Lithology and Petrography FeO, and Zr concentrations of the Pruitt Draw flow make it unique among R[ Grande Ronde Basalt flows. The Teepee Butte Member is typically fine grained to glassy and Although the flows are relatively homogeneous, some compositional aphyric. The one exception is the Pruitt Draw flow, which contains variation does occur (Fig. 4). The greatest variation occurs proximal to the abundant, small (<0.5 cm) plagioclase phenocrysts and sparse olivine

vent. MgO, Ba, Si02, AI2O3, FeO, Ni, Cr, and Cu have the greatest phenocrysts commonly <0.3 cm in size. Teepee Butte flows contain, in variation at any one locality and along the length of a flow. Cu and Ni decreasing abundance, plagioclase, clinopyroxene, olivine, and opaque show a net decrease with distance from the vent area that may reflect minerals (mainly titanomagnetite and copper sulfides). The groundmass

1652 Geological Society of America Bulletin, December 1992

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TABLE 1. TYPE LOCALITY AND REFERENCE SECTIONS FOR THE TEEPEE BUTTE MEMBER Location of Dikes and Vents

Basalt of Limekiln Rapids Basalt of Joseph Creek Basalt of Pruitt Draw Basalt of Limekiln Rapids. A probable dike for the Limekilm Rap-

Type locality ids flow (Fig. 1) occurs in Green Gulch, a tributary to Joseph Creek (Reidel and others, 1992). Tentative flow-dike correlation is based on the SW/NW 14, section 3 SW/NW !i, section 3 NE/NE !4, section 12 T. 6 N, R. 46 E„ T. 6 N., R. 46 E„ T. 30 N„ R. 4 W, similarity of chemical compositions, but erosion does not permit a direct Black Butte, WA, 7.5 Black Butte, WA, 7.5 Wapshilla Creek, ID, physical connection. The distribution pattern of this flow, however, sug- minute USGS quadrangle minute USGS quadrangle 7.5 minute USGS quadrangle gests that the linear vent system that fed it was at least 40 km long. Basalt of Joseph Creek. A 3-km segment of the dike-vent complex Reference locality for the Joseph Creek flow is exposed on the Haberman in the lower

Grande Ronde type section Grande Ronde type Grande Ronde type reaches of Joseph Canyon near the confluence of Joseph Creek and the (Reidel, 1983) section (Reidel, section (Reidel, (Fig. 1). The dike can be traced to the vent complex NW/NW % section 23, 1983), 1983), T. 6 N„ R. 46 E„ NW/NW % section 23, NW/NW », section in a northwest-trending side canyon ~150 m above the valley floor. The Black Butte, WA, 7.5 T. 6 N, R. 46 E„ 23.T.6N, R. 46 E, minute USGS quadrangle Black Butte, WA, 7.5 Black Butte, WA 7,5 distribution pattern for this flow suggests that the linear vent system that minute USGS minute USGS quadrangle fed it was at least 70 km long. quadrangle Near-vent deposits of the Joseph Creek flow (discussed in a section Green Gulch section China Creek section Dog Mountain section below) also are found along the trend of the dike at the type locality for the (Reidel, 1978) (Reidel, 1978) (Anderson, 1987) SW section 2, SE/NW section 32, SE/NE », section Grande Ronde Basalt, about 5 km to the north (Fig. 1). No dike-vent T. 6 N„ R. 46 E„ T.31N..R. 3W„ 32, Black Butte, WA, 7,5 Wapshilla Creek, ID, T. 3 N, R. 9 E., connection, however, is exposed there because of the shallow level of minute USGS quadrangle 7.5 minute USGS Mt. Defiance. WA, erosion. quadrangle 7.5 minute USGS quadrangle Basalt of Pruitt Draw. A dike for the Pruitt Draw flow is exposed in Pruitt Draw and Wapshilla Creek in the Salmon River Canyon about 15 Moses Siding section Skelton Creek section (Camp, 1976) (Reidel, 1978) km east of the Joseph Creek vent locality (Reidel, 1978; Fig. 1). The dike SW 14, section 17, SE \ section 1, T. 11N,R.45E„ T. 30 N„ R. 4 W„ strikes N20°W and can be traced to the Pruitt Draw flow, but exposures Silcotl Island, WA, Wapshilla Creek, ID, do not permit a detailed examination of the flow-dike connection. The 7.5 minute USGS 7.5 minute USGS quadrangle quadrangle distribution pattern for this flow suggests that the linear vent system that fed it was at least 50 km long.

Extent, Volume, and Thickness varies from intersertal to intergranular. Olivine typically constitutes less than 3% of the groundmass and is most abundant in the Pruitt Draw flow. The three flows of the Teepee Butte Member locally consist of several Opaque oxides commonly compose less than 5% of the groundmass. Near flow units each and collectively cover more than 52,000 km2 in Washing- their vents, the Teepee Butte flows are composed of 50% plagioclase and ton, Oregon, and Idaho (Fig. 5) with an estimated volume greater than 30% clinopyroxene (Reidel, 1978), but the relative abundance of plagio- 5,000 km3. The Teepee Butte Member occurs throughout much of the clase and olivine decreases with increasing distance from the vent areas. Clearwater embayment in the eastern part of the Columbia Plateau (Rei-

< CO < < m m so < < X < LM 15 o > m O <

M LIMEKILN RAPIDS FLOW JOSEPH CREEK FLOW + PRUITT DRAW FLOW

Figure 3. MgO and TiOz concentrations for Grande Ronde Basalt flows from the Grande Ronde River area (Reidel, 1983). Compositions plotted by flow or flow unit above the Imnaha Basalt. Rx, Ni, and R2 are magnetostratigraphic units for the Grande Ronde Basalt shown in Figure 2.

Geological Society of America Bulletin, December 1992 1651

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1.60 H 1 r CZJ 120 140 160 180 200 220 o o Zr (ppm) Zr (ppm)

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5.50 O n> 5.25 Hi-. 2a 3 ÊR 5.00 vc BUCKHORN SPRINGS vo 4.75 : MEMBER

O14.50 : 2 : 4.25 :

4.00 -j LIMEKILN RAPIDS FLOW LAVA -POND 3.75 COLLASPE BRECCIA 3.50 T T T 120 140 160 180 200 220 120 140 160 180 200 220 Zr (ppm) Zr (ppm)

Figure 4. Variation diagrams for flows and vents of the Teepee Butte Member and m, Limekiln Rapids flow; *, Joseph Creek flow; +, Pruitt Draw flow; t, Joseph Creek

adjacent flows. (A) TiOz, (B) P2Os, (C) MgO, and (D) Ba plotted against Zr. 2a is two vent tephra; s, Joseph Creek dike selvage; c, lava matrix that partly cements collapse standard deviations of the analytical precision. Symbols: b, Buckhorn Springs member; breccia at the Joseph Creek vent; p, lava pond within the Joseph Creek vent.

