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Map showing publication footprint
  • Usage in publication:
    • Hornbrook formation*
  • Modifications:
    • Named
    • Biostratigraphic dating
  • Dominant lithology:
    • Conglomerate
    • Sandstone
    • Siltstone
    • Mudstone
  • AAPG geologic province:
    • Klamath Mountains province
Publication:

Peck, D.L., Imlay, R.W., and Popenoe, W.P., 1956, Upper Cretaceous rocks of parts of southwestern Oregon and northern California: American Association of Petroleum Geologists Bulletin, v. 40, no. 8, p. 1968-1984. [Available online, with subscription, from AAPG archives: http://www.aapg.org/datasystems or http://search.datapages.com]


Summary:

Named for exposures near Hornbrook, CA. Divided into 6 unnamed members I to VI. Composite type section, members I-III, and lower 70 ft of IV is in NE/4 SE/4 sec.32 to SE/4 SE/4 sec.29 T47N R6W; rest of member IV and all of V is along ridge south of Rancheria Gulch from NW/4 NW/4 sec.29 to middle of SW/4 sec.20 T47N R6W; VI is between Rancheria Gulch and Ditch Creek from SW/4 sec.20 to SE/4 SW/4 sec.17 T47N R6W, Hornbrook 15' quad, Siskiyou Co, CA. Previously assigned to Chico group by Gabb (1869). New name justified on basis of differences in fauna, age, and remoteness from type area of Chico. Discontinuous exposures from northwest-trending band that extends from near Yreka, CA, along valleys of Cottonwood and Bear Creeks to Grave Creek, OR. Grades generally upward from conglomeratic sandstone through interbedded sandstone and siltstone to mudstone. Composite section measures total of 2673 ft. Unconformity exists between members II and III in Hornbrook area. Overlies with angular unconformity Triassic Applegate group in Hornbrook area. Non-conformably overlies metamorphosed (in part) volcanic sedimentary rocks ranging in age from Paleozoic to Late Jurassic. Intruded by granitic rocks of Late Jurassic or Early Cretaceous age. Disconformably underlies clastic sedimentary rocks and volcanic rocks of Eocene age. Age is Late Cretaceous based on fossils of Cenomanian to Campanian age.

Source: GNU records (USGS DDS-6; Menlo GNULEX).


Map showing publication footprint
  • Usage in publication:
    • Hornbrook formation*
  • Modifications:
    • Mapped
  • AAPG geologic province:
    • Klamath Mountains province
Publication:

Wells, F.G., 1956, Geology of the Medford quadrangle, Oregon-California: U.S. Geological Survey Geologic Quadrangle Map, GQ-89, 1 sheet, scale 1:96,000


Summary:

Discontinuous outcrops near Jacksonville, south of Medford, south and southeast of Talent, south and southeast of Ashland, near Foliage, and across CA border below Cottonwood Creek mapped in Medford 15' quad, OR and CA. Unconformably underlies Umpqua formation and Quaternary alluvium; intruded by Upper Jurassic or Lower Cretaceous diorite. Unconformably overlies Applegate group south of Medford. Assigned Late Cretaceous (Cenomanian and Turonian) age on basis of contained fossils (Peck and Imlay, 1956).

Source: GNU records (USGS DDS-6; Menlo GNULEX).


Map showing publication footprint
  • Usage in publication:
    • Hornbrook formation*
  • Modifications:
    • Revised
    • Biostratigraphic dating
  • AAPG geologic province:
    • Klamath Mountains province
Publication:

Jones, D.L., 1959, Stratigraphy of Upper Cretaceous rocks in the Yreka-Hornbrook area, northern California [abs.]: Geological Society of America Bulletin, v. 70, no. 12, pt. 2, p. 1726-1727.


Summary:

Upper Cretaceous rocks of unit in Yreka-Hornbrook area informally divided into (ascending): units A-C. Unit A (700 to 800 ft thick) consists of conglomeratic sandstone in lower part and sandy siltstone in upper part. Unit B (130 ft thick) consists of massive conglomeratic, concretionary, greenish-gray sandstone which unconformably overlies unit A. Unit C (4000 ft thick) consists of dark-gray, marine siltstone with sandstone interbeds and thick sandstone tongue near base. Age of unit A is late Turonian and early Coniacian based on fossils. Unit C has probable age of middle Campanian to Maestrichtian(?) based on rich ammonite fauna and abundant INOCERAMUS.

