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Geologic history of the northern Sierra Nevada
The Laramide Orogeny and uplift of the Sierras
A major consequence is that lithosphere throughout the Far West is "rootless" because deep subcrustal rocks are thought to have been scraped off by the shallow flat slab. This further weakened accreted lithosphere that was already poorly consolidated; and since this time, lithosphere of the Far West has been prone to rifting, particularly behind the Sierras and Cascades.
Ancestral Yuba channels extended well into northwestern Nevada (Garside, 2005). It is within these channels that lag gravels containing California Gold Rush placer deposits are recovered. These were eroded from "mother lode" gold-bearing high grade rocks in the Tahoe region (Garside, 2005). The gravel-filled Yuba channels record a relatively high-gradient braided river system that existed 48-49 Ma. In addition to cobble clasts, channel lag includes abundant wood, including upright stumps. Following a brief retreat, sea level rose a second time ~ 45-46 Ma. During this transgression, gravel-filled channels were backfilled with sand, while channel margins and overbank facies received finer sediment. It is within these overbank and floodplain deposits that leaf, fruit and seed floras of the classic Chalk Bluff flora are preserved. A single radiometric date indicates that these floras are approximately 45.5 Ma. The paleogeography and local geological history of this period and region is discussed in more detail in the overview of the Buckeye Diggings locality of the Chalk Bluff flora later in the field guide.
The end of the Flat Slab period and the post-Eocene Yuba River
Immature volcanic ash filling the channel at La Porte reflects a renewal of highly silicic ignimbrite volcanism beginning in eastern Washington State near the early-middle Eocene boundary and sweeping through central Nevada by middle Miocene time. Called the "ignimbrite storm" this episode reflects catastrophic melting of continental rock thought to have been produced by the north to south tearoff of the flat slab and the subsequent juxtaposition of hot mantle asthenosphere under thin continental lithosphere. Oligocene and early Miocene rocks of Nevada consist of sheet after sheet of ignimbrite storm deposits, which fill the Yuba channels. Few fossils are preserved within them.
Breakoff of the flat slab was followed by renewed high angle subduction and construction of the Cascades volcanic arc. The arc initially formed ~ 40 Ma in Oregon/Washington and built southward over time as the Flat Slab tore. The Cascades arc extended through the northern Sierras by middle Miocene time, perforating Mesozoic rocks of the earlier subduction/accretion complex. Andesitic rocks of the "ancestral" or Western Cascades are abundantly fossiliferous, and include the most complete sequence of Eocene/Oligocene transitional paleofloras in North America. A number of paleofloras of middle Miocene age are preserved in what are interpreted to be debris-dam lake deposits of the southern part of the Western Cascades, which occupied the northern Sierra Nevada region. These include the Gold Lake, Webber Lake, and Mohawk floras found within the Mehrten Formation and its equivalents.
During this time, the spreading center between the Pacific and Farallon plates was rapidly approaching the coast as subduction consumed the Farallon plate (see map at right). The spreading center first collided with the coast in the region of southern California ~ 25 Ma. Once the spreading was consumed, subduction ceased and two triple junctions were formed the Mendocino Triple Junction in the north, and the "Southern triple junction" in the south. As the spreading center continued to be consumed, the two triple junctions moved further north/south, subduction ceased between them, and right lateral shear between the Pacific and North American Plates resulted in the formation of a transform plate boundary now marked by the San Andreas Fault. Subduction of the spreading center gradually restricted the southern terminus of the Cascades Arc northward. It now occurs at the latitude of Cape Mendocino. Mt. Lassen occupies the southern end of the Cascades directly inland.
The transform fault boundary produced pervasive right lateral transtension throughout the region. This dragged more solid crust of the Sierran and western Mexican batholiths northward through the weakened accretionary lithosphere that surrounds them like almonds in a candy bar. As these solid blocks collided, they were dragged westward around each other, while simultaneously rotating and pulling west. The result of this motion was the formation of the California Transverse Ranges and extension of the Great Basin. The Northern Sierras and Klamath/Siskiyou ranges were pulled west, opening the Great Basin. In west central Nevada extension began ~ 16-17 Ma, first forming transtensional sag basins along the "Walker Lane," and later, beginning ~ 9 Ma, producing the fault-bounded horsts observed today. Motion on these earlier faults appears not to have been as extensive as more recent faulting; and the modern basins of the Great Basin began to form in earnest only ~ 5-6 Ma ago.
Abundant paleofloras are preserved within middle Miocene transtensional basins of the Walker Lane. Broad lakes formed within these moderate elevation basins, capturing plant organs, insects, and other fossil material. Abundant diatoms inhabited the lakes, and fossils are largely preserved in diatomite. Fossils preserve a record of deciduous broadleaved and conifer vegetation growing under a mesic temperate climate.
Closely-spaced fallout ash deposits provide 40Ar/39 Ar dates with small errors. Additionally, ash deposits have been widely correlated geochemically, permitting highly resolved inter- and intra-basin correlations. This detailed chronology permits the reconstruction of vegetational and floristic changes over short time periods. Profound summer drought, beginning 15 to 14.5 Ma formed the Mediterranean climate of the modern west, and extirpated mesic and humid plants. The relatives of these plants are now found in southeast Asia, and this event initiated vegetational and floristic trends responsible for the current plants and community structure of the California province. Seasonal drying gradually spread northward, while winter cold built from the north, trapping cold and drought intolerant vegetation in a climatic vise that resulted in the short, opportunistic growing season of today.
Down slope in the northern Sierras, deciduous broadleaved and conifer vegetation was replaced by evergreen hardwood forest. The middle-late Miocene Mehrten Formation consists of ring plane deposits around edifices of the Miocene Western Cascades in the Northern Sierras. Temporary debris-dam lake and floodplain facies preserve the Webber Lake, Gold Lake, Mohawk, Table Mountain, Remington Hill, and other paleofloras discussed in greater detail later in the field guide. During this time the coast built outward from west of Sacramento to the east bay, forming the fossil-rich later Miocene/Pliocene Neroly formation.
Maps adapted from originals on the Paleogeography of the Southwestern US website by Ronald C. Blakey, Northern Arizona University.
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