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124" 123° 122° 121° 120° 119° 118° 117° 116° 115'

Figure 5. Inferred areal extent of the three flows that comprise the Teepee Butte Member. A, the location of the Joseph Creek vent described in this paper.

del, 1978, 1983) and crops out near Hood River in the Columbia River the Cascade Range. Secondary features influencing flow distribution in- Gorge (Anderson, 1987; Reidel and others, 1989b). In the central Colum- clude broad structural basins that caused local ponding (for example, bia Plateau, however, the Teepee Butte Member is covered by more than 3 Pasco Basin), and structural uplifts such as the Yakima Fold Belt and the km of basalt and is encountered only in deep hydrocarbon exploration Blue Mountains (Fig. 1; Reidel, 1984; Reidel and others, 1989a; Beeson boreholes (Reidel and others, 1989b). and others, 1989; Beeson and Tolan, 1990). The thickness of the Teepee Butte Member varies from a minimum of The greater flow thicknesses in the Pasco Basin are the result of lava 30 m at its distal end to more than 200 m in the Pasco Basin, central ponding. Subsidence of the Pasco Basin accompanied by basalt emplace- Columbia Plateau (Fig. 6). The volume of basalt erupted increased with ment and ponding is well established (Reidel, 1984; Reidel and Fecht, each successive flow. The Limekiln Rapids flow covers an estimated 1987; Reidel and others, 1989a, 1989b). 16,000 km2 and has an approximate volume of 840 km3. In the field, no Constructional topography created by the emplacement of the Lime- local flow units have been observed. The Joseph Creek flow is the second kiln Rapids flow influenced both the extent and thickness of the Joseph most voluminous flow, covering -35,000 km2 and having an estimated Creek and Pruitt Draw flows. The flow margins of the Limekiln Rapids volume of 1,850 km3. Locally, this flow may consist of as many as three flow created a topographic barrier which forced the Joseph Creek flow to flow units. The Pruitt Draw flow has the greatest areal extent, covering advance around the older flow. Only after the Joseph Creek flow inun- -52,000 km2 and having an estimated volume of 2,350 km3. Typically, it dated this topographic obstruction was the Pruitt Draw flow able to ad- may have one or two flow units in the eastern part of the province and as vance across both the Joseph Creek and Limekiln Rapids flows. many as five flow units in the western part at Dog Mountain near Hood River, Oregon (Figs. 5 and 6). Physical Appearance and Flow Type In addition to the enormous volume of lava, the Miocene topography was an important factor controlling the extent of the Teepee Butte Flows of the Teepee Butte Member are sheet flows that display little Member and other Columbia River Basalt Group flows as it directed the complex internal structure. The flows generally have blocky to crude path of the advancing lava. The most important features were the Palouse columnar jointing over much of the eastern Columbia Plateau and have Slope, which provided the regional gradient that allowed the flows to poorly developed entablatures and colonnades at the flow margins. Thick move west from the vent area, and the Columbia Trans-arc Lowland (Fig. flow-top breccia composing up to one-third of the flow is locally found 1) that provided a route through the Miocene Cascade Range (Beeson and near the vents. At Dog Mountain in the Columbia River Gorge (Fig. 1), all others, 1989). The Palouse Slope prevented the flows from ponding near of the Pruitt Draw flow units have entablatures and colonnades with the vent, and the Trans-arc Lowland presented lava from ponding against normal vesicular flow tops (Anderson, 1987; Reidel and others, 1989b).

Geological Society of America Bulletin, December 1992 1651

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BASALT OF JOSEPH CREEK J OREGON ' . ••.•:'.,' BASALT OF LIMEKILN RAPIDS oso km

Figure 6. Correlation diagram for flows and flow units of Teepee Butte Member. Inset map shows the location of measured sections and boreholes.

Teepee Butte Member flows in the Pasco Basin are encountered only in some features never before found at other Columbia River Basalt Group boreholes; geophysical logs and drill chips suggest that the flows are rela- vents (Reidel and others, 1987; Reidel and Tolan, 1990). tively massive with thick vesicular flow tops and thin internal vesicular zones. The Dike and Lava Pond The internal structures (for example, jointing patterns, position of vesicular zones) of the Teepee Butte Member flows, as well as most other The Joseph Creek dike is typical of most other Columbia River Grande Ronde Basalt flows, are consistent with emplacement of the flows Basalt Group dikes. About 3 km of a nearly continuous basalt dike is as single or multiple unit sheet flows. The main variation is in presence or exposed in Joseph Canyon. For 300 m on the south side of Joseph Creek, absence of entablature/colonnade jointing, thickness and lateral extent of however, the dike is a series of en echelon segments (Fig. 8A) that accom- the jointing, and presence or absence of flow units. Jointing characteristics modate a change in trend from N16°W to nearly north-south. change rapidly over short distances (10 km), but the number and thickness On the north side of Joseph Canyon, the dike can be traced from just of flow units may remain constant over tens to hundreds of square above Joseph Creek into a vent complex (Fig. 7). The lower portion of the kilometers. dike (where it cuts the Imnaha Basalt) is relatively narrow, averaging The term "flow unit" is commonly used in discussions of the Colum- about 7 m in width. Here the dike displays well-developed horizontal bia River Basalt Group, and so one is tempted to apply Walker's (1971) columnar joints (Fig. 8B, column diameter <0.3 m) and has a fine-grained compound-flow classification. The scale of the flow units in the Teepee texture. The dike margins are marked by a narrow, 1- to 5-cm-wide, glassy Butte Member and other Columbia River Basalt Group flows, however, is selvage (Fig. 8C) accompanied by several dike-parallel vesicular layers orders of magnitude larger than Walker's Hawaiian examples; typically (Figs. 8C, 8D). Narrow vesicular zones (<1 cm) are also found in the flow units in the Teepee Butte Member are extensive, thick basalt flows interior of the dike. rather than thin, overlapping, interconnected lobes of lava. Because of the The widest point in the dike (about 40 m) occurs just below the vent, size of the flow units, one flow unit can easily be mistaken as the total where the jointing pattern gives way to more chaotic columnar jointing output of a single eruption. that consists of larger-diameter (0.5 to >1 m) curvilinear joints. These columns have a coarser texture, with numerous vesicle sheets and vesicle VENT COMPLEX OF THE JOSEPH CREEK FLOW pipe-like features (Fig. 8D). The dike passes through three flows of the Grande Ronde Basalt, two Although the occurrence of Grande Ronde dikes is well established belonging to the Buckhorn Springs member and the Limekiln Rapids flow (Swanson and others, 1979a, 1979b, 1980), the Joseph Creek vent (Fig. 7) (Fig. 9). The dike widens from 40 m at the base of the Limekiln Rapids is undoubtedly the best exposed and preserved vent complex; it contains flow to more than 150 m at the paleoground surface (Figs. 7 and 9).

1656 Geological Society of America Bulletin, December 1992

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Figure 7. Geologic sketch map of the Joseph Creek vent. See Figure 13 for description of measured section A.

