The Precambrian era spans from the formation of the Earth, 4.6 billion years ago, to the Cambrian Explosion 541 million years ago. Precambrian is a term used to describe time between the Earth’s creation and the Cambrian Explosion (542 mya). It contains both the Archaean and Proterozoic Eons. During most of the Archaean Eon, the Earth had a turbulent climate involving tectonic movements, volcanic activity, and heavy and enduring rainfall. It was at this time that the oceans and land masses emerged, as well as an atmosphere containing oxygen. The first life forms were prokaryotes, or single celled bacteria. How these pioneer organisms appeared is still uncertain in the scientific community. During the Proterozoic Eon, eukaryotic cells developed, which contained symbolic organelles contained within the cell and controlled by a nucleus. Additionally, soft-bodied invertebrates developed, as well as Blue Green Algae, which is considered to be the first example of a multi-celled organism. Blue Green Algae existed in large colonies that encompassed most the planet’s oceans.
California During the Precambrian Era:
Geologists know little about this time because the majority of these rocks have been heavily metamorphosed and subject to extensive heat and pressure, altering their state. California was completely underwater as the coastline was still east of it. The Golden State’s oldest rocks are over a billion years old and lie within the eastern deserts and eastern Transverse Ranges. The west coast of the continent was to the east of most of the rest of the state. Geologists infer this from the distribution of these Precambrian rocks.
The Paleozoic Era saw some of the greatest advanced in biological development. The Cambrian Explosion was arguably the greatest diversification of flora and fauna in history. New features such as eyes, the hard shells associated with arthropods, and faster modes of movement similar to fins evolved and were seen in marine life (Anomalocaris, trilobites, Opabinia). Over the course of several periods, the Kingdom Animalia and Plantae diversified greatly (development of fish, vascular plants, amphibians, reptiles, trees, insects, flying animals, etc). Other characteristic changes aside from the Cambrian Explosions include the atmospheric changes of the Carboniferous Period and the Permian Extinction. During the Carboniferous Period, the oxygen levels in the Earth’s atmosphere significantly increased as a result of accelerated plant growth, thus causing insects to adapt with larger body sizes, and more swamp-like biomes to emerge. During the Permian Period, there was a widespread and enduring drought which caused the extinction of roughly 95% of marine species. This mass extinction marked the end of the Paleozoic Era.
California During the Paleozoic Era:
The Antler Orogeny in Nevada occurred about 400 million years ago. An orogeny is when part of the earth’s crust is compressed, causing a mountain range to form. In this case, an island arc rose off shore. Island arc collisions caused by this orogeny continued until 215 million years ago. However, although this theory is widely accepted, there remains alternate hypotheses. Here is one from the USGS. The Sonoma Orogeny starting 215 million years ago formed another island arc whose rocks are now contained in the Sierra Nevada and parts of the Klamath Mountains.
The plate to the left describes the Antler and Sonoma Orogeny events.
The Mesozoic Era is often characterized as the ‘Age of Reptiles’, as most orders of Dinosaurs evolved during this era and constituted most of the apex predators within the planet’s biomes. During the Triassic Period, the first mammal-like reptiles appeared, indicating the development of mammalian features such as fur and the use of breast milk. Many climates were still very arid and hot. Earth’s first bird (Archaeopteryx) evolved during the Jurassic Period and had both reptilian and avian traits. Flowers emerged during the Cretaceous Period, as well as one of the most famous dinosaurs, Tyrannosaurus Rex. The Cretaceous-Paleogene Extinction event, or KT extinction, occurred about 65 million years ago. There are multiple theories surrounding this event that eliminated approximately 80% of the world’s animal life. However, the asteroid theory is the most widely accepted by the scientific community. The theory suggests that an asteroid collided with the planet (on Mexico) resulting in an impact that triggered multiple planet-wide climate changes including tidal waves and a global dust cloud that blocked out sunlight and killed plant life.
California During the Mesozoic Era:
During the Mesozoic, California split further apart from Europe and Pangaea. 210 million years ago marked the Nevadan orogeny, or the uplift of the ancestral Nevadan mountains. This orogeny was caused by the formation of the large section of granitic rocks that are now comprising of most of the Sierra Nevada. By 150 mya (million years ago) the coastline was around the edge of the present day Central Valley and was composed of the Nevadan mountains caused by this uplifting. Intrusive granitic rocks continued to form near the Sierras until about 100 mya.
The Cenozoic Era is the one that we are currently living in, and is considered the ‘Age of Mammals’, as mammals generally came to take the place that dinosaurs held during the Mesozoic Era. Tectonic movements shifted landmasses closer to where they lie today. The Paleogene Period saw the diversification of mammals and plant life similar to modern flora. Neogene Period fossils are very common in California, especially at beaches. This is because the sediments were the most recently deposited so the strata is closer to the surface and easier to find. The Pleistocene Epoch is characterized by the Ice Age, which swept Southward from the Arctic circle and caused a shift in North American, European, and Asiatic life. Many of the species that evolved during this time (2mya-0.01mya) still exist today. Currently we are in the Holocene epoch, which many fear may cause another mass extinction like the Permian or KT extinction. This 6th extinction would be caused by dramatic climate change due to human pollution.
California During the Cenozoic Era:
By the Eocene epoch, large rivers and tributaries flowed through the Western United States. Additionally, the Nevadan mountains that were uplifted in the Mesozoic were gradually reduced to small hills. The San Andreas fault was formed by the contact between the Pacific and North American plates around 30 mya. The below diagram represents the San Andreas Fault.
Around 20 mya, volcanic activity emerged in the Sierra Nevada and Mojave regions. Deep sea basins collected along the present day coast between San Francisco and Orange County in 10 mya. For example, the Monterey Formation is consists of one of these basins. Fossil sites containing Monterey Formation sediments include Gaviota, El Capitan, and Jalama Beach. The rocks in these basins are primarily composed of marine organisms instead of non-aquatic sediments. Around 5 mya, orogenies accelerating, uplifting present day mountain ranges including the Sierras, Transverse Ranges, and Coastal Ranges. Subduction in Northern California formed the volcanoes in Cascades.
“California Geologic History,” (Colorado University) Powerpoint
“A Brief Geologic History of California,” (Sonoma University, July 23, 2003) http://www.sonoma.edu