In Coe Park, the Franciscan Complex has been divided into two distinct units, or terranes. A terrane is a fault-bounded group of rocks that differs from surrounding rocks in composition and age and has had a different history of metamorphism and faulting. The two terranes are called Eylar Mountain and Burnt Hills, for areas north of the park where they were first described. A peculiar mixture of rocks called melange is also present as thin zones within the two terranes and as a broad zone separating them.
Eylar Mountain Terrane
The Eylar Mountain terrane is composed of resistant metagraywacke, slaty mudstone, and minor conglomerate, deposited on chert and greenstone. Much of the metagraywacke was probably derived from the ancestral Sierras, which were then a volcanic highland. The rivers eroding the Sierras carried the material out into the ocean and deposited it on the ocean floor. (Present-day Chile, with its Andes Mountains and offshore trench, is analogous to the ancient Sierras and their trench, where the Franciscan Complex was deposited.)
The landscape of the Eylar Mountain terrane is usually characterized by sharp ridges with steep canyons, typical of the area around park headquarters and the high country south of Mt. Stakes. The terrane supports a variety of trees, including Ponderosa pine at higher elevations.
Lying below the Eylar Mountain terrane is a broad zone of melange, characterized by a pervasively sheared siltstone matrix enclosing rounded knockers of metagraywacke, chert, and greenstone, as well as minor blueschist. The rocks of the melange were sheared, churned, and pulverized during a profound faulting event that occurred when the ocean floor and the rocks deposited on top of it were mixed together. As a result of the faulting, rocks that were once deeply buried (in excess of twenty miles) are found next to rocks that were not buried.
The park's melange has the distinction of containing some of the biggest and most spectacular blueschist knockers known anywhere in the Coast Ranges. Most of these are found in a zone that trends northwesterly from near the Dowdy Ranch through the confluence of Mississippi and Pacheco Creeks, to Pacheco Camp and beyond. Study of the minerals in these blueschists indicates that they formed at temperatures of about 500 degrees C. and at pressures greater than 10 Kb (which equals depths of at least twenty miles). While there is no problem in dragging rocks down to such depths in a subduction zone, there is a big problem in explaining how they were brought back to the surface. Radiometric dating of minerals from these blueschist knockers gives ages of about 160 million years-much older than the surrounding matrix of the melange.
In the field, the melange can be identified not only by its low, rolling topography but also by its extensive grasslands interspersed with oaks. It appears that the melange matrix, because of abundant clays, is so impermeable that only grasses do very well on it.
Burnt Hills Terrane
The Burnt Hills terrane is the youngest Franciscan unit found anywhere in the Diablo Range. It is characterized by thin, gray sandstone layers (usually one to three inches thick) interbedded with darker mudstone (shale). Excellent outcrops of these rocks can be seen along the old jeep trail that extends from the ridge top west of Coit Lake down into Kelly Cabin Canyon. Along this road numerous fossils have been found consisting of clams and ammonites of the Upper Cretaceous Age (about 80 million years ago).
The Burnt Hills terrane is often covered with chamise, buckthorn, and other members of the chaparral community rather than the trees seen on the Eylar Mountain terrane. Sandstone outcrops are often covered with a bright orange lichen.
The presence of abundant grains of granite in the sandstone, rather than the volcanic material of the Eylar Mountain terrane, suggests that by the time the Burnt Hills terrane was being deposited, the ancestral Sierras had been unroofed and their granite, which was below the volcanic material, was being eroded into the trench.
Occasionally, sandstone members with thick beds (one to ten feet) are found, principally along the South Fork of Orestimba Creek and north of the Rooster Comb. These were apparently deposited nearer to the ancient shoreline than the thinner-bedded sandstones seen to the west.
The Rooster Comb deserves more description. This spectacular feature is the result of a thick (100-foot) layer of radiolarian chert that has been tilted up to the vertical by intense folding and faulting. Numerous radiolarians have been extracted from this chert and give ages that are slightly older than the fossils found in the sandstones of the Burnt Hills terrane. The Rooster Comb chert layer can be followed as far west as just north of Paradise Lake. It reappears north of Robison Creek and extends north from there through Hat Spring, all along the east side of Red Creek.