FIGURE 7 EXPLANATION

UNDIFFERENTIATED GRANDE RONDE BASALT ABOVE LIMEKILN RAPIDS FLOW TEEPEE BUTTE MEMBER

PRUITT DRAW FLOW BUCKHORN SPRINGS MEMBER

H JOSEPH CREEK FLOW IMNAHA BASALT FAULT (ARROWS SHOW DIRECTION TEPHRA OF MOVEMENT)

LAVA POND • • LOCATION OF DIKE SECTION

COLLAPSE BRECCIA IIIIII MEASURED SECTION

DIKE CONTOUR INTERVAL - 200 FEET

Outwardly, the jointing pattern of the basalt in this part of the dike of the lava pond and fissure wall (Figs. 7 and 9). The breccia is composed resembles a normal blocky to columnar pattern, typical of basalt flows in of angular clasts, ranging from several centimeters to >1 m (Fig. 10), the area. The texture coarsens from medium grained near the margins to derived from both the Limekiln Rapids flow, which forms the walls of the nearly diabasic in the central portion. We interpret this to be a lava pond fissure, and the Joseph Creek vent. We interpret this debris to have been on the basis of the physical appearance and the composition; the latter is produced by widening of the fissure (see Discussion). discussed in a following section. Lava ponds within vent complexes pre- Faults along the Dike. Two faults with no discernible vertical strati- viously have not been found within the Columbia River Basalt Group. graphic offset occur on the margins of the Joseph Creek dike complex Collapse Breccia. A breccia deposit lies between the eastern margin (Fig. 7). The uppermost fault is exposed in a ravine in the flow above the

Geological Society of America Bulletin, December 1992 1651

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Figure 8. The Joseph Creek dike. (A) A view to the southeast Limekiln Rapids or Pruitt Draw flows were not found. The only signifi- showing en echelon segmentation of the dike and its change in trend as cantly different composition occurs in the selvage zones and the margin of indicated by the letter "B" and arrow. (B) Close-up view of the physi- the dike. The zone of enriched rock at site B-B' is about 3 to 5 cm thick cal appearance of the en echelon dike segments seen in Figure 8A and contains more-evolved compositions that probably represent the first (location "A" in Fig. 8A); "S" indicates wall-rock/dike contact; "J", erupted lava. No flows with this composition have been found in the "K", and "M" denote separate en echelon dike segments. (C) Close- Grande Ronde Basalt, suggesting that it represents only a minor volume. up of dike/wall-rock contact and selvage zone (location "S" in Fig. Similar differences between selvage zones and dike interiors in other 8B); "I", Imnaha Basalt wall rock; "S", the outer edge of the dike's flows of the Columbia River Basalt Group have been described by Ross selvage zone; "V", location of several vesicular zones within the dike; (1983) and Martin (1991). This suggests that a common process such as "D", massive part of dike. (D) vesicle sheets ("V") and pipes in dike assimilation of crustal material (Carlson, 1984) may be operating in Co- at measured dike section B-B' (see Fig. 7). lumbia River Basalt Group magma reservoirs that produced small volumes of initial lavas with a more-evolved composition.

Lava-Pond Composition

vent along the northwest extension of the dike. It is vertical with subhori- The composition of the lava pond is significantly different from that zontal to horizontal striae, suggesting dextral strike-slip movement. The of the Joseph Creek flow (Fig. 4). The lava within the pond has a more- second fault, along the eastern side of the dike, has a curvilinear fault plane evolved composition with depleted compatible elements and enriched in- with subhorizontal to horizontal striae. The strike of this fault is the same compatible elements; only the TÌO2 falls within the normal range of the as that of the dike, but it cannot be traced into the lava pond. The lower Joseph Creek flow. No flows of the Rj magnetostratigraphic unit have this eastern fault may represent part of an initial fault that the dike intruded, or composition, but Wapshilla Ridge member of the R2 magnetostratigraphic a fracture plane that developed during the collapse of the eastern wall of unit (Fig. 2) does have similar compositions (S. P. Reidei, unpub. data). the vent. Numerical modeling of major and trace elements (to be discussed in detail elsewhere) suggests that 30% fractionation of equal proportions of Dike Composition clinopyroxene and plagioclase, with minor olivine (~ 1%) and titanomag- netite, from the Joseph Creek flow can produce the lava pond composi- The Joseph Creek dike was sampled across its width at two locations tion. Major-element analyses were modeled using GENMIX, the and along its exposed length at selected sites to determine (1) the amount petrologie mixing program of LaMaitre (1981); trace elements were mod- of compositional variation, (2) the number of flows erupted, and (3) the eled using a simple fractionation type that employs the Rayleigh relationship between the dike and lava pond. distillation law (Allègre and Minster, 1978). Mineral compositions used in Sample site B-B' is —73 m below the paleoground surface where the the models were obtained from microprobe analyses of the Joseph Creek dike is widest (Figs. 7 and 11), and site A-A' is -122 m below the flow and the lava pond. Minor differences between actual and calculated paleoground surface and 183 m southeast of site B-B'; here the dike is 7 m analyses suggest that the lava pond is Joseph Creek lava that underwent wide. Dike composition is relatively uniform and falls within the Joseph fractionation when it ponded in the fissure, probably at the end of the Creek compositional field (Figs. 4 and 11); compositions similar to the Joseph Creek eruption.

a FIOGEHSBUBÜ MEMBER CAST TEPHRA COMPLEX

Figure 9. Photographic mosaic of the PRUITT DRAW Fl QW Joseph Creek dike and its connection with LIMEKILN HAPIOS FLOW the eastern portion of the Joseph Creek vent. LAVA POND

COLL ASPE BRECCIA

iSÌSfcayi* BUCK HORN SPRINGS MEMBER

DIKE SECTiOM e B — • \ V*- IMNAHA BASAI T DlKE SECTION A-A'

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Pele's tears and bombs occur throughout the eastern rampart but are concentrated at three different levels (Figs. 12 and 13); the thickest ac- cumulation forms the uppermost deposit. Pele's tears range from 1 mm to > 1 cm in size; fragments are black, smooth skinned, and display a range of attenuated shapes including spheres, ovoids, "dumbbells," and the classic pear shapes of Hawaiian Pele's tears (Figs. 14B, 14C). The Pele's tears are typically unaltered glass that stand in relief on red, oxidized, weathered surfaces, giving natural exposures a "pebbly" look (Fig. 14C). The smooth exterior surface of the Pele's tears covers an interior of highly vesiculated glass. The distal extent of these tephra deposits is about 300 m east of the vent. Scattered ribbon and cow-dung bombs are found within the tephra deposits (Fig. 15), and their relative size and number increase with proxim- ity to the vent. Ribbon bombs typically have two forms. One type is a cylindrical ribbon bomb that commonly occurs as broken, rod-shaped segments (Fig. 15A). The second type was still plastic when striking the ground and resembles a coil of flattened rope (Fig. 15B). The second type is generally preserved intact, but the cylindrical ribbon bombs are often broken, apparently too fragile to survive the impact. Less common are cow-dung bombs (Fig. 15C) which have a flattened, disk-like shape. Pahoehoe Flows. Two types of pahoehoe flows are recognized in the vent ramparts: spatter-fed pahoehoe and shelly pahoehoe (Fig. 16). They are interbedded with tephra in the east rampart but form the entire western rampart. Spatter-fed pahoehoe flows are massive to blocky jointed and typi- cally display a distinctive "flow-banded" appearance (Fig. 16A). As the name suggests, these flows formed when molten spatter, created by lava fountaining, fell back to the ground and collected into flows that moved away from the vent. This type of eruption has been observed in Hawaii during the ongoing eruptions at Puu Oo (Wolfe and others, 1988). Al- though most of the spatter was still molten when hitting the ground, some had apparently cooled enough (for example, bombs) to retain the shape they acquired during flight. Upon landing on the molten flow, however, these pyroclasts were incorporated into the flow. The pyroclasts were often partially remelted and deformed by flowing lava, forming welded spatter. Figure 10. Collapse breccia deposit found along the eastern mar- This interpretation is supported by the occurrence of numerous vesicle gin of the lava pond within the Joseph Creek vent. (A) Hammer lies pods and sheets, which, on careful examination, are determined to be upon vesicular lava matrix that appears to have intruded breccia de- stretched and deformed pyroclasts preserved within the flow (Fig. 16B). posit. Breccia deposits contain numerous large blocks of wall rock The shelly pahoehoe flows also have a distinct appearance, typically (left), shelly pahoehoe (right), and vent tephra (upper right). (B) Bar occurring as either a series of thin and discontinuous vesicular layers (Fig. scale (10 cm) rests on block of wall rock set within a matrix of tephra 17), a series of complexly folded and broken layers, or a combination of and basalt fragments. Large block to the right of the bar scale is both. Their internal structures are identical to those seen in shelly pahoe- composed of bedded tephra. hoe produced by some Hawaiian volcanoes (Swanson, 1973; Wentworth and Macdonald, 1953). Using Hawaiian shelly pahoehoe flows as a model, we interpret the thin vesicular layers (Figs. 17 A, 17B) as the collapsed, solidified crust of a Vent Ramparts very gas-rich lava which advanced down a very gentle slope (probably < 1°). The collapse process in Hawaiian examples Tesults from the flow The vent ramparts are asymmetrical (Fig. 12) with respect to the deflating due to degassing (Swanson, 1973). The complexly folded and types of deposits present. The eastern rampart is nearly 30 m high and broken shelly pahoehoe in Figures 17C and 17D probably originated composed of interbedded tephra and lava flows (Fig. 13), whereas the when the solidified surface crust was buckled and billowed by the con- western rampart is about 22 m high and composed wholly of lava flows. tinued lava movement and then subsequently collapsed, possibly when Tephra. Adjacent to the fissure, the eastern rampart is composed of overridden by another flow tongue. Swanson (1973) noted that the tephra separated by relatively thin pahoehoe flows (Figs. 12 and 13). buckled and billowed surface crusts of Hawaiian shelly pahoehoe flows Tephra ranges from ash to bomb size; the larger clasts (bombs) are found are often quite fragile and easily collapsed when any significant weight is proximal to the vent. The tephra deposits are moderately indurated, prob- placed upon them. At the Joseph Creek vent, these complexly folded and ably resulting from some incipient welding as well as minor alteration of broken shelly pahoehoe flows are overlain by other flows or tephra beds; the glassy ash to clay by ground water. The most notable aspect of these either deposit could supply sufficient weight to collapse the billowed sur- deposits is the abundance of coarse ash- to lapilli-sized Pele's tears; this is face crusts. Collapsed shelly pahoehoe flows could also provide an expla- the first report of such pyroclasts at a Columbia River Basalt Group vent nation for a fold-like feature observed within the western rampart, which is (Reidel and Tolan, 1990). composed mostly of shelly pahoehoe flows.

