35

80

75

30

60

70

75

35 35

35

45

15

ls

ls ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

ls

Jdi

Jdi

Jdi

Jmsv

Jmsv

Jmsv

Jmsv

Jmsv

Jmsv

Jmsv

Jmsv

Jmsv

Qoal

Qoal

Qoal

Qoal

Qoal

Qoal Qoal

Qoal

Qoal

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls Qls

Qls

Qls Qls

Qls

Qls

Qls Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qal

Qal

Qal

Qal

Qal

Qal

Qal

Qal

Qal

JPrc

JPrc

JPrc

JPrc

JPrc

JPrc

JPrc

JPrc

JPrc

JPrc

JPrc

JPrc

JPrc

Psv

Psv

Ksfm

Jca

Jca

Jca

Jp

Jp

Jp

Jp

Jp

Jp

Jp

Jp

Jp

Jp Jp

Jp

Jp

Jp

Jg

Jg

Jg

Jg

Tw

Tw

Tw

Tw

Tw

Tw

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

um

ls

um

um

um

um

um

um

um

um

Qt

Qt

Qt

Qt

Ks Ks

rcs

rcs

rcs

rcs

rcs

rcs

Qos

Qos Qos Qos Qos

Qos

Qos

Qos

um

ELTAPOM

PLUTON

GRASSY

MOUNTAIN

PLUTON

Jdi

Jdi

Jdi

Jdi

Jdi

Jdi

Jdi

Jdi

Jdi

Jdi

Jdi

Jdi

Qt

ls

1

2

3

4

6

7

8

10

11

12

13

34

14

15

16

17

18 19

20

21

22

23

24

25

26

27

28

29

31

32

33

5

30

9

3

1

2

3

4 4

Great Valley

 sequence

Weaverville

 Formation

Bear Wallow Fault

South Fork Fault 

Salt Creek Fault

Wilson Point Fault

EASTERN HAYFORK TERRANE

 Outliers of

RATTLESNAKE CREEK TERRANE

WESTERN HAYFORK TERRANE

123°30' 123°25' 123°20' 123°15'

40°45'

40°40'

40°35'

40°30'

WESTERN JURASSIC TERRANE

PICKETT PEAK TERRANE

WHT

WHT

WHT

WHT

WHT

Ironside Mountain

 batholith

WESTERN HAYFORK TERRANE

GENERALIZED TERRANE MAP

The Hyampom 15' quadrangle includes parts of four generally subparallel northwest-trending terranes (the Eastern 

Hayfork, Western Hayfork, Rattlesnake Creek, and Western Jurassic terranes) that were sequentially accreted to 

North America during Jurassic time to become parts of the Klamath Mountains geologic province. Later, during 

Early Cretaceous time, a fifth terrane (Pickett Peak terrane) was accreted, to become part of the Coast Ranges 

geologic province. Closely scattered remnants of the Lower Cretaceous Great Valley sequence overlap assemblage 

are shown diagramatically as a single unit in the northeast corner of the quadrangle. The youngest rocks shown on 

this generalized terrane map are weakly consolidated mid-Tertiary nonmarine strata of the Weaverville Formation 

near the village of Hyampom. Abbreviations: WHT, Western Hayfork terrane. 

 Any use of trade, firm, or product names in this publication is for descriptive purposes only and does not imply endorsement 

by the U.S. Government.

This map was printed on an electronic plotter directly from digital files. Dimensional calibration may vary between electronic 

plotters and between X and Y directions on the same plotter, and paper may change size due to atmospheric conditions; 

therefore, scale and proportions may not be true on plots of this map.

For sale by U.S. Geological Survey, Information Services, Box 25286, Federal Center, Denver, CO 80225, 1-888-ASK-USGS

Digital files available at http://pubs.usgs.gov/sim/3129/

Base: U.S. Geological Survey Hyampom 15-minute 

quadrangle, 1951; polyconic projection

North American Datum 1927.

Reconnaissance Geologic Map of the Hyampom 15’ Quadrangle, Trinity County, California

By

William P. Irwin

2010

CONTOUR INTERVAL 50 FEET

DATUM IS MEAN SEA LEVEL

AREA

OF MAP C A L I F O R N I A

19O

magnetic north

true north

 

Scientific Investigations Map 3129 U.S. Department of the Interior

U.S. Geological Survey

Geology mapped by W.P. Irwin, 1972-1984. 

