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284 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

thickness of about 2,500 feet of similar rocks, including stones, which are arkosic and commonly cross-strati
at least one thin layer of amygdaloidal basalt. fied, occur in the formation. Tan, gray, green, and
Both at the outcrop and in the well the unit is over pink siltstone and clay beds as well as white and gray
lain by rocks representing the Luisian stage (upper vitric and crystal tuff beds are interstratified with
middle Miocene) of Kleinpell. Because of their strati coarser deposits. Some fine-grained units grade later
graphic position below upper middle Miocene rocks and ally into coarser grained beds; this gradation, together
because of the presence of a basalt flow, these beds are with the lenticular nature of the coarser grained beds,
tentatively correlated with the Topanga formation of makes it difficult to trace lithologic units and horizons
middle Miocene age. within the formation. Tuff beds are useful marker
beds even though many of them are less than 4 feet
MINT CANYON FORMATION
thick.
NOMENCLATURE
Thi CKN ESS
Hershey (1902, p. 356-358) gave the name “Mellenia
Jahns (1940, p. 162) ascribed an aggregate thick
series” to the sequence of rocks that includes the Mint
ness of about 4,000 feet to the Mint Canyon formation
Canyon formation. Because the term Mellenia is not
in the Bouquet Canyon region. Oakeshott (1950, p.
a place name, Kew (1924, p. 52) designated these rocks
53) reported the formation to be more than 2,400 feet
the Mint Canyon formation in recognition of their
thick in the area south of the Santa Clara River. A
excellent development in the Mint Canyon region.
complete section is not exposed in the area shown on
Jahns (1939, p. 819) demonstrated that north of the
the geologic map.
area of this report the rocks that Kew named the Mint
Canyon formation consist of two formations rather STRATIGRAPHIC RELATIONS AND AGE
than one; he suggested that the lower part be called the
The first discovery of vertebrate remains in the
Tick Canyon formation and the term Mint Canyon
Mint Canyon formation was made by Kew in 1919
formation be retained for the upper beds. Only rocks
during reconnaissance mapping Following the dis
from the upper part of the Mint Canyon formation as
covery of additional material, Maxson (1930) compared
defined by Kew, and hence also from the Mint Canyon
the fauna with those of other regions, and, after con
formation as restricted by Jahns, are exposed in the area
sidering the position of the Mint Canyon formation
shown on the geologic map (pl. 44). The older Tick
unconformably below marine strata regarded by Wood
Canyon formation of Jahns (1939) is not exposed, nor
ring (1930, p. 155) as the approximate equivalent of
is it known to be present in the subsurface in the
the Cierbo sandstone of northern California, he con
mapped area.
cluded that the Mint Canyon was deposited during
DISTRIBUTION
approximately the middle part of the late Miocene.
The Mint Canyon formation crops out on the north In a critical review of Maxson's work, Stirton (1933,
side of the Santa Clara River valley between Agua p. 569–576) differed with him on the identification of
Dulce and Elizabeth Lake Canyons (outside the the mammalian forms and advocated an early Pliocene
mapped area) and south of the valley between Sand age for the fauna. Subsequent papers by Teilhard de
Canyon and Honby (partly in the mapped area). It is Charden and Stirton (1934), Stirton (1936; 1939),
not found south of the San Gabriel fault. Possible
McGrew and Meade (1938), Lewis (1938), and Maxson
large right-lateral displacement along this fault (Crowell, (1938 a,b) have dealt with the paleontological aspect
1952a) may explain the restriction of the known extent of the age of the Mint Canyon formation. As Reed
of the formation to the area north of the fault.
and Hollister (1936, p. 40, 43) pointed out, the basic
problem in the assignment of an age to the Mint
GENERAL LITHOLOGY
Canyon fauna is the equivalence of the lower Pliocene
The lithologic character of the Mint Canyon forma
of most vertebrate paleontologists and the upper
tion has been discussed in detail by Kew (1924, p. 52–53)
Miocene of most California invertebrate paleontolo
and Jahns (1940, p. 154–163). Jahns (p. 163) noted
gists. There are two major considerations in deter
that “The beds in the lower half of the section are
mining the age of the Mint Canyon formation: (a) the
characteristically fine-grained, thin-bedded, and of
occurrence of Hipparion in the Mint Canyon fauna,
variegated colors, whereas those higher up are more
and (b) the occurrence of marine beds unconformably
irregular, coarser, and subdued in color.” The con
overlying the formation. The presence of Hipparion
glomerates are typically lenticular, cross-stratified,
indicates an early Pliocene age to many vertebrate
poorly sorted, and locally sandy. An abundance of
gneiss, schist, and volcanic clasts is characteristic of the paleontologists. Stirton (1933) adhered to this con
formation. Both well and poorly consolidated sand cept, but Maxson (1930), largely because of the strati