1656 Geological Society of America Bulletin, December 1992

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fS 500 - I- fS 200 I" 450 - I / a ! 190 / k. DIKEB-B' / N 400 - DIKE A-A' FLOW MEAN 180 - DIKE A-A' 350 - S la • im S / k ; niiip a R. I FLOW_ 170 -<0 DIKE B-B I MEAN I 300 J*. I lo

250 I —I— —i— —r~ 160 —I— —I— —I—

10 20 30 40 10 20 30 40

DISTANCE ACROSS DIKE (METERS)

370 1 60 -, DIKE •A-A ' r \ 360 • \ 50

350 - DIKE B-B' flow fri MEAN I flowT ä .. X DIKEB-B' a F -ri « 1 a 40 - 340 - MEANT S \ lo4- 2 V \ 330 DIKE A-A' 30 -L lo I DIKE A-A' 320 -I 20 h I" 310 •S I-

300 — —I— 10 —I— —I— 20 10 30 40 0 10 20 30 40

DISTANCE ACROSS DIKE (METERS)

53.50 -, 5.60

5.40- DIKE B-B' _ 53.00 - / \ s ,/ \ FLOW DIKE A-A' • 5.20 - r x \ MEAN t- H V ^ S / S rV \ •Mo / o 5.00 ¿> 52.50 - o> to 5 V M /FLOW *S DIKE B-B' MEAN 4.80 -| I DIKE A-A' 52.00 - 4.60

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0 10 20 30 40 0 10 20 30 40

S = SELVAGE ZONE DISTANCE ACROSS DIKE (METERS)

Figure 11. Compositional variation across the Joseph Creek site at sites A-A' and B-B' (see Fig. 7). Site B-B' is located 73 m below the paleoground surface level where the dike displays its greatest width. Site A-A' is 183 m southeast of site B-B'. Distances are measured from west side of dike. Two standard deviations (2a) of the analytical precision and mean and la of the Joseph Creek Flow composition (Table 2) are shown. "S", selvage zone samples.

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WEST EAST ROGERSBURG MEMBER

SA1 UNIT OF REIDEL (1983) TEPHRA WEST RAMPART PRUITT DRAW FLOW EAST RAMPART

Figure 12. East-west dia- grammatic cross section through the Joseph Creek vent.

IMNAHA BASALT (HORIZONTAL NOT TO SCALE)

It is interesting to note that the shelly and spatter-fed pahoehoe flows process is related to fissure zones that were wider than average at the start are not indicative of turbulent flow, but rather laminar flow (and possibly of the eruption and became enlarged by melting and erosion, whereas the at low velocity, too). These flows, however, probably were emplaced narrower zones tended to freeze shut. We suggest that this mechanism during the waning phase of eruptive activity. The unusual extent of these might explain the change in dike width. Whether the Joseph Creek fissure flows (>1.5 km downgradient from the vent) suggests that the last phase of system was continuously active along its entire length or developed princi- activity was lava rise-fall cycles in the fissure marked by the emplacement pal centers of eruptive activity cannot be determined on the basis of of shelly pahoehoe flows. existing exposures. It seems unlikely, however, that such a voluminous Extension Cracks. The lower 10 m of the east rampart adjacent to flow was erupted from several point sources along the fissure system; it is the fissure is tilted several degrees toward the lava pond (Fig. 18 A) and is more likely that this vent exposure is only the southern part of one of many cut by numerous extension cracks. The extension cracks penetrate only the active fissure segments. lower two-thirds of the rampart; the largest is 0.3 m wide (Fig. 18B). These extension cracks opened prior to the last episode of fountaining and were Growth and Development of the Joseph Creek Vent filled with tephra. We interpret the tilting and extension cracks to be a later fissure-widening event that occurred when the initial collapse-breccia de- Joseph Creek vent deposits are indicative of a Hawaiian-type erup- posits slumped into the fissure and tilted the rampart. tion. Fountaining and effusion of lava were important parts of the eruptive process. Thick pyroclastic deposits are found only in the eastern rampart, DISCUSSION and deposition was probably influenced by a prevailing westerly wind direction. The west side, however, is downslope; any tephra deposited to Location and Geometry of the Joseph Creek Dike the west could have been either rafted or bulldozed and swept away by erupting lava. A north-south compressive stress regime existing since at least the Evidence suggests that eruptive activity waxed and waned at the vent mid-Miocene is thought to be a controlling factor for the north to north- locality. At least four major episodes of lava fountaining are preserved at west trend of Columbia River Basalt Group dikes (Hooper and Camp, the vent, one in the collapse breccia, and three in the eastern rampart. Each 1981, among others). At the Joseph Creek vent, dextral strike-slip faults episode erupted thin pahoehoe flows accompanied by tephra composed marginal to the dike are consistent with this interpretation. It is not possi- mainly of Pele's tears. The tephra deposits and spatter-fed pahoehoe flows ble to establish the absolute age of the faults, but there is the suggestion that offer direct evidence of repeated fountaining. Based on Hawaiian descrip- faulting occurred prior to the emplacement of the dike and continued after. tions (for example, Swanson, 1973), the shelly pahoehoe flows could have The eastern fault occurs in pre-Teepee Butte Member flows but does not originated in either of two ways. We suggest that most of the shelly penetrate the lava pond or younger flows. The west fault, however, does pahoehoe flows in the western rampart (Fig. 12) were created by the rise penetrate younger flows. of gas-rich lava that overtopped the fissure and vent ramparts; this may or Change in dike width from ~7 m along trend to 40 m at the vent is may not have been accompanied by lava fountaining. Alternatively, these similar to that observed in Hawaiian fissure systems and active eruptions. flows could have been fed by lava fountaining, similar to that observed At Kilauea after the initiation of an eruption, lava fountaining along a during the 1969-1971 Mauna Ulu eruption in Hawaii (Swanson, 1973; fissure usually becomes restricted to one or a few principal point source Swanson and others, 1976). vents (Decker, 1987). Delaney and Pollard (1982) suggested that this Blocks of Pele's tears, shelly pahoehoe, and spatter-fed pahoehoe in