GIS database and digital cartography by W.P. Irwin 

and C.M. Wentworth, 2009-10 

Edited by T.A. Lindquist

Manuscript approved for publication June 8, 2010

 Contact; dashed where approximately located;

 queried where uncertain

 Fault; dashed where approximately located; queried where uncertain;

 dotted where concealed; teeth on upper plate where thrust

Small fold, showing plunge of axis where known

Attitude of inclined bedding

 Inclined Vertical

 Attitude of foliation

 Inclined Vertical 

 Attitude of joints 

50

30

40

20

 Inclined Horizontal

 Attitude of cleavage

MAP SYMBOLS

10

15

Fossil locality

1

Isotopic age locality

40°30'

123°30' 123°25' 123°20'

40°45'

40°40'

40°35'

123°30' 123°25' 123°20' 123°15'

 40°45'

123°15'

40°30'

40°35'

40°40'

R. 6 E. R. 7 E. R. 8 E. R. 12 W.

T. 2 N.

T. 3 N.

T. 4 N.

R. 6 E. R. 7 E. R. 8 E. R. 12 W.

T. 34 N.

T. 33 N.

T. 32 N.

T. 31 N.

 Table 1. Fossil localities in Hyampom quadrangle

Map Sample Lithology* Fossils* Age Reference

No. No. 

1 55-77 C Radiolarians Late Triassic Loc. 77, in Irwin and others, 1982

2 56-77 C Radiolarians Late Triassic (late Loc. 78, in Irwin and others, 1982 

 Karnian? to 

 middle Norian)

3 6-83 C Radiolarians Mesozoic (Triassic) C.D. Blome, written commun., 1983 

4 7-83 C Radiolarians Late Triassic(?) C.D. Blome, written commun., 1983

 or Early Jurassic

 (Hettangian to

 Pliensbachian

5 8-83 C Radiolarians Early or Middle Jurrasic C.D. Blome, written commun., 1983

6 57-77 C Radiolarians Late Triassic Loc. 79, in Irwin and others, 1982

7 58-77 C Radiolarians Mesozoic Loc. 80, in Irwin and others, 1982

 Conodonts Late Triassic Loc. 30, in Irwin and others, 1983

 (CAI 7)

8 59-77 C Radiolarians Triassic or Jurassic Loc. 81, in Irwin and others, 1982

9 12-71 L Conodonts Ordovician to Triassic Loc. 32, in Irwin and others, 1983

 (CAI 7) 

10 16-76 C Radiolarians Middle Triassic Loc. 83, in Irwin and others, 1982

 Vance Creek (Ladinian)

 fauna

11 32-81 C Radiolarians Mesozoic (Jurassic) Loc. 82, in Irwin and others, 1982 

12 6-72 L Conodonts Late Triassic Loc. 33, in Irwin and others, 1983

 (CAI 5-6)

13 5-73 L Conodonts Devonian to Triassic Loc. 31, in Irwin and others, 1983

 (CAI 5)

14 10-82 C Radiolarians Mesozoic (Jurassic?) C.D. Blome, written commun., 1982

15 33-81 C Radiolarians Mesozoic (Jurassic?) Loc. 87, in Irwin and others, 1982;

 9-82 C.D. Blome, written commun., 1982

16 24-81 C Radiolarians Mesozoic (Jurassic?) Loc. 86, in Irwin and others, 1982 

17 30-81 C Radiolarians Mesozoic Loc. 85, in Irwin and others, 1982

18 5-71 L Foraminifers Permian or early Loc. 60, in Irwin and Galanis, 1976

 Mesozoic

19 1-83 C Radiolarians Triassic or Jurassic C.D. Blome, written commun., 1983

20 31-81 C Radiolarians Early Jurassic Loc. 84, in Irwin and others, 1982

 (Pliensbachian) to Middle 

 Jurassic (Bathonian) 

21 11-71 L Foraminifers Permian or early Mesozoic Loc. 61, in Irwin and Galanis, 1976

 Conodonts Ordovician to Triassic Loc. 34, in Irwin and others, 1983

 (CAI 6-7)

22 5-72 L Conodonts Late Triassic Loc. 35, in Irwin and others, 1983

 (CAI 6-7)

23 17-76 C Radiolarians Mesozoic Loc. 88, in Irwin and others, 1982 

24 36-81 C Radiolarians Mesozoic Loc. 89, in Irwin and others, 1982

25 35-81 C Radiolarians Mesozoic Loc. 90, in Irwin and others, 1982

26 2-83 C Radiolarians Early Jurassic (Pliens- C.D. Blome, written commun., 1984

 bachian; Toarcian?)