Source: GNU records (USGS DDS-6; Menlo GNULEX).


Map showing publication footprint
  • Usage in publication:
    • Hornbrook formation
  • Modifications:
    • Not used
  • AAPG geologic province:
    • Klamath Mountains province
Publication:

Jones, D.L., 1960, Lower Cretaceous (Albian) fossils from southwestern Oregon and their paleogeographic significance: Journal of Paleontology, v. 34, no. 1, p. 152-160.


Summary:

Lower Cretaceous rocks in Grave Creek area formerly described as Late Cretaceous by Peck and others (1956) based on Turonian age of ammonites are re-identified as being Albian age fossils. Other Albian species were collected in lower part of section informally called Grave Creek strata, although one acanthoceratid ammonite was found in overlying siltstone in 1901 by G.H. Pease of Placer, OR, in locality about 2 mi upstream (northeast) from Albian localities. [Name "Hornbrook Formation" not mentioned in article]

Source: GNU records (USGS DDS-6; Menlo GNULEX).


Map showing publication footprint
  • Usage in publication:
    • Hornbrook Formation*
  • Modifications:
    • Mapped
  • AAPG geologic province:
    • Klamath Mountains province
Publication:

Peck, D.L. (compiler), and Wells, F.G. (project director), 1961, Geologic map of Oregon west of the 121st Meridian: U.S. Geological Survey Miscellaneous Investigations Series Map, I-325, 1 sheet, scale 1:500,000, Prepared in cooperation with Oregon Dept. Geol. Min. Ind


Summary:

Beds formerly assigned to Chico Formation (Diller, 1893) shown as Hornbrook Formation in Medford and Riddle quads, near Grave Creek, Medford, and Ashland. [Mapped undivided with marine sedimentary rocks of Late Cretaceous age near Humbug Mountain in Gold Beach quad.] In Grave Creek unconformably overlies sedimentary rocks of Galice Formation and volcanic rocks of Galice and Rogue Formations (undivided). In Medford and Ashland areas unconformably underlies alluvium of Quaternary age and unconformably overlies metavolcanic rocks of Applegate Group. Intruded by felsic rocks of Jurassic to Cretaceous age.

Source: GNU records (USGS DDS-6; Menlo GNULEX).


Map showing publication footprint
  • Usage in publication:
    • Hornbrook Formation*
  • Modifications:
    • Mapped
  • AAPG geologic province:
    • Klamath Mountains province
Publication:

Robison, J.H., 1971, Availability and quality of ground water in the Medford area, Jackson County, Oregon: U.S. Geological Survey Hydrologic Investigations Atlas, HA-392, 2 sheets, scale 1:62,500


Summary:

Mapped in Medford area mostly west, south, and southeast of Medford. Wells reveal Dark Hollow area is underlain by Hornbrook up to 300 ft thick beneath hilltop but much less in other places. Unconformably underlies alluvium of Quaternary age. Unconformably overlies Applegate Group. Intruded by granodiorite of Jurassic or Cretaceous age. Assigned Late Cretaceous age.

Source: GNU records (USGS DDS-6; Menlo GNULEX).


Map showing publication footprint
  • Usage in publication:
    • Hornbrook Formation*
  • Modifications:
    • Mapped
  • AAPG geologic province:
    • Klamath Mountains province
Publication:

Robison, J.H., 1972, Availability and quality of ground water in the Ashland quadrangle, Jackson County, Oregon: U.S. Geological Survey Hydrologic Investigations Atlas, HA-421, 1 sheet, scale 1:62,500


Summary:

Mapped in Ashland 15' quad in narrow zone southwest of Ashland and east of Cottonwood Creek south into Siskiyou Co, CA. Unconformably underlies nonmarine sedimentary rocks of Tertiary age formerly assigned to Umpqua Formation. Intruded by quartz diorite of Jurassic or Cretaceous age. Assigned Late Cretaceous age.

Source: GNU records (USGS DDS-6; Menlo GNULEX).