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The uppermost unit. 7.6 m of massive, bedded lapilli with subordinate amounts of bombs.

'1.5 m of shelly pahoehoe. Becomes massive flow rock about 30 m away from vent.

UJ Massive flow unit about 3 m; grades Q laterally toward the dike into shelly o pahoehoe flows and away from vent into CO massive flow rock. Q. UJ 4.9 m of interbedded lapilli, spatter-fed flows and shelly pahoehoe.

1.8 m of well formed shelly pahoehoe flows that are interbedded with lapilli near the top. 1 ' 1.8 m thick unit predominately spatter with I l thin, poorly developed shelly pahoehoe flows. Figure 13. Measured section "A", lo-

Fine grained, basalt flow that can be traced cated on the eastern rampart of the Joseph CO the length of the outcrop (about 60 m). Creek vent (see Fig. 7), showing relationships UJ Merges with massive flow-banded unit between tephra deposits and lava flows. o below farther away from vent. O CO Massive flow-banded unit 2 m thick that Q. can be traced the length of the exposure UJ (60 m). Primarily spatter. Contains welded spatter that grade into flow rock.

0.7 m of thin vesiculated flow rock; welded ^ r spatter.

I i 1.2 m of massive bedded lapilli. Scarce bombs. Pinches out away from vent. CM UJ 0.6 m of shelly pahoehoe flows. Pinches Q out toward canyon rim away from the O vent. CO Q. 1.5 m basalt lava flow. Fine-grained to UJ glassy. Does not overlie early collapse breccia. Can be traced 20 m where it appears to merge with other flow rock.

the collapse breccia record the earliest preserved eruptive activity (episode Compelling evidence for waning eruptive activity between episodes 3 1; Fig. 19A). We infer that following eruptive episode 1, lava drainback and 4 is found in the eastern rampart. Tilting of the eastern rampart nearest occurred and initiated collapse; the eastern side experienced the greatest the fissure produced open extension cracks (Figs. 18B, 19D) that cut amount. Vent walls are inherently unstable and may have failed along deposits from episodes 2 and 3; these cracks are filled with tephra from zones of pre-existing weaknesses. Collapse deposits that are preserved only episode 4 (Fig. 13, unit K; Fig. 19E). The tilted beds and extension cracks along the eastern fissure wall, which strikes 60° to the dike trend, could are confined to the part of the rampart overlying the collapse breccia. We have been in a "flow shadow" and therefore were protected from erosion interpret this to indicate that episodes 3 and 4 were separated by waning by later pulses of lava. Similar deposits along the western fissure wall activity which was accompanied by a lowering of the level of the lava would have been in the main flow path and could have been more easily within the fissure. This caused partial slumping of the earlier collapse eroded. breccia and overlying rampart, opening the extension cracks. Renewed Tephra deposits in the east rampart result from at least three episodes lava fountaining deposited the thickest and most extensive tephra unit of fountaining. The earliest tephra, episode 2, lies directly on the collapse filling the open cracks. breccia with tephra from episodes 3 and 4 overlying it (Fig. 13). Correlations between pahoehoe flows in the western rampart and

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Figure 14. Examples of Pele's tears deposits at the Joseph Creek vent. (A) Interbedded Pele's tears deposits (P) and welded spatter (S) that compose unit I (Fig. 13). (B) Individual Pele's tears collected from the uppermost tephra unit in the east vent rampart (unit K, Fig. 13). (C) Natural exposure of Pele's tears showing pebbly appearance.

tephra deposits and flows in the eastern rampart cannot be established. below the present level of exposure, or fractionation occurred at a deeper Their chemical compositions are identical, and there are no physically level and the evolved liquid fed the lava pond. recognizable horizons. Solidification of the lava pond within the Joseph Creek vent marked the end of Joseph Creek volcanism at this locality. The surface of the lava Closing Phase and Post-Eruptive Events pond had solidified and stabilized prior to emplacement of the Pruitt Draw flow, and so a period of at least several years separated these events. The The closing phase of eruptive activity at the Joseph Creek vent was Pruitt Draw flow only partly flooded the Joseph Creek vent; it filled the marked by the emplacement of a lava pond within the vent (Fig. 19F). low area between the two ramparts and caused minor deformation of the The surface of the lava pond stands at the same level as the ground surface eastern rampart. prior to the Joseph Creek eruption; yet the unique chemical composition of the lava pond (Fig. 4) has not been found in vent deposits or basalt Factors Affecting Flow Size and Extent of Flood-Basalt Flows flows of the R] magnetostratigraphic unit. We infer that no lava flows were erupted from this vent after the change in the pond's composition. There have been no historic eruptions of continental flood . Numerical models suggest that the lava pond within the vent formed The largest historic basaltic fissure eruption was the 1783-1784 Laki by shallow fractionation of clinopyroxene and plagioclase from the Joseph (Skaftar Fires) eruption on Iceland which produced about 20 km3 of Creek lava, but field studies suggest a more complicated history. The lava basalt (Th. Thorardson and S. Self, unpub. data). Still, the Laki eruption pond is coarse grained to diabasic, phenocrysts are sparse, and no cumu- was several orders of magnitude smaller than the eruptions that produced lates are exposed. This suggests that if crystal fractionation did take place, the Teepee Butte Member flows. In this section, we consider factors that either the fractionated minerals were efficiently removed and settled influence the size and extent of flood-basalt flows in the Teepee Butte

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Figure 15. Lava bombs at the Joseph Creek vent. (A) Crescent-shaped ribbon bomb from unit K (Fig. 13). (B) Twisted ribbon bomb from unit K (Fig. 13). (C) Cow-dung bomb from unit C (Fig. 13).