27 32-84 C Radiolarians Mesozoic (Triassic?) C.D. Blome, written commun., 1984

28 34-84 C Radiolarians Late Triassic (late Karnian C.D. Blome, written commun., 1984

 to middle Norian)

29 5-77 C Radiolarians Triassic or Jurassic Loc. 91, in Irwin and others, 1982

30 25-81 C Radiolarians Late Triassic (Norian) to Loc. 92, in Irwin and others, 1982

 Middle Jurassic

31 16-69 L Foraminifers Late Paleozoic Loc. 64, in Irwin and Galanis, 1976 

32 5-74 L Foraminifers Pennsylvanian or early Loc. 124, in Irwin and Galanis, 1976

 Permian

33 13-83 C Radiolarians Mesozoic C.D. Blome, written commun., 1983

34 ------ ------- Fish Miocene LeRoy, 1993

*Abbreviations: C, chert; CAI, color-alteration index; L, limestone 

rcs

?

?

Tw

Qal Qt Qls

JPrc

Ks

Psv

ls

Qoal

Early Cretaceous

Late Jurassic

Middle Jurassic

Early Jurassic

Ksfm

Jg

Qos

Jca Jmsv Jdi

Jp

CORRELATION OF MAP UNITS

UNCONSOLIDATED DEPOSITS

BEDROCK

GREAT VALLEY OVERLAP SEQUENCE

Major unconformity Early Cretaceous (~136 Ma)

TECTONOSTRATIGRAPHIC TERRANES

WEAVERVILLE FORMATION

 Western

 Hayfork

 Eastern

 Hayfork

Rattlesnake

 Creek

Lower Cretaceous

 (Hauterivian)

Miocene and

(or) Oligocene

Holocene and

(or) Pleistocene QUATERNARY

TERTIARY

CRETACEOUS

JURASSIC

TRIASSIC

PERMIAN

Pickett

 Peak

Western

Jurassic

CRETACEOUS

rcs

Jp

Jdi

Jmsv

Ks

Tw

Qoal

Qls

Qal

Qt

JPrc

Psv

ls

Qos

Jg

Jca

Ksfm

um

um

Qos

ls

ls

Younger alluvium (Holocene and (or) Pleistocene)—Unconsolidated silt, sand, and gravel in modern stream 

 channels and on associated low terraces. Alluvium in major streams located in Hayfork terrane has yielded 

 placer gold, but little gold has been produced from streams located in Rattlesnake Creek, Western Jurassic, 

 and Pickett Peak terranes in map area

Terrace deposits (Holocene and (or) Pleistocene)—Thin veneers of sand and gravel on terraces closely 

 related to modern streams

Landslides (Holocene and (or) Pleistocene)—Many landslides in areas of Rattlesnake Creek terrane

 and South Fork Mountain Schist are not shown, although these areas are exceptionally unstable,

 and downslope movement resulting from mass wasting is widespread

BEDROCK

 GREAT VALLEY OVERLAP SEQUENCE

Great Valley sequence (Lower Cretaceous)—Firmly consolidated shale, sandstone, and

 conglomerate that crop out as several small depositional outliers southwest of Big Bar and rest with 

 high angular unconformity on Eastern Hayfork terrane. Stratigraphic thickness about 60 m. Lower

 part of section contains a coal bed that has been mined. Fossil plants are abundant near coal bed. 

