Age of the 

Skooner Gulch 

Formation, Mendocino 

County, California

By WARREN O. ADDICOTT

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1254-C

Early Miocene mollusks from the 

Skooner Gulch Formation near 

Point Arena mark a significant 

northward extension of the 

warm water "Vaqueros" fauna
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

U.S. GOVERNMENT PRINTING OFFICE, WASHINGTON : 1967

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 - Price 10 cents
CONTENTS

Page

Abstract- _________________________________________________________ Cl

Introduction. _____________________________________________________ 1

Geologic setting.__________________________________________________ 1

Faunal assemblage.______________________________________________ 3

Age and position in the provincial chronology.________________________ 6

Zoogeographic significance._________________________________________ 10

References- ___________________ __________________________________ 10

ILLUSTKATIONS

Page 

FIGURE 1. Index map of Schooner Gulch area_ _______________________ C2

2. Photograph showing contact between Skooner Gulch Formation and Iversen Basalt-_______________________________ 4

3. Photograph showing contact between Skooner Gulch and

Gallaway Formations._________________________________ 5

4. Illustrations of fossils from the Skooner Gulch Formation and

Jewett Sand__________________________________________ 8

in

CONTRIBUTIONS TO STRATIGRAPHY

AGE OF THE SKOONER GULCH FORMATION 

MENDOCINO COUNTY, CALIFORNIA

By WARKEN O. ADDICOTT

ABSTRACT

Newly discovered mollusks from the Skooner Gulch Formation near Point 

Arena, Calif, (lat 38.9° N.) are indicative of an early Miocene age. Included are 

Turritella mezana forma lioffmanni Gabb and Chlamys cf. C. hertleini Loel and 

Gorey, species restricted to the "Vaqueros Stage" the standard for the early 

Miocene of the provincial megafaunal chronology. The Skooner Gulch assemblage 

marks a significant northward extension of the warm water "Vaqueros" fauna 

from localities in the Santa Cruz Mountains (lat 37'A° N.).

INTRODUCTION

Marine Miocene megafossils from the Skooner Gulch Formation, a 

200- to 300-foot sandstone unit occurring at the base of a 4,700-foot 

marine Miocene shale and mudstone sequence near Point Arena, Menclocino County, Calif., provide the first direct evidence of age of this 

formation and in addition are a significant new tie between the Pacific 

coast molluscan and foraminiferal chronologies. Weaver (1944, p. 4, 

pi. 11) named and first mapped this formation; although no fossils 

were found, its age was considered to be Oligocene. Mollusks from it 

(USGS Cenozoic loc. M2494) were submitted for identification and 

age determination by E. H. Stinemeyer in June 1965. These specimens, 

plus additional material collected by the writer in November 1965 

(USGS Cenozoic loc. M2638), have yielded a small but significant 

molluscan assemblage that is recorded here for the first time. Previously unreported marine mammal remains of Miocene age from near 

the top of the formation are of further use in determining position in 

the provincial marine chronologies.

GEOLOGIC SETTING

Cretaceous and Tertiary marine elastics crop out in a narrow coastal 

strip west of the San Andreas fault from Fort Ross, about 65 miles

Cl
C2 CONTRIBUTIONS TO STRATIGRAPHY

northwest of San Francisco, northward to Point Arena. Seacliff and 

intertidal zone exposures of the Upper Cretaceous to Miocene rocks of 

this 40-mile belt were mapped by planetable by Weaver (1944, scale, 

1 in. = 1,000 ft). Recently completed mapping of this area (Wentworth, 

1966) is incorporated into the index map showing the type area of the 

Skooner Gulch Formation and the fossil localities (fig. 1).

R 16 W

38°52'

Base from U.S. Geological Survey topographic quadrangle, 

Point Arena, Calif., 1960

FIGURE 1. Map of Schooner Gulch area, Mendocino County, Calif., showing 

relationship of Skooner Gulch Formation (stippled) to Miocene formations. 

Geology compiled from Weaver (1944) and Wentworth (written commun., 

April 1966).

