DESCRIPTION:
Fort Rock Basin, Oregon
Christmas Lake Valley Basin, Oregon

The Fort Rock - Christmas Lake Valley basin is a former lake basin that existed from late Pliocene through late Pleistocene time. The basin is about 64 kilometers long and 40 kilometers wide. Eruptions of basaltic magma occurred along faults that trend diagonally across the basin and adjacent highland, forming maar volcanoes within and on lake margins and forming cinder cones with flows beyond the lake margins (Peterson adn Groh, 1963; Heiken, 1971).

Nearly 40 maars, tuff rings, and tuff cones of Pliocene and Pleistocene age occur in the Fort Rock Basin of south-central Oregon. Most are significantly eroded, allowing excellent exposures of their lithology, bedding, and sedimentary structures; a few retain much of their oringial morphology. The Fort Rock Basin is dry, internally drained, and largely filled with lacustrine sediments which accumulated during the episodic existence of pluvial Fort Rock Lake. This area lies within the extensional environment of the Basin and Range Province and is characterized by numerous normal faults of Pliocene and Pleistocene age that cut volcanic rocks of similar age.

Maar volcanoes are low volcanic cones with broad, bowl-shaped crater. Three general kinds are well-represented in the Fort Rock Basin:

• maar, with a crater floor below original ground level, such as Hole-in-the-Ground;
• tuff ring, with a crater floor at or above original ground level, such as Fort Rock; and
• tuff cone, which is a tall tuff ring, such as Table Rock.

The maar volcanoes of the Fort Rock Basin are the result of the explosive interaction of rising basaltic magma and abundant surface or groundwater. Beyond the basin where surface or shallow groundwater was not available, eruptions produced cinder cones and lava flows. But where water was present, deposits ranging from explosion breccia to thinly bedded layers and massive beds can be seen. ...

The western edge of the maar field is readily accessible from Highway 31, which skirts Moffitt Butte and Big Hole, as well as other freatures. Roads leading east from Highway 31 pass near both Fort Rock and Table Rock.

Big Hole, a circular, 1,820 meter diameter maar crater. Deposits of the tuff ring are 24 to 30 meters thick at the crest and extend 1,800 to 2,500 meters beyond the crater rim. The deposits are thickest on the northeast side, along Highway 31 (Big Hole Butte).

Fort Rock is an isolated tuff ring with spectacular, wave-cut cluffs and terraces. The wave-cut remnant is approximately 1,400 meters in diameter and 60 meters high, and the present crater floor is 6 to 12 meters above the floor of the lake basin. The south rim has been breached by waves of the former Fort Rock Lake providing easy access to the crater. The best developed wave-cut terrace is 20 meters above the floor of the basin.

Fort Rock, with its spectacular wave-cut cliffs, is an isolated maar volcano within a monotonous, flat lake basin (Peterson and Groh, 1963). The wave-cut remnant is 1,360 meters in diameter and 60 meters high, and the present crater floor is 6 to 12 meters above the floor of the lake basin. The south rim has been breached by waves of the former lake, providing easy access to the crater. A wave-cut terrace occurs 20 meters above the floor of Fort Rock Valley.

The maar is composed of orange-brown lapilli-tuff in beds of 1 centimeter to 1 meter thick that can be traced from within the crater to the outer flanks. Gracded beds with accretionary lapilli are common. ...

Fort Rock is typical of most of the smaller, isolated maars of the Fort Rock-Christmas Lake Valley basin, such as Table Mountain, Flat Top, Lost Forest, Green Mountain SW, and Green Mountain S.

Hole-in-the-Ground is a nearly circular maar with a floor 150 meters below and a rim 35 to 65 meters above original ground level. Its diameter from rim to rim is 1,600 meters. The volume of the crater below the original surface is only 60 percent of the volume of ejecta. Only 10 percent of the ejecta is juvenile basaltic material. Most of the ejected material is fine grained, but some of the blocks of older rocks reached dimensions of 8 meters. The largest blocks were hurled distances of up to 3.7 kilometers from the center of the crater. Accretionary lapilli, impact sags, and vesiculated tuffs are well developed.

According to Lorenz (1971):

Hole-in-the-Ground is a volcanic explosion crater or maar located in Central Oregon on the edge of Fort Rock basin. At the time the crater was formed between 13,500 and 18,000 years ago a lake occupied most of the basin and the site of the eruption was close to the water level near the shore. The create is now 112 to 156 meters below the original ground level and is surrounded by a rim that rises another 35 to 65 meters higher. ...

The crater was formed in a few days or weeks by a series of explosions that were triggered when basaltic magma rose along a north-west-trending fissure and came into contact with abudnant ground water at a depth of 300 to 500 meters below the surface. After the initial explosion, repeated slumping and subsidence along a ring-fault let to intermittent closures of the vent, changes in the supply of ground water, and repeated accumulations of pressure in the pipe.