 Presence of both marine and nonmarine fossil mollusks suggests deposition near an Early Cretaceous 

 (Valanginian) shoreline (see Diller, 1908; MacGinitie, 1937; Jones and Irwin, 1971)

TECTONOSTRATIGRAPHIC TERRANES

DESCRIPTION OF MAP UNITS

UNCONSOLIDATED DEPOSITS

Old land surface (Pleistocene)—Remnants of old terrace and colluvium surfaces on south side of 

 Corral Bottom

Mélange (Triassic and Permian)—Slaty argillite and other clastic sedimentary rocks, thin-bedded

 chert, volcaniclastic rocks, sparse recrystallized limestone (ls, where divided), and minor amounts

 of mica schist, amphibolite, and serpentinite. Fossil foraminifers in limestone at fossil localities 31 and

 32 are late Paleozoic in age (table 1). Radiolarians in chert from the mélange unit in Hyampom quadrangle

 have not been dated, but in the same mélange unit outside the quadrangle, they are mostly Late Triassic, and 

 none is clearly of Jurassic age (Irwin and others, 1982) 

Hayfork Bally Meta-andesite (Middle Jurassic)—Layered mafic volcanogenic rocks that range in texture

 from coarse agglomerate to crystal tuff; probably are deposits of oceanic volcanic arc. Commonly

 characterized by euhedral augite and plagioclase crystals in light- to dark-green groundmass. Locally

 includes interlayers of thin-bedded radiolarian chert and dark argillite. Age of meta-andesite is not 

 clearly known, but it is thought to be Middle Jurassic on basis of probable cogenetic relation with

 the Ironside Mountain batholith (170 Ma, revised constant), as well as on K-Ar ages of 159 Ma 

 (M.A. Lanphere, cited in Irwin, 1977; revised constant) and 168 to 177 Ma (Fahan and Wright, 1983) 

 measured on hornblende from meta-andesite

Broken formation of volcanic rocks and mélange (Middle Jurassic to Permian?)—Consists dominantly of 

 sheared and dislocated mafic volcanic rocks, including pillow basalt, flows(?), tuff, volcanic breccia, and 

 volcaniclastic rocks that contain locally interbedded red and gray to black, thin-bedded radiolarian chert; 

 includes minor amounts of intermediate and silicic volcanic rocks, lenses and knockers of limestone (ls, where

 divided), amphibolite, detrital sedimentary rocks (rcs), plutonic rocks (Jp), and serpentinite (um, where divided)

 which is dominantly serpentinized peridotite but includes associated metasomatized mafic rocks of uncertain

 origin as well as minor amounts of sheared-in volcanic rocks and chert. Radiolarians indicate that cherts of 

 both Triassic and Jurassic ages are present (table 1). Color-alteration index (CAI) of conodonts (table 1)

 indicates temperatures of 300° to 500°C (Irwin and others, 1983). Several blocks of radiolarian chert in the

 serpentinite about 4 km southwest of Chaparral Mountain are metamorphosed to blueschist facies, and one

 of these (isotopic-age loc. 1) yielded K-Ar age of 185±8.8 Ma (M.A. Lanphere, written commun., 1982).

 Limestone at fossil localities 12 and 22 contains Late Triassic conodonts and, at localities 18 and 21, early

 Mesozoic or Permian foraminifers (table 1). Continuity of lithic units of Rattlesnake Creek terrane is disrupted

 by faults and obscured by widespread landslides

Galice(?) Formation (Late Jurassic)—Interbedded mudstone, graywacke, and conglomerate that ranges

 in texture from mildly slaty (textural zone 2 of Blake and others, 1967) to semischist (textural zone 2+). 

 Fossils have not been found in the Galice(?) Formation in this quadrangle, but presumably correlative

 rocks in southwestern Oregon contain fossils that indicate Late Jurassic (late Oxfordian and middle 

 Kimmeridgian) age (Imlay, 1959). Age of metamorphism is Late Jurassic (late Kimmeridgian?) on basis of

 K-Ar analysis of samples from isotopic-age localities 2, 3, and 4 on Pelletreau Ridge (respectively,

 148±1.8 Ma, 151±4.4 Ma, and 153±4.5 Ma; Lanphere and others, 1978)

Weaverville Formation (Miocene and (or) Oligocene) —Weakly consolidated nonmarine

 sandstone, conglomerate, and thinly laminated, light-colored clay and tuff. Fossil plants abundant in

 some clay and tuff, and lignitic coal beds present locally. Thickness of stratigraphic section may be

 about 300 m. Age as indicated by fossil plants was previously considered Oligocene(?) (MacGinitie, 

