Bulletin of the Southern California Paleontological Society, vol. 1, no. 8, August 1969, p. 1-6


by    Jack D. Mount¹


      In the Los Angeles basin the late Miocene is represented by the Puente Formation. It is composed generally of shale, siltstone, sandstone and conglomerate (Yerkes and others, 1965, plate 2). The present study is concerned with the exposures of the Puente Formation in the western Repetto. Hills which form the northern boundary of the Los Angeles basin.
      The purpose of this report is to study the petrology and sedimentary structures of the clastic sediments of the Puente Formation and give an interpretation of the sedimentary processes and environment of deposition. This report is offered as an example of how the paleoenvironment of an unfossiliferous sedimentary unit may be determined without the benefit of biological evidence (However, a tooth of the shark Carcharodon sulcidens Agassiz was collected at CSCLA locality number 609.).

Area of Investigation

      The study area is located on the campus of the California State College at Los Angeles in the SE¼, sec. 19, and SW¼, sec. 20, T.1S., R.12W, San Bernardino Base Line and Meridian, Los Angeles Quadrangle (USGS 7½). This investigation is limited to the exposed road cuts along the west side of Campus Road between Cravois Avenue on the north and State College Drive to the south (Fig. 1).

Method of Study

      One sandstone sample was selected for study. It was treated with dilute HCl and after complete disintegration of the CaCO3 cement, the residue was sieved through a nest of seven Tyler standard screens, each, one phi unit different. The material was then weighed so the grain size frequency could be computed. The data on phi statistical measures were calculated following Krumbein and Pettijohn (1938, p.251).
      The mineralogy was obtained by observing grains in oils under the petrographic microscope. A visual estimation was used for the mineral percentage. The petrologic terminology follows Compton (1962, p.217-219).



      The stratigraphic sequence in the study area is composed entirely of the Puente Formation and is approximately 2,000 feet thick. The underlying and overlying formations are not exposed. The formation is characterized and generally consists of light brown to gray or nearly white, thinly bedded siltstone. The siltstone is platy and the laminations are very thin. In some beds the siltstone grades to a silty shale. Occasionally, one foot thick beds occur which are composed of highly indurated concretionary siltstone.
      Interbedded with the siltstone are thick beds (1-4 feet) of light brown to red-brown sandstone. In the lower part of the section, sandstone beds and silty shale beds occur in cyclic repetitions.
      The general sequence is occasionally interrupted by 3 to 4 foot thick beds of gray pebble conglomerate. The clasts are fragments of quartzite, quartz diorite and granodiorite.

Sandstone Lithology

      The clasts in the sandstone and in the pebble conglomerate are subangular to subrounded and have an average or medium sphericity. The cement for these rocks is CaCO3 and is about 1 percent of the whole rock. The exposed rocks are friable but fresh samples are well indurated.

      Grain mineralogy: The average sandstone consists of the following minerals:

K-feldspar     - 10%
Plagioclase     - 30%
Biotite     - 10%
Garnet     - 1%
Epidote     - 1%
Rock fragments     - 8%
Quartz     - 40%
The sandstone would therefore be termed arkosic arenite.
      Grain size analysis: One sample of arenite which is considered typical for the sequence was chosen for grain size analysis. The data for phi statistical analysis is as follows:
Phi mean = 1.24
Phi deviation = 0.36
Phi skewness = -0.13    
Phi median = 1.07
      Sorting designation = well sorted (this is based on the classification of phi deviation values after Friedman (1962, table 4 and figure 6)). In Fig. 2 a histogram, frequency curve and cumulative curve is given for the sample.
      By comparing the phi deviation with Table 1, it is found that the arenite is related to beach or offshore bar sand, as opposed to dune, eolian or river sands. Also, Friedman (1961, p.526) reported that dune and river sands are positively skewed and beach sands negatively skewed. The arenites are, thus, in the beach sand group.

Fig. 1. Geologic map of a portion of the northeast section of the City of Los Angeles, California.


Fig. 2. Sandstone sample - graphs.

AuthorPhi Deviation     Sorting Desig.     Type of Sand    
Krumbein and Sloss (1963)  0.33very welldune
Schlee et al. (1964)  0.34very welleolian
Shepard and Young (1961)-0.35welleolian
Mabesoone (1964)<0.56moderately welleolian
Giles and Pilkey (1965)  0.70moderately welldune
Schlee et al. (1964)  0.31very wellbeach
Inman and Chamberlain (1955)      0.25-0.35very well-wellbeach
Shepard and Young (1961)  0.38wellbeach
Dodge (1965)  0.35-0.63well-moderateoffshore bar
Giles and Pilke (1965)  0.82moderatebeach
Mabesoone (1964)<0.85well-moderatebeach
Friedman (1962)  1.52poorgraywacke
Mabesoone (1964)>0.85moderate-poorriver
Giles and Pilkey (1965)  0.95moderateriver
Hoque (1968)  0.75moderateprotoqtz.
Hoque (1968)  0.80moderatesubgraywacke

Table 1. Sorting characteristics of various types of sand (from Hoque, 1968, p.255).