Member, drawing on our study and published observations of historic that adequate volumes of magma existed, as did the means to transport the eruptions. magma to the surface. Previous studies of Columbia River Basalt Group flows (for example, Heat Loss from Advancing Lava. Heat loss from advancing lava is Shaw and Swanson, 1970; Swanson and others, 1975; Reidel and Fecht, thought to be a major factor controlling the distance of flow (Macdonald, 1987) and historic lava flows (for example, Walker, 1971,1973; Rowland 1972; Walker, 1973; Pieri and Baloga, 1986). The greatest heat loss is and Walker, 1990) suggest that the size and extent of basaltic lava flows typically due to radiation. The effects of heat loss on an advancing lava are principally controlled by (1) the rate and duration of lava discharge, flow can be mitigated or reduced by higher discharge or mode of flowage, (2) the rate of heat loss from the advancing lava, and (3) environmental or a combination of both (Shaw and Swanson, 1970; Macdonald, 1972; conditions. Walker, 1973). Rate and Duration of Lava Discharge. The rate and duration of For the Columbia River Basalt Group, Shaw and Swanson (1970) lava discharge from a vent system are thought to be among the most argued that turbulent flow, a consequence of a high lava discharge, is also a critical aspects that regulate the size of basalt flows. Walker (1973) sug- "worst case" heat-loss scenario, because it may prevent the formation of an gested a direct correlation between the rate of lava eruption and the length insulating crust on the surface of the advancing flow. They suggested, of lava flows. Malin (1980), however, suggested that perhaps a statistically however, that high discharge (>0.6 km3/hr/linear kilometer of active more significant relationship may exist between length and the total vol- fissure and feeder dike widths >3 m) may mitigate this until the active ume of the flow. flow front advances 100 km or more. Shaw and Swanson (1970) argued that the most important criteria The emplacement history of the Huntzinger flow (Reidel and Fecht, for flood-basalt flows on the Columbia Plateau are (1) existence of an 1987) suggests that very little solidification had occurred between em- adequate volume of magma, (2) sufficient magmatic "head pressure" to placement of the first flow and invasion of the ponded flow by later flows. maintain adequate magma supply to the surface, (3) length and width of This implies both (1) an insulating crust to retain the heat and (2) the the actively erupting fissure system, and (4) the duration of the eruption. passage of relatively little time (days to weeks) between the eruption of the Magma supply rates for the Columbia River Basalt Group have been flows that collectively produced the Huntzinger flow. This emplacement estimated to range from those comparable to Hawaii (0.075 km3/yr, mechanism occurred for other flows as well (Reidel and Fecht, 1987; S. P. Swanson and others, 1975; and 0.1 km3/yr, Swanson and others, 1989) to Reidel, unpub. data) and may provide a model for other basalt flows that as high as 1 km3/yr (Reidel and Fecht, 1987). In spite of the uncertainty ponded in the Pasco Basin. of many factors controlling magma supply, the existence of large-volume Environmental Conditions. Environmental factors have been shown flood-basalt flows, such as those within the Teepee Butte Member, is proof to play a role in controlling the extent of historic basalt flows but probably

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Figure 16. Internal features of thin, near- vent pahoehoe flows from the eastern vent rampart (measured section A, Fig. 7). (A) Spatter-fed pahoehoe flows. Flow band- ing could be the result of collapse and "weld- ing" of shelly pahoehoe flows. (B) Deformed pyroclasts (bombs) that fell and were incor- porated into the pahoehoe flow. Pyroclasts were flattened and stretched by reheating and movement of the lava flow. Chalk lines mark the outline of the pyroclasts.

have a much greater influence on flood-basalt flows that travel more than Fold Belt) developed into formidable barriers (Reidel, 1984; Reidel and several hundred kilometers from their source (Shaw and Swanson, 1970; others, 1989a). Large-volume flows with little time between eruptions Fecht and others, 1987; Reidel and Fecht, 1987; Beeson and others, 1979, (Imnaha, Grande Ronde, and Wanapum Basalts) were only slightly influ- 1989). Environmental features include topography that can direct the path enced because drainages and structures were more subdued. of the advancing lava (see section on Extent, Volume, and Thickness), and ground-surface conditions (wet or dry) that can extract heat from the flow. Emplacement of Flood-Basalt Flows in the Teepee Butte Member Topographic features are by far the most important environmental considerations for flows of the Columbia River Basalt Group. Smaller In this section, we propose a model for the emplacement of large- volume flows that erupted after long periods of quiescence (for example, volume flood-basalt flows of the Teepee Butte Member. We consider this Saddle Mountains Basalt) were impacted the most because the river sys- as a general model for large-volume flood-basalt flows in the Columbia tems had sufficient time to establish channels (Swanson and others, 1980; River Basalt Group. Our model draws upon (1) the sequence of events at Fecht and others, 1987), and tectonic structures (for example, the Yakima the Joseph Creek vent; (2) our observations of the regional nature of

1656 Geological Society of America Bulletin, December 1992

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/104/12/1650/3381448/i0016-7606-104-12-1650.pdf by guest on 27 September 2021 Figure 17. Collapsed shelly pahoehoe flows from the Joseph Creek vent. (A) Collapsed shelly pahoehoe flows at a distance of about 1 km west of the Joseph Creek vent locality. Hammer (arrow) at left for scale. (B) Close-up of shelly pahoehoe from the east rampart located about Vi km from the axis of the vent/dike complex. Note calcite-filled vesicles. (C) Pillowed and collapsed shelly pahoehoe flows from the eastern rampart. (D) Uncollapsed shelly pahoehoe "toe" (arrow).

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Figure 18. Filled extension cracks and associated deformation within the eastern portion of the Joseph Creek vent rampart. (A) The extent of the deformation ("T") where tephra is dipping to left, and extension cracks occur. "F", the filled extension crack shown in 18B; "C", collapse breccia underlying the deformed portion of the eastern rampart; "P" and "L", the Pruitt Draw and Limekiln Rapids flows, respectively. "H", Joseph Creek lava pond. "O", overlying flow (SA1 of Reidel, 1983). "J" and "K", units described in Figure 13. (B) Close- up of the filled extension crack ("F"). This extension crack is filled with Pele's tears that are part of unit K shown in Figure 13.

model does not consider special cases of Columbia River flows, including the smaller volume Saddle Mountains Basalt flows (for example, Pomona Member), although the primary difference is the development of river canyons that controlled many flows of the Saddle Mountains Basalt. Two of the most important factors influencing all Columbia River Basalt Group flows are (1) the westward regional paleoslope that provided a gradient away from the vent area and (2) the long, linear fissure systems that strike normal to the paleoslope gradient. These two factors, coupled with the volume and effusion rate of a flow, are the fundamental controls on the overall distribution of all of the flows, including Saddle Mountains Basalt flows. All other factors work to modify the flow distributions. We interpret the fissure system to be segmented, with each segment possibly having periods of intense activity and periods of relative quies- cence. This segmentation allowed the flows to be emplaced in stages. During periods of high lava discharge, pulses would rapidly advance away from the fissure system. As discharge declined, velocity would de- crease, accompanied by a transition from turbulent to laminar flow. Shelly pahoehoe and spatter-fed flows were erupted during these stages. As activ- ity declined along one or more segments, other segments could become active, or the entire fissure could become dormant. As the lava pulse flowed away from an active segment, it moved relatively unhindered down the Palouse Slope to the Pasco Basin. Here, lava would temporarily pond in this structurally low area and thicken as more lava entered the basin. Such lava lakes could be fed by one or more active segments of the fissure system. Flow-front stagnation and deepening of the lava lake would continue in the Pasco Basin until it overflowed the basin. Because of the irregular basin topography created by the shape and size of the bounding anticlinal structures (see Reidel and others, 1989a), continued westward advance of the flows might not occur as a single sheet but as multiple flows each exiting through one or more low spots in the basin. These flow units could then continue as separate flows or could merge back together. As the flows approached the Cascade Range, they were funneled into the Columbia Trans-arc Lowland (Fig. 1). Directing one or more flow units into this relatively narrow lowland could also produce multiple flow units such as that observed at Dog Mountain (Fig. 6). Intermittent discharge along the fissure system could also produce smaller localized flow lobes along the length of the flows. The increased hydraulic pressure could breach stagnated flow margins anywhere along the length of the lava flow. These flow lobes could originate at any position downgradient from the vent and could not, in general, be traced back to the vent.