Overlying the older part of the section mapped as the Gualala Group 

by Weaver (1944), about 8 miles southeast of Point Arena, is an 800- 

foot unit of amygdaloidal basalt (fig. 2) originally named the Skooner 

Gulch Basalt by Weaver (1943), but later renamed the Iversen Basalt 

by Weaver (1944). Unconformably overlying the basalt are mediumto coarse-grained massive sandstones that Weaver originally (1943) 

included in the basal part of his Galaway Beds but which he later 

(1944) named the Skooner Gulch Formation. 1 The type locality of the

1 Although the spelling of the place name has subsequently been corrected to Schooner 

Gulch, Weaver's (1944) spelling of the formation is retained (see Article 12a, 1961 Code 

of Stratigraphic Nomenclature).
SKOONER GULCH FORMATION, MENDOCINO COUNTY, CALIF. C3

formation is here designated as the seacliff and intertidal exposures in 

the east half of sec. 32, T. 12 N., K. 16 W. The basal contact is best exposed in seacliffs about two-thirds of a mile south of the mouth of 

Skooner Gulch (fig. 2). The basal foot or so consists of a very hard 

greenish-gray calcareous sandstone containing scattered poorly preserved mollusks. The contact of the Skooner Gulch with the overlying 

Gallaway Formation of Weaver (1944) is clearly exposed (fig. 3) in 

the intertidal zone about a quarter of a mile south of Schooner Gulch 

where the formation is about 200 feet thick. In this area it is placed at 

the highest stratigraphic occurrence of brown glauconitic fine-grained 

sandstone and is marked by a wave-resistant bench standing higher in 

the intertidal zone than the overlying gray weathering foraminiferal 

mudstone of Weaver's Gallaway Formation which is planed-off into a 

broad reef exposed only during low tide (fig. 3). Near the mouth of 

Schooner Gulch the contact between the Skooner Gulch and Gallaway 

Formations appears to be gradational. Farther south, however, the 

glauconitic and phosphatic sandstones of the upper part of the Skooner 

Gulch Formation are progressively truncated by the overlying gray 

weathering sandy mudstones of the Gallaway Formation. The southernmost exposures of the formation in the intertidal zone at a small 

cove about 3,500 feet south of Schooner Gulch are only about 20-30 

feet thick. The Gallaway consists of about 1,200 feet of foraminiferal 

mudstone with interbedded sandstone and concretionary strata.

The Iverson Basalt of Weaver (1944) and the overlying sandstone 

and foraminiferal mudstone of the Skooner Gulch and Gallaway Formations form a simple westward-dipping homoclinal structure in the 

vicinity of Schooner Gulch that is broken only by minor faults. Excellent continuous exposures of these formations crop out in seacliffs 

and in the intertidal zone from Iverson Point northward for about 2 

miles to near Ross Creek (fig. 1). This stretch includes the type areas 

for each of the formations.

FAUNAL ASSEMBLAGE

Marine megafossils occur in a seacliff 6-12 feet above a gravel beach 

about two-thirds of a mile south of Schooner Gulch (USGS Cenozoic 

Iocs. M2494 and M2638) at the base of massive sandstone mapped as the 

Skooner Gulch Formation by Weaver (1944). The locality (fig. 2) is 

accessible only during a low tide. Poorly preserved mollusks, principally turritellas and small pectinids, are scattered in a 6- to 12-inch 

bed of light-greenish-gray poorly sorted sandstone that unconf ormably 

overlies the Iverson Basalt of Weaver (1944). A few hundred feet
C4 CONTRIBUTIONS TO STRATIGRAPHY

lF'y«

FIGURE 2. Contact between the Skooner Gulch Formation (Tsg) and the underlying Iversen Basalt (Ti) of Weaver (1944) in seacliff about 3,000 feet south of 

mouth of Schooner Gulch. USGS Cenozoic Iocs. M2494 and M2638 are at the 

very base of the Skooner Gulchi Formation about 6-12 feet above the gravel 

beach.

farther south the basal sandstone contains closely packed valves of 

very poorly preserved, indeterminate pelecypods.