 1937); however, more recent study of pollen and spores indicate late Oligocene or early Miocene age 

 for unit (Barnett, 1982). A fossil minnow from lacustrine beds north of Hyampom (fossil loc. 34; table 1)

 is thought to be of Miocene age (LeRoy, 1993). Clay and tuff beds are locally fused and vesiculated by 

 heat from underground fires in coal beds (MacGinitie, 1937; Lydon, 1964; LeRoy, 1993). Red chert 

 pebbles from conglomerate beds in the vicinity of Hyampom contain middle Permian (Guadalupian) 

 radiolarians. Formation generally poorly exposed

Qos

Plutonic rocks (Jurassic?)—Medium- to coarse-grained rocks that range from diorite to granite. They include

 Eltapom and Grassy Mountain plutons and other small unnamed plutons; may include some volcanic rocks

 of intermediate and silicic composition; thought to intrude mafic volcanic rocks of the broken formation (JPrc), 

 but contact relations are unclear, and the plutons may be fault blocks; probably equivalent to rocks isotopically 

 dated as Early Jurassic (193-207 Ma; Wright, 1982) in Dubakella Mountain and Chanchelulla Peak 15' quadrangles 

 (to southeast)

Detrital sedimentary rocks (Jurassic?)—Well-bedded, mildly slaty rocks that vary from mudstone to pebble 

 conglomerate; they locally resemble the Galice(?) Formation (Jg) and may represent tectonic inliers or 

 fault slices of Western Jurassic terrane

Older alluvium (Pleistocene)—Weakly consolidated nonmarine silt, sand, and conglomerate associated 

 with topographically high remnants of old land surfaces. Most of these deposits are at elevations of 3,000

 to 4,000 ft, about 2,000 ft or more higher than that of the South Fork of Trinity River. Deposits are generally

 less indurated than the Weaverville Formation, and bedding generally is near horizontal or dips more gently

 than in Weaverville Formation. Pebbles of red chert in deposits on divide between Eltapom and 

 Buckeye Creeks contain Late Jurassic (Tithonian) radiolarians, implying Coast Ranges provenance

South Fork Mountain Schist (Early Cretaceous)—Mostly schistose metasedimentary rocks

 of textural zone 3 (Blake and others, 1967), represented by quartz-albite-muscovite-chlorite schist that

 commonly contains lawsonite. Formation locally includes the Chinquapin Metabasalt Member (undivided 

 here), which is fine-grained albite-chlorite-actinolite-epidote gneiss that locally contains crossite-rich

 layers. Isotopic analysis by K-Ar and Rb-Sr methods indicates that the metamorphic event that formed the 

 unit occurred 115 to 120 Ma ago during Early Cretaceous time (Lanphere and others, 1978). Age of 

 deposition of sedimentary and volcanic protoliths probably was Late Jurassic and Early Cretaceous. Most

 of the unit is poorly exposed owing to thick cover of slope debris and forest overgrowth 

Chert and argillite unit (Middle Jurassic)—Dominantly thin-bedded radiolarian chert, 

 locally interlayered with argillite and meta-andesite. Generally overlies (conformably?) and locally

 interfingers with the Hayfork Bally Meta-andesite. Radiolarians in the chert generally are recrystallized

 but at fossil locality 33 are identifiable as Mesozoic (table 1)

Western Hayfork Terrane

Eastern Hayfork Terrane

Rattlesnake Creek Terrane

Western Jurassic Terrane

Pickett Peak Terrane

Ironside Mountain batholith (Middle Jurassic)—Mostly medium-grained monzodiorite but also includes

 quartz diorite, minor amounts of gabbro, and pyroxenite. Fine-grained and hybrid(?) varieties are

 common in zone as much as 1 km wide along much of boundary between this unit and the Hayfork

 Bally Meta-andesite. Age of batholith is Middle Jurassic on basis of isotopic ages of about 170 Ma, as

 determined by K-Ar analysis (Lanphere and others, 1968; revised constant) and by U-Pb 

 analysis (Wright, 1982)

WEAVERVILLE FORMATION

 REFERENCES CITED 

Barnett, J.C., 1982, Palynology and paleoecology of the Weaverville Formation (Tertiary), northwestern

 California: Seattle, University of Washington, M.S. thesis, 118 p.

Blake, M.C., Jr., Irwin, W.P., and Coleman, R.G., 1967, Upside-down metamorphic zonation, blueschist facies,

 along a regional thrust in California and Oregon: U.S. Geological Survey Professional Paper 575-C, p. Cl-C9.