Emplacement Times and Discharge

flood-basalt flows throughout Washington, Oregon, and Idaho; and One of the most difficult aspects of understanding flood-basalt (3) the Miocene structural configuration of the Columbia Plateau. We volcanism is estimating the time required to emplace a large-volume flow. argue that these factors are interrelated and combined to produce the Average lava-discharge estimates for large-volume Columbia River Basalt distribution pattern and flow features observed in the flood basalts. Our Group flows have been based on evidence suggesting that the lava was

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WEST EAST

WIND LAVA FOUNTAIN SPATTER-FED PAHOEHOE FLOWS LAVA FOUNTAIN -t- SHELLY PAHOEHOE

TEPHRA AFTER N LIMEKILN RAPIDS FLOW COLLASPE

' » . » ¿ V*1» * * » rnmm

FLOW TOP BRECCIA BUCKHORN SPRINGS MEMBER (A) (g) DRAINBACK-COLLAPSE BRECCIA (C)

WEST EAST

EXTENSION CRACKS

SHELLY AND SPATTER-FED PA^WEHOE FLOWS.

JOSEPH CREEK LAVA POND (D) (E) (F)

Figure 19. Interpretation of the general sequence of events that produced the deposits and features at the Joseph Creek vent. (A) Episode 1. Initial fountaining and deposition of tephra. (B) Drainback and collapse along the fissure wall. Inferred first stand of the lava pond within the fissure. (C) Episodes 2 and 3. Renewed fountaining and deposition of tephra, and spatter-fed and shelly pahoehoe. (D) Decrease in eruptive activity accompanied by lava drainback and fissure-wall collapse. Formation of extension cracks preserved in eastern rampart. (E) Episode 4. Renewed fountaining and infilling of extension cracks with tephra. (F) Formation of final lava pond within the fissure. Pruitt Draw flow emplaced after solidification of the crust on the Joseph Creek lava pond.

rapidly erupted (<30 days) and emplaced as a sheet flow (Shaw and

Swanson, 1970). TABLE 2. TIME FOR EMPLACEMENT OF A SINGLE SHEET FLOW OF 1,000 km3 Table 2 presents an array of simplified estimates for the potential

duration of an eruption that produces a flood-basalt flow with a volume Average discharge rate (-1,000 km3) equivalent to that of a large-volume flow in the Teepee Butte Member. The variable parameters are the average discharge and 0.1* 0.25 0.50 1 2 3

active fissure lengths. The average discharge ranges between that compar- 0.5 km+ 20,000§ 8,000 4,000 2,000 1,000 667 able to sustained discharge for Kilauea Volcano (Swanson, 1972), at the 1 km 10,000 4,000 2,000 1,000 500 334 2 km 5,000 2,000 1,000 500 250 167 low end, and estimated discharges for flood-basalt flows (Shaw and Swan- 5 km 2,000 800 400 200 100 67 10 km 1,000 400 200 100 50 34 son, 1970) at the high end. The range includes the maximum lava- 20 km 500 200 100 50 25 17 discharge rates estimated for the 1783 Laki eruption in Iceland (0.33 50 km 200 80 40 20 10 7 100 km 100 40 20 10 5 3 3 km /day/km of active fissure; Th. Thordarson and S. Self, unpub. data); 150 km 67 27 13 7 3 2 the higher-end discharges for these historic eruptions tend to produce sheet 200 km 50 20 10 5 2.5 1.5

flows (Walker, 1973) and are more applicable to flood-basalt flows on the Noie: Laki maximum discharge rate -0.33 km3/day/km of fissure; rate maintained for only a few hours (Th. Thordarson and S. Self, unpub. data). Columbia Plateau. "km Vday for every kilometer of active fissure length. The estimated time to produce 1,000 km3 of lava ranges from 20,000 ^Active fissure length (km). ^Number of days to erupt 1,000 km3 of lava. days (~55 yr) for the combination of lowest discharge and shortest active

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fissure length to 1.5 days for the highest discharge and longest active fissure volcanism. Lava fountaining created relatively thick deposits of lapilli- length. Petrographic data from other studies of large-volume Columbia sized tephra and bombs. The shelly and spatter-fed pahoehoe flows are River Basalt Group flows provide circumstantial evidence that suggests we interfingered with the tephra at the vent, suggesting that a change from can considerably narrow the range of these estimates. turbulent to laminar flow occurred at the end of the eruptive activity. Petrographic examination of quenched Columbia River Basalt Group 5. Field relationships suggest that several lava rise/fall cycles oc- lava (for example, rinds from pillow lavas) from medial to distal parts of curred within the Joseph Creek vent. Widening of the fissure by wall the flow has shown that the crystallinity is no greater than that of the glassy collapse occurred as lava drained back. selvage zones of the feeder dikes. This strongly suggests that little or no 6. Large-volume flood-basalt flows on the Columbia Plateau were crystal nucleation and growth occurred from the time that the lava was probably emplaced as1 sheet flows erupted from long, en echelon, fissure erupted to when it reached its most distal point (Shaw and Swanson, 1970; systems. Flows advanced relatively unhindered down the Palouse Slope to Swanson and others, 1975; Mangan and others, 1986). This lack of signifi- the Pasco Basin, where they ponded and thickened until they overflowed cant crystal growth has been interpreted to have resulted from the rapid the basin and continued west as either a single flow or as multiple flow eruption and flowage of the lava, with flow-emplacement times on the units. order of a few days to several weeks (Swanson and others, 1975; Mangan 7. Data from the Teepee Butte Member appear to be consistent with and others, 1986; Wright and others, 1989). To achieve such rapid em- previous models that postulate the very rapid emplacement (days to placement times requires a combination of both a long and active fissure weeks) of large-volume flood-basalt flows. The lack of control on impor- 3 system (>20 km) and a high rate of lava discharge (>1 km /day/km of tant factors, including the number of active fissure segments and the actual active fissure) as shown in Table 2. eruption rates, makes these estimates first-order approximations at best. Data collected on the large-volume flows of the Teepee Butte Member appear to be consistent with the rapid-emplacement model postu- ACKNOWLEDGMENTS lated by Shaw and Swanson (1970). For example, we estimate a length of at least 70 km for the Joseph Creek feeder-dike system based upon its We would like to thank the Fred Haberman family for their kindness distribution pattern (Fig. 5). The physical dimensions of the Joseph Creek extended through this study and permission to work on their property; feeder-dike system, although not precisely known, would appear to be of M. H. Beeson for many stimulating discussions concerning factors sufficient size to have been capable of erupting lava at rates required for controlling the emplacement of flood-basalt flows; and Th. Thordarson rapid emplacement. We have not observed any intraflow structures within and S. Self for providing a partial preprint of their paper on the Laki Teepee Butte flows (for example, compound flows, lava tubes) that would eruption. be indicative of low-discharge/longer-duration eruptions. Unfortunately it We gratefully acknowledge the constructive reviews of this paper by is not possible to directly assess all of the factors (for example, location and V. E. Camp, K. Hon, D. A. Swanson, P. R. Hooper, J. E. Schuster, and length of active fissures, rate of lava discharge) bearing on the length of M. E. Ross, but the interpretations presented here are solely the time it actually took to emplace Teepee Butte flows. The important con- responsibility of the authors. Support for this study was provided by U.S. clusion is that these eruptions were orders of magnitude larger than have Department of Energy, Office of Basic Energy Sciences grant number ever been observed. With our present knowledge, we conclude that a rapid DE-FG06-91ER14172 to S. P. Reidel at Washington State University. emplacement time, on the order of days to weeks as originally suggested Support of this study does not necessarily constitute an endorsement by by Shaw and Swanson (1970), for large-volume flow is still a reasonable U.S. Department of Energy of the views expressed in this article. first approximation, but longer times (a month or more) remain a realistic possibility.

