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GEOLOGY OF SOUTHEASTERN VENTURA BASIN 285

graphic relationships of the Mint Canyon formation Canyon formation also unconformably overlies Eocene
to the younger marine rocks, believed the range of the sedimentary rocks and the crystalline basement com
genus extends to the Miocene. plex, considered to be pre-Cretaceous.
Although Kew (1924, p. 52) recorded a marked un
conformity between the marine rocks and the Mint STRATIGRAPHY AND LITHOLOGY IN THE MAPPED AREA
Canyon formation in Haskell Canyon, Stirton (1933,
In the northeastern part of the mapped area, the Mint
p. 569–576) and later Clements (1937, p. 215) indicated
Canyon formation crops out in a predominantly fine
that the relationship is an interfingering or gradational
grained sequence of greenish-gray siltstone units and
one. Jahns (1939, p. 822) was able to demonstrate a
interstratified sandstone, conglomerate, and thin tuff
distinct, though in places slight, angular discordance
beds. The formation has been compressed into a series
between the marine rocks and the Mint Canyon for
of comparatively tight, nearly westward trending folds.
mation. He agreed with Eaton (1939, p. 534) that
Resistant sandstone and conglomerate beds form prom
the Mint Canyon formation thins to the northwest by
inent hills and ridges; less resistant siltstone units form
a loss of basal beds.
a more subdued topography that includes the hum
Grant's examination of additional collections of
mocky landscape typical of landslide areas.
invertebrates from the marine rocks (Maxson, 1938a,
Greenish-gray siltstone units include beds of mud
p. 1716–1717) permitted a more exact determination
stone and claystone and thin beds of sandstone and
than had previously been possible of their age as Neroly
tuff. The siltstone commonly has sharp contacts with
(uppermost Miocene). Kleinpell (1938, p. 71) referred
interstratified sandstone and conglomerate, although at
meager foraminiferal faunas collected from the marine
some places the contacts are gradational. Bedding in
rocks to the Delmontian stage and agreed with the
the siltstone is generally indistinct or contorted due to
concept of the contemporaneity of the marine rocks
the folding of the beds. Gypsum, found along the
and the Mint Canyon formation. Later, M. N.
bedding planes and in fractures, and small bits of
Bramlette examined foraminiferal collections from the
carbonaceous matter are conspicuous in the siltstone.
lower part of the marine sequence and considered the
Fresh-water fossils are abundant in some horizons.
faunas as indicative of a late Mohnian (pre-Delmontian
The fresh-water gastropod Paludestrina imitator Pilsbry
late Miocene) age.
has been identified (Kew, 1924, p. 54) in the formation.
The Mint Canyon formation is undeniably older than
The vitric and crystal tuff beds are massive and white
marine rocks deposited during at least part of late
Miocene time, based on the invertebrate time scale of to gray; they break with a blocky or conchoidal frac
California. Jahns (1940, p. 172) considered the Mint ture. The tuffs are commonly finely laminated, cross
Canyon formation to be of late Miocene age, a desig stratified, and ripple marked.
nation consistent with the stratigraphic position of the Most of the sandstone beds are light tan or yellowish
formation in the marine sequence. He also pointed brown but some are greenish gray. They are arkosic,
out that the faunal gradation within the formation and generally poorly sorted, and show cross-stratifica
indicates that the deposition took place over a consid tion, channeling, and local erosion of beds. Pebbly
erable time interval. sandstone beds and sandstone beds with pebble string
The Mint Canyon formation (restricted) of Jahns ers and lenses are common; the beds are from a few
inches to several tens of feet thick.
(1939) unconformably overlies his Tick Canyon and
older formations. Jahns (1940, p. 174–175) regarded Pebble, cobble, and boulder conglomerates contain
his Tick Canyon formation as late early Miocene or clasts that range from angular to well rounded, but
possibly earliest middle Miocene in age. the majority are subrounded. A count of 430 clasts
The Mint Canyon and Tick Canyon formations of from one pebble conglomerate showed the following
Jahns (1939) overlie with pronounced unconformity percentages of rock types:
beds called the Escondido series by Hershey (1902,
Gneiss----------------------------------------- 58. 5
p. 349–372), referred to the Sespe(?) formation by Volcanic rocks---------------------------------- 34. 8
Kew (1924, p. 38–39), and named Vasquez series by Anorthosite (and related rocks) ------------------- 3. 0
Sharp (1935, p. 314) since Hershey's term is pre Quartzite-------------------------------------- 1. 4
Occupied. The Vasquez is of continental origin, is Schist----------------------------------------- ... 2
Miscellaneous---------------------------------- 2. 1
unfossiliferous, and has been tentatively referred to the
Oligocene by Jahns (1940, p. 170–171). The Mint 100. 0
| Daviess, S. N., 1942, Contact relationship between Mint Canyon formation and The igneous and metamorphic rock types are like
upper Miocene marine beds in eastern Ventura basin, Los Angeles County, Califor
ila: Unpublished M. S. thesis, California Univ. at Los Angeles. those found in the nearby San Gabriel Mountains and