Near the top of the Skooner Gulch Formation is a 15-foot interval of 

fine-grained glauconitic sandstones with abundant nodules of phosphatic material up to 3 inches in diameter. Scattered shark teeth, fish 

vertebrae, and bone fragments occur in these uppermost fine-grained 

sandstones. A cheek tooth of Desmostylus (identified by C. A. 

Repenning, oral commun., Oct. 1966) has also been collected from near 

the top of the formation.

The molluscan assemblage from the base of the Skooner Gulch Formation consists of Turntelld inezcma forma hoffmanni Gabb, Chlamys 

cf. C. hertleini Loel and Corey, Chlamys aif. C. mollita (Reeve), 

Litcinoma sp., a large cardid, and locally abundant, undetermined
SKOONER GULCH FORMATION, MENDOCINO COUNTY, CALIF. C5

FIGURE 3. Contact between the Skooner Gulch Formation and the overlying Gallaway Formation of Weaver (1944) in intertidal zone about a 

quarter of a mile south of Schooner Gulch. Knapsack is lying on contact 

separating glauconitic fine-grained sandstone of the upper part of the Skooner 

Gulch Formation from algae-covered sandy mudstone of the basal part of the 

Gallaway Formation.

270-514 O 67 2
C6 CONTRIBUTIONS TO STRATIGRAPHY

thick-shelled pelecypods. There are also numerous burrows, one-half 

to three-fourths of an inch in diameter, oriented normal to the bedding. 

The pectinids and Turritella are illustrated in figure 4.

Turritella inezana forma hoffmaiwi is represented by a few fairly 

well preserved specimens, the better preserved of which are illustrated 

in figure 4. These specimens have been compared with material collected near the type locality of this form in the Santa Cruz Mountains, 

about 140 miles to the south of Point Arena (USGS Cenozoic loc. 

M2002). Several additional specifically indeterminate turritellas were 

collected from the locality.

Although doubtfully identified, specimens of Chlwnys cf. C. hertleini are very similar to specimens of C. hertleini from central California; the degree of resemblance can be seen in figures 44, B, D, G.

A very finely ribbed pectinid, unlike any known Tertiary species 

from the northeastern Pacific Ocean (fig. 4C), resembles the early Pliocene to Recent Chlamys moTlita (Reeve) (Masuda, 1962, pi. 21, fig. 7; 

pi. 23, figs. 6-8) from Japan. Chlamys mollita is a shallow-water species, 5-10 fathoms (Kira, 1962, p. 121), which lives along the Pacific 

coast of Japan from 31° to 35° N. and in the Sea of Japan (Masuda, 

1962).

AGE AND POSITION IN THE PROVINCIAL 

CHRONOLOGY

Turritella inezana forma hoffmanni and Chlamys cf. C. hertleini 

suggest correlation with the "Vaqueros Stage" of the Pacific coast 

megafaunal chronology (Weaver and others, 1944). The "Vaqueros 

Stage" is considered early Miocene by molluscan paleontologists although some microfossil specialists prefer to call it Oligocene.

The range of Turritella inezana is regarded as coextensive with the 

"Vaqueros Stage" by megafossil workers (Loel and Corey, 1932 2 ; 

Eaton and others, 1941 ; Weaver and others, 1944), and this has long 

been used as the principal index fossil for this chronostratigraphic 

unit in California. The subgeneric group to which this species is assigned first appears in rocks of "Vaqueros" age in California (Merriam, 

1941). It is a distinctly different lineage from the Pacific coast Oligocene turritellas. The variety T. inezana forma liafimanni is closely 

related to T. inezana ss.; it was included in the synonymy of that taxon 

by Loel and Corey (1932). Occurrences of this form within the "Vaqueros Stage" range from what is considered the lower part in the

2 This is the definitive report on the "Vaqueros Stage." Although these authors did not 

use the term "stage," their treatment of their Vaqueros Formation or Vaqueros Horizon is 

more nearly that of a time-stratigraphic unit rather than a rock-stratigraphic unit. Quotation marks are used herein to distinguish the Vaqueros Stage of the Pacific coast megainvertebrate time-stratigraphic classification of Weaver and others (1944) from the 

Vaqueros Formation.
SKOONER GULCH FORMATION, MENDOCINO COUNTY, CALIF. C7

Caliente Range of eastern San Luis Obispo County, Calif. (Eaton 

and others, 1941) to fairly high in the lower Mioceie sequence in the 

Temblor Range of northwestern Kern County (Cf.rneros Sandstone 

Member of the Temblor Formation of Cunningham and Barbat, 1932 ; 

USGS Cenozoic Iocs. M2623 and M262T) .

