The Geologic History of the Diamond Lake Area



The oldest rocks exposed in the Diamond Lake area represent the Eocene, Oligocene, and Miocene Epochs. Therefore, they range from about 60 million to 12 million years in age. These rocks are exposed several miles to the west and northwest of Diamond Lake. More specifically, they make up Trap Mountain, Beartrap Meadows, Black Rock, Rhododendron Ridge, and the intervening canyons and hills to the west of Diamond Lake; and Birds Point, Potter Mountain, Thorn Mountain, Perry Butte, and the intervening country to the northwest of Diamond Lake. These lavas extend many miles to the west of the Diamond Lake area and make up the Western Cascades.

A glance at a topographic map of the Diamond Lake area reveals a decided change in the configuration of the land surface across the boundary between the Western Cascades and the High Cascades. The High Cascades consist of altered conical forms of obvious volcanic origin in various stages of erosion, whereas the Western Cascades consist of an area of irregular ridges and canyons which have been aptly described as a "sea" of peaks and ridges. These ridges and canyons bear no relationship to the original volcanic forms; that is, what is now a topographically high ridge initially might have been a low area between surrounding volcanic peaks. Conversely, what originally might have been a high volcanic peak now might be a deeply eroded canyon. The reason for the marked difference between High Cascade and Western Cascade topography is the greater length of time which has ensued since the formation of the rocks making up the Western Cascades. Erosion has proceeded to wear down these rocks from the instant they were poured out on the surface. During the past 60 million to 12 million years the original volcanic forms have been destroyed, and a more-or-less random network of streams now drains the area. The resulting topographic pattern is termed "mature".

The general nature and composition of the igneous rocks making up the Western Cascades have been described in an earlier section of this report. Suffice it to say here that these rocks, compared with the lavas of the High Cascades, present a more weathered appearance. They are more limonite-stained, more thoroughly altered mineralogically, and contain intrusions of coarser-grained igneous rocks that are absent in the lavas of the High Cascades.

As explained earlier, the Western Cascade lavas underlie the High Cascade lavas in the Diamond Lake area. They simply are overlapped and obscured by the younger High Cascade lavas which, having risen through fractures in the Western Cascade lavas, proceeded to bury the older rocks with fresh flows of molten lava.

The crustal disturbance at the close of the Miocene Epoch, some 12 million years ago, folded and fractured the Western Cascade lava platform. A series of north-south fractures developed along the eastern margin of the Cascade plateau and lava began to well up at various points along these fractures and build new volcanoes. The spacing of the new volcanoes indicates the fracturing of the old lavas did not occur as a single fissure. Mt. Bailey and Mt. Thielsen, for example, which are both High Cascade volcanoes, lie on an east-west line. Viewed in a broader perspective, these two peaks form a portion of a mountain chain that is only a few miles wide, but several hundred miles long. Mt. Bailey and Mt. Thielsen probably grew along separate, but parallel, north-south fractures. Three miles to the north of Mt. Thielsen lie Howlock Mtn. and Tipsoo Peak. Two miles north of Mt. Bailey lie Rodley Butte and another, smaller cinder cone. The north-south lineation of the volcanoes thus is revealed on a small scale with in the larger framework of the Cascade Range.

The fractures in the Diamond Lake area reached deep enough into the earth's crust to tap rocks hot enough to melt if they were not under such great pressure. The fractures reduced this pressure and the rock fused, then began to work its way upward along the fractures. The first eruptions of this lava began at Pig Iron Mountain and Watson Butte, the oldest High Cascade volcanoes in the Diamond Lake area.

The relative ages of the High Cascade volcanoes are established on the basis of the degree to which they have been eroded and glaciated, and where possible to observe, the relationship of their lava flows. The mountain mass consisting of Pig Iron Mountain, Watson Butte and Elephant Mountain is entirely surrounded by the younger intracanyon basalt which obscures the contacts between lavas of those volcanoes and any others. However, the degree of erosion of those mountains suggests they are the oldest of the High Cascade volcanoes. It would be difficult to determine, from topographic form alone, whether or not these masses were volcanoes. However, examination of the rocks reveals a vent existed near the summit of Pig Iron Mountain; hence the mass is included in the High Cascades rather than the older Western Cascades. The High Cascades, which are less than 12 million years old, have retained a more-or-less conical shape despite subsequent erosion.

Petrified — carbonized tree trunk near Loafer Creek.

An interesting feature is exposed on Forest Service Road No. 264.1 about 2-1/2 miles northeast of Toketee Reservoir. This site is on the northern edge of the Pig Iron Mountain—Watson Butte mass, near the junction of Loafer Creek and the North Umpqua River. At this point the road cut exposes a tree trunk whose outer shell is petrified and whose center is carbonized. The trunk measures 3 to 4 feet in diameter and is about 8 feet high. The root system extends another 6 feet below the base of the trunk and appears to be naturally emplaced in a weathered "tuff". Tuff is a rock composed of fragments of volcanic rock. The trunk itself is surrounded by a pale green lava flow containing white feldspar crystals. The rock layer above the truncated tree is an other tuff bed. Scattered through the lava layer surrounding the trunk are carbonized limbs up to 1 foot in diameter and smaller fragments. The majority of these are scattered down-flow from the trunk. The sequence of events leading to the preservation of such a feature must have been as follows: The tree initially was growing on a thin soil zone covering a layer of tuff. Its roots pierced cracks in the rock to a depth of 6 feet or more below the surface. The tree then was engulfed by a lava flow which caused it to break about 8 feet above the ground. Limbs and twigs fell into the flow and were swept down hill by it. The flow was hot enough to sear and carbonize the entire trunk and those limbs that were engulfed by the flow. The tuff bed below the flow was baked and the roots piercing the tuff also were carbonized. The mass cooled and was later covered by another tuff layer. The tree and branches at this time probably were carbonized completely.

Later, silica-bearing solutions worked their way up the contact between the carbonized wood and the lava and began replacing the carbonized wood with silica. This process continued until only the central core of the tree remained carbonized. The result is a petrified silica sheath surrounding the carbonized core. The wood texture is well preserved in both the carbonized and silicified portions of the tree. The lava rock immediately adjacent to the silicified trunk contains minute flecks of pyrite, an iron sulfide mineral. Pyrite commonly forms in the presence of carbon, which reduces the ubiquitous iron to the sulfide form. This is an indication that the outer portion of the tree (now silicified) originally was carbonized, as the center is still.

The limbs and branches which are engulfed entirely by lava are carbonized, not silicified. These fragments are not silicified because they are not in contact with the more porous tuff beds, as is the main trunk. Mineralizing solutions were provided access to the main trunk through the porous tuff beds into which the root system of the tree extended.

It is believed the tuffs and lava encasing the tree are associated with lavas and tuffs of the Pig Iron Mountain and Watson Butte volcanoes, as they are totally unlike the dense, grey, olivine basalts which make up the intracanyon flows. This means the tree is between 12 million and 1 million years old—probably closer to the former.

<<< Previous <<< Contents>>> Next >>>

Last Updated: 01-Jul-2008