The Sonoma orogeny was a period of mountain building in western North America. The exact age and structure of the Sonoma orogeny is controversial. The orogeny is generally thought to have occurred during the

Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last ...

Triassic The Triassic ( ) is a geologic period and system (stratigraphy), system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Year#Abbreviations yr and ya, Mya), to the beginning of the Jurassic Period 201.36 ...

transition, around 250 million years ago, following the Late Devonian

Antler orogeny The Antler orogeny was a tectonic event that began in the early Late Devonian with widespread effects continuing into the Mississippian and early Pennsylvanian. Most of the evidence for this event is in Nevada but the limits of its reach are un ...

. The Sonoma orogeny was one of a sequence of accretionary events along the Cordilleran margin, possibly caused by the closure of the basin between the

island arc Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries. Most island arcs originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle alon ...

of Sonomia and the North American continent. Evidence of this event has been reported throughout western North America, but most distinctly in northwest

Nevada Nevada ( ; ) is a state in the Western region of the United States. It is bordered by Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. Nevada is the 7th-most extensive, ...

. The orogeny was named by Silberling and Roberts, who identified it with the Havallah Formation, originally thought to date to the Pennsylvanian and

ages but has since been revised to include rocks of Late Devonian and Mississippian age. Some geologists dispute whether convergent plate tectonics produced the Sonoma orogeny.

Stratigraphy

The Havallah sequence is the informal name for sequence including the Havallah Formation and other related strata. The Havallah sequence is universally associated with the Sonoma orogeny, but units of the same age range and roughly the same lithic composition, along the western and northern margins of the Havallah, are also relevant. These units include the Inskip Formation in the East Range and a series of formations in the Hot Springs Range. These units consist of

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

felsite Felsite is a very fine-grained volcanic rock that may or may not contain larger crystals. Felsite is a field term for a light-colored rock that typically requires petrographic examination or chemical analysis for more precise definition. Color ...

, bedded

chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a ...

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

, and detrital rocks ranging from conglomerate to

argillite :''"Argillite" may also refer to Argillite, Kentucky.'' Argillite () is a fine-grained sedimentary rock composed predominantly of indurated clay particles. Argillaceous rocks are basically lithified muds and oozes. They contain variable amounts ...

that accumulated in a trough west of the Antler orogenic belt. The Havallah sequence is underlain by the

Golconda thrust The Golconda Thrust is a major oceanic terrane that was thrust over central and northern Nevada, North America, in possibly sometime between the Late Permian and the late Jurassic. It is considered equivalent to the Tobin thrust fault. It contai ...

, and is therefore a component of the Golconda allochthon. Speed observed that there are no arc-derived sedimentary rocks in the Golconda allochthon.

Theories

Dickinson expanded the reach of the Sonoma orogeny as follows: segments of accreted Permian

s, composed of

volcanic A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates a ...

and volcaniclastic strata and paired geotectonically with Sonoma accretionary prisms to the east, are present in the Klamath- Sierran region of the Cordilleran orogen to the south of volcanic cover in the

Pacific Northwest The Pacific Northwest (sometimes Cascadia, or simply abbreviated as PNW) is a geographic region in western North America bounded by its coastal waters of the Pacific Ocean to the west and, loosely, by the Rocky Mountains to the east. Tho ...

. Burchfiel and Davis presented a detailed model relating the Sonoma orogeny to convergent plate tectonic processes. They postulated an east-dipping subduction zone beneath a

magmatic arc Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...

to the west of the continent. In this scheme, a back-arc basin floored by

oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic ...

lay between the arc and the continent. The Sonoma orogeny involved closure of this basin and a process by which the blanket of oceanic sedimentary rocks (the Havallah sequence) was obducted onto the continental shelf via the Golconda thrust. To account for the absence of oceanic crust in the obducted rocks, Davis later proposed the unique concept that the Havallah was separated from the underlying oceanic crust by a process of subduction and obduction. Silberling presented a model similar to that of Burchfiel and Davis Miller and others found that the Schoonover sequence, a northern correlative of the Havallah sequence, was compatible with a

back-arc A back-arc basin is a type of geologic basin, found at some convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found in the western Pacific Ocean. Most o ...

thrusting model for the Sonoma orogeny. Speed offered a distinctly different model involving a

volcanic arc A volcanic arc (also known as a magmatic arc) is a belt of volcanoes formed above a subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc lo ...

above a west-dipping subduction zone to the west of the continent. In this model, an expanse of oceanic crust subducted westward under the volcanic arc, causing the overlying sedimentary rocks of the Havallah sequence to be scraped off the descending plate and forced over the approaching continental slope. Snyder and Brueckner supported the Speed model with detailed lithic descriptions of the Havallah. They interpreted the lithic composition of the Havallah to be the sedimentary floor of an extensive ocean basin. Brueckner and Snyder expressed some uncertainty about the exact time of final emplacement of the

allochthon upright=1.6, Schematic overview of a thrust system. The hanging wall block is (when it has reasonable proportions) called a nappe. If an erosional hole is created in the nappe that is called a window (geology)">window. A klippe is a solitary out ...