Fragmentary specimens of a Chlamys from USG JS Cenozoic locality M2638 are similar to specimens of C. hertleini'Lcel and Corey (fig. 

42?, D) from localities in Kern County, Calif. Ths fragments have 

weakly imbricate primary ribs that alternate with fine secondaries 

(fig. 4J., Cr). Chlamys hertleini is also restricted to the "Vaqueros 

Stage." Loel and Corey (1932, p. 138) originally indicated its range as 

restricted to their middle zone but subsequent evidence has extended 

its range to the upper part of the "Vaqueros Stage" (Addicott, 1965).

Foraminif era have not been found in the Skooner Gulch Formation. 

Microfossil samples collected by the writer from the lower part of the 

overlying Gallaway Formation of Weaver (1944), about 20 feet and 

k220 feet above the glauconitic sandstone at the top of 1 he Skooner Gulch 

Formation (USGS Iocs. Mf851 and Mf852), coi tain fairly large 

faunules referred to the Zemorrian Stage of Kleinpe 1 (1938) by Patsy 

J. Smith (written commun., Dec. 1965). The lowei sample (Mf851) 

contained the following species listed by Kleinpell 1938, p. 137-151) 

as restricted to the lower part of his Zemorrian Sti tge: Nonion affine 

(Reuss), N, ynezianum Kleinpell, Valvulineria casi'asensis gubcasitasensis Kleinpell, Eponides frizzelli Kleinpell, anl Cibicides pseudoungeriamis evolutus Cushman and Hobson. r ?he stratigraphic 

ranges of two of these species, N onion affine and C\ bicides pseudoungerianus evolutus, as well as the range of the zonal index, Umgerina 

gallowayi, have subsequently been extended upward into the lower 

part of the Saucesian Stage (Kleinpell and Weaver, 1933, p. 39). Foraminif era of the lower Zemorrian Uvigerina gaUow* lyi zone wTere first 

reported from localities 50-175 feet stratigraphies! ly above the base 

of the Gallaway Formation on the north side of the r louth of Schooner 

Gulch and from a 210-foot interval in the Gallaway Formation at 

Ross Creek about a mile to the northwest (Kleinpell, 1938, p. 76) . Additional collections from the Gallaway and from the lower part of the 

overlying Point Arena Formation were identified )y Weaver (1944, 

p. 23-25) but were not correlated with the Pacific joast microfaunal 

sequence.

From the stratigraphic relationships of Twrite*la inezana forma 

hoffmanni from the Skooner Gulch Formation to Joraminifera from 

the lower part of the overlying Gallaway Formati >n it is clear that 

the biozone of Turritella inezana includes strata that have been classified as lower Zemorrian. Although this seems to be the first record of
C8 CONTRIBUTIONS TO STRATIGRAPHY

B

D

E F G

FIGURE 4 Mollusks from the Skooner Gulch Formation, Mendocino County, and 

Jewett Sand of Godde (1928), Kern County, Calif. (Explanation on opposite 

page.)
SKOONER GULCH FORMATION, MENDOCINO COUNTY, CALIF. C9

T. inezana occurring with or below lower Zemorrian foraminifers, 

other molluscan guides to the "Vaqueros Stage" such as Lyropecten 

'magnolia and Ostrea vaquerosensis occur stratigraphically below lower 

Zemorrian microfossils in the central California Coast Ranges a few 

miles northeast of Santa Maria (Kleinpell, 1938, p. 73, fig. 14; Loel 

and Corey, 1932, p. 102-103).