Moffitt Butte is a dissected tuff ring, 1,400 meters in diameter and 120 meters high. It is a prominent topographic feature, but is obscured by the forest from the road. Although not associated with a lake basin, as is the case for Big Hole and Hole-in-the-Ground, Moffitt Butte is a tuff ring composed of hyaloclastic tuffs. Rising magma may have encounted permeable aquifers beneath the cone. A line of tuff rings between here and Fort Rock Basin are along a topographic low between Fort Rock and the La Pine Basins.

The crater floor of Moffitt Butte is about 80 meters above the surrounding plain. A parasitic vent, and small tuff ring, 510 meters in diameter, is located on its southwestern flank ... The crater of the parasite vent is filled with lava that issured from a dike on its northwest edge.

The Table Rock maar complex consists of Table Rock (a tuff cone), 2 large tuff rings, and 6 smaller tuff rings and eroded vents. The scomplex forms a north-northwest-elongated oval 5.6 x 8.8 kilometers. The highest point is the top of Table Rock, 395 meters above the basin floor. The complex overlies a 220-meter-thick section of lake sediments, interbedded tuffs, and sands and gravel. ...

Table Rock is a symmetrical tuff cone, approximately 1,500 meters in diameter at its base, tapering to a diameter of approximately 360 meters. The csone is capped with a basalt lava lake now partially eroded. Dikes extend north-northwest and south-southeast of the crater lake, parallel to the long axis of the tuff ring complex. The lava lake and dikes suggest that before eruptions stopped, access of ground or surface water to the magma ceased.

Table Rock is a tuff cone located 14.5 kilometers east of the village of Silver Lake, on the shore of Silver Lake, one of the few remnants of a much larger Pleistocene lake that once filled the Fort Rock-Christmas Lake Valley Basin. The Table Rock tuff cone is part of a maar complex consisting of the cone, two large tuff rings, and six smaller tuff rings or eroded vents. This complex forms an elongate, north-northwest-trending oval 5.6 by 8.8 kilometers. The highest point, about 395 meters above the basin floor, is the crest of Table Rock. ...

Table Rock is an erosional remnant of a tuff cone constructed above lake level on the southern rim of vent 2 (of the Table Rock Complex). ... It is a symmetrical cone about 1,530 meters in diameter at the base, tapering to a diameter of about 360 meters at a height of 360 meters above the surrounding plain. The cone is capped with flat-lying basalt which once filled the crater, but erosion has modified the original cone, exposing the once-ponded basalt lava lake. Dikes extend north and south-southeast of the creater lake, parallel to the long axis of the tuff ring complex. ...

Why is the shape of Table Rock tuff cone different from the broad, low maars of most of the complex? A possible answer is the depth of explosive steam generation where ascending magma interacted with water. The broad, low tuff rings may have resulted from shallow phreatomagmatic explosions where magma nearly reached lake level. Shallow steam explosions would likely produce broad explosion craters, low fragment trajectories and base surges resulting in broad, low rim deposits of the tuff rings. The base of the Table Rock tuff cone is, however, located on the rim of vent 2 above former lake level. About 210 meters below the cone base is a highly permeable aquifer within basalt flows which lies beneath essentially impermeable diatomite beds. In contrast with shallow explosions, phreatomagmatic explosions within the aquifer *may* have been guided by the conduit, resulting in higher angle trajectories and a higer, steep-sided cone.

There is an excellent view of the entire complex and the Fort Rock-Christmas Lake Valley basin from the top of Table Rock.

Connley Hills

A line of domes of intermediate composition. Remnants of a small maar are plastered onto the northernmost dome, near the former lake shoreline. South of the Connley Hills is Hayes Butte, a basaltic shield. These volcanoes stood as an island in the lake that was present here during late Pliocene-Pleistocene time.

Hager Mountain

A 5.9 million-year-old silicic dome on the southwestern rim of the Fort Rock-Christmas Lake Valley Basin (McLeod et.al., 1976)

Hayes Butte

South of the Connley Hills is Hayes Butte, a basaltic shield. These volcanoes stood as an island in the lake that was present here during late Pliocene-Pleistocene time.

Seven-Mile Ridge

A northwest-trending group of five operlapping maars. The complex is 12 kilometers long and 3.2 to 4.8 kilometers wide. The best preserved maars are at the basin edge above the former lake level; they drape over a 61 meter high fault scarp. The two northernmost maars have been eroded by wave action to flat-topped mesas, 9 to 18 meters high.

Silver Lake Graben

The Silver Lake graben is the keystone of a broad arch broken by normal faults; Pliocene age basalt flows exposed in this arch slope northward into and under the lake sediments of the Fort Rock-Christmas Lake Valley Basin. ...

Silver Lake graben, located immediately south of the Table Rock tuff-ring complex, is the keystone of a broad 25-kilometer-wide structural arch that forms the southwest boundary of the lake basin. The normal fault defining the east wall of the graben (through which Highway 32 continues over Picture Rock Pass) is parallel to the long axis of the maar complex and possibly lies beneath it. The fault, however, does not cut any of the tuff deposits.

Return to:
[Fort Rock and Vicinity Menu] ...