SUMMARY AND CONCLUSIONS REFERENCES CITED Allègre, C. J., and Minster, J. F,, 1978, Quantitive models of trace element behavior in magmattc processes: Earth and Planetary Science Letters, v. 38, p. 1-25. The Teepee Butte Member and Joseph Creek vent system provide Anderson, J. L., 1987, Structural geology and ages of deformation of a portion of the southwest Columbia Plateau, Washington and Oregon [Ph.D. thesis]: Los Angeles, California, University of Southern California, 283 p. new insights into the nature of flood-basalt volcanism and the manner in Beeson, M. H., and Tolan, T. L., 1990, The Columbia River Basalt Group in the Cascade Range; a middle Miocene which large-volume flows were erupted and emplaced. Specific to this reference datum for structural analysis: Journal of Geophysical Research, v. 95, no. B12, p. 19,547-19,559. Beeson, M. H., Perttu, R., and Perttu, J., 1979, The origin of the Miocene basalts of coastal Oregon and Washington; an study, we conclude the following. alternative hypothesis: Oregon Geology, v. 41, no. 10, p. 159-166. Beeson, M. H., Tolan, T. L., and Anderson, J. L., 1989, The Columbia River Basalt Group in western Oregon; geologic 1. Basalt flows of the Teepee Butte Member are mostly large-volume structures and other factors that controlled flow emplacement patterns, in Reidel, S. P., and Hooper, P. R., eds., 3 Volcanism and tectonism in the Columbia River flood-basalt province: Boulder, Colorado, Geological Society of sheet flows (>1,000 km ) that erupted from long, linear, fissure systems America Special Paper 239, p. 223-246. and flowed distances greater than 300 km. The flows have as many as five Carlson, R. W., 1984, Isotopie constraints on Columbia River flood-basalt genesis and the nature of the subcontinental mantle: Geochimica et Cosmochimica Acta, v. 48, p. 2357-2372. locally developed flow units that require careful study to distinguish the Decker, R. W., 1987, Dynamics of Hawaiian volcanoes, an overview, in Decker, R. W., Wright, T. L., and Stauffer, P. H., products from individual eruptions. eds., Volcanism in Hawaii: U.S. Geological Survey Professional Paper 1350, Volume 2, p. 997-1018. DeVaney.P.T., andPoUard, D. D., 19S2, Solidification of basaltic magma during flow in a dike: American journal of 2. The distribution of the flows was controlled primarily by a west- Science, v. 282, p. 856-885. Fecht, K. R,, Reidel, S. P., and Tallman, A. M . 1987, Paleodrainage of the Columbia River system on the Columbia ward paleoslope and the length of the fissure system and was modified by Plateau of Washington State; a summary, in Schuster, J. E., ed., Selected papers on the geology of Washington: Washington Division of Geology and Earth Resources Bulletin 77, p. 219-248. topography. Hooper, P. R., and Camp, V. E., 1981, Deformation of the southeast part of the Columbia Plateau: Geology, v. 9, 3. Teepee Butte Member flows are relatively homogeneous and strat- p. 323-328. Hooper, P. R., and Reidel, S. P., 1989, Dikes and vents feeding the Columbia River basalts, in Joseph, N. L., and others, igraphically distinct, with very little compositional variation along the eds., Geologic guidebook for Washington and adjacent areas: Washington Division of Geology and Earth Re- sources Information Circular 86, p. 255-273. length of the flows. Dike selvage zones show significantly evolved compo- LeMaitre, R. W., 1981, GENMIX; a generalized petrologie mixing model program: Computing Geoscience, v. 7, sitions relative to the flow but are of insignificant volume. p. 229-247. Macdonald, G. A., 1972, Volcanoes: Englewood Cliffs, New Jersey, Prentice-Hall, 510 p. 4. Vent deposits at Joseph Creek are characteristic of Hawaiian-type Malin, M. C„ 1980, Lengths of Hawaiian lava flows: Geology, v. 8, p. 306-308.

1656 Geological Society of America Bulletin, December 1992

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P., Fecht, K. R., Hagood, M. C„ and Tolan, T. L., 1989a, The geologic evolution of the central Columbia of America Special Paper 239, p. 1-20. Plateau, in Reidel, S. P., and Hooper, P. R., eds., Volcanism and tectonism in the Columbia River flood-basalt Walker, G.P.L., 1971, Compound and simple lava flows, and flood basalts: Bulletin Volcanologique, v. 35, p. 579- 590. province: Boulder, Colorado, Geological Society of America Special Paper 239, p. 247-264. Walker, G.P.L., 1973, Lengths of lava flows: Royal Society of London Philosophical Transactions, ser. A, v. 274, Reidel, S. P., Tolan, T. L., Hooper, P. R., Beeson, M. H„ Fecht, K. R., Bentley, R. D., and Anderson, J. L„ 1989b, The p. 107-118. Grande Ronde Basalt, Columbia River Basalt Group; stratigraphic descriptions and correlations in Washington, Wentworth, C. K., and Macdonald, G. A., 1953, Structures and forms of basaltic rocks in Hawaii: U.S. Geological Survey Oregon, and Idaho, in Reidel, S. P., and Hooper, P. 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A., 1989, Chemical data for flows and feeder dikes of the Yakima Basalt ern Columbia Plateau: Geological Society of America Bulletin, v. 94, p. 1117-1126. Subgroup, Columbia River Basalt Group, Washington, Oregon, and Idaho, and their bearing on a petrogenetic Rowland, S. K., and Walker, G.P.L., 1990, Pahoehoe and aa in Hawaii; volumetric flow rate and controls on the lava model: U.S. Geological Survey Bulletin 1821,71 p. structure: Bulletin of Volcanology, v. 52, p. 615-628. Shaw, H. R., and Swanson, D. A., 1970, Eruption and flow rates of flood basalt, in Gilmore, E. H„ and Stradling, D. F., eds., Second Columbia River Basalt Symposium, Proceedings: Cheney, Washington, Eastern Washington State College Press, p. 271-299.

Swanson, D. A., 1972, Magma supply rate at Kilauea Volcano. 1952-1971: Science, v. 175, p. 169-170. MANUSCRIPT RECEIVED BY THE SOCIETY DECEMBER 4,1991

Swanson, D. A„ 1973, Pahoehoe flows from the 1969 1971 Mauna Ulu eruption, Kilauea Volcano, Hawaii: Geological REVISED MANUSCRIPT RECEIVED MAY 19,1992

Society of America Bulletin, v. 84, p. 615-626. MANUSCRIPT ACCEPTED MAY 26,1992

Printed in U.S.A.

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