, but emphasized that structures associated with the Sonoma orogeny had a long history from the middle

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...

to the

periods.

Controversy and new ideas

Stewart and others revisited the classic Havallah locality at China Mountain in the Tobin Range, making use of a large number of fossil collections. This locality had been cited by Silberling and Roberts as displaying the best evidence for the existence of the Sonoma orogeny, having been tightly folded and thrust-faulted and overlain unconformably by the Triassic Koipato Formation. However, according to Stewart and others, the Havallah at that location is not tightly folded but is composed of nearly homoclinal strata separated by numerous undated faults sub-parallel to bedding. Stewart and others also cast some doubt as to the nature of the contact with the overlying Koipato Formation indicating on their map of the area that it could be a fault. In the East Range just to the west of China Mountain, Whitebread mapped the contact between strata of Permian age, at the top of the Havallah, and the base of the overlying Koipato as parallel, indicating a lack of evidence for an orogeny at the Permian-Triassic boundary. Ketner, in a complete reversal of the conventional concepts of the Sonoma orogeny, combined all similar deep-water Upper Devonian to Permian sequences in the area of the type Havallah sequence into a single genetic assemblage. Ketner concluded that Paleozoic deposits were compressed in the

Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...

, with the east-verging Golconda thrust in the east, and west-verging thrusts in western parts of the depositional basin. Ketner's work denied the oceanic origin of the Havallah and related sequences, eliminated the necessity for convergent plate tectonics and a far-traveled allochthon, established the age of the Golconda thrust as post-Triassic, and cast doubt on the very existence of the Sonoma orogeny.

