A LATE MIOCENE FLORA FROM THE SOLEMINT AREA, LOS ANGELES COUNTY, CALIFORNIA
by Jack D. Mount¹
During January 1971, Professor Perry Ehlig, Department of
Geology, California State College, Los Angeles (CSCLA), showed me a suite of fossil leaves which he and several students collected while mapping the geology of an area about midway between Solemint and Placerita Canyon which are situated in the northwestern part of Los Angeles County, California. The following month Mr. Phil Watson, a senior geology student at CSCLA, guided me to the fossiliferous exposure.
Axelrod (1940) published a preliminary report on the flora of this area which he collected from three localities (two in Boquet Canyon and one in Sand Canyon). Since the plant material from the new site is fairly abundant, well preserved and has yielded several species not reported by Axelrod, I believe it is interesting and valuable to prepare this brief discussion of the new locality.
The collecting site is near the crest of a small northwest trending ridge about 1½ miles southwest of Solemint (see page 2, upper and middle maps) and adjacent to the Antelope Valley Freeway which is now under construction. This locality is registered as CSCLA Locality 860 and is 1350 feet east and 250 feet north of the SW corner of section 28, T.4 N., R.15 W., Mint Canyon 7.5' Quadrangle (1960 edition) (or Los Angeles County grid system B147,800, N256,250).
CSCLA Locality 860 is in a 4 feet thick rhyolitic tuff bed which is the upper of two volcanic tuff beds in the upper member of the Mint Canyon Formation. The Mint Canyon Formation was named by Kew (1923) and redefined by Jahns (1939) and consists of 4500 feet of normarine conglomerates, sandstones, siltstones and clays. Between Solemint and Placerita Canyon the Mint Canyon Formation is highly folded and faulted (see page 2, lower map). Axelrod's Sand Canyon locality is 2.6 miles east of Locality 860, in a tuff bed at about the same stratigraphic, level as Locality 860.
Flora and Fauna
The flora of the Mint Canyon Formation is listed in Table 1 and consists of 18 plant species. Eight of these plants have been recovered from CSCLA Locality 860. The majority of the plant material from this locality consists of leaves. One small cone,
similar to a juniper or cypress cone was collected and is tentatively identified as Cupressus (?) sp. Axelrod (1939) is very-useful in identifying plants from this flora.
A poorly preserved impression of a freshwater snail similar in shape to Helminthoglypta was uncovered. Future collecting may provide identifiable specimens.
Table 1. The late Miocene flora of the Mint Canyon Formation.
The following plants have been reported by Axelrod (1940). Those taxa with an asterisk have been identified in material from CSCLA Locality 860.
|Ceanothus precuneatus Axelrod||(buck-brush)|
|*Ceanothus n. sp.||(island-lilac)||(fig. la)|
|*Cercocarpus cuneatus Dorf||(mountain-mahogany)||(fig. 1b)|
|Crossosoma n. sp.||(crossosoma)|
|Cyperacites n. sp.||(Sedge Family)|
|Diospyros n. sp.||(persimmon)|
|*Fraxinus edensis Axelrod||(flowering ash)||(fig. 1c)|
|Fremontia lobata Axelrod||(slippery-elm)|
|*Lyonothamnus mohavensis Axelrod||(Mojave ironwood)||(fig. 1d)|
|Mahonia mohavensis Axelrod||(barberry)|
|Platanus paucidentata Dorf||(sycamore)|
|*Quercus convexa Lesquereux||(oak)||(fig. 1e,f)|
|*Quercus dispersa (Lesquereux)||(scrub oak)||(fig. 1g)|
|Quercus lakevillensis Dorf||(live oak)|
|Rhus sonorensis Axelrod||(evergreen sumach)|
|Robinia californica Axelrod||(locust)|
The following taxa occur at CSCLA Locality 860 but were not found at Axelrod's localities.
|Cupressus (?) sp.||(cypress)|
The sediments of the Mint Canyon Formation were deposited in a semi-arid basin with a shallow freshwater lake. Quoting Axelord (1940): "The flora is essentially an oak savanna community whose nearest related modern equivalent species now occur in southern California, southern Arizona, and northern Mexico. At least four habitats contributed to the flora, lake-border and riparian, savanna,, woodland and chaparral. The Mint Canyon climate was largely similar to the present conditions in the region, but differed in having a biseasonal distribution of rainfall and winter temperatures which may have been slightly higher."
The Mint Canyon flora has its greatest relationship with the mid Miocene Tehachapi flora, 13 of the 18 Mint Canyon species are represented in that flora. "The Mint Canyon flora is an integral part of the arid north Mexican vegetation that characterized the Mojave area in middle Miocene time, and which, by the end of the epoch, had migrated westward into southern and central California, northward through the Great Basin, and eastward into the High Plains." (Axelrod, 1940)
Vertebrate fossils in the Mint Canyon Formation indicate an age of late Miocene to early Pliocene. Maxson (1930), Stirton (1933) and Durham, Jahns, and Savage (1954) have reported on these vertebrates. This fauna includes Merychippus, Hipparion, Alticamelus, rhinoceroses, antelopes and carnivores.
Axelrod, D. I., 1939, A Miocene flora from the western border of the Mohave desert: Carnegie Inst. Wash. Pub 516, 129 p.
1940, The Mint Canyon flora of southern California: A preliminary statement: Am. Jour. Sci., vol.238, no.8, p.577-585.
Durham, J. W., Jahns, R. H., and Savage, D. E., 1954, Marine-nonmarine relationships in the Cenozoic section of California: Calif. Div. Mines Bull. 170, ch.3, p.59-71.
Jahns, R. H., 1939, Miocene stratigraphy of the eastern-most Ventura basin, California: A preliminary statement: Am. Jour. Sci. vol. 237, p.818-825.
Kew, W. S. W., 1923, Geologic formations of a part of southern California and their correlation: Am. Assoc. Pet. Geol. Bull., vol.7, p.411-420.
Maxson, J. H., 1930, A Tertiary mammalian fauna from the Mint Canyon Formation of southern California: Carnegie Inst. Wash. Pub. 404.
Stirton, R. A., 1933, Critical review of the Mint Canyon mammalian fauna and its correlative significance: Am. Jour. Sci., vol. 26, p.569.
¹Department of Geology, University of California, Los Angeles, 90024.
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