Diller, J.S., 1908, Strata containing the Jurassic flora of Oregon: Geological Society of America Bulletin, 

 v. 19, p. 367-402.

Fahan, M.R., and Wright, J.E., 1983, Plutonism, volcanism, folding, regional metamorphism and thrust faulting:

 contemporaneous aspects of a major Middle Jurassic orogenic event within the Klamath Mountains, 

 northern California: Geological Society of America Abstracts with Programs, v. 15, no. 5, p. 272-273.

Imlay, R.W., 1959, Succession and speciation of the Pelecypod Aucella, in Shorter contributions to general 

 geology: U.S. Geological Survey Professional Paper 314-G, p. 155-169.

Irwin, W.P., 1977, Review of Paleozoic rocks of the Klamath Mountains, in Stewart, J.H., Stevens, C.H., and

 Fritsche, A.E., eds., Paleozoic paleogeography of the western United States: Society of Economic 

 Paleontologists and Mineralogists, Pacific Coast Paleogeography Symposium I, p. 441-454.

Irwin, W.P., 1985, Reconnaissance geologic map of the Hyampom quadrangle, Trinity County, California:

 U.S. Geological Survey Miscellaneous Field Studies Map MF-1809, scale 1:62,500.

Irwin, W.P., 2009, Geologic map of the Weaverville 15' quadrangle, Trinity County, California: U.S. Geological

 Survey Scientific Investigations Map 3095, scale 1:50,000 (http://pubs.usgs.gov/sim/3095/).

Irwin, W.P., 2010, Reconnaissance geologic map of the Hayfork 15' quadrangle, Trinity County, California: U.S.

 Geological Survey Scientific Investigations Map 3119, scale 1:50,000 (http://pubs.usgs.gov/sim/3119/).

Irwin, W.P., and Galanis, S.P., Jr., 1976, Map showing limestone and selected fossil localities in the Klamath

 Mountains province, California and Oregon: U.S. Geological Survey Miscellaneous Field Studies Map 

 MF-749, scale 1:500,000.

Irwin, W.P., Jones, D.L., and Blome, C.D., 1982, Map showing sampled radiolarian localities in the western

 Paleozoic and Triassic belt, Klamath Mountains, California: U.S. Geological Survey Miscellaneous Field 

 Studies Map MF-1399, scale 1:250,000.

Irwin, W.P., Wardlaw, B.R., and Kaplan, T.A., 1983, Conodonts of the western Paleozoic and Triassic belt,

 Klamath Mountains, California and Oregon: Journal of Paleontology, v. 57, no. 5, p. 1,030-1,039.

Jones, D.L., and Irwin, W.P., 1971, Structural implications of an offset Early Cretaceous shoreline in northern 

 California: Geological Society of America Bulletin, v. 82, no. 4, p. 815-822.

Lanphere, M.A., Blake, M.C., Jr., and Irwin, W.P., 1978, Early Cretaceous metamorphic age of the South Fork

 Mountain Schist in the northern Coast Ranges of California: American Journal of Science, 

 v. 278, no. 6, p. 798-815.

Lanphere, M.A., Irwin, W.P., and Hotz, P.E., 1968, Isotopic age of the Nevadan orogeny and older plutonic and

 metamorphic events in the Klamath Mountains, California: Geological Society of America Bulletin 79,

 no. 8, p. 1,027-1,052.

LeRoy, T.H., 1993, General reconnaissance of the northeast side of Hyampom Valley: Evidence for a late 

 Cenozoic structural basin, southern Klamath Mountains, California: Arcata, Calif., Humboldt State University,

 B.Sc. thesis, 23 p.

Lydon, P.A., 1964, "Silicate slag" from Trinity County: California Division of Mines and Geology, Mineral 

 Information Service, v. 17, no. 5, p. 65-74.

MacGinitie, H.D., 1937, The flora of the Weaverville beds of Trinity County, California, in Eocene flora of 

 western America: Carnegie Institution of Washington Contributions to Paleontology, Publication 465, p. 83-151.

Wright, J.E., 1982, Permo-Triassic accretionary subduction complex, southwestern Klamath Mountains, northern

 California: Journal of Geophysical Research, v. 87, no. B5, p. 3,805-3,818.

