DETAILED MAPPING AND OBSERVATIONS OF A NEW SECTION OF THE SKOOKUM GULCH SCHIST AND TRINITY OPHIOLITE, EASTERN KLAMATH MOUNTAINS, CALIFORNIA

The Yreka Terrane and the underlying Trinity Ophiolite are Neoproterozoic and Paleozoic tectonic elements exposed in the Eastern Klamath Mountains of California. The two structurally lowest units of the Yreka Terrane, the Gregg Ranch Complex and the overlying Skookum Gulch Schist, are well exposed along the drainages surrounding McConaughy Creek, just east of Etna, California.

In the field, the Skookum Gulch Schist appears as a mélange, with blocks, some of which are blueschist-facies metabasalts, enclosed in a variable matrix of quartz-albite schist, greenschist, chlorite schist, chert, and marble. There has been some disagreement as to the relationship between the blocks and matrix, with some interpreting the mélange texture as due to tectonic juxtaposition, while others interpret it as a deformed sequence of otherwise coherent sedimentary rocks. The unit is particularly notable because the Ordovician blueschist-facies metabasalt and Precambrian tonalite date to before convergent tectonics began along the Western North American margin and the age sedimentary zircons may not be compatible with a North American origin.

We present new mapping as well as structural and petrological observations from a previously undescribed section in McConaughy Creek of the Skookum Gulch Schist and Gregg Ranch Complex. Roadcut exposures show gradational contacts between metabasalt, calcschist, and marble, indicating that the apparent block-in-matrix pattern visible in previously reported exposures is the result of dismemberment of a coherent sedimentary sequence by folding and faulting. In addition, passage between blueschist-facies and greenschist-facies metabasalt can be seen in outcrop, indicating compositional control of the preserved mineral assemblage.

In the same area, rocks previously mapped as Gregg Ranch Complex are exposed. However, additional mapping in this section has identified carbonate, conglomerate, gabbro, tonalite, and serpentinized peridotite, which are identical to the structurally highest part of the Trinity Ophiolite exposed at Lover’s Leap to the southeast. This observation indicates that the Trinity Ophiolite likely extends westward some distance beneath the Yreka Terrane.