Map showing publication footprint
  • Usage in publication:
    • Hornbrook Formation*
  • Modifications:
    • Mapped
  • AAPG geologic province:
    • Klamath Mountains province
Publication:

Hotz, P.E., 1977, Geology of the Yreka quadrangle, Siskiyou County, California: U.S. Geological Survey Bulletin, 1436, 72 p., (incl. geologic map, scale 1:62,500)


Summary:

Mapped in Yreka 15' quad, Siskiyou Co, CA. Crops out east of Yreka where unit is approximately 250 m (800 ft) thick. Crops out on Soap Creek Ridge, where unit is about 40 m (130 ft thick). Also crops out in low hills east, northeast, and southeast of Yreka where unit unconformably overlies serpentinite and Schulmeyer Gulch sequences. Resembles coarse clastic rocks in lower section of Hornbrook Formation at type; therefore may be late Turonian and early Coniacian age.

Source: GNU records (USGS DDS-6; Menlo GNULEX).


Map showing publication footprint
  • Usage in publication:
    • Hornbrook Formation
  • Modifications:
    • Mapped
  • AAPG geologic province:
    • Klamath Mountains province
Publication:

Blake, M.C., Jr., Page, N.J., Smith, J.G., Griscom, Andrew, Coleman, R.G., and Loney, R.A., 1982, Geologic cross section from Cape Sebastian to upper Klamath Lake, southwest Oregon, IN Contribution from Plate Margins Working Group, U.S. Geodynamics Committee: Geological Society of America Map and Chart Series, MC-28O, 1 sheet, scale 1:157,480


Summary:

Cross section shows unconformably underlies nonmarine sedimentary rocks and Colestin Formation east of Mt. Ashland (approximately 122 deg 40'W and 42 deg 01'N). Here intruded by Tertiary pluton and unconformably overlies Jurassic-Cretaceous quartz diorite.

Source: GNU records (USGS DDS-6; Menlo GNULEX).


Map showing publication footprint
  • Usage in publication:
    • Hornbrook Formation*
  • Modifications:
    • Revised
  • AAPG geologic province:
    • Klamath Mountains province
Publication:

Nilsen, T.H., 1984, Tectonics and sedimentation of the Cretaceous Hornbrook Formation, Oregon and California, IN Crouch, J.K., and Bachman, S.B., eds., Tectonics and sedimentation along the California margin: Society of Economic Paleontologists and Mineralogists, Pacific Section, Field Trip Guidebook, annual meeting, San Diego, CA, April 18-21, 1984, book 38, p. 101-118.


Summary:

Divided into (ascending): Klamath River Conglomerate Member, Osburger Gulch Sandstone Member, Ditch Creek Siltstone Member, Rocky Gulch Sandstone Member, and Blue Gulch Mudstone Member (which includes Rancheria Gulch Sandstone Beds and Hilt Bed). Formation consists of about 1300 m of nonmarine, shallow-marine, and deep-marine clastic rocks. Most of the members crop out continuously over entire outcrop length of Hornbrook which extends 80 km from Jacksonville area in OR, southeast to Yreka area in Siskiyou Co, CA. Age is Cenomanian to Maestrichtian (Late Cretaceous) based on mollusks, and benthonic and planktonic foraminifers.

Source: GNU records (USGS DDS-6; Menlo GNULEX).


Map showing publication footprint
  • Usage in publication:
    • Hornbrook Formation*
  • Modifications:
    • Overview
    • Age modified
  • AAPG geologic province:
    • Klamath Mountains province
Publication:

Nilsen, T.H., 1993, Stratigraphy of the Cretaceous Hornbrook Formation, southern Oregon and northern California: U.S. Geological Survey Professional Paper, 1521, 89 p., (incl. geologic map, scale 1:62,500)


Summary:

Age of unit is Early and Late Cretaceous based on abundant megafossils and microfossils in its marine units that indicate age of Albian to early Maestrichtian for its main outcrop belt. Measured type sections for each of the members and beds are described. Detailed lithology, depositional environment, and mapping for each of members and beds also included in study.

Source: GNU records (USGS DDS-6; Menlo GNULEX).


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Asterisk (*) indicates published by U.S. Geological Survey authors.

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