The significance of the apparent range extension of T. inezana indicated by its occurrence stratigraphically below f oraminifera referable 

to the lower part of the Zemorrian Stage seems to be clouded somewhat 

by indications that the lower part of the Zemorrian Stage is in need of 

redefinition (Rothwell, 1965). Moreover, Kleinpell and Weaver (1963, 

p. 37) stated that in some parts of central California a deep-water 

facies of the Zemorrian may have persisted into Saucesian time. The 

age of the Gallaway Formation may be affected by this revision as 

Kleinpell and Weaver (1963, p. 43) indicate a "Zemorrian and (or) 

lower Saucesian" age; however, on their correlation chart (fig. 5) an 

early Zemorrian to early Saucesian age is indicated for the "Galloway 

Beds of the Mendocino Coast." After examining foraminiferal assemblages and previously reported faunal lists from the basal part of the 

Gallaway Formation, Richard Pierce (written commun., Dec. 1966) 

concluded that the presence of Siphogenerina multicostata Cushman 

and Jarvis in the samples (Kleinpell, 1938, p. 76) is more suggestive of 

late Zemorrian or Saucesian than of early Zemorrian age.

A cheek tooth of Desnwstylus, collected by the writer and Richard 

Pierce 12 feet below the top of the Skooner Gulch Formation (USGS 

vertebrate loc. M1114), is compatible with the molluscan evidence of 

an early Miocene age for the formation. The well-documented stratiEXPLANATION OF FIGURE 4

A. Chlamys cf. C. hertleini Loel and Corey. X 2. USNM 649592, a rubber cast.

Basal part of Skooner Gulch Formation, USGS Cenozoic loc. M2638. 

B. Chlamys hertleini Loel and Corey. X 1. UCLA 33491.

"Top of Pyramid Hill Sand" [Jewett Sand of Addicott (1965)], Kern

County, Calif., UCLA loc. MB 2000. 

C. Chlamys aff. C. mollita (Reeve). X 2. USNM 649593, a rubber cast.

Same locality as A. 

D. Chlamys hertleini Loel and Corey. X 1. UCMP 32254.

About 170 feet above base of Jewett Sand of Godde (1928), Kern County,

Calif., UCMP loc. B1654. 

E. Turritella inezana forma hoffmanni Gabb. X \%. USNM 649594.

Same locality as A. 

F. Turritella inezana forma hoffmanni Gabb. X 1. USNM 650329.

Basal part of the Skooner Gulch Formation, USGS Cenozoic loc. M2494. 

G. Chlamys cf. C. hertleini Loel and Corey. X 2. USNM 649595, a rubber cast.

Same locality as A.
CIO CONTRIBUTIONS TO STRATIGRAPHY

graphic range of this marine mammal (Mitchell and Eepenning, 1963) 

extends from the "Vaqueros Stage" to the "Neroly Stage" of the molluscan chronology and from the lower Saucesian to the lower Delmontian of the microf ossil chronology.

ZOOGEOGRAPHIC SIGNIFICANCE

The occurrence of a variant of Turritella inesana near Point Arena 

lat 38.9° N.) is a significant northward extension of this species' 

geographic range from localities in the Santa Cruz Mountains (lat 

37.4° N.) ,3 (Gabb, 1866; Arnold, 1906). It also marks the northernmost 

occurrence of the subgenus Torcula (the Turritella dltilira stock of 

Merriam, 1941), a taxonomic unit that is essentially tropical in its 

modern distribution in the eastern Pacific Ocean. The occurrence of 

this subgenus near Point Arena suggests that a subtropical shallowwater marine climate occurred at least this far north during the biochron of Turritella inezana. The small pectinids are compatible with 

this interpretation. Chl-amys liertleini occurs in warm water "Vaqueros" assemblages elsewhere in California. The northwestern Pacific 

Ocean species C. mollita, with which another pectinid is compared, 

lives in a shallow-water subtropical province off the east coast of 

Japan.

REFERENCES

Addicott, W. O., 1965, Miocene macrofossils of the southeastern San Joaquin

Valley, California, in Geological Survey Research 1965: U.S. Geol. Survey

Prof. Paper 525-C, C101-C109, 4 figs. 

American Committee on Stratigraphic Nomenclature, 1961, Code of Stratigraphic

Nomenclature: Am. Assoc. Petroleum Geologists Bull., v. 45, no. 5, p. 645-665. 