Sedimentary Structures

      In the lower part of the section the arenite and pebble conglomerate exhibit well graded bedding. This indicates that the grains were in suspension and able to respond hydrodynamically, Thus, following the genetic classification of Natland (1967, p.476), the arenite can be classified as a turbidite. Using the same classification, the silty shales which are interbedded with the turbidites can be considered hemipelagites indicating an interval of quiet between turbidity flows. If, however, under high magnification the shales exhibit micro-graded bedding, they may be considered as an integral part of the turbidite bed.
      The beds in the study area generally strike east-west and have an average dip of 75° to the north. However, the largest clasts are at the top of each graded arenite or pebble conglomerate bed. This indicates that the beds are overturned, since in graded bedding the particles grade from coarse at the bottom to fine at the top of the bed.
      Current direction structures: The most apparent current direction indicators in the sequence are flamie structures. These features show that the current moved from the northwest to the southeast. Slump or drag folds also occur and indicate either that the beds were deposited on an inclined slope and slumped or the current drag caused the deformation. Since these structures point in the same direction as the flame structures the current drag is the probable action.
      Clasts of silty shale in the arenite beds have an observed orientation or imbrication. It has bed reported that planar and linear clasts tend to dip upcurrent (Compton, 1962, p.227). The observed chips dip to the north or northwest.
      Scour channels and load pockets are apparent in the beds. Their linear trends parallel current directions. In the study area the scour channels appear to be oriented in a north-south direction when exposed in three dimensions.


      The Puente Formation in the study area is characterized by a great thickness of siltstone and silty shales interrupted by turbidite beds. Sediments of the bathyal depth zone are generally composed of gray terrigenous material silt size or finer, as opposed to the coarser clastics of the neritic zone (Dunbar and Rodgers, 1957, p.47-60). Thus, the environment of deposition for the Puente Formation is bathyal (greater than 200 fathoms), close to a slope steep enough to generate a moving mass with enough speed and momentum to produce a turbid flow.
      In the discussion of sedimentary structures it was shown that at various periods in the depositional history of the Puente Formation high energy currents existed which moved beach type sand from an area to the north or northwest. A possible source for the clastics in the study area could be the western San Gabriel Mountains or Verdugo Hills area, since they contain rock types similar to those found in the pebble conglomerate (Woodford and others, 1946, p.547-548).


Compton, R.R.
      1962. Manual of Field Geology, New York, John Wiley & Sons, Inc., 378p.

Dunbar, C.O. and Rodgers, J.
      1957. Principles of Stratigraphy, New York,, John Wiley & Sons, Inc., 356p.

Friedman, G.M.
      1961. Distinction between dune, beach, and river sands from their textural characteristics: Jour. Sed. Petrology, vol. 31, p. 514-529.

      1962. On sorting, sorting coefficients, and the log normality of the grain-size distribution of sandstones: Jour. Geol., vol. 70, p. 737-753.

Hoque, M.U.
      1968. Sedimentologic and paleocurrent study of Mauch Chunk sandstones (Mississippian), south-central and western Pennsylvania: Am. Assoc. Petroleum Geol. Bull., vol. 52, no. 2, p. 246-263.

Krumbein, W.C. and Pettijohn, F.J.
      1938. Manual of Sedimentary Petrography, New York, Appleton Century Co., Inc.

Natland, M.L.
      1967. New classification of water-laid clastic sediments (Abs.): Am. Assoc. Petroleum Geol. Bull., vol. 51, no. 3. pt. 1, p. 476.

Woodford, A.O., Moran, T.G. and Shelton, J.S.
      1946. Miocene conglomerates of Puente and San Jose Hills, California: Am. Assoc. Petroleum Geol. Bull., vol. 30, no. 4. p. 514-560.

Yerkes, R.F., McCulloh, T.H., Schoellhamer, J.E. and Vedder, J.G.
      1965. Geology of the Los Angeles Basin, California - an introduction: U.S. Geol. Survey Prof. Paper 420-A, p. Al-A57.

¹Department of Geology, California State College at Los Angeles, 90032.

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