References

{{reflist, 33em, refs= {{cite book, last=Baldridge, first=W. Scott, title=Geology of the American Southwest : a journey through two billion years of plate-tectonic history, year=2004, publisher=Cambridge Univ. Press, location=Cambridge .a.isbn=0-521-01666-5, page=158, edition=Reprint. {{cite journal, last1=Brueckner, first1=H.K., last2=Snyder, first2=W.S., year=1985, title=Structure of the Havallah sequence, Golconda allochthon, Nevada: Evidence for prolonged evolution in an accretionary prism, journal=Geological Society of America Bulletin, volume=96, issue=9, pages=1113–1130, doi=10.1130/0016-7606(1985)96<1113:sothsg>2.0.co;2 {{cite journal, last1=Burchfiel, first1=B.C., last2=Davis, first2=G.A., year=1972, title=Structural framework and evolution of the southern part of the cordilleran orogen, western United States, journal=American Journal of Science, volume=272, issue=2, pages=97–118, url=http://www.ajsonline.org/content/272/2/97.extract, doi=10.2475/ajs.272.2.97, bibcode=1972AmJS..272...97B {{cite journal, last1=Davis, first1=G.A., year=1973, title=Subduction-obduction model for the Antler and Sonoma orogenies, western Great Basin area, journal=Geological Society of America, volume=5, number=7, page=592 {{cite journal, last=Dickinson, first=W.R., title=Geodynamic interpretation of Paleozoic tectonic trends oriented oblique to the Mesozoic Klamath-Sierran continental margin in California, journal=Geological Society of America Special Papers, volume=347, year=2000, issue=347, pages=209–245, doi=10.1130/0-8137-2347-7.209, isbn=0-8137-2347-7 {{cite map, cartography=Jones, A.E., year=1997, title=Geologic map of the Hot Springs Peak quadrangle and the southeastern part of the Little Poverty quadrangle, Nevada, publisher=Nevada Bureau of Mines, id=Map 14, scale=1:24,000, url=http://www.nbmg.unr.edu/dox/fs14text.pdf, archiveurl=https://web.archive.org/web/20100606222352/http://www.nbmg.unr.edu/dox/fs14text.pdf, archivedate=2010-06-06, accessdate=2014-03-01, url-status=dead {{cite map, cartography=Jones, A.E., year=1997, title=Geologic map of the Delvada Spring quadrangle, Nevada, publisher=Nevada Bureau of Mines, id=Map 13, scale=1:24,000, url=http://www.nbmg.unr.edu/dox/fs13plate.pdf, archiveurl=https://web.archive.org/web/20100606225646/http://www.nbmg.unr.edu/dox/fs13plate.pdf, archivedate=2010-06-06, accessdate=2014-03-01, url-status=dead {{cite book, last=Ketner, first=K.B, year=2008, title=The Inskip Formation, the Harmony Formation, and the Havallah sequence of northwestern Nevada—An interrelated Paleozoic assemblage in the home of the Sonoma orogeny, publisher=U.S. Geological Survey, id=Professional Paper 1757, url=http://pubs.usgs.gov/pp/1757/, accessdate=2014-03-01 {{cite journal, last1=Miller, first1=E.L., last2=Holdsworth, first2=B.K., last3=Whiteford, first3=W.B., last4=Rodgers, first4=D., year=1984, title=Stratigraphy and structure of the Schoonover sequence, northeastern Nevada Implications for Paleozoic plate-margin tectonics, journal=Geological Society of America Bulletin, volume=95, issue=9, pages=1063–1076, bibcode=1984GSAB...95.1063M, doi=10.1130/0016-7606(1984)95<1063:SASOTS>2.0.CO;2 {{cite book, last1=Silberling, first1=N.J., last2=Roberts, first2=R.J., year=1962, title=Pre-Tertiary stratigraphy and structure of northwestern Nevada, publisher=Geological Society of America, id=Special Paper 72, url=https://books.google.com/books?id=8k0cujbsx44C&pg=PP14, accessdate=2014-03-01, isbn=9780813720722 {{cite book, last1=Silberling, first1=N.J., year=1973, chapter=Geologic events during Permian-Triassic time along the Pacific margin of the United States, editor1-last=Logan, editor1-first=A., editor2-last=Hills, editor2-first=L.V., title=The Permian and Triassic systems and their mutual boundary, location=Calgary, Canada, publisher=Alberta Society of Petroleum Geology, pages=345–362, chapter-url=http://archives.datapages.com/data/cspg_sp/data/002/002001/345_cspgsp0020345.htm, accessdate=2014-03-01 {{cite book, last1=Snyder, first1=S.S., last2=Brueckner, first2=H.K., year=1983, chapter=Tectonic evolution of the Golconda allochthon, Nevada: Problems and perspectives, editor1-last=Stevens, editor1-first=C.A., title=Paleozoic and early Mesozoic rocks in microplates of western North America, publisher=Society of Economic Paleontologists and Mineralogists, Pacific Section, pages=103–123, chapter-url=http://archives.datapages.com/data/pac_sepm/047/047001/pdfs/103.htm, accessdate=2014-03-01 {{cite book, last1=Speed, first1=R.C., year=1977, chapter=Island-arc and other paleogeographic terranes of late Paleozoic age in the western Great Basin, editor1-last=Stewart, editor1-first=J.H., editor2-last=Stevens, editor2-first=C.H., editor3-last=Fritsche, editor3-first=A.E., title=Paleozoic paleogeography of the western United States, publisher=Society of Economic Paleontologists and Mineralogists, Pacific Section, id=Pacific coast paleogeography symposium, volume=1, pages=349–362, chapter-url=http://archives.datapages.com/data/pac_sepm/019/019001/pdfs/349.htm {{cite journal, last=Speed, first=R.C., title=Collided Paleozoic microplate in the western United States, journal=Journal of Geology, volume=87, year=1979, issue=3, pages=279–292, url=http://www.muststayawake.com/SDAG/library/Speed1979.pdf, bibcode=1979JG.....87..279S, doi=10.1086/628417 {{cite journal, last1=Stewart, first1=J.H., last2=Murchey, first2=Benita, last3=Jones, first3=D.L., last4=Wardlaw, first4=B.R., year=1986, title=Paleontological evidence for complex tectonic interlayering of Mississippian to Permian deep-water rocks of the Golconda allochthon in Tobin Range, north-central Nevada, journal=Geological Society of America Bulletin, volume=97, issue=9, pages=1122–1132, bibcode=1986GSAB...97.1122S, doi=10.1130/0016-7606(1986)97<1122:PEFCTI>2.0.CO;2 {{cite map, cartography=Whitebread, D.H., year=1994, title=Geologic map of the Dun Glen quadrangle, Pershing County, Nevada, publisher=U.S. Geological Survey, id=Map I-2409, scale=scale 1:48,000, url=https://pubs.er.usgs.gov/publication/i2409, accessdate=2014-03-01 Orogenies of North America Events in the geological history of Earth Permian orogenies Triassic orogenies