 SYNOPSIS

 The Hyampom 15' quadrangle lies west of the Hayfork 15' quadrangle in the southern part of the Klamath 

Mountains geologic province of northern California. It spans parts of four generally northwest-trending tectonostratigraphic terranes of the Klamath Mountains, the Eastern Hayfork, Western Hayfork, Rattlesnake Creek, and 

Western Jurassic terranes, as well as, in the southwest corner of the quadrangle, a small part of the Pickett Peak 

terrane of the Coast Range province. Remnants of the Cretaceous Great Valley overlap sequence that once covered 

much of the pre-Cretaceous bedrock of the quadrangle are now found only as a few small patches in the northeast 

corner of the quadrangle. Fluvial and lacustrine deposits of the mid-Tertiary Weaverville Formation crop out in the 

vicinity of the village of Hyampom.

 The Eastern Hayfork terrane is a broken formation and mélange of volcanic and sedimentary rocks that include 

blocks of chert and limestone. The chert has not been sampled; however, chert from the same terrane in the Hayfork 

quadrangle (Irwin, 2010) contains radiolarians of Permian and Triassic ages, but none clearly of Jurassic age. 

Limestone at two localities contains late Paleozoic foraminifers. Some of the limestone from the Eastern Klamath 

terrane in the Hayfork quadrangle contains faunas of Tethyan affinity. 

 The Western Hayfork terrane is part of an andesitic volcanic arc that was accreted to the western edge of the 

Eastern Hayfork terrane. It consists mainly of metavolcaniclastic andesitic agglomerate and tuff, as well as argillite 

and chert, and it includes the dioritic Ironside Mountain batholith that intruded during Middle Jurassic time (about 

170 Ma). This intrusive body provides the principal constraint on the age of the terrane.

 The Rattlesnake Creek terrane is a mélange consisting mostly of highly dismembered ophiolite. It includes 

slabs of serpentinized ultramafic rock, basaltic volcanic rocks, radiolarian chert of Triassic and Jurassic ages, 

limestone containing Late Triassic conodonts and Permian or Triassic foraminifers, and small exotic(?) plutons. The 

plutons probably are similar to ones to the southeast beyond the quadrangle boundary that yielded isotopic ages 

ranging from 193 Ma to 207 Ma (Wright, 1982). The Rattlesnake Creek terrane contains several areas of wellbedded sedimentary rocks (rcs) that somewhat resemble the Galice(?) Formation and may be inliers of the Western 

Jurassic terrane. 

 The Western Jurassic terrane in the Hyampom quadrangle appears to consist only of a narrow tectonic sliver of 

slaty to semischistose detrital sedimentary rocks of the Late Jurassic Galice(?) Formation. The isotopic age of 

metamorphism of the rocks is about 150 Ma (Lanphere and others, 1978), which probably indicates when the terrane 

was accreted to the Rattlesnake Creek terrane.

 The Pickett Peak terrane, which is the most westerly of the succession of terranes in the Hyampom quadrangle, 

is the accreted eastern margin of the Coast Ranges province. It mainly consists of semischistose and schistose 

metagraywacke of the South Fork Mountain Schist and locally contains the blueschist-facies mineral lawsonite. 

Isotopic analysis indicates a metamorphic age of 120 to 115 Ma (Lanphere and others, 1978). 

 During the Cretaceous period, much of the southern fringe of the Klamath Mountains was onlapped by 

sedimentary strata of the Great Valley sequence. However, much of the onlapping Cretaceous strata has since been 

eroded away, and in the Hyampom quadrangle only a few small remnants are found in the northeast corner near Big 

Bar.

 Near the west edge of the quadrangle, in the vicinity of the village of Hyampom, weakly consolidated fluvial 

and lacustrine rocks and coaly deposits of Oligocene and (or) Miocene age are present. These rocks are similar to 

the Weaverville Formation that occurs in separate sedimentary basins to the east in the Weaverville (Irwin, 2009) 

and Hayfork (Irwin, 2010) 15' quadrangles.

 This map of the Hyampom 15' quadrangle is a digital version of U.S. Geological Survey Miscellaneous Field 

Studies Map MF-1809 (Irwin, 1985), with various improvements and additions. 

50

Mine adit