Arnold, Ralph, 1906, The Tertiary and Quaternary pectens of California: U.S.

Geol. Survey Prof. Paper 47, 264 p., 53 pis. 

Cunningham, G. M., and Barbat, W. F., 1932, Age of the producing horizon at

Kettleman Hills, California: Am. Assoc. Petroleum Geologists Bull., v. 16,

no. 4, p. 417-421. 

Eaton, J. E., Grant, U.S., 4th, and Alien, H. B., 1941, Miocene of Caliente Range

and environs, California: Am. Assoc. Petroleum Geologists Bull., v. 25, no. 2,

p. 193-262. 

Gabb, W. H., 1866, Tertiary invertebrate fossils, in Paleontology: California

Geol. Survey, v. 2, sec. 1, pt. 1, p. 1-38. 

Godde, H. A., 1928, Miocene formations in the east side fields of Kern County:

California Oil Fields, v. 14, no. 1, p. 5-15. 

Kira, Tetsuaki, 1962, Coloured illustrations of the shells of Japan: Osaka, Japan,

Hoikusha, 239 p., 71 pis. [In Japanese.]

3 A record of this species from USGS locality 5450 on the Umpqua River, Douglas County, 

Oreg., by Dall (in Washburne, 1914) is believed to be a misidentiflcation or otherwise in 

error because this locality is in the type section of the middle Eocene Tyee Formation 

(Turner, 1938, p. 19).
SKOONER GULCH FORMATION, MENDOCINO COUNTY, CALIF. Cll

Kleinpell, R. M., 1938, Miocene stratigraphy of California: Tulsa, Okla., Am.

Assoc. Petroleum Geologists, 450 p. 

Kleinpell, R. M., and Weaver, D. W., 1963, Oligocene biostratigraphy of the Santa

Barbara embayment, California: California Univ., Pubs. Geol. Sci., v. 43, 250

p., 38 pis. 

Loel, Wayne, and Corey, W. H., 1932, The Vaqueros formation, lower Miocene of

California; I, Paleontology: California Univ., Dept. Geol. Sci. Bull., v. 22,

no. 3, p. 31^10, pis. 4-65. 

Masuda, Koichiro, 1962, Tertiary Pectinidae of Japan: Tohoku Univ. Sci. Repts.,

ser. 2 (geology), v. 33, no. 2, p. 117-237, pis. 18-27. 

Merriam, C. W., 1941, Fossil Turritellas from the Pacific Coast Region of North

America: California Univ., Dept. Geol. Sci. Bull., v. 26, no. 1, 214 p., 41 pis.,

19 figs. 

Mitchell, E. D., Jr., and Repenning, C. A., 1963, The chronologic and geographic

range of desmostylians: Los Angeles County Mus. Contr. Sci., v. 78, p. 1-20. 

Rothwell, W. T., Jr., 1965, Regional correlation of Pacific Coast Oligocene microfaunas : Am. Assoc. Petroleum Geologists Bull., v. 49, no. 10, p. 1766. 

Turner, F. E., 1938, Stratigraphy and Mollusca of the marine Eocene of western

Oregon : Geol. Soc. America, Spec. Paper 10,130 p., 22 pis. 

Washburne, C. W., 1914, Reconnaissance of the geology and oil prospects of

northwestern Oregon : U.S. Geol. Survey Bull. 590, 111 p. 

Weaver, C. E., 1943, Point Arena-Fort Ross area [California] : California Div.

Mines Bull. 118, p. 628-632. 

 1944, Geology of the Cretaceous (Gualala Group) and Tertiary formations

along the Pacific Coast between Point Arena and Fort Ross, California:

Washington Univ. Pubs. Geology, v. 6, no. 1, 29 p. 

Weaver, C. E., and others, 1944, Correlation of the marine Cenozoic formations of

western North America: Geol. Soc. America Bull., v. 55, no. 5, p. 569-598. 

Wentworth, C. M., 1966. The upper Cretaceous and lower Tertiary rocks of the

Gualala area, northern Coast Ranges, California: Stanford, Calif., Stanford

Univ. Ph. D. thesis.

o