Geology | Introduction | Oldest Rocks | Western Plutonic Belt | Igneous Rocks | Eastern Plutonic Zone
The Oldest Rocks
The oldest rocks in San Diego County are a series of scattered exposures of metamorphosed sedimentary rocks called the Julian Schist. Named after the historic community in east San Diego County, these rocks represent the sediments that accumulated in the shallow seas off the coast of North America nearly 200 million years ago. This thick blanket of sediments extended miles off-shore and wrapped eastward around the then edge of the continent into what is now central Mexico. The sediments have since undergone extensive uplift and erosion so that they now exist only as the scattered exposures depicted in the figure below.
The story of the sediments begins in the shallow seas adjacent to the North American continent. There, muds and sands derived from the Precambrian rocks that now underlie western North America and northern Mexico began to accumulate in layers. As sea level in the ancient Pacific Ocean rose and fell, alternating layers of mud and sand began to form in deeper waters. These sediments accumulated into greater and greater thicknesses until the weight of the overlying material compressed the base of the sediment pile. Remnant ocean waters were squeezed out, grains were compacted, and sedimentary rocks such as mudstone and sandstone were created.
Further to the east, near the San Diego-Imperial County line, massive bodies of white marble are associated with lesser amounts of quartzite and mica schist. These marbles were once ancient limestones that formed in warm, shallow seas adjacent to North America. They are similar to rocks found elsewhere in southern and Baja California that contain an Ordovician fossil assemblage. Although these marbles are generally grouped together with the Julian Schist, they may be much older and may represent a package of sedimentary rocks transported into position by as-yet unrecognized faults.
This realignment occurred at depths of about 8 miles (13 kilometers) and at temperatures approaching 600°C (1100°F). This information, gleaned from many decades of laboratory studies on the behavior of minerals under varying pressures (depth) and temperatures, indicates that these rocks have undergone over 9 miles of uplift to reach their present elevation of nearly 6000 feet above sea level.
These granitic gneisses also contain very small amounts of the mineral zircon which crystallized from the original granite magma. Although zircon has a rather simple chemical formula (ZrSiO4), it commonly contains trace amounts of the radioactive element uranium (U). This element decays at a known rate to a series of secondary elements such as lead (Pb) whose abundance can be measured and then used to calculate the age at which the original granite crystallized.
Several measurements of the secondary elements, made from samples collected along Sunrise Highway, indicate that most of the original granites were solidified during the Jurassic Period between about 160 and 170 million years ago. Geologists interpret this to indicate that subduction of an oceanic crustal plate had created a volcanic arc and its underlying plutonic basement adjacent to the western edge of the North American continent. Remnants of this volcanic-plutonic arc can also be found in the western Sierra Nevada foothills and in smaller mountain ranges scattered throughout the Mojave Desert. The recrystallized and aligned minerals in the rocks in central San Diego County suggests that these granites contain the record of another, more recent geologic event that transformed them from normal granites into granitic gneisses.